Restart method and control device for direct-current line fault of high-voltage direct-current power transmission system

Abstract

The present application relates to the technical field of high-voltage direct-current power transmission, and provides a restart method and control device for a direct-current line fault of a high-voltage direct-current power transmission system. When a ground fault occurs in a direct-current line, the restart method comprises: at a first restart stage, controlling a faulty direct-current pole to be restarted; when the restart frequency is reached but the restart is still not successful, if a dual direct-current pole power command value is greater than the maximum deliverable power of a non-faulty direct-current pole, at a second restart stage, adjusting the dual direct-current pole power command value to be the maximum deliverable power of the non-faulty direct-current pole, and controlling the faulty direct-current pole to be restarted. The controlling the faulty direct-current pole to be restarted comprises: controlling at least one current converter at each of two ends of the ground fault to continue to operate, and determining direct current reference values of the converters; on the basis of the direct current reference values and within a deionization time, controlling the direct current of the converters to be equal or the difference to be smaller than a first current threshold, or controlling the direction current of the converters to be zero; and increasing the direct-current voltage and / or the direct-current power of at least one converter within a restart time.

Description

Method and control device for restarting DC line fault in high-voltage DC transmission system Technical Field

[0001] The present application relates to the technical field of high-voltage direct current (HVDC) transmission, and in particular to a method and a control device for restarting a DC line fault in a high-voltage direct current (HVDC) transmission system. Background Art

[0002] Existing HVDC systems are categorized as conventional DC systems and flexible DC systems. HVDC systems are typically equipped with two DC poles, each of which can form an independent circuit via a ground return or metallic return line. When a ground fault occurs in the DC line of a dual-DC-pole HVDC system, existing technologies only implement a restart phase (the first restart stage) that maintains the dual-DC pole power command at the pre-fault dual-DC pole power level. Due to the power imbalance between the AC and DC systems during the restart process, the number of restarts and the duration must be set based on the surge capacity of the AC system to which the HVDC system is connected. When the restart count reaches the set value, the converter on the faulty DC pole is locked, reverting to single-DC-pole operation. When the dual-DC pole power is high, the time the sending and receiving AC systems of the HVDC system can withstand the power surge is generally very short. If a successful restart fails within a short period, the faulty DC pole will be locked. However, the causes of DC line failures are complex, and some faults cannot be successfully restarted within such a short period. Therefore, under some fault conditions where the arc cannot be extinguished in a short time, there is a contradiction between the ability of the AC system connected to the HVDC transmission system to withstand power shocks and the inability of the HVDC transmission system to be successfully restarted in a short time.

[0003] Existing conventional DC transmission systems include bipolar conventional DC transmission systems and symmetrical monopolar conventional DC transmission systems, while flexible DC transmission systems include bipolar flexible DC transmission systems and symmetrical monopolar flexible DC transmission systems. Bipolar conventional DC transmission systems use grid-commutated converters with two DC poles, each of which can independently form a circuit via a ground return line or a metallic return line. When a ground fault occurs in the DC line of a bipolar conventional DC transmission system, existing technology controls the DC line current to zero by controlling the phase shift of the grid-commutated converter. After a certain de-isolation time, the conventional DC transmission system is restarted by releasing the phase shift. During the de-isolation process, because the grid-commutated converter consumes zero reactive power while the AC filter is not removed, a large amount of reactive power will be generated, which may cause severe overvoltage and a large inrush current to the ground. Furthermore, if a lightning strike occurs again during the phase shift period, the DC line will generate severe overvoltage. If the DC line suffers multiple ground faults when a lightning strike occurs, the phase shift of the grid-commutated converter will not extinguish the arc. The bipolar flexible direct current transmission system uses a voltage source converter and is equipped with two DC poles. Each DC pole can independently form a loop through an earth return line or a metal return line. When a ground fault occurs in the DC line of the bipolar flexible direct current transmission system, the existing technology controls and reduces the converter voltage to make the DC current zero, which will also cause a large impact current in the earth. At the same time, during the restart process, the minimum current limit value of the low-voltage current limiting link is large. If the restart is unsuccessful, it will also cause a large secondary impact on the fault point, which is not conducive to arc extinguishing.

[0004] If a bipolar conventional direct current transmission system or a bipolar flexible direct current transmission system adopts a UHVDC transmission system with high-end and low-end valve groups in series for each pole, and the high-end and low-end valve groups are constructed in stations and connected by a DC line, if the DC line fails, the high-end valve group will not be able to transmit active power by shifting the phase or controlling the DC current to zero.

[0005] Symmetrical monopole conventional DC transmission systems and symmetrical monopole flexible DC transmission systems lack earth return or metallic return lines. Instead, they provide a clamping point at the midpoint of a station through internal grounding. When a ground fault occurs on one DC line, existing technologies, such as phase shifting or controlling to reduce converter voltage, interrupt power on the other DC line, leading to a power outage in the DC transmission system. Summary of the Invention

[0006] In order to solve at least one of the above problems, the present application proposes a method and a control device for restarting a DC line fault in a high-voltage direct current transmission system.

[0007] According to a first aspect of the present application, at least one embodiment of the present application provides a method for restarting a DC line fault in a high-voltage direct current transmission system, wherein the high-voltage direct current transmission system includes at least one rectifier station and at least one inverter station, wherein the rectifier station and the inverter station both include dual DC poles, and each DC pole in the dual DC poles includes at least one converter; the restart method includes: if a ground fault occurs in a DC line connected to one of the DC poles, and the other DC pole is a non-fault DC pole and operates in dual DC pole power control, in a first restart phase, controlling the faulty DC pole of the high-voltage direct current transmission system to restart, and the dual DC poles The power command value of the DC pole is the dual DC pole power before the fault; when the restart number set in the first restart stage is reached and the restart is still not successful, if the dual DC pole power command value is greater than the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole, in the second restart stage, the dual DC pole power command value is adjusted to the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole, and the faulty DC pole of the high-voltage direct current transmission system is controlled to restart; when the restart number set in the second restart stage is reached and the restart is still not successful, the converter of the faulty DC pole is controlled to be locked.

[0008] For example, in some embodiments of the present application, the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole is less than or equal to the short-time overload power, and the short-time overload power is any one of a second-level overload power, a minute-level overload power, or an hour-level overload power.

[0009] For example, in some embodiments of the present application, the step of controlling the maximum transmittable power to be less than or equal to the short-time overload power includes: controlling the DC current of the non-fault DC pole or the dual DC pole to be less than or equal to the short-time overload current, and the short-time overload current is any one of a second-level overload current, a minute-level overload current, or an hour-level overload current.

[0010] For example, in some embodiments of the present application, during the second restart phase, the dual DC pole power instruction value is the maximum deliverable power of the non-fault DC pole or the maximum deliverable power of the dual DC pole and remains unchanged, and is less than or equal to the short-term overload power during the second restart phase.

[0011] For example, in some embodiments of the present application, the number of restarts and time in the first restart phase are determined according to the conditions of the AC system to which the high-voltage direct current transmission system is connected; the number of restarts and time in the second restart phase are determined according to the arc extinguishing time of the fault condition in which the arc cannot be extinguished in the first restart phase.

[0012] For example, in some embodiments of the present application, after adjusting the dual DC pole power instruction value to the maximum deliverable power of the non-fault DC pole or the maximum deliverable power of the dual DC pole, the method further includes: synchronously sending the reduction signal and the reduction power to the safety and stability control system, or / and the frequency emergency control system, or / and the main control system.

[0013] For example, in some embodiments of the present application, the fallback power is equal to the dual DC pole power before the fault minus the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole.

[0014] For example, in some embodiments of the present application, in the first restart phase or the second restart phase, controlling the restart of the faulty DC pole of the HVDC transmission system includes: controlling at least one converter at each end of the faulty DC pole to continue operating; determining DC current reference values ​​of the two converters at both ends of the faulty DC pole based on the needs of the HVDC transmission system, and performing the following restart actions: controlling the DC currents of the two converters to be equal or to have a difference less than a first current threshold value based on the DC current reference values ​​of the two converters, or controlling the DC currents of the two converters to be zero within a de-isolation time; increasing the DC voltage and / or DC power of the two converters within a restart time; or including: controlling at least one converter at both ends of the faulty DC pole to be temporarily blocked or / and the DC current of at least one converter to be zero; attenuating the fault current by utilizing the resistance of the DC line or the resistance of the switched-in converter within a de-isolation time; and controlling at least one converter at both ends of the faulty DC pole to be unlocked and to increase the DC voltage and / or DC power, or / and controlling at least one converter to increase the DC voltage and / or DC power within a restart time.

[0015] For example, in some embodiments of the present application, the requirements of the high-voltage direct current transmission system include: at least one of: active power requirement, reactive power requirement, ground current limit value requirement, current limit value requirement of the fault DC pole, current limit value requirement flowing through the fault point, and AC harmonic suppression requirement.

[0016] For example, in some embodiments of the present application, if the demand of the high-voltage direct current transmission system is one, the demand of the high-voltage direct current transmission system is any one of the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the fault DC pole, the current limit value demand flowing through the fault point and the AC harmonic suppression demand of the rectifier station or the inverter station, and it is determined that the DC current reference values ​​of the two converters at both ends of the fault DC pole are equal.

[0017] For example, in some embodiments of the present application, if the demand of the high-voltage direct current transmission system considers the demands of the rectifier station and the inverter station at the same time, and priorities of different demands are given at the same time; the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the fault DC pole, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand are unified demands, and it is determined that the DC current reference values ​​of the two converters at both ends of the fault DC pole are equal; or, the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the fault DC pole, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand include the demand of the rectifier station and the demand of the inverter station, and it is determined that the DC current reference values ​​of the two converters at both ends of the fault DC pole are unequal and the difference is less than the first current threshold, and the first current threshold is less than the current limit value flowing through the fault point.

[0018] For example, in some embodiments of the present application, increasing the DC voltage of the two converters includes: controlling the converter that was operating under DC voltage control before the fault to operate under DC voltage control, directly giving or giving according to a climbing slope the DC voltage reference value of the two converters as the DC voltage before the fault or the DC voltage that is lower than before the fault; or / and, achieving this by giving the difference in DC current reference values ​​of the two converters.

[0019] For example, in some embodiments of the present application, controlling the DC current of the two converters to be zero includes: if the two converters are grid-commutated converters, it is achieved by controlling the phase shift of the grid-commutated converter of the rectifier station and the trigger angle of the grid-commutated converter of the inverter station to be greater than 90 degrees; if the two converters are voltage source converters, it is achieved by controlling the DC voltage of the two converters to be zero, controlling the DC current of the two converters to be zero, or controlling the two converters to be locked.

[0020] According to a second aspect of the present application, at least one embodiment of the present application provides a flexible restart method for a DC line fault in a high-voltage direct current transmission system, the high-voltage direct current transmission system including at least one rectifier station and at least one inverter station, the rectifier station and the inverter station including a single DC pole or a double DC pole, the DC pole including at least one converter; when a ground fault occurs in the DC line, the flexible restart method includes: controlling at least one converter at each end of the ground fault to continue operating; determining a DC current reference value of two converters at both ends of the ground fault based on the needs of the high-voltage direct current transmission system, the two converters including one converter that continues to operate at each end of the ground fault; controlling the DC currents of the two converters to be equal or to have a difference less than a first current threshold value based on the DC current reference values ​​of the two converters within a first de-isolation time, or controlling the DC currents of the two converters to be zero; after the first de-isolation time, increasing the DC voltage of at least one of the two converters within a first restart time; after the first restart time, if the absolute value of the DC voltage of the DC line is greater than The method comprises the steps of: if the absolute value of the DC voltage of the DC link is less than the first voltage threshold, and the number of restarts is greater than one, continuing to control the DC currents of the two converters to be equal or to have a difference less than the first current threshold based on the DC current reference values ​​of the two converters within a second de-isolation time, or controlling the DC currents of the two converters to be zero, and controlling the two converters to be blocked when the number of restarts is one; if the number of restarts is greater than one, after the second de-isolation time, increasing the DC voltage of at least one of the two converters within a second restart time; if the absolute value of the DC voltage of the DC link is less than the first voltage threshold, and the number of restarts is greater than two, continuing to control the DC currents of the two converters to be equal or to have a difference less than the first current threshold based on the DC current reference values ​​of the two converters within a third de-isolation time, or controlling the DC currents of the two converters to be zero, and controlling the two converters to be blocked when the number of restarts is two; and so on, until the flexible restart of the HVDC transmission system is achieved.

[0021] For example, in some embodiments of the present application, the converter includes at least one of a grid-commutated converter or a voltage source converter.

[0022] For example, in some embodiments of the present application, if at least one of the two converters of the HVDC transmission system is a grid-commutated converter, then after controlling the DC currents of the two converters to be equal or the difference between them is less than a first current threshold based on the DC current reference values ​​of the two converters or before controlling the two converters to be locked, the method further includes: controlling the HVDC transmission system to cut off or put into operation an AC filter connected to the AC system according to the needs of the AC system; if at least one of the two converters of the HVDC transmission system is a voltage source converter, after controlling the DC currents of the two converters to be equal or the difference between them is less than a first current threshold based on the DC current reference values ​​of the two converters or before controlling the two converters to be locked, the method further includes: controlling the reactive power or AC voltage output by the voltage source converter according to the needs of the AC system.

[0023] For example, in some embodiments of the present application, the AC system demand includes reactive power demand and AC voltage limit.

[0024] For example, in some embodiments of the present application, the occurrence of a ground fault in the DC line is determined by detecting a protection action, and the protection includes at least one of: line sudden change protection, line traveling wave protection, line low voltage protection, and line longitudinal differential protection.

[0025] For example, in some embodiments of the present application, the requirements of the HVDC transmission system include: at least one of active power requirement, reactive power requirement, ground current limit value requirement, current limit value requirement of the DC pole where the fault is located, current limit value requirement flowing through the fault point, and AC harmonic suppression requirement; wherein, if the requirements of the HVDC transmission system are more than one, priorities of different requirements are given at the same time; if the requirements of the HVDC transmission system only consider the requirements of the rectifier station, the active power requirement, the reactive power requirement, the ground current limit value requirement, the current limit value requirement of the DC pole where the fault is located, the current limit value requirement flowing through the fault point, and the AC harmonic suppression requirement are the requirements of the rectifier station, and the DC current reference values ​​of the two converters are equal; if the requirements of the HVDC transmission system only consider the requirements of the inverter station, the active power requirement, the reactive power requirement, the ground current limit value requirement, the current limit value requirement of the DC pole where the fault is located, the current limit value requirement flowing through the fault point, and the AC harmonic suppression requirement are the requirements of the rectifier station, and the DC current reference values ​​of the two converters are equal. The current limit value requirement at the fault point and the AC harmonic suppression requirement are the requirements of the inverter station, and the DC current reference values ​​of the two converters are equal; if the requirements of the high-voltage direct current transmission system simultaneously consider the requirements of the rectifier station and the inverter station, when processed uniformly, the active power requirement, the reactive power requirement, the ground current limit value requirement, the current limit value requirement of the DC pole where the fault is located, the current limit value requirement flowing through the fault point, and the AC harmonic suppression requirement are the requirements after unified processing, and the DC current reference values ​​of the two converters are equal; when processed separately, the active power requirement, the reactive power requirement, the ground current limit value requirement, the current limit value requirement of the DC pole where the fault is located, the current limit value requirement flowing through the fault point, and the AC harmonic suppression requirement include the requirements of the rectifier station and the inverter station, the DC current reference values ​​of the two converters are unequal and the difference is less than a first current threshold, and the first current threshold is less than the current limit value flowing through the fault point.

[0026] For example, in some embodiments of the present application, if the DC current reference values ​​of the two converters determined by the demand of the high-voltage direct current transmission system are equal, the DC currents of the two converters are controlled to be equal; if the DC current reference values ​​of the two converters determined by the demand of the high-voltage direct current transmission system are not equal, the DC current difference between the two converters is controlled to be smaller than a first current threshold, and the first current threshold is smaller than a current limit value flowing through the fault point.

[0027] For example, in some embodiments of the present application, determining DC current reference values ​​of the two converters at both ends of the ground fault based on the demand of the HVDC transmission system includes: if the demand of the HVDC transmission system is an active power demand, dividing the active power demand by the sum of the absolute values ​​of the DC voltages of all operating converters of the rectifier station or the inverter station to obtain the DC current reference values ​​of the two converters; or, if the demand of the HVDC transmission system is a reactive power demand and the converter is a six-pulse or twelve-pulse grid-commutated converter, determining the DC current reference values ​​of the two converters based on the reactive power demand, the no-load DC bus voltage, the trigger angle or the turn-off angle, and the commutation angle. The calculation method for the converter of the rectifier station is as follows:

[0028] The calculation method of the converter of the inverter station is as follows:

[0029] Where, I ord is the DC current reference value, Q conv is the reactive power requirement of the six-pulse or twelve-pulse grid-commutated converter, U di0 is the no-load DC bus voltage of a six-pulse or twelve-pulse grid-commutated converter, α is the trigger angle of the converter, μ is the commutation angle of the converter, γ is the turn-off angle of the converter, when the converter is a six-pulse grid-commutated converter, b=1 / 4, when the converter is a twelve-pulse grid-commutated converter, b=1 / 2; or, if the demand of the high-voltage direct current transmission system is a ground current limit value demand, the DC current reference value of each converter of the two converters is greater than the difference between the DC current of another DC pole at the same station and the ground current limit value, and less than the difference between the DC current of another DC pole at the same station and the ground current limit value. the sum of the current limit values; or, if the demand of the HVDC transmission system is a current limit value demand for the DC pole where the fault is located, determining that the DC current reference values ​​of the two converters are less than the current limit value of the DC pole where the fault is located; or, if the demand of the HVDC transmission system is a current limit value demand for flowing through the fault point, determining that the difference between the DC current reference values ​​of the two converters is less than the current limit value flowing through the fault point; or, if the demand of the HVDC transmission system is an AC harmonic suppression demand, converting the AC harmonic suppression demand into a DC current reference value for limiting the two converters.

[0030] For example, in some embodiments of the present application, determining the DC current reference values ​​of the two converters at both ends of the ground fault based on the requirements of the high-voltage direct current transmission system also includes: based on the DC current reference values ​​of the two converters determined according to the active power requirement, the reactive power requirement, the ground current limit value requirement, the current limit value requirement of the DC pole where the fault is located, the current limit value requirement flowing through the fault point, and the AC harmonic suppression requirement, superimposing a current greater than zero and with an absolute value less than 0.1 times the rated DC current on the DC current reference value of at least one of the converters.

[0031] For example, in some embodiments of the present application, the DC current of the converter includes: at least one of the high-voltage bus current, the low-voltage bus current, the pole bus current of the DC pole where the converter is located, or the pole neutral bus current.

[0032] For example, in some embodiments of the present application, the first current threshold is 0.01 to 0.1 times the rated DC current; the first voltage threshold is 0.05 to 1.0 times the rated DC voltage; the first deionization time is 20 to 500 ms; the first restart time is 20 to 300 ms; the second deionization time is 50 to 500 ms; and the second restart time is 20 to 300 ms.

[0033] For example, in some embodiments of the present application, controlling the DC currents of the two converters to be equal or having a difference less than a first current threshold based on the DC current reference values ​​of the two converters includes: respectively controlling the two converters to operate in DC current control based on the DC current reference values ​​of the two converters.

[0034] For example, in some embodiments of the present application, increasing the DC voltage of at least one of the two converters includes: controlling the converter of the two converters that was operating under DC voltage control before the fault to operate under DC voltage control, directly giving or giving according to a climbing slope the DC voltage reference value of the two converters as the DC voltage before the fault or the DC voltage lower than before the fault; or giving the difference in DC current reference values ​​of the two converters; or controlling the converter of the two converters that was operating under DC current control or active power control before the fault to operate under DC current control.

[0035] For example, in some embodiments of the present application, if the two converters are both grid-commutated converters, the difference in the DC current reference values ​​given to the two converters is specifically achieved by reducing the DC current reference value of the DC current controller of the grid-commutated converter that was operating under DC voltage control or maximum trigger angle control before the fault in the two converters.

[0036] For example, in some embodiments of the present application, within the de-ionization time of the flexible restart method, if the DC current of the two converters is controlled to be zero, then within the restart time of the flexible restart method, when the DC voltage of at least one of the two converters is increased, the DC current of one of the two converters is controlled to be zero, and the DC current of the other of the two converters is controlled to be less than 0.25 pu.

[0037] For example, in some embodiments of the present application, controlling the DC current of the other converter of the two converters to be less than 0.25 pu is achieved by reducing the minimum DC current limit of the low-voltage current limiting link of the other converter of the two converters to be less than 0.25 pu.

[0038] For example, in some embodiments of the present application, controlling the DC current of the two converters to be zero includes: if the two converters are grid-commutated converters, it is achieved by controlling the phase shift of the grid-commutated converter of the rectifier station and the trigger angle of the grid-commutated converter of the inverter station to be greater than 90 degrees; if the two converters are voltage source converters, it is achieved by controlling the DC voltage of the two converters to be zero or controlling the DC current of the two converters to be zero.

[0039] For example, in some embodiments of the present application, if the converter is a grid-commutated converter, the converter locking includes: controlling the grid-commutated converter to stop sending trigger pulses, and / or controlling the grid-commutated converter to enter the bypass pair; if the converter is a voltage source converter, the converter locking includes: controlling the voltage source converter to stop sending trigger pulses.

[0040] For example, in some embodiments of the present application, during the de-freeing time of the flexible restart method, when any one of the two converters loses the ability to control DC current due to an AC system fault, if the converter of the rectifier station is a grid-commutated converter, the converter of the rectifier station is controlled to shift phase; if the converter of the inverter station is a grid-commutated converter, the converter trigger angle of the inverter station is controlled to be greater than 90 degrees.

[0041] According to a third aspect of the present application, at least one embodiment of the present application provides a high-voltage direct current transmission system DC line fault restart control device, the high-voltage direct current transmission system includes at least one rectifier station and at least one inverter station, the rectifier station and the inverter station both include dual DC poles, and each DC pole in the dual DC poles includes at least one converter; the control device is used to execute the high-voltage direct current transmission system DC line fault restart control method as described in any one of the first aspects, the control device includes: a first control unit, used to control the high voltage direct current transmission system when a ground fault occurs in the DC line connected to one DC pole and the other DC pole is a non-fault DC pole and operates in dual DC pole power control, in a first restart phase, The faulty DC pole of the DC transmission system is restarted, and the dual DC pole power instruction value is the dual DC pole power before the fault; an adjustment unit is used to adjust the dual DC pole power instruction value to the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole when the number of restarts set in the first restart stage is reached and the restart is still not successful, if the dual DC pole power instruction value is greater than the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole; a second control unit is used to control the restart of the faulty DC pole in the second restart stage; a third control unit is used to control the converter of the faulty DC pole to be locked when the number of restarts set in the second restart stage is reached and the restart is still not successful.

[0042] According to a fourth aspect of the present application, at least one embodiment of the present application provides a high-voltage direct current transmission system DC line fault restart control device, the high-voltage direct current transmission system includes at least one rectifier station and at least one inverter station, the rectifier station and the inverter station each include a dual DC pole, each DC pole in the dual DC pole includes at least one converter; the control device is used to perform the high-voltage direct current transmission system DC line fault flexible restart control method as described in any one of the second aspects, the control device includes: a detection unit, for detecting parameters of the high-voltage direct current transmission system; a fourth control unit, for detecting parameters of the high-voltage direct current transmission system based on the fourth control unit; The method comprises the following steps: determining parameters of a high-voltage direct current transmission system: when a fault occurs in a DC line of a high-voltage direct current transmission system, controlling at least one converter at each end of the ground fault to continue operating; determining DC current reference values ​​of two converters at each end of the ground fault based on the needs of the high-voltage direct current transmission system, wherein the two converters include a converter that continues to operate at each end of the ground fault; controlling the DC currents of the two converters to be equal or to have a difference less than a first current threshold value based on the DC current reference values ​​of the two converters within a first de-isolation time, or controlling the DC currents of the two converters to be zero; and after the first de-isolation time, controlling the DC currents of the two converters to be equal or to have a difference less than a first current threshold value within a first de-isolation time. Increase the DC voltage of at least one of the two converters within the start-up time; after the first restart time, if the absolute value of the DC voltage of the DC line is greater than or equal to the first voltage threshold, continue to increase the DC voltage of the two converters; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, when the number of restarts is greater than 1, continue to control the DC currents of the two converters to be equal or the difference is less than the first current threshold based on the DC current reference values ​​of the two converters within the second de-free time, or control the DC currents of the two converters to be zero, and when the number of restarts is 1, control the two converters Lock; when the number of restarts is greater than 1, after the second de-isolation time, increase the DC voltage of at least one of the two converters within the second restart time; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, when the number of restarts is greater than 2, continue to control the DC currents of the two converters to be equal or the difference is less than the first current threshold based on the DC current reference values ​​of the two converters within the third de-isolation time, or control the DC currents of the two converters to be zero, and when the number of restarts is 2, control the two converters to be locked; and so on, to achieve flexible restart of the HVDC transmission system.

[0043] Beneficial effects: The present application discloses a method for restarting a DC line fault in a high-voltage direct current transmission system. The high-voltage direct current transmission system includes at least one rectifier station and at least one inverter station, each of the rectifier station and the inverter station includes a dual DC pole, and each DC pole in the dual DC pole includes at least one converter. The restart method includes: if a ground fault occurs in a DC line connected to a DC pole, and the other DC pole is a non-fault DC pole and operates in dual DC pole power control, controlling the faulty DC pole of the high-voltage direct current transmission system to restart at the original power; when the restart is not successful after reaching a set number of original power restarts, if the dual DC pole power is greater than the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole, in a second restart phase, adjusting the dual DC pole power command value to the maximum transmittable power, and controlling the faulty DC pole of the high-voltage direct current transmission system to restart at a reduced power; when the restart is still not successful after reaching a set number of restarts, controlling the converter of the faulty DC pole to lock. When a ground fault occurs in the DC line of the HVDC transmission system and the first restart stage cannot guarantee the successful restart of the DC, a second restart stage is added, the power instruction value of the dual DC pole is adjusted to the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole, the deionization time is increased, and the success rate of the DC line restart is further improved. At the same time, the overload capacity of the non-fault DC pole is utilized to maintain more power transmission after a successful restart.

[0044] The present application also discloses a flexible restart method for a DC line fault in a high-voltage direct current transmission system, comprising: controlling at least one converter at each end of a ground fault to continue operating; determining a DC current reference value for two converters at each end of the ground fault based on the needs of the high-voltage direct current transmission system, the two converters including one converter that continues to operate at each end of the ground fault; controlling the DC currents of the two converters to be equal or to have a difference less than a first current threshold based on the DC current reference value; increasing the DC voltage of the two converters within a first restart time after a first deionization time; and continuing to restart or lock according to the above method if the absolute value of the DC voltage of the DC line is less than the first voltage threshold. In the present application, when a ground fault occurs in a DC line of a high-voltage direct current transmission system, the voltage at the ground fault point can be effectively controlled to achieve de-ionization by controlling the DC currents of the two converters at both ends of the ground fault to be equal or to have a difference less than a first current threshold. At the same time, since DC current still flows through the DC line, the grid commutation converter continues to consume reactive power, and the high-voltage direct current transmission system can maintain AC and DC reactive power balance. The present application also has the advantages of preventing the DC line from being struck by severe overvoltage caused by lightning again during the de-ionization period and preventing the failure of de-ionization when multiple faults occur in the DC line due to lightning strikes. In the event of a ground fault in a DC line between the high- and low-end valve group substation construction of an ultra-high voltage direct current system, or a ground fault in a single DC line of a symmetrical single-pole direct current system, the faulty DC line can be used to maintain the transmission of some active power. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] By describing in detail exemplary embodiments thereof with reference to the accompanying drawings, the above and other objects, features and advantages of the present application will become more apparent. The drawings described below are only some embodiments of the present application, and are not intended to limit the present application.

[0046] FIG1 is a flow chart of a method for restarting a DC line fault in a high-voltage DC transmission system according to an embodiment of the present application;

[0047] FIG2 is a schematic diagram of a specific restart phase flow chart of the method for restarting a DC line fault in the HVDC transmission system of FIG1 ;

[0048] 3 is a schematic structural diagram of a DC line fault restart control device for a HVDC power transmission system according to an embodiment of the present application;

[0049] FIG4 is another structural diagram of a device for restarting a DC line fault in a HVDC transmission system according to an embodiment of the present application;

[0050] FIG5 is a schematic diagram of a main circuit of a high-voltage direct current transmission system according to an embodiment of the present application;

[0051] FIG6 is a DC line fault simulation test result in the related art;

[0052] FIG7 is a DC line fault simulation test result of a method for restarting a DC line fault in a HVDC power transmission system according to an embodiment of the present application;

[0053] FIG8 is a schematic diagram of a main circuit of a high-voltage direct current transmission system according to an embodiment of the present application;

[0054] FIG9 is a flow chart of a method for flexible restarting a DC line fault in a high-voltage DC transmission system according to an embodiment of the present application;

[0055] FIG10 is a diagram showing simulation test results when a DC line grounding fault occurs in the UHVDC transmission system shown in FIG8 under the prior art;

[0056] FIG11 is a diagram showing simulation test results when a DC line grounding fault occurs in the UHVDC power transmission system shown in FIG8 according to an embodiment of the present application;

[0057] FIG12 is a diagram showing simulation test results when a DC line between the first high-end valve group and the first low-end valve group is grounded under the prior art;

[0058] FIG13 is a diagram showing simulation test results when a DC line between the first high-end valve group and the first low-end valve group is grounded according to an embodiment of the present application;

[0059] FIG14 is a schematic diagram of the main circuit of another high-voltage direct current transmission system according to an embodiment of the present application;

[0060] FIG15 is a flow chart of another method for flexible restarting of a DC line fault in a HVDC transmission system according to an embodiment of the present application;

[0061] FIG16 is a diagram showing simulation test results of restarting once when a DC line grounding fault occurs in the HVDC transmission system shown in FIG14 under the prior art;

[0062] FIG17 is a diagram showing simulation test results of the HVDC transmission system shown in FIG14 of an embodiment of the present application being restarted once when a DC line grounding fault occurs;

[0063] FIG18 is a diagram showing simulation test results of the HVDC transmission system shown in FIG14 according to an embodiment of the present application being restarted twice when a DC line grounding fault occurs;

[0064] FIG19 is a diagram showing simulation test results of the HVDC transmission system shown in FIG14 according to an embodiment of the present application being locked when a DC line grounding fault occurs;

[0065] FIG20 is a schematic structural diagram of a DC side grounding fault control device for a high-voltage DC transmission system according to an embodiment of the present application. DETAILED DESCRIPTION

[0066] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of this application. In addition, in the description of the present application, the term "including" means "including but not limited to". In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application.

[0067] It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of the embodiments.

[0068] In the related art, HVDC systems are categorized as conventional DC systems and flexible DC systems. HVDC systems are typically equipped with two DC poles, each of which can form an independent circuit via a ground return or metallic return line. When a ground fault occurs in the DC line of a dual-pole HVDC system, existing technologies only provide a restart phase, maintaining the dual-pole power command at the pre-fault dual-pole power level. Due to the power imbalance between the AC and DC systems during the restart process, the number of restarts and the duration must be set based on the surge tolerance of the AC system connected to the HVDC system. When the restart count reaches the set value, the converter on the faulty DC pole is locked out, reverting to single-pole operation. When the dual-pole power is high, the time it takes for the sending and receiving AC systems of the HVDC system to withstand the power surge is very short. If a successful restart fails within a short period, the faulty DC pole will be locked out. However, the causes of DC line failures are complex, and some faults cannot be successfully restarted within such a short period. Therefore, under certain fault conditions, there is a conflict between the AC system's ability to withstand the power surge and the HVDC system's inability to successfully restart within a short period. Conventional DC transmission systems use grid-commutated converters. When a ground fault occurs in a DC line, phase shifting is used to control the current in the DC line to zero. After a certain de-ionization time, the phase shifting is released to restart the conventional DC transmission system. During the de-ionization process, because the grid-commutated converter consumes zero reactive power and the AC filter is not removed, a large amount of residual reactive power will be generated, which may cause severe overvoltage and a large inrush current to the ground.

[0069] In view of this, embodiments of the present application provide a method and a control device for restarting a DC line fault in a high-voltage direct current transmission system, aiming to solve at least one of the above-mentioned technical problems.

[0070] A method and control device for restarting a DC line fault in a high-voltage DC transmission system of the present invention will be described in detail below with reference to Figures 1 to 7. The features of the following embodiments and implementations may be combined with each other unless they conflict.

[0071] A method for restarting a DC line fault in a high-voltage direct current (HVDC) transmission system is disclosed. The HVDC transmission system includes at least one rectifier station 100 and at least one inverter station 200. The rectifier station 100 and the inverter station 200 both include dual DC poles, and each of the dual DC poles includes at least one converter.

[0072] 1 , 2 , and 5 , the restart method includes: if a ground fault occurs in a DC line connected to a DC pole (a faulty DC pole) and the other DC pole is a non-faulty DC pole and is operating in dual-DC pole power control, in a first restart phase, controlling the faulty DC pole of the HVDC transmission system to restart, with the dual-DC pole power command value being the dual-DC pole power before the fault; if the restart fails after a set number of restarts in the first restart phase is reached, if the dual-DC pole power command value is greater than the maximum transmittable power of the non-faulty DC pole, in a second restart phase, adjusting the dual-DC pole power command value to the maximum transmittable power of the non-faulty DC pole; if the dual-DC pole power command value is less than or equal to the maximum transmittable power of the non-faulty DC pole, in the second restart phase, maintaining the dual-DC pole power command value at the dual-DC pole power before the fault, and controlling the faulty DC pole of the HVDC transmission system to restart; and if the restart fails after a set number of restarts in the second restart phase is reached, controlling the converter of the faulty DC pole to lock.

[0073] In some embodiments, a restart method includes: if a ground fault occurs in a DC line connected to a DC pole (a faulty DC pole) and the other DC pole is a non-faulty DC pole and operates in dual DC pole power control, in a first restart phase, controlling the faulty DC pole of the HVDC transmission system to restart, and a dual DC pole power command value is the dual DC pole power before the fault; when a restart attempt is reached after a set number of restarts in the first restart phase and the restart is still unsuccessful, if the dual DC pole power command value is greater than the maximum transmittable power of the dual DC pole, specifically, if the faulty DC pole is restarted by voltage reduction (including reducing the DC voltage or retiring a valve group) and the overload capacity of the dual DC pole is insufficient to bear the DC power loss due to the voltage reduction, in a second restart phase, adjusting the dual DC pole power command value to the maximum transmittable power of the dual DC pole; if the dual DC pole power command value is less than or equal to the maximum transmittable power of the dual DC pole, in the second restart phase, maintaining the dual DC pole power command value at the dual DC pole power before the fault, and controlling the faulty DC pole of the HVDC transmission system to restart.

[0074] In some embodiments, the maximum transmittable power of a non-fault DC pole or the maximum transmittable power of a dual DC pole is subject to an overload limit, and the maximum transmittable power is less than or equal to the short-term overload power, where the short-term overload power is any one of a second-level overload power, a minute-level overload power, or an hour-level overload power. Specifically, the step of controlling the maximum transmittable power to be less than or equal to the short-term overload power includes controlling the maximum DC current of the non-fault DC pole or the dual DC pole to be less than or equal to the short-term overload current, where the short-term overload current is any one of a second-level overload current, a minute-level overload current, or an hour-level overload current.

[0075] In some embodiments, during the second restart phase, the dual DC pole power command value is the maximum deliverable power of the non-fault pole or the maximum deliverable power of the dual DC pole and remains unchanged, and is less than or equal to the short-term overload power during the second restart phase.

[0076] In some embodiments, the number of restarts and the time of the first restart phase are determined according to the conditions of the AC system to which the HVDC transmission system is connected, that is, in the first restart phase, the power shortage of the AC system and the DC system will not affect the safe operation of the power system; the number of restarts and the time of the second restart phase are determined according to the arc extinction time of the fault condition that cannot be arc extinguished in the first restart phase, that is, in the second restart phase, for non-permanent line faults, de-ionization and arc extinguishing can be achieved, while taking into account the short-term overload capacity of the non-fault pole to reduce the power loss caused by the fault.

[0077] In some embodiments, after adjusting the dual DC pole power command value to the maximum transmittable power of the non-faulty DC pole or the maximum transmittable power of the dual DC pole, the method further includes: synchronously transmitting a reduction signal and the reduction power to a safety and stability control system, or / and a frequency emergency control system, or / and a master control system. To maintain active power balance between the AC and DC systems, the safety and stability control system, or / and the frequency emergency control system, or / and the master control system, upon receiving the reduction signal and the reduction power, performs generator removal at the sending end and load removal at the receiving end.

[0078] In some embodiments, the fallback power is equal to the pre-fault dual DC pole power minus the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole. If there is only one restart, the maximum transmittable power of the non-fault DC pole should be selected based on the assumption that the non-fault DC pole can transmit the maximum transmittable power throughout the second restart time. The maximum transmittable power of the dual DC pole should be selected based on the assumption that the dual DC pole can transmit the maximum transmittable power throughout the second restart time.

[0079] More specifically, controlling the restart of the faulty DC pole includes: controlling at least one converter at each end of the faulty DC pole to continue operating; determining the DC current reference values ​​of the two converters at both ends of the faulty DC pole based on the needs of the high-voltage direct current transmission system, and performing the following restarting actions: within the de-ionizing time, controlling the DC currents of the two converters to be equal or the difference to be less than a first current threshold based on the DC current reference values ​​of the two converters, or controlling the DC currents of the two converters to be zero; within the restarting time, increasing the DC voltage and / or DC power of the two converters; or including: controlling at least one converter at both ends of the faulty DC pole to be temporarily locked or / and the DC current of at least one converter to be zero; within the de-ionizing time, using the resistance of the DC line or the resistance of the put-in converter to attenuate the fault current; within the restarting time, controlling at least one converter at both ends of the faulty DC pole to be unlocked and increase the DC voltage and / or DC power, or / and at least one converter to increase the DC voltage and / or DC power. If the absolute value of the DC voltage of the DC line is greater than or equal to the first voltage threshold, the DC voltage of the two converters continues to increase; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, the restart is not successful, and the above restart action is repeated again. If the restart is unsuccessful when the set number of restarts is reached, the two converters are controlled to be locked. In addition, the de-ionization control is not exactly the same according to the characteristics of the converter. For converters that do not have the ability to ride through DC faults, the faulty DC pole is de-ionized by controlling the converter to be temporarily locked. It should be pointed out that controlling the converter to be temporarily locked does not require sending the locking signal and / or the loss power to the safety and stability control system, or / and the frequency emergency control system, or / and the main control system to cut off the generator or cut off the load; when controlling the two converters to be locked, it is necessary to send the locking signal and / or the loss power to the safety and stability control system, or / and the frequency emergency control system, or / and the main control system to cut off the generator or cut off the load.

[0080] It should be noted that the first current threshold value ranges from 0.01 to 0.2 times the rated DC current, and the first voltage threshold value ranges from 0.1 to 0.5 times the rated DC voltage. If the number of restarts in the first restart phase is two, the de-free time in the first restart action is 20 to 500ms; the restart time is 20 to 300ms; the de-free time in the second restart action is 50 to 500ms; and the restart time is 20 to 300ms. If the number of restarts in the first restart phase is three, the de-free time in the third restart action ranges from 80ms to 500ms; the restart time is 20ms to 300ms. If the number of restarts in the second restart phase is one, the de-free time in the first restart action ranges from 80ms to 5000ms; and the restart time is 20ms to 300ms. It should be pointed out that the de-ionization time is related to the voltage level of the DC line and the geographical environment in which it is installed, and is determined according to the project conditions; the time of the first restart phase is determined according to the conditions of the AC system to which the HVDC transmission system is connected, so that it can operate safely and stably under conditions with power shortages; the time of the second restart phase is determined according to the arc extinction time of the DC line under severe working conditions, and the short-term overload capacity of the non-fault pole must also be considered.

[0081] Specifically, if the dual DC pole power before the fault is 6000MW and the maximum transmittable power of the non-fault pole is 5200MW, the dual DC pole power is greater than the maximum transmittable power of the non-fault DC pole. In the second restart phase, the dual DC pole power is controlled to return to the maximum transmittable power of 5200MW; if the dual DC pole power before the fault is 4000MW, in the second restart phase, the dual DC pole power before the fault is still maintained.

[0082] In some embodiments, the converter includes at least one of a grid-commutated converter or a voltage source converter. If the converters in the rectifier and inverter stations are both grid-commutated converters, it is a conventional DC transmission system. If the converters in the rectifier and inverter stations are both voltage source converters, it is a flexible DC transmission system. Voltage source converters include modular multilevel converters based on half-bridge submodules, modular multilevel converters based on a mix of half-bridge and full-bridge submodules, and modular multilevel converters based on full-bridge submodules. If the converters in the rectifier and inverter stations include both grid-commutated converters and voltage source converters, it is a hybrid DC transmission system.

[0083] If at least one of the two converters of the HVDC transmission system is a grid-commutated converter, then after controlling the DC currents of the two converters to be equal or the difference between them to be less than a first current threshold based on the DC current reference values ​​of the two converters, or before controlling the two converters to be locked, the method further includes: controlling the HVDC transmission system to cut off or put into operation an AC filter connected to the AC system according to the needs of the AC system;

[0084] If at least one of the two converters of the high-voltage direct current transmission system is a voltage source converter, after controlling the DC currents of the two converters to be equal or the difference to be less than the first current threshold based on the DC current reference values ​​of the two converters, or before controlling the two converters to lock, it also includes: controlling the reactive power or AC voltage output by the voltage source converter according to the needs of the AC system.

[0085] In some embodiments, a DC line ground fault is detected by detecting line protection. Line protection includes at least one of: line sudden change protection, line traveling wave protection, line low voltage protection, and line longitudinal differential protection. This description uses line sudden change protection and line low voltage protection as examples.

[0086] When a DC line fault occurs, it will cause a DC voltage drop. The speed of the voltage drop varies depending on the fault location. By determining the speed of the voltage drop, the fault on the DC line can be detected. The line sudden change protection action criterion is as follows: dUdl / dt <dUdl_set, |Udl|<Udl_set。

[0087] Where dUdl / dt is the DC voltage mutation per unit time, dUdl_set is the constant value of the DC voltage mutation, Udl is the pole bus voltage, and Udl_set is the constant value of the DC voltage.

[0088] By detecting the DC voltage, if it is found that the DC voltage is continuously low for a certain period of time, and there is no AC system fault or commutation failure, it is determined to be a DC line fault. The line low voltage protection action judgment criteria are as follows: |Udl| <Udl_set1。

[0089] Among them, Udl is the pole bus voltage, and Udl_set1 is the DC voltage constant.

[0090] When a DC line grounding fault is determined by line mutation, line traveling wave, line low voltage and / or line longitudinal differential protection action, at least one converter at each of the two DC poles at both ends of the grounding fault is controlled to continue operating.

[0091] In some embodiments, the requirements of the high-voltage direct current transmission system include: at least one of: active power requirement, reactive power requirement, ground current limit value requirement, fault DC pole current limit value requirement, current limit value requirement flowing through the fault point, and AC harmonic suppression requirement.

[0092] In some embodiments, if the demand of the high-voltage direct current transmission system is one type, the demand of the high-voltage direct current transmission system is any one of the active power demand, reactive power demand, ground current limit value demand, current limit value demand of the fault DC pole, current limit value demand flowing through the fault point, and AC harmonic suppression demand of the rectifier station or the inverter station, and it is determined that the DC current reference values ​​of the two converters at both ends of the fault DC pole are equal.

[0093] In some embodiments, if the demands of the HVDC transmission system consider the demands of the rectifier station and the inverter station at the same time, and priorities are given to different demands at the same time; the active power demand, reactive power demand, ground current limit value demand, current limit value demand of the faulty DC pole, current limit value demand flowing through the fault point, and AC harmonic suppression demand are unified demands, and it is determined that the DC current reference values ​​of the two converters at both ends of the faulty DC pole are equal; or, the active power demand, reactive power demand, ground current limit value demand, current limit value demand of the faulty DC pole, current limit value demand flowing through the fault point, and AC harmonic suppression demand are the demands of the rectifier station and the inverter station respectively, and it is determined that the DC current reference values ​​of the two converters at both ends of the faulty DC pole are unequal and the difference is less than a first current threshold, and the first current threshold is less than the current limit value flowing through the fault point.

[0094] In some embodiments, determining DC current reference values ​​of two converters at both ends of a ground fault based on a demand of a high-voltage direct current transmission system includes: if the demand of the high-voltage direct current transmission system is an active power demand, dividing the active power demand by the sum of the absolute values ​​of the DC voltages of all operating converters of the rectifier station 100 or the inverter station 200 to obtain the DC current reference values ​​of the two converters; or, if the demand of the high-voltage direct current transmission system is a reactive power demand and the converter is a six-pulse or twelve-pulse grid-commutated converter, determining the DC current reference values ​​of the two converters based on the reactive power demand, the no-load DC bus voltage, the trigger angle or the turn-off angle, and the commutation angle.

[0095] If the demand of the HVDC transmission system is a demand for an underground current limit value, the DC current reference value of each converter of the two converters is greater than the difference between the DC current of another DC pole at the same station and the underground current limit value, and is less than the sum of the DC current of another DC pole at the same station and the underground current limit value; or, if the demand of the HVDC transmission system is a demand for a current limit value of the DC pole where the fault is located, the DC current reference values ​​of the two converters are determined to be less than the current limit value of the DC pole where the fault is located; or, if the demand of the HVDC transmission system is a demand for a current limit value flowing through the fault point, the difference between the DC current reference values ​​of the two converters is determined to be less than the current limit value flowing through the fault point; or, if the demand of the HVDC transmission system is a demand for AC harmonic suppression, the AC harmonic suppression demand is converted into a DC current reference value that limits the two converters.

[0096] In some embodiments, determining the DC current reference values ​​of two converters at both ends of a faulty DC pole based on the requirements of a high-voltage direct current transmission system also includes: based on the DC current reference values ​​of the two converters determined based on active power requirements, reactive power requirements, ground current limit requirements, current limit requirements of the DC pole where the fault is located, and current limit requirements flowing through the fault point, superimposing a current value greater than zero and with an absolute value less than 0.1 times the rated DC current on the DC current reference value of at least one of the converters.

[0097] In some embodiments, the DC current of the converter includes at least one of the high-voltage bus current, the low-voltage bus current, the pole bus current of the DC pole where the converter is located, or the pole neutral bus current.

[0098] In some embodiments, increasing the DC voltage of the two converters includes: controlling the converter that was operating under DC voltage control before the fault to operate under DC voltage control, directly setting or setting the DC voltage reference value of the two converters according to a ramp-up slope to the DC voltage before the fault or a DC voltage that is reduced compared to the DC voltage before the fault; or, by setting the difference between the DC current reference values ​​of the two converters. It should be noted that when increasing the DC voltage of the two converters, when the DC voltage is lower than the first voltage threshold, the DC current is limited to prevent a large fault current from being injected into the fault point when the fault persists.

[0099] In some embodiments, controlling the DC current of the two converters to be zero includes: if both converters are grid-commutated converters, this is achieved by controlling the phase shift of the grid-commutated converter of the rectifier station and the trigger angle of the grid-commutated converter of the inverter station to be greater than 90 degrees; if both converters are voltage source converters, this is achieved by controlling the DC voltage of the two converters to be zero, controlling the DC current of the two converters to be zero, or controlling the two converters to be locked.

[0100] It should be pointed out that in the second restart stage of the restart method proposed in the embodiment of the present application, if the overload capacity of the non-fault DC pole changes, the maximum transmittable power of the non-fault DC pole will change. Optionally, the power is gradually reduced according to the maximum transmittable power of the non-fault DC pole, and the reduction signal and the reduced power are sent to the safety and stability control system, or / and the frequency emergency control system, or / and the main control system, and the power loss calculation is performed.

[0101] Accordingly, with reference to FIG3 , an embodiment of the present application further provides a device for controlling a DC line fault in a high-voltage direct current transmission system. The high-voltage direct current transmission system includes at least one rectifier station 100 and at least one inverter station 200. The rectifier station 100 and the inverter station 200 both include dual DC poles, and each DC pole in the dual DC poles includes at least one converter. The control device includes: a first control unit, configured to control the faulty DC pole of the high-voltage direct current transmission system to restart in a first restart phase when a ground fault occurs in a DC line connected to a DC pole (the faulty DC pole) and the other DC pole is a non-faulty DC pole and operates in dual DC pole power control. The dual DC pole power instruction value is the dual DC pole power before the fault; the adjustment unit is used to adjust the dual DC pole power instruction value to the maximum transmittable power or the maximum transmittable power of the dual DC pole when the number of restarts set in the first restart stage is reached and the restart is still not successful, if the dual DC pole power instruction value is greater than the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole; the second control unit is used to control the restart of the faulty DC pole in the second restart stage; the third control unit is used to control the converter locking of the faulty DC pole when the number of restarts set in the second restart stage is reached and the restart is still not successful.

[0102] 4 , in some embodiments, the control device further includes a first detection unit for detecting parameters of the high-voltage direct current transmission system, the parameters including the DC bus current I dl , DC pole neutral bus current I dnc , DC bus voltage U dl and DC pole-neutral bus voltage U dn The first control unit, the second control unit, and the third control unit determine whether a fault occurs in a DC line of the HVDC transmission system based on the parameters of the HVDC transmission system, and control the faulty DC pole of the HVDC transmission system to be restarted or locked.

[0103] As shown in FIG5 , the main circuit of the HVDC transmission system includes a first rectifier station 100, a first inverter station 200, a first DC line 150, a second DC line 160, a rectifier station grounding electrode line 114, and an inverter station grounding electrode line 214. The first rectifier station 100 includes a first DC pole 110 and a second DC pole 120, and the first inverter station 200 includes a third DC pole 210 and a fourth DC pole 220. The converter of each DC pole is a grid-commutated converter or a voltage source converter. The first rectifier station 100 of the HVDC transmission system is configured with a DC pole bus current I dl1 and I dl2 , DC bus voltage U dl1 and U dl2 The first inverter station 200 may also be configured with the same measuring points.

[0104] During positive power transmission, the first rectifier station 100 converts AC power into DC power through the converter of the first DC pole 110 and the converter of the second DC pole 120, and transmits the DC power to the first inverter station 200 through the first DC line 150 and the second DC line 160. The first inverter station 200 converts DC power into AC power through the converter of the third DC pole 210 and the converter of the fourth DC pole 220, and transmits the DC power, thereby achieving positive DC power transmission.

[0105] The HVDC transmission system operates in dual DC pole power control, the DC current command values ​​of the converter of the first DC pole 110 and the converter of the second DC pole 120 are equal, and the set power is the rated power of 12000MW.

[0106] In related art, when a ground fault occurs in a DC line 310 connected to a DC pole (faulty DC pole), the first DC pole 110 and the third DC pole 210 become faulty DC poles, with a fault duration of 2 seconds. The other DC pole is the non-faulty DC pole (including the second DC pole 120 and the fourth DC pole 220) and operates in dual-DC pole power control. The HVDC transmission system only has a first restart phase (as shown in FIG6 ). The number of restarts designed in the simulation experiment is two restarts at full voltage and one restart at reduced voltage, with de-ionization times of 150ms, 200ms, and 200ms, respectively. In the first restart phase, control pole 1 is restarted, and the dual-DC pole power command is the dual-DC pole power command before the fault. The faulty DC pole fails to restart at full voltage for the first restart phase, the second restart at full voltage fails, and the third restart at reduced voltage fails. Since the three restart times set for the first restart phase have not yet been successfully restarted, and considering the limited ability of the AC system to withstand shocks, the converter controlling the faulty DC pole is locked. During the restart process and after the lockout, the converters on the non-faulty DC poles will absorb some of the power lost by the faulty DC pole lockout, placing them in an overloaded state. After the faulty pole lockout, the lockout signal and / or power loss are transmitted to the safety and stability control system, which manages active power balance to ensure safe operation of the power system. The overload capacity of the converters on the non-faulty DC poles is divided into two stages: 3-second overload and 2-hour overload. Within 3 seconds after the fault, the converters on the non-faulty DC poles operate at approximately 7600MW. Then, during the 2-hour overload stage, the converters on the non-faulty DC poles operate at approximately 6400MW, ultimately losing 5600MW.

[0107] In one or more embodiments of the present application, when a ground fault occurs in a DC line 310 connected to a DC pole (faulty DC pole), the fault time is set to 2 seconds. The other DC pole is a non-faulty DC pole and operates in dual DC pole power control. The HVDC transmission system has both a first restart phase and a second restart phase (as shown in FIG7 ). In the simulation experiment, the number of restarts in the first restart phase is designed to be two restarts at full voltage and one restart at reduced voltage, with de-ionization times of 150 ms, 200 ms, and 200 ms, respectively. The number of restarts in the second restart phase is one restart at full voltage, with a de-ionization time of 2 seconds. In the first restart phase, the faulty DC pole is controlled to restart, and the dual DC pole power command is the dual DC pole power before the fault. If the faulty DC pole fails to restart at full voltage for the first time, the second time at full voltage, and the third time at reduced voltage, and the restart fails after reaching the three restart times set in the first restart phase, a reduction signal and a reduction power are sent to the safety and stability control system. The safety and stability control system processes the active power balance to ensure the safe operation of the power system, and then enters the second restart phase. Since the time of the second restart phase is 2s, the maximum transmittable power of the non-fault DC pole is selected as the power value of 3 seconds overload. Since the dual DC pole power instruction value is greater than the maximum transmittable power of the non-fault DC pole, in the second restart phase, the faulty DC pole is controlled to restart, and the dual DC pole power instruction is adjusted to the maximum transmittable power of the non-fault DC pole, 7600MW. Since the fault time is 2s, the faulty DC pole is restarted successfully, and the dual DC pole power operates at 7600MW.

[0108] Comparing the test results of Figures 6 and 7, the total de-ionization time of the faulty DC pole shown in Figure 6 is only 550ms. The reasons for the failure of the DC line are complex. Some faults cannot be successfully restarted in such a short time. The power loss after the restart fails reaches 5600MW. If the de-ionization time is set too long, in the first restart phase, there will always be a large power shortage in the AC system connected to the high-voltage DC transmission system, which seriously affects the safety of the AC system. In one or more embodiments of the present application shown in Figure 7, the total de-ionization time of the faulty DC pole is 2550ms, which greatly improves the restart success rate. At the same time, the 3s overload capacity of the non-faulty DC pole is fully utilized. After the restart is successful, the power loss is reduced to 4400MW, and the earth no longer flows with current. At the same time, the power reduction signal and the reduced power are sent to the safety and stability control system during the whole process. The safety and stability control system synchronously cuts off the generator or cuts off the load to maintain the active power balance, which can ensure the safety of the AC system. In Figures 6 and 7, P d1 is the DC power of the first DC pole 110, P d2 is the DC power of the second DC pole 120, P d is the total DC power of the dual DC poles.

[0109] In summary, the present application provides a method and control device for restarting a DC line fault in a high-voltage direct current (HVDC) transmission system. When a ground fault occurs in the DC line of the HVDC transmission system and the first restart stage cannot guarantee successful DC restart, a second restart stage is added. When the dual DC pole power command value is less than or equal to the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole, the dual DC pole power command value is maintained unchanged. When the dual DC pole power command value is greater than the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole, the dual DC pole power command value is adjusted to the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole. A reduction signal and the reduction power are sent to a safety and stability control system. After the safety and stability control system performs generator or load shedding, the active power of the AC and DC systems is restored to equilibrium. At this point, restarting the DC system will not impact the AC system, thereby increasing the de-ionization time and further improving the success rate of the DC line restart. At the same time, by utilizing the short-term overload capacity of the non-fault DC pole, more active power transmission can be maintained, reducing DC power loss. In addition, if the DC currents of the two converters at both ends of the ground fault are controlled to be equal, the voltage at the ground fault point can be effectively controlled and the voltage oscillation during the line fault can be suppressed, thereby better achieving de-ionization. At the same time, since DC current still flows through the DC line, the converter continues to consume reactive power. The high-voltage DC transmission system based on the grid-commutated converter can maintain the AC and DC reactive power balance and also reduce the current flowing into the earth during the fault.

[0110] Figure 8 is a schematic diagram of the main circuit of a high-voltage direct current transmission system provided in an embodiment of the present application, wherein the high-voltage direct current transmission system is a bipolar conventional direct current transmission system, and each DC pole adopts a structure of dual twelve-pulse grid-commutated converters connected in series, which is widely used in ultra-high voltage direct current transmission projects.

[0111] The HVDC transmission system main circuit includes a first rectifier station 100, a first inverter station 200, a first DC line 150, a second DC line 160, a rectifier station grounding electrode line 114, a rectifier station grounding electrode 115, and an inverter station grounding electrode line 214, a inverter station grounding electrode 215. When the earth return line is in operation, the first rectifier station 100 and the first inverter station 200 are connected to their respective grounding electrodes.

[0112] The first rectifier station 100 includes a first DC pole 110, a second DC pole 120, a first AC filter group 118, a first AC system 140, a first high-end converter transformer incoming line switch 131, a first low-end converter transformer incoming line switch 132, a second low-end converter transformer incoming line switch 133, a second high-end converter transformer incoming line switch 134 and a metal return line transfer switch 113.

[0113] The first DC pole 110 includes a first high-side valve group 111, a first low-side valve group 112, a first high-side converter transformer 116, a first low-side converter transformer 117, a first DC pole neutral bus switch 119, a first DC filter 93, and a first smoothing reactor 91. The first high-side valve group 111 and the first low-side valve group 112 are connected in series.

[0114] The first high-side valve group 111 includes a first high-side converter 1, a first bypass switch 11 of the first high-side valve group, a second bypass switch 12 of the first high-side valve group, a first high-side valve group busbar switch 13, and a first high-side valve group valve group switch 14. The first low-side valve group 112 includes a first low-side converter 2, a first bypass switch 21 of the first low-side valve group, a second bypass switch 22 of the first low-side valve group, a first low-side valve group valve group switch 23, and a first low-side valve group busbar switch 24.

[0115] The first high-end converter 1 and the first low-end converter 2 are grid-commutated converters. The grid-commutated converters include at least one of a six-pulse bridge circuit and a twelve-pulse bridge circuit. The pulsating bridge circuit includes non-disabled half-controlled power semiconductor devices, typically thyristor devices.

[0116] The voltage source converter includes at least one of a two-level converter, a diode-clamped multilevel converter, a modular multilevel converter (MMC), a hybrid multilevel converter (HMC), a cascaded two-level (CTL) converter, and a stacked two-level (STL) converter, wherein the converter includes a fully controllable power semiconductor device that can be turned off. The modular multilevel converter (MMC) includes at least one of a modular multilevel converter (MMC) with a half-bridge submodule structure, a modular multilevel converter (MMC) with a full-bridge submodule structure, and a modular multilevel converter (MMC) with a hybrid of a half-bridge and full-bridge submodule structure.

[0117] The second DC pole 120 includes a second low-end valve group 121, a second high-end valve group 122, a second low-end converter transformer 126, a second high-end converter transformer 127, a second DC pole neutral bus switch 129, a second DC filter 94, and a second smoothing reactor 92. The second low-end valve group 121 and the second high-end valve group 122 are connected in series.

[0118] The second low-side valve group 121 includes a second low-side converter 3, a first bypass switch 31, a second bypass switch 32, a busbar switch 33, and a valve group switch 34. The second high-side valve group 122 includes a second high-side converter 4, a first bypass switch 41, a second bypass switch 42, a valve group switch 43, and a busbar switch 44. The second low-side converter 3 and the second high-side converter 4 are grid-commutated converters.

[0119] The first inverter station 200 includes a third DC pole 210, a fourth DC pole 220, a second AC filter group 218, a second AC system 240, a third high-end converter transformer incoming line switch 231, a third low-end converter transformer incoming line switch 232, a fourth low-end converter transformer incoming line switch 233 and a fourth high-end converter transformer incoming line switch 234.

[0120] The third DC pole 210 includes a third high-end valve group 211, a third low-end valve group 212, a third high-end converter transformer 216, a third low-end converter transformer 217, a third DC pole neutral bus switch 219, a third DC filter 97, and a third smoothing reactor 95. The third high-end valve group 211 and the third low-end valve group 212 are connected in series.

[0121] The third high-side valve group 211 includes a third high-side converter 5, a first bypass switch 51, a second bypass switch 52, a busbar switch 53, and a valve group switch 54. The third low-side valve group 212 includes a third low-side converter 6, a first bypass switch 61, a second bypass switch 62, a valve group switch 63, and a busbar switch 64. The third high-side converter 5 and the third low-side converter 6 are grid-commutated converters.

[0122] The fourth DC pole 220 includes a fourth low-end valve group 221, a fourth high-end valve group 222, a fourth low-end converter transformer 226, a fourth high-end converter transformer 227, a fourth DC pole neutral bus switch 229, a fourth DC filter 98, and a fourth smoothing reactor 96. The fourth low-end valve group 221 and the fourth high-end valve group 222 are connected in series.

[0123] The fourth low-side valve group 221 includes a fourth low-side converter 7, a first bypass switch 71, a second bypass switch 72, a busbar switch 73, and a valve group switch 74. The fourth high-side valve group 222 includes a fourth high-side converter 8, a first bypass switch 81, a second bypass switch 82, a valve group switch 83, and a busbar switch 84, serving as a grid-commutated converter.

[0124] According to some embodiments, the first high-end converter 1, the first low-end converter 2, the second low-end converter 3, the second high-end converter 4, the third high-end converter 5, the third low-end converter 6, the fourth low-end converter 7, and the fourth high-end converter 8 may also be voltage source converters, and the converter transformer type in FIG8 is modified according to the connected voltage source converter. The voltage source converter includes at least one of a two-level converter, a diode-clamped multilevel converter, a modular multilevel converter (MMC), a hybrid multilevel converter (HMC), a cascaded two-level (CTL) converter, and a stacked two-level (STL) converter, wherein the converter includes a fully controllable power semiconductor device that can be turned off. The modular multilevel converter (MMC) includes at least one of a half-bridge submodule structure, a full-bridge submodule structure, and a hybrid of a half-bridge and full-bridge submodule structure. It should be noted that when using a voltage source converter (VSC), the series connection of the high-end and low-end converters at each DC pole must be capable of controlling the DC voltage to zero or negative voltage.

[0125] The various switches mentioned above include at least one of a mechanical switch, a knife switch, a DC circuit breaker, and a thyristor valve group.

[0126] If both the high-side and low-side converters of the DC poles of the first rectifier station 100 and the first inverter station 200 are grid-commutated converters, this constitutes a conventional DC transmission system. If both the high-side and low-side converters of the DC poles of the first rectifier station 100 and the first inverter station 200 are voltage source converters, this constitutes a flexible DC transmission system. Voltage source converters have the ability to regulate voltage to zero or negative voltage, such as modular multilevel converters based on full-bridge submodules or modular multilevel converters based on a mix of half-bridge and full-bridge submodules. If both the first rectifier station 100 and the first inverter station 200 have both grid-commutated converters and voltage source converters, this constitutes a hybrid DC transmission system.

[0127] The first rectifier station 100 is connected to the grounding electrode 115 via a grounding electrode line 114. The first inverter station 200 is connected to the grounding electrode 215 via a grounding electrode line 214. During positive power transmission, the first AC system 140 of the first rectifier station 100 converts AC power into DC power via its first high-side converter 1, first low-side converter 2, second high-side converter 4, and second low-side converter 3. This power is then transmitted to the first inverter station 200 via the first DC line 150 and second DC line 160. The first inverter station 200 then converts DC power into AC power via its third high-side converter 5, third low-side converter 6, fourth high-side converter 8, and fourth low-side converter 7, and transmits the DC power to the second AC system 240 of the first inverter station 200, thereby achieving positive DC power transmission. The converters of the rectifier station generally operate in current control, while the converters of the inverter station generally operate in voltage control or maximum firing angle control (AMAX). It should be pointed out that the maximum firing angle control (AMAX) is only applicable to grid-commutated converters and not to voltage source converters.

[0128] The analog signals collected by the first rectifier station 100 and the first inverter station 200 are: the high-voltage bus current IDC1P and the low-voltage bus current IDC1N on the DC side of the high-end converter, the high-voltage bus current IDC2P and the low-voltage bus current IDC2N on the DC side of the low-end converter, the pole bus current IDL, the pole-neutral bus current IDNC, the DC filter head-end current IZT1, the grounding electrode current IDEL, the pole bus voltage UDL, and the pole-neutral bus voltage UDN.

[0129] DC line ground faults are detected by detecting line voltage surges, traveling waves, line low voltage, and DC current differentials, triggering corresponding protection actions. Protection includes at least one of surge protection, traveling wave protection, line low voltage protection, and line differential protection.

[0130] FIG9 is a flow chart of a flexible restart method for a DC line fault in a HVDC transmission system provided by an embodiment of the present application, illustrating a control flow when a ground fault occurs in the first DC line 150 of the HVDC transmission system shown in FIG8 .

[0131] The converters in the first DC pole 110 of the first rectifier station 100 and the third DC pole 210 of the first inverter station 200 are both grid-commutated converters. Before a ground fault occurs, the bipolar full-valve group operates at rated DC voltage. When a ground fault occurs in the first DC line 150 of the HVDC transmission system, the restart frequency is set to three, including two restarts at full voltage and one restart at reduced voltage. The control flow is as follows.

[0132] In S110, at least one converter in each of the two DC poles at both ends of the ground fault is controlled to continue to operate.

[0133] The DC line grounding fault is determined by the line sudden change, traveling wave protection, line low voltage and / or line longitudinal differential protection action.

[0134] When a DC line fault occurs, it causes a DC voltage drop. The speed of the voltage drop varies depending on the fault location. By determining the speed of the voltage drop, the DC line fault can be detected. The line sudden change protection action criterion is as follows: dUDL / dt <dUDL_set, |UDL|<UDL_set。

[0135] Where, dUDL / dt is the DC voltage mutation per unit time, dUDL_set is the constant value of the DC voltage mutation, UDL is the pole bus voltage, and UDL_set is the constant value of the DC voltage.

[0136] By detecting the DC voltage, if it is found that the DC voltage is low for a certain period of time, and there is no AC system fault or commutation failure, it is determined to be a DC line fault. The line low voltage protection action criteria are as follows: |UDL| <UDL_set1。

[0137] Among them, UDL is the pole bus voltage, and UDL_set1 is the DC voltage constant.

[0138] When a DC line ground fault is detected based on line sudden change, traveling wave protection, line low voltage, and / or line longitudinal differential protection, at least one converter on each of the two DC poles at both ends of the ground fault is controlled to continue operating. In this embodiment, the first high-side converter 1 and the first low-side converter 2, as well as the third high-side converter 5 and the third low-side converter 6, are controlled to continue operating.

[0139] In S120 , direct current reference values ​​of two converters at both ends of the ground fault are determined based on the requirements of the HVDC transmission system, where the two converters include one converter at each end of the ground fault that continues to operate.

[0140] The requirements of the HVDC transmission system include at least one of active power requirement, reactive power requirement, ground current limit value, current limit value of the DC pole where the fault is located, current limit value requirement of the current flowing through the fault point, and AC harmonic suppression requirement. If the HVDC transmission system has more than one requirement, the priorities of different requirements shall be given at the same time.

[0141] If the requirements of the HVDC transmission system only consider the requirements of the rectifier station, the active power requirement, the reactive power requirement, the ground current limit value requirement, the current limit value requirement of the DC pole where the fault is located, the current limit value requirement flowing through the fault point, and the AC harmonic suppression requirement are the requirements of the rectifier station, and the DC current reference values ​​of the two converters are equal;

[0142] If the requirements of the HVDC transmission system only consider the requirements of the inverter station, the active power requirement, the reactive power requirement, the ground current limit value requirement, the current limit value requirement of the DC pole where the fault is located, the current limit value requirement flowing through the fault point, and the AC harmonic suppression requirement are the requirements of the inverter station, and the DC current reference values ​​of the two converters are equal;

[0143] If the demand of the high-voltage direct current transmission system considers the demand of the rectifier station and the inverter station at the same time, when processed uniformly, the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the DC pole where the fault is located, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand are the demands after unified processing, and the DC current reference values ​​of the two converters are equal. When processed separately, the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the DC pole where the fault is located, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand are the demand of the rectifier station and the demand of the inverter station respectively, and the DC current reference values ​​of the two converters are unequal and the difference is less than the first current threshold, and the first current threshold is less than the current limit value flowing through the fault point.

[0144] If the DC current reference values ​​of the two converters determined by the demand of the high-voltage direct current transmission system are equal, the DC currents of the two converters are controlled to be equal; if the DC current reference values ​​of the two converters determined by the demand of the high-voltage direct current transmission system are unequal, the DC current difference of the two converters is controlled to be smaller than a first current threshold, and the first current threshold is smaller than a current limit value flowing through the fault point.

[0145] Determining DC current reference values ​​of two converters at both ends of the ground fault based on the demand of the high voltage direct current transmission system includes:

[0146] If the demand of the HVDC transmission system is an active power demand, the active power demand is divided by the sum of the absolute values ​​of the DC voltages of all operating converters of the rectifier station or the inverter station to obtain the DC current reference values ​​of the two converters; or

[0147] If the demand of the HVDC transmission system is a reactive power demand and the converter is a six-pulse or twelve-pulse grid-commutated converter, the DC current reference values ​​of the two converters are determined based on the reactive power demand, the no-load DC bus voltage, the trigger angle or the turn-off angle, and the commutation angle.

[0148] The calculation method of the converter of the rectifier station is as follows:

[0149] The calculation method of the converter of the inverter station is as follows:

[0150] Where, I ord is the DC current reference value, Q conv is the reactive power requirement of the six-pulse or twelve-pulse grid-commutated converter, U di0 is the no-load DC bus voltage of a six-pulse or twelve-pulse grid-commutated converter, α is the trigger angle of the converter, μ is the commutation angle of the converter, γ is the turn-off angle of the converter, when the converter is a six-pulse grid-commutated converter, b=1 / 4, when the converter is a twelve-pulse grid-commutated converter, b=1 / 2; or,

[0151] If the requirement of the HVDC transmission system is a ground current limit value requirement, the DC current reference value of each of the two converters is greater than the difference between the DC current of another DC pole at the same station and the ground current limit value, and is less than the sum of the DC current of another DC pole at the same station and the ground current limit value; or

[0152] If the requirement of the HVDC transmission system is a current limit value requirement of the DC pole where the fault is located, determining that the DC current reference values ​​of the two converters are less than the current limit value of the DC pole where the fault is located; or

[0153] If the requirement of the HVDC power transmission system is a current limit value requirement for the current flowing through the fault point, determining that the difference between the DC current reference values ​​of the two converters is less than the current limit value for the current flowing through the fault point; or

[0154] If the requirement of the HVDC power transmission system is an AC harmonic suppression requirement, the AC harmonic suppression requirement is converted into a DC current reference value for limiting the two converters.

[0155] Determining DC current reference values ​​of two converters at both ends of the ground fault based on the requirements of the high voltage direct current transmission system further includes:

[0156] Based on the DC current reference values ​​of the two converters determined based on the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the DC pole where the fault is located, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand, a current greater than zero and with an absolute value less than 0.1 times the rated DC current that fluctuates around zero is superimposed on the DC current reference value of at least one of the converters.

[0157] Taking the reactive power demand of the first rectifier station 100 or the first inverter station 200 as an example, the reactive power demand calculation method of the converter is as follows. ord_p2 =0.5×I d_p2 ×Udi0_p2c1 ×(2μ 21 +sin2α 21 -sin2(α 21 +μ 21 )) / (cosα 21 -cos(α 21 +μ 21 ))+0.5×I d_p2 ×U di0_p2c2 ×(2μ 22 +sin2α 22 -sin2(α 22 +μ 22 )) / (cosα 22 -cos(α 22 +μ 22 )), Q ord_p1 =Q ord -Q ord_p2 ,

[0158] Considering that the circuit parameters of the high-end valve group and the low-end valve group of pole I are the same, the reactive power demand of the high-end converter or the low-end converter is 1 / 2 of the reactive power demand of pole I, specifically: Q ord_p1c1 =Q ord_p1c2 =Q ord_p1 / 2,

[0159] According to the reactive power demand of the twelve-pulse converter of the rectifier station, the DC current reference value is calculated as follows. ord_p1 =Q ord_p1c1 / (0.5×U di0_p1c1 ×(2μ 11 +sin2α 11 -sin2(α 11 +μ 11 )) / (cosα 11 -cos(α 11 + μ 11 ))) or I ord_p1 =Q ord_p1c2 / (0.5×U di0_p1c2 ×(2μ 12 +sin2α 12 -sin2(α 12 +μ 12 )) / (cosα 12 -cos(α 12 + μ 12 ))).

[0160] Where, I ord_p1 is the DC current reference value of pole I and pole II, Q ord , Q ord_p1 , Qord_p2 Respectively, the reactive power demand of bipolar, pole I, and pole II, I d_p2 is the DC current of pole II, U di0_p1c1 、U di0_p1c2 、U di0_p2c1 、U di0_p2c2 are the no-load DC voltages of the six-pulse converters in the high-end converter of pole I, the low-end converter of pole I, the high-end converter of pole II, and the low-end converter of pole II, respectively. 11 , α 12 , α 21 , α 22 They are the trigger angles of the high-end converter of pole I, the low-end converter of pole I, the high-end converter of pole II, and the low-end converter of pole II, μ 11 、μ 12 、μ 21 、μ 22 They are the commutation angles of the high-end converter of pole I, the low-end converter of pole I, the high-end converter of pole II, and the low-end converter of pole II.

[0161] The DC current reference values ​​of the first high-side converter 1 and the first low-side converter 2 of the first DC pole 110 of the first rectifier station 100 and the third high-side converter 5 and the third low-side converter 6 of the third DC pole 210 of the first inverter station 200 are given as Iord_p1.

[0162] Taking the active power requirement and ground current limit value requirement of the first rectifier station 100 or the first inverter station 200 as an example, the following formula is shown. ord_p2 =P ord / (U d_p2c1 +U d_p2c2 ),

[0163] I ord_p1 ≥I ord_p2 -I del_lim And I ord_p1 ≤I ord_p2 +I del_lim .

[0164] Where, I ord_p1 , I ord_p2 are the DC current reference values ​​of poles I and II, I del_lim is the ground current limit value, P ord is the active power demand value, U d_p2c1 is the DC voltage of the high-end converter of Pole II, U d_p2c2 is the DC voltage of the low-end converter of Pole II.

[0165] A DC current reference value of the first low-side converter 2 of the first DC pole 110 of the first rectifier station 100 and the third low-side converter 6 of the third DC pole 210 of the first inverter station 200 is given as Iord_p1.

[0166] In S130, the DC currents of the two converters at both ends of the ground fault are controlled to be equal or to have a difference less than a first current threshold based on the DC current reference value. This can be achieved by controlling the two converters to operate in DC current control based on the DC current reference values ​​of the two converters.

[0167] Specifically, the pole bus current IDL of the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 is controlled to be equal to the pole bus current IDL of the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 and is I ord_p1 Alternatively, the difference between the pole bus current IDL of the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the pole bus current IDL of the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 is controlled to be smaller than the first current threshold, such as controlling the pole bus current IDL of the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 to be I ord_p1 , the pole bus current IDL of the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 is I ord_p1 -ΔI ord_p1 , where ΔI ord_p1 is less than the first current threshold.

[0168] According to some embodiments, when the pole bus current IDL of the first high-side converter 1 and the first low-side converter 2 of the first DC pole 110 of the first rectifier station 100 is controlled to be equal to the pole bus current IDL of the third high-side converter 5 and the third low-side converter 6 of the third DC pole 210 of the first inverter station 200, a DC current reference value I of the first high-side converter 1 and the first low-side converter 2 of the first DC pole 110 of the first rectifier station 100 is given. ord_p1 +ΔI r_p1 , the DC current reference value I of the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 ord_p1 , where ΔI r_p1 It is a current that fluctuates around zero and is greater than zero and less than 0.1 times the rated DC current. By superimposing the above pulsating current ΔI r_p1 , which is beneficial to arc extinguishing at the fault point.

[0169] According to some embodiments, the first current threshold value ranges from 0.01 to 0.1 times the rated DC current.

[0170] According to some embodiments, during a fault, the DC currents of the two converters at both ends of the ground fault can be quickly controlled by adjusting the current controller parameters or setting an initial value of the grid-commutated converter trigger angle.

[0171] It should be noted that controlling the DC currents of the two converters at both ends of the ground fault to be equal based on the DC current reference value is more conducive to arc extinguishing at the fault point than controlling the difference to be smaller than the first current threshold.

[0172] In some embodiments, during the de-ionization time of the flexible restart method, if the DC current of the two converters is controlled to be zero, then during the restart time of the flexible restart method, when the DC voltage of at least one of the two converters is increased, the DC current of one of the two converters is controlled to be zero, and the DC current of the other converter is controlled to be less than 0.25 pu.

[0173] In some embodiments, controlling the DC current of the other converter among the two converters to be less than 0.25 pu is achieved by reducing the minimum DC current limit value of the low-voltage current limiting link of the other converter among the two converters to be less than 0.25 pu.

[0174] According to some embodiments, optionally, during the de-freeing time of the flexible restart method, when any one of the two converters loses the ability to control DC current due to an AC system fault, if the converter of the rectifier station is a grid-commutated converter, the converter of the rectifier station controls the phase shift (such as a trigger angle greater than 120 degrees); if the converter of the inverter station is a grid-commutated converter, the converter of the inverter station controls the trigger angle greater than 90 degrees.

[0175] Furthermore, controlling the DC current of the two converters to zero includes: if both converters are grid-commutated converters, this is achieved by controlling the phase shift of the grid-commutated converter of the rectifier station and the trigger angle of the grid-commutated converter of the inverter station to be greater than 90 degrees; if both converters are voltage source converters, this is achieved by controlling the DC voltage of the two converters to zero or controlling the DC current of the two converters to zero.

[0176] Further,

[0177] In S140 , after the first deionization time has passed, the DC voltage of at least one of the two converters is increased within a first restart time.

[0178] Among them, increasing the DC voltage of at least one of the two converters includes: controlling the converter that was operating under DC voltage control before the fault to operate under DC voltage control, directly giving or giving according to the climbing slope the DC voltage reference values ​​of the two converters as the DC voltage before the fault or the DC voltage reduced compared to before the fault; or giving the difference between the DC current reference values ​​of the two converters; or controlling the converter that was operating under DC current control or active power control before the fault to operate under DC current control.

[0179] Furthermore, if both converters are grid-commutated converters, the difference in the DC current reference values ​​of the two converters is specifically achieved by reducing the DC current reference value of the DC current controller of the grid-commutated converter that operates under DC voltage control or maximum trigger angle control before the fault in the two converters.

[0180] Furthermore, if at least one of the two converters in the HVDC transmission system is a grid-commutated converter, then after controlling the DC currents of the two converters to be equal or the difference between them to be less than a first current threshold based on the DC current reference values ​​of the two converters, or before controlling the two converters to be locked, the method further includes: controlling the HVDC transmission system to remove or activate an AC filter connected to the AC system based on the needs of the AC system. If at least one of the two converters in the HVDC transmission system is a voltage source converter, then after controlling the DC currents of the two converters to be equal or the difference between them to be less than a first current threshold based on the DC current reference values ​​of the two converters, or before controlling the two converters to be locked, the method further includes: controlling the reactive power or AC voltage output by the voltage source converter based on the needs of the AC system.

[0181] Furthermore, if the converter is a grid-commutated converter, the converter locking includes: controlling the grid-commutated converter to stop sending trigger pulses, and / or controlling the grid-commutated converter to enter a bypass pair;

[0182] If the converter is a voltage source converter, the converter locking includes: controlling the voltage source converter to stop sending trigger pulses.

[0183] After the first deionization time, the DC voltages of the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are increased within the first restart time.

[0184] If the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100, and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are all grid-commutated converters, and before the fault, the third high-end converter 5 and the third low-end converter 6 are operating under DC voltage control or maximum trigger angle control, increasing the DC voltage of the two converters is achieved by giving the difference between the DC current reference values ​​of the first high-end converter 1, the first low-end converter 2 and the third high-end converter 5, the third low-end converter 6, that is, the DC current reference value of the first high-end converter 1 and the first low-end converter 2 is I ord_p1 The DC current reference value of the third high-end converter 5 and the third low-end converter 6 is I ord_p1 -ΔI, ΔI is the current margin of the DC current controllers of the two stations, such as 0.1 times the rated DC current.

[0185] If the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100, and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are all voltage source converters, and the third high-end converter 5 and the third low-end converter 6 are operating under DC voltage control before the fault, increasing the DC voltage of the two converters is achieved by controlling the third high-end converter 5 and the third low-end converter 6 to operate under DC voltage control, and the DC voltage reference value given to the third high-end converter 5 and the third low-end converter 6 is the DC voltage before the fault or the DC voltage that is lower than before the fault.

[0186] According to some embodiments, the first de-isolation time ranges from 20ms to 500ms; the first restart time ranges from 20ms to 300ms. It should be noted that the de-isolation time is related to the voltage level of the DC line and the geographical environment in which it is installed, and is determined according to the engineering situation. In S150, after the first restart time, if the absolute value of the DC voltage of the DC line is greater than or equal to the first voltage threshold, the DC voltage of the two converters continues to be increased to the rated voltage; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, the DC current of the two converters continues to be controlled to be equal or the difference is less than the first current threshold based on the DC current reference value of the two converters during the second de-isolation time.

[0187] After the first restart time, if the absolute value of the DC voltage of the first DC line 150 is greater than or equal to the first voltage threshold, the DC voltage of the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100, and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are continued to be increased to the rated voltage, and the DC line is restarted successfully; if the absolute value of the DC voltage of the first DC line 150 is less than the first voltage threshold, the pole bus current IDL of the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 is continued to be controlled to be equal to the pole bus current IDL of the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200, or the difference is less than the first current threshold.

[0188] According to some embodiments, the first voltage threshold value ranges from 0.05 to 1.0 times the rated DC voltage.

[0189] The DC current reference value in this step may be recalculated based on the demand of the HVDC power transmission system, or the DC current reference value in step S120 may be used.

[0190] In S160, after the second de-ionization time, the DC voltage of the two converters is increased to the rated voltage within the second restart time; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, the DC current of the two converters is continued to be controlled to be equal or the difference is less than the first current threshold based on the DC current reference value of the two converters.

[0191] The DC current of the converter includes at least one of the high-voltage bus current, the low-voltage bus current, the pole bus current of the DC pole where the converter is located, and the pole neutral bus current.

[0192] After the second de-ionization time and the second restart time, if the absolute value of the DC voltage of the first DC line 150 is greater than or equal to the first voltage threshold, the DC voltage of the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100, and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are continued to be increased to the rated voltage, and the DC line is restarted successfully; if the absolute value of the DC voltage of the first DC line 150 is less than the first voltage threshold, the pole bus current IDL of the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 is continued to be controlled to be equal to the pole bus current IDL of the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 during the third de-ionization time, or the difference is less than the first current threshold.

[0193] According to some embodiments, the second de-isolation time ranges from 50 ms to 500 ms; the second restart time ranges from 20 ms to 300 ms.

[0194] In S170, after the third de-ionization time, the DC voltage of the two converters is increased to the voltage reduction target value within the third restart time; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, the two converters are controlled to be locked.

[0195] After the third de-ionization time, after the third restart time, if the absolute value of the DC voltage of the first DC line 150 is greater than or equal to the first voltage threshold, the DC voltage of the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100, and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 continue to be increased to the voltage reduction target value (such as 0.8 times the rated voltage), and the DC line is restarted successfully; if the absolute value of the DC voltage of the first DC line 150 is less than the first voltage threshold, since the number of restarts is 3, the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are controlled to be locked.

[0196] According to some embodiments, if the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are grid-commutated converters, controlling the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 to be locked is to control the grid-commutated converter to stop sending trigger pulses; or / and to control the grid-commutated converter to be put into bypass. If the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are voltage source converters, the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are controlled to be locked to control the voltage source converter to stop sending trigger pulses.

[0197] According to some embodiments, if the AC system to which the HVDC transmission system is connected is a weak AC system or a new energy access system, and the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are grid-commutated converters, before the first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 are locked, the HVDC transmission system is controlled to disconnect the AC filter connected to the AC system based on the AC system's requirements. The AC system's requirements include reactive power requirements and AC voltage limits.

[0198] The weak AC system is an AC system with an AC / DC short-circuit ratio of less than 3. The short-circuit ratio of the first rectifier station 100 is the ratio of the short-circuit capacity of the first AC system 140 to the rated power of the high-voltage direct current transmission system. The short-circuit ratio of the first inverter station 200 is the ratio of the short-circuit capacity of the second AC system 240 to the rated power of the high-voltage direct current transmission system. The requirements of the first AC system 140 or the second AC system 240 include reactive power requirements and AC voltage limits.

[0199] According to some embodiments, the third de-isolation time ranges from 80 ms to 500 ms; and the third restart time ranges from 20 ms to 300 ms.

[0200] Figure 10 is a diagram showing the results of a simulation test when a DC line grounding fault occurs in the UHVDC system shown in Figure 8 under the prior art. The AC / DC short-circuit ratio in the simulation test is 2.5.

[0201] As shown in Figure 10, the DC voltage UDL is the pole bus voltage of the first DC pole 110 of the first rectifier station 100, the DC current IDL is the pole bus current of the first DC pole 110 of the first rectifier station 100 and the pole bus current of the third DC pole 210 of the first inverter station 200, the fault point current is the current flowing through the DC line ground fault point, the DC power is the DC power of the first DC pole 110 of the first rectifier station 100, and the AC voltage RMS is the maximum value of the three-phase AC voltage RMS of the first AC system 140 of the first rectifier station 100. The rated voltage of the UHVDC transmission system is 800 kV, the rated power is 10,000 MW, and the rated line voltage of the first AC system 140 is 530 kV, with a rated phase voltage of 306 kV.

[0202] Before the fault, the bipolar bus operated at rated power. When a ground fault was detected in the first DC line 150, a large current flowed through the fault point, causing the pole bus voltage UDL of the first DC pole 110 of the first rectifier station 100 to drop. The line protection function was activated, and the first high-side converter 1 and the first low-side converter 2 of the first DC pole 110 of the first rectifier station 100 were phase-shifted, causing the pole bus current IDL to be zero. After a first de-isolation time (150 ms), the first high-side converter 1 and the first low-side converter 2 were phase-shifted, and the pole bus voltage UDL began to increase. However, because the ground fault still existed, a large current flowed through the fault point again, causing the pole bus voltage UDL to drop again during its rise. The first high-side converter 1 and the first low-side converter 2 were phase-shifted again. After a second de-isolation time (200 ms), the first high-side converter 1 and the first low-side converter 2 were phase-shifted, and the pole bus voltage UDL began to increase. Because the ground fault had disappeared, the first DC line 150 was successfully restarted. During the fault process, the electric heat amount at the fault point is 0.32 MJ; the peak effective value of the phase voltage of the first AC system 140 is 382 kV (1.25 pu), and the current of the grounding electrode line is 6250 A (1.0 pu).

[0203] Figure 11 is a diagram showing simulation test results when a DC line ground fault occurs in the UHVDC system shown in Figure 8 according to an embodiment of the present application. The AC / DC short-circuit ratio in the simulation test is 2.5.

[0204] As shown in Figure 11, the DC voltage UDL is the pole bus voltage of the first DC pole 110 of the first rectifier station 100, the DC current IDL is the pole bus current of the first DC pole 110 of the first rectifier station 100 and the pole bus current of the third DC pole 210 of the first inverter station 200, the fault point current is the current flowing through the DC line ground fault point, the DC power is the DC power of the first DC pole 110 of the first rectifier station 100, and the AC voltage RMS is the maximum value of the three-phase AC voltage RMS of the first AC system 140 of the first rectifier station 100. The rated voltage of the UHVDC transmission system is 800 kV, the rated power is 10,000 MW, and the rated line voltage of the first AC system 140 is 530 kV, with a rated phase voltage of 306 kV.

[0205] Before the fault, the bipolar is operated at rated power. When a ground fault is detected in the first DC line 150, a large current flows through the fault point, the pole bus voltage UDL of the first DC pole 110 of the first rectifier station 100 drops, the line sudden change and the traveling wave protection are activated, and the DC current reference values ​​of the two converters are determined to be 0.345pu based on the reactive power balance requirement. The first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 operate in DC current control, and the pole bus current IDL is 0.345pu. The third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 also operate in DC current control, and the pole bus current IDL is 0.345pu. After the first deionization time (1 After a delay of 80 ms, the line low voltage protection is activated. The first high-end converter 1 and the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the third high-end converter 5 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 continue to control the DC current. After the second de-ionization time (200 ms), the DC voltage of the third high-end converter 5 and the third low-end converter 6 is increased, and the pole bus voltage UDL begins to increase. Since the ground fault has disappeared, the first DC line 150 is successfully restarted. During the fault process, the electric heat amount at the fault point is 0.18 MJ; the peak effective value of the phase voltage of the first AC system 140 is 348 kV (1.14 pu), and the current of the grounding electrode line is 4094 A (0.655 pu).

[0206] By comparing FIG10 and FIG11 , the method according to the present application can reduce the overvoltage level of the AC system and reduce the current flowing into the grounding electrode line.

[0207] If the high-end and low-end valve groups of the bipolar UHVDC transmission system shown in Figure 8 are constructed in separate stations and connected via DC lines, that is, the first high-end valve group 111 and the second high-end valve group 122 are at one station, and the first low-end valve group 112 and the second low-end valve group 121 are at another station, the first high-end valve group 111 and the first low-end valve group 112, and the second high-end valve group 122 and the second low-end valve group 121 are respectively connected via DC lines. If a fault occurs in the DC line, the high-end valve group will be unable to transmit DC power by shifting the phase or controlling the DC current to zero.

[0208] 12 is a diagram showing simulation test results under the prior art when a ground fault occurs in the DC line between the first high-end valve group 111 and the first low-end valve group 112. The AC-DC short-circuit ratio in the simulation test is 2.5.

[0209] In Figure 12, the DC voltage UDL is the busbar voltage of the first DC pole 110 of the first rectifier station 100. The DC current is the DC current of the first high-end valve group 111 and the DC current of the first low-end valve group 112. The fault point current is the current flowing through the DC line ground fault point. The DC power is the DC power of the first DC pole 110 of the first rectifier station 100. The AC voltage RMS is the maximum of the three-phase AC voltage RMS values ​​of the AC system connected to the first low-end valve group 112. The rated voltage of the UHVDC transmission system is 800 kV, the rated power is 8000 MW, and the rated line voltage of the AC system connected to the first low-end valve group 112 is 530 kV and the rated phase voltage is 306 kV.

[0210] Before the fault, the bipolar bus operated at rated power. When a ground fault was detected in the DC line between the first high-side valve group 111 and the first low-side valve group 112, a large current flowed through the fault point, causing the pole bus voltage UDL of the first DC pole 110 of the first rectifier station 100 to drop. The line protection was activated, and the first high-side converter 1 and the first low-side converter 2 of the first DC pole 110 of the first rectifier station 100 were phase-shifted, and the DC current was zero. After a first de-isolation time (150 ms), the first high-side converter 1 and the first low-side converter 2 were phase-shifted, and the pole bus voltage UDL began to increase. Because the ground fault still existed, a large current flowed through the fault point again, causing the pole bus voltage UDL to drop again during its rising process. The first high-side converter 1 and the first low-side converter 2 were phase-shifted again. After a second de-isolation time (200 ms), the first high-side converter 1 and the first low-side converter 2 were phase-shifted, and the pole bus voltage UDL began to increase. Because the ground fault had disappeared, the DC line was successfully restarted. During the fault process, the electric heat amount at the fault point is 0.10 MJ; the peak value of the effective phase voltage of the AC system connected to the first low-end valve group 112 is 364 kV (1.19 pu), the current of the grounding electrode line is 5000 A (1.0 pu), and the DC power of the first DC pole 110 is 0 MW (0 p.u.).

[0211] The occurrence of a ground fault in the DC line is determined by detecting a protection action, which includes at least one of line sudden change protection, line traveling wave protection, line low voltage protection, and line longitudinal differential protection.

[0212] 13 is a diagram showing simulation test results when a DC line between the first high-end valve group 111 and the first low-end valve group 112 is grounded, as provided in an embodiment of the present application. The AC / DC short-circuit ratio in the simulation test is 2.5.

[0213] In Figure 13 , the DC voltage UDL is the busbar voltage of the first DC pole 110 of the first rectifier station 100. The DC current is the DC current of the first high-end valve group 111 and the DC current of the first low-end valve group 112. The fault point current is the current flowing through the DC line ground fault point. The DC power is the DC power of the first DC pole 110 of the first rectifier station 100. The AC voltage RMS is the maximum of the three-phase AC voltage RMS values ​​of the AC system connected to the first low-end valve group 112. The rated voltage of the UHVDC transmission system is 800 kV, the rated power is 8000 MW, and the rated line voltage of the AC system connected to the first low-end valve group 112 is 530 kV and the rated phase voltage is 306 kV.

[0214] Before the fault, the bipolar pole operates at rated power. When a ground fault is detected in the DC line between the first high-end valve group 111 and the first low-end valve group 112, a large current flows through the fault point, and the pole bus voltage UDL of the first DC pole 110 of the first rectifier station 100 drops. The line sudden change and traveling wave protection are activated. Based on the requirement of increasing the reactive power consumption of the converter to suppress the AC overvoltage, the DC current reference value of the two converters is determined to be 1.0pu. The first high-end converter 1 of the first DC pole 110 of the first rectifier station 100 operates in DC current control, and the DC current IDC1N is 1.0pu. The first low-end converter 2 also operates in DC The DC current IDC2P is 1.0 pu. After the first de-isolation time (150 ms), the first low-end converter 2 increases the DC voltage. Because the ground fault still exists, current flows through the fault point again. The DC voltage of the first low-end converter 2 is less than the first voltage threshold (0.35 pu). After a delay of 80 ms, the line low-voltage protection is activated. The first high-end converter 1 and the first low-end converter 2 continue to control the DC current. After the second de-isolation time (200 ms), the first low-end converter 2 increases the DC voltage, and the pole bus voltage UDL begins to increase. Because the ground fault has disappeared, the DC line is successfully restarted. During the fault process, the electric heat amount at the fault point is 0.03 MJ; the peak phase RMS voltage of the AC system connected to the first low-end valve group 112 is 318 kV (1.04 pu), the current of the grounded pole line is 0 A (0 p.u.), and the DC power of the first DC pole 110 is 2000 MW (0.5 pu).

[0215] By comparing FIG12 and FIG13 , the method according to the present application can reduce the overvoltage level of the AC system, reduce the current flowing into the grounding electrode line, and maintain 50% of the DC power transmission to the fault electrode during the fault period.

[0216] FIG14 is a schematic diagram of the main circuit of another HVDC transmission system provided in an embodiment of the present application. It is a symmetrical monopole conventional DC transmission system. Each DC pole adopts a structure in which twelve-pulse grid-commutated converters are connected in series. The system can be applied to HVDC transmission projects that do not have grounded electrode lines.

[0217] The HVDC transmission system main circuit includes a second rectifier station 300, a second inverter station 400, a third DC line 350, and a fourth DC line 360. During normal operation, one of the second rectifier station 300 and the second inverter station 400 is grounded internally. For example, the second internal grounding switch 474 of the second inverter station 400 is in the closed position, and the first internal grounding switch 374 of the second rectifier station 300 is in the open position.

[0218] The second rectifier station 300 includes a fifth DC pole 310 , a sixth DC pole 320 , a third AC filter bank 318 , a third AC system 340 , a fifth converter transformer inlet switch 331 , and a sixth converter transformer inlet switch 333 .

[0219] The fifth DC pole 310 includes a fifth valve group 311, a fifth converter transformer 316, a fifth DC pole neutral bus switch 319, a fifth DC filter 393, and a fifth smoothing reactor 391. The fifth valve group 311 includes a fifth converter 301. The fifth converter 301 includes a grid-commutated converter with a twelve-pulse bridge circuit.

[0220] The sixth DC pole 320 includes a sixth valve group 321, a sixth converter transformer 326, a sixth DC pole neutral bus switch 329, a sixth DC filter 394, and a sixth smoothing reactor 392. The sixth valve group 321 includes a sixth converter 302, which includes a twelve-pulse bridge circuit grid-commutated converter.

[0221] Second inverter station 400 includes a seventh DC pole 410, an eighth DC pole 420, a fourth AC filter bank 418, a fourth AC system 440, a seventh converter transformer inlet switch 431, and an eighth converter transformer inlet switch 433. The fifth DC pole 310 and the seventh DC pole 410 are pole I, and the sixth DC pole 320 and the eighth DC pole 420 are pole II.

[0222] The seventh DC pole 410 includes a seventh valve group 411, a seventh converter transformer 416, a seventh DC pole neutral bus switch 419, a seventh DC filter 497, and a seventh smoothing reactor 495. The seventh valve group 411 includes a seventh converter 401, which includes a grid-commutated converter with a twelve-pulse bridge circuit.

[0223] The eighth DC pole 420 includes an eighth valve group 421, an eighth converter transformer 426, an eighth DC pole neutral bus switch 429, an eighth DC filter 498, and an eighth smoothing reactor 496. The eighth valve group 421 includes an eighth converter 402, which includes a twelve-pulse bridge circuit grid-commutated converter.

[0224] According to some embodiments, the fifth converter 301, the sixth converter 302, the seventh converter 401, and the eighth converter 402 in Figure 14 may also be at least one of a grid-commutated converter or a voltage source converter in a six-pulse bridge circuit. The converter transformer type in Figure 14 is modified based on the type of the connected converter. The pulsating bridge circuit includes non-turnoff half-controlled power semiconductor devices, typically thyristor devices.

[0225] The voltage source converter includes at least one of a modular multilevel converter (MMC), a hybrid multilevel converter (HMC), a cascaded two-level (CTL) converter, and a stacked two-level (STL) converter, and the converter includes fully controllable power semiconductor devices that can be turned off. The modular multilevel converter (MMC) includes at least one of a full-bridge submodule structure and a hybrid half-bridge and full-bridge submodule structure.

[0226] If the DC converters of the second rectifier station 300 and the second inverter station 400 are both grid-commutated converters, this constitutes a conventional DC transmission system. If the DC converters of the second rectifier station 300 and the second inverter station 400 are both voltage source converters, this constitutes a flexible DC transmission system. Voltage source converters have the ability to regulate voltage to zero or negative voltage, such as modular multilevel converters based on full-bridge submodules or modular multilevel converters based on a mix of half-bridge and full-bridge submodules. If the second rectifier station 300 and the second inverter station 400 have both grid-commutated converters and voltage source converters, this constitutes a hybrid DC transmission system.

[0227] The second rectifier station 300 is not grounded internally. The second inverter station 400 is grounded internally via the second internal grounding switch 474. During positive power transmission, the third AC system 340 of the second rectifier station 300 converts AC power into DC power via its fifth converter 301 and sixth converter 302, and transmits the power to the second inverter station 400 via the third DC line 350 and fourth DC line 360. The second inverter station 400 converts DC power into AC power via its seventh converter 401 and eighth converter 402, and transmits the power to the fourth AC system 440 of the second inverter station 400, thereby achieving positive DC power transmission. The converters of the rectifier station generally operate in current control, while the converters of the inverter station generally operate in voltage control or maximum firing angle control (AMAX). It should be noted that maximum firing angle control (AMAX) is only applicable to grid-commutated converters and not to voltage source converters.

[0228] The analog signals collected by the second rectifier station 300 and the second inverter station 400 are: the high-voltage bus current IDCP and the low-voltage bus current IDCN on the DC side of the converter, the pole bus current IDL, the DC filter head-end current IZT1, the station grounding current IDGND, the pole bus voltage UDL and the pole-neutral bus voltage UDN.

[0229] FIG15 is a flow chart of another method for flexible restarting of a DC line fault in a HVDC transmission system provided by an embodiment of the present application, illustrating a control flow when a DC side grounding fault occurs in the third DC line 350 of the HVDC transmission system shown in FIG14 .

[0230] The converters in the fifth DC pole 310 of the second rectifier station 300 and the seventh DC pole 410 of the second inverter station 400 are both grid-commutated converters. They operate in bipolar mode before a DC-side ground fault occurs. When a ground fault occurs in the third DC line 350 of the HVDC transmission system, the restart frequency is set to four times: two restarts at full voltage with current, one restart at full voltage without current, and one restart at reduced voltage without current. The control flow is as follows.

[0231] In S210, at least one converter in each of the two DC poles at both ends of the ground fault is controlled to continue to operate.

[0232] The DC line grounding fault is determined by the line sudden change, traveling wave protection, line low voltage and / or line longitudinal differential protection action.

[0233] When a DC line fault occurs, it causes a DC voltage drop. The speed of the voltage drop varies depending on the fault location. By determining the speed of the voltage drop, the DC line fault can be detected. The line sudden change protection action criterion is as follows: dUDL / dt <dUDL_set, |UDL|<UDL_set。

[0234] Where, dUDL / dt is the DC voltage mutation per unit time, dUDL_set is the constant value of the DC voltage mutation, UDL is the pole bus voltage, and UDL_set is the constant value of the DC voltage.

[0235] By detecting the DC voltage, if it is found that the DC voltage is low for a certain period of time, and there is no AC system fault or commutation failure, it is determined to be a DC line fault. The line low voltage protection action criteria are as follows: |UDL| <UDL_set1。

[0236] Among them, UDL is the pole bus voltage, and UDL_set1 is the DC voltage constant.

[0237] When a DC line ground fault is detected based on line sudden change, traveling wave protection, line low voltage, and / or line longitudinal differential protection, at least one converter on each of the two DC poles at both ends of the ground fault is controlled to continue operating. In this embodiment, both the fifth converter 301 and the seventh converter 401 are controlled to continue operating.

[0238] In S220 , direct current reference values ​​of two converters at both ends of the ground fault are determined based on the requirements of the HVDC transmission system, where the two converters include one converter at each end of the ground fault that continues to operate.

[0239] The requirements of the HVDC transmission system include at least one of the following: active power requirement, reactive power requirement, ground current limit, current limit at the DC pole where the fault is located, current limit through the fault point, and AC harmonic suppression requirement. If the HVDC transmission system has more than one requirement, the different requirements are prioritized. Specifically, based on the DC current reference values ​​of the two converters determined based on the active power requirement, reactive power requirement, ground current limit, current limit at the DC pole where the fault is located, current limit through the fault point, and AC harmonic suppression requirement, a current greater than zero and with an absolute value less than 0.1 times the rated DC current can be superimposed on the DC current reference value of at least one of the converters.

[0240] Taking the reactive power demand of the second rectifier station 300 or the second inverter station 400 as an example, the reactive power demand calculation method of the converter is as follows. ord_p2 =0.5×I d_p2 ×U di0_p2 ×(2μ2+sin2α2-sin2(α2+μ2)) / (cosα2-cos(α2+μ2)), Q ord_p1 =Q ord -Q ord_p2 ,

[0241] According to the reactive power demand of the twelve-pulse converter of the rectifier station, the DC current reference value is calculated as follows. ord_p1 =Q ord_p1 / (0.5×U di0_p1 ×(2μ1+sin2α1-sin2(α1+μ1)) / (cosα1-cos(α1+μ1)).

[0242] Where, I ord_p1 is the DC current reference value of pole I and pole II, Q ord , Q ord_p1 , Q ord_p2 Respectively, the reactive power demand of bipolar, pole I, and pole II, I d_p2 is the DC current of pole II, U di0_p1 、U di0_p2 are the no-load DC voltages of the six-pulse converters in the converters of poles I and II, α1 and α2 are the trigger angles of the converters of poles I and II, and μ1 and μ2 are the commutation angles of the converters of poles I and II, respectively.

[0243] Given that the DC current reference value of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 is I ord_p1 .

[0244] Taking the active power requirement and ground current limit value requirement of the second rectifier station 300 or the second inverter station 400 as an example, the following formula is shown. ord_p2 =P ord / U d_p2 ,

[0245] I ord_p1 ≥I ord_p2 -I del_lim And I ord_p1 ≤I ord_p2 +I del_lim .

[0246] Where, I ord_p1 , I ord_p2 are the DC current reference values ​​of poles I and II, I del_lim is the ground current limit value, P ord is the active power demand value, U d_p2 is the DC voltage of the converter at pole II.

[0247] Given that the DC current reference value of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 is I ord_p1 .

[0248] In S230 , the DC currents of the two converters at both ends of the ground fault are controlled to be equal or to have a difference smaller than a first current threshold based on the DC current reference value within the first de-isolation time.

[0249] The pole bus current IDL of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 is controlled to be equal to the pole bus current IDL of the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 and to be Iord_p1. Alternatively, the difference between the pole bus current IDL of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the pole bus current IDL of the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 is controlled to be less than the first current threshold, such as controlling the pole bus current IDL of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 to be I ord_p1 , the pole bus current IDL of the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 is I ord_p1 -ΔI ord_p1 , where ΔI ord_p1 is less than the first current threshold.

[0250] According to some embodiments, the first current threshold value ranges from 0.01 to 0.1 times the rated DC current.

[0251] In S240 , after the first deionization time has passed, the DC voltage of at least one of the two converters is increased within a first restart time.

[0252] After the first deionization time, the DC voltages of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are increased within the first restart time.

[0253] If the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are both grid-commutated converters, and the seventh converter 401 is operating under DC voltage control or maximum firing angle control before the fault, the DC voltage of the two converters can be increased by setting the difference between the DC current reference values ​​of the fifth converter 301 and the seventh converter 401, that is, the DC current reference value of the fifth converter 301 is I ord_p1 , the DC current reference value of the seventh converter 401 is I ord_p1 -ΔI, ΔI is the current margin of the DC current controllers of the two stations.

[0254] If the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are both voltage source converters, and the seventh converter 401 operates under DC voltage control before the fault, increasing the DC voltage of the two converters is achieved by controlling the seventh converter 401 to operate under DC voltage control, and the DC voltage reference value of the seventh converter 401 is given as the DC voltage before the fault or the DC voltage that is lower than before the fault.

[0255] According to some embodiments, the first de-isolation time ranges from 20 ms to 500 ms; the first restart time ranges from 20 ms to 300 ms.

[0256] In S250, after the first restart time, if the absolute value of the DC voltage of the DC line is greater than or equal to the first voltage threshold, continue to increase the DC voltage of the two converters to the rated voltage; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, continue to control the DC currents of the two converters to be equal or the difference to be less than the first current threshold based on the DC current reference values ​​of the two converters during the second de-ionization time.

[0257] After the first restart time, if the absolute value of the DC voltage of the third DC line 350 is greater than or equal to the first voltage threshold, the DC voltage of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 continue to be increased to the rated voltage, and the DC line restarts successfully; if the absolute value of the DC voltage of the third DC line 350 is less than the first voltage threshold, the pole bus current IDL of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 continues to be controlled to be equal to the pole bus current IDL of the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 or the difference is less than the first current threshold.

[0258] According to some embodiments, the first voltage threshold value ranges from 0.05 to 1.0 times the rated DC voltage.

[0259] In S260, after the second de-isolation time, the DC voltage of at least one of the two converters is increased within the second restart time; after the second restart time, if the absolute value of the DC voltage of the DC line is greater than or equal to the first voltage threshold, the DC voltage of the two converters is continued to be increased to the rated voltage; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, the DC currents of the two converters are continued to be controlled to be equal or the difference is less than the first current threshold based on the DC current reference values ​​of the two converters within the third de-isolation time.

[0260] After the second deionization time and the second restart time, if the absolute value of the DC voltage of the third DC line 350 is greater than or equal to the first voltage threshold, continue to increase the DC voltage of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 to the rated voltage; if the absolute value of the DC voltage of the third DC line 350 is less than the first voltage threshold, continue to control the pole bus current IDL of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the pole bus current IDL of the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 to be equal or the difference is less than the first current threshold.

[0261] According to some embodiments, the second de-isolation time ranges from 50 ms to 500 ms; the second restart time ranges from 20 ms to 300 ms.

[0262] In S270, after the third de-ionization time, the DC voltage of at least one of the two converters is increased within the third restart time; after the third restart time, if the absolute value of the DC voltage of the DC line is greater than or equal to the first voltage threshold, the DC voltage of the two converters continues to be increased to the rated voltage; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, the DC current of the two converters is controlled to be zero within the fourth de-ionization time.

[0263] After the third de-ionization time and the third restart time, if the absolute value of the DC voltage of the third DC line 350 is greater than or equal to the first voltage threshold, the DC voltage of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are further increased to the rated voltage. If the absolute value of the DC voltage of the third DC line 350 is less than the first voltage threshold, the pole bus current IDL of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the pole bus current IDL of the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are controlled to be zero. If the fifth converter 301 is a grid-commutated converter, the fifth converter 301 is phase-shifted, i.e., the firing angle of the fifth converter 301 is controlled to be greater than 90 degrees, preferably 120 degrees or 164 degrees. If the seventh converter 401 is a grid-commutated converter, since grid-commutated converters do not experience reverse current flow, no special control is required. The seventh converter 401 is normally controlled to operate under DC voltage control or maximum firing angle control. If the fifth converter 301 and the seventh converter 401 are voltage source converters, the DC current of the converters is controlled to be zero.

[0264] According to some embodiments, if the AC system to which the HVDC transmission system is connected is a weak AC system or a new energy access system with poor tolerance to AC overvoltage, and the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are grid-commutated converters, before controlling the DC current of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 to be zero, the HVDC transmission system is controlled to cut off part of the AC filters connected to the AC system according to the needs of the AC system.

[0265] The above-mentioned weak AC system is an AC system with an AC / DC short-circuit ratio of less than 3. The short-circuit ratio of the second rectifier station 300 is the ratio of the short-circuit capacity of the third AC system 340 to the rated power of the high-voltage direct current transmission system. The short-circuit ratio of the second inverter station 400 is the ratio of the short-circuit capacity of the fourth AC system 440 to the rated power of the high-voltage direct current transmission system. The demand of the third AC system 340 or the fourth AC system 440 includes reactive power demand and AC voltage limit.

[0266] According to some embodiments, the third de-isolation time ranges from 80 ms to 500 ms; and the third restart time ranges from 20 ms to 300 ms.

[0267] According to some embodiments, during the third restart time, the DC voltage of the two converters is increased by controlling the fifth converter 301 to operate in DC current control mode, the DC current of the seventh converter 401 is controlled to zero, and the minimum DC current limit of the low-voltage current limiting link of the fifth converter 301 is reduced, for example, to 0.1 times the rated DC current. At this time, if the absolute value of the DC voltage of the DC line is less than the minimum DC voltage of the low-voltage current limiting link, the DC current command value Iord_p1 of the fifth converter 301 is 0.1 times the rated DC current. By controlling the current flowing through the DC line grounding point to a low value, the heat injected into the DC line grounding point can be reduced if the restart is unsuccessful, facilitating the next arc extinction.

[0268] In S280, after the fourth de-ionization time, the DC voltage of at least one of the two converters is increased within the fourth restart time; after the fourth restart time, if the absolute value of the DC voltage of the DC line is greater than or equal to the first voltage threshold, the DC voltage of the two converters continues to be increased to the step-down target voltage; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, the two converters are controlled to be locked.

[0269] After the fourth de-ionization time and the fourth restart time, if the absolute value of the DC voltage of the third DC line 350 is greater than or equal to the first voltage threshold, the DC voltage of the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 continue to be increased to the voltage reduction target value (such as 0.8 times the rated DC voltage); if the absolute value of the DC voltage of the third DC line 350 is less than the first voltage threshold, the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are controlled to be locked.

[0270] According to some embodiments, if the AC system to which the HVDC transmission system is connected is a weak AC system or a new energy access system, and the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are grid-commutating converters, before the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are locked, the HVDC transmission system is controlled to cut off part of the AC filters connected to the AC system according to the needs of the AC system.

[0271] According to some embodiments, if the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are grid-commutated converters, controlling the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 to be blocked is to control the grid-commutated converter to stop sending trigger pulses; or / and to control the grid-commutated converter to be placed in bypass mode. If the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 are voltage source converters, controlling the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 and the seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 to be blocked is to control the voltage source converter to stop sending trigger pulses.

[0272] According to some embodiments, the fourth de-ionization time ranges from 80 ms to 500 ms; and the fourth restart time ranges from 20 ms to 300 ms.

[0273] According to some embodiments, during the fourth restart time, the DC voltage of the two converters is increased by controlling the fifth converter 301 to operate in DC current control, the DC current of the seventh converter 401 is controlled to be zero, and the minimum DC current limit of the low-voltage current limiting link of the fifth converter 301 is reduced, such as taking a value of 0.1 times the rated DC current. At this time, if the absolute value of the DC voltage of the DC line is less than the minimum DC voltage of the low-voltage current limiting link, the DC current command value Iord_p1 of the fifth converter 301 is 0.1 times the rated DC current.

[0274] 16 , 17 , 18 and 19 are diagrams showing simulation test results of a grid-commutated converter in which the fifth converter 301 , the sixth converter 302 , the seventh converter 401 and the eighth converter 402 in FIG. 14 are six-pulse bridge circuits.

[0275] Figure 16 shows the results of a simulation test of a single restart in the HVDC transmission system shown in Figure 14 under the prior art when a DC line ground fault occurs. The AC / DC short-circuit ratio in the simulation test was 8.3. The second inverter station 400 was internally grounded, while the second rectifier station 300 was not.

[0276] As shown in Figure 16, the positive voltage is the bus voltage of the fifth DC pole 310 of the second rectifier station 300, the negative voltage is the bus voltage of the sixth DC pole 320 of the second rectifier station 300, the DC current is the bus current of the fifth DC pole 310 of the second rectifier station 300 and the bus current of the seventh DC pole 410 of the second inverter station 400, the AC voltage is the phase voltage of the third AC system 340, and the trigger angles are the trigger angles of the fifth converter 301 and the sixth converter 302. The rated voltage of the HVDC transmission system is 200 kV, the rated power is 1200 MW, and the rated line voltage of the third AC system 340 is 230 kV, and the rated phase voltage is 133 kV.

[0277] Before the fault, the system was operating at rated power. When a ground fault was detected on the third DC line 350, a large current flowed through the fault point to the internal grounding point of the second inverter station 400. This caused the pole bus voltage (UDL) of the fifth DC pole 310 of the second rectifier station 300 to drop. The line's sudden change and traveling wave protection activated, causing the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 to shift phase, and the pole bus current (IDL) to zero. After the first de-ionization time (150ms), the fifth converter 301 released the phase shift, and the pole bus voltage (UDL) began to increase. Since the ground fault had disappeared, the first DC line 150 was successfully restarted. During the fault, the peak RMS phase voltage of the third AC system 340 was 148 kV (1.11 pu), and the DC power of the second rectifier station 300 was 0 MW (0 p.u.).

[0278] Figure 17 shows the simulation results of a single restart in the HVDC transmission system shown in Figure 14 when a DC line ground fault occurs, according to an embodiment of the present application. The AC / DC short-circuit ratio in the simulation was 8.3. The second inverter station 400 was internally grounded, while the second rectifier station 300 was not.

[0279] The positive voltage shown in Figure 17 is the bus voltage of the fifth DC pole 310 of the second rectifier station 300, the negative voltage is the bus voltage of the sixth DC pole 320 of the second rectifier station 300, the DC current is the bus current of the fifth DC pole 310 of the second rectifier station 300 and the bus current of the seventh DC pole 410 of the second inverter station 400, the AC voltage is the phase voltage of the third AC system 340, and the firing angles are the firing angles of the fifth converter 301 and the sixth converter 302, respectively. The rated voltage of the HVDC transmission system is 200 kV, the rated power is 1200 MW, and the rated line voltage of the third AC system 340 is 230 kV, and the rated phase voltage is 133 kV.

[0280] Before the fault, the system was operating at rated power. When a ground fault was detected in the third DC line 350, a large current flowed through the fault point and then to the grounding point within the second inverter station 400. The pole bus voltage UDL of the fifth DC pole 310 of the second rectifier station 300 dropped, and the line sudden change and traveling wave protection were activated. Based on the demand for maintaining more active power, the DC current reference value of the two converters was determined to be 1.0 pu. The fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 operated in DC current control with a pole bus current IDL of 1.0 pu. The seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 also operated in DC current control with a pole bus current IDL of 1.0 pu. After the first deionization time (150 ms), the DC current reference value of the seventh converter 401 was reduced by 0.1 pu, thereby increasing the DC voltage of the seventh converter 401. Since the ground fault had disappeared, the third DC line 350 was successfully restarted. During the fault, the peak effective value of the phase voltage of the third AC system 340 is 129 kV (0.97 pu), and the DC power of the second rectifier station 300 is 541 MW (about 0.5 pu).

[0281] By comparing FIG. 16 and FIG. 17 , the method according to the present application can reduce the overvoltage level of the AC system, reduce the current flowing into the grounding electrode line, and maintain about 50% of the DC power transmission during the fault period.

[0282] Figure 18 is a diagram showing the results of two restart simulation tests when a DC line ground fault occurs in the HVDC transmission system shown in Figure 14 according to an embodiment of the present application. The second inverter station 400 is grounded internally, while the second rectifier station 300 is not grounded.

[0283] The positive voltage shown in Figure 18 is the pole bus voltage of the fifth DC pole 310 of the second rectifier station 300, the negative voltage is the pole bus voltage of the sixth DC pole 320 of the second rectifier station 300, the DC currents are the pole bus currents of the fifth DC pole 310 of the second rectifier station 300 and the pole bus currents of the seventh DC pole 410 of the second inverter station 400, respectively. The fault point current is the current at the ground fault point of the third DC line 350. The rated voltage of the HVDC transmission system is 200 kV, the rated power is 1200 MW, and the rated line voltage of the third AC system 340 is 230 kV, and the rated phase voltage is 133 kV.

[0284] Before the fault, the system operates at rated power. When a ground fault is detected in the third DC line 350, a large current flows through the fault point and reaches the grounding point in the second inverter station 400. The pole bus voltage UDL of the fifth DC pole 310 of the second rectifier station 300 drops, and the line mutation and traveling wave protection are activated. Based on the requirements of maintaining active power and suppressing AC harmonics, the DC current reference value of the two converters is determined to be 0.67pu. Within the first de-ionization time (150ms), the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 operates in DC current control, and the pole bus current IDL is 0.67pu. The seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 also operates in DC current control, and the pole bus current IDL After the first de-isolation time, the DC current reference value of the seventh converter 401 is reduced by 0.1 pu, increasing the DC voltage of the seventh converter 401. Because the ground fault still exists, a small current (0.1 pu) flows through the fault point, and the pole bus voltage UDL falls below the first voltage threshold (0.35 pu). After an 80 ms delay, line undervoltage protection is activated. During the second de-isolation time (200 ms), the fifth converter 301 and the seventh converter 401 continue to control the DC current. After the second de-isolation time, the DC current reference value of the seventh converter 401 is reduced by 0.1 pu, increasing the DC voltage of the seventh converter 401. The pole bus voltage UDL begins to increase. Because the ground fault has disappeared, the third DC line 350 is successfully restarted. The sixth DC pole 320 of the second rectifier station 300 uses the faulty third DC line 350 as a current path, with a DC power of 380 MW (approximately 0.32 pu). It should be noted that because the fifth and sixth converters 301 and 302 are six-pulse bridge circuits, inconsistent trigger angles during the de-ionization and restart processes can generate significant fifth and seventh harmonics. Similarly, the seventh and eighth converters 401 and 402 also generate significant fifth and seventh harmonics. If the HVDC transmission system is not equipped with an AC filter to filter out fifth and seventh harmonics, the trigger angles of the fifth and sixth converters 301 and 302 can be controlled to be the same to suppress harmonics, but the trigger angles of the seventh and eighth converters 401 and 402 can only be controlled to be different. In this case, the fifth and sixth converters 301 and 302 act as one of the two converters at either end of the fault.

[0285] Figure 19 is a diagram showing the results of a simulation test of the HVDC transmission system shown in Figure 14, provided in an embodiment of the present application, when a DC line ground fault occurs. The second inverter station 400 is grounded internally, while the second rectifier station 300 is not grounded.

[0286] As shown in FIG19 , the DC voltages are respectively the pole bus voltage of the fifth DC pole 310 of the second rectifier station 300 and the pole bus voltage of the sixth DC pole 320 of the second rectifier station 300; the DC currents are respectively the pole bus current of the fifth DC pole 310 of the second rectifier station 300 and the pole bus current of the seventh DC pole 410 of the second inverter station 400; the trigger angles are respectively the trigger angles of the fifth converter 301 and the sixth converter 302; the DC line fault flag is a DC line fault signal; the ride-through flag is a signal indicating that the DC currents of the fifth converter 301 and the seventh converter 401 are equal during the first de-ionizing time and the second de-ionizing time; the phase shift flag is a signal indicating that the DC currents of the fifth converter 301 and the seventh converter 401 are zero during the third de-ionizing time and the fourth de-ionizing time; and the trip flag is a trip signal. The rated voltage of the HVDC transmission system is 200kV and the rated power is 1200MW. The rated line voltage of the third AC system 340 is 230kV and the rated phase voltage is 133kV.

[0287] Before the fault, it operates at rated power. When a ground fault is detected in the third DC line 350, a large current flows through the fault point, and a large current flows through the fault point to the grounding point in the second inverter station 400. The pole bus voltage UDL of the fifth DC pole 310 of the second rectifier station 300 drops, the line mutation amount and the traveling wave protection are activated, and the DC current reference value of the two converters is determined to be 1.0pu based on the active power demand. Within the first de-ionization time (150ms), the fifth converter 301 of the fifth DC pole 310 of the second rectifier station 300 operates in DC current control, and the pole bus current IDL is 1.0pu. The seventh converter 401 of the seventh DC pole 410 of the second inverter station 400 also operates in DC. Flow control, the pole bus current IDL is 1.0pu; after the first de-ionization time, by reducing the DC current reference value of the seventh converter 401 by 0.1pu, the DC voltage of the seventh converter 401 is increased. Since the ground fault still exists, a small current (0.1pu) flows through the fault point, and the pole bus voltage UDL is less than the first voltage threshold (0.35pu). After a delay of 80ms, the line low voltage protection is activated. Based on the active power demand, the DC current reference value of the two converters is determined to be 1.0pu. Within the second de-ionization time (200ms), the fifth converter 301 and the seventh converter 401 continue to control the DC current. After the second de-ionization time, by reducing the DC current reference value of the seventh converter 401, the DC voltage of the seventh converter 401 is increased. The reference value of the current is 0.1pu, and the DC voltage of the seventh converter 401 is increased. Since the ground fault still exists, a small current (0.1pu) flows through the fault point. The pole bus voltage UDL is less than the first voltage threshold (0.35pu) after a delay of 80ms, and the line low voltage protection is activated. Within the third de-isolation time (150ms), the fifth converter 301 is controlled to shift the phase. After the third de-isolation time, the minimum DC current limit value of the low-voltage current limiting link of the fifth converter 301 and the seventh converter 401 is given to be 0.1pu. The DC current of the fifth converter 301 and the seventh converter 401 is controlled to increase the DC voltage of the seventh converter 401. Since the ground fault still exists, a small current ( 0.1pu), the pole bus voltage UDL is less than the first voltage threshold (0.35pu) after a delay of 80ms, the line low voltage protection is activated, and within the fourth de-isolation time (200ms), the fifth converter 301 is controlled to shift phase. After the fourth de-isolation time, the minimum DC current limit of the low voltage current limiting link of the fifth converter 301 and the seventh converter 401 is given to be 0.1pu, and the DC current of the fifth converter 301 and the seventh converter 401 is controlled to increase the DC voltage of the seventh converter 401. The DC voltage target value is set to the step-down target voltage. Since the ground fault still exists, a small current (0.1pu) flows through the fault point, and the pole bus voltage UDL is less than the first voltage threshold (0.35puAfter a delay of 80ms, the line low voltage protection is activated, controlling the fifth converter 301, the sixth converter 302, the seventh converter 401, and the eighth converter 402 to be locked.

[0288] FIG20 is a schematic structural diagram of a device 500 for controlling a DC line fault in a HVDC transmission system according to an embodiment of the present application. The device includes a second detection unit 510 and a fourth control unit 520 .

[0289] The second detection unit 510 is used to detect parameters of the HVDC transmission system, including the high-voltage bus current IDCP and low-voltage bus current IDCN on the DC side of the converter, the pole bus current IDL, the pole-neutral bus current IDNC, the DC filter head-end current IZT1, the grounding electrode current IDEL, the station grounding current IDGND, the pole bus voltage UDL and the pole-neutral bus voltage UDN, and the requirements of the HVDC transmission system.

[0290] The fourth control unit 520 controls at least one converter at each end of the ground fault to continue operating when it determines based on the parameters of the high-voltage direct current transmission system that a fault occurs in the DC line of the high-voltage direct current transmission system; determines the DC current reference values ​​of the two converters at both ends of the ground fault based on the needs of the high-voltage direct current transmission system, and the two converters include a converter that continues to operate at each end of the ground fault; controls the DC currents of the two converters to be equal or the difference is less than the first current threshold based on the DC current reference values ​​of the two converters within the first de-isolation time; after the first de-isolation time, increases the DC voltages of the two converters within the first restart time; after the first restart time, if the absolute value of the DC voltage of the DC line is greater than or equal to the first voltage threshold, continues to increase the DC voltages of the two converters; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, when restarting When the number of times is greater than 1, the DC currents of the two converters are continued to be controlled to be equal or the difference is less than the first current threshold based on the DC current reference values ​​of the two converters during the second de-isolation time, or the DC currents of the two converters are controlled to be zero. When the number of restarts is 1, the two converters are controlled to be locked. When the number of restarts is greater than 1, after the second de-isolation time, the DC voltages of the two converters are increased during the second restart time. If the absolute value of the DC voltage of the DC line is less than the first voltage threshold, when the number of restarts is greater than 2, the DC currents of the two converters are continued to be controlled to be equal or the difference is less than the first current threshold based on the DC current reference values ​​of the two converters during the third de-isolation time, or the DC currents of the two converters are controlled to be zero. When the number of restarts is 2, the two converters are controlled to be locked. By analogy, the flexible restart of the high-voltage direct current transmission system is realized.

[0291] In summary, the present application provides a flexible restart method and control device for a DC line fault in a high-voltage direct current transmission system. When a ground fault occurs in the DC line of the high-voltage direct current transmission system, by controlling the DC currents of the two converters at both ends of the ground fault to be equal or the difference is less than a first current threshold, the voltage at the ground fault point can be effectively controlled to achieve de-ionization. At the same time, since DC current still flows through the DC line, the grid commutation converter can continue to consume reactive power, and the high-voltage direct current transmission system can maintain AC and DC reactive power balance.

[0292] It should be clearly understood that this application describes how to form and use specific examples, but this application is not limited to any details of these examples. On the contrary, based on the teaching of the content disclosed in this application, these principles can be applied to many other embodiments.

[0293] Furthermore, it should be noted that the aforementioned figures are merely illustrative of the processes included in the methods according to exemplary embodiments of the present application and are not intended to be limiting. It is readily understood that the processes illustrated in the aforementioned figures do not indicate or limit the temporal order of these processes. Furthermore, it is readily understood that these processes may be executed synchronously or asynchronously, for example, in multiple modules.

[0294] While the exemplary embodiments of the present application have been specifically illustrated and described above, it should be understood that the present application is not limited to the detailed structures, configurations, or implementations described herein; rather, the present application is intended to encompass various modifications and equivalent configurations within the spirit and scope of the appended claims.

Claims

1. A method for restarting a DC line fault in a high-voltage DC transmission system, characterized in that: The high-voltage direct current transmission system includes at least one rectifier station and at least one inverter station, wherein the rectifier station and the inverter station each include a double direct current pole, and each direct current pole of the double direct current pole includes at least one converter; the restart method includes: If a ground fault occurs in a DC line connected to one of the DC poles, and the other DC pole is a non-fault DC pole and operates in dual DC pole power control, in a first restart stage, the faulty DC pole of the high voltage DC transmission system is controlled to restart, and the dual DC pole power command value is the dual DC pole power before the fault; When the restart times set in the first restart stage are reached and the restart is still not successful, if the dual DC pole power command value is greater than the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole, in the second restart stage, the dual DC pole power command value is adjusted to the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole, and the faulty DC pole of the high voltage direct current transmission system is controlled to restart; When the restart times set in the second restart stage are reached and the restart is still not successful, the converter of the faulty DC pole is controlled to be locked.

2. A method for restarting a DC line fault in a high-voltage DC transmission system according to claim 1, characterized in that: The maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole is less than or equal to the short-time overload power, and the short-time overload power is any one of the second-level overload power, minute-level overload power or hour-level overload power.

3. A method for restarting a DC line fault in a high-voltage DC transmission system according to claim 2, characterized in that: The step of controlling the maximum transmittable power to be less than or equal to the short-term overload power comprises: The DC current of the non-fault DC pole or the dual DC pole is controlled to be less than or equal to a short-time overload current, wherein the short-time overload current is any one of a second-level overload current, a minute-level overload current or an hour-level overload current.

4. A method for restarting a DC line fault in a high-voltage DC transmission system according to claim 1, characterized in that: In the second restart stage, the dual DC pole power command value is the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole and remains unchanged, and is less than or equal to the short-term overload power in the second restart stage.

5. A method for restarting a DC line fault in a high-voltage DC transmission system according to claim 1, characterized in that: The number of restarts and time in the first restart phase are determined according to the conditions of the AC system to which the high-voltage direct current transmission system is connected; the number of restarts and time in the second restart phase are determined according to the arc extinguishing time of the fault condition in which the arc cannot be extinguished in the first restart phase.

6. A method for restarting a DC line fault in a high-voltage DC transmission system according to claim 1, characterized in that: After adjusting the dual DC pole power instruction value to the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole, the method further includes: The fallback signal and the fallback power are synchronously sent to the safety and stability control system, or / and the frequency emergency control system, or / and the main control system.

7. A method for restarting a DC line fault in a high-voltage DC transmission system according to claim 6, characterized in that: The fallback power is equal to the dual DC pole power before the fault minus the maximum transmittable power of the non-fault DC pole or the maximum transmittable power of the dual DC pole.

8. A method for restarting a DC line fault in a high-voltage DC transmission system according to claim 1, characterized in that: In the first restarting stage or the second restarting stage, controlling the faulty DC pole of the high voltage direct current transmission system to restart includes: Controlling at least one of the converters at each end of the faulty DC pole to continue operating; Determine the DC current reference values ​​of the two converters at both ends of the faulty DC pole based on the demand of the high voltage DC transmission system, and perform the following restarting actions: During the de-ionization time, based on the DC current reference values ​​of the two converters, the DC currents of the two converters are controlled to be equal or the difference is less than a first current threshold, or the DC currents of the two converters are controlled to be zero; During the restart time, increasing the DC voltage and / or DC power of the two converters; Or include: Controlling at least one of the converters at both ends of the faulty DC pole to be temporarily locked or / and controlling the DC current of at least one of the converters to be zero; During the de-ionization time, the fault current is attenuated by using the resistance of the DC line or the resistance of the input converter; During the restart time, at least one of the converters at both ends of the faulty DC pole is controlled to unlock and increase the DC voltage and / or DC power, or / and at least one of the converters increases the DC voltage and / or DC power.

9. A method for restarting a DC line fault in a high-voltage DC transmission system according to claim 8, characterized in that: The requirements of the high-voltage direct current transmission system include: at least one of active power requirement, reactive power requirement, ground current limit value requirement, fault DC pole current limit value requirement, current limit value requirement flowing through a fault point, and AC harmonic suppression requirement.

10. A method for restarting a DC line fault in a high-voltage DC transmission system according to claim 9, characterized in that: If the demand of the high-voltage direct current transmission system is one type, the demand of the high-voltage direct current transmission system is any one of the active power demand of the rectifier station or the inverter station, the reactive power demand, the ground current limit value demand, the current limit value demand of the fault DC pole, the current limit value demand flowing through the fault point and the AC harmonic suppression demand, and it is determined that the DC current reference values ​​of the two converters at both ends of the fault DC pole are equal.

11. A method for restarting a DC line fault in a high-voltage DC transmission system according to claim 9, characterized in that: If the demand of the high-voltage direct current transmission system considers the demand of the rectifier station and the inverter station at the same time, the priorities of different demands are given at the same time; The active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the fault DC pole, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand are unified demands, and it is determined that the DC current reference values ​​of the two converters at both ends of the fault DC pole are equal; Or, the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the faulty DC pole, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand include the demand of the rectifier station and the demand of the inverter station, and determine that the DC current reference values ​​of the two converters at both ends of the faulty DC pole are not equal and the difference is less than the first current threshold, and the first current threshold is less than the current limit value flowing through the fault point.

12. The method for restarting a DC line fault in a high-voltage DC power transmission system according to claim 8, characterized in that: The step of increasing the DC voltage of the two converters comprises: This is achieved by controlling the converter that runs under DC voltage control before the fault to run under DC voltage control, and directly setting or setting the DC voltage reference value of the two converters according to the climbing slope to be the DC voltage before the fault or the DC voltage reduced compared to before the fault; Or / and, it is achieved by giving a difference in DC current reference values ​​of the two converters.

13. The method for restarting a DC line fault in a high-voltage DC power transmission system according to claim 8, characterized in that: The controlling the direct current of the two converters to be zero comprises: If both of the converters are grid-commutated converters, this is achieved by controlling the phase shift of the grid-commutated converter of the rectifier station and the trigger angle of the grid-commutated converter of the inverter station to be greater than 90 degrees; If both of the two converters are voltage source converters, this is achieved by controlling the DC voltage of the two converters to be zero, controlling the DC current of the two converters to be zero, or controlling the two converters to be locked.

14. A flexible restart method for a DC line fault in a high voltage DC transmission system, characterized in that: The high-voltage direct current transmission system comprises at least one rectifier station and at least one inverter station, wherein the rectifier station and the inverter station comprise a single direct current pole or a double direct current pole, and the direct current pole comprises at least one converter; When a ground fault occurs in the DC line, the flexible restart method includes: Controlling at least one converter at each end of the ground fault to continue to operate; Determining a DC current reference value of two converters at both ends of a ground fault based on the demand of the high voltage direct current transmission system, the two converters comprising a converter that continues to operate at each end of the ground fault; Controlling the DC currents of the two converters to be equal or to have a difference less than a first current threshold value based on the DC current reference values ​​of the two converters within the first de-ionization time, or controlling the DC currents of the two converters to be zero; After a first deionization time, increasing a DC voltage of at least one of the two converters within a first restart time; After the first restart time, if the absolute value of the DC voltage of the DC line is greater than or equal to the first voltage threshold, continue to increase the DC voltage of the two converters; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, when the number of restarts is greater than 1, continue to control the DC currents of the two converters to be equal or the difference is less than the first current threshold based on the DC current reference values ​​of the two converters within the second de-ionization time, or control the DC currents of the two converters to be zero, and when the number of restarts is 1, control the two converters to be locked; When the number of restarts is greater than 1, after the second de-ionization time, the DC voltage of at least one of the two converters is increased within the second restart time; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, when the number of restarts is greater than 2, the DC currents of the two converters are continued to be controlled to be equal or the difference is less than the first current threshold based on the DC current reference values ​​of the two converters within the third de-ionization time, or the DC currents of the two converters are controlled to be zero, and when the number of restarts is 2, the two converters are controlled to be locked; And so on, until the flexible restart of the high voltage direct current transmission system is achieved.

15. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 14, characterized in that: The converter comprises at least one of a grid commutated converter or a voltage source converter.

16. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 15, characterized in that: If at least one of the two converters of the high-voltage direct current transmission system is a grid-commutated converter, then after controlling the direct currents of the two converters to be equal or the difference is less than a first current threshold based on the direct current reference values ​​of the two converters or before controlling the two converters to be locked, the method further includes: controlling the high-voltage direct current transmission system to cut off or put into operation an AC filter connected to the AC system according to the needs of the AC system; If at least one of the two converters of the high-voltage direct current transmission system is a voltage source converter, after controlling the DC currents of the two converters to be equal or the difference to be less than a first current threshold based on the DC current reference values ​​of the two converters or before controlling the two converters to be locked, it also includes: controlling the reactive power or AC voltage output by the voltage source converter according to the needs of the AC system.

17. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 16, characterized in that: The AC system requirements include reactive power requirements and AC voltage limitations.

18. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 14, characterized in that: The occurrence of a ground fault in the DC line is determined by detecting a protection action, wherein the protection comprises at least one of line sudden change protection, line traveling wave protection, line low voltage protection, and line longitudinal differential protection.

19. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 14, characterized in that: The requirements of the high-voltage direct current transmission system include: at least one of active power requirements, reactive power requirements, ground current limit value requirements, current limit value requirements of the DC pole where the fault is located, current limit value requirements flowing through the fault point, and AC harmonic suppression requirements; Wherein, if there is more than one demand for the HVDC transmission system, priorities of different demands are given simultaneously; If the demand of the high-voltage direct current transmission system only considers the demand of the rectifier station, the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the DC pole where the fault is located, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand are the demands of the rectifier station, and the DC current reference values ​​of the two converters are equal; If the demand of the high-voltage direct current transmission system only considers the demand of the inverter station, the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the DC pole where the fault is located, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand are the demands of the inverter station, and the DC current reference values ​​of the two converters are equal; If the demand of the high-voltage direct current transmission system considers the demands of the rectifier station and the inverter station at the same time, when processed uniformly, the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the DC pole where the fault is located, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand are the demands after unified processing, and the DC current reference values ​​of the two converters are equal; when processed separately, the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the DC pole where the fault is located, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand include the demand of the rectifier station and the demand of the inverter station, and the DC current reference values ​​of the two converters are not equal and the difference is less than the first current threshold, and the first current threshold is less than the current limit value flowing through the fault point.

20. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 19, characterized in that: If the DC current reference values ​​of the two converters determined by the demand of the high voltage direct current transmission system are equal, controlling the DC currents of the two converters to be equal; If the DC current reference values ​​of the two converters determined by the demand of the high-voltage direct current transmission system are not equal, the DC current difference of the two converters is controlled to be smaller than a first current threshold, and the first current threshold is smaller than a current limit value flowing through a fault point.

21. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 19, characterized in that: Determining direct current reference values ​​of two converters at both ends of the ground fault based on the demand of the high voltage direct current transmission system includes: If the demand of the high-voltage direct current transmission system is an active power demand, the active power demand is divided by the sum of the absolute values ​​of the direct current voltages of all operating converters of the rectifier station or the inverter station to obtain a direct current reference value of the two converters; or, If the demand of the high-voltage direct current transmission system is a reactive power demand and the converter is a six-pulse or twelve-pulse grid-commutated converter, the DC current reference values ​​of the two converters are determined based on the reactive power demand, the no-load DC bus voltage, the trigger angle or the turn-off angle, and the commutation angle, The calculation method of the converter of the rectifier station is as follows: The calculation method of the inverter station converter is as follows: In the formula, I ord is the DC current reference value, Q conv is the reactive power requirement of the six-pulse or twelve-pulse grid-commutated converter, U di0 is the no-load DC bus voltage of a six-pulse or twelve-pulse grid-commutated converter, α is the trigger angle of the converter, μ is the commutation angle of the converter, γ is the turn-off angle of the converter, when the converter is a six-pulse grid-commutated converter, b=1 / 4, when the converter is a twelve-pulse grid-commutated converter, b=1 / 2; or, If the demand of the high-voltage direct current transmission system is a ground current limit value demand, the DC current reference value of each converter of the two converters is greater than the difference between the DC current of another DC pole at the same station and the ground current limit value, and is less than the sum of the DC current of another DC pole at the same station and the ground current limit value; or, If the demand of the high-voltage direct current transmission system is the current limit value demand of the DC pole where the fault is located, determining that the DC current reference values ​​of the two converters are less than the current limit value of the DC pole where the fault is located; or If the requirement of the high-voltage direct current transmission system is a requirement for a current limit value flowing through a fault point, determining that the difference between the direct current reference values ​​of the two converters is less than the current limit value flowing through the fault point; or If the demand of the high-voltage direct current power transmission system is an AC harmonic suppression demand, the AC harmonic suppression demand is converted into a DC current reference value for limiting the two converters.

22. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 21, characterized in that: Determining DC current reference values ​​of two converters at both ends of the ground fault based on the demand of the high voltage direct current transmission system also includes: Based on the DC current reference values ​​of the two converters determined by the active power demand, the reactive power demand, the ground current limit value demand, the current limit value demand of the DC pole where the fault is located, the current limit value demand flowing through the fault point, and the AC harmonic suppression demand, a current greater than zero and with an absolute value less than 0.1 times the rated DC current that fluctuates around zero is superimposed on the DC current reference value of at least one of the converters.

23. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 14, characterized in that: The direct current of the converter includes: At least one of the high-voltage bus current, the low-voltage bus current of the converter, the pole bus current or the pole neutral bus current of the DC pole where the converter is located.

24. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 14, characterized in that: The first current threshold is 0.01 to 0.1 times the rated DC current; the first voltage threshold is 0.05 to 1.0 times the rated DC voltage; the first deionization time is 20 to 500ms; the first restart time is 20 to 300ms; the second deionization time is 50 to 500ms; and the second restart time is 20 to 300ms.

25. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 14, characterized in that: Controlling the DC currents of the two converters to be equal or to have a difference less than a first current threshold based on the DC current reference values ​​of the two converters includes: This is achieved by respectively controlling the two converters to operate in direct current control based on direct current reference values ​​of the two converters.

26. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 15, characterized in that: The step of increasing the DC voltage of at least one of the two converters comprises: This is achieved by controlling the converter among the two converters that is operating under DC voltage control before the fault to operate under DC voltage control, directly giving or giving according to a climbing slope the DC voltage reference value of the two converters as the DC voltage before the fault or a DC voltage reduced compared to before the fault; or by giving the difference in DC current reference values ​​of the two converters; or by controlling the converter among the two converters that is operating under DC current control or active power control before the fault to operate under DC current control.

27. The method for flexible restarting of a DC line fault in a high-voltage DC power transmission system according to claim 26, characterized in that: If the two converters are grid-commutated converters, the difference in the DC current reference values ​​given to the two converters is specifically achieved by reducing the DC current reference value of the DC current controller of the grid-commutated converter that was operating under DC voltage control or maximum firing angle control before the fault in the two converters.

28. The method for flexible restarting of a DC line fault in a high-voltage DC power transmission system according to claim 14, characterized in that: During the de-ionization time of the flexible restart method, if the DC current of the two converters is controlled to be zero, then during the restart time of the flexible restart method, when the DC voltage of at least one of the two converters is increased, the DC current of one of the two converters is controlled to be zero, and the DC current of the other of the two converters is controlled to be less than 0.25pu.

29. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 28, characterized in that: Controlling the DC current of the other converter of the two converters to be less than 0.25 pu is achieved by reducing the minimum DC current limit value of the low-voltage current limiting link of the other converter of the two converters to be less than 0.25 pu.

30. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 15, characterized in that: Controlling the direct current of the two converters to be zero includes: If the two converters are grid-commutated converters, this is achieved by controlling the phase shift of the grid-commutated converter of the rectifier station and the trigger angle of the grid-commutated converter of the inverter station to be greater than 90 degrees; if the two converters are voltage source converters, this is achieved by controlling the DC voltage of the two converters to zero or controlling the DC current of the two converters to zero.

31. The method for flexible restart of a DC line fault in a high-voltage DC power transmission system according to claim 15, characterized in that: If the converter is a grid-commutated converter, the converter locking includes: controlling the grid-commutated converter to stop sending trigger pulses, and / or controlling the grid-commutated converter to be put into bypass; If the converter is a voltage source converter, the converter locking includes: controlling the voltage source converter to stop sending trigger pulses.

32. The method for flexible restarting of a DC line fault in a high-voltage DC power transmission system according to claim 14, characterized in that: During the de-freeing time of the flexible restart method, when any one of the two converters loses the ability to control DC current due to an AC system fault, if the converter of the rectifier station is a grid-commutated converter, the converter of the rectifier station is controlled to shift phase; if the converter of the inverter station is a grid-commutated converter, the converter trigger angle of the inverter station is controlled to be greater than 90 degrees.

33. A DC line fault restart control device for a high voltage DC transmission system, characterized in that: The high-voltage direct current transmission system comprises at least one rectifier station and at least one inverter station, wherein the rectifier station and the inverter station both comprise dual direct current poles, and each of the dual direct current poles comprises at least one converter; the control device is used to execute the direct current line fault restart control method of the high-voltage direct current transmission system according to any one of claims 1 to 13, and the control device comprises: A first control unit is used for controlling the faulty DC pole of the high voltage direct current transmission system to restart in a first restart phase when a ground fault occurs in a DC line connected to one of the DC poles and the other DC pole is a non-faulty DC pole and operates in dual DC pole power control, and the dual DC pole power instruction value is the dual DC pole power before the fault; An adjustment unit is used for adjusting the dual DC pole power command value to the maximum deliverable power of the non-fault DC pole or the maximum deliverable power of the dual DC pole when the restart times set in the first restart stage are reached and the restart is still not successful, if the dual DC pole power command value is greater than the maximum deliverable power of the non-fault DC pole or the maximum deliverable power of the dual DC pole; A second control unit, used for controlling the faulty DC pole to restart in a second restarting phase; The third control unit is used to control the converter of the faulty DC pole to lock when the restart times set in the second restart stage are reached and the restart is still not successful.

34. A DC line fault restart control device for a high voltage DC transmission system, characterized in that: The high-voltage direct current transmission system comprises at least one rectifier station and at least one inverter station, wherein the rectifier station and the inverter station both comprise dual direct current poles, and each of the dual direct current poles comprises at least one converter; the control device is used to execute the flexible restart control method for direct current line fault of the high-voltage direct current transmission system according to any one of claims 14 to 32, and the control device comprises: A detection unit, used for detecting parameters of the high voltage direct current transmission system; a fourth control unit, configured to control at least one converter at each end of the ground fault to continue to operate when determining based on the parameters of the high-voltage direct current transmission system that a fault occurs in a direct current line of the high-voltage direct current transmission system; determine direct current reference values ​​of two converters at each end of the ground fault based on the needs of the high-voltage direct current transmission system, the two converters including a converter that continues to operate at each end of the ground fault; control the direct currents of the two converters to be equal or the difference is less than a first current threshold value based on the direct current reference values ​​of the two converters within a first de-ionization time, or control the direct currents of the two converters to be zero; after the first de-ionization time, increase the direct current voltage of at least one of the two converters within a first restart time; after the first restart time, if the absolute value of the direct current voltage of the direct current line is greater than or equal to the first voltage threshold value, continue to increase the direct current voltage of the two converters; if the absolute value of the direct current voltage of the direct current line is less than a first voltage threshold; when the number of restarts is greater than 1, the DC currents of the two converters are continued to be controlled to be equal or the difference is less than the first current threshold based on the DC current reference values ​​of the two converters during the second de-isolation time, or the DC currents of the two converters are controlled to be zero; when the number of restarts is 1, the two converters are controlled to be locked; when the number of restarts is greater than 1, after the second de-isolation time, the DC voltage of at least one of the two converters is increased during the second restart time; if the absolute value of the DC voltage of the DC line is less than the first voltage threshold, when the number of restarts is greater than 2, the DC currents of the two converters are continued to be controlled to be equal or the difference is less than the first current threshold based on the DC current reference values ​​of the two converters during the third de-isolation time, or the DC currents of the two converters are controlled to be zero; when the number of restarts is 2, the two converters are controlled to be locked; and so on, to achieve flexible restart of the high-voltage direct current transmission system.

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