Flexible restart method and control device for DC line fault in high voltage DC transmission system

Abstract

The present application discloses a flexible restart method and control device for a DC line fault in a high-voltage direct current (HVDC) transmission system. The restart method includes: controlling at least one converter at each end of a ground fault to continue operating; determining a DC current reference value for the two converters at each end of the ground fault based on the needs of the HVDC transmission system, wherein the two converters include one converter that continues to operate at each end of the ground fault; controlling the DC currents of the two converters to be equal based on the DC current reference value; increasing the DC voltage of the two converters within a first restart time after a first de-ionization 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 a first voltage threshold. The restart method can control the voltage at the ground fault point to achieve de-ionization. At the same time, since DC current still flows through the DC line, the converter continues to consume reactive power, and the HVDC transmission system can maintain AC and DC reactive power balance.

Description

Technical Field

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

[0002] The existing HVDC transmission system is divided into conventional DC transmission system and flexible DC transmission system. Among them, the conventional DC transmission system includes bipolar conventional DC transmission system and symmetrical monopole conventional DC transmission system, and the flexible DC transmission system includes bipolar flexible DC transmission system and symmetrical monopole flexible DC transmission system. A bipolar conventional DC transmission system uses a grid-commutated converter with two DC poles, each of which can independently form a loop via an earth return line or a metallic return line. When a ground fault occurs in the DC line of the bipolar conventional DC transmission system, the existing technology controls the DC line current to zero by controlling the phase shift of the grid-commutated converter. After a certain de-ionization time, the conventional DC transmission system is restarted by releasing the phase shift. During the de-ionization process, since the grid-commutated converter consumes zero reactive power and 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. At the same time, if a lightning strike occurs again during the phase shift period, the DC line will generate severe overvoltage. If the DC line has multiple ground faults when the lightning strike occurs, the phase shift of the grid-commutated converter will not be able to 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.

[0003] 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.

[0004] 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

[0005] Purpose of the invention: The embodiments of the present application provide a method and control device for flexible restart of a DC line fault in a high-voltage direct current transmission system, aiming to achieve flexible restart of a DC line grounding fault in a high-voltage direct current transmission system.

[0006] Technical solution: The embodiment of the present application provides a flexible restart method for 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 include a single DC pole or a double DC pole, and the DC pole includes at least one converter;

[0007] When a ground fault occurs in a DC line, the flexible restart method includes:

[0008] Controlling at least one converter at each end of the ground fault to continue operating;

[0009] determining, based on a demand of the HVDC transmission system, DC current reference values ​​for two converters at both ends of the ground fault, the two converters including one converter at each end of the ground fault that continues to operate;

[0010] 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;

[0011] After the first deionization time, increasing the DC voltage of at least one of the two converters within a first restart time;

[0012] After the first restart time, if the absolute value of the DC voltage of the DC link is greater than or equal to the first voltage threshold, the DC voltage of the two converters continues to be increased; if the absolute value of the DC voltage of the DC link is less than the first voltage threshold, when the number of restarts is greater than one, the DC currents of the two converters continue to be controlled 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 during the second de-ionization time, or the DC currents of the two converters are controlled to be zero; when the number of restarts is one, the two converters are controlled to be locked;

[0013] When the number of restarts is greater than one, after a second de-isolation 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 two, the DC currents of the two converters are continued to be controlled 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 the DC currents of the two converters are controlled to be zero; when the number of restarts is two, the two converters are controlled to be locked;

[0014] And so on, until the flexible restart of the HVDC transmission system is achieved.

[0015] In some embodiments, the converter comprises at least one of a grid-commutated converter or a voltage source converter.

[0016] In some embodiments, 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;

[0017] 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.

[0018] In some embodiments, the weak AC system is an AC system with an AC-DC short-circuit ratio less than 3, wherein the AC-DC short-circuit ratio = AC system short-circuit capacity / high voltage direct current transmission system rated power;

[0019] The AC system requirements include reactive power requirements and AC voltage limits.

[0020] In some embodiments, the occurrence of a ground fault in the DC line is determined by detecting a protection action, and the detection protection includes at least one of: line sudden change protection, line traveling wave protection, line low voltage protection, and line longitudinal differential protection.

[0021] In some embodiments, 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 of the current flowing through the fault point, and AC harmonic suppression requirement;

[0022] Wherein, if there is more than one demand for the HVDC transmission system, priorities of the different demands are given simultaneously;

[0023] 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;

[0024] 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;

[0025] 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.

[0026] In some embodiments, 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.

[0027] In some embodiments, determining DC current reference values ​​of two converters at both ends of the ground fault based on the requirements of the HVDC transmission system includes:

[0028] 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

[0029] 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.

[0030] The calculation method of the converter of the rectifier station is as follows:

[0031]

[0032] The calculation method of the converter of the inverter station is as follows:

[0033]

[0034] 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,

[0035] 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

[0036] 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

[0037] 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

[0038] 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.

[0039] In some embodiments, determining the DC current reference values ​​of the two converters at both ends of the ground fault based on the requirements of the HVDC transmission system further includes:

[0040] 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.

[0041] In some embodiments, the DC current of the converter includes:

[0042] 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.

[0043] In some embodiments, 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.

[0044] In some embodiments, 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:

[0045] This is achieved by controlling the two converters to operate in DC current control based on DC current reference values ​​of the two converters.

[0046] In some embodiments, increasing the DC voltage of at least one of the two converters includes:

[0047] This is achieved by controlling the converter that was operating under DC voltage control before the fault among the two converters 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 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 that was operating under DC current control or active power control before the fault among the two converters to operate under DC current control.

[0048] In some embodiments, 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.

[0049] 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.

[0050] 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.

[0051] In some embodiments, controlling the DC current of the two converters to be zero includes:

[0052] 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 reference value of the two converters to zero or controlling the DC current of the two converters to zero.

[0053] In some embodiments, 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;

[0054] If the converter is a voltage source converter, the converter locking includes: controlling the voltage source converter to stop sending trigger pulses.

[0055] In some embodiments, during the de-ionization time, when any one of the two converters loses the ability to control DC current due to an AC system failure, if the converter of the rectifier station is a grid-commutated converter, the converter of the rectifier station is controlled to shift phase (such as a trigger angle greater than 120 degrees); 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.

[0056] Accordingly, an embodiment of the present application further provides a flexible restart control device for a DC line fault in a high-voltage DC transmission system, comprising:

[0057] a detection unit, configured to detect parameters of the HVDC transmission system;

[0058] A control unit is used to execute the above-mentioned flexible restart method.

[0059] Compared with the prior art, this application has the following beneficial effects:

[0060] A flexible restart method for a DC line fault in a high-voltage direct current transmission system of the present application includes: 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.

[0061] Correspondingly, the present application also discloses a flexible restart control device for a DC line fault in a high-voltage direct current transmission system, including a detection unit and a control unit, for implementing the flexible restart method for a DC line fault in a high-voltage direct current transmission system of the present application. Therefore, it can be understood that the control device of the present application has the same beneficial effects as the restart method, which will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0063] Figure 1 This is a schematic diagram of the main circuit of a high-voltage direct current transmission system according to an embodiment of the present application;

[0064] Figure 2 This is a flow chart of a flexible restart method for a DC line fault in a high-voltage DC transmission system according to an embodiment of the present application;

[0065] Figure 3 Under existing technology Figure 1The simulation test results of the UHVDC transmission system when a DC line grounding fault occurs are shown;

[0066] Figure 4 This is an embodiment of the present application Figure 1 The simulation test results of the UHVDC transmission system when a DC line grounding fault occurs are shown;

[0067] Figure 5 This is a simulation test result diagram when a DC line between the first high-end valve group and the first low-end valve group is grounded under the prior art;

[0068] Figure 6 This is a simulation test result diagram of a ground fault in the DC line between the first high-end valve group and the first low-end valve group in an embodiment of the present application;

[0069] Figure 7 This is a schematic diagram of the main circuit of another high-voltage direct current transmission system according to an embodiment of the present application;

[0070] Figure 8 This 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;

[0071] Figure 9 Under existing technology Figure 7 The figure shows the simulation test results of the HVDC transmission system restarting once when a DC line grounding fault occurs;

[0072] Figure 10 This is an embodiment of the present application Figure 7 The figure shows the simulation test results of the HVDC transmission system restarting once when a DC line grounding fault occurs;

[0073] Figure 11 This is an embodiment of the present application Figure 7 The figure shows the simulation test results of the HVDC transmission system restarting twice when a DC line grounding fault occurs;

[0074] Figure 12 This is an embodiment of the present application Figure 7 The figure shows the simulation test results of the HVDC transmission system being locked when a DC line grounding fault occurs;

[0075] Figure 13 This is a 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

[0076] 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.

[0077] It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of the embodiments.

[0078] See also Figure 1 , Figure 1 This is a main circuit diagram 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] According to some embodiments, the first high-side converter 1, the first low-side converter 2, the second low-side converter 3, the second high-side converter 4, the third high-side converter 5, the third low-side converter 6, the fourth low-side converter 7, and the fourth high-side converter 8 may also be voltage source converters. Figure 1The converter transformer type 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 controlled 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 submodule structure of half-bridge and full-bridge. It should be noted that after adopting the voltage source converter, the two converters at the high and low ends of each DC pole connected in series must have the ability to control the DC voltage to zero voltage or negative voltage.

[0093] 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.

[0094] 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.

[0095] 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, not voltage source converters.

[0096] 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.

[0097] 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.

[0098] Figure 2 This is a flow chart of a flexible restart method for a DC line fault in a high-voltage DC transmission system provided by an embodiment of the present application. Figure 1 The control flow shown is when a ground fault occurs in the first DC line 150 of the HVDC transmission system.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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 judging the speed of the voltage drop, the fault on the DC line can be detected. The line sudden change protection action judgment criteria are as follows:

[0103] dUDL / dt <dUDL_set,

[0104] |UDL| <UDL_set。

[0105] 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.

[0106] 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 judged to be a DC line fault. The line low voltage protection action judgment criteria are as follows:

[0107] |UDL| <UDL_set1。

[0108] Among them, UDL is the pole bus voltage, and UDL_set1 is the DC voltage constant.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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;

[0113] 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;

[0114] 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.

[0115] 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.

[0116] 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:

[0117] 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

[0118] 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.

[0119] The calculation method of the converter of the rectifier station is as follows:

[0120]

[0121] The calculation method of the converter of the inverter station is as follows:

[0122]

[0123] 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,

[0124] 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

[0125] 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

[0126] 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

[0127] 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.

[0128] 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 further includes:

[0129] 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.

[0130] Taking the reactive power demand of the first rectifier station 100 or the first inverter station 200 as an example, the reactive power demand of the converter is calculated as follows.

[0131] Q ord_p2 =0.5×I d_p2 ×U di0_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 )),

[0132] Q ord_p1 =Q ord -Q ord_p2 ,

[0133] Considering that the circuit parameters of the high-end valve group and the low-end valve group of pole 1 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 1, specifically:

[0134] Q ord_p1c1 =Q ord_p1c2 =Q ord_p1 / 2,

[0135] Based on the reactive power demand of the twelve-pulse converter of the rectifier station, the DC current reference value is calculated as follows.

[0136] I ord_p1 =Q ord_p1c1 / (0.5×U di0_p1c1 ×(2μ 11 +sin2α 11 -sin2(α11 +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 ))).

[0137] 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_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.

[0138] Given that the DC current reference values ​​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 I ord_p1 .

[0139] 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 equations are shown.

[0140] I ord_p2 =P ord / (U d_p2c1 +Ud_p2c2 ),

[0141] I ord_p1 ≥I ord_p2 -I del_lim And I ord_p1 ≤I ord_p2 +I del_lim .

[0142] 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.

[0143] Given that the DC current reference value of the first low-end converter 2 of the first DC pole 110 of the first rectifier station 100 and the third low-end converter 6 of the third DC pole 210 of the first inverter station 200 is I ord_p1 .

[0144] 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.

[0145] 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.

[0146] 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.

[0147] According to some embodiments, the first current threshold value ranges from 0.01 to 0.1 times the rated DC current.

[0148] 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.

[0149] 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.

[0150] 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.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] Further,

[0155] 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.

[0156] 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.

[0157] 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.

[0158] 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.

[0159] 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;

[0160] If the converter is a voltage source converter, the converter locking includes: controlling the voltage source converter to stop sending trigger pulses.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] 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.

[0165] 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.

[0166] According to some embodiments, the first voltage threshold value ranges from 0.05 to 1.0 times the rated DC voltage.

[0167] 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.

[0168] 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.

[0169] 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.

[0170] 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.

[0171] According to some embodiments, the second de-isolation time ranges from 50 ms to 500 ms; and the second restart time ranges from 20 ms to 300 ms.

[0172] 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.

[0173] 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.

[0174] 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.

[0175] 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.

[0176] 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.

[0177] According to some embodiments, the third de-isolation time ranges from 80 ms to 500 ms; the third restart time ranges from 20 ms to 300 ms.

[0178] Figure 3 Under existing technology Figure 1The figure shows the simulation test results of a UHVDC transmission system with a DC line ground fault. The AC / DC short-circuit ratio in the simulation test was 2.5.

[0179] Figure 3 The DC voltage UDL shown is the busbar voltage of the first DC pole 110 of the first rectifier station 100. The DC current IDL is the busbar current of the first DC pole 110 of the first rectifier station 100 and the busbar 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. 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 UHVDC transmission system has a rated voltage of 800 kV and a rated power of 10,000 MW. The rated line voltage of the first AC system 140 is 530 kV, and the rated phase voltage is 306 kV.

[0180] 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).

[0181] Figure 4 This embodiment of the present application provides Figure 1 The figure shows the simulation test results of a UHVDC transmission system with a DC line ground fault. The AC / DC short-circuit ratio in the simulation test was 2.5.

[0182] Figure 4The DC voltage UDL shown is the busbar voltage of the first DC pole 110 of the first rectifier station 100. The DC current IDL is the busbar current of the first DC pole 110 of the first rectifier station 100 and the busbar 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. 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 UHVDC transmission system has a rated voltage of 800 kV and a rated power of 10,000 MW. The rated line voltage of the first AC system 140 is 530 kV, and the rated phase voltage is 306 kV.

[0183] 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).

[0184] By comparison Figure 3 and Figure 4 The method based on the present application can reduce the overvoltage level of the AC system and reduce the current flowing into the grounding electrode line.

[0185] if Figure 1The high-end and low-end valve groups of the bipolar UHVDC transmission system are constructed at separate stations and connected via DC lines. Specifically, the first high-end valve group 111 and the second high-end valve group 122 are located at one station, while the first low-end valve group 112 and the second low-end valve group 121 are located at another station. The first high-end valve group 111 and the first low-end valve group 112, as well as the second high-end valve group 122 and the second low-end valve group 121, are connected via DC lines. If a DC line fault occurs, the high-end valve group will be unable to transmit DC power by shifting the phase or controlling the DC current to zero.

[0186] Figure 5 This is a simulation test result diagram 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.

[0187] Figure 5 The DC voltage UDL shown 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 value of the three-phase AC voltage RMS of the AC system connected to the first low-end valve group 112. The rated voltage of the UHVDC transmission system is 800 kV and the rated power is 8000 MW. The AC system connected to the first low-end valve group 112 has a rated line voltage of 530 kV and a rated phase voltage of 306 kV.

[0188] 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.).

[0189] 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.

[0190] Figure 6 This 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.

[0191] Figure 6 The DC voltage UDL shown 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 value of the three-phase AC voltage RMS of the AC system connected to the first low-end valve group 112. The rated voltage of the UHVDC transmission system is 800 kV and the rated power is 8000 MW. The AC system connected to the first low-end valve group 112 has a rated line voltage of 530 kV and a rated phase voltage of 306 kV.

[0192] 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).

[0193] By comparison Figure 5 and Figure 6 The method based on 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 pole during the fault period.

[0194] Figure 7 This is a main circuit diagram of another high-voltage direct current transmission system provided in an embodiment of the present application. It is a symmetrical monopole conventional direct current transmission system. Each DC pole adopts a structure in which twelve-pulse grid-commutated converters are connected in series. It can be applied to high-voltage direct current transmission projects that do not have grounded electrode lines.

[0195] 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.

[0196] 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 .

[0197] 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.

[0198] 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.

[0199] 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.

[0200] 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.

[0201] 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.

[0202] According to some embodiments, Figure 7 The fifth converter 301, the sixth converter 302, the seventh converter 401, and the eighth converter 402 may also be at least one of a grid-commutated converter or a voltage source converter of a six-pulse bridge circuit. Figure 7 The converter transformer type is modified according to the type of the connected converter. The pulsating bridge circuit includes a non-turn-off half-controlled power semiconductor device, generally a thyristor device.

[0203] 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.

[0204] 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.

[0205] 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.

[0206] 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.

[0207] Figure 8This is another flow chart of a flexible restart method for a DC line fault in a high-voltage DC transmission system provided by an embodiment of the present application. Figure 7 The control flow shown is when a DC side grounding fault occurs in the third DC line 350 of the HVDC transmission system.

[0208] 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.

[0209] 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.

[0210] 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.

[0211] 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 judging the speed of the voltage drop, the fault on the DC line can be detected. The line sudden change protection action judgment criteria are as follows:

[0212] dUDL / dt <dUDL_set,

[0213] |UDL| <UDL_set。

[0214] 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.

[0215] 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 judged to be a DC line fault. The line low voltage protection action judgment criteria are as follows:

[0216] |UDL| <UDL_set1。

[0217] Among them, UDL is the pole bus voltage, and UDL_set1 is the DC voltage constant.

[0218] 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.

[0219] 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.

[0220] 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.

[0221] Taking the reactive power demand of the second rectifier station 300 or the second inverter station 400 as an example, the reactive power demand of the converter is calculated as follows.

[0222] Q ord_p2 =0.5×I d_p2 ×U di0_p2 ×(2μ2+sin2α2-sin2(α2+μ2)) / (cosα2-cos(α2+μ2)),

[0223] Q ord_p1 =Q ord -Q ord_p2 ,

[0224] Based on the reactive power demand of the twelve-pulse converter of the rectifier station, the DC current reference value is calculated as follows.

[0225] I ord_p1 =Q ord_p1 / (0.5×U di0_p1 ×(2μ1+sin2α1-sin2(α1+μ1)) / (cosα1-cos(α1+μ1)).

[0226] 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 converter of pole I and the converter of pole II, α1 and α2 are the trigger angles of the converter of pole I and the converter of pole II, μ1 and μ2 are the commutation angles of the converter of pole I and the converter of pole II, respectively.

[0227] 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 .

[0228] 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 equations are shown.

[0229] I ord_p2 =P ord / U d_p2 ,

[0230] I ord_p1 ≥I ord_p2 -I del_lim And I ord_p1 ≤I ord_p2 +I del_lim .

[0231] 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, U d_p2 is the DC voltage of the converter at pole II.

[0232] 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 .

[0233] In S230 , the DC currents of 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.

[0234] 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 is I ord_p1Alternatively, 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 smaller 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.

[0235] According to some embodiments, the first current threshold value ranges from 0.01 to 0.1 times the rated DC current.

[0236] 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.

[0237] 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.

[0238] 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.

[0239] 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.

[0240] 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.

[0241] 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.

[0242] 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.

[0243] According to some embodiments, the first voltage threshold value ranges from 0.05 to 1.0 times the rated DC voltage.

[0244] 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.

[0245] 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.

[0246] According to some embodiments, the second de-isolation time ranges from 50 ms to 500 ms; and the second restart time ranges from 20 ms to 300 ms.

[0247] 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.

[0248] 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.

[0249] 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.

[0250] 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.

[0251] According to some embodiments, the third de-isolation time ranges from 80 ms to 500 ms; the third restart time ranges from 20 ms to 300 ms.

[0252] 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, 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 I of the fifth converter 301 is reduced. ord_p1 By controlling the current flowing through the DC line grounding point to a smaller value, the heat injected into the DC line grounding point can be reduced when restarting is unsuccessful, which is beneficial for the next arc extinction.

[0253] 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.

[0254] 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.

[0255] 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.

[0256] 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.

[0257] 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.

[0258] 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 I of the fifth converter 301 is reduced. ord_p1 0.1 times the rated DC current.

[0259] Figure 9 、 Figure 10 、 Figure 11 and Figure 12 yes Figure 7 FIG. 4 is a diagram 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 are six-pulse bridge circuits.

[0260] Figure 9 Under existing technology Figure 7 The figure shows the simulation test results of a single restart in a HVDC transmission system after a DC line ground fault. 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.

[0261] Figure 9 The positive voltage shown 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 triggering angles are the triggering 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.

[0262] 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.).

[0263] Figure 10 This embodiment of the present application provides Figure 7 The figure shows the simulation test results of a single restart in a HVDC transmission system after a DC line ground fault. 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.

[0264] Figure 10The positive voltage shown 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 triggering angles are the triggering 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.

[0265] 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).

[0266] By comparison Figure 9 and Figure 10 The method based on 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.

[0267] Figure 11 This embodiment of the present application provides Figure 7 The figure shows the results of two restart simulation tests when a DC line ground fault occurs in a HVDC transmission system. The second inverter station 400 is grounded internally, while the second rectifier station 300 is not grounded.

[0268] Figure 11The positive voltage shown 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 currents are the bus currents of the fifth DC pole 310 of the second rectifier station 300 and the seventh DC pole 410 of the second inverter station 400, respectively. The fault 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.

[0269] 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.

[0270] Figure 12 This embodiment of the present application provides Figure 7 The figure shows the simulation test results of the HVDC transmission system being locked when a DC line ground fault occurs. The second inverter station 400 is grounded internally, while the second rectifier station 300 is not grounded.

[0271] Figure 12The DC voltages shown 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.

[0272] 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.

[0273] Figure 13 1 is a structural diagram of a DC line fault control device 500 for a high-voltage DC transmission system provided in an embodiment of the present application. The device includes a detection unit 510 and a control unit 520.

[0274] The 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.

[0275] The 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 each end 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 the restart time 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 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 HVDC transmission system is realized.

[0276] 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.

[0277] The above embodiments are merely illustrative of the technical concept of the present application and are not intended to limit the scope of protection of the present application. Persons skilled in the art should understand that any modification or equivalent substitution of the specific embodiments of the present application without departing from the technical concept and scope of the present application shall be encompassed by the claims of the present application.

Claims

1. A flexible restart method for a DC line fault in a high-voltage DC transmission system, characterized by: 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 include a single direct current pole or a double direct current pole, and the direct current pole includes at least one converter; The flexible restart method includes: A preset number of restarts, the preset number of restarts being greater than or equal to 1; When a ground fault occurs in a DC line, controlling at least one converter at each end of the ground fault to continue operating; determining a DC current reference value for two converters at both ends of a ground fault based on a demand of the HVDC transmission system, the two converters including one converter at each end of the ground fault that continues to operate; Performing a de-ionization action: 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 de-ionization time, or controlling the DC currents of the two converters to be zero; Performing a restart action: after performing the de-ionization action, increasing the DC voltage of at least one of the two converters within a restart time; After the 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 voltages of the two converters are continued to be increased, and the converters operate normally; After the restart time, if the absolute value of the DC voltage of the DC link is less than a first voltage threshold, then: If the current restart number is equal to the preset restart number, controlling the two converters to lock; If the current restart number is less than the preset restart number, the de-ionizing action and the restarting action are repeated, and a judgment is performed after each restart time; until the absolute value of the DC voltage of the DC line is greater than or equal to the first voltage threshold, thereby achieving a flexible restart of the high-voltage DC transmission system; or when the absolute value of the DC voltage of the DC line is less than the first voltage threshold and the current restart number reaches the preset restart number, the two converters are controlled to be locked.

2. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 1, characterized in that: The converter includes at least one of a grid-commutated converter or a voltage source converter.

3. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 2, characterized in that: 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; 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.

4. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 3, characterized in that: The AC system requirements include reactive power requirements and AC voltage limits.

5. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 1, characterized in that: The occurrence of a ground fault in the DC line is determined by detecting a protection action, wherein the protection includes at least one of line sudden change protection, line traveling wave protection, line low voltage protection, and line longitudinal differential protection.

6. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 1, characterized in that: 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 of the current flowing through the fault point, and AC harmonic suppression requirement; Wherein, if there is more than one demand for the HVDC transmission system, priorities of the different demands are given simultaneously; 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 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 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.

7. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 6, characterized in that: If the DC current reference values ​​of the two converters determined by the requirements of the HVDC 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 requirements 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 the fault point.

8. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 6, 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 includes: 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 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. The calculation method of the converter of the rectifier station is as follows: The calculation method of the converter of the inverter station is as follows: 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 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 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 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 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.

9. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 8, 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 further includes: 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.

10. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 1, characterized in that: The DC 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.

11. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 1, 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; when the current number of restarts is 1, the de-free time is the first de-free time, the restart time is the first restart time, the first de-free time is 20 to 500ms, and the first restart time is 20 to 300ms; when the current number of restarts is 2, the de-free time is the second de-free time, the restart time is the second restart time, the second de-free time is 50 to 500ms, and the second restart time is 20 to 300ms.

12. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 1, 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 controlling the two converters to operate in DC current control based on DC current reference values ​​of the two converters.

13. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 2, characterized in that: Increasing the DC voltage of at least one of the two converters includes: This is achieved by controlling the converter that was operating under DC voltage control before the fault among the two converters 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 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 that was operating under DC current control or active power control before the fault among the two converters to operate under DC current control.

14. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 13, characterized in that: If the two converters are both 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 firing angle control before the fault among the two converters.

15. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 1, 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 0.25 pu.

16. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 15, characterized in that: 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.

17. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 2, characterized in that: Controlling the DC 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.

18. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 2, 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 enter a bypass pair; If the converter is a voltage source converter, the converter locking includes: controlling the voltage source converter to stop sending trigger pulses.

19. The flexible restart method for a DC line fault in a HVDC power transmission system according to claim 1, 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.

20. A flexible restart control device for a DC line fault in a high-voltage DC transmission system, characterized in that: include: a detection unit, configured to detect parameters of the HVDC transmission system; A control unit, configured to execute the flexible restart method according to any one of claims 1 to 19.

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