Control method of conventional HVDC transmission system providing optimal transient voltage support

Abstract

The present invention discloses a conventional high-voltage direct current transmission system control method for providing optimal transient voltage support, which belongs to the field of LCC-HVDC control technology. Based on the equivalent model of the sending-end AC system, a two-stage method is used to update the DC current instruction in real time, and the instruction can achieve maximum voltage support. At the same time, in order to alleviate the negative impact of modifying the DC current on the inverter side, the shutdown angle instruction is coordinated and modified on the inverter side to maintain the AC voltage on the inverter side within the limit and reduce the risk of subsequent commutation failure. The present invention solves the deficiency that conventional schemes cannot provide effective voltage support under transient voltage disturbances, maximizes the use of the regulation ability of LCC-HVDC to suppress the severity of transient voltage disturbances, and can reduce the impact of voltage disturbances on wind turbines; the present invention takes into account the coupling effect between the rectifier side and the inverter side, which can avoid providing voltage support on the rectifier side to cause serious side effects on the inverter side.

Description

Technical Field

[0001] The present invention belongs to the technical field of LCC-HVDC control, and particularly relates to a control method for a conventional high-voltage direct current transmission system that provides optimal transient voltage support. Background Art

[0002] The conventional high-voltage direct current transmission system (Line commutated converter based high voltage direct current, LCC-HVDC) has become one of the best choices for long-distance and large-capacity transmission of wind power due to its advantages such as low construction cost, small transmission loss, and long transmission distance. However, in such high-proportion wind power DC sending-end systems, faults such as near-area AC short circuits, disconnections, DC blockades, and commutation failures will cause various transient voltage disturbances at the sending end, threatening the normal operation of wind turbines. In severe cases, it will trigger a chain of unit disconnections from the grid, leading to the instability of the entire sending-end power system.

[0003] The transmission capacity of LCC-HVDC is huge, reaching several thousand megawatts, and it has great regulation potential. In theory, it can provide support for suppressing transient voltage disturbances. However, since the LCC-HVDC converter valve is built based on semi-controlled thyristors and cannot achieve decoupled control of active power and reactive power, it is generally considered that its transient voltage control ability is limited and it cannot actively provide millisecond-level transient voltage support. Some studies have improved the low-voltage current limiting control link (Voltage Dependent Current Order Limiter, VDCOL) of the DC control system, which can suppress the continuous "first low then high" transient voltage disturbances caused by commutation failures. However, due to the differences in formation mechanisms, these solutions are not applicable to transient voltage disturbances caused by other faults. In addition, the existing solutions do not consider the coupling effect between the rectifier station and the inverter station, and do not explore how to maximize the control ability of LCC-HVDC, so the control potential of the high-voltage direct current transmission system is not fully utilized. Summary of the Invention

[0004] Aiming at the above deficiencies in the prior art, the control method for a conventional high-voltage direct current transmission system that provides optimal transient voltage support provided by the present invention solves the problems that the traditional method cannot be used under various transient voltage disturbances, does not consider maximizing the voltage support ability of high-voltage direct current, cannot fully utilize the regulation potential of the high-voltage direct current converter station, and will cause errors when high-proportion wind power is connected, and does not consider the coupling effect between the high-voltage direct current rectifier station and the inverter station, resulting in the operation of one side of the AC system exceeding the normal operation range of the system.

[0005] To achieve the above invention object, the technical solution adopted by the present invention is: a control method for a conventional HVDC transmission system providing optimal transient voltage support, including rectifier side control and inverter side control, wherein:

[0006] For the rectifier side control, start the rectifier side control according to the real-time detected AC bus voltage and DC current, and when starting the rectifier side control, introduce the system equivalent parameter estimation to recalculate the DC current command of the rectifier side, and perform the rectifier side control according to it;

[0007] For the inverter side control, based on the influence of the DC current command on the power transmission of the inverter station, start the inverter side control according to the real-time detected AC bus voltage, and when starting the inverter side control, introduce the system equivalent parameter estimation to calculate the extinction angle command, and perform the inverter side control according to it.

[0008] Further, the method of the rectifier side control is specifically as follows:

[0009] SA1. Real-time collect the operation parameters of the DC system and the parameters of the sending-end AC system;

[0010] SA2. For the real-time detected AC bus voltage and DC current in the collected parameters;

[0011] When the AC bus voltage is within the preset interval, no additional rectifier side control is required, and the system is in the normal rectifier control state;

[0012] When the AC bus voltage exceeds the maximum value of the preset interval, a transient voltage rise occurs in the system, start the rectifier side control, and enter step SA3;

[0013] When the AC bus voltage is lower than the minimum value of the preset interval and the DC current is lower than the preset threshold, a transient voltage drop occurs in the system, start the rectifier side control, and enter step SA3;

[0014] When the AC bus voltage is lower than the minimum value of the preset interval and the DC current is greater than or equal to the preset threshold, commutation failure occurs in the system, start the rectifier side control, and enter step SA3;

[0015] SA3. According to the real-time collected voltage and current measurement values, and combined with the equivalent parameters of the wind farm during normal operation, perform Thevenin equivalent parameter estimation on the AC system;

[0016] SA4. Calculate the optimal reactive power command of the rectifier side according to the estimated equivalent parameters and the measured active power;

[0017] SA5. Recalculate the DC reference command of the rectifier side according to the optimal reactive power command, AC voltage, and trigger angle;

[0018] SA6. Control the rectifier side according to the recalculated DC reference command. During the control process, when both the AC bus voltage and the DC current are within the preset intervals and last for the preset duration, it is determined that the rectifier side has returned to the normal operating state, and the control of the rectifier side is exited.

[0019] Further, in the step SA4, the optimal reactive power command is:

[0020]

[0021] The upper limit of the optimal reactive power command is:

[0022]

[0023] In the formula, represents the optimal reactive power, represents the Thevenin equivalent voltage, represents the equivalent reactance of the sending-end AC system, represents the equivalent susceptance of the reactive power compensation device in the rectifier station, and respectively represent the coefficients related and unrelated to the first-order term of the voltage of the active current, and respectively represent the coefficients related and unrelated to the first-order term of the voltage of the reactive current, represents the active power consumed by the rectifier station.

[0024] Further, in the step SA5, the DC reference command is:

[0025]

[0026]

[0027] In the formula, represents the DC current, , , represents the number of six-pulse converters in a converter valve, represents the commutation reactance, represents the AC bus voltage on the rectifier side, represents the converter transformer ratio, represents the firing angle on the rectifier side.

[0028] Further, the method for controlling the inverter side is:

[0029] SB1. Real-time collect the operating parameters of the DC system and the parameters of the receiving-end AC system;

[0030] SB2. Judge whether the AC bus voltage detected in real time among the collected parameters is within the preset interval;

[0031] If so, no additional inverter-side control is required, and the system is in a normal inverter control state;

[0032] If not, start the inverter-side control and enter step SB3;

[0033] SB3. Collect the AC system voltage and current and estimate the Thevenin equivalent parameters of the receiving-end system;

[0034] SB4. Calculate the reference reactive power command for the inverter side according to the estimated equivalent parameters and the measured active power;

[0035] SB5. Calculate the turn-off angle as the turn-off angle command for the inverter side according to the reference reactive power command and the AC voltage and DC current at the common connection point of the inverter side detected in real time;

[0036] SB6. Perform inverter-side control according to the turn-off angle command and detect whether commutation failure occurs during the control process;

[0037] If so, increase the lead trigger angle and enter step SB7;

[0038] If not, enter step SB7;

[0039] SB7. When the AC bus voltage and DC current of the inverter side are both within the preset intervals and last for a preset duration, determine that the inverter side has returned to the normal operating state and exit the inverter-side control; otherwise, return to step SB4.

[0040] Furthermore, in step SB4, the reference reactive power command is:

[0041]

[0042] In the formula, represents the reference reactive power, represents the equivalent susceptance of the reactive power compensation device in the inverter station, represents the AC bus voltage of the inverter side, represents the Thevenin equivalent reactance of the receiving-end AC system, represents the active power of the inverter side, represents the Thevenin equivalent voltage.

[0043] Furthermore, in step SB5, the calculation formula for the turn-off angle is:

[0044]

[0045] In the formula, represents the reference reactive power, represents the DC current, represents the AC bus voltage of the inverter side, Indicates the number of six-pulse converters in a converter valve, Indicates the ratio of the inverter-side converter transformer, Indicates the measured turn-off angle, Indicates the commutation reactance on the inverter side.

[0046] The beneficial effects of the present invention are as follows:

[0047] (1) The present invention proposes a control method for maximizing the transient voltage support ability of a conventional high-voltage direct current (HVDC) transmission system, which can provide optimal transient voltage support for the AC system under various types of transient voltage disturbances. Specifically, based on the equivalent model of the sending-end AC system, a two-stage method is adopted to update the DC current command in real time, and this command can achieve maximum voltage support. At the same time, to alleviate the negative impact of modifying the DC current on the inverter side, the turn-off angle command is coordinated and modified on the inverter side to maintain the inverter-side AC voltage within the limit and reduce the risk of subsequent commutation failures.

[0048] (2) The present invention provides an improved HVDC control strategy, which solves the deficiency that the conventional scheme cannot provide effective voltage support under transient voltage disturbances, maximizes the regulation ability of the line-commutated converter based HVDC (LCC-HVDC) to suppress the severity of transient voltage disturbances, and can reduce the impact of voltage disturbances on wind turbines. In addition, the method of the present invention considers the coupling effect between the rectifier side and the inverter side, and can avoid serious side effects on the inverter side caused by providing voltage support on the rectifier side. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a flowchart of the control method for a conventional HVDC transmission system that provides optimal transient voltage support in the present invention.

[0050] Figure 2 It is a structural diagram of a bipolar HVDC transmission system with a typical wind farm at the sending end in the present invention.

[0051] Figure 3 It is an equivalent circuit diagram of the wind power DC sending-end system in the present invention.

[0052] Figure 4 It is a control block diagram of the rectifier side of the HVDC transmission system in the present invention.

[0053] Figure 5 It is a control block diagram of the inverter side of the HVDC transmission system in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0054] The specific embodiments of the present invention will be described below to facilitate the understanding of those skilled in the art of the present technology. However, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those ordinary skilled in the art of the present technology, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions created using the concept of the present invention are within the scope of protection.

[0055] An embodiment of the present invention provides a control method for a conventional high-voltage direct current (HVDC) transmission system with optimal transient voltage support, as Figure 1 shown, which includes rectifier-side control and inverter-side control, where:

[0056] For the rectifier-side control, the rectifier-side control is determined to be started according to the real-time detected AC bus voltage and DC current. When the rectifier-side control is started, the system equivalent parameter estimation is introduced to recalculate the DC current command of the rectifier side, and the rectifier-side control is performed according to it;

[0057] For the inverter-side control, based on the influence of the DC current command on the power transmission of the inverter station, the inverter-side control is determined to be started according to the real-time detected AC bus voltage. When the inverter-side control is started, the system equivalent parameter estimation is introduced to calculate the extinction angle command, and the inverter-side control is performed according to it.

[0058] The above control method provided in the embodiment of the present invention is for the conventional HVDC transmission system in the wind power DC sending-end system, and improves the control strategies of the HVDC rectifier side and the inverter side. A typical bipolar HVDC transmission system with a wind farm at the sending end is as Figure 2 shown, and its simplified equivalent circuit corresponding to the sending-end system is as Figure 3 shown; in Figure 3 , the wind farm can be equivalent to a current source I wd + jI wq , where I wd and I wq are the active current and the reactive current respectively. Since the electrical distance between the wind farm and the converter station is relatively close, the tie-line impedance R w + jX w can be ignored; the AC system other than the wind farm is represented by Thevenin equivalent, the equivalent voltage is E sr , and the equivalent impedance is R r + jX r ; P r + jQr represents the power provided by the AC system, P dr + jQ dr represents the power absorbed by the DC system, B c is the equivalent admittance of the reactive power compensation device in the converter station, U pr is the voltage at the point of common coupling.

[0059] In the embodiment of the present invention, as Figure 4 shown, the method for rectifier side control is specifically as follows:

[0060] SA1. Real-time collect the operation parameters of the DC system and the parameters of the sending-end AC system;

[0061] Specifically, the collected parameters include the firing angle of the rectifier side of the DC system, the DC current, etc., and the bus AC voltage, etc.;

[0062] SA2. For the AC bus voltage and DC current detected in real time among the collected parameters;

[0063] When the AC bus voltage is within the preset interval, no additional rectifier side control is required, and the system is in the normal rectifier control state; among them, no additional rectifier side control means that it is necessary to start the control method proposed in the present invention according to the subsequent steps and perform it according to the normal rectifier side control method;

[0064] When the AC bus voltage exceeds the maximum value of the preset interval, a transient voltage rise occurs in the system, and the rectifier side control is started to enter step SA3;

[0065] When the AC bus voltage is lower than the minimum value of the preset interval and the DC current is lower than the preset threshold, a transient voltage drop occurs in the system, and the rectifier side control is started to enter step SA3;

[0066] When the AC bus voltage is lower than the minimum value of the preset interval and the DC current is greater than or equal to the preset threshold, commutation failure occurs in the system, and the rectifier side control is started to enter step SA3;

[0067] SA3. Estimate the Thevenin equivalent parameters of the AC system according to the measured voltage and current values collected in real time and in combination with the equivalent parameters of the wind farm during normal operation;

[0068] SA4. Calculate the optimal reactive power command for the rectifier side according to the estimated equivalent parameters and the measured active power;

[0069] SA5. Recalculate the DC reference command for the rectifier side according to the optimal reactive power command, the AC voltage, and the firing angle;

[0070] SA6. Control the rectifier side according to the recalculated DC reference instruction. During the control process, when both the AC bus voltage and the DC current are within the preset intervals and last for the preset duration, it is determined that the rectifier side has returned to the normal operating state, and the control of the rectifier side is exited.

[0071] In this embodiment, in a specific example, in step SA2, the preset interval of the AC bus voltage is [0.9 p.u., 1.1 p.u.], and the preset threshold of the DC current is 1.0 p.u. When the AC bus voltage U pr is within the interval of [0.9 p.u., 1.1 p.u.], the system is considered to be in the normal state, and no additional measures are taken for the DC system; when the AC bus voltage U pr exceeds this interval, it is considered that a fault has occurred in the system, the enable signal Ctrl is set to 1, the HVDC control system switches to the proposed rectifier side control process, and a fault mode judgment is performed; when the measured AC bus voltage U pr is lower than 0.9 p.u. and the DC current I dc <1.0 p.u., it is considered that a voltage sag has occurred; when the AC bus voltage U pr is lower than 0.9 p.u. and the DC current I dc ≥1.0 p.u., it is considered that commutation failure has occurred; when the AC bus voltage U pr is higher than 1.1 p.u., it is considered that a voltage swell has occurred.

[0072] In step SA3 of this embodiment, the equivalent parameters of the AC system are estimated based on the real-time collected AC system voltage and current. Among them, the wind farm is equivalent to a controlled current source, as shown in Equation (1), where K P1 , K P2 , K Q1 , K Q2 are all constant coefficients, which will be different during normal operation and fault ride-through. In this embodiment, only the equivalent parameters of the wind farm during normal operation are required, and the parameters in the fault ride-through state do not need to be considered. During normal operation K P1 , K Q1 , K Q2 can be set to zero, so the difficulty of parameter acquisition is reduced. The Thevenin equivalent parameters include the equivalent voltage E srand equivalent impedance R r + jX r 。

[0073] (1)

[0074] In step SA4 of this embodiment, the optimal reactive power command is:

[0075] (2)

[0076] To ensure a solution for the subsequent equations, the lower limit of the above optimal reactive power command is set to Q dr,min = 0, and the upper limit is set to Q critical , which is obtained from Equation (3). Equation (3) is the condition for ensuring a solution for the system power flow equations and corresponds to the static voltage stability limit. This equation is solved by setting U pr = 1. Due to the monotonic relationship between the AC voltage and reactive power, the solved reactive power can maximize the AC voltage support effect of the HVDC, that is, maximize the voltage when the voltage is below 1.0 p.u. and minimize the voltage when the voltage is above 1.0 p.u. Among them, the upper limit of the optimal reactive power command is:

[0077] (3)

[0078] In the formula, represents the optimal reactive power, represents the Thevenin equivalent voltage, represents the equivalent reactance of the sending-end AC system, represents the equivalent susceptance of the reactive power compensation device in the rectifier station, and respectively represent the coefficients related and unrelated to the first-order term of the active current and voltage, and respectively represent the coefficients related and unrelated to the first-order term of the reactive current and voltage, represents the active power consumed by the rectifier station.

[0079] In step SA5 of this embodiment, the DC reference command is:

[0080] (4)

[0081] Among them, , ,and the other coefficients are:

[0082]

[0083] In the formula, represents the DC current, , , represents the number of six-pulse converters in a commutation valve, represents the commutation reactance, represents the AC bus voltage on the rectifier side, represents the conversion transformer ratio, represents the firing angle on the rectifier side.

[0084] The above equation is derived from the quasi-steady-state equation of LCC-HVDC. This formula only contains I dc one independent variable and can be solved analytically. In order to ensure the continuity of the DC current and not exceed the equipment's tolerance range, in this embodiment, the minimum value of the DC current command I dc,min is set to 0.2 p.u., and the maximum value I dc,max is 1.25 p.u.

[0085] In step SA6 of this embodiment, when the measured DC bus voltage U pr and the DC current satisfy 0.9 < U pr < 1.1 and 0.9 < I dc < 1.1 and last for 50 ms, it is considered that the system resumes normal operation, and the proposed control method exits the operation, and the HVDC transmission system switches to conventional control.

[0086] In the embodiment of the present invention, due to the limitations of the LCC-HVDC's own control, modifying the DC current on the rectifier side will inevitably affect the power transmission of the inverter station, resulting in fluctuations in the receiving-end AC voltage. In the embodiment of the present invention, a control strategy without communication is designed to modify the turn-off angle of the inverter station according to local measurements to maintain the receiving-end AC voltage within a preset reasonable range.

[0087] Specifically, in the embodiment of the present invention, as Figure 5 shown, the method of controlling the inverter side is as follows:

[0088] SB1. Real-time collect the operation parameters of the DC system and the parameters of the receiving-end AC system;

[0089] Specifically, the collected parameters include the turn-off angle and DC current of the inverter side of the DC system, as well as the AC voltage of the commutation bus on the inverter side;

[0090] SB2. Judge whether the AC bus voltage detected in real time among the collected parameters is within a preset interval;

[0091] If so, no additional inverter-side control is required, and the system is in a normal inverter control state. Similarly, the need for no additional inverter-side control here means that the control method proposed in the present invention needs to be started according to the subsequent steps, and the normal inverter-side control method can be followed.

[0092] If not, start the inverter-side control and proceed to step SB3.

[0093] Specifically, the preset range of the AC bus voltage is [0.9 p.u., 1.1 p.u.]. When within this range, the system is considered to be in a normal state. When exceeding the preset range, proceed to step SB3 and start the coordinated inverter-side control.

[0094] SB3: Collect the AC system voltage and current and estimate the equivalent parameters of the AC system, as well as the Thevenin equivalent parameters of the receiving-end system.

[0095] Specifically, estimate the equivalent parameters of the AC system based on the AC system voltage and current collected in the embodiment. For the receiving-end system, since the wind power ratio is relatively low, the Thevenin equivalent can be directly used, and the parameters are the equivalent voltage E si and the equivalent impedance R si + jX si , and its online estimation method is the same as that of the rectifier side.

[0096] SB4: Calculate the reference reactive power command for the inverter side based on the estimated equivalent parameters and the measured active power.

[0097] SB5: Calculate the turn-off angle as the inverter-side turn-off angle command based on the reference reactive power command and the AC voltage and DC current at the point of common coupling of the inverter side detected in real time.

[0098] SB6: Perform inverter-side control according to the turn-off angle command and detect whether commutation failure occurs during the control process.

[0099] If so, increase the lead firing angle and proceed to step SB7.

[0100] If not, proceed to step SB7.

[0101] SB7: When both the inverter-side AC bus voltage and DC current are within the preset range and continue for the preset duration, determine that the inverter side has returned to the normal operating state and exit the inverter-side control; otherwise, return to step SB4.

[0102] In step SB4 of this embodiment, the reference reactive power command is:

[0103] (5)

[0104] In the formula, represents the reference reactive power, represents the equivalent susceptance of the reactive power compensation device in the inverter station, represents the AC bus voltage on the inverter side, represents the Thevenin equivalent reactance of the receiving-end AC system, represents the active power on the inverter side, represents the Thevenin equivalent voltage.

[0105] Specifically, when the AC bus voltage U pi exceeds the upper limit of 1.1 p.u., substitute U pi = 1.1 p.u. into the above formula to solve for the reference reactive power DC; when the AC bus voltage U pi is lower than the lower limit of 0.9 p.u., substitute U pi = 0.9 p.u. The calculated reactive power can maintain the AC voltage on the inverter side within the preset upper or lower limit of the interval.

[0106] In step SB5 of this embodiment, according to the reference reactive power command and the measured U pi and I dc , the calculation formula for the extinction angle is:

[0107] (6)

[0108] In the formula, represents the reference reactive power, represents the DC current, represents the AC bus voltage on the inverter side, represents the number of six-pulse converters in a commutation valve, represents the ratio of the converter transformer on the inverter side, represents the measured extinction angle, represents the commutation reactance on the inverter side.

[0109] In step SB6 of this embodiment, when commutation failure is detected, to reduce the risk of subsequent commutation failure, increase the lead trigger angle β , and the increase amount Δ β is 10°.

[0110] In the present invention, specific embodiments are used to illustrate the principles and implementation manners of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention. At the same time, for those of ordinary skill in the art, based on the idea of the present invention, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation to the present invention.

[0111] Those of ordinary skill in the art will realize that the embodiments described herein are for helping the reader understand the principles of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those of ordinary skill in the art can make various other specific deformations and combinations without departing from the essence of the present invention according to these technical revelations disclosed in the present invention, and these deformations and combinations are still within the protection scope of the present invention.

Claims

1. A conventional high voltage direct current transmission system control method for providing optimal transient voltage support, characterized in that: It includes rectifier side control and inverter side control, where: For the rectifier side control, the rectifier side control is started based on the real-time detected AC bus voltage and DC current. When the rectifier side control is started, the system equivalent parameter estimation is introduced to recalculate the DC current command of the rectifier side, and the rectifier side control is performed based on it; The DC reference instructions are: In the formula, represents the direct current, , , Indicates the number of six-pulse converters in a converter valve, represents the commutation reactance, Indicates the AC bus voltage on the rectifier side, represents the converter transformer ratio, Indicates the trigger angle on the rectifier side; The optimal reactive power command is: Upper limit of optimal reactive power command for: In the formula, represents the optimal reactive power, represents the Thevenin equivalent voltage, represents the equivalent reactance of the AC system at the sending end, Indicates the equivalent susceptance of the reactive power compensation device in the rectifier station. and Respectively represent the coefficients of active current and voltage first-order terms related and unrelated, and The coefficients that represent the reactive current and the voltage first-order term are related and unrelated, respectively. Indicates the active power consumed by the rectifier station; For inverter side control, based on the impact of the DC current command on the power transmission of the inverter station, the inverter side control is started according to the real-time detected AC bus voltage. When starting the inverter side control, the system equivalent parameter estimation is introduced to calculate the reference reactive power command. The shutdown angle command is calculated based on the reference reactive power command, and the inverter side control is performed based on it.

2. The conventional high voltage direct current transmission system control method for providing optimal transient voltage support according to claim 1, characterized in that: The method for controlling the rectifier side is specifically as follows: SA1. Real-time collection of DC system operating parameters and sending-end AC system parameters; SA2: For the AC bus voltage and DC current detected in real time in the acquisition parameters; When the AC bus voltage is within the preset range, no additional rectification side control is required and the system is in a normal rectification control state; When the AC bus voltage exceeds the maximum value of the preset interval, the system experiences a transient voltage rise, starts the rectifier side control, and enters step SA3; When the AC bus voltage is lower than the minimum value of the preset interval and the DC current is lower than the preset threshold, a transient voltage drop occurs in the system, the rectifier side control is started, and the process goes to step SA3; When the AC bus voltage is lower than the minimum value of the preset interval and the DC current is greater than or equal to the preset threshold, the system fails to commutate, starts the rectifier side control, and enters step SA3; SA3, based on the voltage and current measurement values ​​collected in real time and combined with the equivalent parameters of the wind farm during normal operation, the Thevenin equivalent parameters of the AC system are estimated; SA4. Calculate the optimal reactive power instruction on the rectifier side based on the estimated equivalent parameters and the measured active power; SA5. Recalculate the DC reference command on the rectifier side according to the optimal reactive power command, AC voltage and trigger angle; SA6. The rectifier side is controlled according to the recalculated DC reference instruction. During the control process, when the AC bus voltage and the DC current are both in the preset range and last for a preset time, it is determined that the rectifier side has resumed normal operation and the rectifier side control is exited.

3. The conventional high voltage direct current transmission system control method for providing optimal transient voltage support according to claim 1, characterized in that: The inverter side control method is: SB1, real-time collection of DC system operating parameters and receiving end AC system parameters; SB2. Determine whether the AC bus voltage detected in real time in the collected parameters is within a preset range; If yes, no additional inverter-side control is required, and the system is in normal inverter control state; If not, start the inverter side control and go to step SB3; SB3, collect the voltage and current of the AC system and estimate the Thevenin equivalent parameters of the receiving system; SB4. Calculate the reference reactive power command on the inverter side according to the estimated equivalent parameters and the measured active power; SB5. Calculate the shutdown angle as the inverter side shutdown angle instruction according to the reference reactive power instruction and the real-time detected AC voltage and DC current of the common connection point of the inverter side; SB6, control the inverter side according to the shutdown angle command, and detect whether commutation failure occurs during the control process; If yes, then increase the leading trigger angle and go to step SB7; If not, proceed to step SB7; SB7. When the AC bus voltage and DC current on the inverter side are both within the preset range and last for a preset period of time, it is determined that the inverter side has resumed normal operation and the inverter side control is exited; otherwise, return to step SB4.

4. The conventional high voltage direct current transmission system control method for providing optimal transient voltage support according to claim 3, characterized in that: In step SB4, the reference reactive power instruction is: In the formula, represents the reference reactive power, Indicates the equivalent susceptance of the reactive power compensation device in the inverter station. Indicates the AC bus voltage on the inverter side, represents the Thevenin equivalent reactance of the receiving-end AC system, Indicates the active power on the inverter side. Represents the Thevenin equivalent voltage.

5. The conventional high voltage direct current transmission system control method for providing optimal transient voltage support according to claim 3, characterized in that: In step SB5, the calculation formula of the cut-off angle is: In the formula, represents the reference reactive power, represents the direct current, Indicates the AC bus voltage on the inverter side, Indicates the number of six-pulse converters in a converter valve, represents the inverter side converter transformer ratio, Indicates the measurement of the cut-off angle, Indicates the inverter side commutation reactance.

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