A control method for a current source type multi-terminal direct current transmission system

Abstract

The present invention discloses a control method for a current-source multi-terminal DC transmission system, relating to the field of electronic power control technology. In this current-source multi-terminal DC transmission system, the converter station on the rectifier side is an LCC converter station, and the converter station on the inverter side includes at least two parallel CLCC converter stations. The method includes: the converter station on the rectifier side adopts constant current control and fixed minimum trigger angle control; the converter station on the inverter side adopts master-slave control. This method can avoid the occurrence of commutation failure problems, avoid power oscillation, loss, and even power backflow problems caused by fault commutation failure in the current-source multi-terminal DC system, and avoid the low voltage level and power capacity of MMC and the high price.

Description

Technical Field

[0001] The present invention relates to the technical field of electronic power control, and in particular to a control method for a current source type multi-terminal direct current transmission system. Background Art

[0002] Conventional line commutation converters (LCCs) play a vital role in high-voltage direct current (HVDC) transmission systems due to their long-distance, large capacity, low losses, high voltage levels, low manufacturing costs, and mature and reliable technology. They have been widely used in power systems to enable long-distance power transmission and asynchronous grid connections. With the rapid development and gradual grid integration of renewable energy, power systems are increasingly shifting towards multiple energy sources and multiple load centers. To interconnect renewable energy sites across multiple regions and power system load centers, multi-terminal direct current (MTDC) systems are gaining increasing attention and are gradually developing. MTDC systems, through multi-regional meshing, help enhance the reliability, flexibility, cost-effectiveness, and stability of HVDC systems.

[0003] However, the cost of VSC (Voltage Sourced Converter) and MMC (Modular Multi-level Converter) converters is much higher than that of LCC converters, and their smaller capacity limits the transmission power of HVDC. In the event of a fault, expensive and demanding DC circuit breakers are required to cross the fault. Therefore, the use of LCC to form an MTDC system has great potential. In scenarios where the system flow direction is fixed, LCC-MTDC has great application value, and the existing LCC-HVDC project can be directly improved to form an LCC-MTDC, reducing investment costs and construction period. Current source multi-terminal DC transmission system such as Figure 1 As shown, where i dc is the DC current of the DC line.

[0004] Commutation failure is prone to occur during LCC faults, a unique fault mode in LCC-HVDC systems. For current-source DC transmission systems, when an AC fault occurs on the inverter side of the system, commutation failure is prone to occur in the inverter-side LCC converter station. When the other arm of the same phase within the converter triggers conduction, both the upper and lower arms conduct simultaneously, causing a converter short circuit. From a system perspective, commutation failure at the LCC converter station is equivalent to a short circuit to ground at the station. System current is released through the short circuit, causing overcurrent and significant voltage drops in the fault line. In severe cases, the DC voltage can drop to zero, causing system power oscillations and even loss.

[0005] For a current-sourced multi-terminal direct current (HVDC) system (hereinafter referred to as the system), when a commutation failure occurs at an LCC converter station on the inverter side of the system, the current at each terminal of the HVDC system is released from the faulty terminal due to the short circuit grounding. This causes a loss or drop in the transmitted power at each terminal of the HVDC system. Under certain operating conditions, this can trigger power reverse at the receiving end of the HVDC system. This indicates that when a commutation failure occurs at a single LCC converter station in the HVDC system, it can seriously affect the safe and stable operation of the entire HVDC system. Summary of the Invention

[0006] In order to solve the above technical problems existing in the prior art, the present invention provides a control method for a current source type multi-terminal direct current transmission system.

[0007] Specifically, in a current source multi-terminal direct current transmission system, the converter station on the rectifier side is an LCC converter station, and the converter station on the inverter side includes at least two parallel CLCC converter stations;

[0008] The control method includes:

[0009] The converter station on the rectifier side adopts constant current control and fixed minimum firing angle control;

[0010] The converter station on the inverter side adopts master-slave control.

[0011] Preferably, the master-slave control includes:

[0012] One of the CLCC converter stations adopts constant voltage control, and the other CLCC converter stations adopt constant current control.

[0013] Preferably, the constant voltage control specifically includes:

[0014] Measure the DC voltage U on the inverter side d1 , and calculate the DC voltage U d1 and the rated DC voltage U ref1 The difference d1;

[0015] The difference d1 is subjected to PI correction to obtain the trigger angle command A1 of the first CLCC converter station.

[0016] Preferably, the constant current control specifically includes:

[0017] Measure the DC voltage U of the DC line connected to the CLCC converter station d2 and DC current I d2 , and calculate the DC voltage U at the midpoint of the DC line d3 ;

[0018] For DC voltage U d3 Perform low-voltage current limiting control and use the generated current instruction as a constant current control instruction.

[0019] Preferably, the minimum trigger angle control specifically includes:

[0020] Calculate the constant current control instruction minus the rectifier side DC current I d The difference d2;

[0021] The difference d2 is corrected by PI to obtain the inverter angle β1 of the constant current control on the rectifier side;

[0022] By subtracting the inverter angle β1 from π, the trigger angle command A2 for the fixed minimum trigger angle control is obtained.

[0023] Furthermore, it also includes:

[0024] Perform PI correction on the trigger angle command A2 so that A2 is greater than or equal to 5°.

[0025] Furthermore, the CLCC converter station using constant current control is also equipped with a fixed arc extinction angle control, specifically including:

[0026] Calculate the constant current control command and the inverter side DC current I d2 The difference d3;

[0027] The difference d3 is corrected by PI to obtain the inverter angle β2 of constant current control;

[0028] Measure the minimum arc extinction angle β of all inverter bridges in the last power frequency cycle min , calculate the minimum arc extinction angle β min and the preset arc extinction angle β ref The difference d4;

[0029] The difference d4 is limited and then PI correction is performed to obtain the inverter angle β3 for fixed arc extinction angle control;

[0030] The trigger angle command A3 for fixed arc extinction angle control is obtained by subtracting the maximum value of the inverter angle β2 and the inverter angle β3 from π.

[0031] Preferably, the preset arc extinction angle β ref is 15°.

[0032] Preferably, the CLCC converter station includes bridge arms VT1 to VT6;

[0033] The input ends of the bridge arm VT4, the bridge arm VT6 and the bridge arm VT2 are connected to each other, and the output ends of the bridge arm VT1, the bridge arm VT3 and the bridge arm VT5 are connected to each other;

[0034] The input end of bridge arm VT1 is connected to the output end of bridge arm VT4, the input end of bridge arm VT2 is connected to the output end of bridge arm VT5, and the input end of bridge arm VT3 is connected to the output end of bridge arm VT6;

[0035] The input ends of the bridge arm VT1, the bridge arm VT3 and the bridge arm VT5 are all connected to the transformer.

[0036] Preferably, any bridge arm includes a thyristor valve V1, a main full-control valve V2, an auxiliary full-control valve V3 and a high-voltage and low-current thyristor valve V4;

[0037] Main full control valve V2 includes:

[0038] The anode of diode D1 is connected to one end of capacitor C1, and the cathode is connected to the emitter of IGBT1; the other end of capacitor C1 is connected to the collector of IGBT1; resistor R1 is connected in parallel with diode D1;

[0039] Auxiliary full control valve V3 includes:

[0040] The anode of diode D2 is connected to one end of capacitor C2, and the cathode is connected to the emitter of IGBT2; the other end of capacitor C2 is connected to the collector of IGBT2; resistor R2 is connected in parallel with diode D2; variable resistor R3 is connected in parallel with IGBT2;

[0041] In the thyristor valve V1, the anode of the thyristor Thy1 is connected to the collector of IGBT2, and the cathode is connected to the collector of IGBT1;

[0042] In the high-voltage, low-current thyristor valve V4, the anode of thyristor Thy2 is connected to the emitter of IGBT2, and the cathode is connected to the emitter of IGBT1;

[0043] The emitter of IGBT2 is the input end of the bridge arm, and the emitter of IGBT1 is the output end of the bridge arm.

[0044] It can be seen that the technical solution provided by the present invention can fundamentally avoid the occurrence of commutation failure problems, avoid power oscillation, loss and even power reverse caused by fault commutation failure in current source multi-terminal DC systems, and avoid the problems of low voltage level and power capacity as well as high price of MMC. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is the topological structure diagram of the traditional current source multi-terminal DC transmission system.

[0046] Figure 2 This is the topological structure diagram of the phase-commutated converter in the controllable grid.

[0047] Figure 3 This is the topological structure diagram of the bridge arm in the controllable grid commutation converter.

[0048] Figure 4 Schematic diagram of the working principle of the phase-commutated converter in the controllable grid.

[0049] Figure 5 Schematic diagram comparing the inverter side topology structures of the current source multi-terminal DC system before and after improvement.

[0050] Figure 6 (a) shows the fault DC voltage curve when the AC single-phase is grounded at the constant voltage station and the inverter side is LCC.

[0051] Figure 6 (b) shows the fault DC voltage curve when the AC single-phase is grounded at the constant voltage station and the inverter side is CLCC.

[0052] Figure 7 (a) shows the fault DC current curve when the AC single-phase is grounded at the constant voltage station and the inverter side is LCC.

[0053] Figure 7(b) shows the fault DC current curve when the AC single-phase is grounded at the constant voltage station and the inverter side is CLCC.

[0054] Figure 8 (a) shows the fault DC voltage curve when the AC single-phase of the fixed active power station is grounded and the inverter side is LCC.

[0055] Figure 8 (b) shows the fault DC voltage curve when the AC single-phase of the fixed active power station is grounded and the inverter side is CLCC.

[0056] Figure 9 (a) shows the fault DC current curve when the AC single-phase is grounded at a constant voltage station and the inverter side is LCC.

[0057] Figure 9 (b) shows the fault DC current curve when the AC single-phase is grounded at the constant voltage station and the inverter side is CLCC. DETAILED DESCRIPTION

[0058] Hereinafter, the technical solution provided by the present invention will be further elaborated in conjunction with the accompanying drawings.

[0059] The controllable line-commutated converter (CLCC) adds a controllable power electronic device, IGBT (Insulated Gate Bipolar Transistor), to the traditional thyristor commutation converter. This device can force the thyristor valve group to shut down in the event of a fault and provide reverse recovery voltage. In addition, an auxiliary branch is added to transfer the commutation current, forming a topology that combines the thyristor valve group with the IGBT. Figure 2 、 Figure 3 As shown in the figure, this converter still uses the basic six-pulse structure. Each bridge leg (VT1-VT6) consists of a main branch formed by a thyristor valve group and an IGBT valve group connected in series, and an auxiliary branch connected in parallel. To achieve dynamic voltage division during system operation, an RCD damping circuit is also connected in parallel with the IGBT branch. Figure 2 middle, I dc is the DC current input to the CLCC converter, U dc is the DC terminal voltage of the CLCC converter. Figure 3 middle, I VT1 is the current of the input bridge arm, I1 is the main branch current, and I2 is the auxiliary branch current.

[0060] The main branch consists of a thyristor valve V1 and a main fully-controlled valve V2 connected in series. Like the LCC converter, thyristor valve V1 can withstand the high voltages and high currents of system operation, offering high flow and low losses. The main fully-controlled valve V2 actively shuts off during commutation, diverting current to the auxiliary branch and providing reverse recovery voltage for thyristor valve V1. After a certain reverse recovery time, thyristor V1 regains its blocking capability. The auxiliary branch consists of an auxiliary fully-controlled valve V3 and a high-voltage, low-current thyristor valve V4 connected in series. During commutation, the auxiliary branch briefly withstands the low current transferred from the main branch and delays disconnection until thyristor valve V1 regains its reverse blocking capability, ensuring reliable commutation. When non-conducting, the auxiliary branch must withstand the entire bridge arm voltage.

[0061] The working principle of the controllable grid commutated converter is explained by its control signal timing, such as Figure 4 As shown. Sgv1 is the control signal of thyristor valve V1, Sgv2 is the control signal of main full-control valve V2, Sgv3 is the control signal of auxiliary full-control valve V3, and Sgv4 is the control signal of high-voltage and low-current thyristor valve V4. The main logic of control signal timing is as follows:

[0062] At time t0, the thyristor valve V1, the main full-control valve V2 and the auxiliary full-control valve V3 are triggered to conduct, and the main branch of the converter valve flows normally;

[0063] After the main branch conducts for 120°, the bridge arm begins to commutate at time t1;

[0064] During the arm commutation period, when the main branch current drops to a certain level, the main full-control valve V2 is turned off at time t2, and the high-voltage and low-current thyristor valve V4 is turned on, and the main branch current is transferred to the auxiliary branch;

[0065] After a certain delay After that, the auxiliary full-control valve V3 is closed at time t4, and the bridge arm completes the phase change.

[0066] The present invention adopts Figure 3 The controllable grid-commutated converter shown in the figure utilizes a half-controlled thyristor with an IGBT as the controllable device, resulting in a hybrid IGBT-thyristor topology with main and auxiliary branches. Actively shutting down the IGBT in the main branch allows current to be transferred from the main branch to the auxiliary branch. The main branch IGBT provides a reverse recovery voltage to the thyristor, extending the reverse recovery time. Finally, the IGBT in the auxiliary branch is actively shut down to cut the arm current and complete commutation. Forced commutation of the converter is achieved by forcing the IGBT to shut down in the event of a system fault, fundamentally resolving the commutation failure problem caused by AC faults at the receiving end of a current-source multi-terminal DC transmission system.

[0067] In addition, the present invention is based on the CIGRE (Conference International des Grands ReseauxElectriques, International Conference on Large Grids) standard DC transmission model, and builds a current source multi-terminal DC transmission system as shown in Figure 1. Secondly, in the current source multi-terminal DC transmission system, the original conventional LCC converter is replaced with the designed controllable grid commutation converter. Figure 5 shown.

[0068] The controllable grid-commutated converter is composed of the conventional grid-commutated converter topology with the addition of power electronic devices IGBT. In essence, it is still a 6-pulse bridge phase-controlled current source converter, so its control method can still adopt the same constant current, constant voltage or fixed arc extinction angle control as LCC.

[0069] In the current source multi-terminal DC transmission system of the present invention, the rectifier side LCC converter station adopts constant current control and fixed minimum trigger angle control, wherein the constant current control is equipped with a low-voltage current limiting link, and the fixed minimum trigger angle control is based on the constant current control. Due to the multi-terminal parallel connection, the CLCC converter stations at both ends of the inverter side adopt a master-slave control method. Master-slave control is a method in which a master converter station is selected to control the DC voltage, and the remaining converter stations control the power by controlling the DC current flowing through. In the present invention, it is manifested as: the inverter side converter station CLCC1 adopts constant voltage control, and the inverter side converter station CLCC2 adopts constant current control. The two control methods are independent of each other and only control the corresponding converter stations.

[0070] Converter station CLCC1 uses constant voltage control to control the DC voltage of the entire system and serves as a power balancing node to coordinate power changes across the entire system. Converter station CLCC2 uses constant current control to maintain steady-state DC current in the DC transmission lines at each end of the inverter side, and is equipped with fixed arc extinction angle control as a backup.

[0071] For the constant voltage control of converter station CLCC1, the DC voltage U on the inverter side will be measured during operation. d1 The inverter side rated DC voltage U ref1 The subtraction is then fed into the PI correction process to obtain the firing angle command A1 for the inverter-side converter station CLCC1. As a power balancing node, converter station CLCC1 must have a transformer rated capacity 5 to 10 times that of the remaining converter stations in the system to regulate system power. When a system fault occurs, overcurrent may occur, causing system power fluctuations. This 5 to 10 times the rated capacity can be used to transmit the power exceeding the limit through the converter station, achieving system power regulation and preventing system instability caused by system power fluctuations during a fault.

[0072] For constant current control, during operation, the DC voltage U of the DC line connected to the inverter side converter station CLCC2 is measured. d2 and DC current I d2 , calculate the DC voltage U at the midpoint of the DC line d3 , the formula is as follows:

[0073] U d3 =U d2 + I d2 ·X;

[0074] Where X is the DC line midpoint and DC voltage U d2 The sum of the impedance loss of the DC transmission line between the measurement points and the impedance of components such as smoothing reactors connected to the DC line.

[0075] DC voltage U d3 The current command is generated through the low-voltage current limiting link and serves as the constant current control command for both the rectifier and inverter sides. Calculating the DC line midpoint voltage prevents voltage division in devices such as line-end smoothing reactors.

[0076] For the fixed minimum trigger angle control, the current command from the inverter side is used as the constant current control command, minus the actual measured DC current I on the rectifier side. dThe PI correction step then determines the inverter angle β1 for constant current control on the rectifier side. Subtracting β1 from π yields the firing angle command A2 for constant minimum firing angle control. This minimum firing angle control limits the minimum value of the output firing angle command A2 to 5° through the PI correction step, minimizing reactive power loss while leaving room for adjustment to control DC current flow.

[0077] For the backup fixed arc extinction angle control, the constant current control command and the actual measured DC current I on the inverter side d2 After subtraction, it is sent to the PI correction link to generate the inverter angle β2 of constant current control; at the same time, the minimum arc extinction angle β of the two inverter bridges in the past power frequency cycle is measured. min is the actual extinction angle, which is different from the given preset extinction angle β ref Subtract and limit them, and send them to the PI correction link to obtain the inverter angle β3 of the fixed extinction angle control. Finally, select the largest output from the inverter angle β2 obtained by constant current control and the inverter angle β3 obtained by fixed extinction angle control, and subtract them from π to obtain the trigger angle instruction A3 of the inverter side converter station CLCC2. Among them, the preset extinction angle β of the given value is ref Generally, 15° (0.2618) is taken.

[0078] The fixed arc extinction angle control part is also equipped with a current deviation control link, the input of which is the current setting value I on the rectifier side. d0 Subtract the actual DC current I d The deviation value d0 is output as the arc extinction angle increment of the fixed arc extinction angle control on the inverter side. The arc extinction angle increment is output only when the actual current is less than the set value. , and when the actual current is greater than the set value, the output arc extinction angle increment 0. Calculate the trigger angle command A3 and increase the arc extinction angle increment The sum is used as the new trigger angle command A to control the arc extinction angle. 31 The purpose of this link is to make the current return to the given value as soon as possible. The specific value of is calculated based on the ramp transfer function of the deviation value d0.

[0079] To ensure that the CLCC1 converter station operates under constant voltage control during normal operation and avoid being in backup constant current control, its current setting value must be reduced by a current margin. However, the remaining stations must operate under constant current control during normal operation, so no current margin reduction is required. Based on actual DC transmission system operating experience, the current margin is typically 10% of the rated current. Therefore, to implement the master-slave control strategy for multi-terminal DC transmission, in a current-source multi-terminal DC system, only the current margin of CLCC1 needs to be reduced; the current margin does not need to be reduced for the remaining inverter-side converter stations. This ensures that the DC voltage of the entire system is stable and has a certain regulation margin through CLCC1's constant voltage control. CLCC1 also serves as a system power balancing node to regulate system power.

[0080] experiment:

[0081] The current source multi-terminal DC transmission system with LCC converter on the inverter side as shown in Figure 1 is constructed as follows Figure 5 The improved current source multi-terminal DC transmission system with CLCC converter on the inverter side shown in the figure is simulated and verified as an example. The simulation compares the operating characteristics of the constant voltage station LCC1 and the constant voltage station CLCC1 on the inverter side of the system when the LCC and CLCC are used on the inverter side of the system, as shown in Figures 6(a) to 6(b) and Figures 7(a) to 7(b); and the operating characteristics of the constant active power station LCC2 and the constant active power station CLCC2 on the inverter side of the system when the AC single-phase grounding fault occurs, as shown in Figures 8(a) to 8(b) and Figures 9(a) to 9(b). The fault start time is 2.5s and the fault duration is 0.01s.

[0082] By replacing the LCC converter with a CLCC converter on the inverter side of the current source multi-terminal DC transmission system, the commutation failure problem of the LCC converter caused by the system AC fault can be fundamentally avoided, the overcurrent and voltage drop at the fault end of the system are reduced, the voltage and current at the non-fault end of the system are maintained relatively stable, and the problems of system power oscillation, short-term loss and severe power reverse are avoided, thereby ensuring the safe and stable operation of the system and verifying the effectiveness of the technical solution provided by the present invention.

[0083] In summary, the technical solution provided by the present invention can fundamentally avoid the occurrence of commutation failure problems, avoid power oscillation, loss and even power reverse caused by fault commutation failure in current source multi-terminal DC systems, and avoid the problems of low voltage level and power capacity as well as high price of MMC.

[0084] Furthermore, in constant current control, calculating the DC voltage at the midpoint of the DC line can prevent voltage division of devices such as smoothing reactors at the end of the line, thereby improving control accuracy; fixed minimum trigger angle control helps to reduce reactive power loss and retains room for adjusting and controlling DC power flow; fixed arc extinction angle control can further prevent commutation failure and improve the stability of current source multi-terminal DC transmission systems.

Claims

1. A control method for a current source multi-terminal direct current transmission system, characterized in that: In the current source multi-terminal direct current transmission system, the converter station on the rectifier side is an LCC converter station, and the converter station on the inverter side includes at least two parallel CLCC converter stations; The control method includes: The converter station on the rectifier side adopts constant current control and fixed minimum firing angle control; The converter station on the inverter side adopts master-slave control; Among them, LCC is a grid-commutated converter and CLCC is a controllable grid-commutated converter.

2. The control method of a current source multi-terminal direct current transmission system according to claim 1, characterized in that: The master-slave control includes: One of the CLCC converter stations adopts constant voltage control, and the other CLCC converter stations adopt constant current control.

3. The control method of a current source multi-terminal direct current transmission system according to claim 2, wherein: The constant voltage control specifically includes: Measure the DC voltage U on the inverter side d1 , and calculate the DC voltage U d1 and the rated DC voltage U ref1 The difference d1; The difference d1 is subjected to PI correction to obtain the trigger angle command A1 of the first CLCC converter station.

4. A control method for a current source multi-terminal direct current transmission system according to claim 1 or 2, characterized in that: The constant current control specifically includes: Measure the DC voltage U of the DC line connected to the CLCC converter station d2 and DC current I d2 , and calculate the DC voltage U at the midpoint of the DC line d3 ; For DC voltage U d3 Perform low-voltage current limiting control and use the generated current instruction as a constant current control instruction.

5. The control method of a current source multi-terminal direct current transmission system according to claim 4, characterized in that: The minimum trigger angle control specifically includes: Calculate the constant current control instruction minus the rectifier side DC current I d The difference d2; The difference d2 is corrected by PI to obtain the inverter angle β1 of the constant current control on the rectifier side; By subtracting the inverter angle β1 from π, the trigger angle command A2 for the fixed minimum trigger angle control is obtained.

6. The control method of a current source multi-terminal direct current transmission system according to claim 5, characterized in that: Also includes: Perform PI correction on the trigger angle command A2 so that A2 is greater than or equal to 5°.

7. The control method of a current source multi-terminal direct current transmission system according to claim 2, characterized in that: CLCC converter stations using constant current control are also equipped with fixed arc extinction angle control, specifically including: Calculate the constant current control command and the inverter side DC current I d2 The difference d3; The difference d3 is corrected by PI to obtain the inverter angle β2 of constant current control; Measure the minimum arc extinction angle β of all inverter bridges in the last power frequency cycle min , calculate the minimum arc extinction angle β min and the preset arc extinction angle β ref The difference d4; The difference d4 is limited and then PI correction is performed to obtain the inverter angle β3 for fixed arc extinction angle control; The trigger angle command A3 for fixed arc extinction angle control is obtained by subtracting the maximum value of the inverter angle β2 and the inverter angle β3 from π.

8. The control method of a current source multi-terminal direct current transmission system according to claim 7, characterized in that: The preset arc extinction angle β ref is 15°.

9. The control method of a current source multi-terminal direct current transmission system according to claim 1, wherein: The CLCC converter station includes bridge arms VT1 to VT6; The input ends of the bridge arm VT4, the bridge arm VT6 and the bridge arm VT2 are connected to each other, and the output ends of the bridge arm VT1, the bridge arm VT3 and the bridge arm VT5 are connected to each other; The input end of bridge arm VT1 is connected to the output end of bridge arm VT4, the input end of bridge arm VT2 is connected to the output end of bridge arm VT5, and the input end of bridge arm VT3 is connected to the output end of bridge arm VT6; The input ends of the bridge arm VT1, the bridge arm VT3 and the bridge arm VT5 are all connected to the transformer.

10. The control method of a current source multi-terminal direct current transmission system according to claim 9, characterized in that: Any bridge arm includes thyristor valve V1, main full-control valve V2, auxiliary full-control valve V3 and high-voltage and low-current thyristor valve V4; Main full control valve V2 includes: The anode of diode D1 is connected to one end of capacitor C1, and the cathode is connected to the emitter of IGBT1; the other end of capacitor C1 is connected to the collector of IGBT1; resistor R1 is connected in parallel with diode D1; Auxiliary full control valve V3 includes: The anode of diode D2 is connected to one end of capacitor C2, and the cathode is connected to the emitter of IGBT2; the other end of capacitor C2 is connected to the collector of IGBT2; resistor R2 is connected in parallel with diode D2; variable resistor R3 is connected in parallel with IGBT2; In the thyristor valve V1, the anode of the thyristor Thy1 is connected to the collector of IGBT2, and the cathode is connected to the collector of IGBT1; In the high-voltage, low-current thyristor valve V4, the anode of thyristor Thy2 is connected to the emitter of IGBT2, and the cathode is connected to the emitter of IGBT1; The emitter of IGBT2 is the input end of the bridge arm, and the emitter of IGBT1 is the output end of the bridge arm.

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