Control method of current source type multi-terminal direct current power transmission system

Abstract

The invention discloses a control method of a current source type multi-terminal direct current power transmission system, and relates to the technical field of electronic power control. In the current source type multi-terminal direct current power transmission system, a converter station at a rectification side is an LCC converter station, and a converter station at an inversion side comprises at least two CLCC converter stations which are connected in parallel; the method comprises the following steps: a converter station at a rectification side adopts constant current control and constant minimum firing angle control; and the converter station at the inverter side adopts master-slave control. According to the method, the problem of commutation failure can be avoided, the problem of power reverse transmission due to power oscillation and loss caused by fault commutation failure of a current source type multi-terminal direct current system can be avoided, and the problems of low MMC voltage level and power capacity and high price can be avoided.

Description

Technical Field

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

[0002] Due to characteristics such as long distance, large capacity, low loss, high voltage level, low manufacturing cost, and mature and reliable technology, the conventional line commutation converter (LCC) plays an important role in the high-voltage direct current (HVDC) system and has been widely used in the power system to achieve long-distance power transmission and asynchronous grid connection. With the rapid development and gradual grid connection of new energy, the power system is gradually turning towards multi-energy and multi-load centers. To achieve the interconnection between multi-region renewable energy power stations and the load centers of the power system, the multi-terminal direct current (MTDC) system has received increasing attention and is gradually developing. The MTDC system helps to enhance the reliability, flexibility, economy, and stability of the HVDC system through multi-region mesh interconnection.

[0003] However, since the cost of the voltage sourced converter (VSC) and the modular multi-level converter (MMC) is much higher than that of the LCC converter, and their small capacity limits the transmission power of the HVDC. During a fault, an expensive and high-manufacturing-requirement DC circuit breaker needs to be used for fault ride-through. Therefore, there is great potential in using LCC to form an MTDC system. In the scenario where the system power flow direction is fixed, the LCC-MTDC has great application value, and the existing LCC-HVDC project can be directly improved to form an LCC-MTDC, reducing the investment cost and construction period. A current source type multi-terminal DC power transmission system is as Figure 1 shown, where i dc is the DC current of the DC line.

[0004] Commutation failure is likely to occur during an LCC fault, and commutation failure is a unique fault form of the LCC-HVDC system. For a current-source DC transmission system, when an AC fault occurs on the inverter side of the system, commutation failure is likely to occur at the LCC converter station on the inverter side. After the other arm of the same phase inside the converter is triggered and conducted, it will cause the upper and lower arms to conduct simultaneously, triggering a short circuit in the converter. From the perspective of the system, commutation failure at the LCC converter station is equivalent to a short circuit to ground at this station, and the system current will be released from the short circuit at this station, which will cause overcurrent in the faulty line of the system and a large voltage drop. In severe cases, the DC voltage will drop to 0, resulting in system power oscillation and even loss.

[0005] For a current-source multi-terminal DC transmission system (hereinafter referred to as the system), when commutation failure occurs at an LCC converter station at one end of the inverter side of the system due to a fault, due to the short circuit of the converter, the currents at all ends of the multi-terminal DC transmission system will be released from the short circuit to ground at the fault end, resulting in the loss and drop of the transmitted power at all ends of the multi-terminal DC transmission system. Under specific working conditions, it will trigger the reverse power transmission at the receiving end of the multi-terminal DC transmission system. This shows that when commutation failure occurs at a single LCC converter station in the multi-terminal DC transmission system, it will seriously affect the safe and stable operation of the entire multi-terminal DC transmission system. Summary of the Invention

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

[0007] Specifically, in a current-source multi-terminal DC transmission system, the converter station on the rectifier side is an LCC converter station, and the converter stations on the inverter side include at least two parallel CLCC converter stations; The control method includes: The converter station on the rectifier side adopts constant current control and constant minimum trigger angle control; The converter stations on the inverter side adopt master-slave control.

[0008] Preferably, the master-slave control includes: One of the CLCC converter stations adopts constant voltage control, and the remaining CLCC converter stations adopt constant current control.

[0009] Preferably, the constant voltage control specifically includes: Measure the DC voltage U d1 on the inverter side, and calculate the difference d d1 between the DC voltage U ref1 and the rated DC voltage U 1 on the inverter side; Perform PI correction on the difference d 1 to obtain the trigger angle command A 1 of the first CLCC converter station.

[0010] Preferably, the constant current control specifically includes: Measuring the DC voltage U of the DC line connected to the CLCC converter station d2 and the DC current I d2 , and calculating the DC voltage U at the midpoint of the DC line d3 ; Performing low-voltage current limiting control on the DC voltage U d3 and using the generated current command as the constant current control command.

[0011] Preferably, the fixed minimum firing angle control specifically includes: Calculating the difference d obtained by subtracting the rectifier-side DC current I d from the constant current control command 2 ; Performing PI correction on the difference d 2 to obtain the inverter angle β of the rectifier-side constant current control 1 ; Subtracting the inverter angle β from π 1 to obtain the firing angle command A of the fixed minimum firing angle control 2 .

[0012] Furthermore, it further includes: Performing PI correction on the firing angle command A 2 to make A 2 greater than or equal to 5°.

[0013] Furthermore, the CLCC converter station adopting constant current control is also configured with fixed extinction angle control, which specifically includes: Calculating the difference d between the constant current control command and the inverter-side DC current I d2 3 ; Performing PI correction on the difference d 3 to obtain the inverter angle β of the constant current control 2 ; Measuring the minimum extinction angle β of all inverter bridges in the previous power frequency cycle min , calculating the difference d between the minimum extinction angle β min and the preset extinction angle β ref ; 4 Limiting the difference d and then performing PI correction to obtain the inverter angle β of the fixed extinction angle control 4 ; 3 Subtracting the maximum value of the inverter angle β from π and the inverter angle β 2 and the inverter angle β 3 to obtain the firing angle command A of the fixed extinction angle control 3 .

[0014] Preferably, the preset extinction angle β​ref is 15°.

[0015] Preferably, the CLCC converter station includes arm VT1 to arm VT6; The input ends of arm VT4, arm VT6 and arm VT2 are connected to each other, and the output ends of arm VT1, arm VT3 and arm VT5 are connected to each other; The input end of arm VT1 is connected to the output end of arm VT4, the input end of arm VT2 is connected to the output end of arm VT5, and the input end of arm VT3 is connected to the output end of arm VT6; The input ends of arm VT1, arm VT3 and arm VT5 are all connected to the transformer.

[0016] Preferably, any arm includes a thyristor valve V1, a main fully-controlled valve V2, an auxiliary fully-controlled valve V3 and a high-voltage small-current thyristor valve V4; The main fully-controlled valve V2 includes: The anode of diode D1 is connected to one end of capacitor C1, 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; The auxiliary fully-controlled valve V3 includes: The anode of diode D2 is connected to one end of capacitor C2, 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 thyristor Thy1 is connected to the collector of IGBT2, and the cathode is connected to the collector of IGBT1; In the high-voltage small-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; Wherein, the emitter of IGBT2 is the input end of the arm, and the emitter of IGBT1 is the output end of the arm.

[0017] It can be seen that the technical solution provided by the present invention can fundamentally avoid the occurrence of commutation failure problems, avoid the power oscillation, loss and even power reverse transmission problems caused by faulty commutation failure in the current-source multi-terminal DC system, and avoid the problems of low MMC voltage level, low power capacity and high price. Description of the Drawings

[0018] Figure 1 is the topological structure diagram of the traditional current-source multi-terminal DC transmission system.

[0019] Figure 2 is the topological structure diagram of the controllable grid-commutated converter.

[0020] Figure 3 It is the topological structure diagram of the bridge arm in the controllable line-commutated converter (CLCC).

[0021] Figure 4 It is the schematic diagram of the working principle of the controllable line-commutated converter (CLCC).

[0022] Figure 5 It is the schematic diagram for comparing the topological structures of the inverter side of the current source multi-terminal DC system before and after improvement.

[0023] Figure 6(a) is the fault DC voltage curve when the AC single-phase grounding occurs at the constant voltage station and the inverter side is the LCC.

[0024] Figure 6(b) is the fault DC voltage curve when the AC single-phase grounding occurs at the constant voltage station and the inverter side is the CLCC.

[0025] Figure 7(a) is the fault DC current curve when the AC single-phase grounding occurs at the constant voltage station and the inverter side is the LCC.

[0026] Figure 7(b) is the fault DC current curve when the AC single-phase grounding occurs at the constant voltage station and the inverter side is the CLCC.

[0027] Figure 8(a) is the fault DC voltage curve when the AC single-phase grounding occurs at the constant active power station and the inverter side is the LCC.

[0028] Figure 8(b) is the fault DC voltage curve when the AC single-phase grounding occurs at the constant active power station and the inverter side is the CLCC.

[0029] Figure 9(a) is the fault DC current curve when the AC single-phase grounding occurs at the constant voltage station and the inverter side is the LCC.

[0030] Figure 9(b) is the fault DC current curve when the AC single-phase grounding occurs at the constant voltage station and the inverter side is the CLCC. Specific implementation manners

[0031] Hereinafter, the technical solutions provided by the present invention will be further elaborated in detail with reference to the accompanying drawings.

[0032] Based on the thyristors of the traditional line-commutated converter, the controllable line-commutated converter (CLCC) adds a controllable power electronic device IGBT (Insulated Gate Bipolar Transistor), which can realize the forced turn-off of the thyristor valve group during faults and provide a reverse recovery voltage. In addition, an auxiliary branch is added to transfer the commutation current, forming a topological structure in which the thyristor valve group and the IGBT are connected in parallel, such as Figure 2 、 Figure 3As shown. The converter still adopts the basic structure of 6-pulse. Each bridge arm (VT1-VT6) is composed of a main branch and an auxiliary branch in parallel, where the main branch is formed by the series connection of a thyristor valve group and an IGBT valve group. To achieve dynamic voltage sharing during system operation, an RCD damping circuit is also connected in parallel to the IGBT branch. Figure 2 In it, I dc is the DC current input to the CLCC converter, and U dc is the DC terminal voltage of the CLCC converter. Figure 3 In it, I VT1 is the current of the input bridge arm, I 1 is the main branch current, and I 2 is the auxiliary branch current.

[0033] The main branch is composed of the series connection of thyristor valve V1 and main fully-controlled valve V2. Thyristor valve V1 is the same as that of the LCC converter, which can withstand the high voltage and large current during system operation, and has the advantages of high current-carrying capacity and low loss. The main fully-controlled valve V2 can be actively turned off during commutation, transfer the current to the auxiliary branch, and provide a reverse recovery voltage for thyristor valve V1. After maintaining a certain reverse recovery time, thyristor V1 restores its blocking ability. The auxiliary branch is composed of the series connection of auxiliary fully-controlled valve V3 and high-voltage and small-current thyristor valve V4. During commutation, the auxiliary branch briefly bears the small current transferred from the main branch and delays to disconnect after thyristor valve V1 restores its reverse blocking ability, so as to ensure reliable commutation. When not conducting, the auxiliary branch needs to bear the voltage of the entire bridge arm.

[0034] The working principle of the controllable line-commutated converter is illustrated by the timing of its control signals, as Figure 4 shown. Sgv1 is the control signal of thyristor valve V1, Sgv2 is the control signal of main fully-controlled valve V2, Sgv3 is the control signal of auxiliary fully-controlled valve V3, and Sgv4 is the control signal of high-voltage and small-current thyristor valve V4. The main logic of the control signal timing is as follows: At time t 0 , thyristor valve V1, main fully-controlled valve V2, and auxiliary fully-controlled valve V3 are triggered and conducted, and the main branch of the commutation valve conducts normally; After the main branch has conducted for 120°, commutation of the bridge arm starts at time t 1 ; During the commutation of the bridge arm, when the main branch current drops to a certain level, at time t 2 , main fully-controlled valve V2 is turned off, high-voltage and small-current thyristor valve V4 is conducted, and the main branch current is transferred to the auxiliary branch; After a certain time delay , at time t 4 , auxiliary fully-controlled valve V3 is turned off, and the commutation of the bridge arm is completed.

[0035] The present invention adopts as shown in Figure 2, Figure 3The controllable line-commutated converter shown is formed by adding a controllable device IGBT to a semi-controlled thyristor, constituting a topology of parallel connection of IGBT and thyristor with main and auxiliary branches. By actively turning off the IGBT in the main branch, the current can be transferred from the main branch to the auxiliary branch, and the IGBT in the main branch provides a reverse recovery voltage for the thyristor, extending the reverse recovery time. Finally, the IGBT in the auxiliary branch is actively turned off to cut off the arm current and complete commutation. When a system fault occurs, forced commutation of the converter is achieved by forced turn-off of the controllable device IGBT, fundamentally solving the commutation failure problem caused by the AC fault at the receiving end of the current-source multi-terminal DC power transmission system.

[0036] In addition, based on the CIGRE (Conference International des Grands Reseaux Electriques, International Conference on Large High-Voltage Electric Systems) standard DC power transmission model, the current-source multi-terminal DC power transmission system shown in Fig. 1 is built. Secondly, in the current-source multi-terminal DC power transmission system, the original conventional LCC converter is replaced with the designed controllable line-commutated converter as Figure 5 shown.

[0037] The controllable line-commutated converter is formed by adding a power electronic device IGBT on the basis of the topology of the conventional line-commutated converter. In essence, it still belongs to a 6-pulse bridge phase-controlled current-source converter, so its control method can still adopt the same constant current, constant voltage or constant extinction angle control as the LCC.

[0038] In the current-source multi-terminal DC power transmission system of the present invention, the rectifier-side LCC converter station adopts constant current control and constant minimum trigger angle control. Among them, the constant current control is equipped with a low-voltage current limiting link, and the constant minimum trigger angle control is based on the constant current control. Due to the multi-terminal parallel situation, the two inverter-side CLCC converter stations adopt the master-slave control method. The master-slave control means that 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.

[0039] The converter station CLCC1 adopts constant voltage control to control the DC voltage of the whole system and, at the same time, acts as a power balance node to coordinate the power changes of the whole system. The converter station CLCC2 adopts constant current control to maintain the steady-state DC current of the DC transmission lines at each end of the inverter side, and is equipped with constant extinction angle control as a backup.

[0040] For the constant voltage control of the converter station CLCC1, during operation, the DC voltage U d1Subtract from the set rated DC voltage U at the inverter side and send it to the PI correction link to obtain the trigger angle command A of the converter station CLCC1 at the inverter side ref1 1 As the converter station CLCC1 serving as the power balance node, to realize the function of regulating the power of its regulation system, the rated capacity of the transformer in the converter station CLCC1 needs to be set to 5 to 10 times that of the other converter stations in the system. When a fault occurs in the system, an overcurrent situation will occur, resulting in a certain oscillation of the system power. At this time, the power exceeding the limit of the oscillation can be transmitted through this converter station with a rated capacity of 5 to 10 times to realize the regulation of the system power and avoid the instability of the system caused by the oscillation of the system power during a fault.

[0041] For constant current control, during operation, according to the measured DC voltage U of the DC line connected to the converter station CLCC2 at the inverter side d2 and the DC current I d2 , calculate the DC voltage U at the midpoint of this DC line d3 , and the formula is as follows: U d3 =U d2 + I d2 ·X; In the formula, X is the sum of the impedance value of the DC transmission line between the midpoint of this DC line and the measurement point of the DC voltage U d2 and the impedance values of components such as smoothing reactors connected to this DC line.

[0042] The DC voltage U d3 generates a current command through the low-voltage current limiting link and serves as the constant current control command for both the rectifier side and the inverter side at the same time. Among them, calculating the midpoint voltage of the DC line can prevent the voltage division of devices such as smoothing reactors at the end of the line.

[0043] For the fixed minimum trigger angle control, take the current command transmitted from the inverter side as the constant current control command, subtract the actually measured DC current I at the rectifier side d and then obtain the inverter angle β of the constant current control at the rectifier side through the PI correction link 1 , subtract the inverter angle β 1 from π, and then obtain the trigger angle command A of the fixed minimum trigger angle control 2 . The fixed minimum trigger angle control limits the minimum value of the output trigger angle command A 2 to 5° through the PI correction link, aiming to reduce the reactive power loss and leave room for regulating the DC power flow.

[0044] For the backup fixed extinction angle control, subtract the constant current control command from the actually measured DC current I at the inverter side d2 and send it to the PI correction link to generate the inverter angle β of the constant current control​2 ; meanwhile, by taking the minimum extinction angle β of the two inverter bridges within the past power frequency cycle obtained through measurement min as the actual extinction angle, subtracting it from the given preset extinction angle β ref , performing amplitude limiting, and then sending it into the PI correction link to obtain the commutation angle β for the fixed extinction angle control 3 . Finally, select the larger one from the commutation angle β 2 obtained from the constant current control and the commutation angle β 3 obtained from the fixed extinction angle control, subtract it from π to obtain the trigger angle command A 3 of the converter station CLCC2 on the inverter side. Among them, the preset extinction angle β of the given value ref is generally taken as 15° (0.2618).

[0045] The fixed extinction angle control part is also equipped with a current deviation control link. The input is the current set value I d0 on the rectifier side minus the deviation value d d of the actual DC current I 0 , and the output is the extinction angle increment for the fixed extinction angle control on the inverter side. Only when the actual current is less than the set value will the extinction angle increment be output , while when the actual current is greater than the set value, the output extinction angle increment is 0. Calculate the sum of the trigger angle command A 3 and the increased extinction angle increment , and use the sum as the new trigger angle command A 31 for the fixed extinction angle control. The purpose of this link is to quickly bring the current back to the given value. Among them, the specific value of the extinction angle increment is obtained based on the ramp transfer function calculation of the deviation value d 0 .

[0046] To ensure that the CLCC1 converter station is under constant voltage control in the normal state and avoid it being in the backup constant current control, it is necessary to reduce its current set value by a current margin. While the other stations need to be under constant current control during normal operation, so there is no need to subtract the current margin. According to the actual operation experience of the DC transmission system, the current margin is usually 10% of the rated current value. Therefore, to implement the master-slave control strategy of the multi-terminal DC transmission, in the current-source multi-terminal DC system, only CLCC1 needs to subtract the current margin, and the other converter stations on the inverter side do not need to subtract the current margin, which can ensure the stability of the DC voltage of the entire system and have a certain regulation margin through the constant voltage control of CLCC1. At the same time, CLCC1 will also act as the system power balance node to regulate the system power.

[0047] Experiment: For the current-source multi-terminal DC transmission system with the LCC converter adopted on the inverter side as shown in Figure 1 establishedFigure 5 Taking the improved current source type multi-terminal DC power transmission system with a CLCC converter on the inverter side as shown as an example for simulation verification, the operating characteristics of the system when a single-phase AC grounding fault occurs at the constant voltage station LCC1 and the constant voltage station CLCC1 on the inverter side of the system are simulated and compared under the conditions where the LCC and CLCC are respectively adopted 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 system when a single-phase AC grounding fault occurs at the constant active power station LCC2 and the constant active power station CLCC2 on the inverter side of the system, as shown in Figures 8(a) to 8(b) and Figures 9(a) to 9(b). The fault start time is 2.5 s for all cases, and the fault duration is 0.01 s for all cases.

[0048] After replacing the LCC converter with a CLCC converter on the inverter side of the current source type multi-terminal DC power transmission system, the commutation failure problem of the LCC converter caused by the AC fault of the system 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 power oscillation, short-term power loss, and power reverse transmission in severe cases are avoided, thus ensuring the safe and stable operation of the system and verifying the effectiveness of the technical solution provided by the present invention.

[0049] In summary, the technical solution provided by the present invention can fundamentally avoid the generation of the commutation failure problem, avoid the problems of power oscillation, loss, and even power reverse transmission caused by the fault commutation failure of the current source type multi-terminal DC system, and avoid the problems of low MMC voltage level, low power capacity, and high price.

[0050] Furthermore, in the constant current control, calculating the DC voltage at the midpoint of the DC line can prevent the voltage division of devices such as the smoothing reactor at the end of the line, and improve the control accuracy; the constant minimum trigger angle control helps to reduce the reactive power loss and reserves room for adjusting and controlling the DC power flow; the constant extinction angle control can further prevent commutation failure and improve the stability of the current source type multi-terminal DC power transmission system.

Claims

1. A control method for a current source multi-terminal direct current transmission system, characterized in that: In the current source type 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 comprises: 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.

2. A control method for 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, characterized in that: The constant voltage control specifically includes: Measure the DC voltage U on the inverter side d1 , and calculate the DC voltage U d1 With inverter side 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 command as a constant current control command.

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 command 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. A control method for a current source multi-terminal direct current transmission system as claimed in claim 5, characterized in that: Also includes: The trigger angle command A2 is PI corrected 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: The CLCC converter station adopting constant current control is also equipped with fixed arc extinction angle control, which includes: 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 With 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 of the fixed arc extinction angle control; The trigger angle command A3 of the 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. A control method for a current source multi-terminal direct current transmission system as claimed in 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, characterized in that: 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 the bridge arm VT1 is connected to the output end of the bridge arm VT4, the input end of the bridge arm VT2 is connected to the output end of the bridge arm VT5, and the input end of the bridge arm VT3 is connected to the output end of the 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. A control method for a current source multi-terminal direct current transmission system according to claim 9, characterized in that: 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; Main full control valve V2 includes: The anode of the diode D1 is connected to one end of the capacitor C1, and the cathode is connected to the emitter of the IGBT1; the other end of the capacitor C1 is connected to the collector of the IGBT1; the resistor R1 is connected in parallel with the diode D1; Auxiliary full control valve V3 includes: The anode of the diode D2 is connected to one end of the capacitor C2, and the cathode is connected to the emitter of the IGBT2; the other end of the capacitor C2 is connected to the collector of the IGBT2; the resistor R2 is connected in parallel with the diode D2; the variable resistor R3 is connected in parallel with the 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 and low-current thyristor valve V4, the anode of the thyristor Thy2 is connected to the emitter of IGBT2, and the cathode is connected to the emitter of IGBT1; Among them, 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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