Control method and system for active turn-off of direct-current power transmission system

Abstract

The invention discloses a control method and system for active turn-off of a direct-current power transmission system. During the recovery period of the thyristor of the converter valve, when the bridge arm voltage of the converter valve is positively biased and the full-control device of the converter valve completes the recovery process, the full-control device preferentially bears the positively biased voltage; or in the recovery period or the commutation process of the full-control device of the converter valve, when the voltage of the bridge arm of the converter valve is positively biased and the current does not exceed the maximum turn-off current of the full-control device, the corresponding bridge arm is turned off, and forced commutation is carried out; in any one of the conditions, independent valve control is configured in each converter valve, an active turn-off strategy of the converter valves is set in pole control or valve control, and thyristors and full-control devices in the converter valves are arranged according to a set proportion. According to the technical scheme, the thyristor can be in a zero-voltage recovery stage, and more recovery time is won for the thyristor, so that commutation failure is avoided, and various adverse effects caused by commutation failure are overcome.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission, and in particular to a control method and system for active shutdown of a direct current transmission system. Background Art

[0002] As more and more high voltage direct current (HVDC) projects are put into operation, my country's power grid has developed into a large-scale, large-capacity AC / DC hybrid power grid, and the electrical connection between AC and DC systems has become increasingly close. In this context, the importance of safe and stable operation of HVDC projects to ensure the stable operation of large power grids and the reliable supply of national electricity is self-evident.

[0003] At present, most of the converter valves in domestic UHV DC transmission projects use line commutated converter valves (LCC). LCC uses semi-controlled thyristors (which can only be controlled to open but not closed) as the main power devices. Commutation failure is prone to occur under AC grid faults, harmonics and other working conditions. Commutation failure is one of the most common faults in HVDC projects and is an issue that deserves great attention. Commutation failure refers to the failure of the converter valve to restore its blocking ability within a period of time under the action of reverse voltage after it exits operation, and reopens after the valve voltage changes to the forward direction. Once commutation failure occurs, it will lead to adverse consequences such as excessive DC current, reduced DC transmission power, malfunction of relay protection, DC bias of converter transformer, shortened converter valve life, and unstable voltage of weak AC system on the inverter side. If the control is improper or the fault is serious, it may lead to continuous commutation failure. Summary of the invention

[0004] In order to solve the above problems, the inventors have made the present invention, and provide a control method and system for active shutdown of a direct current transmission system through specific implementation methods.

[0005] In a first aspect, an embodiment of the present invention provides a control method for active shutdown of a direct current transmission system, comprising the following steps:

[0006] During the recovery period of the converter valve thyristor, when the converter valve bridge arm voltage is forward biased and the converter valve fully-controlled device has completed the recovery process, the fully-controlled device is preferentially subjected to the forward bias voltage;

[0007] Or, during the recovery period or commutation process of the fully-controlled device of the converter valve, when the voltage of the bridge arm of the converter valve is positively biased and the current does not exceed the maximum shutoff current of the fully-controlled device, the corresponding bridge arm is shut off to perform forced commutation;

[0008] In any of the above situations, independent valve control is configured in each converter valve, the active shutdown strategy of the converter valve is set in pole control or valve control, and the thyristor and fully controlled device in the converter valve are set according to a set ratio.

[0009] Optionally, the control method for active shutdown of a DC power transmission system further includes the following steps:

[0010] A closed-loop current controller and a voltage controller are respectively arranged on the rectifier side and the inverter side of the DC transmission system. The rectifier side controls the current and the inverter side controls the voltage. The current margin is subtracted from the current command on the inverter side. The current controller on the inverter side reduces the arc extinction angle until it reaches the angle value allowed by the voltage controller to maintain the voltage.

[0011] Optionally, the control method for active shutdown of a DC power transmission system further includes the following steps:

[0012] When the AC voltage on the inverter side decreases or the voltage controller on the inverter side can no longer play a control role, the inverter side will be operated in the fixed arc extinction angle control mode.

[0013] Optionally, the control method for active shutdown of a DC power transmission system further includes the following steps:

[0014] In the fixed arc extinction angle control mode, when the DC current is between the inverter side current constant and the rectifier side current constant, the trigger angle is reduced according to the DC current change value, so that the external characteristic of the inverter side with respect to voltage and current becomes a positive slope or zero slope.

[0015] Optionally, the control method for active shutdown of a DC power transmission system further includes the following steps:

[0016] When the AC voltage is abnormal, the rectifier side is controlled according to the minimum trigger angle, and the closed-loop current controller on the inverter side adjusts the current to compensate for the current command drop when the current control is transferred to the inverter side with the current margin.

[0017] Optionally, the control method for active shutdown of a DC power transmission system further includes the following steps:

[0018] A low-voltage current limiting control link is configured on the rectifier side and the inverter side of the DC transmission system respectively, so as to limit the DC current command when the DC voltage decreases.

[0019] In a second aspect, an embodiment of the present invention provides a control system for active shutdown of a DC power transmission system, including: a bipolar control system, a pole control, and a valve control;

[0020] The bipolar control system is configured in two monopolar layer control hosts respectively;

[0021] The control hosts of the bipolar control system, pole control and valve control adopt dual redundant configuration, and are respectively equipped with independent protection action outlet judgment devices;

[0022] Each converter is equipped with independent valve control, and the control hosts of the valve control of converters with the same pole communicate directly with each other to coordinate the control of the trigger angle and the tap change of the converter transformer.

[0023] Optionally, the control system for active shutdown of the DC power transmission system also includes an AC / DC system integrated system and a high-voltage DC system; the control system for active shutdown of the DC power transmission system is hierarchically controlled, and each control layer is, in sequence, the AC / DC system level, the regional level, the high-voltage DC bipole, the high-voltage DC monopole and the converter unit pole; the AC / DC system integrated system is located at the AC / DC system level, the high-voltage DC system is located at the regional level, the bipole control system is located at the high-voltage DC bipole, the pole control is located at the high-voltage DC monopole, and the valve control is located at the converter unit pole.

[0024] Optionally, the dual redundant configuration adopts a three-out-of-two redundancy scheme, and the protection action outlet judgment device adopts a three-out-of-two judgment scheme; the thyristors and fully-controlled devices in each converter are set according to a set ratio; the pole-controlled control host is used to realize the calculation and distribution of power / current instructions, coordination of current instructions between converter stations, converter station reactive power control, bipolar / pole zone sequence control, and commutation failure detection and evaluation; the valve-controlled control host is used to realize the control and valve triggering of the corresponding converter, including adjusting the DC current, DC voltage and arc extinction angle, unlocking and locking the converter, and generating the opening pulse and closing pulse of the fully-controlled device.

[0025] Optionally, the pole-controlled control host transmits the calculated current command to the pole-controlled control host. When all pole-controlled control hosts fail, the corresponding valve-controlled control host automatically executes the current command latching and performs closed-loop regulation.

[0026] The beneficial effects of the above technical solution provided by the embodiment of the present invention include at least:

[0027] A control method for active shutdown of a direct current transmission system, wherein during the recovery period of a converter valve thyristor, when the converter valve bridge arm voltage is positively biased and the converter valve fully-controlled device has completed the recovery process, the fully-controlled device preferentially bears the forward bias voltage, so that the thyristor is in a zero-voltage recovery stage, and more recovery time is gained for the thyristor, thereby avoiding commutation failure. During the recovery period of the converter valve fully-controlled device or the commutation process, when the converter valve bridge arm voltage is positively biased and the current does not exceed the maximum shutoff current of the fully-controlled device, the corresponding bridge arm is shut off, and forced commutation is performed, thereby overcoming various adverse effects caused by commutation failure.

[0028] A control system for active shutdown of a direct current transmission system can avoid disturbances to both poles caused by bipolar layer equipment failures as much as possible, ensure the independence of converter operation and maintenance, improve system operation stability, and reduce the impact of equipment failures on system operation.

[0029] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings.

[0030] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0032] Figure 1 This is a schematic diagram of a hybrid connection of IGCT and thyristor in a converter valve according to an embodiment of the present invention;

[0033] Figure 2 It is a schematic diagram of correction of the external characteristics of an embodiment of the present invention;

[0034] Figure 3 A schematic diagram of hierarchical configuration of a control system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0036] In order to solve the problems existing in the prior art, an embodiment of the present invention provides a control method and system for active shutdown of a direct current transmission system.

[0037] An embodiment of the present invention provides a control method for active shutdown of a direct current transmission system, comprising the following steps:

[0038] During the recovery period of the converter valve thyristor, when the converter valve bridge arm voltage is forward biased and the converter valve fully-controlled device has completed the recovery process, the fully-controlled device is preferentially subjected to the forward bias voltage;

[0039] Or, during the recovery period or commutation process of the fully-controlled device of the converter valve, when the voltage of the bridge arm of the converter valve is positively biased and the current does not exceed the maximum shutoff current of the fully-controlled device, the corresponding bridge arm is shut off to perform forced commutation;

[0040] Different from the traditional LCC DC transmission system, in any of the above cases, an independent valve control is configured in each converter valve, and the active shutdown strategy of the converter valve is set in the pole control (Pole Control and Protection, PCP for short, pole control protection system) or valve control (commutation unit control system), and the thyristors and fully controlled devices in the converter valve are set according to a set ratio. Fully controlled devices can control both opening and closing, including but not limited to IGBT, IGCT, etc. The traditional LCC DC transmission system uses a semi-controlled thyristor, which can only control opening but not closing. The method proposed in this embodiment is based on a DC transmission system that can be actively shut down. Since the fully controlled device can control the shutdown, a DC transmission system that can be actively shut down can be obtained by partially or completely replacing the thyristors in the LCC converter valve with fully controlled devices.

[0041] In a hybrid commutated converter (HCC) valve in which thyristors and fully controlled devices are connected in series, IGCT is used as the fully controlled device. Figure 1 As shown, Figure 1 In the process, the IGCT valve group and the thyristor valve group are connected in series, and a dynamic voltage-equalizing circuit, a static voltage-equalizing resistor, and a separate lightning arrester are connected in parallel at both ends of the IGCT device. The shutdown capability of a single IGCT is limited. If the proportion is too low, it may not effectively shut down the large current. Therefore, it can be replaced in a certain proportion according to the actual needs of the project, and the proportion of fully controlled devices is set in the range from 50% to 100%. Taking advantage of the IGCT's shorter recovery time and active current shut-off capability, it can be used for converter valve modification to achieve the purpose of resisting commutation failure.

[0042] In a DC transmission system that can be actively shut down, for an HCC converter valve based on a hybrid of IGCT and thyristor, if the bridge arm voltage is forward biased in advance during the thyristor recovery period, and the IGCT has completed the recovery process at this time, then the IGCT can bear this part of the positive voltage first, so that the thyristor can be in the zero-voltage recovery stage, buying more recovery time for it, thereby avoiding commutation failure.

[0043] In a DC transmission system that can be actively shut down, if the bridge arm voltage is positively biased during the IGCT recovery period or even during the commutation process, commutation failure is inevitable. In this case, the active shutdown capability of the IGCT can be used to shut down the bridge arm that should be shut down before the current exceeds the maximum shutoff current of the IGCT, so that the commutation bridge arm can complete forced commutation, thereby overcoming the commutation failure.

[0044] In some optional embodiments, the control method for active shutdown of the DC power transmission system further includes the following steps: respectively configuring a closed-loop current controller and a voltage controller on the rectifier side and the inverter side of the DC power transmission system, controlling the current on the rectifier side and controlling the voltage on the inverter side, subtracting the current margin from the inverter side current command, and the inverter side current controller reducing the arc extinction angle until it reaches the angle value allowed by the voltage controller to maintain the voltage. The arc extinction angle (or extinction angle), usually represented by γ, corresponds to the time required to establish the forward voltage. After the current is turned on, the converter valve also needs an extinction period to restore the ability to support the forward voltage. The extinction angle γ is related to the turn-off time of the thyristor.

[0045] Specifically, closed-loop current controllers and voltage controllers are provided on both the rectifier and inverter sides, but with different operating parameters. Through the coordination of the controllers on both sides, the rectifier side controls the current and the inverter side controls the voltage under normal operating conditions. This control method is achieved by subtracting a current margin from the current command on the inverter side. The current margin value is 10% of the rated current value, such as Figure 2 As shown, the effective command on the inverter side is lower than that on the rectifier side. Figure 2 In the figure, the horizontal axis represents the DC current and the vertical axis represents the DC voltage. Because the closed-loop current controller tries to establish the specified current, the current controller on the inverter side will try to reduce its arc extinction angle until it reaches the angle value allowed by the voltage controller to maintain the voltage. In short, the converter station with a high current command operates as a rectifier station, and the other station operates as an inverter station.

[0046] In some optional embodiments, the control method for active shutdown of a DC power transmission system further includes the following steps: when the AC voltage on the inverter side decreases or the voltage controller on the inverter side can no longer play a control role, the inverter side is operated in a fixed arc extinction angle control mode.

[0047] Specifically, in the fixed arc extinction angle control mode, the arc extinction angle of most DC projects is set within the range of 15° to 18°. The arc extinction angle can be directly measured, but cannot be directly controlled. It can only be adjusted indirectly by changing the trigger angle of the converter. The arc extinction angle is not only related to the trigger angle on the inverter side, but also depends on the commutation voltage and DC current. The trigger angle, also called the conduction angle, is usually expressed as α, which refers to the time delay from the valve voltage becoming positive to the thyristor or fully controlled device being turned on.

[0048] In the fixed arc extinction angle control mode, according to the characteristics of DC transmission, the following formula can be obtained:

[0049] U d =U di0 cosγ-(d x -d r )I d

[0050] Among them, U d Indicates the DC voltage on the inverter side, U di0 represents the ideal no-load DC voltage, γ represents the arc extinction angle, dx represents the equivalent commutation reactance, dr represents the equivalent commutation resistance, I d Indicates the DC current on the inverter side.

[0051] From the above formula, we can see that for a constant γ angle, when the DC current I d When it increases, the DC voltage U d Therefore, when the inverter side operates at a fixed arc extinction angle γ, it has a negative resistance characteristic at low frequency. The negative resistance characteristic of the inverter side is not conducive to the stability of the control system. The negative resistance characteristic refers to the characteristic that when the current of some circuits or electronic components increases, the voltage decreases instead. When the current of a general resistor increases, the voltage also increases. The negative resistance characteristic is opposite to the characteristic of the resistor.

[0052] Based on the analysis of the above formula, in order to improve the stability of the control system, in some optional embodiments, the control method for active shutdown of the DC transmission system also includes the following steps: in the fixed arc extinction angle control mode, when the DC current is between the inverter side current constant and the rectifier side current constant, the trigger angle is reduced according to the DC current change value, so that the external characteristics of the inverter side with respect to voltage and current become a positive slope or a zero slope.

[0053] Specifically, a modified constant γ angle control is adopted on the inverter side to modify the characteristics of the inverter end so that it has a positive slope or zero slope in transient conditions, that is, Figure 2 As shown between the AB segments, when the DC current is between the inverter side current constant and the rectifier side current constant, the trigger angle is reduced according to the current difference, so that the external characteristic of the inverter end becomes a positive slope, that is, when the inverter side operates at the fixed arc extinction angle γ, it no longer exhibits negative resistance characteristics at low frequencies. When the current increases, the voltage will also increase, thereby eliminating the negative resistance characteristics in the fixed arc extinction angle control mode and improving the stability of the control system.

[0054] In some optional embodiments, the control method for active shutdown of a DC power transmission system further includes the following steps: when the AC voltage is abnormal, Figure 2As shown, the rectifier side is controlled according to the minimum trigger angle, and the closed-loop current controller on the inverter side adjusts the current to compensate for the current command drop when the current control is transferred to the inverter side with the current margin. Figure 2 As shown in Figure 2, when the AC voltage is abnormal, the inverter side may obtain current control. At this time, the rectifier side operates at the minimum trigger angle α. min The closed-loop current regulator on the inverter side controls the current, and the current margin compensation can compensate for the current command drop equal to the current margin when the current control is transferred to the inverter side.

[0055] In some optional embodiments, the control method for active shutdown of the DC power transmission system further includes the following steps: configuring a low-voltage current limiting control link on the rectifier side and the inverter side of the DC power transmission system respectively, and limiting the DC current command when the DC voltage is reduced to help the DC system to recover quickly and controllably after an AC / DC fault.

[0056] In the above method of this embodiment, during the recovery period of the converter valve thyristor, when the converter valve bridge arm voltage is positive biased and the converter valve fully-controlled device has completed the recovery process, the fully-controlled device is preferentially subjected to the forward bias voltage, so that the thyristor can be in the zero-voltage recovery stage, and more recovery time can be gained for the thyristor, thereby avoiding commutation failure. During the recovery period of the converter valve fully-controlled device or the commutation process, when the converter valve bridge arm voltage is positive biased and the current does not exceed the maximum turn-off current of the fully-controlled device, the corresponding bridge arm is turned off, and forced commutation is performed to overcome various adverse effects caused by commutation failure.

[0057] Those skilled in the art can change the above sequence without departing from the protection scope of the present disclosure.

[0058] Another embodiment of the present invention provides a control system for active shutdown of a direct current transmission system, comprising:

[0059] Bipolar control system, pole control and valve control;

[0060] The bipolar control system is configured in two monopolar layer control hosts respectively;

[0061] The control hosts of the bipolar control system, pole control and valve control adopt dual redundant configuration, and are respectively equipped with independent protection action outlet judgment devices;

[0062] Each converter is equipped with independent valve control, and the control hosts of the valve control of converters with the same pole communicate directly with each other to coordinate the control of the trigger angle and the tap change of the converter transformer.

[0063] The bipolar in this embodiment refers to a monopole composed of two positive and negative voltages and a common grounding electrode. When the bipolar is in balanced operation, the current of the two poles forms a loop, and no current flows through the grounding electrode. The monopole is composed of a 12-pulse or 6-pulse converter on the rectifier side, a DC line, a 12-pulse or 6-pulse converter on the inverter side, a grounding electrode line and related equipment to complete all the equipment for DC transmission. The monopole has an earth return line and a metal return line operation mode.

[0064] In some optional embodiments, the control system for active shutdown of the DC power transmission system also includes an AC / DC system integrated system and a high-voltage DC system; the control system for active shutdown of the DC power transmission system is hierarchically controlled, and each control layer is, in sequence, the AC / DC system level, the regional level, the high-voltage DC bipole, the high-voltage DC monopole and the converter unit pole; the AC / DC system integrated system is located at the AC / DC system level, the high-voltage DC system is located at the regional level, the bipole control system is located at the high-voltage DC bipole, the pole control is located at the high-voltage DC monopole, and the valve control is located at the converter unit pole.

[0065] In some optional embodiments, the dual redundant configuration adopts a three-out-of-two redundant scheme, and the protection action exit judgment device adopts a three-out-of-two judgment scheme; the thyristors and fully controlled devices in each converter are set according to a set ratio; the pole-controlled control host is used to realize the calculation and distribution of power / current instructions, the coordination of current instructions between converter stations, the reactive power control of converter stations, the bipolar / polar region sequential control, and the phase change failure detection and evaluation; the valve-controlled control host is used to realize the control and valve triggering of the corresponding converter, including adjusting the DC current, DC voltage and arc extinction angle, unlocking and locking the converter, and generating the opening pulse and closing pulse of the fully controlled device. Among them, the three-out-of-two redundant scheme refers to configuring three groups of identical devices. When the outputs of two or more groups of devices are the same, the results are output to the next link, thereby improving the stability of the system. The three-out-of-two judgment scheme is similar to this. Only when the judgment results of at least two groups of devices in the three groups are the same, the judgment results are output, thereby reducing the situation of judgment errors.

[0066] In some optional embodiments, the pole-controlled control host transmits the calculated current command to the pole-valve-controlled control host. When all pole-controlled control hosts fail, the corresponding valve-controlled control host automatically executes the current command latch and performs closed-loop regulation.

[0067] Specifically, in a control system for active shutdown of an HCC commutation system, the components are as follows: Figure 3As shown in the figure, the control functions are divided into system level, regional level, DC bipolar (converter station) level, DC unipolar level, and converter unit (converter valve) level. The DC bipolar (converter station) level and DC unipolar level are collectively called the pole layer. The DC bipolar (converter station) level is also called the bipolar layer. The bipolar layer functions are respectively configured in the pole layer hosts of the two poles (referring to the two unipolar poles that make up the bipolar) to achieve, which can avoid the disturbance of the bipolar layer equipment failure to the two poles at the same time as much as possible; as shown in the figure, the control functions are divided into system level, regional level, DC bipolar (converter station) level, DC unipolar level, and converter unit (converter valve) level. The DC bipolar (converter station) level and the DC unipolar level are collectively called the pole layer. The DC bipolar (converter station) level is also called the bipolar layer. The bipolar layer functions are respectively configured in the pole layer hosts of the two poles (referring to the two unipolar poles that make up the bipolar) to achieve, which can avoid the disturbance of the two poles at the same time due to the bipolar layer equipment failure as much as possible; as shown in the figure, the control functions are divided into system level, regional level, regional level, and DC bipolar (converter station) level, regional ... Figure 3 As shown, each valve-controlled control host independently controls a converter, and each converter is equipped with an independent control and protection host to ensure the independence of converter operation and maintenance.

[0068] Specifically, in a control system for active shutdown of HCC commutation system, the control host adopts dual redundant configuration; the protection host adopts a three-out-of-two redundant solution, and the pole layer and the converter layer are respectively configured with independent three-out-of-two devices to judge the protection action outlet. The layered design requires the control and protection hosts within and between layers to coordinate to realize the overall function. The hosts communicate through high-speed optical fiber Ethernet, divided by pole layer and converter layer, and dual redundant design.

[0069] Specifically, in a control system for active shutdown of an HCC commutation system, optical fiber Ethernet direct communication is used between control hosts of two converters of the same pole, and coordinated control of trigger angles and converter transformer taps is performed between the converters.

[0070] Specifically, in a control system for active shutdown of HCC commutation system, the control function configuration includes: the pole control host realizes the calculation and distribution of power / current instructions, the coordination of current instructions between stations, the reactive power control of the converter station, the bipolar / polar area sequence control, the commutation failure detection and evaluation and other functions; the converter control LAN host realizes the control and valve triggering functions of the converter, including the adjustment of DC current, DC voltage, arc extinction angle, the unlocking and locking of the converter, the generation of the opening pulse and the closing pulse of the IGCT, etc.; wherein, LAN refers to the local area network formed by the communication connection between the converter control hosts. The pole control host refers to the control host of the pole control.

[0071] The pole control host transmits the calculated current command to the inverter control host of the pole. When the redundant pole control hosts all fail, the inverter control host automatically executes the current command latch to maintain the minimum closed-loop regulation function, thereby avoiding forced shutdown of the pole. The inverter control host refers to the valve-controlled control host.

[0072] In this embodiment, it is possible to avoid disturbances to both poles caused by bipolar layer equipment failures as much as possible, ensure the independence of converter operation and maintenance, improve system operation stability, and reduce the impact of equipment failures on system operation.

[0073] Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention shall still fall within the patent scope of the present invention.

Claims

1. A control method for active shutdown of a DC power transmission system, characterized in that: The following steps are involved: During the recovery period of the converter valve thyristor, when the converter valve bridge arm voltage is forward biased and the converter valve fully-controlled device has completed the recovery process, the fully-controlled device is preferentially subjected to the forward bias voltage; Or, during the recovery period or commutation process of the fully-controlled device of the converter valve, when the voltage of the bridge arm of the converter valve is positively biased and the current does not exceed the maximum shutoff current of the fully-controlled device, the corresponding bridge arm is shut off to perform forced commutation; In any of the above situations, independent valve control is configured in each converter valve, the active shutdown strategy of the converter valve is set in pole control or valve control, and the thyristor and fully controlled device in the converter valve are set according to a set ratio.

2. The method according to claim 1, characterized in that The control method for active shutdown of a DC power transmission system further comprises the following steps: A closed-loop current controller and a voltage controller are respectively arranged on the rectifier side and the inverter side of the DC transmission system. The rectifier side controls the current and the inverter side controls the voltage. The current margin is subtracted from the current command on the inverter side. The current controller on the inverter side reduces the arc extinction angle until it reaches the angle value allowed by the voltage controller to maintain the voltage.

3. The method according to claim 2, characterized in that The control method for active shutdown of a DC power transmission system further comprises the following steps: When the AC voltage on the inverter side decreases or the voltage controller on the inverter side can no longer play a control role, the inverter side will be operated in the fixed arc extinction angle control mode.

4. The method according to claim 3, characterized in that The control method for active shutdown of a DC power transmission system further comprises the following steps: In the fixed arc extinction angle control mode, when the DC current is between the inverter side current constant and the rectifier side current constant, the trigger angle is reduced according to the DC current change value, so that the external characteristic of the inverter side with respect to voltage and current becomes a positive slope or zero slope.

5. The method according to claim 2, characterized in that The control method for active shutdown of a DC power transmission system further comprises the following steps: When the AC voltage is abnormal, the rectifier side is controlled according to the minimum trigger angle, and the closed-loop current controller on the inverter side adjusts the current to compensate for the current command drop when the current control is transferred to the inverter side with the current margin.

6. The method according to claim 2, characterized in that The control method for active shutdown of a DC power transmission system further comprises the following steps: A low-voltage current limiting control link is configured on the rectifier side and the inverter side of the DC transmission system respectively, so as to limit the DC current command when the DC voltage decreases.

7. A control system for active shutdown of a DC power transmission system, characterized in that: include: Bipolar control systems, pole control and valve control; The bipolar control system is configured in two monopolar layer control hosts respectively; The control hosts of the bipolar control system, pole control and valve control adopt dual redundant configuration, and are respectively equipped with independent protection action outlet judgment devices; Each converter is equipped with independent valve control, and the control hosts of the valve control of converters with the same pole communicate directly with each other to coordinate the control of the trigger angle and the tap change of the converter transformer.

8. The system according to claim 7, characterized in that The control system for active shutdown of a DC power transmission system also includes an AC / DC system integrated system and a high voltage DC system; The control system for active shutdown of the DC power transmission system is hierarchically controlled, and each control layer is, in sequence, an AC / DC system level, a regional level, a high-voltage DC bipolar level, a high-voltage DC monopolar level, and a converter unit level; The AC / DC system integrated system is located at the AC / DC system level, the HVDC system is located at the regional level, the bipolar control system is located at the HVDC bipolar, the pole control is located at the HVDC monopole, and the valve control is located at the converter unit pole.

9. The system according to claim 7, characterized in that The dual redundant configuration adopts a three-out-of-two redundancy scheme, and the protection action exit judgment device adopts a three-out-of-two judgment scheme; The thyristors and fully controlled devices in each converter are set according to the set ratio; The control host of the pole control is used to realize the calculation and distribution of power / current instructions, the coordination of current instructions between converter stations, the reactive power control of converter stations, the bipolar / polar area sequence control, and the detection and evaluation of commutation failure; The valve-controlled control host is used to realize the control and valve triggering of the corresponding converter, including adjusting the DC current, DC voltage and arc extinction angle, unlocking and locking the converter, and generating the opening pulse and closing pulse of the fully controlled device.

10. The system according to claim 9, characterized in that The pole-controlled control host transmits the calculated current command to the pole-controlled control host. When the pole-controlled control hosts all fail, the corresponding valve-controlled control hosts automatically execute the current command latch and perform closed-loop regulation.

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