Control system and method for zero direct current voltage operation

Abstract

The invention relates to the technical field of electric power systems, and discloses a control system and method for zero direct current voltage operation, and the system is directly connected with a switching module through a direct current fault detection module, a steady state operation module and a direct current fault ride-through module, so that when the direct current fault detection module detects that a flexible direct current system has a fault, the direct current fault ride-through module is switched on; the second modulation signal output by the direct-current fault ride-through module can be quickly switched and received, it is ensured that the flexible direct-current system can safely ride through in the fault period, and equipment damage and system collapse are avoided. Meanwhile, after the flexible direct current system recovers normal operation, the first modulation signal output by the steady-state operation module can be quickly switched back, and stable operation of the system is maintained. Meanwhile, the modularized multi-level converter valve and the thyristor valve in the controllable assembly are connected in parallel to share the total direct current of the system, so that the transient through-current capability of the controllable assembly is matched with that of a diode valve, and the short-time overload capability of the converter valve is improved.

Description

Technical Field

[0001] This invention relates to the field of power system technology, and in particular to a control system and method for zero DC voltage operation. Background Technology

[0002] In recent years, onshore wind resources have become increasingly limited and development is nearing saturation, while offshore wind power resources are abundant, especially in the deep sea, which holds vast untapped potential. Developing deep-sea wind power requires addressing the challenge of long-distance transmission. Currently, flexible DC transmission technology is commonly used in engineering projects. The wind farm's output is connected to an offshore converter station via a step-up transformer, rectified, and then transmitted via submarine DC cables to an onshore converter station before being connected to the onshore AC grid. Offshore converter stations are typically modular multilevel converters (MMCs) based on half-bridge or full-bridge submodules, enabling bidirectional power flow and supporting black start of wind farms and DC transmission of wind energy.

[0003] Using flexible DC transmission to transmit offshore wind power meets the needs of long-distance wind energy transmission and black start of wind farms, and is one of the more mature solutions in current engineering applications. However, in the control method of the offshore MMC in this wind power transmission system, a constant DC voltage control is used for the MMC valve in the hybrid topology. The DC voltage of the MMC valve is superimposed with the DC voltage of the diode valve in series to form the total DC side voltage of the offshore converter station. Since the MMC and the diode valve are connected in series, the same DC current flows through the MMC and the diode valve. However, the short-time current carrying capacity of the diode is significantly higher than that of the MMC, which will seriously affect the overall overcurrent capacity of the offshore converter valve. This results in the offshore converter valve's tolerance to sudden current surges being limited, increasing the risk of equipment damage, and thus affecting the stability and reliability of the entire wind power transmission system. Summary of the Invention

[0004] In view of this, in order to solve the above-mentioned technical problems, the present invention provides a control system and method for zero DC voltage operation.

[0005] The first aspect of the present invention provides a control system for zero DC voltage operation, comprising: a DC fault detection module, a steady-state operation module, a DC fault ride-through module, a switching module, and a converter valve;

[0006] The DC fault detection module, the steady-state operation module, and the DC fault ride-through module are all electrically connected to the input terminal of the switching module.

[0007] The DC fault detection module is used to detect faults in the flexible DC system and send the fault detection results to the switching module.

[0008] The steady-state operation module is used to generate a first modulation signal to maintain the stable operation state of the flexible DC system during normal operation of the flexible DC system, and send the first modulation signal to the switching module;

[0009] The DC fault ride-through module is used to generate a second modulation signal to guide the flexible DC system through the fault when a fault occurs in the flexible DC system, and to send the second modulation signal to the switching module.

[0010] The switching module is used to switch between receiving the first modulation signal or the second modulation signal according to the received fault detection result, and send the received modulation signal to the converter valve.

[0011] The converter valve includes a first diode valve assembly, a controllable assembly, and a second diode valve assembly connected in series. The controllable assembly is electrically connected to the output terminal of the switching module. The controllable assembly includes a modular multilevel converter valve based on a full-bridge submodule and a thyristor valve, with the thyristor valve connected in parallel with the modular multilevel converter valve.

[0012] Preferably, the steady-state operation module includes a DC current distribution circuit, a first reactive power synchronization control circuit, a first active power control circuit, a DC capacitor voltage control circuit, a first q-axis voltage control circuit, a first dq-axis current control module, and a coordinate inverse transformation module;

[0013] The DC current distribution circuit is used to subtract the DC current setpoint of the steady-state operation module from the DC current feedback value of the converter valve to obtain a first difference value, and to calculate the first DC component of the modulation signal of the converter valve by processing the first difference value through a PI regulator.

[0014] The first reactive power synchronization control loop is used to subtract the reactive power setpoint of the steady-state operation module from the AC reactive power feedback value of the converter valve to obtain a second difference value. The second difference value is then passed through a low-pass filter to obtain the first wind farm AC voltage angular frequency adjustment amount. The first wind farm AC voltage angular frequency adjustment amount is then added to the AC voltage rated angular frequency to obtain the first synchronization angular frequency of the converter valve. The first synchronization angular frequency is then passed through an integration circuit to obtain the first wind farm AC voltage synchronization phase.

[0015] The first active power control loop is used to calculate the difference between the active power setpoint of the steady-state operation module and the active power feedback value of the converter valve to obtain a third difference value, and output the third difference value as a first d-axis voltage setpoint value through a P regulator. The first d-axis voltage setpoint value is then calculated as a difference between the first d-axis voltage setpoint value and the d-axis voltage feedback value of the converter valve to obtain a fourth difference value, and output the fourth difference value as a first d-axis current setpoint value through a PI regulator.

[0016] The DC capacitor voltage control circuit is used to calculate the difference between the given value of the average capacitor voltage of the steady-state operation module and the feedback value of the average capacitor voltage of the converter valve to obtain a fifth difference value. The fifth difference value is then passed through a PI regulator to obtain a second d-axis current given value. The first d-axis current given value and the second d-axis current given value are then superimposed to obtain a third d-axis current given value.

[0017] The first q-axis voltage control loop is used to calculate the difference between the q-axis voltage setpoint of the steady-state operation module and the q-axis voltage feedback value of the converter valve to obtain a sixth difference value, and output the sixth difference value as the first q-axis current setpoint value through the PI regulator;

[0018] The first dq-axis current control module is used to generate a first dq-axis current modulation signal based on the third d-axis current setpoint and the first q-axis current setpoint, and to transmit the first dq-axis current modulation signal to the coordinate inverse transformation module;

[0019] The coordinate inverse transformation module is used to perform coordinate inverse transformation processing based on the received first dq axis current modulation signal and the first wind farm AC voltage synchronization phase to generate the first AC component of the converter valve modulation signal, and to superimpose the first AC component of the converter valve modulation signal and the first DC component of the converter valve modulation signal to obtain the first modulation signal.

[0020] Preferably, the thyristor valve has a multi-stage thyristor series structure.

[0021] Preferably, the DC current setpoint of the steady-state operation module is determined according to the desired current ratio between the modular multilevel converter valve and the thyristor valve.

[0022] Preferably, the DC fault ride-through module includes a DC current control loop, a DC voltage control loop, a second reactive power synchronization control loop, a second active power control loop, a second q-axis voltage control loop, a dq-axis current inner loop module, and a second dq-axis current control module.

[0023] The DC current control loop is used to calculate the difference between the DC current setpoint of the DC fault ride-through module and the DC current feedback value of the converter valve to obtain a seventh difference value. The seventh difference value is calculated by a PI regulator to output a DC current component. The DC current component is multiplied by the DC current enable loop to obtain a DC current enable component. The DC current enable component is then limited and output as the second DC component of the modulation signal of the converter valve.

[0024] The DC voltage control loop is used to calculate the difference between the DC voltage setpoint of the DC fault ride-through module and the DC voltage feedback value of the converter valve to obtain an eighth difference value. The eighth difference value is processed by a PI regulator to output a DC voltage component. The DC voltage component is multiplied by the DC voltage enable loop to obtain a DC voltage enable component. The DC voltage enable component is then limited and output as the third DC component of the modulation signal of the converter valve.

[0025] The second reactive power synchronization control loop is used to calculate the difference between the reactive power setpoint of the DC fault ride-through module and the AC reactive power feedback value of the converter valve to obtain the ninth difference value. The ninth difference value is then passed through a low-pass filter to obtain the second wind farm AC voltage angular frequency adjustment amount. The second wind farm AC voltage angular frequency adjustment amount is then added to the AC voltage rated angular frequency to obtain the second synchronization angular frequency of the converter valve. The second synchronization angular frequency is then passed through an integration circuit to obtain the second wind farm AC voltage synchronization phase.

[0026] The second active control loop is used to calculate the difference between the active power setpoint of the DC fault ride-through module and the active power feedback value of the converter valve to obtain the tenth difference value, and output the tenth difference value as the second d-axis voltage setpoint value through the P regulator. The second d-axis voltage setpoint value is then calculated as the difference between the second d-axis voltage setpoint value and the d-axis voltage feedback value of the converter valve to obtain the eleventh difference value, and output the eleventh difference value as the fourth d-axis current setpoint value through the PI regulator.

[0027] The second q-axis voltage control loop is used to calculate the difference between the q-axis voltage setpoint of the DC fault ride-through module and the q-axis voltage feedback value of the converter valve to obtain the twelfth difference value, and output the twelfth difference value as the second q-axis current setpoint value through the PI regulator;

[0028] The dq-axis current inner loop module is used to generate a second dq-axis current modulation signal based on the fourth d-axis current setpoint and the second q-axis current setpoint, and to transmit the second dq-axis current modulation signal to the second dq-axis current control module.

[0029] The second dq-axis current control module is used to perform coordinate inverse transformation processing based on the received second dq-axis current modulation signal and the synchronous phase of the second wind farm AC voltage to generate a second AC component of the modulation signal of the converter valve, and to superimpose the second AC component of the modulation signal of the converter valve and the fourth DC component of the modulation signal of the converter valve to obtain the second modulation signal; wherein, the fourth DC component is obtained by superimposing the second DC component and the third DC component.

[0030] Preferably, the system further includes a first isolating switch, a second isolating switch, a first transformer, a second transformer, a third transformer, a first AC circuit breaker, a second AC circuit breaker, and a third AC circuit breaker;

[0031] The first diode valve assembly is connected in parallel with the first isolation blade;

[0032] The second diode valve assembly is connected in parallel with the second isolation blade;

[0033] The first diode valve assembly is connected in parallel to the AC busbar of the wind farm via the first transformer and the first AC circuit breaker in sequence.

[0034] The second diode valve assembly is connected in parallel with the AC busbar of the wind farm via the second transformer and the second AC circuit breaker in sequence;

[0035] The controllable component is connected in parallel to the AC busbar of the wind farm via the third transformer and the third AC circuit breaker.

[0036] In a second aspect, the present invention also provides a control method for a control system operating with zero DC voltage as described in the first aspect, comprising:

[0037] Detect faults in flexible DC systems and obtain fault detection results;

[0038] Based on the fault detection results, a modulation signal is determined by combining the first modulation signal and the second modulation signal, and the modulation signal is sent to the converter valve, so that the converter valve performs the operation of the flexible DC system under normal operation or fault stable operation and fault crossing according to the modulation signal; wherein, the first modulation signal is generated to maintain the stable operation state of the flexible DC system when the flexible DC system is operating normally; the second modulation signal is generated to guide the flexible DC system to cross the fault when a fault occurs in the flexible DC system.

[0039] Preferably, the generation process of the first modulated signal includes:

[0040] The first difference is obtained by subtracting the DC current setpoint of the steady-state operation module from the DC current feedback value of the converter valve. The first difference is then processed by a PI regulator to obtain the first DC component of the modulation signal of the converter valve.

[0041] The reactive power setpoint of the steady-state operation module is subtracted from the AC reactive power feedback value of the converter valve to obtain a second difference value. The second difference value is then passed through a low-pass filter to obtain the first wind farm AC voltage angular frequency adjustment amount. The first wind farm AC voltage angular frequency adjustment amount is then added to the AC voltage rated angular frequency to obtain the first synchronization angular frequency of the converter valve. The first synchronization angular frequency is then passed through an integral element to obtain the first wind farm AC voltage synchronization phase.

[0042] The active power setpoint of the steady-state operation module is subtracted from the active power feedback value of the converter valve to obtain a third difference value. The third difference value is output as the first d-axis voltage setpoint value through the P regulator. The first d-axis voltage setpoint value is subtracted from the d-axis voltage feedback value of the converter valve to obtain a fourth difference value. The fourth difference value is output as the first d-axis current setpoint value through the PI regulator.

[0043] The fifth difference is obtained by subtracting the given value of the average capacitor voltage of the steady-state operation module from the feedback value of the average capacitor voltage of the converter valve. The fifth difference is then passed through a PI regulator to obtain the second d-axis current given value. The first d-axis current given value and the second d-axis current given value are then superimposed to obtain the third d-axis current given value.

[0044] The difference between the q-axis voltage setpoint of the steady-state operation module and the q-axis voltage feedback value of the converter valve is used to obtain the sixth difference value. The sixth difference value is then output as the first q-axis current setpoint value through the PI regulator.

[0045] Based on the third d-axis current setpoint and the first q-axis current setpoint, a first dq-axis current modulation signal is generated;

[0046] The coordinate inverse transformation is performed based on the first dq axis current modulation signal and the first wind farm AC voltage synchronous phase to generate the first AC component of the converter valve modulation signal. The first AC component of the converter valve modulation signal and the first DC component of the converter valve modulation signal are then superimposed to obtain the first modulation signal.

[0047] Preferably, the DC current setpoint of the steady-state operation module is determined according to the desired current ratio between the modular multilevel converter valve and the thyristor valve.

[0048] Preferably, the generation process of the second modulated signal includes:

[0049] The difference between the DC current setpoint of the DC fault ride-through module and the DC current feedback value of the converter valve is obtained as the seventh difference value. The seventh difference value is calculated by the PI regulator to output the DC current component. The DC current component is multiplied by the DC current enable circuit to obtain the DC current enable component. The DC current enable component is then limited and output as the second DC component of the modulation signal of the converter valve.

[0050] The difference between the DC voltage setpoint of the DC fault ride-through module and the DC voltage feedback value of the converter valve is obtained to get the eighth difference value. The eighth difference value is calculated by the PI regulator to output the DC voltage component. The DC voltage component is multiplied by the DC voltage enable circuit to obtain the DC voltage enable component. After the DC voltage enable component is limited, it is output as the third DC component of the modulation signal of the converter valve.

[0051] The reactive power setpoint of the DC fault ride-through module is subtracted from the AC reactive power feedback value of the converter valve to obtain the ninth difference value. The ninth difference value is then passed through a low-pass filter to obtain the second wind farm AC voltage angular frequency adjustment amount. The second wind farm AC voltage angular frequency adjustment amount is then added to the AC voltage rated angular frequency to obtain the second synchronization angular frequency of the converter valve. The second synchronization angular frequency is then passed through an integral element to obtain the second wind farm AC voltage synchronization phase.

[0052] The difference between the active power setpoint of the DC fault ride-through module and the active power feedback value of the converter valve is used to obtain the tenth difference value. The tenth difference value is output as the second d-axis voltage setpoint value through the P regulator. The difference between the second d-axis voltage setpoint value and the d-axis voltage feedback value of the converter valve is used to obtain the eleventh difference value. The eleventh difference value is output as the fourth d-axis current setpoint value through the PI regulator.

[0053] The difference between the q-axis voltage setpoint of the DC fault ride-through module and the q-axis voltage feedback value of the converter valve is used to obtain the twelfth difference value. The twelfth difference value is then output as the second q-axis current setpoint value via a PI regulator.

[0054] Based on the fourth d-axis current setpoint and the second q-axis current setpoint, a second dq-axis current modulation signal is generated;

[0055] The coordinate inverse transformation is performed based on the second dq axis current modulation signal and the second wind farm AC voltage synchronization phase to generate the second AC component of the converter valve modulation signal. The second AC component of the converter valve modulation signal and the fourth DC component of the converter valve modulation signal are then superimposed to obtain the second modulation signal. The fourth DC component is obtained by superimposing the second DC component and the third DC component.

[0056] As can be seen from the above technical solution, this invention directly connects the DC fault detection module, steady-state operation module, and DC fault ride-through module to the switching module. This allows for rapid switching to receive the second modulation signal output by the DC fault ride-through module when the DC fault detection module detects a fault in the flexible DC system, ensuring the flexible DC system can safely ride through the fault and preventing equipment damage and system collapse. Simultaneously, after the flexible DC system resumes normal operation, it can quickly switch back to the first modulation signal output by the steady-state operation module to maintain stable system operation. This design significantly improves the reliability of the flexible DC system.

[0057] Meanwhile, by using a modular multilevel converter valve and a thyristor valve in parallel within the controllable component to share the total DC current of the system, the transient current-carrying capacity of the controllable component is matched with that of the diode valve, improving the short-term overload capacity of the converter valve. Furthermore, by employing a parallel connection of the modular multilevel converter valve and the thyristor valve, in the event of a DC bipolar short-circuit fault, a positive voltage pulse is output from the modular multilevel converter valve to achieve rapid turn-off of the thyristor valve. After the thyristor turns off, the modular multilevel converter valve can block the DC fault current. Therefore, the proposed controllable component can balance transient current-carrying capacity and rapid fault current blocking capability, improving the stability and reliability of the entire wind power transmission system. Attached Figure Description

[0058] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0059] Figure 1 A schematic diagram of a control system operating at zero DC voltage, provided as an embodiment of the present invention;

[0060] Figure 2 This is a schematic diagram of the main circuit structure of a flexible DC system provided in an embodiment of the present invention;

[0061] Figure 3 This is a control structure diagram of the steady-state operation module provided in an embodiment of the present invention;

[0062] Figure 4 This is a control structure diagram of the DC fault ride-through module provided in an embodiment of the present invention;

[0063] Figure 5 A flowchart of a control method for zero DC voltage operation provided in an embodiment of the present invention. Detailed Implementation

[0064] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0065] like Figure 1 and Figure 2 As shown, this application provides a control system for zero DC voltage operation, including: a DC fault detection module 10, a steady-state operation module 20, a DC fault ride-through module 30, a switching module 40, and a converter valve 50;

[0066] The DC fault detection module 10, the steady-state operation module 20, and the DC fault ride-through module 30 are all electrically connected to the input terminal of the switching module 40.

[0067] The DC fault detection module 10 is used to detect faults in the flexible DC system and send the fault detection results to the switching module 40.

[0068] The DC fault detection module 10 uses a well-known industry-standard low-voltage overcurrent criterion for detecting faults in the flexible DC system. Specifically, when the DC-side voltage is lower than a preset low-voltage threshold and the current exceeds a preset overcurrent threshold, a fault is determined to have occurred in the flexible DC system. This criterion offers advantages such as accurate detection and rapid response, enabling timely detection of faults in the system. In practical applications, the low-voltage and overcurrent thresholds can be reasonably set according to the specific parameters and operating requirements of the flexible DC system to ensure the reliability and effectiveness of the detection.

[0069] The steady-state operation module 20 is used to generate a first modulation signal to maintain the stable operation of the flexible DC system during normal operation, and send the first modulation signal to the switching module 40.

[0070] The DC fault ride-through module 30 is used to generate a second modulation signal to guide the flexible DC system through the fault when a fault occurs in the flexible DC system, and to send the second modulation signal to the switching module 40.

[0071] The switching module 40 is used to switch between receiving the first modulation signal or the second modulation signal according to the received fault detection result, and send the received modulation signal to the converter valve 50.

[0072] Specifically, when the received fault detection result indicates normal operation, the first modulation signal is received and the second modulation signal is received but not received, and the first modulation signal is sent to the converter valve 50. Similarly, when the received fault detection result indicates a fault, the second modulation signal is received and the first modulation signal is received but not received, and the second modulation signal is sent to the converter valve 50.

[0073] The converter valve 50 includes a first diode valve assembly, a controllable assembly, and a second diode valve assembly connected in series. The controllable assembly is electrically connected to the output terminal of the switching module 40. The controllable assembly includes a modular multilevel converter valve based on a full-bridge submodule and a thyristor valve, with the thyristor valve connected in parallel with the modular multilevel converter valve.

[0074] Among them, the thyristor valve is a multi-stage thyristor series structure.

[0075] Among them, such as Figure 2 As shown, the converter valve 50 includes a first diode valve assembly, a controllable assembly, and a second diode valve assembly connected in series. The control system for zero DC voltage operation also includes a first isolation switch, a second isolation switch, a first transformer, a second transformer, a third transformer, a first AC circuit breaker, a second AC circuit breaker, and a third AC circuit breaker.

[0076] The first diode valve assembly is connected in parallel with the first isolation blade K1;

[0077] The second diode valve assembly is connected in parallel with the second isolation blade K2;

[0078] The first diode valve assembly is connected in parallel to the AC bus of the wind farm via the first transformer T1 and the first AC circuit breaker CB1 in sequence.

[0079] The second diode valve assembly is connected in parallel to the AC bus of the wind farm via the second transformer T2 and the second AC circuit breaker CB2.

[0080] The controllable components are connected in parallel to the AC busbar of the wind farm via the third transformer T3 and the third AC circuit breaker CB3.

[0081] Among them, such as Figure 2 As shown, the first diode valve assembly includes series-connected measurement and control units DRU1 and DRU2, and the second diode valve assembly includes series-connected measurement and control units DRU3 and DRU4. The transformer ratios T1 and T2 can be 66kV / 166kV, and the transformer ratio of T3 can be 66kV / 35kV. In steady state, the typical DC voltage of the offshore converter station can be selected to be stabilized at approximately ±400kV. During steady-state operation, the switching module 40 uses the output of the steady-state operation module 20 as the modulation signal for the controllable components. During DC fault ride-through, the switching module 40 uses the output of the DC fault ride-through module 30 as the modulation signal for the controllable components.

[0082] It should be noted that in this embodiment, the DC fault detection module, steady-state operation module, and DC fault ride-through module are all directly connected to the switching module. This allows the DC fault detection module to quickly switch to receiving the second modulation signal output by the DC fault ride-through module when it detects a fault in the flexible DC system, ensuring the flexible DC system can safely ride through the fault and preventing equipment damage and system collapse. Simultaneously, after the flexible DC system resumes normal operation, it can quickly switch back to the first modulation signal output by the steady-state operation module to maintain stable system operation. This design significantly improves the reliability of the flexible DC system.

[0083] Meanwhile, by using a modular multilevel converter valve and a thyristor valve in parallel within the controllable component to share the total DC current of the system, the transient current-carrying capacity of the controllable component is matched with that of the diode valve, improving the short-term overload capacity of the converter valve. Furthermore, by employing a parallel connection of the modular multilevel converter valve and the thyristor valve, in the event of a DC bipolar short-circuit fault, a positive voltage pulse is output from the modular multilevel converter valve to achieve rapid turn-off of the thyristor valve. After the thyristor turns off, the modular multilevel converter valve can block the DC fault current. Therefore, the proposed controllable component can balance transient current-carrying capacity and rapid fault current blocking capability, improving the stability and reliability of the entire wind power transmission system.

[0084] In some embodiments, the steady-state operation module includes a DC current distribution circuit, a first reactive power synchronization control circuit, a first active power control circuit, a DC capacitor voltage control circuit, a first q-axis voltage control circuit, a first dq-axis current control module, and a coordinate inverse transformation module;

[0085] The DC current distribution circuit is used to calculate the difference between the DC current setpoint of the steady-state operation module and the DC current feedback value of the converter valve to obtain the first difference value. The first difference value is then processed by the PI regulator to obtain the first DC component of the modulation signal of the converter valve.

[0086] The first reactive power synchronization control loop is used to calculate the difference between the reactive power setpoint of the steady-state operation module and the AC reactive power feedback value of the converter valve to obtain the second difference value. The second difference value is then passed through a low-pass filter to obtain the AC voltage angular frequency adjustment of the first wind farm. The AC voltage angular frequency adjustment of the first wind farm is then added to the AC voltage rated angular frequency to obtain the first synchronization angular frequency of the converter valve. The first synchronization angular frequency is then passed through an integral element to obtain the AC voltage synchronization phase of the first wind farm.

[0087] The first active power control loop is used to calculate the difference between the active power setpoint of the steady-state operation module and the active power feedback value of the converter valve to obtain the third difference value. The third difference value is output as the first d-axis voltage setpoint value through the P regulator. The first d-axis voltage setpoint value is calculated as the difference between the first d-axis voltage setpoint value and the d-axis voltage feedback value of the converter valve to obtain the fourth difference value. The fourth difference value is output as the first d-axis current setpoint value through the PI regulator.

[0088] The DC capacitor voltage control loop is used to calculate the difference between the given value of the average capacitor voltage of the steady-state operation module and the feedback value of the average capacitor voltage of the converter valve to obtain the fifth difference value. The fifth difference value is then passed through a PI regulator to obtain the second d-axis current setpoint value. The first d-axis current setpoint value and the second d-axis current setpoint value are then superimposed to obtain the third d-axis current setpoint value.

[0089] The first q-axis voltage control loop is used to calculate the difference between the q-axis voltage setpoint of the steady-state operation module and the q-axis voltage feedback value of the converter valve to obtain the sixth difference value. The sixth difference value is then output as the first q-axis current setpoint value via a PI regulator.

[0090] The first dq-axis current control module is used to generate a first dq-axis current modulation signal based on the third d-axis current setpoint and the first q-axis current setpoint, and transmit the first dq-axis current modulation signal to the coordinate inverse transformation module;

[0091] The coordinate inverse transformation module is used to perform coordinate inverse transformation processing based on the received first dq axis current modulation signal and the synchronous phase of the first wind farm AC voltage to generate the first AC component of the converter valve modulation signal, and to superimpose the first AC component of the converter valve modulation signal and the first DC component of the converter valve modulation signal to obtain the first modulation signal.

[0092] When the output of the steady-state operation module is selected as valid by the selection module, the output thyristor trigger pulse of the steady-state operation module is 1, which is connected to the thyristor valve in the controllable component. The MMC outputs a small negative DC voltage close to 0 to keep the thyristor valve conducting.

[0093] For example, such as Figure 3 As shown, the DC current distribution circuit controls the ratio of DC current flowing through the MMC valve and the thyristor valve. Specifically, it sets the DC current setpoint I of the steady-state operation module. ref1 DC current feedback value I of the converter valve dc1 The difference is calculated to obtain the first difference value. This first difference value is then processed by a PI controller to obtain the first DC component U of the modulation signal of the converter valve. com .

[0094] The DC current setpoint of the steady-state operation module is determined according to the desired current ratio between the modular multilevel converter valve and the thyristor valve, that is:

[0095] I ref1 = P ref / U dcref / N

[0096] In the formula, N is the distribution ratio of DC current between the MMC and the parallel thyristor valves, typically taken as 2, so that the MMC valve and the parallel thyristor valve each carry 1 / 2 of the total DC current. dcref As the reference voltage, P ref This is the reference power.

[0097] The first reactive power synchronization control loop can compensate for the reactive power on the AC side of the diode valve. Specifically, it adjusts the reactive power setpoint Q of the steady-state operation module. ref The second difference is obtained by subtracting the AC reactive power feedback value Q from the converter valve. The second difference is then passed through a low-pass filter to obtain the AC voltage angular frequency adjustment of the first wind farm. The AC voltage angular frequency adjustment of the first wind farm is then added to the AC voltage rated angular frequency w0 to obtain the first synchronization angular frequency w of the converter valve. The first synchronization angular frequency is then passed through an integrator to obtain the AC voltage synchronization phase φ of the first wind farm.

[0098] Among them, the reactive power setpoint Q ref The reactive power consumption is determined by the diode valve. A typical low-pass filter stage can use 1 / (T) s The s+1) stage, where s is the Laplace operator, typically with a time constant T. s It can be 0.02.

[0099] The first active power control loop is used to input the active power setpoint (P) of the steady-state operation module. ref =0) and the active power feedback value P of the converter valve fbk The difference is calculated to obtain the third difference value. This third difference value is then passed through the P regulator and output as the first d-axis voltage setpoint U. dref The first d-axis voltage setpoint U dref The d-axis voltage feedback value U of the converter valve dfbk The difference is calculated to obtain the fourth difference value, which is then output as the first d-axis current setpoint I via a PI regulator. dref1 .

[0100] The DC capacitor voltage control loop sets the given value U of the average capacitor voltage of the steady-state operation module. c_ref (U) c_ref The feedback value U is set to the average value of the capacitor voltage of the converter valve, along with the rated voltage of the MMC valve submodule in the controllable components (typically 2.1kV). c_ref The difference is calculated to obtain the fifth difference value. This fifth difference value is then passed through a PI controller to obtain the second d-axis current setpoint I. dcref2 The first d-axis current setpoint I dcref1Second d-axis current setpoint I dcref2 By superimposing the values, the third d-axis current reference value I is obtained. dcref ;

[0101] The first q-axis voltage control loop sets the q-axis voltage setpoint U of the steady-state operation module. qref = 0 and the q-axis voltage feedback value U of the converter valve q The difference is calculated to obtain the sixth difference value, which is then output as the first q-axis current setpoint I via a PI regulator. qref ;

[0102] The first dq-axis current control module is a well-known dq-axis current inner loop structure in flexible DC transmission converters. It generates the first dq-axis current modulation signal u based on the third d-axis current setpoint and the first q-axis current setpoint. cd u cq And modulate the first dq axis current signal u cd u cq Transmitted to the coordinate inverse transformation module;

[0103] The first dq-axis current control module controls the third d-axis current by setting the value I. dcref and the first q-axis current setpoint I qref The signals are compared with the corresponding feedback values, processed by a proportional-integral (PI) regulator, and then used to generate the first dq-axis current modulation signal u for control. cd u cq This control method can precisely regulate the current, ensuring the stability and accuracy of the system during steady-state operation.

[0104] The coordinate inverse transformation module, based on the received first dq-axis current modulation signal u cd u cq The first AC component u of the modulation signal of the converter valve is generated by performing an inverse coordinate transformation on the phase φ of the AC voltage synchronized with the first wind farm. ap ,u bp ,u cp ,u an ,u bn ,u cn And the first AC component u of the modulation signal of the converter valve ap ,u bp ,u cp ,u an ,u bn ,u cn The first DC component U of the modulation signal of the converter valve com The signals are superimposed to obtain the first modulation signal, which is the modulation signal of the 6 bridge arms of the MMC in the controllable component.

[0105] In this application, the DC voltage is basically 0 in steady state, which can reduce the number of MMC valve cascade modules to a certain extent. At the same time, it avoids the coupling relationship between the DC voltage of the MMC valve and the modulation ratio of the MMC in the existing control methods. The AC modulation is mainly determined by the transformer ratio and reactive power output capability of the MMC valve, and is decoupled from the DC voltage control.

[0106] In some embodiments, the DC fault ride-through module includes a DC current control loop, a DC voltage control loop, a second reactive synchronous control loop, a second active power control loop, a second q-axis voltage control loop, a dq-axis current inner loop module, and a second dq-axis current control module.

[0107] The DC current control loop is used to calculate the difference between the DC current setpoint of the DC fault ride-through module and the DC current feedback value of the converter valve to obtain the seventh difference value. The seventh difference value is calculated by the PI regulator to output the DC current component. The DC current component is multiplied by the DC current enable loop to obtain the DC current enable component. After the DC current enable component is limited, it is output as the second DC component of the modulation signal of the converter valve.

[0108] The DC voltage control loop is used to calculate the difference between the DC voltage setpoint of the DC fault ride-through module and the DC voltage feedback value of the converter valve to obtain the eighth difference value. The eighth difference value is calculated by the PI regulator to output the DC voltage component. The DC voltage component is multiplied by the DC voltage enable loop to obtain the DC voltage enable component. After the DC voltage enable component is reduced and limited, it is output as the third DC component of the modulation signal of the converter valve.

[0109] The second reactive power synchronization control loop is used to calculate the difference between the reactive power setpoint of the DC fault ride-through module and the AC reactive power feedback value of the converter valve to obtain the ninth difference value. The ninth difference value is then passed through a low-pass filter to obtain the AC voltage angular frequency adjustment of the second wind farm. The AC voltage angular frequency adjustment of the second wind farm is then added to the AC voltage rated angular frequency to obtain the second synchronization angular frequency of the converter valve. The second synchronization angular frequency is then passed through an integration loop to obtain the AC voltage synchronization phase of the second wind farm.

[0110] The second active control loop is used to calculate the difference between the active power setpoint of the DC fault ride-through module and the active power feedback value of the converter valve to obtain the tenth difference value. The tenth difference value is output as the second d-axis voltage setpoint value through the P regulator. The second d-axis voltage setpoint value is calculated as the difference between the second d-axis voltage setpoint value and the d-axis voltage feedback value of the converter valve to obtain the eleventh difference value. The eleventh difference value is output as the fourth d-axis current setpoint value through the PI regulator.

[0111] The second q-axis voltage control loop is used to calculate the difference between the q-axis voltage setpoint of the DC fault ride-through module and the q-axis voltage feedback value of the converter valve to obtain the twelfth difference value. The twelfth difference value is then output as the second q-axis current setpoint value via a PI regulator.

[0112] The dq-axis current inner loop module is used to generate a second dq-axis current modulation signal based on the fourth d-axis current setpoint and the second q-axis current setpoint, and transmit the second dq-axis current modulation signal to the second dq-axis current control module.

[0113] The second dq-axis current control module is used to perform coordinate inverse transformation processing based on the received second dq-axis current modulation signal and the synchronous phase of the second wind farm AC voltage to generate the second AC component of the converter valve modulation signal. The second AC component of the converter valve modulation signal and the fourth DC component of the converter valve modulation signal are superimposed to obtain the second modulation signal. The fourth DC component is obtained by superimposing the second DC component and the third DC component.

[0114] For example, such as Figure 4 As shown, the DC current control loop sets the DC current setpoint I of the DC fault ride-through module. dc_ref (I) dc_ref = 0) and the DC current feedback value I of the converter valve dc1 The difference is calculated to obtain the seventh difference value. This seventh difference value is then processed by a PI controller to output the DC current component. This DC current component is multiplied by the DC current enable circuit ENIdc (which is 1 when enabled and 0 when disabled) to obtain the DC current enable component. This DC current enable component is then subjected to a limiting operation (typically limited to [-1p.u., 0p.u.]), and the output is the second DC component U, which is the modulation signal of the converter valve. com1 ;

[0115] The DC voltage control loop sets the DC voltage setpoint U of the DC fault ride-through module. dc_ref (U) dc_ref A short-duration, small positive DC voltage is provided (i.e., the voltage setpoint is a pulse signal with adjustable width, used to quickly reduce the thyristor current in the controllable component to 0, thereby reliably turning it off), and the DC voltage feedback value U of the commutator valve is also provided. dc_fbk The difference is calculated to obtain the eighth difference value. This eighth difference value is then processed by a PI controller to output a DC voltage component. This DC voltage component is multiplied by the DC voltage enable circuit ENIdc (which is 1 when enabled and 0 when disabled) to obtain the DC voltage enable component. This DC voltage enable component is then reduced and limited (typically limited to [0p.u., 0.05pu]) to output the third DC component U, which is the modulation signal of the converter valve. com2 ;

[0116] The second reactive power synchronization control loop will control the reactive power setpoint Q of the DC fault ride-through module. refThe difference between the AC reactive power feedback value Q of the converter valve is used to obtain the ninth difference value. The ninth difference value is then passed through a low-pass filter to obtain the AC voltage angular frequency adjustment of the second wind farm. The AC voltage angular frequency adjustment of the second wind farm is then added to the AC voltage rated angular frequency w0 to obtain the second synchronization angular frequency w of the converter valve. The second synchronization angular frequency w is then passed through an integral element to obtain the AC voltage synchronization phase φ of the second wind farm.

[0117] The second active power control loop will control the active power setpoint P of the DC fault ride-through module. ref (P) ref =0) and the active power feedback value P of the converter valve fbk The difference is calculated to obtain the tenth difference value. This tenth difference value is then passed through the P regulator and output as the second d-axis voltage setpoint U. dref The second d-axis voltage setpoint U dref The d-axis voltage feedback value U of the converter valve dfbk The difference is calculated to obtain the eleventh difference value, which is then output as the fourth d-axis current setpoint U via a PI controller. dref ;

[0118] The second q-axis voltage control loop will control the q-axis voltage setpoint U of the DC fault ride-through module. qref (U) qref =0) and the q-axis voltage feedback value U of the converter valve q The difference is calculated to obtain the twelfth difference value, which is then output as the second q-axis current setpoint I via a PI regulator. qref2 ;

[0119] The dq-axis current inner loop module, based on the fourth d-axis current setpoint U dref Second q-axis current setpoint I qref2 Generate the second dq-axis current modulation signal u cd1 u cq1 And modulate the second dq axis current signal u cd1 u cq1 Transmitted to the second dq axis current control module;

[0120] The second dq-axis current control module controls the second dq-axis current modulation signal u based on the received second dq-axis current modulation signal u. cd1 u cq1 The second AC component u of the modulation signal of the converter valve is generated by performing an inverse coordinate transformation on the synchronous phase φ of the AC voltage of the second wind farm. ap ,u bp ,u cp ,u an ,u bn ,u cn And the second AC component u of the modulation signal of the converter valve ap ,u bp ,ucp ,u an ,u bn ,u cn The fourth DC component U of the modulation signal of the converter valve com The components are superimposed to obtain the second modulation signal; wherein, the fourth DC component is the second DC component U. com1 and the third DC component U com3 The result is obtained by superposition.

[0121] Once the DC fault ride-through module is enabled (the enabling condition is the typical DC-side low-voltage overcurrent criterion for a DC transmission system), the control system operates in the following sequence:

[0122] (1) The steady-state operation module blocks the thyristor valve trigger pulse in the controllable components, and the thyristor trigger pulse is 0.

[0123] (2) Enable the DC voltage control loop and set the enable flag ENV for this loop. dc =1, set the DC voltage setpoint in the DC voltage control circuit to a short-term small positive voltage, the MMC valve in the controllable component outputs a small positive voltage pulse, causing the thyristor valve branch current in the controllable component to drop rapidly to 0, and the thyristor valve group to turn off.

[0124] (3) Execute simultaneously with step (2), shield the DC current control loop, set the loop enable flag ENIdc = 0, and reset the PI regulator in the loop.

[0125] (4) Enable the DC current control loop and set the enable flag of the loop to ENI. dc =1. Set the DC current setpoint to 0 to suppress the rise of short-circuit current.

[0126] (5) Execute simultaneously with step (4) to shield the DC voltage control circuit, set the enable flag ENVdc=0 for the circuit, and reset the PI regulator in the circuit.

[0127] Based on the same inventive concept, this application also provides a control method for realizing the control system of zero DC voltage operation as described above.

[0128] like Figure 5 As shown, this application also provides a control method for a control system operating with zero DC voltage as described in the above embodiments, comprising:

[0129] Step S1: Detect faults in the flexible DC system and obtain fault detection results.

[0130] Step S2: Based on the fault detection results, determine the modulation signal by combining the first modulation signal and the second modulation signal, and send the modulation signal to the converter valve so that the converter valve performs the actions of the flexible DC system under normal operation or fault stable operation and fault crossing according to the modulation signal; wherein, the first modulation signal is generated to maintain the stable operation state of the flexible DC system when the flexible DC system is operating normally; the second modulation signal is generated to guide the flexible DC system to cross the fault when a fault occurs in the flexible DC system.

[0131] In some embodiments, the process of generating the first modulated signal includes:

[0132] The first difference is obtained by subtracting the DC current setpoint of the steady-state operation module from the DC current feedback value of the converter valve. The first difference is then processed by the PI regulator to obtain the first DC component of the modulation signal of the converter valve.

[0133] The reactive power setpoint of the steady-state operation module is subtracted from the AC reactive power feedback value of the converter valve to obtain the second difference value. The second difference value is passed through a low-pass filter to obtain the AC voltage angular frequency adjustment of the first wind farm. The AC voltage angular frequency adjustment of the first wind farm is then added to the AC voltage rated angular frequency to obtain the first synchronization angular frequency of the converter valve. The first synchronization angular frequency is then passed through an integral element to obtain the AC voltage synchronization phase of the first wind farm.

[0134] The active power setpoint of the steady-state operation module is subtracted from the active power feedback value of the converter valve to obtain the third difference value. The third difference value is output as the first d-axis voltage setpoint value through the P regulator. The first d-axis voltage setpoint value is subtracted from the d-axis voltage feedback value of the converter valve to obtain the fourth difference value. The fourth difference value is output as the first d-axis current setpoint value through the PI regulator.

[0135] The fifth difference is obtained by subtracting the given value of the average capacitor voltage of the steady-state operation module from the feedback value of the average capacitor voltage of the converter valve. The fifth difference is then passed through a PI regulator to obtain the second d-axis current setpoint. The first d-axis current setpoint and the second d-axis current setpoint are then superimposed to obtain the third d-axis current setpoint.

[0136] The difference between the q-axis voltage setpoint of the steady-state operation module and the q-axis voltage feedback value of the converter valve is used to obtain the sixth difference value. The sixth difference value is then output as the first q-axis current setpoint value through the PI regulator.

[0137] Based on the third d-axis current setpoint and the first q-axis current setpoint, a first dq-axis current modulation signal is generated;

[0138] The coordinate inverse transformation is performed based on the first dq axis current modulation signal and the first wind farm AC voltage synchronous phase to generate the first AC component of the converter valve modulation signal. The first AC component of the converter valve modulation signal and the first DC component of the converter valve modulation signal are then superimposed to obtain the first modulation signal.

[0139] In some embodiments, the DC current setpoint of the steady-state operation module is determined according to the desired current ratio between the modular multilevel converter valve and the thyristor valve.

[0140] In some embodiments, the process of generating the second modulation signal includes:

[0141] The difference between the DC current setpoint of the DC fault ride-through module and the DC current feedback value of the converter valve is obtained as the seventh difference value. The seventh difference value is calculated by the PI regulator to output the DC current component. The DC current component is multiplied by the DC current enable circuit to obtain the DC current enable component. After the DC current enable component is limited, the second DC component of the modulation signal of the converter valve is output.

[0142] The difference between the DC voltage setpoint of the DC fault ride-through module and the DC voltage feedback value of the converter valve is obtained as the eighth difference value. The eighth difference value is calculated by the PI regulator to output the DC voltage component. The DC voltage component is multiplied by the DC voltage enable circuit to obtain the DC voltage enable component. After the DC voltage enable component is reduced and subjected to a limiting operation, it is output as the third DC component of the modulation signal of the converter valve.

[0143] The reactive power setpoint of the DC fault ride-through module is subtracted from the AC reactive power feedback value of the converter valve to obtain the ninth difference value. The ninth difference value is then passed through a low-pass filter to obtain the second wind farm AC voltage angular frequency adjustment amount. The second wind farm AC voltage angular frequency adjustment amount is then added to the AC voltage rated angular frequency to obtain the second synchronization angular frequency of the converter valve. The second synchronization angular frequency is then passed through an integral element to obtain the second wind farm AC voltage synchronization phase.

[0144] The difference between the active power setpoint of the DC fault ride-through module and the active power feedback value of the converter valve is used to obtain the tenth difference value. The tenth difference value is output as the second d-axis voltage setpoint value through the P regulator. The difference between the second d-axis voltage setpoint value and the d-axis voltage feedback value of the converter valve is used to obtain the eleventh difference value. The eleventh difference value is output as the fourth d-axis current setpoint value through the PI regulator.

[0145] The difference between the q-axis voltage setpoint of the DC fault ride-through module and the q-axis voltage feedback value of the converter valve is used to obtain the twelfth difference value. The twelfth difference value is then output as the second q-axis current setpoint value through the PI regulator.

[0146] The second dq-axis current modulation signal is generated based on the fourth d-axis current setpoint and the second q-axis current setpoint.

[0147] Based on the second dq axis current modulation signal and the synchronous phase of the second wind farm AC voltage, a coordinate inverse transformation is performed to generate the second AC component of the converter valve modulation signal. The second AC component of the converter valve modulation signal and the fourth DC component of the converter valve modulation signal are then superimposed to obtain the second modulation signal. The fourth DC component is obtained by superimposing the second DC component and the third DC component.

[0148] Those skilled in the art will understand that, for the sake of convenience and brevity, the specific working process of the method described above can be referred to the corresponding process in the aforementioned system embodiments, and will not be repeated here.

[0149] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0150] It should be understood that although the steps in the flowcharts of the embodiments described above are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the embodiments described above may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0151] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A control system for zero DC voltage operation, characterized in that, include: DC fault detection module, steady-state operation module, DC fault ride-through module, switching module, and converter valve; The DC fault detection module, the steady-state operation module, and the DC fault ride-through module are all electrically connected to the input terminal of the switching module. The DC fault detection module is used to detect faults in the flexible DC system and send the fault detection results to the switching module. The steady-state operation module is used to generate a first modulation signal to maintain the stable operation state of the flexible DC system during normal operation of the flexible DC system, and send the first modulation signal to the switching module; The DC fault ride-through module is used to generate a second modulation signal to guide the flexible DC system through the fault when a fault occurs in the flexible DC system, and to send the second modulation signal to the switching module. The switching module is used to switch between receiving the first modulation signal or the second modulation signal according to the received fault detection result, and to send the received modulation signal to the converter valve. The converter valve includes a first diode valve assembly, a controllable assembly, and a second diode valve assembly connected in series. The controllable assembly is electrically connected to the output terminal of the switching module. The controllable assembly includes a modular multilevel converter valve based on a full-bridge submodule and a thyristor valve, with the thyristor valve connected in parallel with the modular multilevel converter valve.

2. The control system for zero DC voltage operation according to claim 1, characterized in that, The steady-state operation module includes a DC current distribution circuit, a first reactive power synchronization control circuit, a first active power control circuit, a DC capacitor voltage control circuit, a first q-axis voltage control circuit, a first dq-axis current control module, and a coordinate inverse transformation module; The DC current distribution circuit is used to subtract the DC current setpoint of the steady-state operation module from the DC current feedback value of the converter valve to obtain a first difference value, and to calculate the first DC component of the modulation signal of the converter valve by processing the first difference value through a PI regulator. The first reactive power synchronization control loop is used to subtract the reactive power setpoint of the steady-state operation module from the AC reactive power feedback value of the converter valve to obtain a second difference value. The second difference value is then passed through a low-pass filter to obtain the first wind farm AC voltage angular frequency adjustment amount. The first wind farm AC voltage angular frequency adjustment amount is then added to the AC voltage rated angular frequency to obtain the first synchronization angular frequency of the converter valve. The first synchronization angular frequency is then passed through an integration circuit to obtain the first wind farm AC voltage synchronization phase. The first active power control loop is used to calculate the difference between the active power setpoint of the steady-state operation module and the active power feedback value of the converter valve to obtain a third difference value, and output the third difference value as a first d-axis voltage setpoint value through a P regulator. The first d-axis voltage setpoint value is then calculated as a difference between the first d-axis voltage setpoint value and the d-axis voltage feedback value of the converter valve to obtain a fourth difference value, and output the fourth difference value as a first d-axis current setpoint value through a PI regulator. The DC capacitor voltage control circuit is used to calculate the difference between the given value of the average capacitor voltage of the steady-state operation module and the feedback value of the average capacitor voltage of the converter valve to obtain a fifth difference value. The fifth difference value is then passed through a PI regulator to obtain a second d-axis current given value. The first d-axis current given value and the second d-axis current given value are then superimposed to obtain a third d-axis current given value. The first q-axis voltage control loop is used to calculate the difference between the q-axis voltage setpoint of the steady-state operation module and the q-axis voltage feedback value of the converter valve to obtain a sixth difference value, and output the sixth difference value as the first q-axis current setpoint value through the PI regulator; The first dq-axis current control module is used to generate a first dq-axis current modulation signal based on the third d-axis current setpoint and the first q-axis current setpoint, and to transmit the first dq-axis current modulation signal to the coordinate inverse transformation module; The coordinate inverse transformation module is used to perform coordinate inverse transformation processing based on the received first dq axis current modulation signal and the first wind farm AC voltage synchronization phase to generate the first AC component of the converter valve modulation signal, and to superimpose the first AC component of the converter valve modulation signal and the first DC component of the converter valve modulation signal to obtain the first modulation signal.

3. The control system for zero DC voltage operation according to claim 1, characterized in that, The thyristor valve has a multi-stage thyristor series structure.

4. The control system for zero DC voltage operation according to claim 2, characterized in that, The DC current setpoint of the steady-state operation module is determined according to the desired current ratio between the modular multilevel converter valve and the thyristor valve.

5. The control system for zero DC voltage operation according to claim 1, characterized in that, The DC fault ride-through module includes a DC current control loop, a DC voltage control loop, a second reactive synchronous control loop, a second active power control loop, a second q-axis voltage control loop, a dq-axis current inner loop module, and a second dq-axis current control module. The DC current control loop is used to calculate the difference between the DC current setpoint of the DC fault ride-through module and the DC current feedback value of the converter valve to obtain a seventh difference value. The seventh difference value is calculated by a PI regulator to output a DC current component. The DC current component is multiplied by the DC current enable loop to obtain a DC current enable component. The DC current enable component is then limited and output as the second DC component of the modulation signal of the converter valve. The DC voltage control loop is used to calculate the difference between the DC voltage setpoint of the DC fault ride-through module and the DC voltage feedback value of the converter valve to obtain an eighth difference value. The eighth difference value is processed by a PI regulator to output a DC voltage component. The DC voltage component is multiplied by the DC voltage enable loop to obtain a DC voltage enable component. The DC voltage enable component is then limited and output as the third DC component of the modulation signal of the converter valve. The second reactive power synchronization control loop is used to calculate the difference between the reactive power setpoint of the DC fault ride-through module and the AC reactive power feedback value of the converter valve to obtain the ninth difference value. The ninth difference value is then passed through a low-pass filter to obtain the second wind farm AC voltage angular frequency adjustment amount. The second wind farm AC voltage angular frequency adjustment amount is then added to the AC voltage rated angular frequency to obtain the second synchronization angular frequency of the converter valve. The second synchronization angular frequency is then passed through an integration circuit to obtain the second wind farm AC voltage synchronization phase. The second active control loop is used to calculate the difference between the active power setpoint of the DC fault ride-through module and the active power feedback value of the converter valve to obtain the tenth difference value, and output the tenth difference value as the second d-axis voltage setpoint value through the P regulator. The second d-axis voltage setpoint value is then calculated as the difference between the second d-axis voltage setpoint value and the d-axis voltage feedback value of the converter valve to obtain the eleventh difference value, and output the eleventh difference value as the fourth d-axis current setpoint value through the PI regulator. The second q-axis voltage control loop is used to calculate the difference between the q-axis voltage setpoint of the DC fault ride-through module and the q-axis voltage feedback value of the converter valve to obtain the twelfth difference value, and output the twelfth difference value as the second q-axis current setpoint value through the PI regulator; The dq-axis current inner loop module is used to generate a second dq-axis current modulation signal based on the fourth d-axis current setpoint and the second q-axis current setpoint, and to transmit the second dq-axis current modulation signal to the second dq-axis current control module. The second dq-axis current control module is used to perform coordinate inverse transformation processing based on the received second dq-axis current modulation signal and the synchronous phase of the second wind farm AC voltage to generate a second AC component of the modulation signal of the converter valve, and to superimpose the second AC component of the modulation signal of the converter valve and the fourth DC component of the modulation signal of the converter valve to obtain the second modulation signal; wherein, the fourth DC component is obtained by superimposing the second DC component and the third DC component.

6. The control system for zero DC voltage operation according to claim 1, characterized in that, It also includes a first isolating switch, a second isolating switch, a first transformer, a second transformer, a third transformer, a first AC circuit breaker, a second AC circuit breaker, and a third AC circuit breaker; The first diode valve assembly is connected in parallel with the first isolation blade; The second diode valve assembly is connected in parallel with the second isolation blade; The first diode valve assembly is connected in parallel to the AC busbar of the wind farm via the first transformer and the first AC circuit breaker in sequence. The second diode valve assembly is connected in parallel with the AC busbar of the wind farm via the second transformer and the second AC circuit breaker in sequence; The controllable component is connected in parallel to the AC busbar of the wind farm via the third transformer and the third AC circuit breaker.

7. A control method for a control system operating with zero DC voltage as described in any one of claims 1 to 6, characterized in that, include: Detect faults in flexible DC systems and obtain fault detection results; Based on the fault detection results, a modulation signal is determined by combining the first modulation signal and the second modulation signal, and the modulation signal is sent to the converter valve, so that the converter valve performs the operation of the flexible DC system under normal operation or fault stable operation and fault crossing according to the modulation signal; wherein, the first modulation signal is generated to maintain the stable operation state of the flexible DC system when the flexible DC system is operating normally; the second modulation signal is generated to guide the flexible DC system to cross the fault when a fault occurs in the flexible DC system.

8. The control method according to claim 7, characterized in that, The generation process of the first modulated signal includes: The first difference is obtained by subtracting the DC current setpoint of the steady-state operation module from the DC current feedback value of the converter valve. The first difference is then processed by a PI regulator to obtain the first DC component of the modulation signal of the converter valve. The reactive power setpoint of the steady-state operation module is subtracted from the AC reactive power feedback value of the converter valve to obtain a second difference value. The second difference value is then passed through a low-pass filter to obtain the first wind farm AC voltage angular frequency adjustment amount. The first wind farm AC voltage angular frequency adjustment amount is then added to the AC voltage rated angular frequency to obtain the first synchronization angular frequency of the converter valve. The first synchronization angular frequency is then passed through an integral element to obtain the first wind farm AC voltage synchronization phase. The active power setpoint of the steady-state operation module is subtracted from the active power feedback value of the converter valve to obtain a third difference value. The third difference value is output as the first d-axis voltage setpoint value through the P regulator. The first d-axis voltage setpoint value is subtracted from the d-axis voltage feedback value of the converter valve to obtain a fourth difference value. The fourth difference value is output as the first d-axis current setpoint value through the PI regulator. The fifth difference is obtained by subtracting the given value of the average capacitor voltage of the steady-state operation module from the feedback value of the average capacitor voltage of the converter valve. The fifth difference is then passed through a PI regulator to obtain the second d-axis current given value. The first d-axis current given value and the second d-axis current given value are then superimposed to obtain the third d-axis current given value. The difference between the q-axis voltage setpoint of the steady-state operation module and the q-axis voltage feedback value of the converter valve is used to obtain the sixth difference value. The sixth difference value is then output as the first q-axis current setpoint value through the PI regulator. Based on the third d-axis current setpoint and the first q-axis current setpoint, a first dq-axis current modulation signal is generated; The coordinate inverse transformation is performed based on the first dq axis current modulation signal and the first wind farm AC voltage synchronous phase to generate the first AC component of the converter valve modulation signal. The first AC component of the converter valve modulation signal and the first DC component of the converter valve modulation signal are then superimposed to obtain the first modulation signal.

9. The control method according to claim 8, characterized in that, The DC current setpoint of the steady-state operation module is determined according to the desired current ratio between the modular multilevel converter valve and the thyristor valve.

10. The control method according to claim 7, characterized in that, The generation process of the second modulated signal includes: The difference between the DC current setpoint of the DC fault ride-through module and the DC current feedback value of the converter valve is obtained as the seventh difference value. The seventh difference value is calculated by the PI regulator to output the DC current component. The DC current component is multiplied by the DC current enable circuit to obtain the DC current enable component. The DC current enable component is then limited and output as the second DC component of the modulation signal of the converter valve. The difference between the DC voltage setpoint of the DC fault ride-through module and the DC voltage feedback value of the converter valve is obtained to get the eighth difference value. The eighth difference value is calculated by the PI regulator to output the DC voltage component. The DC voltage component is multiplied by the DC voltage enable circuit to obtain the DC voltage enable component. After the DC voltage enable component is limited, it is output as the third DC component of the modulation signal of the converter valve. The reactive power setpoint of the DC fault ride-through module is subtracted from the AC reactive power feedback value of the converter valve to obtain the ninth difference value. The ninth difference value is then passed through a low-pass filter to obtain the second wind farm AC voltage angular frequency adjustment amount. The second wind farm AC voltage angular frequency adjustment amount is then added to the AC voltage rated angular frequency to obtain the second synchronization angular frequency of the converter valve. The second synchronization angular frequency is then passed through an integral element to obtain the second wind farm AC voltage synchronization phase. The difference between the active power setpoint of the DC fault ride-through module and the active power feedback value of the converter valve is used to obtain the tenth difference value. The tenth difference value is output as the second d-axis voltage setpoint value through the P regulator. The difference between the second d-axis voltage setpoint value and the d-axis voltage feedback value of the converter valve is used to obtain the eleventh difference value. The eleventh difference value is output as the fourth d-axis current setpoint value through the PI regulator. The difference between the q-axis voltage setpoint of the DC fault ride-through module and the q-axis voltage feedback value of the converter valve is used to obtain the twelfth difference value. The twelfth difference value is then output as the second q-axis current setpoint value via a PI regulator. Based on the fourth d-axis current setpoint and the second q-axis current setpoint, a second dq-axis current modulation signal is generated; The coordinate inverse transformation is performed based on the second dq axis current modulation signal and the second wind farm AC voltage synchronization phase to generate the second AC component of the converter valve modulation signal. The second AC component of the converter valve modulation signal and the fourth DC component of the converter valve modulation signal are then superimposed to obtain the second modulation signal. The fourth DC component is obtained by superimposing the second DC component and the third DC component.

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