Fault processing method for offshore wind power direct current parallel sending-out system through DRU-MMC

Abstract

The invention discloses a fault processing method for an offshore wind power direct current parallel sending-out system through a DRU-MMC. The fault processing method comprises the steps that direct current energy consumption devices are arranged at direct current outlets of respective land MMC converter stations of a DRU-HVDC system and an MMC-HVDC system; when the alternating current power grid at the receiving end has an alternating current fault, the direct current energy consumption device is controlled to adopt a direct current voltage suppression strategy; when the DRU-HVDC system and the MMC-HVDC system respectively have direct current faults, different locking and circuit breaker disconnection time sequence control strategies are adopted, and in the direct current fault ride-through period, a direct current suppression strategy and a direct current voltage suppression strategy are controlled to be adopted by a direct current energy consumption device of the DRU-HVDC system. Therefore, the fault ride-through capability of the DRU-MMC direct current parallel sending-out system of the offshore wind power can be effectively improved, and stable operation of the system is ensured.

Description

Technical Field

[0001] This invention belongs to the field of power system transmission and distribution technology, specifically relating to a fault handling method for an offshore wind power parallel DC transmission system via DRU-MMC. Background Technology

[0002] With near-shore resources nearing depletion, offshore wind power projects are increasingly moving towards deep-sea areas. Flexible DC transmission technology based on MMC (Multilevel Modular Converter) offers advantages such as high technological maturity, strong operational flexibility, and passive system operation capabilities. However, the converter stations themselves have high investment costs and are difficult to operate and maintain. Furthermore, the construction of offshore platforms is challenging, resulting in high costs. To improve the economics of deep-sea wind power transmission systems, Siemens proposed an offshore wind power transmission solution based on DRU (Diode Rectifier Unit). This solution can significantly reduce the size and weight of offshore converter platforms, thereby substantially reducing the cost of deep-sea wind power DC transmission projects. However, under current technological conditions, DRUs have the following limitations: First, DRUs are uncontrolled rectifier converters and cannot provide voltage support for offshore wind farms, requiring the wind turbines themselves to have grid connection capabilities; second, DRUs have a single power transmission direction and lack power reverse transmission capabilities, making it impossible to provide black-start power for offshore wind farms. To address the issues of wind power grid integration and black start problems in pure DRU solutions, both domestic and international efforts are actively seeking DC transmission system solutions that combine MMC and DRU on the rectifier side.

[0003] In the existing technology, some scholars have proposed a heterogeneous DC transmission system for deep-sea wind power based on the parallel operation of flexible DC and DRU DC (reference [R. Li, L. Yu, L. Xu and GP Adam, "Coordinated Control of Parallel DR-HVDC and MMC-HVDC Systems for Offshore Wind Energy Transmission," in IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 8, no. 3, pp. 2572-2582, Sept. 2020]), which can improve the technical and economic efficiency from the perspective of multiple deep-sea wind power DC transmission systems. However, with this scheme, when a DC fault occurs in the MMC-HVDC (High Voltage Direct Current) system, the offshore MMC converter station will be quickly shut down. At this time, the active power output from the offshore wind farm will flow to the offshore DRU converter station, causing the DC current of the DRU-HVDC system to increase. Since the DRU converter station itself has strong overcurrent withstand capability, the transient process may trigger the overcurrent protection of the receiving-end MMC converter station, thereby causing the system to shut down. In addition, when a fault occurs in the receiving-end AC grid, causing a voltage drop, surplus power will be generated in the DC system, resulting in DC overvoltage.

[0004] Currently, a common method for addressing surplus power is to install DC energy dissipation devices at the DC outlet of the receiving-end converter station (e.g., in the literature [Qi Lei, Wu Sihang, Shi Dan, et al. DC energy dissipation technology for offshore wind power transmission systems [J / OL]. High Voltage Engineering, 1-22]). When the DC voltage rises to the operating threshold, the DC energy dissipation device activates to dissipate the surplus power. However, this scheme only responds to the increase in DC voltage and cannot detect the overcurrent in the DRU-HVDC system. When a fault in the MMC-HVDC causes power to be transferred to the DRU-HVDC, the receiving-end MMC may trip due to overcurrent protection, but the energy dissipation device cannot activate because the voltage is not exceeded, leading to a system-wide shutdown.

[0005] Therefore, in order to avoid overcurrent and overvoltage after system failure, it is urgent to study fault handling methods applicable to offshore wind power transmission systems via DRU-MMC DC parallel transmission to ensure the safe and stable operation of the system. Summary of the Invention

[0006] In view of the above, the present invention provides a fault handling method for an offshore wind power parallel DC transmission system via DRU-MMC, which can overcome the problem of overcurrent and overvoltage leading to system shutdown when DC faults and AC faults of the receiving-end grid occur in the prior art. It effectively suppresses overcurrent and overvoltage caused by system faults, enables the system to reliably overcome DC faults and AC faults of the receiving-end grid, and improves the stability and reliability of the system.

[0007] A fault handling method for an offshore wind power parallel DC transmission system via DRU-MMC includes the following steps: (1) Install DC energy dissipation devices at the DC outlet of the onshore MMC converter station of each DRU-HVDC system and MMC-HVDC system; (2) When an AC fault occurs in the receiving-end AC power grid, a DC voltage suppression strategy is adopted to control the DC energy consumption devices of the DRU-HVDC system and the MMC-HVDC system respectively; (3) When a DC fault occurs in the DRU-HVDC system, the fault ride-through control strategy of the grid-connected wind farm-MMC DC system shall be executed; (4) When a DC fault occurs in the MMC-HVDC system, the fault ride-through control strategy of the grid-type wind farm-DRU DC system is executed.

[0008] The offshore wind power transmission system via DRU-MMC parallel DC transmission includes a first offshore wind farm based on grid-connected wind turbines, a second offshore wind farm based on grid-connected wind turbines, an offshore MMC converter station, an offshore DRU converter station, a first onshore MMC converter station, and a second onshore MMC converter station. The first offshore wind farm is connected to a first AC bus via a first AC collection submarine cable, and the second offshore wind farm is connected to a second AC bus via a second AC collection submarine cable. The first AC bus is connected to the second AC bus via a connecting submarine cable. The AC side of the offshore DRU converter station is connected to the first AC bus via a converter transformer, and the DC side of the offshore DRU converter station is connected to the DC side of the first onshore MMC converter station via a first DC submarine cable. The AC side of the offshore MMC converter station is connected to the second AC bus via a converter transformer. The DC side of the offshore MMC converter station is connected to the DC side of the second onshore MMC converter station via a second DC submarine cable. The AC sides of the first and second onshore MMC converter stations are connected to their respective receiving-end AC power grids via converter transformers. The offshore MMC converter station uses V / f control (the ratio of output voltage to frequency is constant), while both the first and second onshore MMC converter stations use constant DC voltage control. The offshore DRU converter station and the first onshore MMC converter station are connected to form a DRU-HVDC system, and the offshore MMC converter station and the second onshore MMC converter station are connected to form an MMC-HVDC system.

[0009] Furthermore, the specific implementation method of the fault ride-through control strategy of the grid-connected wind farm-MMC DC system in step (3) is as follows: 5ms after the fault occurs, the onshore MMC converter station in the DRU-HVDC system is locked. 20ms after the onshore MMC converter station is locked, the offshore wind farm connected to the DRU-HVDC system is locked. After a delay of 80ms, the AC circuit breaker of the offshore wind farm, the AC circuit breaker of the offshore DRU converter station, and the AC circuit breaker of the submarine cable of the connecting line near the offshore DRU converter station are disconnected.

[0010] Further, the specific implementation of the fault ride-through control strategy of the grid-type wind farm-DRU DC system in step (4) is as follows: within 5ms after the fault occurs, the offshore MMC converter station and the onshore MMC converter station in the MMC-HVDC system are locked; after the two MMC converter stations are locked for 20ms, the offshore wind farm connected to the MMC-HVDC system is locked; after a delay of 80ms, the AC circuit breaker of the offshore wind farm, the AC circuit breaker of the offshore MMC converter station, and the AC circuit breaker of the submarine cable of the connecting line near the offshore MMC converter station are disconnected; during the DC fault ride-through, the DC voltage suppression strategy and the DC current suppression strategy are used in parallel to control the DC energy consumption device of the DRU-HVDC system.

[0011] Furthermore, the specific implementation method of the DC voltage suppression strategy is as follows: A1. Monitor the DC voltage at the DC output of the onshore MMC converter station. U dc ; A2. When DC voltage U dc Rise to the preset action threshold U dcH At that time, the DC power dissipation device at the DC outlet of the onshore MMC converter station is put into operation to dissipate the surplus power; A3. When DC voltage U dc Drop to the preset exit threshold U dcL At that time, the DC power consumption device at the DC outlet of the onshore MMC converter station is disconnected; A4. Repeat the above input and output steps until the DC voltage is reached. U dc Return to steady state.

[0012] Furthermore, the specific implementation of the DC current suppression strategy is as follows: B1. Monitor the DC current injected into the onshore MMC converter station. I dc ; B2. When DC current Idc Exceeding the preset action threshold I dcH At that time, the DC power dissipation device at the DC outlet of the onshore MMC converter station is put into operation to dissipate the surplus power and shunt it; B3. When direct current I dc Return to the preset exit threshold I dcL At that time, the DC power consumption device at the DC outlet of the onshore MMC converter station is disconnected; B4. Repeat the above input and output steps until the direct current is reached. I dc Return to steady state.

[0013] Furthermore, the DC power dissipation device consists of multiple power dissipation valves and a power dissipation resistor connected in series, and the power dissipation valves are composed of several half-bridge sub-modules connected in series.

[0014] Furthermore, the action threshold I dcH The preset value is 1.4 times the rated DC current, and the exit threshold is reached. I dcL The preset value is 1.05 times the rated DC current.

[0015] Furthermore, the action threshold U dcH The preset value is 1.1 times the rated DC voltage, and the exit threshold is set. U dcL The preset value is 1.02 times the rated DC voltage.

[0016] A computer device includes a memory and a processor, wherein the memory stores a computer program and the processor executes the computer program to implement the above-mentioned fault handling method for the offshore wind power transmission system via DRU-MMC DC parallel transmission.

[0017] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-described fault handling method for the offshore wind power parallel DC transmission system via DRU-MMC.

[0018] Based on the above technical solution, the present invention can effectively suppress overvoltage and overcurrent caused by DC faults and AC system faults at the receiving end of offshore wind power transmitted through the DRU-MMC DC parallel transmission system, enabling the system to reliably overcome DC faults and AC system faults at the receiving end, improving the stability and reliability of the system, and reducing the system operation and maintenance costs. Attached Figure Description

[0019] Figure 1This is a schematic diagram of the structure of the offshore wind power transmission system via DRU-MMC DC parallel transmission.

[0020] Figure 2 This is a schematic diagram of the control strategy for the DC energy consumption device in the DRU-HVDC system of the present invention.

[0021] Figure 3 This is a schematic diagram of the DC fault handling process of the MMC-HVDC system of the present invention.

[0022] Figure 4 The following is a simulation waveform diagram of a land-based AC system failure according to an embodiment of the present invention, wherein (a) is the DC voltage of the DRU-HVDC system, (b) is the DC current of the DRU-HVDC system, (c) is the frequency of the marine AC system, (d) is the output power of the DRU, (e) is the power flowing from the MMC to the DRU, and (f) is the AC voltage of the DRU.

[0023] Figure 5 The following is a simulation waveform diagram of a DC fault in the MMC-HVDC system according to an embodiment of the present invention, where (a) is the output power of the MMC, (b) is the power flowing from the MMC to the DRU, (c) is the output power of the DRU, (d) is the DC current of the DRU-HVDC system, (e) is the AC current of the DRU, and (f) is the AC voltage of the DRU.

[0024] Figure 6 The following is a simulation waveform diagram of a DC fault in the DRU-HVDC system according to an embodiment of the present invention, wherein (a) is the DC voltage of the DRU-HVDC system, (b) is the power flowing from the MMC to the DRU, (c) is the DC voltage of the MMC-HVDC system, (d) is the DC current of the MMC-HVDC system, (e) is the AC current of the MMC, and (f) is the AC voltage of the MMC. Detailed Implementation

[0025] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] This embodiment provides a fault handling method for an offshore wind power parallel DC transmission system via DRU-MMC, which is applicable to systems such as... Figure 1The offshore wind power system shown is a DRU-MMC parallel DC transmission system. This system includes two offshore wind farms, one offshore DRU converter station, one offshore MMC converter station, and two onshore MMC converter stations. Offshore wind farm 1 is connected to the AC bus of the offshore DRU converter station via AC collecting submarine cable 1; offshore wind farm 2 is connected to the AC bus of the offshore MMC converter station via AC collecting submarine cable 2; a connecting submarine cable connects the AC buses of the offshore DRU converter station and the offshore MMC converter station; and a DC submarine cable connects the DC sides of the offshore DRU converter station and the offshore MMC converter station, transmitting power in parallel to different receiving-end onshore MMC converter stations. Offshore wind farm 1 consists of grid-connected wind turbines, while offshore wind farm 2 consists of grid-connected wind turbines. The offshore MMC converter station uses V / f control, and both onshore MMC converter stations use constant DC voltage control.

[0027] This embodiment addresses the fault handling method for the aforementioned system, including the following steps: (1) A DC energy dissipation device is installed at the DC outlet of the receiving end MMC converter station of the DRU-HVDC system and the MMC-HVDC system respectively; the DC energy dissipation device consists of multiple energy dissipation valves and an energy dissipation resistor connected in series, and the energy dissipation valve is composed of several half-bridge sub-modules connected in series.

[0028] (2) When an AC fault occurs in the receiving-end AC power grid, the DC energy-consuming devices of the DRU-HVDC system and the MMC-HVDC system are controlled by a DC energy-consuming device control strategy that includes DC voltage suppression control. Specifically: A1. Monitor the DC voltage at the DC output of the receiving-end MMC converter station. U dc ; A2. When DC voltage U dc Rise to the preset action threshold U dcH At that time, the DC power dissipation device at the DC outlet of the receiving-end MMC converter station is put into operation to dissipate the surplus power; A3. When DC voltage U dc Drop to the preset exit threshold U dcL At that time, the DC power consumption device at the DC output of the receiving-end MMC converter station is disconnected; A4. Repeat the above input and output steps until the DC voltage returns to a steady state.

[0029] The preset action threshold in this embodiment U dcH The preset exit threshold is 1.1 times the rated DC voltage. U dcL It is 1.02 times the rated DC voltage.

[0030] (3) When a DC fault occurs in the DRU-HVDC system, perform the following operations: 5ms after the fault occurs, lock the MMC converter station on the receiving end of the DRU side; 20ms after the MMC converter station on the receiving end of the DRU side is locked, lock the offshore wind farm 1; after a delay of 80ms, disconnect the AC circuit breaker of the offshore wind farm 1, the AC circuit breaker of the offshore DRU converter station, and the AC circuit breaker of the tie line near the offshore DRU converter station.

[0031] (4) such as Figure 3 As shown, when a DC fault occurs in the MMC-HVDC system, the following operations are performed: within 5ms after the fault occurs, the MMC converter stations at both ends of the faulty line are locked; 20ms after the MMC converter stations at both ends of the faulty line are locked, offshore wind farm 2 is locked; after a delay of 80ms, the AC circuit breaker of offshore wind farm 2, the AC circuit breaker of the offshore MMC converter station, and the AC circuit breaker of the tie line near the offshore MMC converter station are disconnected.

[0032] During DC fault ride-through, the DC power consumption devices controlling the DRU-HVDC system employ a control strategy that includes DC current suppression control and DC voltage suppression control, such as... Figure 2 As shown, DC current suppression control and DC voltage suppression control are executed in parallel, and triggering either control strategy will cause the DC power consumption device of the DRU-HVDC system to be put into operation.

[0033] The specific process of the DC current suppression control strategy is as follows: B1. Monitor the DC current injected into the receiving-end MMC converter station. I dc ; B2. When the DC current exceeds the preset action threshold I dcH At that time, the DC power dissipation device at the DC output of the receiving-end MMC converter station is put into operation to dissipate the surplus power and divert the current. B3. When the DC current drops back to the preset exit threshold I dcL At that time, the DC power consumption device at the DC output of the receiving-end MMC converter station is disconnected; B4. Repeat the above steps of adding and removing the current until the DC current returns to a steady state.

[0034] The preset action threshold in this embodiment I dcH The preset exit threshold is 1.4 times the rated DC current. I dcL It is 1.05 times the rated DC current.

[0035] To verify the effectiveness of the fault handling method of the present invention, we built a system in PSCAD / EMTDC as follows: Figure 1 The simulation model of the offshore wind power transmission system via DRU-MMC DC parallel transmission is shown, and simulation analysis of onshore AC system faults and DC faults is performed. The relevant parameters of the system are shown in Table 1. The rated frequency of the offshore AC system is 50Hz.

[0036] Table 1 (1) Onshore receiving-end AC grid fault: Assume the system has entered steady state before t=3.5 seconds. At t=3.5 seconds, a three-phase metallic ground fault occurs in the AC system at the receiving end of the DRU side. After 0.1 seconds, the fault is cleared, and the system response is as follows. Figure 4 As shown. During the fault, the power surplus caused the DRU DC voltage to rise rapidly; when the voltage reached 1.10 pu, the DC power dissipation device activated, effectively suppressing the voltage rise and maintaining it below 1.20 pu, as... Figure 4 As shown in (a) above. Simultaneously, the reduction in DRU output power will cause additional power to flow to the MMC via the submarine cable link, such as... Figure 4 As shown in (e) above. At this time, the active power-voltage amplitude controller of the MMC converter station will respond to the change in active power and actively increase the AC side voltage amplitude, as shown in... Figure 4 As shown in (f) in the diagram. Simultaneously, to maintain the reactive power balance of the marine AC system, the frequency of the marine AC system will also fluctuate. After the fault is cleared, the system gradually returns to a stable state.

[0037] (2) DC fault in MMC-HVDC system: Assuming the system is in steady state before t=3.495 seconds, a positive ground fault occurs at the DC outlet of the offshore MMC converter station at t=3.495 seconds. The system response is as follows: Figure 5 As shown. Before the MMC lockout and the grid-type wind turbine lockout, the power flowing from the tie line to the DRU surges, causing the DRU output power to increase, and the AC and DC currents to rise accordingly; when the DC current on the MMC side rises to 1.4 pu, the DC power dissipation device is activated, and when the current drops to 1.05 pu, the DC power dissipation device is deactivated. Figure 5 As can be seen in (d) during this period, the DC current on the DRU side will rise to 1.93 pu, while the DC current on the MMC side is successfully limited to within 1.50 pu, which verifies the effectiveness of the DC current suppression control strategy based on DC power consumption device of the present invention.

[0038] (3) DC fault in DRU-HVDC system: Assuming the system is in steady state before t=3.495 seconds, a positive ground fault occurs at the DC outlet of the offshore DRU converter station at t=3.495 seconds. The system response is as follows: Figure 6 As shown. In the initial stage of the fault (0~25ms), as the active power output from the DRU continues to decrease, some power from offshore wind farm 1 flows to the MMC converter station through the tie line, while the DC voltage of the DRU drops rapidly. Due to the strong coupling characteristics of the AC and DC side voltages of the DRU, this voltage drop effectively suppresses the rise in the voltage of the offshore AC system and the DC voltage of the MMC-HVDC system. After offshore wind farm 1 is blocked (25~105ms), the DRU still maintains some power transmission capacity, causing the power from offshore wind farm 2 to be fed back into the DRU through the tie line. At this time, the MMC converter station actively reduces the AC side voltage amplitude to maintain system stability, while the excess reactive power generated by the AC side filter of the DRU flows to the MMC converter station through the tie line. After the circuit breaker is disconnected, the offshore AC system retains only the grid-connected wind farm and the MMC converter station. At this time, the AC voltage amplitude and system frequency will return to the steady state value. Therefore, the MMC can achieve fault ride-through without additional control. Simulation results can also verify this. No protective blocking action occurred during the fault ride-through of the offshore MMC.

[0039] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A fault handling method for an offshore wind power parallel DC transmission system via DRU-MMC, characterized in that, Includes the following steps: (1) Install DC energy dissipation devices at the DC outlet of the onshore MMC converter station of each DRU-HVDC system and MMC-HVDC system; (2) When an AC fault occurs in the receiving-end AC power grid, a DC voltage suppression strategy is adopted to control the DC energy consumption devices of the DRU-HVDC system and the MMC-HVDC system respectively; (3) When a DC fault occurs in the DRU-HVDC system, the fault ride-through control strategy of the grid-connected wind farm-MMC DC system shall be executed; (4) When a DC fault occurs in the MMC-HVDC system, the fault ride-through control strategy of the grid-type wind farm-DRU DC system is executed.

2. The fault handling method for the offshore wind power parallel DC transmission system via DRU-MMC according to claim 1, characterized in that: The offshore wind power transmission system via DRU-MMC parallel DC transmission includes a first offshore wind farm based on grid-connected wind turbines, a second offshore wind farm based on grid-connected wind turbines, an offshore MMC converter station, an offshore DRU converter station, a first onshore MMC converter station, and a second onshore MMC converter station. The first offshore wind farm is connected to a first AC bus via a first AC collecting submarine cable. The second offshore wind farm is connected to a second AC bus via a second AC collecting submarine cable. The first AC bus is connected to the second AC bus via a connecting submarine cable. The AC side of the offshore DRU converter station is connected to the first AC bus via a converter transformer. The DC side of the offshore DRU converter station is connected to the first onshore MMC converter station via a first DC submarine cable. The DC side of the offshore MMC converter station is connected to the second AC bus via a converter transformer. The DC side of the offshore MMC converter station is connected to the DC side of the second onshore MMC converter station via a second DC submarine cable. The AC sides of the first and second onshore MMC converter stations are connected to their respective receiving-end AC power grids via converter transformers. The offshore MMC converter station uses V / f control, while both the first and second onshore MMC converter stations use constant DC voltage control. The connection between the offshore DRU converter station and the first onshore MMC converter station forms the DRU-HVDC system, and the connection between the offshore MMC converter station and the second onshore MMC converter station forms the MMC-HVDC system.

3. The fault handling method for the offshore wind power parallel DC transmission system via DRU-MMC according to claim 1, characterized in that: The specific implementation method of the fault ride-through control strategy of the grid-connected wind farm-MMC DC system in step (3) is as follows: 5ms after the fault occurs, the onshore MMC converter station in the DRU-HVDC system is locked. 20ms after the onshore MMC converter station is locked, the offshore wind farm connected to the DRU-HVDC system is locked. After a delay of 80ms, the AC circuit breaker of the offshore wind farm, the AC circuit breaker of the offshore DRU converter station, and the AC circuit breaker of the submarine cable of the connecting line near the offshore DRU converter station are disconnected.

4. The fault handling method for the offshore wind power parallel DC transmission system via DRU-MMC according to claim 1, characterized in that: The specific implementation method of the fault ride-through control strategy of the grid-type wind farm-DRU DC system in step (4) is as follows: within 5ms after the fault occurs, the offshore MMC converter station and the onshore MMC converter station in the MMC-HVDC system are locked; after the two MMC converter stations are locked for 20ms, the offshore wind farm connected to the MMC-HVDC system is locked; after a delay of 80ms, the AC circuit breaker of the offshore wind farm, the AC circuit breaker of the offshore MMC converter station, and the AC circuit breaker of the submarine cable of the connecting line near the offshore MMC converter station are disconnected; during the DC fault ride-through, the DC voltage suppression strategy and the DC current suppression strategy are used in parallel to control the DC energy consumption device of the DRU-HVDC system.

5. The fault handling method for the offshore wind power parallel DC transmission system via DRU-MMC according to claim 1 or 4, characterized in that: The specific implementation method of the DC voltage suppression strategy is as follows: A1. Monitor the DC voltage at the DC output of the onshore MMC converter station. U dc ; A2. When DC voltage U dc Rise to the preset action threshold U dcH At that time, the DC power dissipation device at the DC outlet of the onshore MMC converter station is put into operation to dissipate the surplus power; A3. When DC voltage U dc Drop to the preset exit threshold U dcL At that time, the DC power consumption device at the DC outlet of the onshore MMC converter station is disconnected; A4. Repeat the above input and output steps until the DC voltage is reached. U dc Return to steady state.

6. The fault handling method for the offshore wind power parallel DC transmission system via DRU-MMC according to claim 4, characterized in that: The specific implementation method of the DC current suppression strategy is as follows: B1. Monitor the DC current injected into the onshore MMC converter station. I dc ; B2. When DC current I dc Exceeding the preset action threshold I dcH At that time, the DC power dissipation device at the DC outlet of the onshore MMC converter station is put into operation to dissipate the surplus power and shunt it; B3. When direct current I dc Return to the preset exit threshold I dcL At that time, the DC power consumption device at the DC outlet of the onshore MMC converter station is disconnected; B4. Repeat the above input and output steps until the direct current is reached. I dc Return to steady state.

7. The fault handling method for the offshore wind power parallel DC transmission system via DRU-MMC according to claim 1, characterized in that: The DC power dissipation device consists of multiple power dissipation valves and a power dissipation resistor connected in series. The power dissipation valves are composed of several half-bridge sub-modules connected in series.

8. The fault handling method for the offshore wind power parallel DC transmission system via DRU-MMC according to claim 6, characterized in that: The action threshold I dcH The preset value is 1.4 times the rated DC current, and the exit threshold is reached. I dcL The preset value is 1.05 times the rated DC current.

9. The fault handling method for the offshore wind power parallel DC transmission system via DRU-MMC according to claim 5, characterized in that: The action threshold U dcH The preset value is 1.1 times the rated DC voltage, and the exit threshold is set. U dcL The preset value is 1.02 times the rated DC voltage.

Images