A control method for offshore wind power transmitted through multi-terminal flexible DC

Abstract

The present invention discloses a control method for offshore wind power transmission via multi-terminal flexible direct current. The present invention is used for an offshore wind power transmission via multi-terminal flexible direct current system, which includes an offshore wind farm, a wind power converter station, and a multi-terminal flexible direct current system. The control method adopted includes: adjusting the power instruction of the receiving-end converter station of the multi-terminal flexible direct current system according to the power evacuation control strategy of the wind farm; implementing the wind power converter station startup control strategy; when a DC fault occurs in the offshore wind power transmission via multi-terminal flexible direct current system, starting the multi-terminal flexible direct current system DC fault handling strategy and the wind power converter station DC fault ride-through strategy. The present invention effectively improves the stability, reliability, and flexibility of the offshore wind power transmission via multi-terminal flexible direct current system, while reducing the impact of DC faults on the offshore wind power transmission via multi-terminal flexible direct current system, and enhancing the adaptability and recovery capability of the offshore wind power transmission via multi-terminal flexible direct current system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission and distribution of electric power systems, and specifically relates to a control method for transmitting offshore wind power via multi-terminal flexible direct current (DC) transmission. Background Art

[0002] Offshore wind power faces a series of challenges during grid connection and transmission. In particular, in the process of large-scale wind power grid connection and efficient transmission, how to ensure system stability, improve wind power transmission capacity and fault tolerance has become an urgent problem to be solved.

[0003] Traditional AC transmission suffers from inherent drawbacks such as high transmission losses and poor stability in long-distance, high-capacity scenarios. Flexible DC transmission technology, with its advantages of low manufacturing complexity, low switching losses, and high waveform quality, has become an effective means of integrating offshore wind power into the grid. Multi-terminal flexible DC transmission systems (MTVDCs), with their multi-point access and flexible power allocation, can flexibly distribute power to multiple onshore receiving nodes, making them a preferred solution for clustered offshore wind power transmission.

[0004] However, current multi-terminal flexible direct current transmission systems still face numerous challenges in integrating offshore wind power into the grid. First, there is a certain risk of system overload. Due to the volatile power output of wind farms, improper control can lead to system overload and affect grid stability. Second, offshore wind power transmission via multi-terminal flexible direct current transmission systems also faces the challenge of DC fault isolation and recovery. Once a DC fault occurs, how to quickly and effectively locate and isolate the fault and restore system operation is key to improving system reliability. Furthermore, it is crucial to consider how to rationally dispatch the power of each converter station under different wind power output conditions to ensure load matching during system operation and avoid overload or power shortages.

[0005] Therefore, for offshore wind power transmission systems via multi-terminal flexible direct current (DC / DC) systems, a universal and effective control method is urgently needed to optimize the system's power control, fault handling, and converter station coordination mechanism, improve the system's stability, reliability, and adaptability, and achieve efficient transmission of offshore wind power. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the defects of the above-mentioned existing technologies and provide a control method for offshore wind power transmission via multi-terminal flexible direct current, which realizes offshore wind power power evacuation while avoiding system overload, effectively improves the stability, reliability and flexibility of the offshore wind power transmission system via multi-terminal flexible direct current, enhances the adaptability and recovery capability of the transmission system, and realizes highly reliable transmission of offshore wind power via multi-terminal flexible direct current.

[0007] To this end, the present invention adopts the following technical solution: a control method for offshore wind power transmission via multi-terminal flexible direct current, which is used in an offshore wind power transmission system via multi-terminal flexible direct current, the system comprising an offshore wind farm, a wind power converter station and a multi-terminal flexible direct current system, the offshore wind farm being grid-connected via the wind power converter station, the wind power converter station being connected to the DC bus of a receiving-end converter station in the multi-terminal flexible direct current system via a DC line, the receiving-end converter station adopting constant power control; one of the other receiving-end converter stations adopting constant voltage control, and the remaining receiving-end converter stations adopting constant power control; the control method comprises:

[0008] Step 1) Adjust the power command of the receiving-end converter station of the multi-terminal flexible direct current system according to the wind farm power evacuation control strategy;

[0009] Step 2) Implement the wind power converter station startup control strategy;

[0010] Step 3) When a DC fault occurs in the offshore wind power transmission system through the multi-terminal flexible DC system, the DC fault handling strategy of the multi-terminal flexible DC system and the DC fault ride-through strategy of the wind power converter station are activated;

[0011] The multi-terminal flexible DC system fault handling strategy achieves rapid recovery of the multi-terminal flexible DC system through the four steps of "blocking all receiving converter stations - fault line selection - fault isolation - restart";

[0012] The DC fault ride-through strategy of the wind power converter station achieves lock-free fault ride-through of the wind power converter station through four processes: "circuit breaker disconnection - DC energy consumption device voltage stabilization - power limitation - reconnection".

[0013] Furthermore, in step 1), the wind farm power evacuation control strategy is as follows: in order to prevent the DC line from being overloaded after the offshore wind farm is connected to the multi-terminal flexible DC system, the constant power receiving end converter station needs to manually adjust the power command to achieve controllable evacuation of wind power, and the constant voltage receiving end converter station automatically balances the system power under wind power fluctuations; when the constant voltage receiving end converter station exits operation due to a fault, a constant power receiving end converter station is required to take over the constant voltage to achieve constant voltage control.

[0014] Furthermore, the power command value of the constant power receiving converter station must meet the following constraints:

[0015]

[0016] Where, P WT,N is the rated output power of the offshore wind farm; P sum is the sum of the power command values adjusted by the fixed power receiving converter station; S ni and P refi Respectively iThe rated capacity of the fixed power receiving converter station and the adjusted power command value, i =1,2,3,4;S n5 is the rated capacity of the constant voltage receiving-end converter station; S nc and P refc They are respectively the rated capacity and power command values of the constant power receiving end converter station that takes over the constant voltage after the constant voltage receiving end converter station stops operating for some reason.

[0017] Furthermore, in step 2), the wind power converter station startup control strategy is as follows: when the wind power converter station is started, the wind power converter station first completes the pole connection and closes the DC circuit breaker of the wind power converter station; then the receiving-end converter station connected to the wind power converter station controls the bus interval DC resonant switch, and the multi-terminal flexible DC system provides charging energy for the wind power converter and the DC cable, and the wind power converter and the DC cable automatically enter the startup process.

[0018] Furthermore, in step 3), if a DC fault occurs on a wind power branch, the DC circuit breaker on the wind power converter station side is quickly disconnected, and the faulty branch is cut off before the protection of the receiving-end converter station is activated. Then the wind power converter station is locked, and the receiving-end converter station connected to the wind power converter station opens the lead-out line interval DC resonant switch.

[0019] Furthermore, in step 3), if a DC fault occurs in the multi-terminal flexible DC system, the protection of each station is activated, all receiving-end converter stations are locked, the wind farm converter station is not locked and the DC circuit breaker is opened, and the DC energy consumption device on the DC side of the wind power converter station is automatically activated according to the DC voltage overvoltage level of the wind power converter station to maintain the operation of the wind farm and the wind power converter station.

[0020] Furthermore, in the DC fault handling strategy of the multi-terminal flexible DC system in step 3),

[0021] All receiving-end converter stations are locked out: when a DC fault occurs in a multi-terminal flexible DC system, the protection of each receiving-end converter station is activated, and all receiving-end converter stations are locked out. The wind power converter station is not locked out, and the circuit breaker on the DC side of the wind power converter station is disconnected.

[0022] Fault line selection means that after a DC fault occurs, each receiving converter station determines the fault location based on the DC current changes, including the terminal DC lines, tie lines, and DC busbars of the multi-terminal flexible DC system.

[0023] Fault isolation means: determining the location of the DC fault by fault line selection. If the fault occurs on the terminal DC line of the multi-terminal flexible DC system, the receiving converter station at the fault line terminal will be shut down and the circuit breakers at both ends of the faulty line will be disconnected. If the fault occurs on the tie line of the multi-terminal flexible DC system, the receiving converter stations at both ends of the tie line will perform fault line selection and disconnect the circuit breakers at both ends of the tie line, dividing the multi-terminal flexible DC system into two sections for operation. If the fault occurs on the DC busbar of the multi-terminal flexible DC system, the corresponding DC switches of the lines connected to the DC busbar will be disconnected to isolate the fault.

[0024] Restart means: when a DC fault occurs on the terminal DC line or DC bus of the multi-terminal flexible DC system, it is determined whether there is a receiving-end converter station with constant voltage control in the intact system after fault isolation. If not, the remaining receiving-end converter stations will take over the constant voltage control through cross-communication, and the receiving-end converter station with the largest capacity has the highest priority; when a DC fault occurs on the interconnection line of the multi-terminal flexible DC system, the multi-terminal flexible DC system will be divided into two areas, and the receiving-end converter stations in the area connected to the wind power branch will take over the constant voltage control through cross-communication, and the receiving-end converter station with the largest capacity in the area will have priority to take over; then, the multi-terminal flexible DC system will first unlock the locked voltage receiving-end converter station to establish DC voltage, and then unlock other fixed-power receiving-end converter stations that have not exited.

[0025] Furthermore, if the multi-terminal flexible DC system is divided into two areas after fault isolation, the constant power receiving-end converter station in the area connected to the wind power branch needs to increase the constant power instruction value according to the power output after the wind farm is restored, so as to avoid overload of the constant voltage receiving-end converter station in the area.

[0026] Furthermore, in the wind power converter station DC fault ride-through strategy of step 3),

[0027] Breaker disconnection means that when a DC fault occurs, the wind power converter station does not lock and opens the DC circuit breaker on the wind power branch line;

[0028] The voltage stabilization of DC energy consumption devices means that when the DC circuit breaker of the wind power branch is disconnected, the DC energy consumption devices on the DC side of the wind power converter station are automatically put into operation according to the DC voltage overvoltage level of the wind power converter station, thereby maintaining the operation of the wind farm and the wind power converter station;

[0029] Power limitation means that when an N-1 DC fault occurs on the DC side of a multi-terminal flexible DC system, the system's carrying capacity may fall below the rated power of the wind farm. Therefore, wind farm power limitation is required to prevent the multi-terminal flexible DC system from overloading.

[0030] Reconnection means: after the DC voltage of the multi-terminal flexible DC system is restored, the wind power converter station is informed through communication, and the wind power converter station then controls the DC circuit breaker to restore the power output of the wind farm; if no signal is received within the set time, the wind power converter station actively locks the converter, the wind farm stops operating, and the receiving-end converter station connected to the wind power converter station opens the lead-out line interval resonant switch.

[0031] Furthermore, in step 3), the power limitation shall be implemented in the following manner to avoid overload of the multi-terminal flexible DC system: when an N-1 fault occurs on the DC side of the multi-terminal flexible DC system, the protection in the wind power converter station senses the fault and controls the DC circuit breaker to disconnect. At this time, the control and protection system of the receiving-end converter station withdraws the wind power collection lines one by one according to the power situation until the wind farm inputs the maximum active power of the multi-terminal flexible DC system. P max Satisfies the following formula: .

[0032] Based on the above technical solution, the present invention has the following beneficial effects:

[0033] The present invention effectively improves the stability, reliability and flexibility of the offshore wind power multi-terminal flexible direct current transmission system, while reducing the impact of faults on the transmission system, enhancing the adaptability and recovery capability of the transmission system, and providing key technical support for the access of a high proportion of new energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a structural diagram of the offshore wind power multi-terminal flexible direct current transmission system;

[0035] Figure 2 This is a flow chart of the control method for offshore wind power transmission via multi-terminal flexible direct current transmission according to the present invention;

[0036] Figure 3 This is a flow chart of the DC fault handling strategy for the multi-terminal flexible DC system of the present invention;

[0037] Figure 4 This is a flow chart of the DC fault ride-through strategy for a wind power converter station according to the present invention;

[0038] Figure 5 This is a schematic diagram of the system power flow when the wind farm is connected at full power according to an embodiment of the present invention;

[0039] Figure 6 The DC circuit of the embodiment of the present invention Power waveform of wind power converter station during fault;

[0040] Figure 7 The DC circuit of the embodiment of the present invention DC voltage waveform of wind power converter station during fault;

[0041] Figure 8The DC circuit of the embodiment of the present invention Schematic diagram of system power flow after fault exit;

[0042] Figure 9 The DC circuit of the embodiment of the present invention Schematic diagram of system power flow after fault exit. DETAILED DESCRIPTION

[0043] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific implementation methods.

[0044] This embodiment provides a control method for offshore wind power transmission via multi-terminal flexible direct current. The method is applied to offshore wind power transmission via multi-terminal flexible direct current system. Figure 1 As shown, the system includes an offshore wind farm, a wind turbine converter station MMC6, five receiving converter stations, and five AC power grids. The five receiving converter stations are the first receiving converter station MMC1, the second receiving converter station MMC2, the third receiving converter station MMC3, the fourth receiving converter station MMC4, and the fifth receiving converter station MMC5. The five AC power grids are the first AC power grid S1, the second AC power grid S2, the third AC power grid S3, the fourth AC power grid S4, and the fifth AC power grid S5.

[0045] The offshore wind farm WT includes n wind power collection lines; the first to fourth receiving-end converter stations MMC1-4 adopt fixed active power control, the fifth receiving-end converter station MMC5 adopts fixed DC voltage control, and the wind power converter station MMC6 adopts island control; the offshore wind farm is connected to the grid through the wind power converter station MMC6.

[0046] The DC side of the wind power converter station MMC6 is equipped with a DC starting resistor, a bypass switch, a DC circuit breaker DCCB and a DC energy consumption device; the DC busbar interval of the first receiving converter station MMC1 is equipped with a DC resonant switch; the wind power converter station MMC6 is connected to the DC line The first receiving-end converter station MMC1 is connected to the DC bus, the second receiving-end converter station MMC2 and the fourth receiving-end converter station MMC4 are connected to the DC bus through the DC line and The third receiving terminal converter station MMC3 and the fifth receiving terminal converter station MMC5 are connected to the DC bus of the first receiving terminal converter station MMC1; and Connected to the DC bus of the fourth receiving converter station MMC4; DC line - DC circuit breakers are configured at both ends of the line; the AC side of the receiving-end converter station is connected to the AC grid, that is, the AC side of the first receiving-end converter station MMC1 is connected to the first AC grid S1, and so on.

[0047] The control method for offshore wind power transmission via multi-terminal flexible direct current is as follows: Figure 2 As shown, the steps are as follows:

[0048] Step (1): Adjust the power instructions of the converter station of the multi-terminal flexible direct current system according to the power evacuation control strategy of the wind farm.

[0049] Specifically, the implementation method of the wind farm power evacuation control strategy is as follows: in order to prevent the DC line from being overloaded after the wind farm is connected to the multi-terminal flexible DC system, the constant power receiving end converter station needs to manually adjust the power instruction to achieve controllable evacuation of wind power, and the constant voltage receiving end converter station can automatically balance the system power under the condition of wind power fluctuations; when the constant voltage receiving end converter station exits operation due to a fault, a constant power receiving end converter station is required to take over the constant voltage to achieve constant voltage control.

[0050] The power command value of the constant power receiving converter station must meet the following constraints:

[0051]

[0052] Where, P WT,N is the rated output power of the offshore wind farm; P sum is the sum of the power command values adjusted by the fixed power receiving converter station; S ni and P refi Respectively i The rated capacity of the fixed power receiving converter station and the adjusted power command value, i =1,2,3,4;S n5 is the rated capacity of the constant voltage receiving-end converter station; S nc and P refc They are the rated capacity and power command value of the constant power receiving end converter station that takes over the constant voltage after the constant voltage receiving end converter station is out of operation for some reason. Before the multi-terminal flexible DC system is connected to the wind power branch, its DC line ~ The transmission capacity is S n2 、S n3 、S n1 +S n2 、S n5 .

[0053] Step (2): Implement the wind power converter station startup control strategy.

[0054] Specifically, the implementation method of the wind power converter station startup control strategy is as follows: when the wind power converter station is started, the wind power converter station first completes the pole connection and closes the wind power converter station DC circuit breaker; then the first receiving-end converter station MMC1 controls the busbar interval DC resonant switch to charge the wind power converter and DC cable as a whole and automatically enters the startup process.

[0055] Step (3): When a DC fault occurs in the offshore wind power transmission system through the multi-terminal flexible DC transmission system, the DC fault handling strategy of the multi-terminal flexible DC system and the DC fault ride-through strategy of the wind power converter station are activated. The DC fault handling strategy of the multi-terminal flexible DC system achieves rapid recovery of the multi-terminal flexible DC system through the four processes of "five-station blocking - fault line selection - fault isolation - restart". The DC fault ride-through strategy of the wind power converter station achieves fault ride-through without blocking of the wind power converter station through the four processes of "circuit breaker disconnection - DC energy consumption stabilization - power limitation - reconnection".

[0056] In step (3), the DC fault handling strategy of the multi-terminal flexible DC system is as follows: Figure 3 shown.

[0057] If a DC fault occurs on the wind power branch, the DC circuit breaker on the wind power converter station side will be quickly disconnected, and the faulty branch will be cut off before the protection of the receiving end converter station is activated. Then the wind power converter station will be locked, and the first receiving end converter station MMC1 connected to the wind power converter station will open the DC resonant switch between the lead lines.

[0058] If a DC fault occurs in a multi-terminal flexible DC system, the protection of each station will be activated, all receiving-end converter stations will be locked, but the wind farm converter station will not be locked and the DC circuit breaker will be opened. The DC energy consumption device on the DC side of the wind power converter station will be automatically activated according to the DC voltage overvoltage level of the wind power converter station to maintain the operation of the wind farm and wind power converter station.

[0059] Specifically, the five-station locking process in the DC fault handling strategy of the multi-terminal flexible DC system refers to: when a DC fault occurs in the multi-terminal flexible DC system, the protection of each receiving-end converter station is activated and all receiving-end converter stations are locked; the wind power converter station is not locked, and the circuit breaker on the DC side of the wind power converter station is disconnected.

[0060] Specifically, the fault line selection process in the DC fault handling strategy of the multi-terminal flexible DC system refers to: after a DC fault occurs, each receiving-end converter station determines the location of the fault based on the change in DC current, including the terminal DC line of the multi-terminal flexible DC system, the interconnecting line of the multi-terminal flexible DC system and the DC busbar of the multi-terminal flexible DC system.

[0061] Specifically, the fault isolation process in the DC fault handling strategy of the multi-terminal flexible DC system refers to: determining the location of the DC fault by fault line selection. If the fault occurs in the DC line of the terminal ( 、 、 ), the receiving converter station at the fault line terminal will be shut down and the circuit breakers at both ends of the fault line will be disconnected; if the fault occurs on the tie line If the fault occurs on the DC bus, the receiving-end converter station will select the fault line and then disconnect the circuit breakers at both ends of the interconnecting line, and the multi-terminal flexible DC system will be divided into two areas for operation; if the fault occurs on the DC bus, the corresponding DC switch of the line connected to the DC bus will be opened to isolate the fault.

[0062] Specifically, the restart process in the DC fault handling strategy of the multi-terminal flexible DC system refers to: when a DC fault occurs in the DC line of the terminal or the DC busbar of the receiving-end converter station, it is determined whether there is a constant voltage converter station in the intact system after fault isolation. If not, the remaining receiving-end converter stations will achieve constant voltage takeover through cross-communication, and the receiving-end converter stations with larger capacity have higher priority; when a DC fault occurs in the interconnecting line, the multi-terminal flexible DC system will be divided into two areas, and the receiving-end converter stations in the area connected to the wind power branch will achieve constant voltage takeover through cross-communication, and the receiving-end converter station with the largest capacity in the area will have priority to take over. Subsequently, the multi-terminal flexible DC system first unlocks the locked voltage receiving-end converter station to establish DC voltage, and then unlocks other fixed power receiving-end converter stations that have not exited.

[0063] If the multi-terminal flexible DC system is divided into two areas after fault isolation, the constant power receiving-end converter station in the area connected to the wind power branch needs to increase the constant power command value according to the power output after the wind farm is restored, so as to avoid overload of the constant voltage receiving-end converter station in the area.

[0064] In step (3), the DC fault ride-through strategy of the wind power converter station is as follows: Figure 4 shown.

[0065] Specifically, the circuit breaker disconnection process in the DC fault ride-through strategy of the wind power converter station refers to: when a DC fault occurs, the wind power converter station MMC6 does not lock and opens the wind power branch DC circuit breaker.

[0066] Specifically, the DC energy consumption device voltage stabilization process in the DC fault ride-through strategy of the wind power converter station refers to: when the DC circuit breaker of the wind power branch is disconnected, the DC energy consumption device on the DC side of the wind power converter station is automatically put into operation according to the DC voltage overvoltage level of the wind power converter station, thereby maintaining the operation of the wind farm and the wind power converter station.

[0067] Specifically, the power limitation process in the DC fault ride-through strategy of the wind power converter station refers to: when an N-1 DC fault occurs on the DC side of the multi-terminal flexible DC system, it may cause the system carrying capacity to be lower than the rated power of the wind farm, and the wind farm power needs to be limited to avoid system overload.

[0068] When an N-1 fault occurs on the DC side of the multi-terminal flexible DC system, the protection in the wind power converter station can sense the fault and control the DC circuit breaker to disconnect. At this time, the converter station control and protection system will exit the wind power collection line one by one according to the power situation until the wind farm inputs the maximum active power of the multi-terminal flexible DC system. P max Satisfies the following formula:

[0069] .

[0070] Specifically, the reconnection process within the wind converter station DC fault ride-through strategy involves waiting for the DC voltage of the multi-terminal flexible DC system to recover, notifying the wind converter station via communication. The wind converter station then controls the DC circuit breaker to restore power delivery from the wind farm. If no signal is received within a set time, the wind converter station actively locks the converter, shutting down the wind farm. The first receiving-end converter station, MMC1, opens the resonant switch between the outgoing conductors.

[0071] In order to verify the effectiveness of the control method of the present invention, the following Figure 1 The simulation model of the offshore wind power transmission system through multi-terminal flexible direct current is shown. The offshore wind farm is connected to the wind power converter station MMC6 via four 66kV three-core submarine cables, transmitting 56MW, 56MW, 70MW, and 70MW of wind power respectively, with a total installed capacity of 252MW. The parameters of each receiving converter station are shown in Table 1:

[0072] Table 1

[0073]

[0074] According to the wind farm power evacuation control strategy of the present invention, the power command constraint of the fixed power converter station is performed, and a feasible operation mode is obtained: the first receiving end converter station MMC1 is set to absorb active power 60MW, the second receiving end converter station MMC2 is set to absorb active power 60MW, the third receiving end converter station MMC3 is set to absorb active power 50MW, the fourth receiving end converter station MMC4 is set to absorb active power 60MW, and the fifth receiving end converter station MMC5 uses constant voltage control to balance the system power. Then the system power flow when the wind farm is connected at full power is as follows: Figure 5 shown.

[0075] Assume that the offshore wind power is transmitted through the multi-terminal flexible direct current transmission system and a DC line occurs at t=3s. l 5 single-pole grounding fault, after the fault occurs, the protection of each station is activated, the five receiving-end converter stations are locked, the new energy station is not locked and opens the DC circuit breaker and starts energy consumption; the fifth receiving-end converter station MMC5 opens the DC circuit breaker. Then the fourth receiving-end converter station MMC4 and the fifth receiving-end converter station MMC5 determine that the DC line Fault, the fourth receiving converter station MMC4 opens the corresponding DC switch. Considering the line fault, one 66kV collector line will be synchronously exited, and the total wind power power will not exceed 196MW. After the fifth receiving converter station MMC5 exits, the fourth receiving converter station MMC4 will take over at a constant voltage. After the fault is isolated, the fourth receiving converter station MMC4 will be unlocked first, and then the other receiving converter stations will be unlocked respectively. After the DC voltage of the multi-terminal flexible DC system is restored, the wind power converter station MMC6 will close the DC circuit breaker and resume power transmission. DC line The power and DC voltage waveforms of the wind power converter station during a fault are as follows: Figure 6 and Figure 7 As shown, Figure 6 In the equation, Ps6 represents the active power input to the wind power converter station, and Qs6 represents the reactive power input to the wind power converter station; Figure 7 In the figure, Udc6 represents the DC side voltage of the wind power converter station, and the horizontal axis t represents seconds; The system flow after the failure exit is as follows Figure 8 shown.

[0076] Assume that the offshore wind power is transmitted through the multi-terminal flexible direct current transmission system and a DC line occurs at t=3s. Single-pole ground fault, after the fault occurs, the protection of each station is activated, the five receiving-end converter stations are locked, and the new energy station is not locked and opens the DC circuit breaker and starts energy consumption. Then the first receiving-end converter station MMC1 and the fourth receiving-end converter station MMC4 determine that the DC line Fault, the first receiving end converter station MMC1 and the fourth receiving end converter station MMC4 open the corresponding DC switches, the offshore wind power is divided into two areas through the multi-terminal flexible direct current transmission system, and the first receiving end converter station MMC1 takes over the constant voltage. Considering the line fault, one 66kV collector line will be synchronously withdrawn, and the total wind power power will not exceed 196MW. Due to the line After the exit, the multi-terminal flexible direct current system is divided into two areas. To prevent the offshore wind power from being overloaded when it is transmitted through the multi-terminal flexible direct current system, the power command value of the second receiving converter station MMC2 needs to be increased to 98MW. After the fault is isolated, the first receiving converter station MMC1 and the fourth receiving converter station MMC4 are unlocked first, followed by the unlocking of the other receiving converter stations. The new energy station closes the DC circuit breaker and restores power transmission. DC line The system flow after the failure exit is as follows Figure 9 shown.

[0077] The above description of the embodiments is intended to facilitate understanding and application of the present invention by those skilled in the art. It is apparent that 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 requiring creative effort. Therefore, the present invention is not limited to the above embodiments. Any improvements or modifications made by those skilled in the art based on the disclosure of the present invention should fall within the scope of protection of the present invention.

Claims

1. A control method for offshore wind power transmission via multi-terminal flexible direct current (DC) transmission, which is used in an offshore wind power transmission system via multi-terminal flexible DC transmission. The system includes an offshore wind farm, a wind power converter station, and a multi-terminal flexible DC system. The offshore wind farm is grid-connected via the wind power converter station, which is connected to the DC busbar of a receiving-end converter station in the multi-terminal flexible DC system via a DC line. The receiving-end converter station adopts constant power control. One of the other receiving-end converter stations adopts constant voltage control, and the other receiving-end converter stations adopt constant power control; the feature is that: The control method includes: Step 1) Adjust the power command of the receiving-end converter station of the multi-terminal flexible direct current system according to the wind farm power evacuation control strategy; Step 2) Implement the wind power converter station startup control strategy; Step 3) When a DC fault occurs in the offshore wind power transmission system through the multi-terminal flexible DC system, the DC fault handling strategy of the multi-terminal flexible DC system and the DC fault ride-through strategy of the wind power converter station are activated; The multi-terminal flexible DC system fault handling strategy achieves rapid recovery of the multi-terminal flexible DC system through the four steps of "blocking all receiving converter stations - fault line selection - fault isolation - restart"; The wind power converter station DC fault ride-through strategy achieves lockout-free fault ride-through through the four steps of "circuit breaker disconnection - DC energy consumption device voltage stabilization - power limitation - reconnection"; In the multi-terminal flexible DC system DC fault handling strategy of step 3), All receiving-end converter stations are locked: When a DC fault occurs in a multi-terminal flexible DC system, the protection of each receiving-end converter station is activated and each station is locked. However, the wind power converter station is not locked and the circuit breaker on the DC side of the wind power converter station is disconnected. Fault line selection: After a DC fault occurs, each receiving converter station determines the fault location based on the DC current changes. Fault isolation: The location of the DC fault is determined through fault line selection. If the fault occurs on the terminal DC line of the multi-terminal flexible DC system, the receiving converter station at the faulty line terminal will be shut down. If the fault occurs on the tie line of the multi-terminal flexible DC system, the receiving converter stations at both ends of the tie line will perform fault line selection and disconnect the circuit breakers at both ends of the tie line. If the fault occurs on the DC busbar of the multi-terminal flexible DC system, the corresponding DC switches of the lines connected to the DC busbar will be disconnected to isolate the fault. Restart: When a DC fault occurs on a terminal DC line or DC bus, determine whether there is a receiving-end converter station with constant voltage control in the intact system after fault isolation. If not, the remaining receiving-end converter stations will take over the constant voltage control through cross-communication; when a DC fault occurs on the interconnection line, the multi-terminal flexible DC system will be divided into two areas, and the receiving-end converter stations in the area connected to the wind power branch will take over the constant voltage control through cross-communication.

2. The control method according to claim 1, characterized in that: In step 1), the wind farm power evacuation control strategy is as follows: to prevent the DC line from being overloaded after the offshore wind farm is connected to the multi-terminal flexible DC system, the constant power receiving end converter station needs to manually adjust the power command to achieve controllable evacuation of wind power, and the constant voltage receiving end converter station automatically balances the system power under wind power fluctuations; when the constant voltage receiving end converter station exits operation due to a fault, a constant power receiving end converter station is required to take over the constant voltage to achieve constant voltage control.

3. The control method according to claim 2, characterized in that: The power command value of the fixed power receiving converter station must meet the following constraints: Where, P WT,N is the rated output power of the offshore wind farm; P sum is the sum of the power command values adjusted by the fixed power receiving converter station; S ni and P refi Respectively i The rated capacity of the fixed power receiving converter station and the adjusted power command value, i =1,2,3,4;S n5 is the rated capacity of the constant voltage receiving-end converter station; S nc and P refc They are respectively the rated capacity and power command values of the constant power receiving end converter station that takes over the constant voltage after the constant voltage receiving end converter station stops operating for some reason.

4. The control method according to claim 1, wherein: In step 2), the wind power converter station startup control strategy is as follows: when the wind power converter station is started, the wind power converter station first completes the pole connection and closes the DC circuit breaker of the wind power converter station; then the receiving-end converter station connected to the wind power converter station controls the busbar interval DC resonant switch to close, and the multi-terminal flexible DC system provides charging energy for the wind power converter and the DC cable, and the wind power converter and the DC cable automatically enter the startup process.

5. The control method according to claim 1, characterized in that: In step 3), if a DC fault occurs on a wind power branch, the DC circuit breaker on the wind power converter station side is quickly disconnected, and the faulty branch is cut off before the protection of the receiving-end converter station is activated. Then the wind power converter station is locked, and the receiving-end converter station connected to the wind power converter station opens the lead-out line interval DC resonant switch.

6. The control method according to claim 1, characterized in that: In step 3), if a DC fault occurs in the multi-terminal flexible DC system, the protection of each station is activated, all receiving-end converter stations are locked, the wind farm converter station is not locked and the DC circuit breaker is opened, and the DC energy consumption device on the DC side of the wind power converter station is automatically activated according to the DC voltage overvoltage level of the wind power converter station to maintain the operation of the wind farm and the wind power converter station.

7. The control method according to claim 1, characterized in that: If the multi-terminal flexible DC system is divided into two areas after fault isolation, the constant power receiving-end converter station in the area connected to the wind power branch needs to increase the constant power command value according to the power output after the wind farm is restored, so as to avoid overload of the constant voltage receiving-end converter station in the area.

8. The control method according to claim 1, characterized in that: In the wind power converter station DC fault ride-through strategy of step 3), Breaker disconnection means that when a DC fault occurs, the wind power converter station does not lock and opens the DC circuit breaker on the wind power branch line; DC energy consumption device voltage stabilization means that when the DC circuit breaker of the wind power branch is disconnected, the DC energy consumption device on the DC side of the wind power converter station is automatically put into operation according to the DC voltage overvoltage level of the wind power converter station, thereby maintaining the operation of the wind farm and wind power converter station; Power limitation means that when an N-1 DC fault occurs on the DC side of a multi-terminal flexible DC system, the system's carrying capacity may fall below the rated power of the wind farm. Therefore, wind farm power limitation is required to prevent the multi-terminal flexible DC system from overloading. Reconnection means: after the DC voltage of the multi-terminal flexible DC system is restored, the wind power converter station is informed through communication, and the wind power converter station then controls the DC circuit breaker to restore the power output of the wind farm; if no signal is received within the set time, the wind power converter station actively locks the converter, the wind farm stops operating, and the receiving-end converter station connected to the wind power converter station opens the lead-out line interval resonant switch.

9. The control method according to claim 3, characterized in that: In step 3), the power limitation shall be implemented in the following way to avoid overload of the multi-terminal flexible DC system: when an N-1 fault occurs on the DC side of the multi-terminal flexible DC system, the protection in the wind power converter station senses the fault and controls the DC circuit breaker to disconnect. At this time, the control and protection system of the receiving-end converter station withdraws the wind power collection lines one by one according to the power situation until the wind farm inputs the maximum active power of the multi-terminal flexible DC system. P max Satisfies the following formula: .

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