Control method for flexibly and directly sending out offshore wind power through multiple terminals

Abstract

The invention discloses a control method for flexibly and directly sending out offshore wind power through multiple terminals. The control method is used for an offshore wind power multi-terminal flexible direct output system, the system comprises an offshore wind power plant, a wind power converter station and a multi-terminal flexible direct system, and the adopted control method comprises the steps that a power instruction of the converter station at the receiving end of the multi-terminal flexible direct system is adjusted according to a wind power plant power evacuation control strategy; implementing a wind power converter station starting control strategy; and when the offshore wind power passes through the multi-terminal flexible direct-current output system and has a direct-current fault, a multi-terminal flexible direct-current system direct-current fault processing strategy and a wind power converter station direct-current fault ride-through strategy are started. According to the invention, the stability, reliability and flexibility of the multi-terminal flexible and direct delivery system of the offshore wind power are effectively improved, the influence of a direct current fault on the multi-terminal flexible and direct delivery system of the offshore wind power is reduced, and the adaptability and recovery capability of the multi-terminal flexible and direct delivery system of the offshore wind power are enhanced.

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 sending offshore wind power via multi-terminal flexible direct current. Background Art

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

[0003] The traditional AC transmission mode has inherent defects such as high transmission loss and poor stability in long-distance and large-capacity scenarios. Flexible DC transmission technology has the characteristics of low manufacturing difficulty, low switching loss and high waveform quality, and has become an effective way to connect offshore wind power to the grid. Among them, the multi-terminal flexible DC transmission system (referred to as the multi-terminal flexible DC system) can flexibly distribute electric energy to multiple onshore receiving nodes with its multi-point access and flexible power distribution characteristics, becoming the preferred solution for clustered transmission of offshore wind power.

[0004] However, the current multi-terminal flexible direct current transmission system still faces many problems in the process of offshore wind power grid connection. First, there is a certain risk of system overload. Since the power output of wind farms is volatile, if it is not properly controlled, it may cause system overload and affect the stability of the power grid. Secondly, the offshore wind power transmission system through multi-terminal flexible direct current also has the problem 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 the key to improving system reliability. In addition, it is also necessary to focus on how to reasonably 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 shortage.

[0005] Therefore, for offshore wind power transmission systems via multi-terminal flexible direct current (DC / DC) transmission, 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 prior art 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 capacity of the transmission system, so as to realize high-reliability 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 sending offshore wind power through multi-terminal flexible direct current, which is used for an offshore wind power sending system through a 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 is connected to the grid through the wind power converter station, the wind power converter station is connected to the DC bus of a receiving-end converter station in the multi-terminal flexible direct current system through 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 control method comprises: 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 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 "locking all receiving-end converter stations - fault line selection - fault isolation - restart"; The DC fault ride-through strategy of the wind power converter station achieves the wind power converter station fault ride-through without locking through the four processes of "circuit breaker disconnection - DC energy consumption device voltage stabilization - power limitation - reconnection".

[0008] Furthermore, in the 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 control to achieve constant-voltage control.

[0009] Furthermore, the power command value of the fixed power receiving converter station must meet the following constraints:

[0010] In the formula, 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 end 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 exits operation for some reason.

[0011] Furthermore, in the 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.

[0012] Furthermore, in the 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 receiving-end converter station protection 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.

[0013] Furthermore, in the 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.

[0014] Furthermore, in the DC fault handling strategy of the multi-terminal flexible DC system in step 3), All receiving-end converter stations are locked, which means that 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; Fault line selection means that after a DC fault occurs, each receiving-end converter station determines the location of the fault by the change of 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; Fault isolation means: the location of the DC fault is determined by fault line selection. If the fault occurs on the terminal DC line of the multi-terminal flexible DC system, the receiving-end converter station at the terminal of the faulty line will be shut down and the circuit breakers at both ends of the faulty line will be disconnected. If the fault occurs on the interconnection line of the multi-terminal flexible DC system, the receiving-end converter stations at both ends of the interconnection line will perform fault line selection and disconnect the circuit breakers at both ends of the interconnection line, and the multi-terminal flexible DC system will be divided into two areas for operation. If the fault occurs on the DC bus of the multi-terminal flexible DC system, the corresponding DC switch of the line connected to the DC bus will be disconnected to isolate the fault. 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 the fault is isolated. If not, the remaining receiving-end converter stations will be taken over by 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 be taken over by 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 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.

[0015] 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 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.

[0016] Furthermore, 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 pulls open the DC circuit breaker on the wind power branch line; 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; Power limitation means that when an N-1 DC fault occurs on the DC side of a multi-terminal flexible DC system, the system carrying capacity may be lower than the rated power of the wind farm, and the wind farm power must be limited to avoid overloading the multi-terminal flexible DC system. 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 delivery 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.

[0017] Furthermore, in step 3), the power limitation shall be implemented in the following manner to avoid overloading 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 exits 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: .

[0018] Based on the above technical solution, the present invention has the following beneficial effects: 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

[0019] Figure 1 This is a structural diagram of the offshore wind power multi-terminal flexible direct current transmission system; Figure 2 This is a flow chart of a control method for sending offshore wind power via multi-terminal flexible direct current according to the present invention; Figure 3 It is a flow chart of the DC fault processing strategy of the multi-terminal flexible DC system of the present invention; Figure 4 This is a flow chart of a DC fault ride-through strategy for a wind power converter station according to the present invention; 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; Figure 6 The DC circuit of the embodiment of the present invention Power waveform of wind power converter station during fault; Figure 7 The DC circuit of the embodiment of the present invention DC voltage waveform of wind power converter station during fault; Figure 8 The DC circuit of the embodiment of the present invention Schematic diagram of system power flow after failure exit; Fig. 9 The DC circuit of the embodiment of the present invention Schematic diagram of system power flow after fault exit. DETAILED DESCRIPTION

[0020] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0021] This embodiment provides a control method for offshore wind power transmission via multi-terminal flexible direct current. The method is applied to an offshore wind power transmission system via multi-terminal flexible direct current. Figure 1 As shown, the system includes an offshore wind farm, a wind power converter station MMC6, five receiving-end converter stations and five AC power grids. The five receiving-end converter stations are respectively the first receiving-end converter station MMC1, the second receiving-end converter station MMC2, the third receiving-end converter station MMC3, the fourth receiving-end converter station MMC4, and the fifth receiving-end converter station MMC5. The five AC power grids are respectively 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.

[0022] The offshore wind farm WT comprises 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.

[0023] 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, and 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-end converter station MMC3 and the fifth receiving-end converter station MMC5 are connected to the DC busbar of the first receiving-end converter station MMC1 through the DC line and Connected to the DC bus of the fourth receiving converter station MMC4; DC line - Both ends of the line are equipped with DC circuit breakers; 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.

[0024] The control method of offshore wind power transmission via multi-terminal flexible direct current is as follows: Figure 2 As shown, the steps are as follows: Step (1): adjusting the power command of the converter station of the multi-terminal flexible direct current system according to the wind farm power evacuation control strategy.

[0025] 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 command to achieve controllable evacuation of wind power, and the constant-voltage receiving-end converter station can automatically balance 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 control to achieve constant-voltage control.

[0026] The power command value of the fixed power receiving end converter station must meet the following constraints:

[0027] In the formula, P WT,N is the rated output power of the offshore wind farm; P sumis 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 end 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 fixed power receiving end converter station that takes over the fixed voltage after the fixed 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 .

[0028] Step (2): Implement the wind power converter station startup control strategy.

[0029] Specifically, the implementation method of the wind power converter station startup control strategy is: 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 bus interval DC resonant switch to charge the wind power converter and DC cable as a whole and automatically enter the startup process.

[0030] Step (3): When a DC fault occurs in the multi-terminal flexible DC transmission system of offshore wind power, 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 realizes 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 realizes the wind power converter station non-blocking fault ride-through through the four processes of "circuit breaker disconnection-DC energy consumption voltage stabilization-power limitation-reconnection".

[0031] In step (3), the DC fault handling strategy of the multi-terminal flexible DC system is as follows: Figure 3 shown.

[0032] If a DC fault occurs on a 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-out lines.

[0033] 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.

[0034] 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.

[0035] 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 through the change of 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 bus of the multi-terminal flexible DC system.

[0036] 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 faulty 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 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 interconnection 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 disconnected to isolate the fault.

[0037] 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 bus of the receiving-end converter station, it is determined whether there is a constant voltage converter station in the sound system after the fault is isolated. 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 the DC fault occurs at 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 achieve constant voltage takeover through cross-communication, and the receiving-end converter station with the largest capacity in the area has priority to take over. Subsequently, the multi-terminal flexible DC system first unlocks the locked voltage receiving-end converter station to establish a DC voltage, and then unlocks other fixed-power receiving-end converter stations that have not exited.

[0038] 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 of the wind farm after recovery, so as to avoid overload of the constant voltage receiving-end converter station in the area.

[0039] In step (3), the wind power converter station DC fault ride-through strategy is as follows: Figure 4 shown.

[0040] Specifically, the circuit breaker disconnection process in the wind power converter station DC fault ride-through strategy refers to: when a DC fault occurs, the wind power converter station MMC6 does not lock and pulls open the wind power branch DC circuit breaker.

[0041] Specifically, the DC energy consuming 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 consuming 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.

[0042] 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, the system carrying capacity may be lower than the rated power of the wind farm, and the wind farm power needs to be limited to avoid system overload.

[0043] 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: .

[0044] Specifically, the reconnection process in the DC fault ride-through strategy of the wind power converter station refers to: 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 controls the DC circuit breaker to restore the power delivery 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 first receiving-end converter station MMC1 opens the lead-out line interval resonant switch.

[0045] In order to verify the effectiveness of the control method of the present invention, the following Figure 1The simulation model of the offshore wind power transmission system through multi-terminal flexible direct current is shown in Figure 1. The offshore wind farm is connected to the wind power converter station MMC6 through four 66kV three-core submarine cables, which transmit 56MW, 56MW, 70MW, and 70MW wind power respectively, with a total installed capacity of 252MW. The parameters of each receiving converter station are shown in Table 1: Table 1

[0046] 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 adopts constant voltage control to balance the system power. Then the system flow when the wind farm is connected at full power is as follows: Figure 5 shown.

[0047] Assume that the offshore wind power is transmitted through the multi-terminal flexible direct current transmission system and a direct current 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 the DC circuit breaker is opened and energy consumption is started; 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-end converter station MMC4 opens the corresponding DC switch. Considering the line fault, one 66kV collector line will be synchronously withdrawn, and the total wind power power shall not exceed 196MW. After the fifth receiving-end converter station MMC5 withdraws, the fourth receiving-end converter station MMC4 will take over at a constant voltage. After the fault is isolated, the fourth receiving-end converter station MMC4 will be unlocked first, and then the other receiving-end 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 delivery. 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 figure, 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.

[0048] Assume that the offshore wind power is transmitted through the multi-terminal flexible direct current transmission system and a direct current line occurs at t=3s. Single-pole grounding 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 the DC circuit breaker is opened and energy consumption is started. 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 fixed voltage. Considering the line fault, one 66kV collector line will be synchronously withdrawn, and the total wind power power does not exceed 196MW. Due to the line After the exit, the multi-terminal flexible direct current system is divided into two areas. In order to prevent the offshore wind power from being overloaded through the multi-terminal flexible direct current transmission system, the power command value of the second receiving-end converter station MMC2 needs to be increased to 98MW. After the fault is isolated, the first receiving-end converter station MMC1 and the fourth receiving-end converter station MMC4 are unlocked first, and then the other receiving-end converter stations are unlocked respectively. 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 Fig. 9 shown.

[0049] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art to the present invention based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A control method for sending offshore wind power via multi-terminal flexible direct current, which is used in an offshore wind power sending 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 is connected to the grid via the wind power converter station, the wind power converter station is connected to the DC bus of a receiving-end converter station in the multi-terminal flexible direct current system via a DC line, and 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 characteristic is that: The control method comprises: 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 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 "locking all receiving-end converter stations - fault line selection - fault isolation - restart"; The DC fault ride-through strategy of the wind power converter station achieves the non-locking fault ride-through of the wind power converter station through the four processes of "circuit breaker disconnection-DC energy consumption device voltage stabilization-power limitation-reconnection".

2. The control method according to claim 1, characterized in that: In the 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 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.

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: In the formula, 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 end 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 value of the constant power receiving end converter station which 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, characterized in that: In the 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.

5. The control method according to claim 1, characterized in that: In the 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 receiving-end converter station protection 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 the 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: In the DC fault handling strategy of the multi-terminal flexible DC system in 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; 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 location of the fault based on the change in DC current; Fault isolation: The location of the DC fault is determined 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 terminal of the faulty line 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 disconnect the circuit breakers at both ends of the tie line after fault line selection; If the fault occurs on the DC bus of the multi-terminal flexible DC system, the corresponding DC switch of the line connected to the DC bus is disconnected to isolate the fault; Restart: When a DC fault occurs on the 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 be taken over by 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 station in the area connected to the wind power branch will be taken over by constant voltage control through cross communication.

8. The control method according to claim 7, 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 of the wind farm after recovery, so as to avoid overload of the constant voltage receiving-end converter station in the area.

9. 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 pulls open the DC circuit breaker on the wind power branch line; 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; Power limitation means that when an N-1 DC fault occurs on the DC side of a multi-terminal flexible DC system, the system carrying capacity may be lower than the rated power of the wind farm, and the wind farm power must be limited to avoid overloading the multi-terminal flexible DC system. 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 delivery 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.

10. 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 overloading 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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