Surplus power balance method and apparatus during direct-current fault of flexible direct-current power transmission system

Abstract

A surplus power balance method and apparatus during a direct-current fault of a flexible direct-current power transmission system, which method and apparatus use high-speed switches to replace direct-current circuit breakers and direct-current energy dissipation apparatuses, and provide a surplus power balancing strategy during a direct-current fault of a flexible direct-current power transmission system. Surplus power balancing after a direct-current fault does not need to depend on direct-current circuit breakers and direct-current energy dissipation apparatuses, and the surplus power balancing after a direct-current fault can be realized by using direct-current high-speed switches and corresponding control strategies, such that the cost is greatly reduced, thereby solving the technical problems of the construction cost being high and same not being conducive to popularization and application since the existing offshore wind power flexible direct-current power transmission systems need to use direct-current circuit breakers and direct-current energy dissipation apparatuses to realize surplus power balancing after a direct-current fault.

Description

Method and device for balancing surplus power during DC fault of flexible DC power transmission system

[0001] The present application claims priority to the Chinese patent application No. 202411090979.6, filed on August 9, 2024, and entitled "Method and device for balancing surplus power during DC fault of flexible DC power transmission system", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of flexible DC power transmission, and in particular to a method and device for balancing surplus power during DC fault of a flexible DC power transmission system. BACKGROUND

[0003] The existing offshore wind power flexible DC power transmission system is shown in FIG. 4. The offshore wind power flexible DC power transmission system includes a wind farm, a power collection system, an offshore converter station and a land converter station. The offshore converter station and the land converter station are connected by a DC cable. After landing, the DC cable continues to be transmitted to the load center through a DC overhead line. Since the failure probability of the DC overhead line is much higher than that of the DC cable, the DC overhead line fault clearing needs to be considered. In the offshore wind power flexible DC power transmission system of FIG. 4, the cable-overhead line conversion uses a DC circuit breaker (DCCB). After a DC fault occurs, the DC circuit breaker is used to disconnect the DC overhead line, and a DC energy consumption device is used to balance the surplus power. The DC circuit breaker and the DC energy consumption device are high in cost. For example, in a ±500kV / 2000MW system, the cost of two DC circuit breakers and one 2000MW DC energy consumption device is close to 300 million yuan, which is not conducive to popularization and application. SUMMARY

[0004] The present application provides a method and device for balancing surplus power during DC fault of a flexible DC power transmission system, which is used to solve the technical problem that the existing offshore wind power flexible DC power transmission system needs to use a DC circuit breaker and a DC energy consumption device to balance the surplus power after a DC fault, which is high in construction cost and not conducive to popularization and application.

[0005] Therefore, the present application provides, in a first aspect, a method for balancing surplus power during DC fault of a flexible DC power transmission system, which is applied to an offshore wind power flexible DC power transmission system. The offshore wind power flexible DC power transmission system includes an offshore converter station and a land converter station. The offshore converter station is connected to the land converter station through a DC cable, a DC high-speed switch and a DC overhead line connected in series. The offshore converter station and the land converter station both adopt a topology structure with DC fault clearing capability. The method for balancing surplus power during DC fault of the flexible DC power transmission system includes the following steps:

[0006] S1, when the offshore converter station detects that a DC fault occurs, control the grid-side converter in all wind turbine converters of the sending end to be blocked, and the DC voltage of the wind turbine converter is maintained stable by the DC energy consumption device in the wind turbine converter;

[0007] S2, according to the topological structure of the offshore wind power flexible DC power transmission system, control the DC port voltage and / or DC port current of the offshore converter station and the onshore converter station to be 0;

[0008] S3, after the DC fault is cleared, gradually restore the DC voltage of the onshore converter station within a first preset time, wherein when the DC voltage of the onshore converter station starts to be restored, the offshore converter station notifies the wind turbine converter of the sending end to gradually restore to the operating power before the DC fault within a second preset time, and the second preset time is not less than the first preset time.

[0009] Optionally, before step S3, the method further comprises:

[0010] determining whether there is a grid-side converter that has not been successfully blocked in all wind turbine converters, if yes, obtaining the total power output of the grid-side converter that has not been successfully blocked, unlocking a corresponding number of the blocked grid-side converters according to the total power output of the grid-side converter that has not been successfully blocked, and controlling all the unlocked grid-side converters to absorb power in reverse from the power collection system of the offshore wind power flexible DC power transmission system, wherein the power absorbed in reverse by the unlocked grid-side converters is equal to the total output of the grid-side converter that has not been successfully blocked.

[0011] Optionally, the offshore converter station and the onshore converter station both adopt a full half-bridge hybrid topological structure.

[0012] Optionally, step S2 specifically comprises:

[0013] if the flexible DC power transmission system adopts a symmetric single-machine topology, control the DC port voltage of the onshore converter station to be 0 and the DC port current of the offshore converter station to be 0, and if the flexible DC power transmission system adopts a bipolar topology, control the DC port current of the offshore converter station and the onshore converter station to be 0.

[0014] The second aspect of the present application provides a surplus power balancing device during DC fault of a flexible DC power transmission system, which is applied to an offshore wind power flexible DC power transmission system, the offshore wind power flexible DC power transmission system comprising an offshore converter station and an onshore converter station, the offshore converter station being connected to the onshore converter station through a DC cable, a DC high-speed switch and a DC overhead line connected in series, the offshore converter station and the onshore converter station both adopting a topological structure with DC fault clearing capability, and the surplus power balancing device during DC fault of the flexible DC power transmission system comprising the following modules:

[0015] The converter blocking module is used for controlling the grid-side converter in all wind turbine converters to be blocked when the offshore converter station detects that a DC fault occurs, and the DC voltage of the wind turbine converter is maintained stable by the DC energy consumption device in the wind turbine converter.

[0016] The voltage and current control module is used for controlling the DC port voltage and / or DC port current of the offshore converter station and the onshore converter station to be reduced to 0 according to the topological structure of the offshore wind power flexible DC power transmission system.

[0017] The recovery module is used for gradually recovering the DC voltage of the onshore converter station within a first preset time after the DC fault is cleared, wherein when the DC voltage of the onshore converter station starts to recover, the offshore converter station informs the wind turbine converter at the sending end to gradually recover to the operating power before the DC fault within a second preset time, and the second preset time is not less than the first preset time.

[0018] Further comprising:

[0019] The judgment module is used for judging whether there is a grid-side converter that fails to be blocked in all wind turbine converters before the DC voltage of the onshore converter station starts to recover after the blocking instruction of the grid-side converter in all wind turbine converters is sent, and if yes, the power output sum of the grid-side converter that fails to be blocked is obtained, a corresponding number of the blocked grid-side converters are unlocked again according to the power output sum of the grid-side converter that fails to be blocked, and all the unlocked grid-side converters are controlled to reversely absorb power from the power collection system of the offshore wind power flexible DC power transmission system, and the power reversely absorbed by the unlocked grid-side converters is equal to the output sum of the grid-side converter that fails to be blocked.

[0020] Optionally, the offshore converter station and the onshore converter station both adopt a full-half bridge hybrid topological structure.

[0021] Optionally, the voltage and current control module is specifically used for:

[0022] If the flexible DC power transmission system adopts a symmetric single-machine topology, the DC port voltage of the onshore converter station is controlled to be reduced to 0, and the DC port current of the offshore converter station is controlled to be reduced to 0, and if the flexible DC power transmission system adopts a bipolar topology, the DC port current of the offshore converter station and the onshore converter station are both controlled to be reduced to 0.

[0023] The third aspect of the present application provides a flexible DC power transmission system surplus power balancing device during a DC fault, the device comprising a processor and a memory:

[0024] The memory is used for storing program codes and transmitting the program codes to the processor.

[0025] The processor is configured to execute the method for balancing surplus power during DC fault of the flexible HVDC system according to the instructions in the program code.

[0026] The fourth aspect of the present application provides a computer readable storage medium for storing program code for executing the method for balancing surplus power during DC fault of the flexible HVDC system according to any one of the first aspect.

[0027] From the above technical solutions, the method for balancing surplus power during DC fault of the flexible HVDC system has the following advantages:

[0028] The method for balancing surplus power during DC fault of the flexible HVDC system provided by the present application uses a DC high-speed switch to replace a DC circuit breaker and a DC energy consumption device, and provides a surplus power balancing strategy during DC fault of the flexible HVDC system. When the offshore converter station detects a DC fault, the grid-side converter in all wind turbine converters is controlled to be blocked, and the DC energy consumption device in the wind turbine converter is used to maintain the DC voltage of the wind turbine converter stable. According to the topological structure of the flexible HVDC system, the DC port voltage and / or DC port current of the offshore converter station and the onshore converter station are controlled to be reduced to 0. After the DC fault is cleared, the DC voltage of the onshore converter station is gradually restored within a first preset time. When the DC voltage of the onshore converter station starts to recover, the offshore converter station notifies the sending-end wind turbine converter to gradually restore to the operating power before the DC fault within a second preset time, and the second preset time is not less than the first preset time. The surplus power balancing after the DC fault does not need to rely on the DC circuit breaker and the DC energy consumption device, but can achieve the surplus power balancing after the DC fault by using the DC high-speed switch and the corresponding control strategy, thereby greatly reducing the cost, and solving the technical problem that the existing offshore wind power flexible HVDC system needs to use the DC circuit breaker and the DC energy consumption device to achieve the surplus power balancing after the DC fault, which has a high construction cost and is not conducive to popularization and application. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other related drawings can also be obtained without creative labor.

[0030] Fig. 1 is a flowchart of a method for balancing surplus power during DC fault of a flexible HVDC system according to an embodiment of the present application;

[0031] Fig. 2 is a schematic diagram of a circuit structure of a flexible HVDC system for offshore wind power provided in an embodiment of the present application;

[0032] Fig. 3 is a schematic diagram of a structure of a surplus power balancing device during DC fault of a flexible HVDC system provided in an embodiment of the present application;

[0033] Fig. 4 is a schematic diagram of a circuit structure of a prior flexible HVDC system for offshore wind power. DETAILED DESCRIPTION

[0034] In order to make the personnel in the technical field better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts fall within the scope of the present application.

[0035] For the convenience of understanding, please refer to Figs. 1 and 2. The present application provides an embodiment of a method for balancing surplus power during DC fault of a flexible HVDC system, which is applied to a flexible HVDC system for offshore wind power. The flexible HVDC system for offshore wind power includes an offshore converter station and a land converter station. The offshore converter station is connected to the land converter station through a DC cable, a DC high-speed switch (HSS) and a DC overhead line connected in series. The offshore converter station and the land converter station both adopt a topology structure with DC fault clearing capability, which can be a full-half bridge hybrid topology structure. The method includes the following steps:

[0036] Step S1, when the offshore converter station detects a DC fault, control all grid-side converters in the wind turbine converters at the sending end to be locked, and maintain the DC voltage of the wind turbine converters by the DC energy consumption device in the wind turbine converters.

[0037] It should be noted that there is a direct and fast communication channel between the offshore converter station and the wind turbine converter. The DC energy consumption device in the wind turbine converter is called within 1 ms. When the offshore converter station detects a DC fault, an instruction is sent to the wind turbine converter through fast communication. All grid-side converters of the wind turbine converters at the sending end are locked, and the DC energy consumption device in the wind turbine converter maintains the DC voltage of the wind turbine converter stable. In the flexible HVDC system for offshore wind power, the sending end is one end of the offshore converter station, and the receiving end is one end of the land converter station.

[0038] Step S2, control the DC port voltage and / or DC port current of the offshore converter station and the land converter station to be reduced to 0 according to the topology structure of the flexible HVDC system for offshore wind power.

[0039] It should be noted that after or at the same time when the offshore converter station controls the grid-side converter in all wind turbine converters to be blocked, the flexible DC power transmission system controls the DC port voltage and / or the DC port current of the offshore converter station and the onshore converter station to be 0 according to the topology structure adopted by the flexible DC power transmission system. Specifically, if the flexible DC power transmission system adopts a symmetric single-machine topology, the DC port voltage of the onshore converter station is controlled to be 0, and the DC port current of the offshore converter station is controlled to be 0. If the flexible DC power transmission system adopts a bipolar topology, the offshore converter station and the onshore converter station control the DC port current of each to be 0, respectively.

[0040] Step S3, after the DC fault is cleared, the DC voltage of the onshore converter station is gradually restored within a first preset time. When the DC voltage of the onshore converter station starts to be restored, the offshore converter station informs the sending-end wind turbine converter to gradually restore to the operating power before the DC fault within a second preset time, and the second preset time is not less than the first preset time.

[0041] It should be noted that after the DC fault is reliably cleared, the onshore converter station controls the DC voltage to be gradually restored within a first preset time. When the DC voltage starts to be restored, the offshore converter station informs the sending-end wind turbine converter to gradually restore to the operating power before the fault within a second preset time, and the second preset time is not less than the first preset time.

[0042] In one embodiment, since the flexible DC power transmission system needs to communicate with hundreds of wind turbines at the same time, in actual engineering implementation, part of the wind turbines may not be able to respond to the requirement of blocking the grid-side converter due to communication interruption, internal equipment abnormality, etc., and still exist a part of surplus power to charge the offshore converter station, which may cause module overvoltage. Therefore, it is necessary to call the wind turbine converter which can normally communicate to additionally absorb part of the surplus power. Specifically, after step S1 and before step S3, it further includes:

[0043] It is judged whether there is a grid-side converter which is not successfully blocked in all wind turbine converters, if yes, the total power output of the grid-side converter which is not successfully blocked is obtained, a corresponding number of the blocked grid-side converters are unlocked again according to the total power output of the grid-side converter which is not successfully blocked, all the unlocked grid-side converters are controlled to absorb power in reverse from the power collection system of the offshore wind power flexible DC power transmission system, and the power absorbed in reverse by the unlocked grid-side converters is equal to the total output of the grid-side converter which is not successfully blocked. The power collection system of the offshore wind power flexible DC power transmission system is a power collection device for transmitting AC power output by a wind farm to the offshore converter station, which is the part on the left side of the offshore converter station in FIG. 4, including AC / DC and DC / AC. The wind farm includes a plurality of wind turbine generators WT, the wind turbine generators output AC power, and the AC power is transmitted to the power collection system. The power collection system is used for voltage conversion processing of the received AC power and transmission to the offshore converter station.

[0044] The application provides a method for balancing surplus power during DC fault of a flexible HVDC system, which uses a DC high-speed switch to replace a DC circuit breaker and a DC energy consumption device, and provides a strategy for balancing surplus power during DC fault of a flexible HVDC system. When the offshore converter station detects a DC fault, the grid-side converter in all wind turbine converters is controlled to be blocked, the DC voltage of the wind turbine converter is maintained stable by the DC energy consumption device in the wind turbine converter, the DC port voltage and / or the DC port current of the offshore converter station and the onshore converter station are controlled to be reduced to 0 according to the topology structure of the flexible HVDC system, and after the DC fault is cleared, the DC voltage of the onshore converter station is gradually restored within a first preset time. When the DC voltage of the onshore converter station starts to be restored, the sending-end wind turbine converter is informed by the offshore converter station to gradually restore to the operating power before the DC fault within a second preset time, and the second preset time is not less than the first preset time. The balancing of surplus power after the DC fault does not need to rely on the DC circuit breaker and the DC energy consumption device, but can be realized by using the DC high-speed switch and the corresponding control strategy, so that the cost is greatly reduced, and the technical problem that the existing offshore wind power flexible HVDC system needs to use the DC circuit breaker and the DC energy consumption device to realize the balancing of surplus power after the DC fault, the construction cost is high, and the application is not conducive is solved.

[0045] For ease of understanding, please refer to FIG. 3. An embodiment of a device for balancing surplus power during DC fault of a flexible HVDC system is provided in the application, which is applied to an offshore wind power flexible HVDC system. The offshore wind power flexible HVDC system includes an offshore converter station and an onshore converter station. The offshore converter station is connected to the onshore converter station through a DC cable, a DC high-speed switch and a DC overhead line connected in series. The offshore converter station and the onshore converter station both adopt a topology structure with DC fault clearing capability, which can be a full-half bridge hybrid topology structure. The device includes the following modules:

[0046] A converter blocking module is used to control the grid-side converter in all wind turbine converters to be blocked when the offshore converter station detects a DC fault, and the DC voltage of the wind turbine converter is maintained stable by the DC energy consumption device in the wind turbine converter.

[0047] A voltage and current control module is used to control the DC port voltage and / or the DC port current of the offshore converter station and the onshore converter station to be reduced to 0 according to the topology structure of the flexible HVDC system.

[0048] A restoration module is used to control the DC voltage of the onshore converter station to be gradually restored within a first preset time after the DC fault is cleared. When the DC voltage of the onshore converter station starts to be restored, the sending-end wind turbine converter is informed by the offshore converter station to gradually restore to the operating power before the DC fault within a second preset time, and the second preset time is not less than the first preset time.

[0049] Further comprising:

[0050] The judgment module is configured to, after issuing the grid-side converter blocking instruction in all the fan converters, judge whether there is a grid-side converter that has not been successfully blocked in all the fan converters before the DC voltage of the land converter station starts to recover, and if so, obtain the total power output of the grid-side converter that has not been successfully blocked, unlock a corresponding number of the blocked grid-side converters according to the total power output of the grid-side converter that has not been successfully blocked, control all the unlocked grid-side converters to reversely absorb power from the power collection system, and the power reversely absorbed by the unlocked grid-side converters is equal to the total output of the grid-side converter that has not been successfully blocked.

[0051] The voltage and current control module is specifically configured to:

[0052] If the flexible DC power transmission system adopts a symmetric single-machine topology, the DC port voltage of the land converter station is controlled to be 0, and the DC port current of the offshore converter station is controlled to be 0, and if the flexible DC power transmission system adopts a bipolar topology, the DC port currents of the offshore converter station and the land converter station are controlled to be 0.

[0053] The application further provides an embodiment of a flexible DC power transmission system surplus power balancing device during a DC fault,

[0054] The device comprises a processor and a memory:

[0055] The memory is configured to store program code and transmit the program code to the processor.

[0056] The processor is configured to execute the flexible DC power transmission system surplus power balancing method during a DC fault according to the instructions in the program code.

[0057] The application further provides an embodiment of a computer readable storage medium for storing program code, and the program code is used to execute the flexible DC power transmission system surplus power balancing method during a DC fault.

[0058] The flexible DC power transmission system surplus power balancing device, the device and the computer readable storage medium provided in the application are used to execute the flexible DC power transmission system surplus power balancing method provided in the application, and the principle and the technical effects obtained are the same as those of the flexible DC power transmission system surplus power balancing method provided in the application, and will not be described here.

[0059] The terms "first", "second", "third", "fourth", and the like in the description of the application and in the claims of the foregoing drawings are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order. It is to be understood that the use of such terms is not made with the intention of limiting the scope of the application to the preferred embodiments which are described herein. Also, the term "comprising" is not used excluding the presence of elements other than the ones expressly listed, and the "a" or "an" before the citation of an element does not exclude the presence of a plurality of such elements. Moreover, it is to be understood that the use of the term "including" or "containing" does not exclude the presence of other elements or steps than those expressly listed.

[0060] The above description is only used to illustrate the technical solutions of the present application, and is not limited to the same. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can be modified, or some technical features can be replaced by equivalent ones. Such modifications or replacements do not change the essence of the corresponding technical solutions, and are within the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for balancing surplus power during DC fault of a flexible HVDC power transmission system, applied to an offshore wind power flexible HVDC power transmission system, characterized in that, The offshore wind power flexible DC power transmission system comprises an offshore converter station and a land converter station, the offshore converter station is connected with the land converter station through DC cables, DC high-speed switches and DC overhead lines connected in series, the offshore converter station and the land converter station both adopt a topology structure with DC fault clearing capability, and the surplus power balancing method of the flexible DC power transmission system during a DC fault comprises the following steps: S1, when the offshore converter station detects that a DC fault occurs, controlling all grid-side converters in the sending-end wind turbine converter to be locked, and maintaining the DC voltage of the wind turbine converter stable by a DC energy consumption device in the wind turbine converter; S2, according to the topology structure of the offshore wind power flexible DC power transmission system, controlling the DC port voltage and / or the DC port current of the offshore converter station and the land converter station to be reduced to 0; S3, after the DC fault is cleared, gradually restoring the DC voltage of the land converter station within a first preset time, wherein when the DC voltage of the land converter station starts to be restored, the offshore converter station informs the sending-end wind turbine converter to gradually restore the operating power before the DC fault within a second preset time, and the second preset time is not less than the first preset time.

2. The method of claim 1, wherein, After step S1 and before step S3, the method further comprises: judging whether there is a grid-side converter that has not been successfully locked in all wind turbine converters, if yes, obtaining the total power output of the grid-side converter that has not been successfully locked, unlocking a corresponding number of locked grid-side converters according to the total power output of the grid-side converter that has not been successfully locked, and controlling all the unlocked grid-side converters to reversely absorb power from the power collection system of the offshore wind power flexible DC power transmission system, wherein the power reversely absorbed by the unlocked grid-side converters is equal to the total output of the grid-side converter that has not been successfully locked.

3. The method of claim 1, wherein, The offshore converter station and the land converter station both adopt a full-half bridge hybrid topology structure.

4. The surplus power balancing method of the flexible DC power transmission system during a DC fault according to claim 1, wherein step S2 specifically comprises: if the flexible DC power transmission system adopts a symmetric single-machine topology, controlling the DC port voltage of the land converter station to be reduced to 0 and the DC port current of the offshore converter station to be reduced to 0, and if the flexible DC power transmission system adopts a bipolar topology, controlling the DC port current of the offshore converter station and the land converter station to be reduced to 0.

5. A device for balancing surplus power during DC fault of a flexible DC power transmission system, applied to an offshore wind power flexible DC power transmission system, characterized in that, The offshore wind power flexible DC power transmission system comprises an offshore converter station and a land converter station, the offshore converter station is connected with the land converter station through DC cables, DC high-speed switches and DC overhead lines connected in series, the offshore converter station and the land converter station both adopt a topology structure with DC fault clearing capability, and the surplus power balancing device of the flexible DC power transmission system during a DC fault comprises the following modules: a converter locking module, configured to, when the offshore converter station detects that a DC fault occurs, control all grid-side converters in the sending-end wind turbine converter to be locked, and maintain the DC voltage of the wind turbine converter stable by a DC energy consumption device in the wind turbine converter; a voltage and current control module, configured to, according to the topology structure of the offshore wind power flexible DC power transmission system, control the DC port voltage and / or the DC port current of the offshore converter station and the land converter station to be reduced to 0; and a voltage and current control module, configured to, according to the topology structure of the offshore wind power flexible DC power transmission system, control the DC port voltage and / or the DC port current of the offshore converter station and the land converter station to be reduced to 0. The recovery module is configured to gradually restore the DC voltage of the onshore converter station within a first preset time after the DC fault is cleared, and the sending-end wind turbine converter is gradually restored to the operating power before the DC fault within a second preset time by the offshore converter station when the DC voltage of the onshore converter station starts to be restored, and the second preset time is not less than the first preset time.

6. The flexible DC power transmission system DC fault period excess power balancing device according to claim 5, characterized by, Further comprising: The judgment module is configured to determine whether there is a grid-side converter that has not been successfully blocked in all wind turbine converters before the DC voltage of the onshore converter station starts to be restored after the blocking instruction of the grid-side converter in all wind turbine converters is sent, and if so, to obtain the total power output of the grid-side converter that has not been successfully blocked, to re-unblock the corresponding number of grid-side converters that have been blocked according to the total power output of the grid-side converter that has not been successfully blocked, and to control all the re-unblocked grid-side converters to reversely absorb power from the power collection system of the offshore wind power flexible DC power transmission system, and the power reversely absorbed by the re-unblocked grid-side converter is equal to the total output of the grid-side converter that has not been successfully blocked.

7. The flexible DC power transmission system surplus power balancing device during DC fault of claim 5, wherein, Both the offshore converter station and the onshore converter station adopt a full half-bridge hybrid topology structure.

8. The device according to claim 5, wherein the voltage and current control module is specifically configured to: If the flexible DC power transmission system adopts a symmetric single-machine topology, the DC port voltage of the onshore converter station is controlled to be 0 and the DC port current of the offshore converter station is controlled to be 0, and if the flexible DC power transmission system adopts a bipolar topology, the DC port currents of the offshore converter station and the onshore converter station are both controlled to be 0.

9. A flexible DC power transmission system DC fault period surplus power balancing device, characterized by, The device comprises a processor and a memory: The memory is configured to store program code and transmit the program code to the processor; The processor is configured to execute the method according to any one of claims 1-4 according to instructions in the program code.

10. A computer-readable storage medium, characterized in that, The computer readable storage medium is configured to store program code, and the program code is configured to execute the method according to any one of claims 1-4.

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