Overvoltage calculation method for offshore wind power power frequency and low frequency hybrid power transmission system

Abstract

The invention discloses a method for calculating overvoltage of an offshore wind power power frequency and low frequency hybrid power transmission system, which comprises the following steps: establishing an electromagnetic transient simulation model of the offshore wind power power frequency and low frequency hybrid power transmission system, selecting typical working conditions of the offshore wind power power frequency and low frequency hybrid power transmission system, and calculating the overvoltage of the system according to a control protection strategy of the system; correct action logic is set for key equipment in the system, overvoltage waveforms of key positions of the system are obtained through simulation calculation, representative overvoltage and corresponding working conditions of the key positions of the system are obtained after all working conditions are summarized, and a guidance basis can be provided for insulation cooperation design of offshore wind power power frequency and low frequency hybrid power transmission projects.

Description

Technical Field

[0001] This invention belongs to the field of new energy power generation, specifically relating to a method for calculating overvoltage in a hybrid power frequency and low frequency transmission system for offshore wind power. Background Technology

[0002] Currently, the construction of offshore wind farms is gradually shifting from short-distance, small-capacity operations to large-scale, deep-sea operations. When offshore wind farms are located beyond a certain distance from the shore, the traditional power frequency AC transmission method suffers from excessive reactive power consumption due to the equivalent capacitance of the submarine cable. In this case, using flexible low-frequency transmission is a more economical and effective solution.

[0003] Because low-frequency power transmission technology is not yet mature and key low-frequency equipment has not yet been mass-produced and applied, the construction cost of low-frequency power transmission projects is relatively high, and it only has a cost advantage under specific capacity and transmission distance conditions. The transmission distance and transmission capacity of offshore wind farms are often limited by a variety of factors, which makes the application scenarios of using low-frequency power transmission alone very limited. Adopting a hybrid power frequency and low-frequency power transmission scheme can expand the application scope of offshore wind power low-frequency power transmission technology.

[0004] However, the topology and control and protection strategies of offshore power frequency and low frequency hybrid transmission systems are quite complex. The system includes various power electronic devices such as M3C converters, energy-consuming devices, and wind turbine converters, as well as multiple switching devices such as low frequency and power frequency circuit breakers. This leads to complex system operating conditions, and the overvoltage mechanism and calculation method are still unclear. Existing research has not yet proposed an overvoltage calculation method for offshore power frequency and low frequency hybrid transmission systems. There is an urgent need to propose an overvoltage calculation method for offshore wind power power frequency and low frequency hybrid transmission systems to provide guidance for the insulation coordination design of the project. Summary of the Invention

[0005] The purpose of this invention is to address the complex operating conditions of offshore power frequency and low frequency hybrid transmission systems, the unclear overvoltage mechanism and calculation methods, and the fact that existing research has not yet proposed a method for calculating overvoltage in offshore power frequency and low frequency hybrid transmission systems. This invention provides a method for calculating overvoltage in offshore wind power power frequency and low frequency hybrid transmission systems.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0007] A method for calculating overvoltage in a hybrid power frequency and low frequency transmission system for offshore wind power is characterized by: establishing an electromagnetic transient simulation model of the hybrid power frequency and low frequency transmission system for offshore wind power; setting up three-phase grounding short-circuit fault conditions, two-phase grounding short-circuit fault conditions, two-phase phase-to-phase short-circuit fault conditions, and single-phase grounding short-circuit fault conditions on the onshore power frequency bus, the offshore low-frequency transmission cable, the valve side of the onshore power frequency connection transformer, the valve side of the onshore low-frequency connection transformer, and the valve side of the bridge arm reactor; setting up open line closing condition, fault-free load shedding condition, and single-phase grounding fault load shedding condition in the power frequency transmission system; and setting up correct operating logic for the M3C converter, power frequency circuit breaker, low-frequency circuit breaker, and wind turbine side circuit breaker in the system according to the control and protection strategy of the hybrid power frequency and low frequency transmission system for offshore wind power, and simulating and calculating the overvoltage values ​​at key locations in the system; and selecting the maximum overvoltage value among all the above conditions to obtain the representative overvoltage of the hybrid power frequency and low frequency transmission system for offshore wind power. Specifically, the following steps are included:

[0008] Establish an electromagnetic transient simulation model for a hybrid power frequency and low frequency transmission system for offshore wind power;

[0009] Three-phase grounding short-circuit fault, two-phase grounding short-circuit fault, two-phase phase-to-phase short-circuit fault, and single-phase grounding short-circuit fault conditions were set at the power frequency bus on the land grid side. The energy consumption device did not operate. When the submodule voltage exceeded the overvoltage protection threshold, the M3C converter was locked. The overvoltage value at the key location of the system was obtained by simulation calculation.

[0010] Three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault were set at the low-frequency transmission submarine cable. After the fault occurred, the M3C converter was locked after the first protection action time. After the M3C converter was locked, the power frequency side circuit breaker of the M3C converter opened after the first circuit breaker action time. After the M3C converter was locked, the low-frequency side circuit breaker of the M3C converter opened after the second circuit breaker action time. The circuit breaker on the wind turbine side of the low-frequency transmission submarine cable did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation.

[0011] Three-phase grounding short-circuit fault conditions were set at the valve side of the onshore power frequency connection transformer, the valve side of the onshore low frequency connection transformer, and the valve side of the bridge arm reactor, respectively. After the fault occurred, the M3C converter was locked after the second protection action time. After the M3C converter was locked, the power frequency side circuit breaker of the M3C converter opened after the first circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the second circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation.

[0012] Two-phase ground fault and two-phase-to-phase fault conditions were set at the valve side of the onshore power frequency connection transformer, the valve side of the onshore low frequency connection transformer, and the valve side of the bridge arm reactor, respectively. After the fault occurred, the M3C converter was locked after the third protection action time. After the M3C converter was locked, the power frequency side circuit breaker of the M3C converter opened after the first circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the second circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation.

[0013] Single-phase ground fault conditions were set at the valve side of the onshore power frequency connection transformer, the valve side of the onshore low frequency connection transformer, and the valve side of the bridge arm reactor. After the fault occurred, the M3C converter was locked after the fourth protection action time. After the M3C converter was locked, the power frequency side circuit breaker of the M3C converter opened after the first circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the second circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation.

[0014] The overvoltage values ​​at key locations in the power frequency wind turbine transmission system were obtained by simulation calculations under the following conditions: closed-line operation, no-fault load shedding operation, and single-phase ground fault load shedding operation.

[0015] Summarize the overvoltage values ​​at key locations of the system under all the above operating conditions, select the maximum overvoltage value as the representative overvoltage at that location, and the corresponding operating condition is the representative overvoltage operating condition.

[0016] Furthermore, the overvoltages at key locations in the system include the grid-side voltage of the power frequency connection transformer, the valve-side voltage of the power frequency connection transformer, the grid-side voltage of the low-frequency connection transformer, the valve-side voltage of the low-frequency connection transformer, the valve-side voltage of the bridge arm reactor, the low-frequency outgoing submarine cable voltage, the low-frequency collecting submarine cable voltage, the power frequency outgoing submarine cable voltage, and the power frequency collecting submarine cable voltage.

[0017] The beneficial effects of this invention are:

[0018] By adopting the technical solution of this invention, typical operating conditions of a hybrid power frequency and low frequency transmission system for offshore wind power are selected. Based on the system's control and protection strategy, the correct action logic is set for the key equipment in the system. The overvoltage waveforms at key locations in the system are obtained through simulation calculation. After summarizing all operating conditions, representative overvoltages and corresponding operating conditions at key locations in the system are obtained. This can provide guidance for the insulation coordination design of hybrid power frequency and low frequency transmission projects for offshore wind power. Attached Figure Description

[0019] Figure 1 This is a typical topology diagram of the offshore wind power power frequency and low frequency hybrid transmission system of the present invention.

[0020] Figure 2 This is a typical topology diagram of the M3C converter in this invention.

[0021] Figure 3 This is the voltage waveform on the power frequency grid side under a single-phase ground fault in an embodiment of the present invention.

[0022] Figure 4 This is the low-frequency grid-side voltage waveform under a low-frequency grid-side single-phase ground fault in an embodiment of the present invention.

[0023] Figure 5 This is the low-frequency grid-side voltage waveform under a three-phase ground fault on the power frequency valve side in an embodiment of the present invention.

[0024] Figure 6 This is the low-frequency grid-side voltage waveform under a three-phase ground fault on the low-frequency valve side in an embodiment of the present invention. Detailed Implementation

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

[0026] like Figure 1 As shown in this embodiment of the invention, the offshore wind power frequency and low-frequency hybrid transmission system includes: an offshore frequency wind turbine 1, an offshore low-frequency wind turbine 2, a frequency collection submarine cable 3, a frequency collection submarine cable circuit breaker 4, a low-frequency collection submarine cable 5, a low-frequency collection submarine cable circuit breaker 6, an offshore frequency 66kV circuit breaker 7, an offshore low-frequency 66kV circuit breaker 8, an offshore frequency step-up transformer 9, an offshore low-frequency step-up transformer 10, an offshore frequency 220kV circuit breaker 11, an offshore frequency high-voltage reactor 12, and an offshore low-frequency 220kV circuit breaker 13. 13. kV circuit breaker; 14. Submarine cable for power frequency transmission; 15. Submarine cable for low frequency transmission; 16. Onshore power frequency high voltage reactor; 17. Onshore low frequency 220kV circuit breaker; 18. Onshore low frequency connecting transformer; 19. Onshore low frequency 66kV circuit breaker; 20. M3C converter; 21. Onshore starting resistor; 22. Onshore power frequency 66kV circuit breaker; 23. Onshore power frequency connecting transformer; 24. Onshore power frequency 220kV circuit breaker; 25. Onshore power frequency station transformer; 26. Onshore 220kV power frequency busbar; 27. Energy consumption device.

[0027] Offshore power frequency wind turbine 1 is connected to offshore power frequency step-up transformer 9 via power frequency collection cable 3, and then via offshore power frequency 66kV circuit breaker 7. After being stepped up to 220kV, it is connected to power frequency transmission cable 14 via offshore power frequency 220kV circuit breaker 11. Offshore power frequency high-voltage reactor 12 is also connected to power frequency transmission cable 14. Power frequency collection cable circuit breakers 4 are installed on both sides of power frequency collection cable 3; offshore low frequency wind turbine 2 is connected to low frequency collection cable 9. After cable 5 is collected, it is connected to the offshore low-frequency step-up transformer 10 via the offshore low-frequency 66kV circuit breaker 8. After being stepped up to 220kV, it is connected to the low-frequency transmission submarine cable 15 via the offshore low-frequency 220kV circuit breaker 13. Low-frequency collection submarine cable circuit breakers 6 are installed on both sides of the low-frequency collection submarine cable 5. After the low-frequency transmission submarine cable 15 lands, it is connected to the onshore low-frequency connection transformer 18 via the onshore low-frequency 220kV switch 17. After being stepped down to 66kV, it is connected to the onshore low-frequency connection transformer 18. The low-frequency 66kV circuit breaker 19 is connected to the M3C converter 20. After the M3C converter 20 converts the frequency to the power frequency, it is connected to the onshore power frequency connection transformer 23 through the onshore starting resistor 21 and the onshore power frequency 66kV circuit breaker 22. After being stepped up to 220kV, it is connected to the onshore 220kV power frequency bus 26 through the onshore power frequency 220kV circuit breaker 24. After the power frequency transmission submarine cable 14 lands, it is connected to the onshore power frequency bus 26 through the onshore power frequency 220kV circuit breaker 24. The 220kV power frequency bus 26 is connected to the onshore power frequency high voltage reactor 16, which is connected to the power frequency transmission submarine cable 14. The onshore power frequency station transformer 25 is connected to the onshore 220kV power frequency bus 26 through the onshore power frequency 220kV circuit breaker 24 as the onshore station power supply. The onshore 220kV power frequency bus 26 is connected to the onshore AC power grid. The onshore AC energy dissipation device 27 is connected between the onshore low frequency 66kV circuit breaker 19 and the onshore low frequency connecting transformer 18.

[0028] like Figure 2 As shown, in this embodiment of the invention, the M3C converter consists of three frequency conversion modules, each of which contains three bridge arm branches. The entire M3C converter consists of nine bridge arm branches. Each bridge arm branch is composed of multiple cascaded full-bridge sub-modules and bridge arm reactors 28 connected in series. The three-phase AC systems on both sides of the M3C converter are connected through the bridge arm branches. Each phase of one system is connected to the three phases of the other system through three bridge arm branches.

[0029] In this embodiment of the invention, the overvoltage calculation method for a hybrid power frequency and low frequency transmission system for offshore wind power specifically includes the following steps:

[0030] First, according to Figure 1 The system shown is based on electromagnetic transient simulation software, which establishes an electromagnetic transient simulation model of a hybrid power frequency and low frequency transmission system for offshore wind power.

[0031] At point 26 on the 220kV power frequency bus, three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions were simulated. Energy dissipation device 27 did not operate. When the submodule voltage exceeded the overvoltage protection threshold, M3C converter 20 was locked. The overvoltage values ​​at key system locations were calculated using the aforementioned simulation model. These overvoltages at key system locations include the grid-side voltage of the onshore power frequency connection transformer 23, the valve-side voltage of the onshore power frequency connection transformer 23, the grid-side voltage of the onshore low-frequency connection transformer 18, the valve-side voltage of the onshore low-frequency connection transformer 18, the valve-side voltage of the bridge arm reactor 28, the voltage of the low-frequency outgoing submarine cable 15, the voltage of the low-frequency collecting submarine cable 5, the voltage of the power frequency outgoing submarine cable 14, and the voltage of the power frequency collecting submarine cable 3.

[0032] Three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault were respectively set at point 15 of the low-frequency transmission submarine cable. After the fault occurred, the M3C converter 20 was locked after the first protection action time. After the M3C converter was locked, the first circuit breaker action time was passed, and the onshore power frequency 220kV circuit breaker 24 was opened. After the M3C converter was locked, the second circuit breaker action time was passed, and the onshore low-frequency 220kV circuit breaker 17 was opened. The offshore low-frequency 220kV circuit breaker 13 did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation using the above simulation model.

[0033] Three-phase ground fault conditions were set at the valve side of onshore power frequency connection transformer 23, the valve side of onshore low frequency connection transformer 18, and the valve side of bridge arm reactor 28. After the fault occurred, the M3C converter 20 was locked after the second protection action time. After the M3C converter was locked, the onshore power frequency 220kV circuit breaker 24 opened after the first circuit breaker action time. After the M3C converter was locked, the onshore low frequency 220kV circuit breaker 17 opened after the second circuit breaker action time. The offshore low frequency 220kV circuit breaker 13 did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation using the above simulation model.

[0034] Two-phase ground fault and two-phase phase-to-phase fault conditions were respectively set at the valve side of the onshore power frequency connection transformer 23, the valve side of the onshore low frequency connection transformer 18, and the valve side of the bridge arm reactor 28. After the fault occurred, the M3C converter 20 was blocked after the third protection action time. After the M3C converter was blocked, the onshore power frequency 220kV circuit breaker 24 was opened after the first circuit breaker action time. After the M3C converter was blocked, the onshore low frequency 220kV circuit breaker 17 was opened after the second circuit breaker action time. The offshore low frequency 220kV circuit breaker 13 did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation using the above simulation model.

[0035] A single-phase ground fault was set at the valve side of the onshore power frequency connection transformer 23, the valve side of the onshore low frequency connection transformer 18, and the valve side of the bridge arm reactor 28. After the fault occurred, the M3C converter 20 was locked after the fourth protection action time. After the M3C converter was locked, the onshore power frequency 220kV circuit breaker 24 opened after the first circuit breaker action time. After the M3C converter was locked, the onshore low frequency 220kV circuit breaker 17 opened after the second circuit breaker action time. The offshore low frequency 220kV circuit breaker 13 did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation using the above simulation model.

[0036] Simulations were performed on the power frequency wind turbine transmission system under three conditions: closed-loop operation, no-fault load shedding, and single-phase ground fault load shedding. The overvoltage values ​​at key locations in the system were calculated. The closed-loop operation involved closing the onshore 220kV power frequency circuit breaker 24 while the onshore power frequency bus 26 was energized. The no-fault load shedding operation involved opening the onshore 220kV power frequency circuit breaker 24 under the system's rated operating conditions. The single-phase ground fault load shedding operation involved opening the onshore 220kV power frequency circuit breaker 24 after a single-phase ground fault occurred in the power frequency outgoing submarine cable 14.

[0037] Summarize the overvoltage values ​​at key locations of the system under all the above operating conditions, select the maximum overvoltage value as the representative overvoltage at that location, and the corresponding operating condition is the representative overvoltage operating condition.

[0038] To further illustrate the specific implementation effects of the present invention, simulation calculation waveforms under several typical working conditions are given in conjunction with the above specific embodiments.

[0039] like Figure 3 The figure shows the voltage waveform on the power frequency grid side under a single-phase ground fault in an embodiment of the present invention. When a single-phase ground fault occurs at point 26 of the 220kV power frequency bus on land, the voltage of the faulted phase drops to 0, while the voltage of the non-faulted phase remains at its rated value, and no obvious overvoltage is generated on the power frequency grid side.

[0040] like Figure 4 The figure shows the voltage waveform of the low-frequency grid side under a single-phase ground fault in an embodiment of the present invention. When a single-phase ground fault occurs in the low-frequency transmission cable 15 at sea, the voltage of the fault phase drops to 0. After the M3C converter is blocked, the voltage of the non-fault phase is significantly distorted, and the voltage amplitude reaches more than 400kV, which is a representative operating condition of low-frequency grid side overvoltage.

[0041] like Figure 5The figure shows the low-frequency grid-side voltage waveform under a three-phase ground fault on the power frequency valve side in an embodiment of the present invention. When a three-phase ground fault occurs on the valve side of the onshore power frequency connection transformer 23, the M3C converter is locked, and the active power output by the offshore wind turbine charges the low-frequency transmission submarine cable, resulting in overvoltage on the low-frequency grid side. This is another representative operating condition of low-frequency grid-side overvoltage.

[0042] like Figure 6 The figure shows the low-frequency grid-side voltage waveform under a three-phase ground fault on the low-frequency valve side in an embodiment of the present invention. When a three-phase ground fault occurs on the valve side of the onshore low-frequency connection transformer 18, the M3C converter is locked. Since the three-phase ground fault occurs on the low-frequency valve side, the low-frequency grid-side voltage is also close to 0, and no obvious overvoltage is generated.

[0043] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to the above embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A method for calculating overvoltage in a hybrid power frequency and low frequency transmission system for offshore wind power, characterized in that: An electromagnetic transient simulation model of a hybrid power frequency and low frequency transmission system for offshore wind power was established. Three-phase grounding fault, two-phase grounding fault, two-phase phase-to-phase short-circuit fault, and single-phase grounding fault conditions were simulated on the onshore power frequency bus, the offshore low-frequency transmission cable, the valve side of the onshore power frequency connection transformer, the valve side of the onshore low-frequency connection transformer, and the valve side of the bridge arm reactor. On the power frequency transmission system, closed-circuit, fault-free load shedding, and single-phase grounding fault load shedding conditions were simulated. Based on the control and protection strategy of the hybrid power frequency and low frequency transmission system for offshore wind power, the correct operating logic was set for the M3C converter, power frequency circuit breaker, low-frequency circuit breaker, and wind turbine-side circuit breaker in the system. The overvoltage values ​​at key locations in the system were calculated through simulation. By traversing all the above conditions, the maximum overvoltage value was selected to obtain the representative overvoltage of the hybrid power frequency and low frequency transmission system for offshore wind power.

2. The overvoltage calculation method for a hybrid power frequency and low frequency transmission system for offshore wind power according to claim 1, characterized in that, Specifically, the following steps are included: Establish an electromagnetic transient simulation model for a hybrid power frequency and low frequency transmission system for offshore wind power; Three-phase grounding short-circuit fault, two-phase grounding short-circuit fault, two-phase phase-to-phase short-circuit fault, and single-phase grounding short-circuit fault conditions were set at the power frequency bus on the land grid side. The energy consumption device did not operate. When the submodule voltage exceeded the overvoltage protection threshold, the M3C converter was locked. The overvoltage value at the key location of the system was obtained by simulation calculation. Three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault were set at the low-frequency transmission submarine cable. After the fault occurred, the M3C converter was locked after the first protection action time. After the M3C converter was locked, the power frequency side circuit breaker of the M3C converter opened after the first circuit breaker action time. After the M3C converter was locked, the low-frequency side circuit breaker of the M3C converter opened after the second circuit breaker action time. The circuit breaker on the wind turbine side of the low-frequency transmission submarine cable did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation. Three-phase grounding short-circuit fault conditions were set at the valve side of the onshore power frequency connection transformer, the valve side of the onshore low frequency connection transformer, and the valve side of the bridge arm reactor, respectively. After the fault occurred, the M3C converter was locked after the second protection action time. After the M3C converter was locked, the power frequency side circuit breaker of the M3C converter opened after the first circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the second circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation. Two-phase ground fault and two-phase-to-phase fault conditions were set at the valve side of the onshore power frequency connection transformer, the valve side of the onshore low frequency connection transformer, and the valve side of the bridge arm reactor, respectively. After the fault occurred, the M3C converter was locked after the third protection action time. After the M3C converter was locked, the power frequency side circuit breaker of the M3C converter opened after the first circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the second circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation. Single-phase ground fault conditions were set at the valve side of the onshore power frequency connection transformer, the valve side of the onshore low frequency connection transformer, and the valve side of the bridge arm reactor. After the fault occurred, the M3C converter was locked after the fourth protection action time. After the M3C converter was locked, the power frequency side circuit breaker of the M3C converter opened after the first circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the second circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The overvoltage values ​​at the key locations of the system were obtained by simulation calculation. The overvoltage values ​​at key locations in the power frequency wind turbine transmission system were obtained by simulation calculations under the following conditions: closed-line operation, no-fault load shedding operation, and single-phase ground fault load shedding operation. Summarize the overvoltage values ​​at key locations of the system under all the above operating conditions, select the maximum overvoltage value as the representative overvoltage at that location, and the corresponding operating condition is the representative overvoltage operating condition.

3. The overvoltage calculation method for a hybrid power frequency and low frequency transmission system for offshore wind power according to claim 1, characterized in that: The overvoltages at key locations in the system include the grid-side voltage of the power frequency connection transformer, the valve-side voltage of the power frequency connection transformer, the grid-side voltage of the low-frequency connection transformer, the valve-side voltage of the low-frequency connection transformer, the valve-side voltage of the bridge arm reactor, the low-frequency outgoing submarine cable voltage, the low-frequency collecting submarine cable voltage, the power frequency outgoing submarine cable voltage, and the power frequency collecting submarine cable voltage.

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