Method for calculating short-circuit current of offshore wind power power frequency and low frequency hybrid power transmission system

Abstract

The invention discloses a short-circuit current calculation method for an offshore wind power power frequency and low frequency hybrid power transmission system, and the method comprises the following steps: building an electromagnetic transient simulation model of the offshore wind power power frequency and low frequency hybrid power transmission system, selecting typical fault conditions of the offshore wind power power frequency and low frequency hybrid power transmission system, and calculating the short-circuit current of the offshore wind power power frequency and low frequency hybrid power transmission system according to a control protection strategy of the system. Correct action logic is set for key equipment in the system, short-circuit current waveforms of key positions of the system are obtained through simulation calculation, representative short-circuit currents 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 electrical equipment type selection 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 short-circuit current 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 relatively 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 short-circuit current calculation principle is not yet clear. Existing research has not yet proposed a method for calculating the short-circuit current of offshore power frequency and low frequency hybrid transmission systems. There is an urgent need to propose a method for calculating the short-circuit current of offshore wind power power frequency and low frequency hybrid transmission systems to provide guidance for the selection and design of electrical equipment in the project. Summary of the Invention

[0005] The purpose of this invention is to address the problems of complex operating conditions and unclear short-circuit current calculation principles in offshore power frequency and low-frequency hybrid transmission systems, and the lack of a proposed method for calculating short-circuit current in such systems in existing research. This invention provides a method for calculating short-circuit current 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 short-circuit current 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 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 at the offshore low-frequency collection submarine cable, the low-frequency transmission submarine 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 three-phase grounding short-circuit fault conditions at the offshore power frequency collection submarine cable, the power frequency transmission submarine cable, and the onshore grid-side power frequency bus; and setting the 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 short-circuit current values ​​at key locations in the system; and obtaining the representative short-circuit current of the hybrid power frequency and low-frequency transmission system for offshore wind power by traversing all the above conditions and selecting the maximum short-circuit current value.

[0008] Furthermore, the specific steps include:

[0009] Establish an electromagnetic transient simulation model for a hybrid power frequency and low frequency transmission system for offshore wind power;

[0010] 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 were respectively set on the low-frequency collecting submarine cable. After the fault occurred, the circuit breakers on both sides of the faulty low-frequency collecting submarine cable were disconnected after the first circuit breaker operated. The short-circuit current value at the key location of the system was obtained by simulation calculation.

[0011] 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 were respectively set up on 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 second circuit breaker action time. After the M3C converter was locked, the low-frequency side circuit breaker of the M3C converter opened after the third circuit breaker action time. The circuit breaker on the wind turbine side of the low-frequency transmission submarine cable did not operate. The short-circuit current value at the key location of the system was obtained by simulation calculation.

[0012] Three-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, 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 second circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the third circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The short-circuit current values ​​at the key locations of the system were obtained by simulation calculation.

[0013] Two-phase ground fault and two-phase-to-phase 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 third protection action time. After the M3C converter was locked, the power frequency side circuit breaker of the M3C converter opened after the second circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the third circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The short circuit current value at the key location of the system was obtained by simulation calculation.

[0014] 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 second circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the third circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The short-circuit current values ​​at the key locations of the system were obtained by simulation calculation.

[0015] Three-phase grounding short-circuit fault conditions were set up on the power frequency collection submarine cable and the power frequency transmission submarine cable respectively. After the fault occurred, the circuit breakers on both sides of the faulty submarine cable were disconnected after the fourth circuit breaker operated. The short-circuit current values ​​at the key locations of the system were obtained by simulation calculation.

[0016] A three-phase grounding fault condition is set on the power frequency bus on the land grid side. After the fault occurs, the energy consumption device does not operate. When the submodule voltage is higher than the overvoltage protection threshold, the M3C converter is locked. The short-circuit current value at the key position of the system is obtained by simulation calculation.

[0017] Summarize the short-circuit current values ​​at key locations of the system under all the above operating conditions, select the maximum short-circuit current value among them as the representative short-circuit current at that location, and the corresponding operating condition is the representative short-circuit current operating condition.

[0018] Furthermore, the short-circuit currents at key locations in the system include the M3C converter arm current, the grid-side current of the power frequency connection transformer, the valve-side current of the power frequency connection transformer, the valve-side current of the low frequency connection transformer, the low frequency outgoing submarine cable current, the low frequency collecting submarine cable current, the power frequency outgoing submarine cable current, and the power frequency collecting submarine cable current.

[0019] The beneficial effects of this invention are:

[0020] By adopting the technical solution of this invention, typical fault conditions of the hybrid power frequency and low frequency transmission system for offshore wind power are selected. According to the control and protection strategy of the system, the correct action logic is set for the key equipment in the system. The short-circuit current waveform at the key location of the system is obtained by simulation calculation. After summarizing all the conditions, the representative short-circuit current at the key location of the system and the corresponding conditions are obtained. This can provide guidance for the selection of electrical equipment for hybrid power frequency and low frequency transmission projects for offshore wind power. Attached Figure Description

[0021] 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.

[0022] Figure 2 This is a typical topology diagram of the M3C converter in this invention.

[0023] Figure 3 This is the power frequency grid side current waveform under a three-phase grounding fault in an embodiment of the present invention.

[0024] Figure 4 This is the low-frequency grid-side current waveform under a low-frequency grid-side three-phase grounding fault in an embodiment of the present invention.

[0025] Figure 5 This is the current waveform on the power frequency valve side under a three-phase ground fault in an embodiment of the present invention.

[0026] Figure 6 This is the low-frequency valve side current waveform under a three-phase ground fault on the low-frequency valve side in an embodiment of the present invention. Detailed Implementation

[0027] 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.

[0028] like Figure 1As 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.

[0029] 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.

[0030] like Figure 2As 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.

[0031] In this embodiment of the invention, the method for calculating the short-circuit current of a hybrid power frequency and low-frequency transmission system for offshore wind power specifically includes the following steps:

[0032] 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.

[0033] Three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions were respectively set up on the low-frequency collecting submarine cable 5. After the fault occurred and the first circuit breaker operated for a period of time, the low-frequency collecting submarine cable circuit breakers 6 on both sides of the faulty submarine cable were disconnected. The short-circuit current values ​​at the critical locations of the system were obtained by simulation calculation using the above simulation model. The short-circuit currents at the critical locations of the system include the arm current of the M3C converter 20, the current on the grid side of the onshore power frequency connection transformer 23, the current on the valve side of the onshore power frequency connection transformer 23, the current on the valve side of the onshore low-frequency connection transformer 18, the current of the low-frequency sending submarine cable 15, the current of the low-frequency collecting submarine cable 5, the current of the power frequency sending submarine cable 14, and the current of the power frequency collecting submarine cable 3.

[0034] 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 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 land-based 220kV power frequency circuit breaker 24 opened after the second circuit breaker action time. After the M3C converter was locked, the land-based low-frequency 220kV circuit breaker 17 opened after the third circuit breaker action time. The marine low-frequency 220kV circuit breaker 13 did not operate. The short-circuit current values ​​at the key locations of the system were obtained by simulation calculation using the above simulation model.

[0035] 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 was opened after the second circuit breaker action time. After the M3C converter was locked, the onshore low frequency 220kV circuit breaker 17 was opened after the third circuit breaker action time. The offshore low frequency 220kV circuit breaker 13 did not operate. The short-circuit current values ​​at the key locations of the system were obtained by simulation calculation using the above simulation model.

[0036] Two-phase ground fault and two-phase phase-to-phase fault short circuit 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 locked after the third protection action time. After the M3C converter was locked, the onshore power frequency 220kV circuit breaker 24 was opened after the second circuit breaker action time. After the M3C converter was locked, the onshore low frequency 220kV circuit breaker 17 was opened after the third circuit breaker action time. The offshore low frequency 220kV circuit breaker 13 did not operate. The short circuit current values ​​at the key locations of the system were obtained by simulation calculation using the above simulation model.

[0037] 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 operation time. After the M3C converter was locked, the onshore power frequency 220kV circuit breaker 24 opened after the second circuit breaker operation time. After the M3C converter was locked, the onshore low frequency 220kV circuit breaker 17 opened after the third circuit breaker operation time. The offshore low frequency 220kV circuit breaker 13 did not operate. The short-circuit current values ​​at the key locations of the system were obtained by simulation calculation using the above simulation model.

[0038] Three-phase grounding short-circuit fault conditions were set at the submarine cable 3 for power frequency collection and the submarine cable 14 for power frequency transmission. After the fault occurred, the 220kV power frequency circuit breaker 11 at sea and the 220kV power frequency circuit breaker 24 on land were disconnected after the fourth circuit breaker operated. The short-circuit current values ​​at the key locations of the system were obtained by simulation calculation using the above simulation model.

[0039] 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 set up respectively. The energy consumption device 27 did not operate. When the submodule voltage exceeded the overvoltage protection threshold, the M3C converter 20 was locked. The short-circuit current value at the key location of the system was obtained by simulation calculation using the above simulation model.

[0040] Summarize the short-circuit current values ​​at key locations of the system under all the above operating conditions, select the maximum short-circuit current value among them as the representative short-circuit current at that location, and the corresponding operating condition is the representative short-circuit current operating condition.

[0041] To further illustrate the specific implementation effects of the present invention, simulation waveforms of short-circuit current under several typical operating conditions are given in conjunction with the above specific embodiments.

[0042] like Figure 3 The figure shows the current waveform on the power frequency grid side under a three-phase ground fault in an embodiment of the present invention. When a three-phase ground fault occurs at point 26 of the 220kV power frequency bus on land, a large short-circuit current flows into the short-circuit location from the power frequency grid, with an amplitude of 50kA. This condition is a representative condition of the short-circuit current on the power frequency grid side.

[0043] like Figure 4 The figure shows the low-frequency grid-side current waveform under a three-phase grounding fault in an embodiment of the present invention. When a three-phase grounding fault occurs in the offshore low-frequency transmission cable 15, the short-circuit current output by the low-frequency grid side is not large before the M3C converter is locked out, and becomes 0 after the M3C converter is locked out.

[0044] like Figure 5 The figure shows the current waveform of the power frequency valve side under a three-phase ground fault 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. The short-circuit current on the power frequency valve side mainly flows in from the power frequency grid side through the power frequency connection transformer. This condition is a representative condition of the short-circuit current on the power frequency valve side.

[0045] like Figure 6 The figure shows the low-frequency valve side current waveform under a three-phase ground fault 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. The short-circuit current on the low-frequency valve side mainly flows in from the wind turbine side. Since the wind turbine grid-side converter will limit the output current, the short-circuit current is not large.

[0046] 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 the short-circuit current of 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 at the offshore low-frequency collection cable, low-frequency transmission cable, onshore power frequency connection transformer valve side, onshore low-frequency connection transformer valve side, and bridge arm reactor valve side. A three-phase grounding fault was also simulated at the offshore power frequency collection cable, power frequency transmission cable, and onshore grid-side power frequency bus. 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 short-circuit current values ​​at key locations in the system were calculated through simulation. By traversing all the above conditions, the maximum short-circuit current value was selected to obtain the representative short-circuit current of the hybrid power frequency and low frequency transmission system for offshore wind power.

2. The method for calculating short-circuit current in 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 were respectively set on the low-frequency collecting submarine cable. After the fault occurred, the circuit breakers on both sides of the faulty low-frequency collecting submarine cable were disconnected after the first circuit breaker operated. The short-circuit current value at the key location of the system was obtained by simulation calculation. 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 were respectively set up on 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 second circuit breaker action time. After the M3C converter was locked, the low-frequency side circuit breaker of the M3C converter opened after the third circuit breaker action time. The circuit breaker on the wind turbine side of the low-frequency transmission submarine cable did not operate. The short-circuit current value at the key location of the system was obtained by simulation calculation. Three-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, 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 second circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the third circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The short-circuit current values ​​at the key locations of the system were obtained by simulation calculation. Two-phase ground fault and two-phase-to-phase 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 third protection action time. After the M3C converter was locked, the power frequency side circuit breaker of the M3C converter opened after the second circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the third circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The short circuit current value at the key location of the system was 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 second circuit breaker action time. After the M3C converter was locked, the low frequency side circuit breaker of the M3C converter opened after the third circuit breaker action time. The circuit breaker on the low frequency transmission submarine cable wind turbine side did not operate. The short-circuit current values ​​at the key locations of the system were obtained by simulation calculation. Three-phase grounding short-circuit fault conditions were set up on the power frequency collection submarine cable and the power frequency transmission submarine cable respectively. After the fault occurred, the circuit breakers on both sides of the faulty submarine cable were disconnected after the fourth circuit breaker operated. The short-circuit current values ​​at the key locations of the system were obtained by simulation calculation. A three-phase grounding fault condition is set on the power frequency bus on the land grid side. After the fault occurs, the energy consumption device does not operate. When the submodule voltage is higher than the overvoltage protection threshold, the M3C converter is locked. The short-circuit current value at the key position of the system is obtained by simulation calculation. Summarize the short-circuit current values ​​at key locations of the system under all the above operating conditions, select the maximum short-circuit current value among them as the representative short-circuit current at that location, and the corresponding operating condition is the representative short-circuit current operating condition.

3. The method for calculating short-circuit current in a hybrid power frequency and low-frequency transmission system for offshore wind power according to claim 1, characterized in that: The short-circuit currents at key locations in the system include the M3C converter arm current, the grid-side current of the power frequency connection transformer, the valve-side current of the power frequency connection transformer, the valve-side current of the low frequency connection transformer, the low frequency outgoing submarine cable current, the low frequency collecting submarine cable current, the power frequency outgoing submarine cable current, and the power frequency collecting submarine cable current.

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