Overvoltage calculation method for offshore high-voltage and medium-voltage hybrid direct current transmission system

Abstract

The application discloses a kind of offshore high-voltage and medium-voltage hybrid DC transmission system overvoltage calculation method, the electromagnetic transient simulation model of offshore high-voltage and medium-voltage hybrid DC transmission system is established, the typical working condition of offshore high-voltage and medium-voltage hybrid DC transmission system is selected, according to the control protection strategy of system, the correct action logic of key equipment in system is set, the overvoltage waveform of key position of system is obtained by simulation calculation, after all conditions are summarized, the representative overvoltage of key position of system and corresponding condition can be obtained, can provide guidance basis for the insulation coordination design of offshore high-voltage and medium-voltage hybrid DC transmission project.

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 offshore high-voltage and medium-voltage DC transmission system. Background Technology

[0002] To fully utilize marine resources and vigorously develop offshore industries such as long-distance offshore operation and maintenance platforms and marine ranches, it is necessary to propose highly reliable power supply technologies for long-distance offshore platforms.

[0003] When offshore platforms are located beyond a certain distance from shore, traditional AC power supply methods suffer from excessive reactive power consumption due to the equivalent capacitance of the submarine cable. In such cases, DC power supply becomes a more economical and effective solution. To integrate and utilize marine resources, the future development direction is to consider the unified construction of power supply projects for offshore operation and maintenance platforms and offshore wind power transmission projects. While offshore wind power typically uses high-voltage DC transmission, offshore operation and maintenance platforms, with their relatively smaller power loads, generally benefit from medium-voltage DC transmission. In this case, using an offshore high-voltage converter station as a relay station to step down the voltage before supplying power to the offshore operation and maintenance platform via a medium-voltage DC submarine cable can significantly reduce the length of the medium-voltage DC submarine cable, making it the preferred solution for long-distance offshore platform power supply systems.

[0004] However, the aforementioned systems simultaneously include both high-voltage direct current (HVDC) and medium-voltage direct current (MTDC) transmission systems. The topology and control and protection strategies of offshore HVDC hybrid transmission systems are quite complex. The systems contain various power electronic devices such as high-voltage modular multilevel converters (MMC) and medium-voltage cascaded three-level converters (CNPC), as well as multiple high-voltage and medium-voltage circuit breakers and other switching equipment. This leads to complex system operating conditions, and the overvoltage mechanism and calculation methods are still unclear. Existing research has not yet proposed an overvoltage calculation method for offshore HVDC hybrid transmission systems. Summary of the Invention

[0005] The purpose of this invention is to provide a method for calculating overvoltage in a hybrid offshore high-voltage and medium-voltage DC transmission system, which can provide a reliable design basis for insulation coordination in offshore high-voltage and medium-voltage hybrid DC transmission projects.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A method for calculating overvoltage in a hybrid offshore high-voltage and medium-voltage DC transmission system is characterized by: establishing an electromagnetic transient simulation model of the hybrid offshore high-voltage and medium-voltage DC transmission system; setting up three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions in the onshore high-voltage AC grid, offshore wind farm AC grid, offshore sending-end medium-voltage AC bus, receiving-end medium-voltage AC bus, onshore high-voltage converter station valve side, and offshore high-voltage converter station valve side of the electromagnetic transient simulation model; and setting up three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions in the high-voltage DC submarine cable and the medium-voltage DC submarine cable. The system sets up single-pole grounding fault and double-pole grounding fault conditions. Based on the control and protection strategies of the offshore high-voltage and medium-voltage hybrid DC transmission system, it sets the correct operating logic for the onshore high-voltage MMC converter, offshore high-voltage MMC converter, sending-end medium-voltage CNPC converter, receiving-end medium-voltage CNPC converter, high-voltage circuit breaker, and medium-voltage circuit breaker in the electromagnetic transient simulation model. The simulation calculates the overvoltage values ​​at key locations in the system. By traversing all the above conditions, the maximum overvoltage value is selected to obtain the representative overvoltage of the offshore high-voltage and medium-voltage hybrid DC transmission system.

[0007] Furthermore, the overvoltage calculation method for the offshore high-voltage and medium-voltage hybrid DC transmission system specifically includes the following steps: Establish an electromagnetic transient simulation model for a hybrid offshore high-voltage and medium-voltage DC transmission system; In the electromagnetic transient simulation model, the onshore high-voltage AC power grid is configured with three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions. When the DC bus voltage exceeds the overvoltage protection threshold, the onshore high-voltage MMC converter and the offshore high-voltage MMC converter are locked, and the high-voltage circuit breaker is opened. Then, the sending-end medium-voltage CNPC converter and the receiving-end medium-voltage CNPC converter are locked, and the medium-voltage circuit breaker is opened. The overvoltage values ​​at key locations of the system are obtained through simulation calculation. In the electromagnetic transient simulation model of the offshore wind farm AC power grid, three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions are respectively set. After the fault occurs, the wind farm circuit breakers on both sides of the fault line are disconnected after the first protection action time. The overvoltage values ​​at the key locations of the system are obtained by simulation calculation. Three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault are respectively set on the valve side of the onshore high-voltage converter station and the valve side of the offshore high-voltage converter station in the electromagnetic transient simulation model. After the fault occurs, the onshore high-voltage MMC converter and the offshore high-voltage MMC converter are locked after the second protection action time, and the high-voltage circuit breaker is opened. Then the sending end medium-voltage CNPC converter and the receiving end medium-voltage CNPC converter are locked, and the medium-voltage circuit breaker is opened. The overvoltage value at the key position of the system is obtained by simulation calculation. In the electromagnetic transient simulation model, the sending-end medium-voltage AC bus and the receiving-end medium-voltage AC bus are respectively set to three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions. After the fault occurs, after the third protection action time, the sending-end medium-voltage CNPC converter and the receiving-end medium-voltage CNPC converter are locked, the medium-voltage circuit breaker is opened, the high-voltage and medium-voltage system connection circuit breaker is opened, the onshore high-voltage MMC converter, the offshore high-voltage MMC converter and the high-voltage circuit breaker do not operate, and the overvoltage value at the key location of the system is obtained by simulation calculation. In the electromagnetic transient simulation model, the high-voltage DC submarine cable is set to single-pole grounding fault condition and double-pole grounding fault condition respectively. After the fault occurs, after the fourth protection action time, the onshore high-voltage MMC converter and the offshore high-voltage MMC converter are locked, the high-voltage circuit breaker is opened, and then the sending end medium-voltage CNPC converter and the receiving end medium-voltage CNPC converter are locked, the medium-voltage circuit breaker is opened, and the overvoltage value at the key position of the system is obtained by simulation calculation. In the electromagnetic transient simulation model, the medium-voltage DC submarine cable is set to single-pole grounding fault and double-pole grounding fault conditions respectively. After the fault occurs, after the fifth protection action time, the sending-end medium-voltage CNPC converter and the receiving-end medium-voltage CNPC converter are locked, the medium-voltage circuit breaker is opened, the high-voltage and medium-voltage system connection circuit breaker is opened, the onshore high-voltage MMC converter, the offshore high-voltage MMC converter and the high-voltage circuit breaker do not operate, and the overvoltage value at the key location of the system is obtained by simulation calculation. 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.

[0008] Furthermore, the overvoltages at key locations in the system include the grid-side voltage of the onshore high-voltage MMC converter, the valve-side voltage of the onshore high-voltage MMC converter, the grid-side voltage of the offshore high-voltage MMC converter, the valve-side voltage of the offshore high-voltage MMC converter, the voltage of the high-voltage DC submarine cable, the grid-side voltage of the sending-end medium-voltage CNPC converter, the valve-side voltage of the sending-end medium-voltage CNPC converter, the grid-side voltage of the receiving-end medium-voltage CNPC converter, the valve-side voltage of the receiving-end medium-voltage CNPC converter, and the voltage of the medium-voltage DC submarine cable.

[0009] By adopting the technical solution of this invention and the method of this invention, typical operating conditions of offshore high-voltage and medium-voltage hybrid DC transmission systems 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 overvoltage waveforms at the key locations of the system are obtained through simulation calculation. After summarizing all operating conditions, the representative overvoltages and corresponding operating conditions at the key locations of the system are obtained. This can provide a reliable design basis for the insulation coordination design of offshore high-voltage and medium-voltage hybrid DC transmission projects. Attached Figure Description

[0010] Figure 1 This is a typical topology diagram of the offshore high-voltage and medium-voltage hybrid DC transmission system of the present invention.

[0011] Figure 2 This is a typical topology diagram of the high-voltage MMC converter in this invention.

[0012] Figure 3 This is a typical topology diagram of the medium-voltage CNPC converter in this invention.

[0013] Figure 4 This is the overvoltage waveform on the valve side of the medium-voltage CNPC converter at the sending end under a single-pole grounding fault of a medium-voltage DC submarine cable in an embodiment of the present invention.

[0014] Figure 5 This is the valve-side voltage waveform of the offshore high-voltage MMC converter under a single-pole grounding fault in a high-voltage DC submarine cable according to an embodiment of the present invention.

[0015] Figure 6 This is the overvoltage waveform of the medium-voltage CNPC converter at the sending end of the offshore wind farm AC grid under a three-phase grounding fault in an embodiment of the present invention. Detailed Implementation

[0016] 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.

[0017] like Figure 1 As shown, in this embodiment of the invention, the offshore high-voltage and medium-voltage hybrid DC transmission system includes: an onshore high-voltage connection transformer 1, a high-voltage circuit breaker 2, an onshore high-voltage MMC converter 3, a high-voltage DC submarine cable 4, an offshore high-voltage MMC converter 5, an offshore high-voltage connection transformer 6, a high-voltage and medium-voltage system connection circuit breaker 7, a step-down transformer 8, a medium-voltage circuit breaker 9, a sending-end medium-voltage connection transformer 10, a sending-end medium-voltage CNPC converter 11, a medium-voltage DC submarine cable 12, a receiving-end medium-voltage CNPC converter 13, a receiving-end medium-voltage connection transformer 14, a wind farm circuit breaker 15, and a wind farm collection submarine cable 16.

[0018] The onshore high-voltage connection transformer 1 is connected to the onshore high-voltage AC grid and the onshore high-voltage MMC converter 3 via high-voltage circuit breaker 2. The onshore high-voltage MMC converter 3 is connected to the offshore high-voltage MMC converter 5 via high-voltage DC submarine cable 4. The offshore high-voltage MMC converter 5 is connected to the offshore high-voltage connection transformer 6 via high-voltage circuit breaker 2. The offshore high-voltage connection transformer 6 is connected to the offshore AC bus via high-voltage circuit breaker 2. The offshore wind turbine is connected to the offshore AC bus via wind farm circuit breaker 15 and wind farm collection submarine cable 16. (The last sentence appears to be incomplete and possibly refers to a different process.) The medium-voltage connection transformer 14 is connected to the offshore operation and maintenance platform and the receiving-end medium-voltage CNPC converter 13 respectively through the medium-voltage circuit breaker 9. The receiving-end medium-voltage CNPC converter 13 is connected to the sending-end medium-voltage CNPC converter 11 through the medium-voltage DC submarine cable 12. The sending-end medium-voltage CNPC converter 11 is connected to the sending-end medium-voltage connection transformer 10 through the medium-voltage circuit breaker 9. The sending-end medium-voltage connection transformer 10 is connected to the step-down transformer 8 through the medium-voltage circuit breaker 9. The step-down transformer 8 is connected to the offshore AC bus through the high-voltage and medium-voltage system connection circuit breaker 7.

[0019] In this embodiment of the invention, the topology of the high-voltage MMC converter is as follows: Figure 2 As shown, it contains a total of three phases and six bridge arms. Each bridge arm consists of multiple sub-modules connected in series, and the bridge arm reactor is located on the DC side of the series sub-module.

[0020] In this embodiment of the invention, the topology of the medium-voltage CNPC converter is as follows: Figure 3 As shown, a topology is adopted in which multiple NPC converters are connected in parallel on the AC side and in series on the DC side.

[0021] In this embodiment of the invention, the overvoltage calculation method for a hybrid offshore high-voltage and medium-voltage DC transmission system specifically includes the following steps: 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 offshore high-voltage and medium-voltage DC transmission system.

[0022] In the electromagnetic transient simulation model of the onshore high-voltage AC power grid, three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions are respectively set. When the DC bus voltage exceeds the overvoltage protection threshold, the onshore high-voltage MMC converter 3 and the offshore high-voltage MMC converter 5 are locked, the high-voltage circuit breaker 2 is opened, and then the sending-end medium-voltage CNPC converter 11 and the receiving-end medium-voltage CNPC converter 13 are locked, and the medium-voltage circuit breaker 9 is opened. The overvoltage values ​​at the key locations of the system are obtained by simulation calculation using the above simulation model. The overvoltages at key locations in the system include the grid-side voltage of the onshore high-voltage MMC converter, the valve-side voltage of the onshore high-voltage MMC converter, the grid-side voltage of the offshore high-voltage MMC converter, the valve-side voltage of the offshore high-voltage MMC converter, the voltage of the high-voltage DC submarine cable, the grid-side voltage of the sending-end medium-voltage CNPC converter, the valve-side voltage of the sending-end medium-voltage CNPC converter, the grid-side voltage of the receiving-end medium-voltage CNPC converter, the valve-side voltage of the receiving-end medium-voltage CNPC converter, and the voltage of the medium-voltage DC submarine cable.

[0023] In the electromagnetic transient simulation model of the offshore wind farm AC power grid, three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions are respectively set. After the fault occurs, the wind farm circuit breaker 15 on both sides of the fault line is disconnected after the first protection action time. The overvoltage value at the key location of the system is obtained by simulation calculation.

[0024] On the valve side of the onshore high-voltage converter station in the electromagnetic transient simulation model ( Figure 1 The location between the onshore high-voltage connection transformer 1 and the onshore high-voltage MMC converter 3, and the valve side of the offshore high-voltage converter station ( Figure 1 The onshore high-voltage MMC converter 5 and the offshore high-voltage connecting transformer 6 are respectively set to three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions. After the fault occurs, the onshore high-voltage MMC converter 3 and the offshore high-voltage MMC converter 5 are locked after the second protection action time, and the high-voltage circuit breaker 2 is opened. Then the sending end medium-voltage CNPC converter 11 and the receiving end medium-voltage CNPC converter 13 are locked, and the medium-voltage circuit breaker 9 is opened. The overvoltage value at the key position of the system is obtained by simulation calculation using the above simulation model.

[0025] In the electromagnetic transient simulation model, the medium-voltage AC bus at the sending end ( Figure 1 The busbar between the medium-voltage step-down transformer 8 and the sending-end medium-voltage connection transformer 10), and the receiving-end medium-voltage AC busbar ( Figure 1The intermediate-voltage connection transformer 14 at the receiving end and the busbar between the offshore maintenance platform are respectively set to three-phase grounding short-circuit fault condition, two-phase grounding short-circuit fault condition, two-phase phase-to-phase short-circuit fault condition, and single-phase grounding short-circuit fault condition. After the fault occurs, after the third protection action time, the sending end intermediate-voltage CNPC converter 11 and the receiving end intermediate-voltage CNPC converter 13 are blocked, the intermediate-voltage circuit breaker 9 is opened, the high-voltage and intermediate-voltage system connection circuit breaker 7 is opened, and the onshore high-voltage MMC converter 3, the offshore high-voltage MMC converter 5 and the high-voltage circuit breaker 2 do not operate. The overvoltage value at the key position of the system is obtained by simulation calculation using the above simulation model.

[0026] In the electromagnetic transient simulation model, the high-voltage DC submarine cable 4 is set to have single-pole grounding fault and double-pole grounding fault conditions respectively. After the fault occurs, after the fourth protection action time, the onshore high-voltage MMC converter 3 and the offshore high-voltage MMC converter 5 are locked, the high-voltage circuit breaker 2 is opened, and then the sending end medium-voltage CNPC converter 11 and the receiving end medium-voltage CNPC converter 13 are locked, the medium-voltage circuit breaker 9 is opened, and the overvoltage value at the key position of the system is obtained by simulation calculation using the above simulation model.

[0027] In the electromagnetic transient simulation model, the medium-voltage DC submarine cable 12 is set to have both single-pole grounding fault and double-pole grounding fault conditions. After the fault occurs, after the fifth protection action time, the sending-end medium-voltage CNPC converter 11 and the receiving-end medium-voltage CNPC converter 13 are locked, the medium-voltage circuit breaker 9 is opened, the high-voltage and medium-voltage system connection circuit breaker 7 is opened, and the onshore high-voltage MMC converter 3, the offshore high-voltage MMC converter 5 and the high-voltage circuit breaker 2 do not operate. The overvoltage values ​​at the key locations of the system are obtained by simulation calculation using the above simulation model.

[0028] 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.

[0029] 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.

[0030] like Figure 4 The figure shows the overvoltage waveform on the valve side of the medium-voltage CNPC converter at the sending end under a single-pole grounding fault of the medium-voltage DC submarine cable in an embodiment of the present invention. When a single-pole grounding fault occurs in the medium-voltage DC submarine cable 12, the voltage on the valve side of the medium-voltage CNPC converter at the sending end will generate a further DC bias, resulting in an overvoltage on the valve side of the medium-voltage CNPC converter at the sending end.

[0031] like Figure 5The figure shows the voltage waveform of the valve side of the offshore high-voltage MMC converter under a single-pole grounding fault of the high-voltage DC submarine cable in an embodiment of the present invention. When a single-pole grounding fault occurs in the high-voltage DC submarine cable 4, the valve side voltage of the offshore high-voltage MMC converter will be biased by DC, resulting in overvoltage on the valve side of the offshore high-voltage MMC converter.

[0032] like Figure 6 The figure shows the overvoltage waveform on the grid side of the medium-voltage CNPC converter at the sending end under a three-phase ground fault in the AC grid of an offshore wind farm, as illustrated in this embodiment of the invention. When a three-phase ground fault occurs in the AC grid of an offshore wind farm, the grid-side voltage of the medium-voltage CNPC converter at the sending end drops synchronously. When the circuit breakers on both sides of the faulted line open and the AC grid voltage of the offshore wind farm recovers, an overvoltage will occur on the grid side of the medium-voltage CNPC converter at the sending end.

[0033] 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 offshore high-voltage and medium-voltage DC transmission system, characterized in that: An electromagnetic transient simulation model of a hybrid offshore high-voltage and medium-voltage DC transmission system 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 in the onshore high-voltage AC grid, offshore wind farm AC grid, offshore sending-end medium-voltage AC bus, receiving-end medium-voltage AC bus, onshore high-voltage converter station valve side, and offshore high-voltage converter station valve side. Single-pole grounding fault and double-pole grounding fault conditions were also simulated in the high-voltage and medium-voltage DC submarine cables. The simulation model is designed to simulate overvoltage conditions and, based on the control and protection strategies of the offshore high-voltage and medium-voltage hybrid DC transmission system, sets the correct operating logic for the onshore high-voltage MMC converter, offshore high-voltage MMC converter, sending-end medium-voltage CNPC converter, receiving-end medium-voltage CNPC converter, high-voltage circuit breaker, and medium-voltage circuit breaker in the electromagnetic transient simulation model. The simulation calculation yields the overvoltage values ​​at key locations in the system. By traversing all the above operating conditions, the maximum overvoltage value is selected to obtain the representative overvoltage of the offshore high-voltage and medium-voltage hybrid DC transmission system.

2. The overvoltage calculation method for a hybrid offshore high-voltage and medium-voltage DC transmission system according to claim 1, characterized in that, Specifically, the following steps are included: Establish an electromagnetic transient simulation model for a hybrid offshore high-voltage and medium-voltage DC transmission system; In the electromagnetic transient simulation model, the onshore high-voltage AC power grid is configured with three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions. When the DC bus voltage exceeds the overvoltage protection threshold, the onshore high-voltage MMC converter and the offshore high-voltage MMC converter are locked, and the high-voltage circuit breaker is opened. Then, the sending-end medium-voltage CNPC converter and the receiving-end medium-voltage CNPC converter are locked, and the medium-voltage circuit breaker is opened. The overvoltage values ​​at key locations of the system are obtained through simulation calculation. In the electromagnetic transient simulation model of the offshore wind farm AC power grid, three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions are respectively set. After the fault occurs, the wind farm circuit breakers on both sides of the fault line are disconnected after the first protection action time. The overvoltage values ​​at the key locations of the system are obtained by simulation calculation. Three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault are respectively set on the valve side of the onshore high-voltage converter station and the valve side of the offshore high-voltage converter station in the electromagnetic transient simulation model. After the fault occurs, the onshore high-voltage MMC converter and the offshore high-voltage MMC converter are locked after the second protection action time, and the high-voltage circuit breaker is opened. Then the sending end medium-voltage CNPC converter and the receiving end medium-voltage CNPC converter are locked, and the medium-voltage circuit breaker is opened. The overvoltage value at the key position of the system is obtained by simulation calculation. In the electromagnetic transient simulation model, the sending-end medium-voltage AC bus and the receiving-end medium-voltage AC bus are respectively set to three-phase ground fault, two-phase ground fault, two-phase phase-to-phase fault, and single-phase ground fault conditions. After the fault occurs, after the third protection action time, the sending-end medium-voltage CNPC converter and the receiving-end medium-voltage CNPC converter are locked, the medium-voltage circuit breaker is opened, the high-voltage and medium-voltage system connection circuit breaker is opened, the onshore high-voltage MMC converter, the offshore high-voltage MMC converter and the high-voltage circuit breaker do not operate, and the overvoltage value at the key location of the system is obtained by simulation calculation. In the electromagnetic transient simulation model, the high-voltage DC submarine cable is set to single-pole grounding fault condition and double-pole grounding fault condition respectively. After the fault occurs, after the fourth protection action time, the onshore high-voltage MMC converter and the offshore high-voltage MMC converter are locked, the high-voltage circuit breaker is opened, and then the sending end medium-voltage CNPC converter and the receiving end medium-voltage CNPC converter are locked, the medium-voltage circuit breaker is opened, and the overvoltage value at the key position of the system is obtained by simulation calculation. In the electromagnetic transient simulation model, the medium-voltage DC submarine cable is set to single-pole grounding fault and double-pole grounding fault conditions respectively. After the fault occurs, after the fifth protection action time, the sending-end medium-voltage CNPC converter and the receiving-end medium-voltage CNPC converter are locked, the medium-voltage circuit breaker is opened, the high-voltage and medium-voltage system connection circuit breaker is opened, the onshore high-voltage MMC converter, the offshore high-voltage MMC converter and the high-voltage circuit breaker do not operate, and the overvoltage value at the key location of the system is obtained by simulation calculation. 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 offshore high-voltage and medium-voltage hybrid DC transmission systems according to claim 1, characterized in that: The overvoltages at key locations in the system include the grid-side voltage of the onshore high-voltage MMC converter, the valve-side voltage of the onshore high-voltage MMC converter, the grid-side voltage of the offshore high-voltage MMC converter, the valve-side voltage of the offshore high-voltage MMC converter, the voltage of the high-voltage DC submarine cable, the grid-side voltage of the sending-end medium-voltage CNPC converter, the valve-side voltage of the sending-end medium-voltage CNPC converter, the grid-side voltage of the receiving-end medium-voltage CNPC converter, the valve-side voltage of the receiving-end medium-voltage CNPC converter, and the voltage of the medium-voltage DC submarine cable.

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