Comprehensive overvoltage suppression method based on offshore wind power output system through intermediate frequency MMC (Modular Multilevel Converter)

Abstract

The invention discloses a comprehensive overvoltage suppression method based on an intermediate-frequency MMC sending-out system of offshore wind power, namely, when overvoltage is caused by surplus power of a direct-current system due to a fault, an overvoltage suppression method combining an active step-down method of an offshore alternating-current system and a switching method of a direct-current energy consumption device based on a half-bridge sub-module is adopted. When the AC voltage of the grid-connected point of the wind power plant is reduced, a slope strategy is adopted to control the switching of an energy consumption device sub-module, and the design of an energy consumption resistor depends on the power level of the wind power plant after voltage reduction; when a permanent fault occurs to cause overvoltage, a converter station split-phase locking overvoltage suppression method is adopted, and locking judgment of three-phase bridge arms is mutually independent. According to the invention, the overvoltage suppression problem of the offshore wind power flexible direct output system under different faults is solved, the expansibility is strong, the implementation is simple, low-cost and high-reliability output of offshore wind power can be realized, and the method has a certain engineering application value.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission and distribution of electric power systems, and in particular relates to a comprehensive overvoltage suppression method based on an offshore wind power transmission system via a medium frequency MMC. Background Art

[0002] As offshore wind power resources are exhausted, offshore wind power projects are gradually moving towards the mid-sea area with richer and more stable resources. Compared with high-voltage AC, the flexible DC transmission system based on modular multilevel converter (MMC) technology has shown good application prospects in large-scale offshore wind power transmission due to its lower manufacturing difficulty, lower switching loss and higher waveform quality. Flexible DC transmission has advantages in both technology and economy, and offshore wind power using flexible DC transmission will be the future development direction.

[0003] When an AC power grid or DC transmission line fails, the safety and stability of the offshore wind power flexible direct current transmission system will be seriously affected; the AC and DC overvoltage problems caused by the fault will not only threaten the safe and stable operation of the system, but also put forward higher requirements on the withstand voltage and insulation performance of equipment and lines, thus significantly increasing the construction cost of the project. Therefore, the research on the fault overvoltage suppression strategy of the offshore wind power flexible direct current transmission system has attracted widespread attention.

[0004] For the offshore wind power flexible direct current transmission system, the offshore converter station generally adopts Vf control, so the power flowing into the DC system will be determined by the output power of the wind farm; when the onshore converter station or AC power grid fails, the power transmission of the onshore converter station is blocked, and there will be surplus power in the DC system in a short time, causing the DC voltage to rise rapidly, endangering the safety of the system. At present, the commonly used method to solve the surplus power is to install energy dissipation devices on the DC side of the onshore converter station. At present, most of the DC energy dissipation devices in the project adopt a centralized arrangement scheme in which high-power switching devices are directly connected in series and then connected in series with energy dissipation resistors. This scheme has high control requirements for power devices and is prone to large impacts during the switching process of the device; at the same time, in order to fully dissipate the surplus power in the DC system, the design of the DC energy dissipation device needs to match the rated power of the offshore wind farm, which has problems such as high cost, large footprint and high heat dissipation requirements. Another way to solve the surplus power problem is to actively reduce the AC voltage at the offshore wind farm grid connection point to temporarily reduce the power sent to the DC system; however, in order not to affect the normal operation of the wind turbines, the reduction in AC voltage must be limited to a certain range, and the onshore AC system may suffer serious faults resulting in a complete loss of power delivery capacity. Therefore, this method cannot completely solve the power surplus problem of the DC system.

[0005] For offshore wind power flexible direct current transmission systems, when permanent faults occur (including direct current cable faults and faults in converter stations, etc.), the converter station needs to be locked to suppress overvoltage and protect converter station devices. The commonly used converter station locking scheme is to directly lock all submodules, but in the event of an asymmetric fault, directly locking all phase submodules will affect the normal operation of the non-fault phase, thereby expanding the scope of the fault and making the overvoltage more serious.

[0006] Therefore, for large-scale wind power transmission systems via medium-frequency MMC DC, a more universal and effective overvoltage suppression strategy is urgently needed to overcome the defects of existing technologies. Summary of the invention

[0007] In view of the above, the present invention provides a comprehensive overvoltage suppression method based on the offshore wind power transmission system via a medium frequency MMC, which can solve the overvoltage suppression problem of the offshore wind power flexible direct current transmission system under different faults. It has strong scalability and simple implementation, and can realize low-cost and high-reliability transmission of offshore wind power, and has certain engineering application value.

[0008] A comprehensive overvoltage suppression method based on an offshore wind power transmission system via a medium frequency MMC, wherein the transmission system is composed of an offshore wind farm, an offshore rectifier station, an onshore inverter station and a receiving-end power grid connected in sequence, the offshore rectifier station and the onshore inverter station are connected via a DC submarine cable, and both the offshore rectifier station and the onshore inverter station use MMC;

[0009] When a fault occurs in the receiving power grid, causing a power surplus in the DC system (including the DC side of the offshore rectifier station, the DC submarine cable, and the DC side of the onshore inverter station) causing overvoltage, the overvoltage is suppressed by combining the active voltage reduction method of the offshore AC system with the switching method of the DC energy-consuming device based on the half-bridge submodule; when a permanent fault occurs, causing the DC system to overvoltage, the MMC phase-by-phase locking method is used to suppress the overvoltage.

[0010] Furthermore, when a short-term fault occurs in the receiving-end power grid, the power transmission of the onshore inverter station is blocked. If the power transmitted into the DC system by the offshore rectifier station does not match the power transmitted by the onshore inverter station, a power surplus will be generated in the DC system, causing the DC voltage to rise and a DC overvoltage to occur. The active voltage reduction law of the offshore AC system actively reduces the AC voltage amplitude of the offshore wind farm grid connection point through the offshore rectifier station to temporarily reduce the output power of the offshore wind farm.

[0011] Furthermore, the specific implementation method of the active voltage reduction method of the offshore AC system is as follows: when the DC voltage U dc Exceeding the maximum DC voltage U HWhen the fault occurs, the AC voltage command value of the offshore rectifier station is reduced to 0.2pu (per unit); when the fault is cleared, the AC voltage command value is restored to 1.0pu.

[0012] If a serious fault occurs in the receiving power grid, causing the onshore inverter station to lose all power output capacity, the active voltage reduction method of the offshore AC system cannot completely solve the power surplus problem of the DC system. Therefore, it is also necessary to use a DC energy consumption device switching method based on a half-bridge submodule to suppress overvoltage.

[0013] Furthermore, the specific implementation method of the DC energy consumption device switching method is: firstly, a DC energy consumption device is installed on the DC side of the onshore inverter station, and when the DC voltage U dc The voltage rises above the upper threshold U H1 When the DC energy consumption device is triggered, all the internal half-bridge sub-modules will be cut off, making the DC voltage U dc All of them are applied to the energy dissipation resistor to dissipate the surplus power of the DC system; when the DC voltage U dc Drops below the voltage threshold U L When the DC energy consumption device is turned on, all the half-bridge sub-modules inside will be turned on until the DC voltage U dc rises above the voltage threshold U H2 After that, the DC energy dissipation device is put into the energy dissipation resistor again to limit the DC voltage U dc Fluctuates within a certain range.

[0014] Furthermore, the DC energy dissipation device is composed of a plurality of energy dissipation valves and an energy dissipation resistor connected in series, and the energy dissipation valve is composed of a plurality of half-bridge sub-modules connected in series.

[0015] Furthermore, the calculation expression of the resistance value R of the energy dissipation resistor is as follows:

[0016]

[0017] Where: P wind To reduce the output power of the offshore wind farm after the AC voltage command value, U av It is the average DC voltage when the DC energy consuming device is put into operation during the fault period (1.03 to 1.05 times the rated DC voltage).

[0018] Furthermore, the maximum DC voltage U H The voltage threshold upper limit U is set to 1.05 to 1.1 times the rated DC voltage. H1 is set to 1.1 to 1.2 times the rated DC voltage. H2 The voltage threshold lower limit U is set to 1.05 to 1.07 times the rated DC voltage. LSet to 1.01 to 1.03 times the rated DC voltage.

[0019] Furthermore, the half-bridge submodules in the DC energy consumption device adopt a ramp control strategy, that is, the half-bridge submodules are cut off or put into operation one by one according to a predetermined slope.

[0020] When a permanent fault (such as a ground fault in a DC line or a fault in a converter station) causes overvoltage, it is necessary to lock the MMC in phases to suppress the overvoltage and reduce the impact of the fault phase on the normal operation of the non-fault phase.

[0021] Furthermore, the MMC phase-by-phase locking method is to lock each of the three-phase bridge arms of the MMC independently. If the fault current of a phase is greater than twice the rated current of the submodule, a shutdown control signal will be sent to all submodules of this phase, and all submodules of this phase will enter a locked state. The submodules of the remaining two phases will be locked after 10ms, and the circuit breaker on the MMC AC side will trip 100ms after the fault occurs to protect the safety of the MMC equipment.

[0022] Furthermore, the operating frequency of the AC side of the offshore rectifier station is a medium frequency, that is, in the range of 100 to 200 Hz, and the operating frequency of the AC side of the onshore inverter station is 50 Hz.

[0023] In a second aspect, the present invention provides a comprehensive overvoltage suppression system based on an offshore wind power transmission system via a medium frequency MMC, comprising:

[0024] Active voltage reduction control unit, used to suppress overvoltage by active voltage reduction method of offshore AC system when a fault occurs in the receiving-end power grid and causes overvoltage in the DC system due to power surplus;

[0025] The energy consumption device switching unit is used to suppress overvoltage by switching the DC energy consumption device based on the half-bridge submodule when a fault occurs in the receiving-end power grid, resulting in a power surplus in the DC system and causing overvoltage;

[0026] The MMC phase-separated locking unit is used to suppress overvoltage by means of MMC phase-separated locking when a permanent fault occurs and causes overvoltage in the DC system.

[0027] In a third aspect, the present invention provides a computer device, comprising a memory and a processor, wherein the memory stores a computer program, and the processor is used to execute the computer program to implement the above-mentioned comprehensive overvoltage suppression method based on the offshore wind power transmission system via a medium frequency MMC.

[0028] In a fourth aspect, the present invention provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-mentioned comprehensive overvoltage suppression method based on the offshore wind power transmission system via a medium frequency MMC.

[0029] When a fault occurs in the receiving power grid, resulting in a power surplus in the DC system and causing overvoltage, the present invention uses a combination of an active voltage reduction method of the offshore AC system and a DC energy consumption device switching method based on a half-bridge submodule to suppress overvoltage; when a permanent fault occurs, resulting in an overvoltage in the DC system, the present invention uses an MMC phase-splitting interlocking method to suppress overvoltage, which can solve the overvoltage suppression problem of the offshore wind power flexible direct current transmission system under different faults, and can achieve low-cost and high-reliability transmission of offshore wind power, with high engineering application value. In addition, the present invention has strong versatility, and in theory, the comprehensive overvoltage suppression method is applicable to a variety of offshore wind power direct current transmission systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic diagram of the topological structure of the offshore wind power flexible direct current transmission system including direct current energy consumption devices.

[0031] Figure 2 This is a control flow diagram of the active voltage reduction method for the offshore AC system.

[0032] Figure 3 Schematic diagram of the topological structure of the DC energy consumption device.

[0033] Figure 4 This is a control flow diagram of the DC energy consumption device switching method.

[0034] Figure 5 This is a schematic diagram of the DC voltage change when the DC energy consumption device is switched on and off.

[0035] Figure 6 This is a schematic diagram of phase-by-phase locking in the MMC converter station.

[0036] Figure 7 The DC voltage simulation waveform diagram under the overvoltage suppression method combining AC voltage reduction and DC energy-consuming switching, where (a) corresponds to the case where the instantaneous unbalanced power of the DC system is 55MW, and (b) corresponds to the case where the instantaneous unbalanced power of the DC system is 28MW.

[0037] Figure 8 The DC voltage simulation waveforms under the overvoltage suppression methods of phase-by-phase locking and full-by-phase locking of the converter station are shown in Figure 1, where (a) corresponds to full-by-phase locking and (b) corresponds to phase-by-phase locking.

[0038] Fig. 9 The present invention is a schematic diagram of the structure of an electronic device implemented according to the method of the present invention. DETAILED DESCRIPTION

[0039] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0040] In this implementation, the topology of the offshore wind power flexible direct current transmission system containing direct current energy consumption devices is as follows: Figure 1 As shown in Figure 1, the output power of the wind turbine is sent to the offshore rectifier station (sending end MMC) through the offshore AC collection system, and then transmitted to the onshore inverter station through the DC transmission system containing DC energy consumption devices, thereby achieving AC grid connection at the receiving end. Both the rectifier station and the inverter station are MMC converter stations, the rated voltage of the DC submarine cable is ±200kV, the rated power is 250WM, and the remaining system parameters are shown in Table 1:

[0041] Table 1

[0042]

[0043]

[0044] The comprehensive overvoltage suppression method adopted in this embodiment includes the following steps:

[0045] (1) When a fault occurs and causes surplus power in the DC system, resulting in overvoltage, an overvoltage suppression method is adopted that combines the active voltage reduction method of the offshore AC system and the switching method of the DC energy dissipation device based on the half-bridge submodule.

[0046] (2) When a permanent fault occurs and causes overvoltage, the overvoltage suppression method of phase-by-phase locking of the converter station is adopted.

[0047] like Figure 2 As shown in the figure, the control logic of the active voltage reduction method of the offshore AC system is as follows:

[0048] When the MMC DC side voltage U dc Exceeding the maximum DC voltage U H When the fault is cleared, the command value of the offshore AC system voltage is reduced to 0.2pu; when the fault is cleared, the command value of the offshore AC system voltage is restored to 1.0pu. The initial action voltage U that triggers the active voltage reduction of the offshore AC system H Set to 1.1 times the rated DC voltage, i.e. 220kV.

[0049] When the AC voltage at the grid connection point of the wind farm is 1.0pu, the rated output power of the wind farm is 250MW; when the AC voltage command value of the outer loop control of the offshore station MMC drops to 0.2pu, after system testing, it can be obtained that the active power output of the wind farm at this time is about 55MW.

[0050] If a serious fault occurs in the AC side power grid of the onshore converter station, causing the converter station to completely lose its power output capacity, the overvoltage suppression method of active voltage reduction of the offshore AC system cannot solve the power surplus problem of the DC system. There is still a power surplus of 55MW, so it is necessary to supplement it with an overvoltage suppression method of the DC energy consumption device based on the half-bridge submodule.

[0051] like Figure 3 As shown, the DC energy dissipation device is composed of a plurality of energy dissipation valves and a centralized DC energy dissipation resistor, and the energy dissipation valve is composed of a plurality of half-bridge sub-modules connected in series.

[0052] The half-bridge submodule in the energy-consuming valve presents three states with different combinations of the upper and lower IGBTs: ① Locked state: both the upper IGBT and the lower IGBT are in the off state; ② Cut-off state: the upper IGBT is turned off, the lower IGBT is turned on, and the system DC voltage is directly applied to the energy-consuming resistor to discharge the system surplus power; ③ Put-in state: the upper IGBT is turned on, the lower IGBT is turned off, at this time the bridge arm current must flow through the module sub-capacitor, and the sub-module output voltage is the capacitor voltage.

[0053] When the energy-consuming device is in the cut-off state, the half-bridge sub-modules in the energy-consuming valve are all in the locked state, the energy-consuming valve bears all the DC voltage, and the voltage on both sides of the energy-consuming resistor is approximately zero; when the energy-consuming device is in the operation state, the half-bridge sub-modules in the energy-consuming valve will adopt a ramping control strategy and switch between the cut-off state and the operation state according to a predetermined slope.

[0054] Install DC energy dissipation devices on the DC side of the onshore converter station, such as Figure 4 As shown, when the DC voltage rises above the upper threshold U H1 After that, the DC energy dissipation device will be triggered to work. After the DC energy dissipation device is triggered, all half-bridge sub-modules will be cut off, and the DC voltage will be applied to the DC energy dissipation resistor to dissipate the surplus power on the DC side; when the DC voltage is lower than the lower threshold U L The device will stop consuming energy temporarily, and the voltage will rise again to U H2 Then put the resistor back in to limit the DC voltage fluctuation within a certain range, such as Figure 5 shown.

[0055] In order to avoid false triggering of the DC energy dissipation device during normal system operation, the initial operating voltage U H1 It needs to be higher than the rated DC voltage, so it is set to 1.1 times the rated DC voltage in this test system; the upper and lower limit voltages U H2 and U L Set to 1.07 times and 1.02 times the rated DC voltage.

[0056] The value of the centralized DC energy dissipation resistor depends on the output power of the wind farm after voltage reduction, and its calculation expression is as follows:

[0057]

[0058] Where: P windThe output power of the wind farm after the offshore AC system is stepped down is 55MW after the system is measured. av is the average DC voltage when the DC energy dissipation device is put into operation during the fault period, which is set to 1.05 times the rated DC voltage. Therefore, it can be calculated that the value of the centralized DC energy dissipation resistor is 3200Ω.

[0059] When permanent faults such as DC line grounding faults and converter station faults cause overvoltage, phase-by-phase locking of the MMC is required to suppress the overvoltage and reduce the impact of the fault phase on the normal operation of the non-fault phase.

[0060] The three-phase bridge arms of the MMC perform the blocking strategy independently. If the fault current of a phase is greater than twice the rated current of the submodule, a shutdown control pulse will be sent to all submodules of the phase, and all submodules of the phase will enter the blocking state. Figure 6 As shown; the submodules of the remaining phases that are not overcurrent will be locked after 10ms. The circuit breaker on the AC side of the converter station will trip 100ms after the fault occurs to protect the safety of the converter equipment.

[0061] Method (1) is verified based on the simulation model built based on the parameters in Table 1. Before the fault occurred, the offshore converter station operated in the fixed AC voltage control mode, and the offshore wind farm generated 250MW of power. Assuming that the onshore AC power grid failed at 1.0s, the power transmission of the onshore converter station was blocked, causing overvoltage in the DC line. When the DC voltage rose to 1.1pu, the AC voltage at the wind farm grid connection point was reduced to 0.2pu, and the DC energy consumption device was triggered. An overvoltage suppression method combining AC voltage reduction and DC energy consumption switching is now adopted, and simulation runs are carried out under the two conditions of 55MW and 28MW of instantaneous unbalanced power in the DC system. After collecting DC voltage data, the results are plotted as shown in the figure. Figure 7 shown.

[0062] When the instantaneous unbalanced power of the DC system is 55MW, the DC energy consumption device will always remain in the state of being put into operation during the fault period. dc It was successfully controlled within 1.12pu, achieving overvoltage suppression and fault ride-through of the entire system. When the instantaneous unbalanced power of the DC system was 28MW, the switching action of the DC energy consumption device was most frequent. After the DC energy consumption device was triggered, U dc It was successfully controlled between the design values ​​of 1.02pu and 1.07pu, effectively achieving the overvoltage suppression of the system. Therefore, the overvoltage suppression method combining AC voltage reduction and DC energy consumption switching, by maximizing the regulation of MMC and the wind farm itself, further reduced the selection pressure of centralized energy consumption resistors, and achieved an ideal overvoltage suppression effect.

[0063] Method (2) is verified based on the simulation model built based on the parameters in Table 1. Before the fault occurred, the offshore converter station was operating in the fixed AC voltage control mode, and the offshore wind farm generated 250MW of power. Assuming that a single-phase grounding fault occurred on the valve side of the offshore converter station at 1.0s, the AC circuit breaker was disconnected after 100ms. The overvoltage suppression method of the converter station phase-by-phase locking and the conventional full locking method are used for simulation operation. After collecting the DC voltage data, the results are plotted as shown in the figure. Figure 8 As shown; it can be seen from the figure that when a single-phase short circuit fault occurs on the converter transformer valve side, the phase-separated locking overvoltage suppression method is more effective in suppressing overvoltage than the conventional full locking method. The overvoltage reduction is obvious, which can reduce the insulation level of the converter station equipment and play a significant role in maintaining the safety and stability of the system and improving the economy of the system.

[0064] The embodiment of the present invention further provides a comprehensive overvoltage suppression system based on an offshore wind power transmission system via a medium frequency MMC, comprising:

[0065] Active voltage reduction control unit, used to suppress overvoltage by active voltage reduction method of offshore AC system when a fault occurs in the receiving-end power grid and causes overvoltage in the DC system due to power surplus;

[0066] The energy consumption device switching unit is used to suppress overvoltage by switching the DC energy consumption device based on the half-bridge submodule when a fault occurs in the receiving-end power grid, resulting in a power surplus in the DC system and causing overvoltage;

[0067] The MMC phase-separated locking unit is used to suppress overvoltage by means of MMC phase-separated locking when a permanent fault occurs and causes overvoltage in the DC system.

[0068] Regarding the system in the above embodiment, the specific manner in which each unit performs operations has been described in detail in the embodiment of the method, and will not be elaborated here.

[0069] Accordingly, the present invention also provides an electronic device, including a processor and a memory, wherein the memory is used to store a computer program, and the processor is used to execute the computer program to implement the above-mentioned comprehensive overvoltage suppression method based on the offshore wind power transmission system via the medium frequency MMC. Fig. 9 The figure shows a hardware structure of any electronic device with data processing capability in which the comprehensive overvoltage suppression system based on the offshore wind power transmission system via the medium frequency MMC provided by the embodiment of the present invention is located. Fig. 9 In addition to the processor, memory and network interface shown, the electronic device with data processing capability in which the system in the embodiment is located may also include other hardware according to the actual function of the electronic device, which will not be described in detail.

[0070] Accordingly, the present invention also provides a computer-readable storage medium on which computer instructions are stored, and when the instructions are executed by the processor, the above-mentioned comprehensive overvoltage suppression method based on the offshore wind power transmission system via the medium frequency MMC is implemented. The computer-readable storage medium can be the internal storage unit of the aforementioned electronic device with data processing capability, such as a hard disk or memory; or it can be other external storage devices, such as a plug-in hard disk, a smart memory card (SmartMedia Card, SMC), an SD card, a flash card (Flash Card), etc. equipped on the device. The computer-readable storage medium can also include both the internal storage unit of the electronic device with data processing capability and the external storage device, which is used to store the computer program and other programs and data required by the electronic device with data processing capability, and can also be used to temporarily store data that has been output or is to be output.

[0071] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art to the present invention based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A comprehensive overvoltage suppression method based on an offshore wind power transmission system via a medium frequency MMC, characterized in that: When a fault occurs in the receiving power grid, causing a power surplus in the DC system and causing overvoltage, the overvoltage is suppressed by combining the active voltage reduction method of the offshore AC system with the switching method of the DC energy consumption device based on the half-bridge submodule; when a permanent fault occurs, causing the DC system to overvoltage, the MMC phase-by-phase locking method is used to suppress the overvoltage.

2. The comprehensive overvoltage suppression method according to claim 1, characterized in that: When a short-term fault occurs in the receiving-end power grid, the power transmission of the onshore inverter station is blocked. If the power transmitted into the DC system by the offshore rectifier station does not match the power transmitted by the onshore inverter station, a power surplus will be generated in the DC system, resulting in a DC voltage rise and a DC overvoltage. The active voltage reduction law of the offshore AC system actively reduces the AC voltage amplitude of the offshore wind farm grid connection point through the offshore rectifier station to temporarily reduce the output power of the offshore wind farm; the transmission system includes an offshore wind farm, an offshore rectifier station, an onshore inverter station and a receiving-end power grid. The offshore rectifier station is connected to the onshore inverter station through a DC submarine cable, and both the offshore rectifier station and the onshore inverter station use MMC.

3. The comprehensive overvoltage suppression method according to claim 2, characterized in that: The specific implementation method of the active voltage reduction method of the offshore AC system is as follows: when the DC voltage U dc Exceeding the maximum DC voltage U H When the fault occurs, the AC voltage command value of the offshore rectifier station is reduced to 0.2pu; when the fault is cleared, the AC voltage command value is restored to 1.0pu.

4. The comprehensive overvoltage suppression method according to claim 3, characterized in that: The specific implementation method of the DC energy consumption device switching method is: firstly, a DC energy consumption device is installed on the DC side of the onshore inverter station, and when the DC voltage U dc The voltage rises above the upper threshold U H1 When the DC energy consumption device is triggered, all the internal half-bridge sub-modules will be cut off, making the DC voltage U dc All of them are applied to the energy dissipation resistor to dissipate the surplus power of the DC system; when the DC voltage U dc Drops below the voltage threshold U L When the DC energy consumption device is turned on, all the half-bridge sub-modules inside will be turned on until the DC voltage U dc rises above the voltage threshold U H2 After that, the DC energy dissipation device is put into the energy dissipation resistor again to limit the DC voltage U dc Fluctuates within a certain range.

5. The comprehensive overvoltage suppression method according to claim 4, characterized in that: The DC energy dissipation device comprises a plurality of energy dissipation valves connected in series and an energy dissipation resistor, wherein the energy dissipation valve is composed of a plurality of half-bridge submodules connected in series.

6. The comprehensive overvoltage suppression method according to claim 5, characterized in that: The calculation expression of the resistance value R of the energy dissipation resistor is as follows: Where: P wind To reduce the output power of the offshore wind farm after the AC voltage command value, U av It is the average DC voltage when the DC energy consumption device is put into operation during the fault period.

7. The comprehensive overvoltage suppression method according to claim 4, characterized in that: The maximum DC voltage U H The voltage threshold upper limit U is set to 1.05 to 1.1 times the rated DC voltage. H1 is set to 1.1 to 1.2 times the rated DC voltage. H2 The voltage threshold lower limit U is set to 1.05 to 1.07 times the rated DC voltage. L Set to 1.01 to 1.03 times the rated DC voltage.

8. The comprehensive overvoltage suppression method according to claim 5, characterized in that: The half-bridge submodules in the DC energy consumption device adopt a ramp control strategy, that is, the half-bridge submodules are cut off or put into operation one by one according to a predetermined slope.

9. The comprehensive overvoltage suppression method according to claim 2, characterized in that: The MMC phase-by-phase locking method is to lock each of the three-phase bridge arms of the MMC independently. If the fault current of a phase is greater than twice the rated current of the submodule, a shutdown control signal will be sent to all submodules of this phase, and all submodules of this phase will enter the locked state. The submodules of the remaining two phases will be locked after 10ms, and the circuit breaker on the MMC AC side will trip 100ms after the fault occurs to protect the safety of the MMC equipment.

10. The comprehensive overvoltage suppression method according to claim 2, characterized in that: The operating frequency of the AC side of the offshore rectifier station is a medium frequency, that is, in the range of 100 to 200 Hz, and the operating frequency of the AC side of the onshore inverter station is 50 Hz.

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