A method and system for suppressing zero sequence fluctuation of a decoupled power distribution network

Abstract

The present application belongs to the field of direct current power distribution network, and particularly relates to a zero sequence fluctuation suppression method and system for a non-coupled variable power distribution network. According to different characteristics of fault stations and non-fault stations, different zero sequence voltage feedforward suppression modes are set, so that the present method can be applied in the case that the current converter is a fault station or a non-fault station, and thus can be applied to more zero sequence fluctuation suppression scenarios. The converter configured with the present suppression method can eliminate zero sequence fluctuation under various working conditions, and ensure that the non-fault station is not affected by the zero sequence fluctuation. Two different suppression conditions are set for the zero sequence voltage suppression of the non-fault station, and can be flexibly selected according to the actual situation. And the zero sequence fluctuation suppression strategy is only completed at the selected one suppression end, only one of the converters is set as a structure including full-bridge sub-modules as the suppression end, and the remaining converters can all adopt pure half-bridge sub-modules, so as to reduce the number of full-bridge sub-modules, and to reduce the system land occupation and cost.

Description

Technical Field

[0001] This invention belongs to the field of DC distribution networks, specifically relating to a method and system for suppressing zero-sequence fluctuations in a disconnected distribution network. Background Technology

[0002] Traditional medium-voltage distribution networks generally adopt a "closed-loop design, open-loop operation" mode, lacking the capabilities for power flow regulation, load balancing, and continuous load transfer. Open-loop operation results in poor power supply reliability, while closed-loop operation suffers from excessive short-circuit current and electromagnetic loop problems. By using DC converters to achieve flexible regional interconnection of the AC grid, on the one hand, the ring connection improves power supply reliability and limits the propagation range of AC faults; on the other hand, through the control of the DC converters, the short-circuit current of the grid can be effectively limited, protecting the safety of grid equipment. Simultaneously, the power flow of the regional grid is evenly distributed, improving the utilization rate of grid equipment.

[0003] In existing flexible DC transmission technologies based on modular multilevel converters (MMCs), MMCs typically use coupling transformers to connect to the AC grid. When an asymmetrical fault occurs in the AC grid, the coupling transformer can isolate the zero-sequence voltage between the grid and the converter valve, preventing the zero-sequence voltage from being conducted to the DC side and other connected AC systems, resulting in high system reliability. However, the configuration of coupling transformers significantly increases the cost and footprint of flexible DC distribution systems. For compact design requirements, a flexible DC medium-voltage distribution network without coupling transformers is needed. However, without coupling transformers, the zero-sequence component of the AC system cannot be naturally isolated. The zero-sequence voltage on the fault side will cause fluctuations in the neutral point voltage on the DC side of the converter, and this will be transmitted through... Figure 1 The medium-voltage flexible loop-closing device shown transmits the fault to other connected AC systems, expanding the impact range of AC system failures.

[0004] In the existing technology, the medium-voltage flexible loop-connected device includes two MMC converters, each MMC corresponding to one converter station. The existing zero-sequence voltage fluctuation elimination is usually only for the faulty station, which is equivalent to the default that zero-sequence voltage fluctuation is eliminated only at the faulty station, without considering how to deal with the impact of zero-sequence voltage fluctuation at the non-faulty station. Moreover, since the faulty station and the non-faulty station have different fault characteristics, the zero-sequence voltage fluctuation elimination method for the faulty station is not applicable when the converter station is a non-faulty station. Therefore, if the faulty station fails to eliminate the zero-sequence voltage fluctuation in time, the non-faulty station will be greatly affected by the zero-sequence voltage fluctuation. Summary of the Invention

[0005] The purpose of this invention is to provide a method and system for suppressing zero-sequence fluctuations in a disconnected power distribution network, which solves the problem that existing zero-sequence fluctuation voltage elimination methods are not applicable to non-faulty stations, causing non-faulty stations to be affected by zero-sequence fluctuation voltages when faulty stations have not eliminated them.

[0006] To achieve the above objectives, this invention provides a method for suppressing zero-sequence fluctuations in a disconnected distribution network. The method uses an MMC converter with a full-bridge submodule within a dual- or multi-terminal flexible DC system as the suppression terminal. Zero-sequence fluctuation suppression is performed at this suppression terminal, which includes zero-sequence voltage feedforward suppression. If the suppression terminal is a non-faulty station, the feedforward amount for the zero-sequence voltage feedforward suppression is determined based on the zero-sequence component of the DC side neutral point voltage or the zero-sequence component of the faulty station's AC voltage. The feedforward amount is superimposed onto a voltage modulation wave, and a trigger pulse is generated based on the superimposed voltage modulation wave and output to the submodule at the suppression terminal to suppress zero-sequence voltage fluctuations.

[0007] The term "faulted station" refers to an MMC converter that experiences an asymmetrical fault on the AC side, while "non-faulted station" refers to an MMC converter that does not experience an asymmetrical fault on the AC side.

[0008] The beneficial effects of the above technical solution are as follows: Based on the propagation characteristics of zero-sequence fluctuations in non-faulty stations during asymmetrical faults in AC systems, a targeted zero-sequence voltage feedforward suppression method is set, making the zero-sequence fluctuation suppression method of the present invention applicable even when the current MMC converter is a non-faulty station, fully ensuring that it is not affected by zero-sequence fluctuations; two different and replaceable suppression conditions are set for the zero-sequence voltage feedforward suppression of non-faulty stations, which can be flexibly selected according to actual conditions to further adapt to different operating conditions; and only one of the MMC converters in the dual-terminal or multi-terminal flexible DC system needs to be set as the suppression end with a structure including a full-bridge submodule for configuring and implementing the zero-sequence fluctuation suppression method described in this embodiment, then the remaining MMC converters in the flexible DC system can all adopt a pure half-bridge submodule structure, thereby reducing the number of full-bridge submodules configured in the entire system, and thus reducing the system footprint and cost.

[0009] Furthermore, if the feedforward amount for zero-sequence voltage feedforward suppression is determined based on the zero-sequence component of the AC voltage at the fault station, then the value of the feedforward amount is the product of the zero-sequence component of the AC voltage at the fault station and the first feedforward coefficient.

[0010] Furthermore, the first feedforward coefficient is obtained as follows:

[0011]

[0012] Where k is the first feedforward coefficient, m0 is the no-load regulation, and u vm U is the peak value of the rated AC phase voltage.dc This is the rated DC voltage on the DC side.

[0013] Furthermore, if the feedforward amount for zero-sequence voltage feedforward suppression is determined based on the zero-sequence component of the DC-side neutral point voltage, then the value of the feedforward amount is the product of the zero-sequence component of the DC-side neutral point voltage and the second feedforward coefficient, where the second feedforward coefficient is -1.

[0014] Furthermore, if the suppression terminal is a fault station, the feedforward amount of the zero-sequence voltage feedforward suppression is determined based on the zero-sequence component of the AC voltage of the suppression terminal itself; the feedforward amount is superimposed on the voltage modulation wave, and a trigger pulse is generated based on the superimposed voltage modulation wave and output to the submodule of the suppression terminal to suppress the zero-sequence fluctuation voltage.

[0015] The beneficial effects of the above technical solution are as follows: Based on the characteristics of the fault station, a zero-sequence voltage feedforward suppression method suitable for the fault station is set, so that the zero-sequence fluctuation suppression method of the present invention can be applied to both non-faulty and faulty MMC converters. Therefore, it can be applied to more zero-sequence fluctuation suppression scenarios. MMC converters equipped with the zero-sequence fluctuation suppression method of the present invention can eliminate zero-sequence fluctuation voltage and current under various operating conditions, further ensuring that converters equipped with the zero-sequence fluctuation suppression method are not affected by zero-sequence fluctuation voltage.

[0016] Furthermore, if the suppression terminal is a faulty station, then the value of the feedforward quantity is the product of the zero-sequence component of the AC voltage of the suppression terminal itself and the third feedforward coefficient.

[0017] Furthermore, the method for obtaining the third feedforward coefficient is as follows:

[0018]

[0019] Where m0 is the third feedforward coefficient, u vm U is the peak value of the rated AC phase voltage. dc This is the rated DC voltage on the DC side.

[0020] Furthermore, the zero-sequence fluctuation suppression also includes zero-sequence current closed-loop control suppression. The control quantity of the zero-sequence current closed-loop control suppression is determined by the difference between the zero-sequence component of the AC current at the suppression end and the zero-sequence current reference value. The control quantity is superimposed on the voltage modulation wave, and a trigger pulse is generated based on the superimposed voltage modulation wave and output to the submodule at the suppression end to suppress the zero-sequence fluctuation current.

[0021] The zero-sequence current reference value is the target value of the zero-sequence component of the AC current at the suppression end to be achieved by the zero-sequence current closed-loop control suppression.

[0022] Furthermore, the current closed-loop control employs a PR controller.

[0023] This invention also provides a zero-sequence fluctuation suppression system for a disconnected distribution network, wherein the disconnected distribution network is a dual-terminal or multi-terminal flexible DC system. The zero-sequence fluctuation suppression system includes at least one MMC converter with a full-bridge submodule; it also includes a processor for executing program instructions to implement the zero-sequence fluctuation suppression method for the disconnected distribution network described above. The zero-sequence fluctuation suppression system can achieve the same beneficial effects as the zero-sequence fluctuation suppression method for the disconnected distribution network described above. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the topology of the double-end back-to-back medium-pressure flexible ring-closing device in the background art of this invention;

[0025] Figure 2 This is a block diagram of the zero-sequence fluctuation suppression strategy for non-faulty stations in an embodiment of the zero-sequence fluctuation suppression method for disconnected substations of the present invention;

[0026] Figure 3 This is a block diagram of the zero-sequence fluctuation suppression strategy for faulted stations in an embodiment of the zero-sequence fluctuation suppression method for disconnected power distribution networks of the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0028] Example of a zero-sequence fluctuation suppression method for disconnected power distribution networks:

[0029] This embodiment provides a technical solution for zero-sequence fluctuation suppression in a disconnected distribution network. The zero-sequence fluctuation suppression method uses one of the MMC converters in a dual-terminal or multi-terminal flexible DC system, which includes a full-bridge submodule, as the suppression end, and performs zero-sequence fluctuation suppression at this suppression end. The zero-sequence fluctuation suppression in this embodiment includes zero-sequence voltage feedforward suppression and zero-sequence current closed-loop control suppression, and suppresses both zero-sequence fluctuation voltage and zero-sequence fluctuation current.

[0030] Specifically, based on the AC side electrical equations of the flexible DC converter, the zero-sequence expression of the converter can be obtained:

[0031]

[0032] Where u o For the zero-sequence component of AC voltage, i o For the zero-sequence component of alternating current, u o e is the zero-sequence component of the DC-side neutral point voltage. oThe zero-sequence component of the modulation wave in the flexible DC converter indicates that the suppression methods for zero-sequence fluctuations differ between faulty and non-faulty stations. Based on the different propagation characteristics of zero-sequence fluctuations in faulty and non-faulty stations, the specific methods of zero-sequence voltage feedforward suppression can be divided into two types:

[0033] ① If the suppression terminal is a non-faulty station, the feedforward amount of the zero-sequence voltage feedforward suppression is determined based on the zero-sequence component of the DC side neutral point voltage or the zero-sequence component of the AC voltage of the faulty station. The zero-sequence component of the DC side neutral point voltage can be obtained directly at the DC side neutral point or by collecting the pole-to-ground voltage at the DC pole of the ground terminal. The theoretical values ​​of the zero-sequence component of the DC side neutral point voltage and the zero-sequence component of the DC pole-to-ground voltage are equal, so the values ​​can be used interchangeably. The zero-sequence component of the AC voltage of the faulty station is obtained at the faulty station.

[0034] Reference Figure 2 , Figure 2 middle u o For the zero-sequence component of the AC voltage at the faulty station, i o To suppress the zero-sequence component of the terminal alternating current, u o To suppress the zero-sequence component of the DC-side neutral point voltage (the zero-sequence component of the DC pole-to-ground voltage), e o To suppress the zero-sequence component of the modulation wave in the flexible DC converter, k is the feedforward coefficient; if the feedforward amount for zero-sequence voltage suppression is determined based on the zero-sequence component of the AC voltage at the fault station, the value of the feedforward amount is the zero-sequence component u of the AC voltage at the fault station. o The product of the first feedforward coefficient and the first feedforward coefficient is k = -m0. In this embodiment, m0 is the no-load modulation degree, which is a positive number used to match the zero-sequence component superimposed on the voltage modulation wave with the zero-sequence component of the system AC voltage, thereby effectively suppressing zero-sequence fluctuations. The first feedforward coefficient is obtained as follows:

[0035]

[0036] Where k is the first feedforward coefficient, m0 is the no-load regulation, and u vm U is the peak value of the rated AC phase voltage. dc This is the rated DC voltage on the DC side.

[0037] If the feedforward quantity for zero-sequence voltage feedforward suppression is based on the zero-sequence component u of the DC side neutral point voltage... o If determined, the value of the feedforward quantity is the product of the zero-sequence component of the DC side neutral point voltage and the second feedforward coefficient, which corresponds to k = -1.

[0038] ② If the suppression terminal is a faulty station, the feedforward amount for the zero-sequence voltage feedforward suppression is determined based on the zero-sequence component of the AC voltage at the suppression terminal itself; refer to Figure 3 , Figure 3 middle u oTo suppress the zero-sequence component of the terminal AC voltage, i o To suppress the zero-sequence component of the terminal alternating current, u o To suppress the zero-sequence component of the DC-side neutral point voltage (the zero-sequence component of the DC pole-to-ground voltage), e o To suppress the zero-sequence component of the modulation wave of the flexible DC converter, the third feedforward coefficient is m0. In this embodiment, the m0 corresponding to the third feedforward coefficient and the first feedforward coefficient are both no-load modulation degrees, and the calculation methods are the same.

[0039] Regardless of whether it's a faulty or non-faulty station, after obtaining the feedforward amount for zero-sequence voltage feedforward suppression, this feedforward amount is superimposed on the voltage modulation wave. In this embodiment, the voltage modulation wave is represented in the form of zero-sequence voltage, which is the zero-sequence component e of the aforementioned flexible DC converter modulation wave. o The submodule generates a trigger pulse based on the superimposed voltage modulation wave and outputs it to the suppression terminal to suppress zero-sequence fluctuation voltage.

[0040] In this embodiment, "faulted station" refers to an MMC converter (converter station) experiencing an asymmetrical fault on the AC side, i.e., an MMC converter (converter station) connected to the faulty AC system. "Non-faulted station" refers to an MMC converter (converter station) where no asymmetrical fault has occurred on the AC side. Furthermore, the zero-sequence fluctuation suppression method in this embodiment is applicable to operating conditions where an asymmetrical fault occurs on the AC side.

[0041] Reference Figure 2 and Figure 3 The control quantity for zero-sequence current closed-loop control suppression is determined by the difference between the zero-sequence component of the AC current at the suppression end and the zero-sequence current reference value. Here, the zero-sequence current reference value is the target value of the zero-sequence component of the AC current at the suppression end to be achieved by the zero-sequence current closed-loop control suppression. In this embodiment, the input zero-sequence current reference value is 0. The PR controller is used to realize the zero-sequence current closed-loop control. The output control quantity makes the difference between the zero-sequence component of the AC current at the suppression end and the zero-sequence current reference value approach 0, that is, the zero-sequence component of the AC current at the suppression end approaches 0. The output control quantity is also superimposed on the voltage modulation wave. A trigger pulse is generated based on the superimposed voltage modulation wave and output to the sub-module at the suppression end to suppress the zero-sequence fluctuating current.

[0042] like Figure 1 The double-ended back-to-back medium-voltage flexible loop device shown corresponds to a double-ended flexible DC system, with one MMC converter (converter station) on each side. SM1, SM2, etc., are the various sub-modules within the converter, and both converters are directly connected to the AC grid. Due to the zero-sequence ripple suppression method, the zero-sequence component e of the modulation wave output by the flexible DC converter... oThe zero-sequence ripple suppression strategy must be implemented through a full-bridge submodule, but the cost of a full-bridge submodule is significantly higher than that of a half-bridge submodule. Therefore, considering the overall cost, this embodiment only implements the zero-sequence ripple suppression strategy at a selected suppression end. Thus, only one MMC converter in a dual-terminal or multi-terminal flexible DC system needs to be configured as the suppression end, including a structure with a full-bridge submodule (further considering cost, this structure can be a hybrid half-bridge and full-bridge submodule structure, not a pure full-bridge submodule structure) to configure and implement the zero-sequence ripple suppression method described in this embodiment. Then, the remaining MMC converters in the flexible DC system can all adopt a pure half-bridge submodule structure, thereby reducing the number of full-bridge submodules configured in the entire system, and thus reducing the system footprint and cost.

[0043] Example of a zero-sequence fluctuation suppression system for disconnected power distribution networks:

[0044] This embodiment provides a technical solution for a zero-sequence fluctuation suppression system in a disconnected substation. The disconnected substation is a dual-terminal or multi-terminal flexible DC system. The zero-sequence fluctuation suppression system includes at least one MMC converter with a full-bridge submodule, used as the suppression terminal to carry the output that suppresses zero-sequence fluctuations. It also includes a processor to execute program instructions to implement the zero-sequence fluctuation suppression method as described in the above embodiment of the zero-sequence fluctuation suppression method for disconnected substations. Since the specific principle and operation of this zero-sequence fluctuation suppression system have been described in detail in the above embodiment of the zero-sequence fluctuation suppression method for disconnected substations, they will not be repeated here.

[0045] This invention has the following characteristics:

[0046] 1) Based on the propagation characteristics of zero-sequence fluctuations in non-faulted stations during AC system asymmetric faults, different zero-sequence voltage feedforward suppression methods are specifically set. This solves the problem of mutual conduction of zero-sequence voltage and zero-sequence current between AC systems connected to the flexible loop device caused by unconnected transformers, and reduces the impact range of AC asymmetric faults under unconnected transformers. At the same time, it makes the zero-sequence fluctuation suppression method of the present invention applicable to both faulted and non-faulted MMC converters. Therefore, it can be applied to more zero-sequence fluctuation suppression scenarios. MMC converters equipped with the zero-sequence fluctuation suppression method of the present invention can eliminate zero-sequence fluctuation voltage and current under various operating conditions, ensuring that they are not affected by zero-sequence fluctuations.

[0047] 2) Two different and replaceable suppression conditions are set for zero-sequence voltage feedforward suppression of non-faulty stations, which can be flexibly selected according to the actual situation to further adapt to different operating conditions.

[0048] 3) The zero-sequence ripple suppression strategy is performed only at one selected suppression end. Only one MMC converter of the dual-terminal or multi-terminal flexible DC system needs to be set as the suppression end with a structure including a full-bridge submodule. This is used to configure and perform the zero-sequence ripple suppression method described in this embodiment. Then, the remaining MMC converters of the flexible DC system can adopt a pure half-bridge submodule structure, thereby reducing the number of full-bridge submodules configured in the entire system, and thus reducing the system footprint and cost.

[0049] It should be understood that the specific embodiments described above are merely illustrative or explanatory of the principles of the invention and do not constitute a limitation thereof. Therefore, any modifications, equivalent substitutions, improvements, etc., made without departing from the spirit and scope of the invention should be included within the protection scope of the invention. Furthermore, the appended claims are intended to cover all variations and modifications falling within the scope and boundaries of the appended claims, or equivalent forms of such scope and boundaries.

Claims

1. A method for suppressing zero-sequence fluctuations in a disconnected power distribution network, characterized in that, Using one of the MMC converters in a dual- or multi-terminal flexible DC system, which includes a full-bridge submodule, as the suppression terminal, zero-sequence ripple suppression is performed at the suppression terminal. The zero-sequence ripple suppression includes zero-sequence voltage feedforward suppression. If the suppression terminal is a non-faulty station, the feedforward amount of the zero-sequence voltage feedforward suppression is the product of the zero-sequence component of the AC voltage of the faulty station and the first feedforward coefficient, or the product of the zero-sequence component of the DC side neutral point voltage and the second feedforward coefficient. The first feedforward coefficient is negative of the no-load modulation degree, used to match the zero-sequence component superimposed on the voltage modulation wave with the zero-sequence component of the system AC voltage; the second feedforward coefficient is -1; if the suppression terminal is a fault station, the feedforward amount of the zero-sequence voltage feedforward suppression is the product of the zero-sequence component of the AC voltage of the suppression terminal itself and the third feedforward coefficient, which is the no-load modulation degree. The feedforward quantity is superimposed on the voltage modulation wave, and a trigger pulse is generated based on the superimposed voltage modulation wave and output to the suppression terminal submodule to suppress the zero-sequence fluctuation voltage. Faulty stations and non-faulty stations refer to MMC converters that have experienced AC side faults and those that have not, respectively.

2. The method for suppressing zero-sequence fluctuations in a disconnected power distribution network according to claim 1, characterized in that, The first feedforward coefficient is obtained in the following way: Where k is the first feedforward coefficient, m0 is the no-load regulation, and u vm U is the peak value of the rated AC phase voltage. dc This is the rated DC voltage on the DC side.

3. The method for suppressing zero-sequence fluctuations in a disconnected power distribution network according to claim 1, characterized in that, The method for obtaining the third feedforward coefficient is as follows: Where m0 is the third feedforward coefficient, which is also the no-load regulation; u vm U is the peak value of the rated AC phase voltage. dc This is the rated DC voltage on the DC side.

4. The method for suppressing zero-sequence fluctuations in a disconnected power distribution network according to claim 1 or 2, characterized in that, The zero-sequence fluctuation suppression also includes zero-sequence current closed-loop control suppression. The control quantity of the zero-sequence current closed-loop control suppression is determined by the difference between the zero-sequence component of the AC current at the suppression end and the zero-sequence current reference value. The control quantity is superimposed on the voltage modulation wave, and a trigger pulse is generated based on the superimposed voltage modulation wave and output to the sub-module at the suppression end to suppress the zero-sequence fluctuation current. The zero-sequence current reference value is the target value of the zero-sequence component of the AC current at the suppression end to be achieved by the zero-sequence current closed-loop control suppression.

5. The method for suppressing zero-sequence fluctuations in a disconnected power distribution network according to claim 4, characterized in that, The current closed-loop control uses a PR controller.

6. A zero-sequence fluctuation suppression system for a disconnected distribution network, wherein the disconnected distribution network is a dual-terminal or multi-terminal flexible DC system, characterized in that, The zero-sequence fluctuation suppression system includes at least one MMC converter with a full-bridge submodule; it also includes a processor for executing program instructions to implement the zero-sequence fluctuation suppression method for disconnected distribution networks as described below: Using one of the MMC converters in a dual- or multi-terminal flexible DC system, which includes a full-bridge submodule, as the suppression terminal, zero-sequence ripple suppression is performed at the suppression terminal. The zero-sequence ripple suppression includes zero-sequence voltage feedforward suppression. If the suppression terminal is a non-faulty station, the feedforward amount of the zero-sequence voltage feedforward suppression is the product of the zero-sequence component of the AC voltage of the faulty station and the first feedforward coefficient, or the product of the zero-sequence component of the DC side neutral point voltage and the second feedforward coefficient. The first feedforward coefficient is negative of the no-load modulation degree, used to match the zero-sequence component superimposed on the voltage modulation wave with the zero-sequence component of the system AC voltage; the second feedforward coefficient is -1; if the suppression terminal is a fault station, the feedforward amount of the zero-sequence voltage feedforward suppression is the product of the zero-sequence component of the AC voltage of the suppression terminal itself and the third feedforward coefficient, which is the no-load modulation degree. The feedforward quantity is superimposed on the voltage modulation wave, and a trigger pulse is generated based on the superimposed voltage modulation wave and output to the suppression terminal submodule to suppress the zero-sequence fluctuation voltage. Faulty stations and non-faulty stations refer to MMC converters that have experienced AC side faults and those that have not, respectively.

7. The zero-sequence fluctuation suppression system for disconnected power distribution networks according to claim 6, characterized in that, The first feedforward coefficient is obtained in the following way: Where k is the first feedforward coefficient, m0 is the no-load regulation, and u vm U is the peak value of the rated AC phase voltage. dc This is the rated DC voltage on the DC side.

8. The zero-sequence fluctuation suppression system for disconnected power distribution networks according to claim 6, characterized in that, The method for obtaining the third feedforward coefficient is as follows: Where m0 is the third feedforward coefficient, which is also the no-load regulation; u vm U is the peak value of the rated AC phase voltage. dc This is the rated DC voltage on the DC side.

9. The zero-sequence fluctuation suppression system for disconnected power distribution networks according to claim 6 or 7, characterized in that, The zero-sequence fluctuation suppression also includes zero-sequence current closed-loop control suppression. The control quantity of the zero-sequence current closed-loop control suppression is determined by the difference between the zero-sequence component of the AC current at the suppression end and the zero-sequence current reference value. The control quantity is superimposed on the voltage modulation wave, and a trigger pulse is generated based on the superimposed voltage modulation wave and output to the sub-module at the suppression end to suppress the zero-sequence fluctuation current. The zero-sequence current reference value is the target value of the zero-sequence component of the AC current at the suppression end to be achieved by the zero-sequence current closed-loop control suppression.

10. The zero-sequence fluctuation suppression system for disconnected power distribution networks according to claim 9, characterized in that, The current closed-loop control uses a PR controller.

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