An end-to-end coordinated suppression method and system for zero sequence fluctuation of a decoupled power distribution network

Abstract

The present application belongs to the field of the non-tie-in power distribution network, and particularly relates to a kind of end-to-end coordinated suppression method and system of non-tie-in power distribution network zero sequence fluctuation.The present application suppresses a part of zero sequence fluctuation voltage at the fault station, and suppresses the remaining zero sequence fluctuation voltage at the non-fault station;Through end-to-end coordination suppression, the part of zero sequence fluctuation voltage that is not eliminated due to control delay and other factors at the fault station can be naturally eliminated, and the influence of control delay of zero sequence fluctuation suppression control at the fault station on the suppression effect of zero sequence fluctuation voltage at the non-fault station is avoided;According to the different characteristics of the fault station and the non-fault station, different suppression strategies are set, so that the converter at the fault station or the non-fault station can participate in the coordinated suppression of zero sequence fluctuation voltage under the working condition, and all suppression ends can achieve the effect of end-to-end coordination by configuring the same set of suppression methods;And two replaceable feedforward quantity calculation methods are set for the non-fault station, which can form a more accurate coordination relationship with the zero sequence voltage feedforward quantity calculation method of the fault station.

Description

Technical Field

[0001] This invention belongs to the field of disconnected power distribution networks, and specifically relates to an inter-terminal coordination suppression method and system for zero-sequence fluctuations in disconnected power distribution networks. Background Technology

[0002] Traditional medium-voltage distribution networks typically achieve flexible regional interconnection of the AC grid through converters, effectively limiting short-circuit currents and protecting grid equipment. Simultaneously, the power flow across the regional grid is evenly distributed, improving equipment utilization. For example, flexible DC transmission networks based on MMC (Multi-Mode Controller)... Figure 1 The diagram shows a topology for a dual-end medium-voltage flexible loop device used to connect various AC systems. It includes two back-to-back MMC converters, each of which is equivalent to a converter station. The MMCs are typically connected to the AC grid via a connecting transformer. This connecting transformer can isolate the zero-sequence voltage between the grid and the converter when an asymmetrical fault occurs in the AC grid, thus avoiding fluctuations in the DC neutral point voltage of the converter caused by the zero-sequence voltage on the fault side, thereby achieving the effect of fault isolation.

[0003] Because connecting transformers significantly increase the cost and footprint of flexible DC distribution systems, existing technologies have begun to study distribution networks without connecting transformers. However, without connecting transformers, the zero-sequence component of the AC system cannot be isolated, allowing zero-sequence voltage to be conducted to the DC side of the faulty grid section and other connected AC systems, negatively impacting the unfaulty grid sections. In existing technologies, to avoid the impact of zero-sequence components of the AC system on unfaulty grid sections, configurations for eliminating zero-sequence fluctuation voltage are implemented for flexible DC distribution networks without connecting transformers. Since the fault characteristics of faulty and non-faulty stations differ, the method for eliminating zero-sequence fluctuation voltage is only effective when the MMC (Mechanical Management Control) configured for this method is located at the faulty station of the corresponding faulty AC system. Furthermore, although zero-sequence fluctuation voltage elimination at the faulty station can also eliminate zero-sequence fluctuations at non-faulty stations, the zero-sequence fluctuation suppression control at the faulty station may experience control delays relative to the actual zero-sequence fluctuation situation at non-faulty stations, resulting in poor elimination of zero-sequence fluctuations at non-faulty stations, which may still be affected by residual zero-sequence fluctuations. Summary of the Invention

[0004] The purpose of this invention is to provide an inter-terminal coordination suppression method and system for zero-sequence fluctuations in unconnected substations, which solves the problem that the existing zero-sequence fluctuation suppression method, which only works at faulted stations, will have a control delay relative to the actual zero-sequence fluctuation situation at non-faulted stations, resulting in poor elimination effect of zero-sequence fluctuations at non-faulted stations.

[0005] To achieve the above objectives, the present invention provides an inter-terminal coordinated suppression method for zero-sequence fluctuations in a disconnected distribution network. This method uses all MMC converters of a dual-terminal or multi-terminal flexible DC system as suppression terminals, and coordinates the suppression of zero-sequence fluctuations through cooperation between these suppression terminals. Each suppression terminal includes a full-bridge submodule.

[0006] The zero-sequence fluctuation coordinated suppression includes zero-sequence voltage feedforward coordinated suppression: for the suppression terminal belonging to the fault station, the feedforward amount of the corresponding zero-sequence voltage feedforward coordinated suppression is determined according to the zero-sequence component of the AC voltage of the suppression terminal itself; the feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to each suppression terminal belonging to the fault station is superimposed on the voltage modulation wave corresponding to the suppression terminal, and a trigger pulse is generated according to the superimposed voltage modulation wave and output to the sub-module of the suppression terminal to suppress a portion of the zero-sequence fluctuation voltage;

[0007] For the suppression terminal belonging to the non-faulty station, the feedforward amount of the corresponding zero-sequence voltage feedforward coordinated suppression is determined according to the zero-sequence component of the DC neutral point voltage of the suppression terminal or the zero-sequence component of the AC voltage of the faulty station; the feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to each suppression terminal belonging to the non-faulty station is superimposed on the voltage modulation wave corresponding to the suppression terminal, and a trigger pulse is generated according to the superimposed voltage modulation wave and output to the sub-module of the suppression terminal to suppress the remaining zero-sequence fluctuation voltage after suppression by the suppression terminal belonging to the faulty station;

[0008] The term "faulty station" refers to an MMC converter that has experienced a fault on the AC side, while "non-faulty station" refers to an MMC converter that has not experienced a fault on the AC side.

[0009] The beneficial effects of the above technical solution are as follows: the inter-terminal coordinated suppression, which suppresses a portion of the zero-sequence fluctuation voltage at the faulty station and the remaining portion at the non-faulty station, can directly eliminate all the remaining zero-sequence fluctuation voltage at the non-faulty station. This also eliminates the portion of zero-sequence fluctuation voltage that was missed due to factors such as control delay at the faulty station. Therefore, it avoids the impact of control delay issues related to the zero-sequence fluctuation suppression control at the faulty station on the zero-sequence fluctuation voltage suppression effect at the non-faulty station. The suppression effect is significantly improved compared to relying solely on zero-sequence fluctuation voltage suppression at the faulty station. Furthermore, different suppression strategies are set according to the different characteristics of the faulty and non-faulty stations, so that MMC converters equipped with this zero-sequence fluctuation suppression method at each end of the dual-terminal or multi-terminal flexible DC system can participate in the coordinated suppression of zero-sequence fluctuation voltage under both faulty and non-faulty station conditions. The effect of inter-terminal coordination can be achieved by configuring the same set of zero-sequence fluctuation suppression methods at all suppression ends.

[0010] Furthermore, the feedforward amount corresponding to the zero-sequence voltage feedforward coordinated suppression of the suppression terminal belonging to the fault station is the product of the zero-sequence component of the AC voltage of the suppression terminal itself, the first feedforward coefficient, and the first suppression coefficient; the first suppression coefficient is greater than 0 and less than 1.

[0011] Furthermore, if the feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to the suppression terminal of the non-faulty station is determined according to the zero-sequence component of the AC voltage of the faulty station, then the value of the feedforward amount is the product of the zero-sequence component of the AC voltage of the faulty station, the second feedforward coefficient, and the second suppression coefficient; the second suppression coefficient is greater than 0 and less than 1; the sum of the first suppression coefficient and the second suppression coefficient is equal to 1.

[0012] The beneficial effect of the above technical solution is that by linking the values ​​of the first feedforward coefficient and the second feedforward coefficient, a more precise coordination relationship can be formed between the zero-sequence voltage feedforward suppression of faulty stations and non-faulty stations.

[0013] Furthermore, if the feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to the suppression terminal of the non-faulty station is determined based on the zero-sequence component of the DC side neutral point voltage of the suppression terminal, then the value of the feedforward amount is the product of the zero-sequence component of the DC side neutral point voltage and the third feedforward coefficient, wherein the third feedforward coefficient is -1.

[0014] Furthermore, the first feedforward coefficient is obtained as follows:

[0015]

[0016] Where m0 is the first 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.

[0017] Furthermore, the second feedforward coefficient is obtained as follows:

[0018]

[0019] Where k is the second 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 coordination 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 current reference value and the zero-sequence component of the AC current at the suppression end. 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 zero-sequence current closed-loop control suppression employs a PR controller.

[0023] This invention also provides an inter-terminal coordination suppression system for zero-sequence fluctuations in a disconnected power distribution network. The disconnected power distribution network is a dual-terminal or multi-terminal flexible DC system, and the MMC converters of the dual-terminal or multi-terminal flexible DC system all include a full-bridge submodule. The system also includes a processor for executing program instructions to implement the inter-terminal coordination suppression method for zero-sequence fluctuations in a disconnected power distribution network as described above. This inter-terminal coordination suppression system for zero-sequence fluctuations can achieve the same beneficial effects as the aforementioned inter-terminal coordination suppression method for zero-sequence fluctuations in a disconnected power distribution network. Attached Figure Description

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

[0025] Figure 2 This is a block diagram of the overall strategy for inter-terminal coordination suppression of zero-sequence fluctuations in the embodiment of the method for suppressing zero-sequence fluctuations in a disconnected power distribution network according to the present invention. Detailed Implementation

[0026] 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.

[0027] Example of an inter-terminal coordination method for suppressing zero-sequence fluctuations in disconnected distribution networks:

[0028] This embodiment provides an inter-terminal coordination suppression method for zero-sequence fluctuations in a disconnected distribution network. This method uses all MMC converters in a dual-terminal or multi-terminal flexible DC system as suppression terminals, and achieves coordinated suppression of zero-sequence fluctuations through cooperation between these suppression terminals. The dual-terminal or multi-terminal flexible DC system includes a corresponding number of MMC converters as converter stations, such as... Figure 1 As shown, the dual-ended flexible DC system contains two MMC converters. Since the output of zero-sequence ripple coordination suppression in this embodiment needs to be carried by a full-bridge submodule, all MMC converters (i.e., all suppression terminals) in the dual-ended or multi-ended flexible DC system include a full-bridge submodule. In this embodiment, the fault station refers to the MMC converter that has failed on the AC side, and the non-fault station refers to the MMC converter that has not failed on the AC side.

[0029] The zero-sequence fluctuation coordinated suppression in this embodiment includes zero-sequence voltage feedforward coordinated suppression and zero-sequence current closed-loop control suppression. The zero-sequence voltage feedforward coordinated suppression requires cooperation between various suppression terminals. That is, a portion of the zero-sequence fluctuation voltage is suppressed at the faulty station, and the remaining portion of the zero-sequence fluctuation voltage is suppressed at the non-faulty station. Such inter-terminal coordinated suppression can directly eliminate all the remaining zero-sequence fluctuation voltage at the non-faulty station, and naturally also eliminates the portion of zero-sequence fluctuation voltage that was missed due to factors such as control delay at the faulty station. Therefore, it can avoid the impact of control delay and other issues on the zero-sequence fluctuation suppression effect at the non-faulty station caused by the zero-sequence fluctuation suppression control performed at the faulty station relative to the actual zero-sequence fluctuation situation at the non-faulty station. The suppression effect is significantly improved compared to relying solely on the zero-sequence fluctuation voltage suppression at the faulty station.

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

[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 represents the zero-sequence component of the DC-side neutral point voltage. o The zero-sequence component of the modulation wave of the flexible DC converter is shown. It can be seen that the suppression methods of zero-sequence fluctuations differ between faulty and non-faulty stations. Therefore, based on the different propagation characteristics of zero-sequence fluctuations in faulty and non-faulty stations during asymmetrical faults in the AC system, the zero-sequence fluctuation suppression method in this embodiment sets different suppression strategies accordingly. This allows MMC converters equipped with this zero-sequence fluctuation suppression method at each end of a dual-terminal or multi-terminal flexible DC system to participate in the coordinated suppression of zero-sequence fluctuation voltage under both faulty and non-faulty station conditions, achieving the effect of inter-terminal coordination.

[0033] Reference to the overall strategy for coordinating and suppressing zero-sequence fluctuations Figure 2 , Figure 2 middleu 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 dcp To suppress the DC pole-to-ground voltage, the corresponding zero-sequence component u is obtained through a bandpass filter. o e oTo suppress the zero-sequence component of the modulation wave of the flexible DC converter, this embodiment uses FS logic to determine the appropriate suppression strategy based on the situation of the suppression terminal. When FS=1, the suppression strategy corresponding to the faulty station is used; when FS=0, the suppression strategy corresponding to the non-faulty station is used. For the suppression terminal belonging to the faulty station, the feedforward amount of its corresponding zero-sequence voltage feedforward coordinated suppression is determined based on the zero-sequence component of the AC voltage of the suppression terminal itself. The feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to the suppression terminal belonging to the faulty station is superimposed on the voltage modulation wave corresponding to the suppression terminal. A trigger pulse is generated based on the superimposed voltage modulation wave and output to the submodule of the suppression terminal to suppress a portion of the zero-sequence voltage fluctuation. The value of this feedforward amount is the zero-sequence component u of the AC voltage of the suppression terminal itself. o The first feedforward coefficient m0 ​​and the first suppression coefficient k u The product of the zero-sequence component of the AC voltage at the suppression terminal and the first feedforward coefficient corresponds to the magnitude of the feedforward that can completely eliminate the zero-sequence ripple voltage. The first suppression coefficient k u This first suppression coefficient represents the proportion of zero-sequence fluctuation voltage eliminated at the fault station relative to all zero-sequence fluctuation voltages that need to be eliminated. This first suppression coefficient is greater than 0 and less than 1.

[0034] In this embodiment, the first feedforward coefficient m0 ​​corresponds to the no-load modulation index. This control modulation index 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 specific calculation method is as follows:

[0035]

[0036] Where m0 is the no-load regulation degree, 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] For the suppression terminal belonging to a non-faulty station, the feedforward amount of its corresponding zero-sequence voltage feedforward coordinated suppression is determined based on the zero-sequence component of the DC neutral point voltage of the suppression terminal or the zero-sequence component of the AC voltage of the faulty station. The feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to each suppression terminal belonging to a non-faulty station is superimposed on the voltage modulation wave corresponding to the suppression terminal. A trigger pulse is generated based on the superimposed voltage modulation wave and output to the submodule of the suppression terminal to suppress the remaining zero-sequence fluctuation voltage after suppression by the suppression terminal belonging to the faulty station. If the feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to the suppression terminal belonging to a non-faulty station is determined based on the zero-sequence component of the AC voltage of the faulty station, then the value of the corresponding feedforward amount is the zero-sequence component u of the AC voltage of the faulty station. oThe product of the second feedforward coefficient and the second suppression coefficient; in this embodiment, the second feedforward coefficient is -m0, which is equal in magnitude and opposite in direction to the first feedforward coefficient; the product of the zero-sequence component of the AC voltage at the fault station and the second feedforward coefficient corresponds to the magnitude of the feedforward amount that can completely eliminate the zero-sequence fluctuation voltage, and the second suppression coefficient represents the proportion of the zero-sequence fluctuation voltage eliminated at the non-fault station to all the zero-sequence fluctuation voltages that need to be eliminated, that is, the remaining zero-sequence fluctuation voltage after elimination at the fault station. Therefore, the second suppression coefficient is greater than 0 and less than 1, and the sum of the first suppression coefficient and the second suppression coefficient is equal to 1. In this embodiment, the value of the second suppression coefficient is 1-k. u .

[0038] If the feedforward amount for zero-sequence voltage feedforward coordination suppression corresponding to the suppression end of a non-faulty station is determined based on the zero-sequence component of the DC side neutral point voltage of the suppression end, then the value of the feedforward amount is the product of the zero-sequence component of the DC side neutral point voltage and the third feedforward coefficient, where the third feedforward coefficient is -1. Since the voltage parameters on the DC side of the suppression end can directly reflect the remaining situation of the zero-sequence fluctuation voltage after elimination at the faulty station, it is not necessary to limit the size of the feedforward amount by adding an additional suppression coefficient to achieve the effect of matching the feedforward amount with the remaining zero-sequence fluctuation voltage after elimination at the faulty station.

[0039] Zero-sequence current closed-loop control suppression does not require coordination between suppression terminals, refer to Figure 2 The control quantity for zero-sequence current closed-loop control suppression at each suppression terminal is determined solely by the difference between the zero-sequence current reference value and the zero-sequence component of the AC current at that suppression terminal. The determined control quantity is superimposed onto 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 terminal to suppress the zero-sequence ripple current. Here, the zero-sequence current reference value is the target value of the zero-sequence component of the AC current at the suppression terminal to be achieved by the zero-sequence current closed-loop control suppression. In this embodiment, to achieve the effect of eliminating the zero-sequence ripple current as much as possible, the zero-sequence current reference value is set to 0. The control effect of the control quantity itself is to make the difference between the zero-sequence current reference value and the zero-sequence component of the AC current at the suppression terminal as close to 0 as possible. Therefore, in this embodiment, the goal is actually to control the zero-sequence current to 0, and this goal is dynamically achieved through zero-sequence current closed-loop control suppression. The specific zero-sequence current closed-loop control suppression process is implemented using a PR controller.

[0040] In this embodiment, the voltage modulation waves described above are all represented in the form of zero-sequence voltage, that is, the zero-sequence component e of the modulation wave of the flexible DC converter described above. oThe DC-side neutral point voltage corresponding to the zero-sequence component of the DC-side neutral point voltage required to obtain the feedforward quantity can be obtained directly at the DC-side neutral point, or it can be obtained by collecting the pole-to-ground voltage at the DC pole of the ground terminal. Since the DC-side neutral point voltage and its zero-sequence component correspond to the theoretical values ​​of the DC pole-to-ground voltage and its zero-sequence component, the values ​​can be used interchangeably. The zero-sequence component of the AC voltage at the fault station is obtained at the fault station.

[0041] Example of an inter-terminal coordination suppression system for zero-sequence fluctuations in a disconnected distribution network:

[0042] This embodiment provides a technical solution for an inter-terminal coordination suppression system for zero-sequence fluctuations in a disconnected substation. The disconnected substation is a dual-terminal or multi-terminal flexible DC system, and the MMC converters of this dual-terminal or multi-terminal flexible DC system all include a full-bridge submodule to carry the trigger pulse output corresponding to suppressing zero-sequence fluctuations. The inter-terminal coordination suppression system also includes a processor for executing program instructions to implement the inter-terminal coordination suppression method as described in the above embodiment of the inter-terminal coordination suppression method for zero-sequence fluctuations in a disconnected substation. Since the specific principles and operation of this inter-terminal coordination suppression system have been described in detail in the above embodiment of the inter-terminal coordination suppression method for zero-sequence fluctuations in a disconnected substation, they will not be repeated here.

[0043] This invention has the following characteristics:

[0044] 1) Zero-sequence voltage feedforward coordinated suppression is achieved through the cooperation between various suppression terminals. A portion of the zero-sequence fluctuation voltage is suppressed at the faulty station, and the remaining portion of the zero-sequence fluctuation voltage is suppressed at the non-faulty station. This inter-terminal coordinated suppression can directly eliminate all the remaining zero-sequence fluctuation voltage at the non-faulty station, and naturally also eliminates the portion of zero-sequence fluctuation voltage that was missed due to factors such as control delay at the faulty station. Therefore, it can avoid the impact of control delay and other issues on the zero-sequence fluctuation suppression effect at the non-faulty station caused by the zero-sequence fluctuation suppression control performed at the faulty station relative to the actual zero-sequence fluctuation situation at the non-faulty station. The suppression effect is significantly improved compared to relying solely on the zero-sequence fluctuation voltage suppression at the faulty station.

[0045] 2) Based on the different propagation characteristics of zero-sequence fluctuations in faulty and non-faulty stations during asymmetrical faults in AC systems, the zero-sequence fluctuation suppression method of the present invention sets different suppression strategies accordingly. This allows MMC converters equipped with the zero-sequence fluctuation suppression method at each end of a dual-terminal or multi-terminal flexible DC system to participate in the coordinated suppression of zero-sequence fluctuation voltage under both faulty and non-faulty station conditions. The effect of inter-terminal coordination can be achieved by configuring the same set of zero-sequence fluctuation suppression methods at all suppression ends.

[0046] 3) Two different and replaceable feedforward calculation methods are set for zero-sequence voltage feedforward suppression of non-faulty stations. Both can form a relatively accurate matching relationship with the zero-sequence voltage feedforward calculation method of faulty stations, and can be flexibly selected according to the actual situation.

[0047] 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 inter-terminal coordination and suppression of zero-sequence fluctuations in a disconnected power distribution network, characterized in that, Using all MMC converters of a dual- or multi-terminal flexible DC system as suppression terminals, zero-sequence ripple is coordinated and suppressed through the cooperation between each suppression terminal; each suppression terminal includes a full-bridge submodule. The zero-sequence fluctuation coordinated suppression includes zero-sequence voltage feedforward coordinated suppression: for a suppression terminal belonging to a faulty station, the feedforward amount of its corresponding zero-sequence voltage feedforward coordinated suppression is determined based on the zero-sequence component of the AC voltage of the suppression terminal itself; the feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to each suppression terminal belonging to a faulty station is superimposed on the voltage modulation wave corresponding to the suppression terminal, and a trigger pulse is generated based on the superimposed voltage modulation wave and output to the sub-module of the suppression terminal to suppress a portion of the zero-sequence fluctuation voltage; the value of the feedforward amount is the product of the zero-sequence component of the AC voltage of the suppression terminal itself, the first feedforward coefficient, and the first suppression coefficient; the first feedforward coefficient is greater than 0 and less than 1, and is obtained as follows: Where m0 is the first feedforward coefficient, u vm U is the peak value of the rated AC phase voltage. dc The rated DC voltage on the DC side; For the suppression terminal belonging to the non-faulty station, the feedforward amount of the corresponding zero-sequence voltage feedforward coordinated suppression is determined according to the zero-sequence component of the DC neutral point voltage of the suppression terminal or the zero-sequence component of the AC voltage of the faulty station; the feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to each suppression terminal belonging to the non-faulty station is superimposed on the voltage modulation wave corresponding to the suppression terminal, and a trigger pulse is generated according to the superimposed voltage modulation wave and output to the sub-module of the suppression terminal to suppress the remaining zero-sequence fluctuation voltage after suppression by the suppression terminal belonging to the faulty station; The value of the feedforward quantity is the product of the zero-sequence component of the AC voltage at the fault station, the second feedforward coefficient, and the second suppression coefficient, or the product of the zero-sequence component of the DC neutral point voltage and -1; the second suppression coefficient is greater than 0 and less than 1; the sum of the first suppression coefficient and the second suppression coefficient is equal to 1; the second feedforward coefficient is obtained as follows: Where k is the second feedforward coefficient, u vm U is the peak value of the rated AC phase voltage. dc The rated DC voltage is the DC side voltage; the fault station refers to the MMC converter that has a fault on the AC side, and the non-fault station refers to the MMC converter that has not a fault on the AC side.

2. The method for inter-terminal coordination suppression of zero-sequence fluctuations in disconnected distribution networks according to claim 1, characterized in that, The DC-side neutral point voltage corresponding to 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.

3. The method for inter-terminal coordination suppression of zero-sequence fluctuations in disconnected distribution networks according to claim 1, characterized in that, The zero-sequence component of the AC voltage at the fault station is obtained at the fault station.

4. The method for inter-terminal coordination suppression of zero-sequence fluctuations in disconnected distribution networks according to claim 1, characterized in that, The zero-sequence fluctuation coordination 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 current reference value and the zero-sequence component of the AC current at the suppression end. 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 inter-terminal coordination and suppression of zero-sequence fluctuations in disconnected distribution networks according to claim 4, characterized in that, The zero-sequence current reference value is 0.

6. The method for inter-terminal coordination suppression of zero-sequence fluctuations in disconnected distribution networks according to claim 4, characterized in that, The zero-sequence current closed-loop control suppression uses a PR controller.

7. An inter-terminal coordination suppression system for zero-sequence fluctuations in a disconnected distribution network, wherein the disconnected distribution network is a dual-terminal or multi-terminal flexible DC system, characterized in that, The MMC converters of the dual-terminal or multi-terminal flexible DC systems all include a full-bridge submodule; they also include a processor for executing program instructions to implement an inter-terminal coordination suppression method for zero-sequence fluctuations in disconnected distribution networks. The inter-terminal coordination suppression method for zero-sequence fluctuations in the disconnected distribution network uses all MMC converters of a dual-terminal or multi-terminal flexible DC system as suppression terminals, and performs coordinated suppression of zero-sequence fluctuations through cooperation between each suppression terminal; each suppression terminal includes a full-bridge sub-module. The zero-sequence fluctuation coordinated suppression includes zero-sequence voltage feedforward coordinated suppression: for a suppression terminal belonging to a faulty station, the feedforward amount of its corresponding zero-sequence voltage feedforward coordinated suppression is determined based on the zero-sequence component of the AC voltage of the suppression terminal itself; the feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to each suppression terminal belonging to a faulty station is superimposed on the voltage modulation wave corresponding to the suppression terminal, and a trigger pulse is generated based on the superimposed voltage modulation wave and output to the sub-module of the suppression terminal to suppress a portion of the zero-sequence fluctuation voltage; the value of the feedforward amount is the product of the zero-sequence component of the AC voltage of the suppression terminal itself, the first feedforward coefficient, and the first suppression coefficient; the first feedforward coefficient is greater than 0 and less than 1, and is obtained as follows: Where m0 is the first feedforward coefficient, u vm U is the peak value of the rated AC phase voltage. dc The rated DC voltage on the DC side; For the suppression terminal belonging to the non-faulty station, the feedforward amount of the corresponding zero-sequence voltage feedforward coordinated suppression is determined according to the zero-sequence component of the DC neutral point voltage of the suppression terminal or the zero-sequence component of the AC voltage of the faulty station; the feedforward amount of the zero-sequence voltage feedforward coordinated suppression corresponding to each suppression terminal belonging to the non-faulty station is superimposed on the voltage modulation wave corresponding to the suppression terminal, and a trigger pulse is generated according to the superimposed voltage modulation wave and output to the sub-module of the suppression terminal to suppress the remaining zero-sequence fluctuation voltage after suppression by the suppression terminal belonging to the faulty station; The value of the feedforward quantity is the product of the zero-sequence component of the AC voltage at the fault station, the second feedforward coefficient, and the second suppression coefficient, or the product of the zero-sequence component of the DC neutral point voltage and -1; the second suppression coefficient is greater than 0 and less than 1; the sum of the first suppression coefficient and the second suppression coefficient is equal to 1; the second feedforward coefficient is obtained as follows: Where k is the second feedforward coefficient, u vm U is the peak value of the rated AC phase voltage. dc The rated DC voltage is the DC side voltage; the fault station refers to the MMC converter that has a fault on the AC side, and the non-fault station refers to the MMC converter that has not a fault on the AC side.

8. The inter-terminal coordination suppression system for zero-sequence fluctuations in a disconnected power distribution network according to claim 7, characterized in that, The DC-side neutral point voltage corresponding to 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.

9. The inter-terminal coordination suppression system for zero-sequence fluctuations in a disconnected power distribution network according to claim 7, characterized in that, The zero-sequence component of the AC voltage at the fault station is obtained at the fault station.

10. The inter-terminal coordination suppression system for zero-sequence fluctuations in a disconnected power distribution network according to claim 7, characterized in that, The zero-sequence fluctuation coordination 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 current reference value and the zero-sequence component of the AC current at the suppression end. 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.

11. The inter-terminal coordination suppression system for zero-sequence fluctuations in a disconnected distribution network according to claim 10, characterized in that, The zero-sequence current reference value is 0.

12. The inter-terminal coordination suppression system for zero-sequence fluctuations in a disconnected power distribution network according to claim 10, characterized in that, The zero-sequence current closed-loop control suppression uses a PR controller.

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