A fault discrimination and zero sequence suppression method and system for a coupling-free power transmission and distribution network

Abstract

The present application belongs to the field of direct current distribution network, and particularly relates to a fault discrimination and zero sequence suppression method and system for a non-coupling variable distribution network. The present application can accurately and quickly locate the fault station by using the characteristic of the amplitude difference of the zero sequence component of the AC voltage of each MMC converter caused by the zero sequence current closed-loop control. The condition for distinguishing the fault station and the non-fault station is obtained by comprehensively comparing the working conditions of each end, the working conditions of each end can be referenced to each other, the fault recognition and positioning is not delayed or misjudged due to the inadaptation of the distinguishing condition to the actual working condition, and the reliability of the fault discrimination is improved. According to the different characteristics of the fault station and the non-fault station, different zero sequence voltage feed-forward suppression modes are set, and two different and replaceable suppression conditions are set for the zero sequence voltage feed-forward suppression of the non-fault station, which can be flexibly selected according to the actual situation, and further adapt to different working conditions.

Description

Technical Field

[0001] This invention belongs to the field of DC distribution networks, specifically relating to a fault identification and zero-sequence suppression method and system for disconnected substations and distribution networks. Background Technology

[0002] Medium-voltage DC distribution networks typically employ flexible DC transmission technology based on modular multilevel converters (MMCs). MMCs are usually connected to the AC grid using a connecting transformer, but this increases the cost and footprint of the flexible DC distribution system. To save costs and simplify the structure, existing technologies have begun to research flexible medium-voltage DC distribution networks without connecting transformers. However, when an asymmetrical fault occurs in the AC grid, the connecting 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. But if the connecting transformer is removed, the zero-sequence component of the AC system cannot be isolated, and the zero-sequence voltage on the fault side will cause fluctuations in the neutral point voltage on the DC side of the converter, which will then be transmitted to other connected AC systems, expanding the impact of the AC system fault.

[0003] In the prior art, in a dual-end medium-voltage flexible DC distribution network system, a medium-voltage flexible loop-closing device connecting various AC systems is as follows: Figure 1 As shown, it includes two MMC converters, also known as converter stations. The converter corresponding to the end where the AC fault occurs is the fault station. Existing zero-sequence voltage fluctuation elimination usually only targets the fault station. The zero-sequence voltage elimination strategy at the fault station can eliminate the impact of zero-sequence fluctuations from the source. The determination of the fault station is also made independently at each MMC converter based on the fault conditions. However, fault identification and location performed separately at each MMC converter may result in fault conditions that are not adapted to the actual operating conditions, especially in multi-terminal unconnected medium-voltage flexible DC distribution network systems. This makes it difficult to detect the fault in time and locate the fault station to implement the corresponding zero-sequence voltage fluctuation elimination strategy. Therefore, it is impossible to cut off the transmission of zero-sequence fluctuations in time, resulting in a large impact time and range of zero-sequence fluctuations; or it may lead to misjudgment, reducing the reliability of fault identification and zero-sequence fluctuation suppression. Summary of the Invention

[0004] The purpose of this invention is to provide a fault identification and zero-sequence suppression method and system for disconnected power distribution networks, which solves the problem that existing fault identification and location methods may have fault conditions that are not adapted to actual working conditions, resulting in low reliability of fault identification and zero-sequence fluctuation suppression.

[0005] In order to achieve the above object, the application provides a fault discrimination method for a non-connected variable power distribution network, wherein a zero sequence current closed-loop control strategy is included in the non-connected variable power distribution network, the zero sequence current closed-loop control strategy generates corresponding control quantity superimposed to a voltage modulation wave, a trigger pulse is generated according to the superimposed voltage modulation wave and output to a sub-module at a suppression end, so as to suppress the zero sequence fluctuation current, and during the operation of the non-connected variable power distribution network, the zero sequence current closed-loop control strategy is always used for zero sequence current closed-loop control at each MMC converter.

[0006] The fault discrimination method has the following steps:

[0007] 1) The amplitude of the AC voltage zero sequence component at each MMC converter is obtained and compared;

[0008] 2) If the AC voltage zero sequence component at any MMC converter is greater than a set amplitude threshold value, it is determined whether the amplitude of the AC voltage zero sequence component meets a fault condition, if yes, it is determined that an asymmetric fault occurs at the AC side of the corresponding MMC converter, and the MMC converter is a fault station;

[0009] The fault condition includes a first fault condition and a second fault condition, as long as it is determined that the amplitude of the AC voltage zero sequence component at the MMC converter meets any one of the first fault condition or the second fault condition, it is determined that the fault condition is met;

[0010] The first fault condition is that the amplitude variation of the AC voltage zero sequence component at the MMC converter within a set time compared with the AC voltage zero sequence components at other MMC converters is maximum;

[0011] The second fault condition is that the amplitude difference between the AC voltage zero sequence component at the MMC converter and the AC voltage zero sequence components at other MMC converters is greater than a set amplitude difference threshold value, and the amplitude of the AC voltage zero sequence component at the MMC converter compared with the AC voltage zero sequence components at other MMC converters is maximum.

[0012] The beneficial effects of the above technical solution are that the amplitude difference of the AC voltage zero sequence component at each MMC converter caused by the zero sequence current closed-loop control can be used to accurately and quickly locate the fault station, and the conditions for distinguishing the fault station and the non-fault station are obtained by comprehensively comparing the working conditions at each end, the working conditions at each end can be referenced to each other, the fault recognition and positioning are not delayed or misjudged due to the inadaptation of the distinguishing conditions to the actual working conditions, and the reliability of the fault discrimination is improved.

[0013] Further, the set amplitude threshold value is determined by the normal value of the AC voltage under the steady-state operation of the non-connected variable power distribution network.

[0014] Further, the set amplitude difference threshold is determined by the amplitude of the AC voltage zero sequence component under the AC asymmetric fault of the unlinked variable power distribution network.

[0015] The application further provides a fault discrimination system of an unlinked variable power distribution network, which comprises a processor for executing program instructions to realize the above-mentioned fault discrimination method of the unlinked variable power distribution network.

[0016] The fault discrimination system can realize the same beneficial effects as the above-mentioned fault discrimination method of the unlinked variable power distribution network.

[0017] The application further provides a zero sequence suppression method of an unlinked variable power distribution network, wherein a zero sequence current closed-loop control strategy is included in the unlinked variable power distribution network, the zero sequence current closed-loop control strategy generates a corresponding control amount superimposed on a voltage modulation wave, a trigger pulse is generated according to the superimposed voltage modulation wave and output to a sub-module at the suppression end to suppress the zero sequence fluctuation current, and during the operation of the unlinked variable power distribution network, the zero sequence fluctuation current is always suppressed according to the zero sequence current closed-loop control strategy at each MMC converter;

[0018] The zero sequence suppression method comprises the following steps:

[0019] 1) The amplitude of the AC voltage zero sequence component at each MMC converter is obtained and compared;

[0020] 2) If the AC voltage zero sequence component at any MMC converter is greater than the set amplitude threshold, it is determined whether the amplitude of the AC voltage zero sequence component meets the fault condition, if yes, it is determined that the asymmetric fault occurs at the AC side of the corresponding MMC converter, and the MMC converter is the fault station; otherwise, it is determined that the MMC converter is a non-fault station;

[0021] The fault condition comprises a first fault condition and a second fault condition, as long as it is determined that the amplitude of the AC voltage zero sequence component at the MMC converter meets any one of the first fault condition or the second fault condition, it is determined that the fault condition is met;

[0022] The first fault condition is that the amplitude variation of the AC voltage zero sequence component at the MMC converter compared with the AC voltage zero sequence components at other MMC converters within a set time is the largest;

[0023] The second fault condition is that the amplitude difference between the AC voltage zero sequence component at the MMC converter and the AC voltage zero sequence components at other MMC converters is greater than the set amplitude difference threshold, and the amplitude of the AC voltage zero sequence component at the MMC converter compared with the AC voltage zero sequence components at other MMC converters is the largest;

[0024] 3) judging out the fault station, and suppressing the zero sequence fluctuation voltage according to the zero sequence fluctuation voltage suppression strategy.

[0025] The technical scheme has the beneficial effects that: the amplitude difference of the AC voltage zero sequence component of each MMC converter caused by the zero sequence current closed-loop control is utilized to accurately and quickly locate the fault station, and the condition for distinguishing the fault station and the non-fault station is obtained by comprehensively comparing the working conditions of each end, the working conditions of each end can be referenced to each other, the fault recognition and positioning are not delayed or misjudged due to the condition not adapting to the actual working condition, and the reliability of the fault discrimination is improved. Further, the set amplitude threshold is determined by the normal value of the AC voltage in the steady state operation of the unconnected variable power distribution network.

[0026] Further, the set amplitude difference threshold is determined by the amplitude of the AC voltage zero sequence component in the AC asymmetric fault of the unconnected variable power distribution network.

[0027] Further, the set amplitude threshold is determined by the normal value of the AC voltage in the steady state operation of the unconnected variable power distribution network.

[0028] Further, when the zero sequence fluctuation voltage is suppressed according to the zero sequence fluctuation voltage suppression strategy, different zero sequence fluctuation voltage suppression strategies are adopted for the fault station and the non-fault station.

[0029] The technical scheme has the beneficial effects that: two different and replaceable suppression conditions are set for the zero sequence voltage feedforward suppression of the non-fault station, and the conditions can be flexibly selected according to the actual situation, and the conditions are further adapted to different working conditions.

[0030] Further, the zero sequence fluctuation voltage suppression strategy is zero sequence voltage feedforward suppression: at the non-fault station, the zero sequence voltage feedforward suppression is performed according to the zero sequence component of the DC side neutral point voltage of the non-fault station or the zero sequence component of the AC voltage of the fault station; and at the fault station, the zero sequence voltage feedforward suppression is performed according to the zero sequence component of the AC voltage of the fault station itself.

[0031] The technical scheme has the beneficial effects that: different zero sequence voltage feedforward suppression modes are set according to the different characteristics of the fault station and the non-fault station, the problem of mutual conduction of the zero sequence voltage and the zero sequence current between the AC systems connected by the flexible loop device due to the unconnected variable is solved, the influence range of the AC asymmetric fault under the unconnected variable is reduced, the zero sequence fluctuation suppression method of the application can be applied to the fault station or the non-fault station, and thus the zero sequence fluctuation suppression method of the application can be applied to more zero sequence fluctuation suppression scenes, the MMC converter configured with the zero sequence fluctuation suppression method of the application can eliminate the zero sequence fluctuation voltage and current under various working conditions, and the MMC converter is ensured not to be affected by the zero sequence fluctuation.

[0032] The embodiment also provides a zero sequence suppression system of the decoupled power distribution network, which comprises a processor configured to execute program instructions to implement the zero sequence suppression method of the decoupled power distribution network.

[0033] The zero sequence suppression system can achieve the same beneficial effects as the zero sequence suppression method of the decoupled power distribution network. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 a topological structure diagram of a medium-voltage flexible loop closing device connecting each alternating current system in the background art of the present application;

[0035] Figure 2 a logic block diagram of a fault discrimination method in an embodiment of the zero sequence suppression method of the decoupled power distribution network of the present application;

[0036] Figure 3 a zero sequence fluctuation suppression strategy block diagram in an embodiment of the zero sequence suppression method of the decoupled power distribution network of the present application. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present application clearer and more apparent, the present application will be further described in detail below in combination with the drawings and embodiments.

[0038] An embodiment of a fault discrimination method of a decoupled power distribution network:

[0039] The embodiment provides a technical solution of a fault discrimination method of a decoupled power distribution network, which comprises a zero sequence current closed-loop control strategy. The zero sequence current closed-loop control strategy generates corresponding control quantities superimposed on a voltage modulation wave. Trigger pulses are generated according to the superimposed voltage modulation wave and output to a sub-module at the suppression end to suppress zero sequence fluctuation current. During operation of the decoupled power distribution network, zero sequence current closed-loop control is performed at each MMC converter according to the zero sequence current closed-loop control strategy.

[0040] The steps of the fault discrimination method are as follows:

[0041] 1) The amplitudes of the zero sequence components of the alternating current voltages at each MMC converter are obtained and compared.

[0042] 2) If the zero sequence component of the alternating current voltage at any MMC converter is greater than a set amplitude threshold value, it is determined whether the amplitude of the zero sequence component of the alternating current voltage satisfies a fault condition. If the fault condition is satisfied, it is determined that an asymmetric fault occurs at the alternating current side of the corresponding MMC converter, and the MMC converter is a fault station. The set amplitude threshold value is determined by the normal value of the alternating current voltage under the steady-state operation of the decoupled power distribution network.

[0043] The fault condition includes a first fault condition and a second fault condition, and as long as it is judged that the amplitude of the AC voltage zero sequence component at the MMC converter meets any one of the first fault condition or the second fault condition, it is determined that the fault condition is met;

[0044] The first fault condition is that the amplitude variation of the AC voltage zero sequence component at the MMC converter in a set time compared with the AC voltage zero sequence components at other MMC converters is the largest;

[0045] The second fault condition is that the amplitude difference between the AC voltage zero sequence component at the MMC converter and the AC voltage zero sequence components at other MMC converters is greater than a set amplitude difference threshold, and the amplitude of the AC voltage zero sequence component at the MMC converter compared with the AC voltage zero sequence components at other MMC converters is the largest; the set amplitude difference threshold is determined by the amplitude of the AC voltage zero sequence component under the AC asymmetric fault of the unconnected variable power distribution network.

[0046] In the embodiment, the zero sequence current closed-loop control strategy is realized by a zero sequence current closed-loop controller; since the zero sequence current closed-loop controller in the embodiment is always put into during the steady-state operation of the flexible DC system corresponding to the unconnected variable power distribution network, when the AC asymmetric fault occurs, the zero sequence voltage is greatly increased, at this time, the zero sequence current closed-loop controller can output a certain zero sequence modulation wave through closed-loop automatic control to realize the suppression of part of the neutral point zero sequence voltage and the zero sequence current, so that the amplitude of the AC zero sequence voltage of the non-fault station is reduced; and the amplitude of the AC zero sequence voltage of the fault station is basically unchanged due to the restriction of the fault point potential. Therefore, the amplitude difference of the AC voltage zero sequence component of each MMC converter can be used to quickly locate the fault station without relying on the conventional fault discrimination method of the medium voltage power distribution network; and the condition for distinguishing the fault station and the non-fault station is obtained by comprehensively comparing the working conditions of each end, the working conditions of each end can be referenced to each other, avoiding the fault recognition and positioning not timely or misjudgment due to the inadaptation of the distinguishing condition to the actual working condition, and improving the reliability of the fault discrimination.

[0047] When the zero sequence current closed-loop controller is always put into, the amplitude of the AC voltage zero sequence component at the fault station is necessarily relatively large, thereby it can be determined that the power distribution network system has a fault; in the embodiment, if the amplitude of the AC voltage zero sequence component is greater than a set amplitude threshold, it is determined that there is a fault in the power distribution network; the corresponding set amplitude threshold is determined by the normal value of the AC voltage under the steady-state operation of the unconnected variable power distribution network, and to ensure the sensitivity, the set amplitude threshold in the embodiment is selected within 15%-30% of the normal value of the AC voltage under the steady-state operation of the unconnected variable power distribution network, and in other embodiments, it can also be specifically set according to the actual situation.

[0048] If it is judged according to the above conditions that there is no fault in the coupled variable power distribution network, the amplitude difference characteristics of the zero sequence component of the AC voltage of each MMC converter can be embodied by the following different judgment conditions:

[0049] ① Since the zero sequence current closed-loop control strategy at the fault station cannot reduce the zero sequence component of the AC voltage at this station, the amplitude of the zero sequence component of the AC voltage at the fault station must be greater than that at the non-fault station, so the position of the fault station in the fault power distribution system can be determined; therefore, the first fault condition in this embodiment embodies the amplitude change characteristics of the fault station itself, realizes the identification of the fault station, that is, the end with the largest amplitude change and the largest change amount is determined as the fault station, and the other ends with smaller amplitude change amounts are determined as non-fault stations.

[0050] ② Since the zero sequence current closed-loop control strategy cannot reduce the zero sequence component of the AC voltage at the fault station, but can successfully reduce the zero sequence component of the AC voltage at the non-fault station, the fault can also be distinguished by comparing the amplitude relationship of the zero sequence component of the AC voltage at each end, the difference between the amplitude of the zero sequence component of the AC voltage at the fault station and the amplitude of the zero sequence component of the AC voltage at the non-fault station must be greater than the normal fluctuation of the amplitude of the zero sequence component of the AC voltage of the flexible DC system, and the amplitude of the zero sequence component of the AC voltage at the fault station is relatively the largest, so the existence of the fault in the power distribution system and the position of the fault station can be determined; therefore, the second fault condition in this embodiment embodies the amplitude relationship characteristics of the entire power distribution network, realizes the identification of the fault station, that is, the end with the largest amplitude and the largest amplitude relationship is determined as the fault station.

[0051] Fault discrimination system for unconnected variable power distribution network embodiment:

[0052] The embodiment provides a technical scheme of a fault discrimination system for an unconnected variable power distribution network, which comprises a processor for executing program instructions to realize the fault discrimination method in the fault discrimination method embodiment for the unconnected variable power distribution network described above. Since the specific principles and operation modes of the fault discrimination system have been fully described in the fault discrimination method embodiment for the unconnected variable power distribution network described above, they will not be repeated here.

[0053] Zero sequence suppression method for unconnected variable power distribution network embodiment:

[0054] The embodiment provides a technical scheme of a zero sequence suppression method of a decoupled variable power distribution network, the decoupled variable power distribution network comprising a zero sequence current closed-loop control strategy, the zero sequence current closed-loop control strategy generates corresponding control quantities superimposed to a voltage modulation wave, trigger pulses are generated according to the superimposed voltage modulation wave and output to a sub-module at a suppression end, so as to suppress the zero sequence fluctuation current, and during operation of the decoupled variable power distribution network, the zero sequence fluctuation current is always suppressed according to the zero sequence current closed-loop control strategy at each MMC converter;

[0055] The zero sequence suppression method comprises the following steps:

[0056] 1) The amplitude of the AC voltage zero sequence component at each MMC converter is obtained and compared;

[0057] 2) If the AC voltage zero sequence component at any MMC converter is greater than a set amplitude threshold value, it is determined whether the amplitude of the AC voltage zero sequence component satisfies a fault condition, if yes, it is determined that the asymmetric fault occurs on the AC side of the corresponding MMC converter, and the MMC converter is a fault station; otherwise, the MMC converter is a non-fault station; the set amplitude threshold value is determined by the normal value of the AC voltage under the steady-state operation of the decoupled variable power distribution network

[0058] The fault condition comprises a first fault condition and a second fault condition, as long as it is determined that the amplitude of the AC voltage zero sequence component at the MMC converter satisfies any one of the first fault condition or the second fault condition, it is determined that the fault condition is satisfied;

[0059] The first fault condition is that the amplitude variation of the AC voltage zero sequence component at the MMC converter compared with the AC voltage zero sequence components at other MMC converters within a set time is maximum;

[0060] The second fault condition is that the amplitude difference between the AC voltage zero sequence component at the MMC converter and the AC voltage zero sequence components at other MMC converters is greater than a set amplitude difference threshold value, and the amplitude of the AC voltage zero sequence component at the MMC converter compared with the AC voltage zero sequence components at other MMC converters is maximum; the set amplitude difference threshold value is determined by the amplitude of the AC voltage zero sequence component under the AC asymmetric fault of the decoupled variable power distribution network; for example Figure 2 The figure shows a fault discrimination method logic block diagram corresponding to the second fault condition of the double-ended flexible DC system, wherein u o is the AC voltage zero sequence component, u oset is the set amplitude threshold value, u o1_ph and u o2_ph are the amplitudes of the AC voltage zero sequence components at two ends, Δu oset is the set amplitude difference threshold value, and Δu oset is a value greater than 0.

[0061] In this embodiment, the zero-sequence current closed-loop control strategy is realized by a zero-sequence current closed-loop controller; since the zero-sequence current closed-loop controller in this embodiment is always put into operation during the steady-state operation of the flexible HVDC system corresponding to the unconnected variable power distribution network, on this basis, when an AC asymmetric fault occurs, the zero-sequence voltage increases significantly, at this time, the zero-sequence current closed-loop controller can output a certain zero-sequence modulation wave through closed-loop automatic control to realize the suppression of part of the neutral point zero-sequence voltage and zero-sequence current, thereby reducing the amplitude of the AC zero-sequence voltage of the non-fault station; while the amplitude of the AC zero-sequence voltage of the fault station is basically unchanged due to the restriction of the fault point potential. Therefore, the amplitude difference of the AC voltage zero-sequence component of each MMC converter can be used to quickly locate the fault station without relying on the conventional fault discrimination method of the medium-voltage power distribution network; and the condition for distinguishing the fault station and the non-fault station is obtained by comprehensively comparing the operating conditions of each end, the operating conditions of each end can be referenced to each other, avoiding the fault recognition and positioning not timely or misjudgment due to the inadaptation of the distinguishing condition to the actual operating condition, and improving the reliability of fault discrimination.

[0062] Under the condition that the zero-sequence current closed-loop controller is always put into operation, the amplitude of the AC voltage zero-sequence component at the fault station must be relatively large, thereby it can be determined that there is a fault in the power distribution network system; in this embodiment, if the amplitude of the AC voltage zero-sequence component is greater than a set amplitude threshold, it is determined that there is a fault in the power distribution network; the corresponding set amplitude threshold is determined by the normal value of the AC voltage under the steady-state operation of the unconnected variable power distribution network, to ensure the sensitivity, the set amplitude threshold in this embodiment is selected within 15%-30% of the normal value of the AC voltage under the steady-state operation of the unconnected variable power distribution network, in other embodiments, it can also be set according to the actual situation.

[0063] If it has been determined that there is a fault in the unconnected variable power distribution network according to the above condition, the amplitude difference characteristic of the AC voltage zero-sequence component of each MMC converter can be embodied by the following different judgment conditions:

[0064] ① Since the zero-sequence current closed-loop control strategy at the fault station cannot reduce the AC voltage zero-sequence component at this station, the amplitude of the AC voltage zero-sequence component at the fault station must be greater than that at the non-fault station, thereby it can be determined that the position of the fault station in the power distribution network system with the fault; therefore, the first fault condition in this embodiment embodies the amplitude variation characteristic of the fault station itself to realize the judgment and identification of the fault station, that is, the end with the largest amplitude variation and the largest variation amount is determined as the fault station, and the other ends with smaller amplitude variation amounts are determined as non-fault stations.

[0065] ② Since the zero-sequence current closed-loop control strategy cannot reduce the zero-sequence component of the AC voltage at the fault station, but can successfully reduce the zero-sequence component of the AC voltage at the non-fault station, fault discrimination can also be performed by comparing the amplitude relationship of the zero-sequence components of the AC voltages at each end. The difference between the amplitude of the zero-sequence component of the AC voltage at the fault station and the amplitude of the zero-sequence component of the AC voltage at the non-fault station is necessarily greater than the normal fluctuation of the zero-sequence component of the AC voltage of the flexible DC system, and the amplitude of the zero-sequence component of the AC voltage at the fault station is relatively the largest. Thus, it can be determined that there is a fault in the distribution network system and the location of the fault station. Therefore, the second fault condition in this embodiment realizes the judgment and identification of the fault station by embodying the amplitude relationship characteristics of the overall distribution network, that is, the determination of the amplitude relationship anomaly and the largest amplitude as the fault station.

[0066] 3) After the fault station is determined, the zero-sequence fluctuation voltage is suppressed according to the zero-sequence fluctuation voltage suppression strategy.

[0067] When the zero-sequence fluctuation voltage is suppressed according to the zero-sequence fluctuation voltage suppression strategy, different zero-sequence fluctuation voltage suppression strategies are adopted for the fault station and the non-fault station. In this embodiment, the zero-sequence fluctuation voltage suppression strategy adopted is zero-sequence voltage feedforward suppression. At the non-fault station, zero-sequence voltage feedforward suppression is performed according to the zero-sequence component of the DC side neutral point voltage of the non-fault station or the zero-sequence component of the AC voltage of the fault station. At the fault station, zero-sequence voltage feedforward suppression is performed according to the zero-sequence component of the AC voltage of the fault station itself.

[0068] Referring to Figure 3 , Figure 3 u o1 and u o2 are zero-sequence voltage feedforward of the fault station and the non-fault station, respectively, i o is the zero-sequence component of the AC current at the suppression end, e o is the zero-sequence component of the SMES modulation wave at the suppression end, and k1 and k2 are zero-sequence voltage feedforward coefficients of the fault station and the non-fault station, respectively.

[0069] If the suppression end is the fault station, the feedforward amount of the zero-sequence voltage feedforward suppression is determined according to the zero-sequence component of the AC voltage of the suppression end itself. The value of the feedforward amount is the product of the zero-sequence component of the AC voltage of the fault station u o and k1, and the value range of k1 is 0-m0, where m0 is the no-load modulation degree, which is used to match the zero-sequence component superimposed on the voltage modulation wave with the zero-sequence component of the system AC voltage, so as to achieve the purpose of effectively suppressing the zero-sequence fluctuation.

[0070] If the suppression end is a non-fault station, the feedforward amount of the zero sequence voltage feedforward suppression is determined according to the zero sequence component of the DC side neutral point voltage or the zero sequence component of the AC voltage of the fault station, and the value of the feedforward amount is the product of the zero sequence component of the DC side neutral point voltage and k2, or the product of the zero sequence component of the AC voltage of the fault station and k2; wherein the zero sequence component of the DC side neutral point voltage can be directly obtained at the DC side neutral point, or can be obtained by collecting the pole-to-ground voltage at the DC pole line end, and the zero sequence component of the DC side neutral point voltage is equal to the theoretical value of the zero sequence component of the DC pole-to-ground voltage, so the value can be universal; the zero sequence component of the AC voltage of the fault station is obtained at the fault station.

[0071] If the feedforward amount of the zero sequence voltage feedforward suppression is determined according to the zero sequence component of the AC voltage of the fault station, the value of k2 is in the range of -m0~0, wherein m0 is the no-load regulation degree; if the feedforward amount of the zero sequence voltage feedforward suppression is determined according to the zero sequence component of the DC side neutral point voltage (the zero sequence component of the DC pole-to-ground voltage), the value of k2 is -1.

[0072] Whether it is a fault station or a non-fault station, after obtaining the feedforward amount of the zero sequence voltage feedforward suppression, the feedforward amount is superimposed on the voltage modulation wave, and in the embodiment, the voltage modulation wave is represented in the form of zero sequence voltage, that is, the above-mentioned zero sequence component e of the MMC modulation wave is o According to the superimposed voltage modulation wave, a trigger pulse is generated and output to the sub-module of the suppression end to suppress the zero sequence fluctuation voltage. In order to carry the output of the zero sequence fluctuation suppression, the zero sequence fluctuation suppression system in the embodiment at least includes an MMC converter including a full-bridge sub-module, which is used as an output end of the zero sequence fluctuation suppression of the MMC modulation wave.

[0073] Embodiment of zero sequence suppression system of unconnected variable power distribution network:

[0074] The embodiment provides a technical scheme of a zero sequence fluctuation suppression system of an unconnected variable power distribution network, and the unconnected variable power distribution network is a double-ended or multi-ended flexible DC system. The zero sequence fluctuation suppression system in the embodiment at least includes an MMC converter including a full-bridge sub-module, which is used as a suppression end of the zero sequence fluctuation suppression to carry the output of the zero sequence fluctuation suppression; and further includes a processor for executing program instructions to realize the zero sequence fluctuation suppression method in the above-mentioned embodiment of the zero sequence fluctuation suppression method of the unconnected variable power distribution network. Since the specific principle and operation process of the zero sequence fluctuation suppression system have been described in detail in the above-mentioned embodiment of the zero sequence fluctuation suppression method of the unconnected variable power distribution network, they will not be repeated here.

[0075] The application has the following characteristics:

[0076] 1) The amplitude difference of the zero sequence component of the AC voltage of each MMC converter caused by the zero sequence current closed-loop control can be used to accurately and quickly locate the fault station, and the condition for distinguishing the fault station and the non-fault station is obtained by comprehensive comparison of the working conditions of each end, the working conditions of each end can be referenced to each other, so as to avoid the fault recognition and positioning not timely or misjudgment caused by the condition not adapting to the actual working condition, and the reliability of fault discrimination is improved.

[0077] 2) According to the different characteristics of the fault station and the non-fault station, different zero sequence voltage feedforward suppression modes are set, which can solve the mutual conduction problem of zero sequence voltage and zero sequence current between the AC systems connected by the flexible loop device, reduce the influence range of AC asymmetric fault under no connected transformer, and make the zero sequence fluctuation suppression method of the present application applicable to the case that the current MMC converter is a fault station or a non-fault station, so it can be applied to more zero sequence fluctuation suppression scenes.

[0078] 3) Two different and replaceable suppression conditions are set for the zero sequence voltage feedforward suppression of the non-fault station, which can be flexibly selected according to the actual situation, and further adapt to different working conditions.

[0079] It should be understood that the above specific embodiments of the present application are only used for illustrative or explanatory purposes of the principles of the present application, and do not constitute a limitation of the present application. Therefore, any modification, equivalent replacement, improvement, etc. made without departing from the spirit and scope of the present application shall be included in the protection scope of the present application. In addition, the appended claims of the present application are intended to cover all variations and modifications falling within the scope and boundary of the appended claims, or the equivalent forms of such scope and boundary.

Claims

1. A fault diagnosis method for a disconnected power distribution network, characterized in that, The disconnected substation includes a zero-sequence current closed-loop control strategy. The zero-sequence current closed-loop control strategy generates a corresponding control quantity that is superimposed on the voltage modulation wave. 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 fluctuating current. During the operation of the disconnected substation, each MMC converter always performs zero-sequence current closed-loop control according to the zero-sequence current closed-loop control strategy. The fault diagnosis method has the following steps: 1) Obtain and compare the amplitude of the zero-sequence component of the AC voltage at each MMC converter; 2) If the zero-sequence component of the AC voltage at any MMC converter is greater than the set amplitude threshold, then it is further determined whether the amplitude of the zero-sequence component of the AC voltage meets the fault conditions. If it does, it is determined that an asymmetrical fault has occurred on the AC side of the corresponding MMC converter, and the MMC converter is a fault station. The fault conditions include a first fault condition and a second fault condition. As long as the amplitude of the zero-sequence component of the AC voltage at the MMC converter is determined to meet either the first fault condition or the second fault condition, the fault condition is determined to be met. The first fault condition is: the amplitude change of the zero-sequence AC voltage component at this MMC converter is the largest compared with the zero-sequence AC voltage component at other MMC converters within a set time period. The second fault condition is: the amplitude difference between the zero-sequence AC voltage component at this MMC converter and the zero-sequence AC voltage component at other MMC converters is greater than the set amplitude difference threshold, and the zero-sequence AC voltage component at this MMC converter has the largest amplitude compared to the zero-sequence AC voltage component at other MMC converters.

2. The fault diagnosis method for disconnected substations and distribution networks according to claim 1, characterized in that, The set amplitude threshold is determined by the normal value of AC voltage under steady-state operation of an unconnected power distribution network.

3. The fault diagnosis method for disconnected substations and distribution networks according to claim 1, characterized in that, The set amplitude difference threshold is determined by the amplitude of the zero-sequence component of AC voltage under AC asymmetric fault in a disconnected power distribution network.

4. A fault diagnosis system for a disconnected power distribution network, characterized in that, The fault identification system includes a processor for executing program instructions to implement the fault identification method for disconnected power distribution networks as described in any one of claims 1-3.

5. A zero-sequence suppression method for a disconnected distribution network, characterized in that, The disconnected distribution network includes a zero-sequence current closed-loop control strategy. The zero-sequence current closed-loop control strategy generates a corresponding control quantity that is superimposed on the voltage modulation wave. 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. During the operation of the disconnected distribution network, each MMC converter always suppresses the zero-sequence fluctuation current according to the zero-sequence current closed-loop control strategy. The zero-order suppression method has the following steps: 1) Obtain and compare the amplitude of the zero-sequence component of the AC voltage at each MMC converter; 2) If the zero-sequence component of the AC voltage at any MMC converter is greater than the set amplitude threshold, then it is further determined whether the amplitude of the zero-sequence component of the AC voltage meets the fault conditions. If it does, it is determined that an asymmetrical fault has occurred on the AC side of the corresponding MMC converter, and the MMC converter is a fault station. Otherwise, the MMC converter is considered a non-faulty station; The fault conditions include a first fault condition and a second fault condition. As long as the amplitude of the zero-sequence component of the AC voltage at the MMC converter is determined to meet either the first fault condition or the second fault condition, the fault condition is determined to be met. The first fault condition is: the amplitude change of the zero-sequence AC voltage component at this MMC converter is the largest compared with the zero-sequence AC voltage component at other MMC converters within a set time period. The second fault condition is: the amplitude difference between the zero-sequence AC voltage component at this MMC converter and the zero-sequence AC voltage component at other MMC converters is greater than the set amplitude difference threshold, and the zero-sequence AC voltage component at this MMC converter has the largest amplitude compared with the zero-sequence AC voltage component at other MMC converters. 3) After identifying the faulty station, suppress the zero-sequence voltage fluctuation according to the zero-sequence voltage fluctuation suppression strategy.

6. The zero-sequence suppression method for disconnected distribution networks according to claim 5, characterized in that, The set amplitude threshold is determined by the normal value of AC voltage under steady-state operation of an unconnected power distribution network.

7. The zero-sequence suppression method for disconnected distribution networks according to claim 5, characterized in that, The set amplitude difference threshold is determined by the amplitude of the zero-sequence component of AC voltage under AC asymmetric fault in a disconnected power distribution network.

8. The zero-sequence suppression method for disconnected distribution networks according to any one of claims 5-7, characterized in that, When suppressing zero-sequence voltage fluctuations according to the zero-sequence voltage fluctuation suppression strategy, different zero-sequence voltage fluctuation suppression strategies are adopted for faulty stations and non-faulty stations.

9. The zero-sequence suppression method for disconnected distribution networks according to claim 8, characterized in that, The zero-sequence voltage fluctuation suppression strategy is zero-sequence voltage feedforward suppression: at non-faulty stations, zero-sequence voltage feedforward suppression is performed based on the zero-sequence component of the DC side neutral point voltage of the non-faulty station or the zero-sequence component of the AC voltage of the faulty station. At the fault station, zero-sequence voltage feedforward suppression is performed based on the zero-sequence component of the fault station's own AC voltage.

10. A zero-sequence suppression system for a disconnected power distribution network, characterized in that, The zero-sequence suppression system includes a processor for executing program instructions to implement the zero-sequence suppression method for disconnected power distribution networks as described in any one of claims 5-9.

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