Power distribution network phase sequential grounding fault identification method based on phase current abrupt variable characteristics

Abstract

This invention relates to a method for identifying successive grounding faults in a distribution network based on the characteristics of phase current mutations. Applied to multiple monitoring points along the line, the method includes the following steps: When any monitoring point identifies the first single-phase grounding fault, the remaining monitoring points enter the successive grounding fault identification stage and perform the following actions: extracting the mutation characteristics of the three-phase and zero-sequence currents; calculating the effective value of each phase mutation and the vector sum of the mutations of the two non-faulty phases; determining the line experiencing a successive single-phase grounding fault; when the first faulted line trips and is disconnected, supplementary judgment is made based on the change in the three-phase current mutation at the moment of disconnection; for the identified successive faulted lines, the effective value and duration of the zero-sequence current are continuously monitored; tripping is executed when the successive fault protection criteria are met; if the zero-sequence current returns to the normal range, the successive grounding fault identification stage is exited; thereby achieving local identification and distributed protection of cross-line single-phase grounding faults of the same and different phases.

Description

Technical Field

[0001] This invention relates to the field of power distribution network technology, and in particular to a method for identifying successive grounding faults in power distribution networks based on the characteristics of phase current abrupt changes. Background Technology

[0002] In single-phase grounding faults in low-current grounding systems, successive grounding faults sometimes occur. Specifically, after a single-phase grounding fault is detected, another single-phase grounding fault occurs in the system before the zero-sequence voltage returns to normal. Single-phase grounding faults are relatively common in distribution networks, while successive grounding faults are less common but not uncommon.

[0003] Among the successive grounding fault types, there are two categories: successive grounding faults of different phases and successive grounding faults of the same phase. The former is generated when a grounding fault occurs in one phase, which will cause the voltage of the other two phases to rise, and then insulation breakdown will occur at the weak insulation parts of the two phases. The latter is generated when there are multiple insulation damages in the same phase line, which will eventually lead to multiple simultaneous grounding or successive grounding faults in that phase.

[0004] In recent years, the detection methods for successive grounding faults have attracted extensive research from relevant experts and scholars.

[0005] CN107703416A discloses a method and system for locating secondary single-phase grounding fault sections in a low-current grounding system. This method measures the bus zero-sequence voltage and the zero-sequence current at the feeder's starting end, calculates the ground capacitance parameters based on these electrical quantities, and then determines the feeder segment point where the secondary fault occurred based on the abrupt change points of the capacitance parameter's positive and negative characteristics-time curve, thereby locating the secondary fault section. The core of this algorithm lies in the need to accurately measure the voltage frequency first, and then apply windowing to the DFT formula to achieve accurate calculation of the ground capacitance parameters. However, under fault conditions, accurate measurement of the voltage frequency presents significant difficulties and safety risks, easily leading to a substantial increase in the error of the calculated ground capacitance parameters.

[0006] CN117686838A discloses a method and system for identifying phase-sequence faults across lines in a power distribution system. The technical solution involves: real-time acquisition of the bus zero-sequence voltage sampling value and calculation of its amplitude; simultaneous real-time acquisition of the zero-sequence current of each line and calculation of its corresponding amplitude; and then completion of fault identification through a preset succession criterion. The first succession criterion proposed in this solution includes parameters R1 and R2 (corresponding to the transition resistances of the first and second grounding points of the successive faults, respectively). These parameters need to be adjusted based on the actual on-site operating conditions. However, due to the difficulty in accurately obtaining the on-site transition resistance, the engineering implementation of this criterion is quite challenging.

[0007] CN117686839A discloses a method and system for continuous detection of secondary single-phase grounding faults in power distribution lines. This method acquires real-time samples of the zero-sequence voltage of the power distribution line bus and the zero-sequence current of each line, calculates the amplitude and phase of the zero-sequence current of each line, and then identifies the fault based on a preset voltage-current secondary criterion. However, the secondary criterion formula of this scheme is complex and requires tuning in conjunction with multiple system parameters and grounding resistance values, making the overall tuning process quite difficult.

[0008] CN117741505A discloses a method and system for detecting successive faults in distribution networks based on kurtosis and the Teager energy operator. It extracts the intrinsic mode components of the zero-sequence current of the line after a fault and combines kurtosis characteristics with the Teager energy operator to complete the successive fault discrimination. However, this algorithm relies on the high-frequency components in the original signal and is only applicable to metallic or low-resistance grounding fault scenarios. If the successive fault is a high-resistance grounding fault, the proportion of the high-frequency component of the zero-sequence current will be significantly reduced, which will lead to a significant decrease in the reliability of the algorithm for fault identification.

[0009] CN120468575A discloses a method for detecting secondary cross-line faults in a neutral-point non-effectively grounded distribution network. This method establishes an equivalent circuit model of secondary cross-line faults with different names, solves the analytical expression of the fault electrical quantities, and then identifies the fault based on the changes in zero-sequence voltage and zero-sequence current. The main technical difficulty of this scheme is that it involves a large number of setting parameters, and the parameter setting at the engineering level is quite difficult.

[0010] CN120761774A discloses a method and system for selecting fault lines in successive grounding faults based on cosine similarity and Euclidean distance. This method utilizes the model differences between faulty and healthy lines, quantifying the characteristic differences between the first faulty line and subsequent faulty lines relative to healthy lines using cosine similarity and Euclidean distance, thereby achieving fault line selection. However, this method relies on mutual comparison between faulty and healthy lines and is only applicable to grounding fault selection at the substation level; it cannot achieve autonomous fault identification and assessment at the distribution line level.

[0011] In their article "Method for Selecting Lines for Successive Grounding Faults at Two Points in Distribution Network Lines" (Ouyang Jianna, Yu Xiaoyong, Zhou Yangjun, et al. Method for Selecting Lines for Successive Grounding Faults at Two Points in Distribution Network Lines [J]. Southern Power Grid Technology, 2020, 14(06): 81-89), Ouyang Jianna et al. pointed out that for successive grounding faults at the same phase, if the first fault is a metallic or low-resistance grounding and the second fault is a high-resistance grounding, existing line selection devices cannot accurately identify the second grounding line, and the aforementioned patent documents have not proposed an effective solution for this type of working condition.

[0012] Meanwhile, the aforementioned patented solutions share a common flaw: the application scenarios of existing technologies are mostly limited to the substation interior, requiring the substation's overall electrical information to identify successive grounding faults in its feeder lines. Solutions for identifying successive grounding faults in external distribution terminals are rarely mentioned. At the signal acquisition level, existing solutions all rely on zero-sequence voltage and zero-sequence current signals. However, a large number of existing switchgear, ring main units, and cable branch boxes in the distribution network are not equipped with zero-sequence PTs (or three-phase PTs), but only have VV-connected Uab and Ucb line voltage acquisition circuits, making it impossible to obtain zero-sequence voltage signals. This makes the aforementioned fault identification methods based on zero-sequence voltage unsuitable for application in such scenarios.

[0013] In recent years, the phase current method has been gradually applied to the field of single-phase grounding fault identification. For example, CN113219300A discloses a method for sensing single-phase grounding faults in distribution networks based on the transient and steady-state characteristics of phase current. This method can effectively identify the first single-phase grounding fault. However, as of now, there are no reports on related technologies for identifying successive single-phase grounding faults based on phase current characteristics.

[0014] When a permanent single-phase ground fault occurs at a point in the distribution network, the voltage of the non-faulty phases will rise, putting other parts of the network at risk of recurring single-phase ground faults. Therefore, it is necessary not only to quickly identify the initially faulty line and execute protection tripping operations, but also to quickly and accurately identify and handle any subsequent single-phase ground faults occurring on other healthy lines in the network during the duration of the initial fault.

[0015] Regarding equipment technical requirements, the latest technical standards of the State Grid Corporation of China for 2021-2023 do not mention the relevant technical requirements for the network access professional testing outline for distribution terminals (feeder terminals / station terminals) corresponding to the main equipment of 10kV distribution network (including primary and secondary integrated pole-mounted circuit breakers and primary and secondary integrated ring network cabinets).

[0016] Furthermore, on the one hand, there are differences in understanding among those skilled in the art; on the other hand, the applicant studied a large number of documents and patents when making this invention, but due to space limitations, not all details and contents were listed in detail. However, this does not mean that the present invention does not possess the features of these prior art. On the contrary, the present invention already possesses all the features of the prior art, and the applicant reserves the right to add relevant prior art to the background art. Summary of the Invention

[0017] To address the shortcomings of existing technologies, this invention provides a method for identifying successive grounding faults in distribution networks based on the characteristics of phase current abrupt changes. This method aims to overcome the limitations of existing technologies that rely on global electrical information (bus zero-sequence voltage and zero-sequence current of each feeder) for successive grounding fault identification. It solves the technical pain point that existing technologies can only identify successive grounding faults at the feeder terminals within the substation using a traditional centralized line selection method, and cannot independently complete local identification of successive faults at feeder terminals within the substation or at each distribution terminal along the distribution line. Furthermore, addressing the industry-wide technical challenge of existing technologies being unable to identify successive grounding faults of the same phase where "the first is metallic / low-resistance grounding and the second is high-resistance grounding," this invention eliminates the dependence on global signals and achieves successive fault identification across all scenarios by collecting only local three-phase current signals. Specifically, for the scenario where a permanent single-phase ground fault occurs at a monitoring point in a distribution network, this method not only enables accurate identification of the first single-phase ground fault in the distribution network, but also allows for local identification of various successive ground faults (including successive ground faults of different and the same phases) at other monitoring points when other distribution lines in the distribution network enter an abnormal operating state due to the first fault. This successive ground fault identification method can be deployed in distributed devices within the distribution network, enabling existing equipment (switchgear, ring main unit bays, pole-mounted circuit breakers) to possess the core function of local successive ground fault identification when the distribution network is in an abnormal state where the first ground fault persists.

[0018] This invention, through in-depth analysis of the phase current variation characteristics during successive grounding faults, develops a successive grounding fault identification technology based on local current information, and constructs a distributed fault identification scheme based on this technology. This scheme is not only a significant technological supplement to the field of relay protection by extending the phase current method to the identification of single-phase grounding faults in distribution networks, but also provides key technical support for the rapid operation and maintenance of single-phase grounding faults in distribution networks, possessing significant engineering application value and practical promotion significance.

[0019] In a first aspect, this invention discloses a method for identifying successive grounding faults in a distribution network based on the characteristics of phase current abrupt changes, applicable to multiple monitoring points along the line, comprising the following steps: Once any monitoring point identifies the first single-phase ground fault line based on the local three-phase current and the zero-sequence current synthesized from it, the remaining monitoring points enter the successive ground fault identification stage and execute: (1) Based on the continuous cycle three-phase current sampling data, extract the abrupt change characteristics of the three-phase and zero-sequence currents, calculate the effective value of the abrupt change of each phase and the vector sum of the abrupt changes of the two non-fault phases; (2) When the correlation and effective value difference between the vector sum of the non-faulted phase change and the zero-sequence current change in any cycle satisfy the successive fault criterion, it is determined that a successive single-phase ground fault has occurred on the line. (3) When the first fault line trips and is cleared, a supplementary judgment is made based on the change in the three-phase current at the moment of clearing. When the effective value difference between the change in the fault phase and the non-fault phase meets the subsequent fault criterion after tripping, it is still judged as a successive single-phase ground fault. (4) For the identified successive fault lines, continuously monitor the effective value and duration of the zero-sequence current. If the successive fault protection criteria are met, trip the circuit breaker. If the zero-sequence current returns to the normal range, exit the successive ground fault identification stage. Thus, without requiring bus zero-sequence voltage and based on local three-phase current information, local identification and distributed protection of successive single-phase grounding faults of the same and different phases across the line can be achieved.

[0020] According to a preferred embodiment, a method for extracting the abrupt change characteristics of three-phase and zero-sequence current includes extracting the current abrupt change waveform of the latest cycle by subtracting the corresponding sampling point of two cycles ago from the current latest sampling point, and performing a low-pass filter of 0~600Hz on the abrupt change waveform to retain the power frequency and characteristic frequency band information, thereby forming the current abrupt change characteristic waveform.

[0021] According to a preferred embodiment, the correlation between the vector sum of the non-faulty phase mutation and the zero-sequence current mutation is characterized by calculating the correlation coefficient ρ between the two, which is calculated based on the sampling data of the waveform of the non-faulty phase mutation vector sum and the waveform of the zero-sequence current mutation within 1 to y cycles after the fault.

[0022] According to a preferred embodiment, the effective value difference is the difference between the effective value of the fault phase current mutation and the maximum value of the effective values ​​of the two non-fault phase current mutations, or the difference between the effective value of the vector sum of the non-fault phase mutations and the effective value of the zero-sequence current mutation.

[0023] According to a preferred embodiment, the successive fault criterion includes at least one of the following: "successive fault phase current mutation characteristic 1" criterion and "successive fault phase current mutation characteristic 2" criterion. The "successive fault phase current mutation characteristic 1" criterion is: when the effective value of the phase current mutation of a certain phase in the distribution network is greater than two specific comparison values ​​at the same time, it is determined that a successive ground fault has occurred below the monitoring point. The two specific comparison values ​​are the sum of the characteristic difference value and the effective value of the phase current mutation in the other two phases, respectively. The "successive fault phase current mutation characteristic 2" criterion is: when both the conditions of "correlation coefficient ρ is less than the correlation coefficient set value" and "maximum phase current mutation is greater than the current set value" are met at the same time, it is determined that a successive single-phase ground fault has occurred below the monitoring point.

[0024] According to a preferred embodiment, when making supplementary judgments based on the change in the three-phase current at the moment of disconnection, the change in the three-phase current at the moment of disconnection of the first single-phase ground fault line tripping is extracted, and the difference between the effective values ​​of the change in the fault phase and the non-fault phase is calculated. When the difference satisfies the criterion for subsequent faults after tripping, it is determined that there are still ongoing successive single-phase ground faults.

[0025] According to a preferred embodiment, satisfying the successive fault protection criterion means that the effective value of the zero-sequence current is greater than the preset zero-sequence current setting value I. set2 And the duration of this state exceeds the preset fault duration setting value T. set2 .

[0026] According to a preferred embodiment, after entering the successive ground fault identification stage, the waveform of the first fault recording is first analyzed, and it is determined whether the "characteristic 1 of the sudden change in phase current of successive fault" or "characteristic 2 of the sudden change in phase current of successive fault" is met within the time range covered by the recording data. If it is met, it is immediately determined that a successive single-phase ground fault has occurred below the monitoring point.

[0027] According to a preferred embodiment, the initial fault recording waveform is obtained as follows: when the effective value of the zero-sequence current mutation is greater than the starting setpoint ΔI... 0set At that time, taking the current sampling point as the reference point, the three-phase current and zero-sequence current data of x cycles before the reference point to y cycles after the reference point are saved to form a waveform recording dataset.

[0028] According to a preferred embodiment, after a successive single-phase ground fault is determined, if the zero-sequence current decreases to a preset unbalanced current setting value I during subsequent monitoring... set3 The following is reported as a transient successive ground fault, and the system is restored to normal operation. Among them, I set3 It can be taken as 1.5 times the maximum unbalanced current of this line.

[0029] Secondly, this invention discloses a successive grounding fault identification system for a distribution network, which is installed at multiple monitoring points along the distribution network, including: The mutation feature extraction module is used to generate mutation features of the three-phase and zero-sequence currents based on continuous cycle sampling of the local three-phase phase currents, and to calculate the effective value of the mutation of each phase and the mutation vector sum composed of two non-faulty phases. The successive fault determination module is used to determine the successive single-phase grounding faults at other monitoring points in any cycle after the first single-phase grounding fault is identified by any monitoring point. This determination is based on the correlation between the vector sum of the sudden change and the zero-sequence sudden change, as well as the effective value difference. The post-trip compensation judgment module is used to perform subsequent fault supplement judgment based on the change in the three-phase current at the moment of the first fault line tripping and the effective value difference of the change in the change in the fault phase and the non-fault phase when the fault line trips and is cleared. The distributed protection module is used to continuously monitor the effective value and duration of the zero-sequence current of lines identified as having successive faults, and to issue a trip command when the successive fault protection criteria are met, or to exit successive fault identification after the zero-sequence current returns to normal. This enables each monitoring point to achieve local identification and distributed protection of successive single-phase grounding faults of the same and different phases across the line without the need for bus zero-sequence voltage and based solely on local three-phase current.

[0030] Thirdly, this invention discloses a distribution network successive grounding fault identification system, deployed at multiple monitoring points along the line, including: The state switching unit is used to switch other monitoring points from normal monitoring state to successive fault identification state after the first single-phase ground fault is identified at any monitoring point. The local current feature acquisition unit is used to continuously sample the local three-phase phase current in the identification state and generate current feature quantities that reflect the fault development trend. The successive fault decision unit is used to independently determine whether a successive single-phase ground fault has occurred under local conditions based on the correspondence between the current characteristics of each monitoring point and the information of the first fault. The protection action unit is used to trip the circuit breaker according to the successive fault protection criteria when a successive fault is determined, or to return the monitoring point to normal monitoring state when the current characteristic value returns to normal. By constructing a collaborative identification framework based on state switching for multiple monitoring points, distributed identification and protection of successive grounding faults can be achieved at each monitoring point without the participation of bus zero-sequence voltage.

[0031] Fourthly, the present invention discloses a tripping transient analysis device for identifying successive single-phase ground faults, comprising: The transient change extraction module is used to collect the three-phase current of the line at the moment of the first single-phase ground fault line tripping and clearing, and to extract the transient change of each phase. The post-trip determination module is used to determine whether a successive single-phase ground fault has occurred on this line based on whether the difference between the transient change in the sudden change of the faulty phase and the non-faulty phase meets the successive fault criterion after the trip.

[0032] Fifthly, this invention discloses a distributed successive grounding fault protection system, applied to multiple monitoring terminals along a distribution network. The system includes: The local current measurement unit is used to acquire the three-phase current of this line and generate current characteristic quantities that represent the fault development process. The local decision unit is used to independently determine whether a successive single-phase grounding fault has occurred on the line based on the current characteristics of the terminal after the first single-phase grounding fault information is broadcast by any terminal. Autonomous protection units are used to directly trip the line when the successive fault protection criteria are met, without relying on bus zero-sequence voltage or centralized fault criteria.

[0033] The beneficial technical effects of this invention include: 1) Based on the method of this invention, relying solely on local three-phase current information, it is possible not only to reliably identify the first single-phase grounding fault, but also to accurately identify subsequent single-phase grounding faults, providing a complete and effective technical solution for the full life cycle handling of single-phase grounding faults in distribution networks.

[0034] 2) The effective period of the successive single-phase grounding fault identification criterion proposed in this invention is precisely defined as "after the first grounding fault is assessed and the line to which this monitoring point belongs is determined to be a sound line". By clarifying the effective time range of the criterion, the engineering applicability and identification reliability of the successive grounding fault identification criterion are greatly improved.

[0035] 3) The deployment method of the present invention is highly flexible. It can be implemented in the form of distributed independent devices or by upgrading the program version of existing power distribution devices to complete the functional integration. Therefore, it has great engineering promotion and application value.

[0036] 4) The method of the present invention can cover all types of grounding fault combination scenarios, including all working condition combinations of the first grounding fault from the full impedance range from metallic grounding to high-resistance grounding, and the subsequent grounding fault from the full impedance range from metallic grounding to high-resistance grounding, and has extremely strong working condition universality; the lower limit of its fault identification performance depends on the minimum detectable threshold of the current change.

[0037] 5) In response to the long-standing technical pain point in the industry that "the first fault is metallic or low-resistance grounding and the second fault is high-resistance grounding, it is impossible to effectively identify it", this invention provides a successive high-resistance grounding fault identification scheme with practical engineering value. Attached Figure Description

[0038] Figure 1 This is an overall flowchart of a preferred embodiment of the present invention; Figure 2 This is a system simulation model of a preferred embodiment provided by the present invention; Figure 3 This is a simulation waveform diagram of a cross-line phase-to-phase grounding fault where the first fault is a metallic grounding and subsequent faults are also metallic grounding, according to a preferred embodiment of the present invention. Figure 4This is a simulation waveform diagram of a cross-line phase-to-phase grounding fault where the first fault is metallic grounding and the subsequent faults are high-resistance grounding, according to a preferred embodiment of the present invention. Figure 5 This is a simulation waveform diagram of a cross-line phase-to-phase grounding (including the tripping action of the first fault) where the first fault is a metallic grounding and the subsequent faults are high-resistance grounding, according to a preferred embodiment of the present invention. Figure 6 This is a simulation waveform diagram of a cross-line phase-to-phase grounding fault where the first fault is a high-resistance grounding fault and the subsequent faults are metallic grounding, according to a preferred embodiment of the present invention. Figure 7 This is a simulation waveform diagram of a cross-line phase-to-phase grounding fault where the first fault is a high-resistance grounding fault and subsequent faults are also high-resistance grounding, according to a preferred embodiment of the present invention. Figure 8 This is a simulation waveform diagram of a cross-line dissimilar phase grounding where the first fault is a metallic grounding and subsequent faults are also metallic grounding, according to a preferred embodiment of the present invention. Figure 9 This is a simulation waveform diagram of a cross-line dissimilar phase grounding with the first fault being metallic grounding and the subsequent faults being high-resistance grounding, according to a preferred embodiment of the present invention. Figure 10 This is a simulation waveform diagram of a cross-line dissimilar phase grounding with the first fault being high-resistance grounding and the subsequent faults being metallic grounding, according to a preferred embodiment of the present invention. Figure 11 This is a simulation waveform diagram of a cross-line dissimilar phase grounding where the first fault is a high-resistance grounding and the subsequent faults are also high-resistance grounding, according to a preferred embodiment of the present invention. Detailed Implementation

[0039] The following is a detailed description with reference to the accompanying drawings. It should be noted that, in order to distinguish the variables in different identification stages, the present invention uses different expressions for the variables in different stages to avoid confusion, such as (phase A, phase B, phase C) and (phase a, phase b, phase c), as well as the "characteristic 1 of sudden change in current of successive fault phases" criterion, the formula for the correlation coefficient in different steps, etc.

[0040] This invention discloses a method for identifying successive grounding faults in a distribution network based on the characteristics of phase current abrupt changes, applicable to multiple monitoring points along the line, including the following steps: Once any monitoring point identifies the first single-phase ground fault line based on the local three-phase current and the zero-sequence current synthesized from it, the remaining monitoring points enter the successive ground fault identification stage and execute: (1) Based on the continuous cycle three-phase current sampling data, extract the abrupt change characteristics of the three-phase and zero-sequence currents, calculate the effective value of the abrupt change of each phase and the vector sum of the abrupt changes of the two non-fault phases; (2) When the correlation and effective value difference between the vector sum of the non-faulty phase change and the zero-sequence current change in any cycle satisfy the successive fault criterion, it is determined that a successive single-phase ground fault has occurred on the line. (3) When the first fault line trips and is cleared, a supplementary judgment is made based on the change in the three-phase current at the moment of clearing. When the effective value difference between the change in the fault phase and the non-fault phase meets the subsequent fault criterion after tripping, it is still judged as a successive single-phase ground fault. (4) For the identified successive fault lines, continuously monitor the effective value and duration of the zero-sequence current. If the successive fault protection criteria are met, trip the circuit breaker. If the zero-sequence current returns to the normal range, exit the successive ground fault identification stage. Thus, without requiring bus zero-sequence voltage and based on local three-phase current information, local identification and distributed protection of successive single-phase grounding faults of the same and different phases across the line can be achieved.

[0041] In this invention, the first fault, "first single-phase ground fault", is defined as a single-phase ground fault occurring on a certain distribution line in the distribution network. The second fault, "successive single-phase ground fault", is defined as a single-phase ground fault occurring on another distribution line (i.e., a cross line) in the distribution network after the first single-phase ground fault (the same phase as the first ground fault) or a different phase (different phase from the first ground fault) after the first single-phase ground fault.

[0042] The following is a detailed analysis of the variation law of the phase current change when the first single-phase ground fault and subsequent single-phase ground faults occur. The neutral point grounding method of the distribution network is grounded through the arc suppression coil (the law is basically similar for systems that use the neutral point ungrounded method).

[0043] Preferably, when a single-phase ground fault occurs in the distribution network (assuming phase A (the faulty phase) is grounded), the expression for the sudden change in three-phase current for a healthy line is as follows: .

[0044] In the above formula, △I a , △I b , △I c These are the current surges in phases A, B, and C, respectively. m To ensure the capacitance of each phase to ground in the line is adequate, ΔU0 is the zero-sequence voltage change.

[0045] For a faulty line, assuming phase A is grounded, the expression for the sudden change in three-phase current is as follows: .

[0046] In the above formula, C J For each phase-to-ground capacitance of the faulty line, i fi is the current at the fault point. C∑ To ensure the sum of the non-faulty phase-to-ground capacitance currents of the line and the faulty line, i L This is the inductance current of the arc suppression coil.

[0047] The sound circuit identification criterion constructed in this invention is as follows: .

[0048] As can be seen from the above formula, the correlation between "constructed waveform feature 1" and "zero-sequence sudden change waveform 2" is in phase at the moment of the fault (sound line rule 1). The criterion is to realize the reliable identification of sound line features by amplifying the value of the sudden change when a single-phase ground fault occurs.

[0049] The fault line identification criterion constructed in this invention is as follows: .

[0050] At the moment of failure, i f Mainly i C∑ Gradually becoming i C∑ +i L Therefore, the characteristics of the fault instant can be expressed as follows: .

[0051] As can be seen from the above formula, the correlation between “constructed waveform feature 1” and “zero-sequence sudden change waveform 2” is inverse at the moment of the fault (fault line law 2). By amplifying the value of the sudden change during a single-phase ground fault, the characteristics of the fault line can be reliably identified.

[0052] Preferably, after a single-phase ground fault occurs in the distribution network (assuming phase A (faulty phase) is grounded), the line where the first single-phase ground fault occurred has been identified, and other healthy lines in the distribution network are likely to experience successive ground faults (the purpose of this invention is to identify such successive single-phase ground faults).

[0053] For a healthy line, when the second fault does not occur as a successive ground fault on the healthy line, the expression for the sudden change in three-phase current is as follows: .

[0054] In the above formula, ΔU'0 represents the second zero-sequence voltage change during a successive ground fault. The change in phase current is generated by ΔU'0 when other lines experience successive ground faults.

[0055] However, for a healthy line, the three-phase current mutations are identical regardless of whether it is the initial single-phase ground fault or a subsequent single-phase ground fault. The only difference is that, compared to the initial single-phase ground fault, the magnitude of the current mutation is related to ΔU'0 during a subsequent single-phase ground fault. Since the three-phase current mutations of a healthy line still satisfy Rule 1 for healthy lines, reliable identification of the characteristics of a healthy line can be achieved through relevant calculations.

[0056] On a healthy transmission line, when the second fault occurs as a phase A successive ground fault, the expression for the sudden change in three-phase current is as follows: .

[0057] In the above formula, C J To improve the phase-to-ground capacitance of each phase of the line, i C∑ To ensure the sum of the non-faulty phase-to-ground capacitance currents of the line and the faulty line, i L U'0 is the inductance current of the arc suppression coil, and ΔU'0 is the second zero-sequence voltage change during a successive ground fault.

[0058] The characteristics of successive faults are related to the magnitude of ΔU'0 (corresponding to the current successive fault): if ΔU'0 is large, i f The system cannot change abruptly, and the above-mentioned fault line rule 2 still applies; however, when ΔU'0 is small, the system has no obvious transient process, and the influence of the inductor current will decrease accordingly—at the moment of successive single-phase ground faults, i f From other states to primarily i C∑ +i L (Because the arc suppression coil is operating in an overcompensated state, i) C∑ +i L Presenting sensuality, and - (i C∑ +i L (Then it is capacitive), therefore the characteristics of the fault instant can be expressed as follows: .

[0059] As can be seen from the above formula, there is a difference in value between "non-fault phase sudden change waveform 1" and "fault phase sudden change waveform 2" (fault line rule 3 (only applicable to successive grounding faults)). The difference between the two is the distribution current of the total grounding current in this successive grounding fault line, and satisfies the condition that the effective value of the fault phase sudden change is greater than the effective value of the non-fault phase sudden change.

[0060] Through mathematical derivation and verification, in cross-line phase-sequential grounding faults, the grounding current distribution between the initial grounding fault and subsequent grounding faults satisfies the following relationship: .

[0061] In the above formula, i f1For the initial ground fault grounding current, i f2 For the grounding current of successive grounding faults, R f1 For the initial grounding fault grounding resistance, R f2 For successive grounding faults, the grounding resistance is used.

[0062] From the above relationship, it can be clearly stated that in the scenario of the first single-phase ground fault, the grounding resistance R of the first ground fault is... f1 The larger the value, the corresponding grounding current i f2 The larger the resistance, the more significant the difference in electrical characteristics between the subsequent grounding fault and the initial grounding fault when the initial grounding fault is a high-resistance grounding fault, thus making it relatively easier to accurately identify the subsequent grounding fault.

[0063] When the initial ground fault is a metallic ground fault (in practice, a ground fault with a resistance of approximately 30Ω is considered a metallic ground fault), the system enters a single-phase ground fault operation state. If a subsequent ground fault occurs at this time, and it is a high-resistance ground fault (e.g., a ground resistance reaching 3000Ω), the fault current i f2 The amplitude will be approximately the initial fault current i f1 1 / 100, resulting in i f2 The value is extremely small, making it difficult to detect and identify. Due to the excessively low fault current amplitude, existing fault identification mechanisms often fail to effectively distinguish this abnormal state from unbalanced currents or measurement noise during normal operation, leading to missed or false diagnoses. Furthermore, under the condition of an initial metallic grounding, the system's ability to identify successive grounding faults is typically limited by a grounding resistance upper limit of approximately 300Ω. f2 Approximately i f1 The fault current is only 1 / 10 of the rated value, which is still within the critical level of the identifiable range. As the grounding resistance further increases, the fault current decays rapidly, and the sensitivity of the criterion decreases significantly. Therefore, when the initial grounding is metallic, if subsequent grounding faults are of a high-resistance nature (typically, the grounding resistance exceeds 1000Ω), the existing protection devices cannot reliably detect and accurately judge the fault current due to its severely insufficient amplitude. This has become the main technical bottleneck restricting the identification of successive high-resistance grounding faults. This invention provides the following solution to this fault situation.

[0064] In the event of a first metallic ground fault, this type of fault is more easily and accurately identified by the system due to its obvious fault characteristics, such as a large fault current and stable electrical quantity changes. Based on this, the technical solution proposed in this invention can reliably activate the protection logic and execute corresponding tripping actions to disconnect the distribution line experiencing the first single-phase ground fault. After the faulty line is instantaneously disconnected, the total ground fault current in the system, originally shared by two or more paths, will be reconfigured: the original total ground fault current will be replaced by the fault current i at the initial fault point.f1 The fault current i at the successive fault points f2 Together, they form a parallel split-current configuration; however, after the initial faulty line is disconnected, only i remains in the system. f2 The remaining ground fault current causes a significant change in the overall ground fault current (as shown in the equation below). This change will cause a redistribution of the phase currents of the relevant lines in the system, manifested as abrupt changes or amplitude variations in the phase currents of specific lines. This invention is based on an in-depth analysis of the dynamic response process of this electrical quantity. By real-time monitoring and extraction of the changes in phase current before and after fault clearance, corresponding criteria are constructed to effectively identify subsequent ground faults still existing in the system. This method fully utilizes the transient response characteristics of the system's electrical quantities at the moment of fault clearance, avoiding direct detection of weak, high-resistance fault currents, and improving the ability to identify subsequent faults and the reliability of operation under complex grounding conditions.

[0065] .

[0066] The following characteristics of change can be obtained from the above formula: .

[0067] In the above formula, ΔU”0 represents the instantaneous zero-sequence voltage change of the distribution line after the first single-phase ground fault is cleared during a successive ground fault, and Δi f This represents the change in grounding current analyzed above.

[0068] As can be seen from the above, this situation also satisfies rule 3 for faulty lines.

[0069] Regarding the specific calculation process at the moment of the first fault line trip, the successive fault judgment module is in a transient feature extraction state when the current is reconfigured due to the first fault clearing, so as to calculate the difference between the effective value of the sudden change in the fault phase and the effective value of the sudden change in the non-fault phase. Specifically, this module captures the transient change generated at the moment of clearing, and when the difference of the calculated effective values ​​meets the preset successive fault judgment criteria after the trip, it is determined that the system still has a successive single-phase ground fault.

[0070] Preferably, for a healthy line, when the second fault does not occur during a subsequent ground fault on the healthy line, the expression for the sudden change in three-phase current is as follows: .

[0071] In the above formula, △U'0 represents the second zero-sequence voltage change during a successive ground fault.

[0072] However, for a healthy line, the three-phase current mutations are identical regardless of whether it is the initial single-phase ground fault or a subsequent single-phase ground fault. The only difference is that, compared to the initial single-phase ground fault, the magnitude of the current mutation is related to ΔU'0 during a subsequent single-phase ground fault. Since the three-phase current mutations of a healthy line still satisfy Rule 1 for healthy lines, reliable identification of the characteristics of a healthy line can be achieved through relevant calculations.

[0073] On a healthy transmission line, when the second fault occurs as a phase B successive ground fault, mathematical derivation and verification show that the ground current distribution between the initial ground fault and the successive ground fault satisfies the following relationship: .

[0074] In the above formula, E represents the equivalent zero-sequence voltage source at the grounding point.

[0075] During a fault, the expression for the sudden change in three-phase current is as follows: .

[0076] As can be seen from the above formula, when a successive ground fault occurs, a significant fault current i will be generated between the faulty phase and the non-faulty phase. f Differences. For cross-line out-of-phase grounding faults, even in R f1 With R f2 Even when the sum reaches 5000 ohms, a current change of approximately 1A may still occur. This current change typically has a sufficient amplitude and is not easily masked by noise. Therefore, by identifying the difference in phase current changes between the faulty and non-faulty phases, the existence of a successive grounding fault can be effectively determined, thereby achieving accurate fault detection.

[0077] Based on the above analytical principles, such as Figure 1 As shown, the method for identifying successive grounding faults in a distribution network based on the characteristics of phase current abrupt changes, according to the present invention, may specifically include the following steps: S1. When the system is powered on, it collects the line voltage, the phase current of the three-phase measurement winding, and the zero-sequence current synthesized from the physical vector of the three-phase phase current in real time. The current is sampled by frequency tracking through the zero-crossing phase of the line voltage.

[0078] S2. Extract the zero-sequence current mutation in real time and calculate its effective value. When the fault initiation conditions are met, start the first fault recording, perform the first single-phase grounding fault analysis, and execute steps S3, S4, and S5 in sequence.

[0079] S3. If the system disturbance is identified based on the "characteristic 1 of sudden change in phase current during the first fault", return to step S1.

[0080] S4. If the faulty line is identified according to the "characteristic 1 of sudden change in phase current of the first fault", then the first single-phase ground fault is determined and marked below the monitoring point. Then, the zero-sequence current and duration are continuously monitored. If the tripping condition is met, the protection trips, the process is terminated, and the process returns to step S1.

[0081] S5. If the line is identified as healthy according to the "characteristic 1 of sudden change in phase current of the first fault" criterion, mark the "sound line successive grounding fault identification stage" state and start the successive single-phase grounding fault identification. Analyze the waveform of the first fault recording again to determine whether the "characteristic 1 of sudden change in phase current of the successive fault" or "characteristic 2 of sudden change in phase current of the successive fault" criterion is met within the range of the waveform of the first fault recording. If it is met, determine and mark that a successive single-phase grounding fault has occurred below the monitoring point and execute step S6; if it is not met, execute step S7.

[0082] S6. After the monitoring point is identified as a successive single-phase ground fault, the zero-sequence current and duration are continuously monitored. If the tripping conditions are met, the protection trip is executed, the process is terminated, and the process returns to step S1. If the conditions are not met, it is considered that a transient successive single-phase ground fault has occurred below the monitoring point, the "sound line successive ground fault identification stage" is restored, and step S7 is executed.

[0083] S7. Continuously monitor the zero-sequence current. If the zero-sequence current drops below the set value, it is considered that the faulty line of the first single-phase ground fault in the distribution network has been tripped and cleared by the protection. Then exit the "successive single-phase ground fault identification stage of healthy lines" state and return to step S1. If not, execute step S8.

[0084] S8. Continuously monitor the phase current change. If the phase current change exceeds the set value, start recording the successive ground fault waveform and determine whether the "successive fault phase current change characteristic 1" or "successive fault phase current change characteristic 2" criteria are met. If the criteria are met, determine and mark that a successive single-phase ground fault has occurred below the monitoring point and execute step S6. If not, return to step S7.

[0085] Preferably, step S1 may include the following sub-steps: S1-1 System Power On: Real-time acquisition of the effective value of the line voltage. When the effective value exceeds the set value and meets the power-on delay time, the system is powered on and other processes begin to be executed.

[0086] .

[0087] In the above formula, U on For the voltage of the upper wire, T on The effective value of the power-on delay time is calculated using the root mean square method (the same applies below).

[0088] S1-2. Using the three-phase measurement winding current as the three-phase phase current acquisition object: By using the 0.5-level three-phase measurement winding phase current (instead of the protection winding phase current) configured on-site as the three-phase phase current acquisition object of this invention, the accuracy of the phase current acquisition data is guaranteed from the source.

[0089] S1-3. Acquiring the zero-sequence current synthesized from the physical vectors of the three-phase phase currents: The zero-sequence current is obtained by simultaneously acquiring the current synthesized from the physical vectors of the secondary phase currents of the three-phase measuring windings. The acquired zero-sequence current satisfies the following equation: .

[0090] In the above formula, I a I b I c These represent the currents for phases A, B, and C, respectively. This effectively avoids the problem of decreased zero-sequence current accuracy caused by phase errors in the secondary current sensor when calculating using software vector sum.

[0091] S1-4. Current frequency tracking sampling through line voltage zero-crossing phase: The zero-crossing sampling point of the line voltage is calculated in real time. Based on the software, the half-cycle timing length of the line voltage zero-crossing sampling point at two intervals is calculated in real time. This is used to control the AD timing sampling interval and realize current frequency tracking sampling.

[0092] Furthermore, the frequency-following tracking sampling method is as follows: After the system is powered on, the line voltage (U) is acquired in real time. ab or U cb The counter value is reset to zero each time the line voltage crosses zero, and then restarted. The counter value N is then retrieved at the next zero-crossing. half N half This refers to the half-cycle timing duration. By continuously monitoring the half-cycle timing duration for p cycles, the values ​​N of the counters for p consecutive cycles are taken. half Calculate the arithmetic mean and use it as the half-cycle timing length N' half The timing sampling interval T of the AD converter is controlled by the following formula. AD : .

[0093] In the above formula, m is the number of sampling points per cycle.

[0094] The three-phase phase current and zero-sequence current are acquired in real time by voltage-controlled frequency-following sampling AD. This enables full-cycle sampling of phase current and zero-sequence current, providing a mathematical guarantee for the accurate extraction of abrupt changes in phase current and zero-sequence current characteristics in this invention.

[0095] Preferably, step S2 may include the following sub-steps: S2-1. Extracting Zero-Sequence Current Sudden Change: Every T1, the waveform of the zero-sequence current sudden change for the current cycle is extracted by subtracting the corresponding sampling point from the current latest sampling point by 2 cycles ago. The aforementioned "every T1 time interval" is the minimum processing time interval for the device processor system to start the algorithm. The value of T1 can be 5ms to 10ms to ensure that fault start analysis is not missed when actual disturbances occur. In particular, T1 can be set to 5ms to extract the zero-sequence current mutation at regular intervals, thus ensuring that the device processor system can start data analysis for any disturbance occurring on the distribution network within each cycle (20ms). Specifically, the extraction formula is as follows: .

[0096] In the above formula, I0(i) is the current zero-sequence current sample value corresponding to the i-th sampling point, I0(i-2m) is the zero-sequence current sample value corresponding to the (i-2m)-th sampling point, k is the latest sampling point number, and m is the number of sampling points per cycle.

[0097] Furthermore, the effective value of the zero-sequence current mutation waveform of the latest cycle is calculated. And determine whether the following fault start conditions are met by using the following formula: .

[0098] In the above formula, △I 0set It can be set to 1~2A.

[0099] If the conditions are met, the initial fault recording will be initiated, and the recorded data will be stored in the following manner: After a delay of y cycles, using the current sampling point k as the reference point, collect the current data (including three-phase current and zero-sequence current) from x cycles before the reference point to y cycles after the reference point, thus completing the saving of the waveform data. Preferably, current data can be taken from 4 cycles before the reference point to 8 cycles after the reference point.

[0100] S2-2. Extracting characteristic quantities for the initial single-phase ground fault assessment: 1) Record the waveform The precise initiation point k0 of the zero-sequence current surge is found using the second-order difference method. Based on the value of k0, the surge waveforms of each current (including three-phase currents and zero-sequence current) in one cycle after the fault are extracted. The extraction method for the current surge waveform is as follows: .

[0101] In the above formula, m is the number of sampling points per cycle, and k0 is the precise start point of the zero-sequence current mutation.

[0102] 2) Perform low-pass filtering (0-600Hz) on the waveforms of each current surge, retaining the power frequency and characteristic frequency band information, to construct the characteristic waveform of the current surge. .

[0103] 3) Calculation The effective value of the characteristic waveform of the three-phase phase current sudden change was selected, and the maximum value was chosen. And the corresponding phase sequence number (a, b, or c), and then add the characteristic waveform vectors of the phase current change of the other two phases to construct the characteristic change waveform (vector sum of non-fault phases). ,in, Specifically, it can be calculated using the following formula: .

[0104] S2-3. Initial Single-Phase Ground Fault Assessment (hereinafter referred to as Initial Assessment), the specific method is as follows: Based on characteristic mutation waveform and zero-order mutation waveform ( The principle of similarity is used to identify single-phase grounding faults, and the similarity identification parameter ρ is calculated using the following correlation coefficient formula: .

[0105] The vector sum of non-faulty phase abrupt changes and the zero-sequence current abrupt change can establish a waveform reverse polarity mapping relationship under the condition of weak current amplitude caused by high-resistance grounding, so that the judgment result is fixed to the "faulty line" conclusion. When the amplitude characteristics are not obvious, the correlation coefficient criterion is locked with the successive faults while retaining the waveform reverse characteristics, thus overcoming the defect of existing technology that relies solely on amplitude and is prone to failure when dealing with high-resistance faults; while under non-faulty load disturbances, the three-phase currents are assigned to healthy lines under the condition that the vector sum is in the same direction.

[0106] Furthermore, the results of the initial single-phase ground fault assessment are classified using the following criterion: "Characteristic 1 of the sudden change in phase current during the initial fault". .

[0107] In the above formula, ρ set1 ρ set2 The preset threshold for similarity identification parameters, where ρ set1 ρ is the similarity threshold corresponding to the faulty line. set2 To improve the similarity threshold for the corresponding lines; △I max I represents the maximum value of the phase current abrupt change. set1 I set2 I set3 The preset threshold for the phase current abrupt change, where I set1 A preset threshold is set for the sudden change in phase current corresponding to the faulty line, Iset2 I set3 To ensure the accuracy of the phase current surge parameters for the corresponding circuit, a preset threshold is established. Preset I set1 and I set2 Its function is to: require that the phase current mutation amount be greater than a preset threshold before similarity judgment can be determined, which can effectively improve the reliability of the criterion. Generally, I can be preset. set1 and I set2 It is 1A. Preset I set3 The physical meaning is that when the maximum phase current change is small, it is directly judged as a sound circuit, and I can generally be preset. set3 The value is 0.5A. Based on a large amount of simulation test data, ρ set1 ρ set2 They can be preset to -0.85 and +0.85 respectively.

[0108] The physical essence of the above criteria is as follows: For a faulty line, the vector sum of the sudden changes in the current of the non-faulty phases and the sudden change in the zero-sequence current have a clear opposite phase relationship in the first half-wave of the fault; for a healthy line, the vector sum of the sudden changes in the current of the non-faulty phases and the sudden change in the zero-sequence current have a clear in-phase phase relationship in the first half-wave of the fault; if the electrical quantities of the line do not meet the above phase rules, it can be determined as a system disturbance.

[0109] Preferably, step S4 is the follow-up handling process for the faulty line as determined by the initial assessment. Specifically, after determining the line to be faulty, the effective value of the zero-sequence current of one cycle following the precise starting point k0 in the waveform recording data can be extracted and calculated, and marked as I. 0fault .

[0110] Furthermore, the implementation method of protection tripping is as follows: Immediately after determining that a faulty line is being constructed, a timer is started to record the cumulative fault duration t0, and the zero-sequence current is continuously collected. During the duration of the single-phase ground fault, the zero-sequence current will continue to exist. The effective value of the zero-sequence current is continuously calculated. If the following successive fault protection criteria are met, protection tripping is executed: .

[0111] In the above formula, I0 is the effective value of the zero-sequence current obtained from real-time calculation, and I set1 T is the zero-sequence current setting value (i.e., the preset protection trip current threshold), t0 is the fault duration accumulation time, and T is the zero-sequence current setting value. set1 This is the fault duration setting value (i.e., the pre-set protection trip time threshold).

[0112] In the above formula, I set1 The value can be based on the effective value of the zero-sequence current (I) calculated when the initial judgment result of step S4 is a faulty line. 0fault ) Determine. For example, I set1 The value can be taken as 0.9 I 0fault A set value is set, with 0.9 as the reliability coefficient. Physically, this represents the effective value (characteristic quantity) of the zero-sequence current under continuous fault conditions.

[0113] After the protection trips, the process terminates and returns to step S1.

[0114] It is worth noting that when the identification process is initiated, the successive fault identification logic operates in a fault-tolerant monitoring state while the initial single-phase ground fault persists, enabling real-time correlation between other monitoring points and potential successive fault characteristics. This ensures that the system is activated only under specific unbalanced boundary conditions of the power grid, thus distinguishing it from conventional single-fault monitoring logic.

[0115] Preferably, step S5 may include the following sub-steps: S5-1. Marking the new status: Mark the "sound line successive grounding fault identification stage" status, calculate the effective value of the zero-sequence current of one cycle after the precise starting point k0 in the waveform data, and mark it as I. 0step value.

[0116] S5-2. Extract the characteristic value of the phase current sudden change within the waveform range of the first fault recording: 1) Based on the precise starting point k0 value, extract the waveforms of the sudden changes in phase current and zero-sequence current within 1 to y cycles after the fault. .

[0117] Starting from the precise starting point k0, by subtracting the sampling points at the same position in the two cycles preceding k0 from the sampling points at each sampling point within each cycle, the complete phase current abrupt change waveforms for the 1 to y cycles following the precise starting point k0 are extracted sequentially according to cycle numbers 1 to y. The extraction method is as follows: .

[0118] 2) Perform low-pass filtering (0-600Hz) on the waveforms of each current surge, retaining the power frequency and characteristic frequency band information, to construct the characteristic waveform of the current surge. .

[0119] 3) Calculate the effective value of each phase per cycle within the 1~y cycle range using the following formula: .

[0120] In the above formula, RMS△I A '(n), RMS△I B '(n), RMS△I C '(n) are the effective values ​​of the current mutations in phases A, B, and C within the nth cycle.

[0121] 4) Calculation The effective value of the characteristic waveform of the three-phase phase current sudden change was selected, and the maximum value was chosen. And the corresponding phase sequence number (a, b, or c), and then add the characteristic waveform vectors of the phase current change of the other two phases to construct the characteristic change waveform (vector sum of non-fault phases). ,in, Specifically, it can be calculated using the following formula: .

[0122] 5) Based on the characteristic mutation waveform and the zero-order mutation waveform ( The principle of similarity is used to identify single-phase grounding faults. The similarity identification parameter ρ is calculated using the following correlation coefficient formula: .

[0123] 6) The criterion for "characteristic 1 of sudden change in phase current during successive faults" is as follows: .

[0124] In the above formula, k set1 The characteristic difference value can be 1A.

[0125] If, in any cycle from 1 to y, a certain cycle satisfies criterion 1 above, then it is determined and marked that a successive single-phase grounding fault has occurred below the monitoring point.

[0126] 7) The criterion for "characteristic 2 of sudden change in phase current during successive faults" is as follows: .

[0127] In the above formula, ρ set1 The physical meaning is that the zero-sequence abrupt change waveform of a successively grounded fault line and the vector sum waveform of the non-faulted phases have opposite characteristics, that is, the calculated correlation coefficient value is significantly negative, and a typical value can be taken as -0.85; I set1 The minimum eigenvalue can be 1A, which means that ρ<ρ is only executed when the maximum value of the mutation is greater than this value. set1 The criteria are determined to prevent the risk of the criteria becoming invalid due to too small a mutation amount.

[0128] If criterion 2 above is met, then it is determined and marked that a successive single-phase grounding fault has occurred below the monitoring point.

[0129] Preferably, after determining a successive single-phase ground fault in step S5, the effective value of the zero-sequence current of that cycle can be extracted and calculated when the criterion of "successive fault phase current change characteristic 1" or "successive fault phase current change characteristic 2" is met, and marked as I. 0fault1 .

[0130] Preferably, after determining a successive single-phase ground fault in step S5, step S6 continuously monitors the zero-sequence current and its duration to determine whether the tripping conditions are met, and determines the subsequent execution plan based on the determination result. Specifically, after determining a successive single-phase ground fault, a timer can be started immediately to record the cumulative fault duration t0, and the zero-sequence current can be continuously collected. During the duration of the successive ground fault, the zero-sequence current will be maintained, and the effective value of the zero-sequence current will be continuously calculated.

[0131] Firstly, if the following criteria are met, a permanent successive ground fault will be reported, and the protection circuit will be tripped, terminating the process and returning to step S1: .

[0132] In the above formula, I set2 The value can be based on the zero-sequence current value (I) calculated in step S5 when a successive ground fault is determined. 0fault1 ) Determine. For example, I set2 The value can be taken as 0.9 I 0fault1 A set value is set, with 0.9 as the reliability coefficient. Physically, this represents the effective value (characteristic quantity) of the zero-sequence current under continuous fault conditions.

[0133] Secondly, if the following criteria are met, it indicates that the zero-sequence current has returned to normal during this period due to the complete disappearance of single-phase ground faults (tripping of the initial ground fault protection and disappearance of subsequent ground faults), and is reported as a transient successive ground fault, then return to step S1: .

[0134] In the above formula, I set3 The value can be 1.5 times the maximum unbalanced current of this line.

[0135] Thirdly, if neither of the above two criteria is met, it indicates that during this period, the disappearance of the successive grounding fault caused the zero-sequence current to recover to the effective value I of the "sound line successive grounding fault identification stage". 0step The system is reported as a transient successive ground fault and step S7 is executed.

[0136] Preferably, in step S7, the zero-sequence current is continuously monitored, and the effective value of the zero-sequence current is calculated in real time.

[0137] When the following criteria are met, it indicates that the zero-sequence current has returned to its normal value, and there are currently no single-phase grounding faults at other monitoring points in the distribution network (the line where the first single-phase grounding fault occurred in the distribution network has already been tripped and disconnected by protection). In this case, the "Identification Stage of Successive Single-Phase Grounding Faults in Healthy Lines" state will exit, and the process will return to step S1: .

[0138] In the above formula, I set4 The value can be based on the effective value of the zero-sequence current I collected in real time by the monitoring point during normal system operation. normal Determined. For example, I set4 The value can be determined according to 1.1 I normal Set a reliability coefficient of 1.1. When the zero-sequence current collected in real time after a fault is less than the set value, it indicates that the zero-sequence current has returned to normal.

[0139] If the above criteria are not met, proceed to step S8.

[0140] Preferably, in step S8, the waveform of the latest three-phase current and the zero-sequence current change is extracted by subtracting the corresponding sampling point two cycles ago from the current latest sampling point. The extraction formula is as follows: .

[0141] In the above formula, k is the current latest sampling point number, and m is the number of sampling points per cycle.

[0142] Furthermore, the waveforms of each current surge are low-pass filtered from 0 to 600 Hz to retain power frequency and characteristic frequency band information, thus forming the characteristic waveforms of current surges. Then, calculate the effective values ​​of the above three-phase current change waveforms in sequence. .

[0143] Further calculation The effective value of the characteristic waveform of the three-phase phase current sudden change was selected, and the maximum value was chosen. And the corresponding phase sequence number (a, b, or c), and then add the characteristic waveform vectors of the phase current change of the other two phases to construct the characteristic change waveform (vector sum of non-fault phases). ,in, Specifically, it can be calculated using the following formula: .

[0144] Based on characteristic mutation waveform and zero-order mutation waveform ( The principle of similarity is used to identify single-phase grounding faults. The similarity identification parameter ρ is calculated using the following correlation coefficient formula: .

[0145] Preferably, the criterion for "characteristic 1 of sudden change in phase current during successive faults" is as follows: .

[0146] In the above formula, k set1 The characteristic difference value can be 1A.

[0147] Preferably, the criterion for "characteristic 2 of sudden change in phase current during successive faults" is as follows: .

[0148] In the above formula, ρ set1 The physical meaning is that the zero-sequence abrupt change waveform of a successively grounded fault line and the vector sum waveform of the non-faulted phases have opposite characteristics, that is, the calculated correlation coefficient value is significantly negative, and a typical value can be taken as -0.85; I set1 The minimum eigenvalue can be 1A, which means that ρ<ρ is only executed when the maximum value of the mutation is greater than this value. set1 The criteria are determined to prevent the risk of the criteria becoming invalid due to too small a mutation amount.

[0149] If either of the above two criteria is met, then a successive single-phase grounding fault is determined and marked as occurring below the monitoring point, and step S6 is executed; otherwise, the process returns to step S7.

[0150] The embodiments of the present invention verify the technical solution using the Matlab simulation platform. The system simulation model is as follows: Figure 2 As shown in the table below, the simulated circuit parameters are as follows: The inductance L of the arc suppression coil in the arc suppression coil grounding system is calculated using the following formula: .

[0151] In the above formula, ω0 represents the power frequency, and C 0∑ The system capacitance to ground is represented by v, which represents the compensation degree. In this invention, v = -0.08 can be used.

[0152] Preferably, the total capacitance of the system to ground can be calculated by the following formula: Total capacitance of the system to ground = zero-sequence capacitance and capacitive reactance to ground per unit length of overhead line (e.g., 0.006Ω / km) The total length of overhead lines in the system plus the zero-sequence capacitance and capacitive reactance to ground per unit length of cable (e.g., 0.276Ω / km). The total length of the cables in the system.

[0153] The specific simulation settings are explained below: Examples 1 to 5 all simulated the cross-line phase-to-phase grounding fault. The fault parameters were set as follows: at 0.085s, a single-phase grounding fault of phase A occurred in the middle section (F1) of line L1-2 (the first single-phase grounding fault); at 0.145s, a single-phase grounding fault of phase A occurred in the middle section (F2) of line L4-2 (the successive single-phase grounding fault).

[0154] Examples 6 to 9 all simulate cross-line phase-to-phase grounding faults with different names. The fault parameters are set as follows: at 0.085s, a single-phase grounding fault of phase A occurs in the middle section (F1) of line L1-2 (the first single-phase grounding fault); at 0.145s, a single-phase grounding fault of phase B occurs in the middle section (F2) of line L4-2 (the successive single-phase grounding fault).

[0155] If the simulation is set to a sampling rate of 200 points per cycle (i.e., a sampling frequency of 10kHz), then... Figures 3-11 The relationship between the number of sampling points and the corresponding time is: Sampling point number = (Time t(ms) / 20ms) 200.

[0156] Example 1 The fault conditions in Example 1 are as follows: the grounding resistance of the first single-phase grounding fault is 30Ω, and the subsequent single-phase grounding faults are cross-line same-phase grounding faults with a grounding resistance of 30Ω.

[0157] The criterion parameters were calculated through simulation, as shown in the table below: Depend on Figure 3 It can be seen that when the first single-phase ground fault occurs in the distribution network, the phase current of line L1 shows a significant abrupt change; when subsequent single-phase ground faults occur, the amplitude of the abrupt change in phase A current of line L4 changes significantly.

[0158] Under the condition of initial grounding fault, the fault identification method of the present invention can accurately determine that line L1 is a faulty line and lines L2 to L4 are healthy lines by using the "characteristic 1 of sudden change in current of initial fault phase". When successive grounding faults occur in the distribution network, the "characteristic 1 of sudden change in current of successive fault phase phase" can further accurately identify line L4 as a newly added faulty line, while lines L2 and L3 can still be accurately determined as healthy lines.

[0159] Example 2 The fault conditions in Example 2 are as follows: the grounding resistance of the first single-phase grounding fault is 30Ω, and the subsequent single-phase grounding faults are cross-line same-phase grounding faults with a grounding resistance of 2000Ω.

[0160] The criterion parameters were calculated through simulation, as shown in the table below: Depend on Figure 4 It can be seen that when the first single-phase ground fault occurs in the distribution network, the phase current of line L1 shows a significant abrupt change; when subsequent single-phase ground faults occur, the amplitude of the phase current change of phase A of line L4 remains basically unchanged. This is the core reason why conventional fault identification technology cannot effectively identify this type of successive fault.

[0161] Under the condition of initial grounding fault, the fault identification method of the present invention can accurately determine that line L1 is a faulty line and lines L2 to L4 are healthy lines by using the "characteristic 1 of sudden change in phase current of initial fault" criterion. When the distribution network experiences the above-mentioned successive grounding faults, if identification is performed solely by the "characteristic 1 of sudden change in phase current of successive faults" criterion, lines L2 to L4 will all be determined as healthy lines. That is, the successive faults of line L4 cannot be correctly identified as faulty lines. For such fault scenarios where conventional criteria fail, the present invention can complete targeted handling through the supplementary identification mechanism set in Example 3.

[0162] Example 3 The fault conditions in Example 3 are as follows: the grounding resistance of the initial single-phase grounding fault is 30Ω, and the subsequent single-phase grounding faults are cross-line same-phase grounding faults with a grounding resistance of 2000Ω. The protection trips on the line with the initial fault 0.22 seconds after the fault.

[0163] The criterion parameters were calculated through simulation, as shown in the table below: Depend on Figure 5 It can be seen that after the L1 line trips, the amplitude of phase A of the L4 line begins to change due to the L1 line tripping. This feature can be used to further accurately identify the L4 line as the faulty line.

[0164] Example 4 The fault conditions in Example 4 are as follows: the grounding resistance of the first single-phase grounding fault is 2000Ω, and the subsequent single-phase grounding faults are cross-line same-phase grounding faults with a grounding resistance of 30Ω.

[0165] The criterion parameters were calculated through simulation, as shown in the table below: Depend on Figure 6 It can be seen that when the first single-phase ground fault occurs in the distribution network, the phase current of line L1 shows a significant abrupt change; when subsequent single-phase ground faults occur, the phase current of phase A of line L4 and its amplitude both show significant changes.

[0166] Under the condition of initial grounding fault, the fault identification method of the present invention can accurately determine that line L1 is a faulty line and lines L2 to L4 are healthy lines by using the "characteristic 1 of sudden change in phase current of initial fault" criterion. When successive grounding faults occur in the distribution network, line L4 can be accurately identified as a faulty line by using either the "characteristic 1 of sudden change in phase current of successive fault" or the "characteristic 2 of sudden change in phase current of successive fault" criterion. Lines L2 and L3 can also be accurately determined as healthy lines.

[0167] Example 5 The fault conditions in Example 5 are as follows: the grounding resistance of the first single-phase grounding fault is 2000Ω, and the subsequent single-phase grounding faults are cross-line same-phase grounding faults with a grounding resistance of 2000Ω.

[0168] The criterion parameters were calculated through simulation, as shown in the table below: Depend on Figure 7 It can be seen that when the first single-phase ground fault occurs in the distribution network, the phase current of line L1 shows a significant abrupt change; when subsequent single-phase ground faults occur, the amplitude of the phase current of phase A of line L4 changes significantly.

[0169] Under the condition of initial grounding fault, the fault identification method of the present invention can accurately determine that line L1 is a faulty line and lines L2 to L4 are healthy lines by using the "characteristic 1 of sudden change in current of initial fault phase". When successive grounding faults occur in the distribution network, the "characteristic 1 of sudden change in current of successive fault phase phase" can not only accurately identify line L4 as a faulty line, but also accurately determine lines L2 and L3 as healthy lines.

[0170] Example 6 The fault conditions in Example 6 are as follows: the grounding resistance of the first single-phase grounding fault is 30Ω, and the subsequent single-phase grounding faults are cross-line opposite-named phase grounding faults with a grounding resistance of 30Ω.

[0171] The criterion parameters were calculated through simulation, as shown in the table below: Depend on Figure 8 It can be seen that when the first single-phase ground fault occurs in the distribution network, the phase current of line L1 shows a significant abrupt change; when subsequent single-phase ground faults occur, the amplitude of the phase current of phase A of line L4 changes significantly.

[0172] Under the condition of initial grounding fault, the fault identification method of the present invention can accurately determine that line L1 is a faulty line and lines L2 to L4 are healthy lines by using the "characteristic 1 of sudden change in current of initial fault phase". When successive grounding faults occur in the distribution network, the "characteristic 1 of sudden change in current of successive fault phase phase" can accurately identify line L4 as a faulty line and accurately determine lines L2 and L3 as healthy lines.

[0173] Example 7 The fault conditions in Example 7 are as follows: the grounding resistance of the first single-phase grounding fault is 30Ω, and the subsequent single-phase grounding fault is a cross-line opposite-named phase grounding fault with a grounding resistance of 2000Ω.

[0174] The criterion parameters were calculated through simulation, as shown in the table below: Depend on Figure 9 It can be seen that when the first single-phase ground fault occurs in the distribution network, the phase current of line L1 shows a significant abrupt change; when subsequent single-phase ground faults occur, the amplitude of the phase current of phase A of line L4 changes significantly.

[0175] Under the condition of initial grounding fault, the fault identification method of the present invention can accurately determine that line L1 is a faulty line and lines L2 to L4 are healthy lines by using the "characteristic 1 of sudden change in current of initial fault phase". When successive grounding faults occur in the distribution network, the "characteristic 1 of sudden change in current of successive fault phase phase" can accurately identify line L4 as a faulty line and accurately determine lines L2 and L3 as healthy lines.

[0176] Example 8 The fault conditions in Example 8 are as follows: the grounding resistance of the first single-phase grounding fault is 2000Ω, and the subsequent single-phase grounding faults are cross-line opposite-named phase grounding faults with a grounding resistance of 30Ω.

[0177] The criterion parameters were calculated through simulation, as shown in the table below: Depend on Figure 10 It can be seen that when the first single-phase ground fault occurs in the distribution network, the phase current mutation of line L1 exhibits a significant abrupt change; when subsequent single-phase ground faults occur, the amplitude of the phase current mutation of phase A of line L4 changes significantly. Based on the data shown in the table above, in this embodiment, the characteristic difference value k... set1 It can be set to 1A. The effective value of the A-phase mutation amount of device 4 (46.64A) is significantly greater than that of the B-phase (3.50A) and k. set1 The sum of these is also greater than that of phase C (3.50A) and k. set1 The sum of these values ​​satisfies the "characteristic 1 of sudden change in phase current during the first fault" criterion, therefore it is determined to be a fault.

[0178] Under the condition of initial grounding fault, the fault identification method of the present invention can accurately determine that line L1 is a faulty line and lines L2 to L4 are healthy lines by using the "characteristic 1 of sudden change in phase current of initial fault" criterion. When successive grounding faults occur in the distribution network, the L4 line can be accurately identified as a faulty line by using the "characteristic 1 of sudden change in phase current of successive fault" or "characteristic 2 of sudden change in phase current of successive fault" criterion, and lines L2 and L3 can also be accurately determined as healthy lines.

[0179] Example 9 The fault conditions in Example 9 are as follows: the grounding resistance of the first single-phase grounding fault is 2000Ω, and the subsequent single-phase grounding faults are cross-line opposite-named phase grounding faults with a grounding resistance of 2000Ω.

[0180] The criterion parameters were calculated through simulation, as shown in the table below: Depend on Figure 11 It can be seen that when the first single-phase ground fault occurs in the distribution network, the phase current of line L1 shows a significant abrupt change; when subsequent single-phase ground faults occur, the amplitude of the phase current of phase A of line L4 changes significantly.

[0181] Under the condition of initial grounding fault, the fault identification method of the present invention can accurately determine that line L1 is a faulty line and lines L2 to L4 are healthy lines by using the "characteristic 1 of sudden change in current of initial fault phase". When successive grounding faults occur in the distribution network, the "characteristic 1 of sudden change in current of successive fault phase phase" can accurately identify line L4 as a faulty line and accurately determine lines L2 and L3 as healthy lines.

[0182] It should be noted that, Figures 3-11 The original three-phase current abrupt change characteristic waveforms are displayed; the “non-fault phase abrupt change vector sum” is a logic waveform synthesized in real time by the device based on the above three-phase waveforms. Although it is not drawn separately in the figure, its numerical characteristics are reflected in the correlation coefficient calculation results in the tables of each embodiment.

[0183] It should be noted that the specific embodiments described above are exemplary. Those skilled in the art can devise various solutions inspired by the disclosure of this invention, and these solutions all fall within the scope of this invention and its protection. Those skilled in the art should understand that this specification and its accompanying drawings are illustrative and not intended to limit the scope of the claims. The scope of protection of this invention is defined by the claims and their equivalents. This specification contains multiple inventive concepts; terms such as "preferredly," "according to a preferred embodiment," or "optionally" indicate that the corresponding paragraph discloses an independent concept. The applicant reserves the right to file divisional applications based on each inventive concept.

Claims

1. A method for identifying successive grounding faults in a distribution network based on the characteristics of phase current abrupt changes, applied to multiple monitoring points along the line, characterized in that... Includes the following steps: Once any monitoring point identifies the first single-phase ground fault line based on the local three-phase current and the zero-sequence current synthesized from it, the remaining monitoring points enter the successive ground fault identification stage and execute: (1) Based on the continuous cycle three-phase current sampling data, extract the abrupt change characteristics of the three-phase and zero-sequence currents, calculate the effective value of the abrupt change of each phase and the vector sum of the abrupt changes of the two non-fault phases; (2) When the correlation and effective value difference between the vector sum of the non-faulty phase change and the zero-sequence current change in any cycle satisfy the successive fault criterion, it is determined that a successive single-phase ground fault has occurred on the line. (3) When the first fault line trips and is cleared, a supplementary judgment is made based on the change in the three-phase current at the moment of clearing. When the effective value difference between the change in the fault phase and the non-fault phase meets the subsequent fault criterion after tripping, it is still judged as a successive single-phase ground fault. (4) For the identified successive fault lines, continuously monitor the effective value and duration of the zero-sequence current, and execute the trip when the successive fault protection criteria are met; If the zero-sequence current returns to the normal range, the successive grounding fault identification stage is exited. Thus, without requiring bus zero-sequence voltage and based on local three-phase current information, local identification and distributed protection of successive single-phase grounding faults of the same and different phases across the line can be achieved.

2. The method according to claim 1, characterized in that, The method for extracting the abrupt change characteristics of three-phase and zero-sequence currents includes extracting the current abrupt change waveform of the latest cycle by subtracting the corresponding sampling point of two cycles ago from the current latest sampling point, and performing low-pass filtering of the abrupt change waveform from 0 to 600 Hz to retain the power frequency and characteristic frequency band information, thus forming the current abrupt change characteristic waveform.

3. The method according to claim 1, characterized in that, The correlation between the vector sum of the non-faulty phase mutation and the zero-sequence current mutation is characterized by calculating the correlation coefficient ρ between the two. The correlation coefficient ρ is calculated based on the sampling data of the waveform of the non-faulty phase mutation vector sum and the waveform of the zero-sequence current mutation within 1 to y cycles after the fault.

4. The method according to claim 1, characterized in that, The effective value difference is the difference between the effective value of the fault phase current mutation and the maximum value of the effective values ​​of the two non-fault phase current mutations, or the difference between the effective value of the vector sum of the non-fault phase mutations and the effective value of the zero-sequence current mutation.

5. The method according to claim 3, characterized in that, The successive fault criteria include at least one of the following: "successive fault phase current abrupt change characteristic 1" criterion and "successive fault phase current abrupt change characteristic 2" criterion. Among them, the criterion for "characteristic 1 of phase current mutation in successive faults" is: when the effective value of the phase current mutation in a certain phase in the distribution network is greater than two specific comparison values ​​at the same time, it is determined that a successive ground fault has occurred below the monitoring point. The two specific comparison values ​​are the sum of the characteristic difference value and the effective value of the phase current mutation in the other two phases, respectively. Among them, the criterion for "characteristic 2 of successive fault phase current change" is: when both the conditions of "correlation coefficient ρ is less than the correlation coefficient set value" and "maximum phase current change is greater than the current set value" are met, it is determined that a successive single-phase grounding fault has occurred below the monitoring point.

6. The method according to claim 1, characterized in that, When making supplementary judgments based on the change in the three-phase current at the moment of disconnection, the change in the three-phase current at the moment of disconnection of the first single-phase ground fault line tripping is extracted, and the difference between the effective values ​​of the change in the fault phase and the non-fault phase is calculated. When the difference satisfies the criterion for subsequent faults after tripping, it is determined that there are still ongoing successive single-phase ground faults.

7. The method according to claim 1, characterized in that, Meeting the successive fault protection criterion means that the effective value of the zero-sequence current is greater than the preset zero-sequence current setting value I. set2 And the duration of this state exceeds the preset fault duration setting value T. set2 .

8. The method according to claim 5, characterized in that, After entering the successive grounding fault identification stage, the waveform of the first fault is analyzed first. Within the time range covered by the waveform data, it is determined whether the "characteristic 1 of the sudden change in phase current of successive fault" or "characteristic 2 of the sudden change in phase current of successive fault" is met. If it is met, it is immediately determined that a successive single-phase grounding fault has occurred below the monitoring point.

9. The method according to claim 8, characterized in that, The method for obtaining the initial fault waveform is as follows: when the effective value of the zero-sequence current surge is greater than the starting setpoint ΔI... 0set At that time, taking the current sampling point as the reference point, the three-phase current and zero-sequence current data of x cycles before the reference point to y cycles after the reference point are saved to form a waveform recording dataset.

10. The method according to claim 1, characterized in that, After a single-phase ground fault is identified, if the zero-sequence current decreases to the preset unbalanced current setting value I during subsequent monitoring... set3 The following is reported as a transient successive ground fault, and the system is restored to normal operation. Among them, I set3 It can be taken as 1.5 times the maximum unbalanced current of this line.

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