Power distribution network single-phase earth fault identification method based on phase current break variable characteristics

Abstract

The invention relates to a method for identifying a single-phase earth fault of a power distribution network based on a phase current break variable characteristic, which comprises the following steps of: acquiring a three-phase current signal according to a set theoretical sampling frequency, calculating an effective value of the three-phase current signal, and completing three-phase current power-on judgment when specific conditions are met at the same time; carrying out frequency tracking sampling frequency real-time sampling monitoring on the zero-sequence current, extracting a zero-sequence current break variable difference current curve, and carrying out fault starting condition judgment on the difference current curve; extracting three-phase current fault component waveforms from a fault starting point, and then performing low-pass filtering processing on the waveforms to obtain effective frequency band characteristic waveforms and power frequency component characteristic waveforms; and calculating the fault characteristic quantity, and executing different single-phase grounding fault positioning algorithms according to different types of neutral point grounding systems. According to the method, the single-phase grounding fault of the power distribution network is identified by adopting the technical principle of definite physical rules through phase current and zero sequence current signals.

Description

Technical Field

[0001] The present invention relates to the technical field of distribution networks, and in particular to a method for identifying single-phase grounding faults in distribution networks based on the characteristics of sudden changes in phase current. Background Art

[0002] In many switch cabinets, ring main units, and cable branch boxes in distribution networks, zero-sequence PTs (or three-phase PTs) are not installed, resulting in the inability of the device to obtain zero-sequence voltage signals. In recent years, researchers have tried to use only current signals for fault identification, complete single-phase grounding fault identification by analyzing the transient or steady-state process of phase current, and use the phase asymmetry method to complete single-phase grounding fault location relying on phase current signals. Effectively identifying single-phase grounding faults has been a research and application hotspot in recent years.

[0003] CN105914718A discloses a grounding fault protection method based on sudden changes in phase current. This invention patent judges the faulty line by obtaining the sudden change of transient phase current and calculating the inter-phase correlation coefficient. However, directly calculating the correlation coefficient of the sampled values of the sudden change of transient phase current has low signal-to-noise ratio and poor anti-interference ability because it completely depends on the actual sampled values.

[0004] CN111999674A discloses a method for detecting and locating single-phase grounding faults in distribution lines based on changes in phase current. This invention patent synthesizes zero-sequence current from three-phase currents and locates the fault by judging the polarity and amplitude of the changes in three-phase currents. By self-producing and synthesizing zero-sequence current, the self-error of synthesizing zero-sequence current and extracting the fault component of phase current is significantly increased when the load current is large, and it is difficult to start for high-resistance grounding faults. At the same time, when looking for the maximum value, polarity, and magnitude of the changes in three-phase currents within two cycles at the current point after the fault, for arc grounding, there is a high possibility of misjudging the polarity point.

[0005] CN113219300A discloses a method for perceiving single-phase grounding faults in distribution networks based on the transient and steady states of phase current. This invention patent analyzes faults through steady-state characteristics and transient characteristics respectively. For resistive grounding faults, the steady-state characteristic positioning result is adopted, and for arc grounding faults, the transient characteristics are used to locate the result according to the sudden change direction of three-phase currents. Simply relying on transient or steady-state characteristics for analysis often has blind spots, and the sudden change direction is often difficult to accurately obtain in practice, reducing the overall reliability of the algorithm.

[0006] CN115792504A discloses a method and system for locating single-phase grounding faults in a distribution network based on the sudden change of phase current. This invention patent calculates the effective values of the sudden changes of the three-phase currents according to the waveforms of the sudden changes of the phase currents before and after the grounding fault, and sorts them; calculates the ratio of the effective values of the sudden changes, and locates the single-phase grounding fault of the distribution network line according to the magnitude relationship between the ratio and the ratio setting value. When an arc grounding fault actually occurs, simply relying on the effective value ratio calculation method, it is often very difficult to set the setting value, and it is difficult to apply in practice.

[0007] In recent years, the application of the phase current method (phase asymmetry method) to single-phase grounding fault identification has the problem that when reclosing onto a permanent fault, the fault characteristics are mixed with inrush current and three-phase non-synchronization, which often leads to frequent refusal or misoperation after reclosing. Therefore, this invention urgently needs a method for accurately and efficiently identifying single-phase grounding faults in a distribution network.

[0008] In addition, on the one hand, there are differences in the understanding of those skilled in the art; on the other hand, although the applicant has studied a large number of documents and patents when making this invention, due to space limitations, all details and content are not listed in detail. However, this does not mean that this invention does not possess the features of these prior arts. On the contrary, this invention already possesses all the features of the prior arts, and the applicant reserves the right to add relevant prior arts in the background art. Summary of the Invention

[0009] Aiming at the deficiencies of the prior art, this invention provides a method for identifying single-phase grounding faults in a distribution network based on the characteristics of sudden changes in phase current, aiming to solve the defects in the prior art related to the phase current method in single-phase grounding fault identification technology, such as simply relying on sampling values to calculate correlation coefficients, directions, polarities and other characteristics, and relying on transient or steady-state single characteristics for fault identification. Especially for the distribution network neutral point grounded through an arc suppression coil system, the problem of difficult effective identification brought by the current use of the phase current method for fault identification. The identification method of this invention can be particularly applicable to the application scenario of a distribution network without zero-sequence voltage signal. Through phase current and zero-sequence current signals, and using a technical principle based on clear physical laws, single-phase grounding fault identification of the distribution network is realized.

[0010] This invention discloses a method for identifying single-phase grounding faults in a distribution network based on the characteristics of sudden changes in phase current, which may include one or more of the following steps:

[0011] S1. Collect three-phase current signals according to the set theoretical sampling frequency, and calculate their effective values. When specific conditions are met simultaneously, the power-on judgment of the three-phase current is completed;

[0012] S2. Realize frequency tracking sampling by controlling the PWM output through real-time calculation of the phase angle difference of the three-phase currents;

[0013] S3. Perform real-time sampling and monitoring on the three-phase current and zero-sequence current with a frequency-tracking sampling frequency, extract the differential current curve of the sudden change in zero-sequence current, and jointly determine the starting condition for the over-limit of the differential current curve and the zero-sequence current curve;

[0014] S4. After performing low-pass filtering on the differential current curve of the sudden change in zero-sequence current, find and mark the fault starting point;

[0015] S5. Extract the fault component waveforms of the three-phase current starting from the fault starting point, and then perform low-pass filtering on these waveforms respectively to obtain the effective frequency band characteristic waveforms and power frequency component characteristic waveforms;

[0016] S6. Calculate the fault characteristic quantities, including but not limited to the energy characteristic value of the effective frequency band characteristic waveform, the difference value of the decaying DC component, and the instantaneous phase angle of the power frequency component characteristic waveform and the curve of the change in its phase difference;

[0017] S7. Execute different single-phase grounding fault location algorithms according to different types of neutral grounding systems;

[0018] S8. Implement the tripping logic locally;

[0019] S9. Implement accelerated protection actions for permanent faults after automatic reclosing;

[0020] S10. If no fault is detected, return to S2 in the process and continue to monitor the system status.

[0021] Preferably, collect the three-phase current signals according to the set theoretical sampling frequency and calculate their effective values. When the specific condition of "the effective values of the three-phase current signals are all greater than or equal to the power-on current setting value, and the time variable is greater than or equal to the power-on delay time setting value" is met, the power-on judgment of the three-phase current is completed.

[0022] Preferably, perform real-time sampling and monitoring on the three-phase current and zero-sequence current with a frequency-tracking sampling frequency, regularly extract the zero-sequence current waveforms within a specified time window (N cycles), subtract the sampling points of the previous cycle from the sampling points of the subsequent cycle of the N-cycle zero-sequence current waveforms starting from the first cycle in turn to extract the differential current curve of the sudden change in zero-sequence current, and judge the fault starting condition for the differential current curve. The fault starting conditions include detecting whether there is an effective value greater than the preset threshold I in each cycle of the differential current curve of the sudden change in zero-sequence current, and the average value of the effective values of the zero-sequence current in multiple consecutive cycles is greater than another preset threshold I. 0SET1 and the average value of the effective values of the zero-sequence current in multiple consecutive cycles is greater than another preset threshold I. 0SET2 .

[0023] Preferably, the three-phase current fault component waveforms are extracted by subtracting the corresponding sampling points of the previous cycle from each sampling point of the temporarily stored three-phase current waveforms starting from the fault starting point, and then these waveforms are respectively low-pass filtered by low-pass filters with two different cut-off frequencies to obtain the effective frequency band characteristic waveforms and power frequency component characteristic waveforms.

[0024] Preferably, fault characteristic quantities are calculated based on the effective frequency band characteristic waveforms and power frequency component characteristic waveforms. Among them, the calculated fault characteristic quantities include the energy eigenvalue of the effective frequency band characteristic waveform, the difference value of the decaying DC component, and the instantaneous phase angle of the power frequency component characteristic waveform and the curve of the change in its phase difference. By calculating the root mean square value (RMS) of the effective frequency band waveform in one cycle after the fault starting point, the maximum value and its corresponding phase sequence are found as the reference phase to identify the energy difference between the fault phase and the non-fault phase. The decaying DC component of the fault component of each phase current is calculated using the power frequency component characteristic waveform, and the difference value of the decaying DC component between the fault phase and the non-fault phase is constructed to determine whether there is a significant difference in the decaying DC component. The instantaneous phase angle of the power frequency characteristic waveform after the fault starting point is quickly calculated based on the peak-valley point estimation method. According to the calculated instantaneous phase angle, the curve of the change in the phase difference of the power frequency component characteristic waveform between the fault phase and the non-fault phase is constructed to provide the basic judgment data for fault location.

[0025] Preferably, according to the fault characteristic quantities, different single-phase grounding fault location algorithms are executed according to different types of neutral grounding systems. For an ungrounded neutral system, its fault location criterion needs to meet the following characteristics: a) The magnitudes of the steady-state power frequency components of the fault components of the phase currents of the fault phase and the non-fault phase are different, and the directions are opposite; b) There is a large difference in the effective energy (steady-state power frequency and transient components) of the fault components of the phase currents of the fault phase and the non-fault phase. For a system with a Peterson coil grounded at the neutral point, its fault location criterion needs to meet the following characteristics: a) The effective energy of the fault component of the phase current of the fault phase > the effective energy of the fault component of the phase current of the non-fault phase; b) During the transient process after the fault occurs, there is an obvious phase change process (from being opposite in phase gradually transitioning to being in phase or nearly in phase) in the power frequency phases of the fault components of the phase currents between the fault phase and the non-fault phase; c) During the transient process after the fault occurs, there may be a decaying DC component in the fault component of the phase current of the fault phase.

[0026] Compared with the prior art, the advantages of the present invention are as follows:

[0027] 1. The present invention adopts a frequency-tracking sampling method based on phase difference calculation for sampling, which can extract the fault components of the phase current more accurately than the conventional fixed-frequency sampling method. Especially in the case of large three-phase load currents, the recognition ability of high-resistance grounding faults is significantly improved.

[0028] 2. The present invention constitutes a new fault startup condition by combining the calculation of the startup variable using the sudden change current difference curve of zero-sequence current and the startup with the over-limit value of zero-sequence current, which has stronger sensitivity and anti-interference ability, can effectively avoid misjudgment caused by load jitter, and at the same time the algorithm is simple and easy to implement.

[0029] 3. The method for extracting the instantaneous phase angle of the characteristic waveform of the power frequency component and the method for calculating the curve of the change amount of the phase difference proposed by the present invention have small calculation amount, simple algorithm, clear physical concept, and can quickly calculate the instantaneous phase of the non-standard sine wave in the fault transient process. Since the peak and valley signals are extracted instead of the zero-crossing signals for cycle number calibration, it can effectively accommodate the influence of the decaying DC component on the calculation result, so the algorithm has strong adaptability.

[0030] 4. The method for extracting the decaying DC component from the characteristic waveform of the power frequency component proposed by the present invention has stronger anti-interference ability of the calculation result due to the absence of high-frequency component interference after low-pass filtering.

[0031] 5. When the present invention is applied to the ungrounded neutral system and the arc suppression coil grounded neutral system, a unified low-pass filtering algorithm is adopted. The constructed effective frequency band characteristic waveform retains the fault power frequency characteristics and the possible high-frequency characteristics of the first capacitance section of the fault, and the constructed power frequency component characteristic waveform retains the characteristics of the phase change amount of the power frequency component and the decaying DC component characteristics. Based on this, a unified feature quantity for identifying low-resistance, high-resistance, and arc grounding faults is established, which improves the adaptability of the algorithm.

[0032] 6. The fault identification technology that combines the energy eigenvalue of the effective frequency band characteristic waveform between the fault phase and the non-fault phase and the curve characteristic of the phase difference change amount of the power frequency component is applied to the fault identification algorithm of the arc suppression coil grounded neutral system as criterion 1. At the same time, the fault identification technology that combines the energy eigenvalue of the effective frequency band characteristic waveform and the difference value characteristic of the decaying DC component is added as criterion 2. The calculation is simple, the physical process is clear, and it effectively avoids the problems such as difficult setting value, easy misoperation, and poor reliability caused by the existing algorithm relying only on calculating the correlation coefficient method using the transient or steady-state fault component waveforms. The algorithm implementation is simple and easy to achieve.

[0033] 7. The fault identification technology that combines the energy eigenvalue of the effective frequency band characteristic waveform between the fault phase and the non-fault phase and the curve characteristic of the phase difference change amount of the power frequency component proposed by the present invention is applied to the fault identification algorithm of the ungrounded neutral system, with simple calculation and clear physical process.

[0034] 8. The present invention adopts the reclosure action judgment condition after reclosing on a permanent fault and the fault reset process, which can effectively avoid the problems of refusal to operate and misoperation caused by inrush current or three-phase asynchronous closing when reclosing on a permanent fault in the prior art. At the same time, by identifying the zero-sequence current memory fault quantity, rapid protection action is achieved.

[0035] 9. The present invention uses zero-sequence current and three-phase current, without the need for zero-sequence voltage signals. The algorithm is simple and reliable, and is easy to integrate on-site. This algorithm can be conveniently implanted into various existing distribution terminals without zero-sequence voltage acquisition signals. Without modifying the hardware, only by adding the algorithm of the present invention, each distribution terminal can independently realize the judgment of single-phase grounding faults, and has self-booting properties. Therefore, it has profound significance and broad application prospects. Description of the Drawings

[0036] Figure 1 is the step flowchart of the identification method provided by the present invention;

[0037] Figure 2 is the calculation chart of the difference in abrupt change of adjacent cycle quantities for frequency tracking sampling and fixed-frequency sampling provided by the present invention;

[0038] Figure 3 is the full-current characteristic waveform diagram of the three-phase current fault components of the fault line and the healthy line in the neutral-point non-grounding system when a 2000-ohm high-resistance grounding fault occurs provided by the present invention;

[0039] Figure 4 is the full-current characteristic waveform diagram of the three-phase current fault components of the fault line and the healthy line in the neutral-point non-grounding system when an arc grounding fault occurs provided by the present invention;

[0040] Figure 5 is the full-current characteristic waveform diagram of the three-phase current fault components of the fault line and the healthy line in the neutral-point arc suppression coil grounding system when a 5000-ohm high-resistance grounding fault occurs provided by the present invention;

[0041] Figure 6 is the full-current characteristic waveform diagram of the three-phase current fault components of the fault line and the healthy line in the neutral-point arc suppression coil grounding system when a 1000-ohm high-resistance grounding fault occurs provided by the present invention;

[0042] Figure 7 is the full-current characteristic waveform diagram of the three-phase current fault components of the fault line and the healthy line in the neutral-point arc suppression coil grounding system when an arc grounding fault occurs provided by the present invention;

[0043] Figure 8 is the curve of the instantaneous phase angle of the power frequency characteristic waveform of the phase current fault component of the fault line and the change amount of the phase difference between the fault phase and the non-fault phase in the neutral-point arc suppression coil grounding system when a 5000-ohm high-resistance grounding fault occurs provided by the present invention;

[0044] Figure 9 is the curve of the instantaneous phase angle of the power frequency characteristic waveform of the phase current fault component of the fault line and the change amount of the phase difference between the fault phase and the non-fault phase in the neutral-point arc suppression coil grounding system when an arc grounding fault occurs provided by the present invention. Detailed implementation manners

[0045] The following is a detailed description with reference to the accompanying drawings.

[0046] The present invention discloses a method for identifying single-phase grounding faults in a distribution network based on the characteristics of sudden changes in phase current. To better understand the operating principles of the various steps included in the identification method of the present invention and the technical details behind them, it is necessary to first introduce the variation law of the phase current fault component when a single-phase grounding fault occurs. When a single-phase grounding fault occurs in a distribution network, the phase current fault components of the healthy line and the faulty line show significant differences. For a healthy line, its three-phase current fault components consist of a steady-state power frequency component and a transient free oscillation component, and the magnitudes of the fault components between the phases are close to the same, and the directions are in-phase or nearly in-phase. However, on the faulty line, the current fault component of the faulty phase not only includes the above components, but also additional current changes will occur due to the fault, such as differences in the steady-state power frequency component, reverse direction, the phase change process from reverse direction to in-phase direction, and obvious differences in effective energy. In addition, in a system with a neutral point grounded through an arc suppression coil, the current of the faulty phase may also exhibit obvious decaying DC component characteristics, which helps to further distinguish the faulty line from the healthy line.

[0047] When a single-phase grounding fault occurs in a distribution network, the phase current fault components on the healthy line and the faulty line show different characteristics. These characteristics are crucial for accurately identifying the faulty line.

[0048] Specifically, after the fault occurs, the three-phase phase current fault components of the healthy line are expressed as follows:

[0049]

[0050] In the above formulas (1), (2), and (3), Δi kA , Δi kB , Δi kC respectively represent the phase current fault components of phases A, B, and C of the healthy line, i kA ′, i kB ′, i kC ′ are the currents of phases A, B, and C of the healthy line after the fault, i kA , i kB , i kC are the currents of phases A, B, and C of the healthy line before the fault, c k is the capacitance to ground of each phase of the healthy line, U0 is the neutral point offset voltage of the system, and t is the time variable, representing the time starting from the moment of fault occurrence.

[0051] For the faulty line, assuming the fault occurs in phase C, its three-phase phase current fault components can be expressed as:

[0052]

[0053] In the above formulas (4), (5), and (6), Δi JA , Δi JB , Δi JC are the phase current fault components of the faulty line in phases A, B, and C respectively, i JA ′, i JB ′, i JC ′ are the currents in phases A, B, and C of the faulty line after the fault, i JA , i JB , i JC are the currents in phases A, B, and C of the faulty line before the fault, c J is the capacitance to ground of each phase of the faulty line, i f is the current flowing through the fault point, and t is the time variable, representing the time starting from the moment of the fault.

[0054] Preferably, for a sound line in an ungrounded neutral system, the full current expression of the phase current fault component can be written as:

[0055]

[0056] In the above formula (7), i Ck ″, i Ck ′ are the steady-state power frequency component and the transient free oscillation component respectively, I ck is the amplitude of the current in each phase of the sound line, ω is the power frequency angular frequency, is the initial phase angle, ω f is the angular frequency of the transient free oscillation component, σ is the transient attenuation coefficient, and t is the time variable, representing the time starting from the moment of the fault.

[0057] This indicates that the phase current fault component of the sound line consists of a steady-state power frequency component and a transient free oscillation component, and the magnitudes of the fault components between each phase are close to being the same, and the directions are in-phase or close to in-phase.

[0058] Preferably, for a faulty line in an ungrounded neutral system, the full current expression of the phase current fault component of its non-faulty phase can be written as:

[0059]

[0060] In the above formula (8), i CJ ″, i CJ ′ are the steady-state power frequency component and the transient free oscillation component respectively, I cJ is the amplitude of the current in each phase of the faulty line, ω is the power frequency angular frequency, is the initial phase angle, ω fis the angular frequency of the transient free oscillation component, σ is the transient decay coefficient, and t is the time variable, representing the time starting from the fault occurrence moment.

[0061] For a faulty line in an ungrounded neutral system, the expression for the total current of the fault component of the phase current of the faulty phase can be written as:

[0062]

[0063] In the above formula (9), I cJ is the amplitude of the current of phase C of the faulty line, ω is the power frequency angular frequency, is the initial phase angle, ω f is the angular frequency of the transient free oscillation component, σ is the transient decay coefficient, t is the time variable, representing the time starting from the fault occurrence moment, I ∑c is the amplitude of the current of the non-faulty phases in the whole network, ω fc is the angular frequency of the transient free oscillation component of the non-faulty phases in the whole network, σ c is the transient decay coefficient of the non-faulty phases in the whole network.

[0064] The above formula (9) shows that the fault component of the phase current is composed of a steady-state power frequency component and a transient free oscillation component. The steady-state power frequency component is the vector sum of the steady-state power frequency components of the faulty phase current and the steady-state power frequency components of the non-faulty phases in the whole network. The transient free oscillation component is the vector sum of the transient free oscillation components of the faulty phase current and the transient free oscillation components of the non-faulty phases in the whole network. Both the steady-state power frequency component and the transient free oscillation component play a role of assisting and increasing in the fault component of the faulty phase current in an opposite phase.

[0065] Preferably, when the distribution network is an ungrounded neutral system, the characteristics of the fault component of the phase current can include the following two characteristics. Among them, characteristic 1 is the law of the fault components of the three-phase currents of the sound lines in the ungrounded neutral system, and characteristic 2 is the law of the fault components of the three-phase currents of the faulty lines in the ungrounded neutral system.

[0066] Furthermore, the law of characteristic 1 is that the magnitudes of the steady-state power frequency components of the fault components of the phase currents of the faulty phase and the non-faulty phases are close to the same, and the directions are in the same phase or close to the same phase; the difference in the effective energy (steady-state power frequency and transient components) of the fault components of the phase currents of the faulty phase and the non-faulty phases is small.

[0067] Furthermore, the law of characteristic 2 is that the magnitudes of the steady-state power frequency components of the fault components of the phase currents of the faulty phase and the non-faulty phases are different, and the directions are opposite; the directions of the steady-state power frequency components of the fault components of the phase currents of the non-faulty phases and the non-faulty phases are close to the same. The difference in the effective energy (steady-state power frequency and transient components) of the fault components of the phase currents of the faulty phase and the non-faulty phases is large.

[0068] Preferably, according to Feature 1 and Feature 2, the criterion for judging the faulty line in an ungrounded neutral system can be obtained, that is, the following features need to be satisfied:

[0069] ① The effective energy of the fault component of the phase current of the faulty phase > the effective energy of the fault component of the phase current of the non-faulty phase;

[0070] ② The phase angle of the steady-state power-frequency component of the fault component of the phase current of the faulty phase and the non-faulty phase is continuously opposite, and the phase angle of the steady-state power-frequency component of the fault component of the phase current of the non-faulty phase and the non-faulty phase is nearly the same.

[0071] Preferably, when the neutral point is grounded through an arc suppression coil, the fault component of the phase current after a single-phase ground fault occurs in the distribution network exhibits specific characteristics, which are crucial for accurately identifying the faulty line.

[0072] Specifically, the access of the neutral arc suppression coil makes the current i flowing through the fault point f not only include the capacitive current to the ground, but also increase the steady-state power-frequency component i of the inductance of the arc suppression coil L ″ and the transient free oscillation component i of the inductance of the arc suppression coil L ′, as shown in the following formula (10):

[0073]

[0074] In the above formula (10), I Lm is the amplitude of the inductive current of the arc suppression coil, ω is the power-frequency angular frequency, is the initial phase angle, τ L is the time constant of the arc suppression coil.

[0075] Preferably, for the case where the distribution network is a system with the neutral point grounded through an arc suppression coil, the full-current expressions of the fault component of the phase current of the sound line and the full-current expressions of the fault component of the phase current of the non-faulty phase of the faulty line are the same as the above formulas (7) and (8), while the full-current expression of the fault component of the phase current of the faulty line - faulty phase will increase i L ″ + i L ′, as shown in the following formula (11):

[0076]

[0077] In the above formula (11), I cJ is the amplitude of the phase current of the faulty line C, ω is the power-frequency angular frequency, is the initial phase angle, ω f is the angular frequency of the transient free oscillation component, σ is the transient attenuation coefficient, t is the time variable, representing the time starting from the moment of the fault, I ∑c is the amplitude of the non-faulty phase current of the whole network, ω fcis the angular frequency of the transient free oscillation component of the non-fault phases in the whole network, σ c is the transient attenuation coefficient of the non-fault phases in the whole network, I Lm is the amplitude of the inductive current of the arc suppression coil, τ L is the time constant of the arc suppression coil.

[0078] Furthermore, in the above formula (11), when the value of is the largest, there will be a decaying DC component; when the value of is the smallest, there will be no transient free oscillation component of the arc suppression coil inductance.

[0079] Preferably, when the distribution network is a system with the neutral point grounded through an arc suppression coil, the characteristics of the phase current fault component may include the following two characteristics. Among them, characteristic 3 is the law of the three-phase phase current fault component of the sound line in the arc suppression coil grounding system, and characteristic 4 is the law of the three-phase phase current fault component of the fault line in the arc suppression coil grounding system.

[0080] Furthermore, the law of characteristic 3 is that the magnitudes of the steady-state power-frequency components of the phase current fault components of the fault phase and the non-fault phases are close to the same, and the directions are in-phase or close to in-phase; the difference in the effective energy (steady-state power-frequency and transient components) of the phase current fault components between the fault phase and the non-fault phases is small.

[0081] Furthermore, the law of characteristic 4 is that since the arc suppression coil generally operates in an over-compensation mode, after complete compensation, the steady-state power-frequency component in the full-current expression of the phase current fault component of the fault phase of the fault line will eventually be in-phase or close to in-phase with the non-fault steady-state power-frequency component. However, due to the relatively large time constant of the arc suppression coil circuit, the compensation current of the arc suppression coil is extremely small in the first half cycle after the fault occurs. Therefore, the steady-state power-frequency component of the phase current fault component of the fault phase of the fault line will have an obvious phase change process during this period (from the moment of the fault, it gradually changes from anti-phase to in-phase or close to in-phase after the arc suppression coil starts to work). Further, the phase change process refers to the process in which the phase angle of the steady-state power-frequency component of the phase current fault component of the fault phase relative to the non-fault phase gradually changes from anti-phase to in-phase or close to in-phase during the fault occurrence, and this phenomenon is the obvious phase change process. During the fault occurrence, when the transient free oscillation component of the fault component is at ( or a small angle), the phase current fault component of the fault phase has the characteristic of a decaying DC component, and its waveform obviously has a unidirectional offset. The phase current fault component of the non-fault phase hardly has a decaying DC component, and this phenomenon is the decaying DC component phenomenon. And during the transition process, the effective energy magnitudes of the phase current fault components of the fault phase and the non-fault phase are different and the difference is large. The directions of the steady-state power-frequency components of the phase current fault components between the non-fault phases are close to the same.

[0082] Preferably, based on Feature 3 and Feature 4, the criterion for fault line determination in a neutral point grounded through an arc suppression coil system can be obtained, that is, the following features need to be satisfied:

[0083] ① The effective energy of the fault component of the phase current in the fault phase > the effective energy of the fault component of the phase current in the non-fault phase;

[0084] ② During the transient process after the fault occurs, there will be an obvious phase change process between the fault components of the phase currents in the fault phase and the non-fault phase in terms of the power frequency phase angle (gradually transitioning from being out of phase to being in phase or close to being in phase), and the power frequency phase angles of the fault components of the phase currents between the non-fault phases are directly close to being the same, without this phase change process;

[0085] ③ During the transient process after the fault occurs, there may be a decaying DC component in the fault component of the phase current in the fault phase.

[0086] Based on the above analysis principle, as Figure 1 shown, a method for identifying single-phase grounding faults in a distribution network based on the full current characteristics of the sudden change of phase current of the present invention may include the following steps:

[0087] S1. Monitoring of three-phase current power-on

[0088] Collect three-phase current signals according to the theoretical sampling frequency (fixed-frequency sampling) and calculate the effective values. When the following specific conditions are simultaneously satisfied, the monitoring of three-phase current power-on is completed:

[0089] RMSI A ≥I ON

[0090] RMSI B ≥I ON

[0091] RMSI C ≥I ON

[0092] t≥t ON ,

[0093] where RMSI A , RMSI B , RMSI C represent the effective values of the three-phase current signals, and the calculation method of the effective value adopts the root mean square value calculation method, I ON is the power-on current setting value, t ON is the power-on delay time setting value, and t is a time variable, representing the time starting from the moment when the effective values of the three-phase current signals simultaneously satisfy the power-on current setting value.

[0094] According to a preferred embodiment, in step S1, I ON can be approximately 2A or higher, tON Generally, it can be set to 5 seconds.

[0095] S2. Three-phase current frequency-tracking sampling

[0096] Calculate the phase angle difference of the three-phase current in real time and control the PWM output to achieve frequency-tracking sampling.

[0097] Preferably, after the power-on monitoring of the three-phase current in step S1 is completed, the three-phase current signals are collected at regular intervals according to the theoretical sampling frequency f s The phase angles of the three-phase currents I A 、I B 、I C are calculated respectively, and the phase angle differences between two adjacent full cycles are calculated:

[0098]

[0099] where are the phase angle differences of the A, B, and C phase currents between the current cycle and the previous cycle respectively, are the full-cycle phase angles of the A, B, and C phase currents calculated by the Fourier algorithm with x as the calculation starting point respectively, are the phase angles of the A, B, and C phase currents of the previous full cycle calculated by the Fourier algorithm with x - m as the calculation starting point respectively, and m is the number of sampling points per cycle.

[0100] Furthermore, the average value of the above three-phase current phase angle differences can be calculated by the following formula:

[0101]

[0102] Preferably, through dynamic feedback control, the PWM signal output for frequency-tracking sampling is controlled, and the new sampling frequency f s ′ control method is as follows:

[0103] Reduce the PWM output frequency, f s ′ = f s -Δf s ;

[0104] Increase the PWM output frequency, f s ′ = f s +Δf s 。

[0105] Update f s ′ in this way continuously to achieve frequency-tracking sampling.

[0106] According to a preferred embodiment, in step S2, m can take 128 or 256 points according to industry requirements to achieve a sampling frequency of f s = 6.4k or 12.8k.

[0107] According to a preferred embodiment, in step S2, the phase angle difference calculation can be performed every 15 cycles (0.3 seconds) regularly. According to the above control method, frequency tracking can generally be completed within three steps of coarse tuning, fine tuning, and micro tuning (0.3 seconds × 3 = 0.9 seconds), and the tracking sampling frequency f s ′ is achieved to perform full-cycle sampling on the sampling points.

[0108] According to a preferred embodiment, in step S2, after the three-phase current frequency-tracking sampling is completed, the following technical indicators are met: within the rated current range (such as the distribution network load current of 600A), a stable current of 600A is applied, and the calculation effective value change error between the previous and the next two full cycles is ≤ 0.25A in calculation, so as to ensure the accurate extraction of the fault component when the minimum grounding current may be about 1A in the case of a high-resistance grounding fault in the distribution network.

[0109] As Figure 2 (a) shows, when using frequency-tracking sampling (the power grid randomly jitters within the range of 49.95Hz to 50.05Hz), when the three-phase current is 600A, the calculation effective value change error between the previous and the next two full cycles is ≤ 0.25A (peak-to-peak value ± 0.5A) in calculation.

[0110] As Figure 2 (b) shows, when using fixed-frequency sampling (such as f s = 6.4k or 12.8k), when the three-phase current is 600A (49.95Hz), the calculation effective value change error between the previous and the next two full cycles is > 3.5A (peak-to-peak value ± 6A) in calculation.

[0111] As Figure 2 (c) shows, when using fixed-frequency sampling (such as f s = 6.4k or 12.8k), when the three-phase current is 600A (50.05Hz), the calculation effective value change error between the previous and the next two full cycles is > 3.6A (peak-to-peak value ± 6A) in calculation.

[0112] From Figure 2 the comparison between (a) and (b), (c), it can be clearly seen that after using frequency-tracking sampling, the calculation error of the effective value change amount is reduced by one order of magnitude compared with fixed-frequency sampling.

[0113] S3. Fault startup judgment

[0114] S3.1. The zero-sequence current is sampled according to the frequency-tracking sampling frequency fs ′Real-time sampling and monitoring

[0115] S3.2. Extract the zero-sequence current sudden change differential current curve

[0116] Preferably, the zero-sequence current waveform i0(x) within a specified time window (N cycles) is extracted at regular intervals, and the zero-sequence current sudden change differential current curve within the specified time window (N cycles) is calculated.

[0117] Further, the zero-sequence current sudden change differential current curve can be calculated in the following manner: Starting from the first cycle of the N-cycle zero-sequence current waveform i0(x), the sampling points of the subsequent cycle are subtracted from the corresponding sampling points of the previous cycle in sequence. The newly constructed N-cycle waveform is defined as the zero-sequence current sudden change differential current curve Δi0(x), as shown below:

[0118] Δi0(x) = 0 for x ∈ (0, m - 1)

[0119] Δi0(x) = i0(x) - i0(x - m) for x ∈ (m, Nm - 1) and N ≥ 2.

[0120] S3.3. Zero-sequence current sudden change start judgment

[0121] For the zero-sequence current sudden change differential current curve Δi0(x), the effective value of each cycle is calculated in sequence from start to end according to the full cycle, and it is detected whether there is an effective value greater than the preset threshold in each cycle of the zero-sequence current sudden change differential current curve in sequence, that is, the fault start condition 1 shown below:

[0122] ΔI 0i ≥I 0SET1 ,

[0123] where, ΔI 0i is the effective value of the i-th cycle of the currently calculated zero-sequence current sudden change differential current curve Δi0(x), and I 0SET1 is the fault start threshold set for the fault start condition 1.

[0124] S3.4. Zero-sequence current effective value start judgment

[0125] If the fault start condition 1 is satisfied, the effective value of the zero-sequence current for M consecutive cycles starting from the i-th cycle is calculated in the following manner, and the average value of the M effective values of the zero-sequence current is calculated:

[0126]

[0127] where, I 0ave is the average value of the effective values of the zero-sequence current for M consecutive cycles starting from the i-th cycle, I 0j is the effective value of the zero-sequence current signal of the j-th cycle, M is the number of consecutive cycles, and i is the index of the starting cycle.

[0128] Further, it is determined whether the average value satisfies the following fault startup condition 2:

[0129] I 0ave ≥I 0SET2 ,

[0130] wherein, I 0SET2 is the fault startup threshold set for the fault startup condition 2.

[0131] Further, if the fault startup condition 2 is satisfied, it indicates that a single-phase grounding fault has occurred in the distribution network. At this time, the zero-sequence current and three-phase current within the specified time window in the above step S3.2 are recorded and temporarily stored (typically, the data of the first 4 and the last 8 cycles of the index of the starting cycle are saved for subsequent fault analysis and judgment), and the single-phase grounding fault detection method is started; if the fault startup condition 2 is not satisfied, the return process is executed, that is, step S10.

[0132] According to a preferred embodiment, in step S3.1, the zero-sequence current can be collected by directly collecting the zero-sequence current through a zero-sequence current transformer or by calculating and synthesizing the three-phase phase currents. As a preferred example, we directly collect the zero-sequence current through a zero-sequence current transformer to minimize the problems of uncontrollable zero-sequence current error and large error caused by collecting the zero-sequence current by calculating and synthesizing the three-phase phase currents.

[0133] According to a preferred embodiment, in step S3, the specified time window can be extracted regularly in synchronization with step S2, and the fault startup judgment can be performed every 15 cycles regularly.

[0134] According to a preferred embodiment, in step S3, I 0SET1 and I 0SET2 can both take the value of 1A.

[0135] S4. Fault startup point search

[0136] The zero-sequence current sudden change differential current curve Δi0(x) is low-pass filtered (the cut-off frequency is f s ), and the zero-sequence differential current low-pass filtered curve Δi0(x)′ is obtained. The zero-sequence differential current low-pass filtered curve Δi0(x)′ is used to search for the fault startup point according to the second-order difference quotient method and is marked:

[0137] Δi0(i)′ = F{Δi0(i) * g(t)},

[0138] Among them, Δi0(i)' is the differential current value of the sudden change in zero-sequence current at the i-th sampling point after low-pass filtering, F is the filtering operation, Δi0(i) is the differential current value of the sudden change in zero-sequence current at the i-th sampling point, g(t) is the filtering function, and low-pass filtering data processing is performed (cutoff frequency is f s ).

[0139] According to a preferred embodiment, in step S4, low-pass filtering can be performed using an FIR filter, and the cutoff frequency f s can be selected according to the cutoff frequencies for different lengths and reference lines in relevant reference documents. For example, it can be selected as 600 Hz.

[0140] S5. Extract the full-current characteristic waveforms of the three-phase current fault components

[0141] S5.1. Extract the waveforms of the three-phase current fault components

[0142] Extract the waveforms of the three-phase current fault components Δi A (i), Δi B (i), Δi C (i) within 2 cycles after the fault starting point of the three-phase current waveforms. The extraction method is as follows: Subtract the corresponding sampling points in the 1st cycle before the fault starting point from the sampling points in the 1st cycle starting from the fault starting point of the temporarily stored three-phase current waveforms, and subtract the corresponding sampling points in the 1st cycle starting from the fault starting point from the sampling points in the 2nd cycle starting from the fault starting point. The mathematical expression of the extraction method is as follows:[[]]

[0143] Δi A (i) = i A (x) - i A (x - (i / m + 1)*m)

[0144] Δi B (i) = i B (x) - i B (x - (i / m + 1)*m)

[0145] Δi C (i) = i C (x) - i C (x - (i / m + 1)*m)

[0146] i ∈ (0, 2m - 1),

[0147] where x is the corresponding sampling point of the fault starting point, and the value range is 2 cycles.

[0148] S5.2. Extract the effective frequency band characteristic waveforms of the three-phase current fault components

[0149] Perform low-pass filtering on the above 2-cycle waveforms of the three-phase current fault components (cutoff frequency is fs1 ), perform mode 1 - low - pass filtering (cut - off frequency \(f\) s1 = 600 Hz) on the fault - component waveform to obtain the effective - frequency - band characteristic waveforms of the two - cycle three - phase phase - current fault components as follows:

[0150] Δi A (i)' = F{Δi A (i)*g(t)}

[0151] Δi B (i)' = F{Δi B (i)*g(t)}

[0152] Δi C (i)' = F{Δi C (i)*g(t)},

[0153] where, Δi A (i)', Δi B (i)', Δi C (i)' are respectively the effective - frequency - band characteristic waveforms of the two - cycle A, B, and C phase - current fault components, F is the filtering operation, and g(t) is the filtering function, performing low - pass filtering data processing (cut - off frequency \(f\) s1 ).

[0154] S5.3. Extract the power - frequency - component characteristic waveforms of the three - phase current fault components

[0155] Perform low - pass filtering (cut - off frequency \(f\) s2 ) on the above - mentioned two - cycle three - phase current fault - component waveforms, and perform mode 2 - low - pass filtering (cut - off frequency \(f\) s2 = 100 Hz) on the fault - component waveforms to obtain the power - frequency - component characteristic waveforms of the two - cycle three - phase phase - current fault components as follows:

[0156] Δi A (i)'' = F{Δi A (i)*g(t)}

[0157] Δi B (i)'' = F{Δi B (i)*g(t)}

[0158] Δi C (i)'' = F{Δi C (i)*g(t)},

[0159] where, Δi A (i)'', Δi B (i)'', Δi C(i)″ are the power frequency component characteristic waveforms of the phase current fault components of two cycles of phases A, B, and C respectively. F is the filtering operation, g(t) is the filtering function, and low-pass filtering data processing is performed (the cut-off frequency is f s2 ).

[0160] According to a preferred embodiment, in step S5, "Method 1 - Low-pass filtering (the cut-off frequency is f s1 = 600 Hz), obtaining the effective frequency band characteristic waveforms of the phase current fault components of two cycles of three phases" includes the power frequency component and the free transient component, provides characteristic waveforms for the fault recognition algorithm, and the constructed effective frequency band characteristic waveforms of the three-phase current fault components retain the power frequency fault characteristics and possible high-frequency fault characteristics of the first capacitance section.

[0161] According to a preferred embodiment, in step S5, "Method 2 - Low-pass filtering (the cut-off frequency is f s2 = 100 Hz), obtaining the power frequency component characteristic waveforms of the phase current fault components of two cycles of three phases" includes the power frequency component and the decaying DC component information, and provides characteristic waveforms for the fault recognition algorithm.

[0162] Preferably, after a fault occurs in the distribution line, according to the above step S5, the full current characteristic waveforms of the fault components are extracted, and then the healthy line characteristics and the fault line are analyzed through subsequent steps.

[0163] S6. Calculate the fault characteristic quantity

[0164] S6.1. Calculate the energy eigenvalue of the effective frequency band characteristic waveform

[0165] Through Δi A (i)', Δi B (i)', Δi C (i)' effective frequency band characteristic waveforms (one cycle starting from the fault starting point), calculate the energies I a_RMS 、I b_RMS 、I c_RMS of the effective frequency band characteristic waveforms of the phase currents of phases A, B, and C. Further, I a_RMS 、I b_RMS 、I c_RMS can be calculated and obtained by the root mean square value RMS method. Among them, by finding the maximum value in {I a_RMS 、I b_RMS 、I c_RMS}, it is recorded as I max_RMS , and the phase sequence (A or B or C) corresponding to its maximum value is marked as the reference phase.

[0166] If phase A is the fault phase, k1 and k2 can be calculated in the following manner:

[0167]

[0168] If phase B is the faulty phase, k1 and k2 can be calculated in the following manner:

[0169]

[0170] If phase C is the faulty phase, k1 and k2 can be calculated in the following manner:

[0171]

[0172] By constructing the effective band characteristic waveform energy eigenvalue, calculating whether there is a fault energy difference between the faulty phase and the non-faulty phases can provide the basic judgment data for the effective band characteristic waveform energy eigenvalue algorithm based on the phase current fault component proposed in the present invention.

[0173] S6.2, Decaying DC component extraction and eigenvalue calculation

[0174] For the power frequency component characteristic waveform Δi A (i)″, Δi B (i)″, Δi C (i)″ of the phase current fault component, quickly calculate the decaying DC components of the phase current fault components of phases A, B, and C according to the method of calculating the average value by arithmetic sum, as shown in the following formula:

[0175]

[0176] where m is the number of sampling points per cycle, and calculate the decaying DC component of the power frequency component characteristic waveform.

[0177] Furthermore, construct the decaying DC component difference value according to the following calculation:

[0178] (1) If phase A is the faulty phase, the decaying DC component difference value is:

[0179] D = D A -k * (D B + D C + D min );

[0180] (2) If phase B is the faulty phase, the decaying DC component difference value is:

[0181] D = D B -k * (D C + D A + D min );

[0182] (3) If phase C is the faulty phase, the decaying DC component difference value is:

[0183] D = D C -k * (DA +D B +D min ),

[0184] In the above formula, D min is the minimum value set to prevent the original D A , D B , D C data from being too small, and k is the difference value multiple.

[0185] By constructing the difference value of the decaying DC component, it is calculated whether there is an obvious difference in the decaying DC component between the fault phase and the non-fault phase, which can provide basic judgment data for the feature algorithm based on the difference value of the decaying DC component proposed by the present invention.

[0186] According to a preferred embodiment, in step S6.2, k can be taken as 10, and D min can be taken as 0.5 A

[0187] S6.3, Calculation of the instantaneous phase angle of the power frequency component characteristic waveform

[0188] The power frequency component characteristic waveform Δi A (i)″, Δi B (i)″, Δi C (i)″ is a non-standard sine waveform. At the same time, because there is an obvious phase change process, there is a decaying DC component, and the transient process frequency is not 50 Hz, and is the number of non-integral cycle sampling points. Therefore, the method of using the traditional Fourier algorithm to calculate the phase angle is not feasible. Based on this, the present invention proposes a fast calculation method of the instantaneous phase angle based on the estimation of the peak and trough points to realize the fast calculation of the instantaneous phase angle of the power frequency characteristic waveform of the phase current fault component, and finally calculate the phase difference change curve of the power frequency component characteristic waveforms of the fault phase and the non-fault phase.

[0189] Further, the fast calculation method of the instantaneous phase angle based on the estimation of the peak and trough points is as follows: The point with the sampling point number x a is defined as the peak point, and the phase angle of the peak point is 90°. The point with the sampling point number x b is defined as the trough point, and the phase angle of the trough point is 270°. Starting from the first peak and trough points after the fault point, each peak and trough point in the power frequency component characteristic waveform is sequentially marked, and then the instantaneous phase angles of other points are calculated according to the following rules:

[0190] a) First, calculate the number n of sampling points between the peak and trough points, that is, n = x b -x a , and then calculate the phase angle i of the point x

[0191]

[0192] b) Calculate the number n of sampling points between the trough and the next peak, i.e., n = x a -x b , and then calculate the phase angle of the point x i between the trough - the next peak point

[0193]

[0194] Through the above rules, calculate the instantaneous phase angles of the power frequency characteristic waveforms of the fault components of the three phases A, B, and C curve.

[0195] S6.4. Calculation of the curve of the change amount of the phase difference of the power frequency component characteristic waveform

[0196] After calculating the curve of the phase angles of the power frequency characteristic waveforms of the three-phase phase current fault components in step S6.3 , the curve of the change amount of the phase difference of the power frequency component characteristic waveform can be calculated according to the following rules:

[0197] a) If phase A is the fault phase, construct the curve of the change amount of the phase difference of the power frequency component characteristic waveform in the following way:

[0198]

[0199] b) If phase B is the fault phase, construct the curve of the change amount of the phase difference of the power frequency component characteristic waveform in the following way:

[0200]

[0201] c) If phase C is the fault phase, construct the curve of the change amount of the phase difference of the power frequency component characteristic waveform in the following way:

[0202]

[0203] In the above formula, are respectively the curve of the change amount of the phase difference of the power frequency component characteristic waveform between the fault phase - non-fault phase and the curve of the change amount of the phase difference of the power frequency component characteristic waveform between the non-fault phase - non-fault phase.

[0204] Furthermore, the constructed curve of the change amount of the phase difference of the power frequency characteristic waveform can provide basic judgment data for the eigenvalue algorithm based on the curve of the change amount of the phase difference of the power frequency component characteristic waveform proposed by the present invention.

[0205] S7. Execute different single-phase grounding fault recognition algorithms according to different neutral grounding methods

[0206] S7.1. Fault location criterion for the isolated neutral system

[0207] Generate the fault location criterion for the isolated neutral system based on Feature 1 and Feature 2. Among them, the criterion for the faulty line needs to meet the following features:

[0208] a) The magnitudes of the steady-state power-frequency components of the fault components of the phase currents in the faulty phase and the non-faulty phases are different, and the directions are opposite.

[0209] b) There is a large difference in the effective energy (steady-state power-frequency and transient components) of the fault components of the phase currents in the faulty phase and the non-faulty phases.

[0210] Specifically, the mathematical expression of the fault criterion for the isolated neutral system is:

[0211] I max_RMS ≥I set

[0212] k1≥k set and k2≥k set

[0213]

[0214] where I max_RMS is the maximum effective value current, I set is the set threshold current, k1 and k2 are two coefficients used to judge the difference in the effective energy of the fault components of the phase currents in the faulty phase and the non-faulty phases, and k set is the set threshold coefficient, is the curve of the phase difference change amount, and are the upper and lower limits of the set range of the curve of the phase difference change amount range, and are the upper and lower limits of the set range of the curve of the phase difference change amount range.

[0215] According to a preferred implementation, in step S7.1, after a large number of simulation tests and data calculation and statistics, the recommended setting parameters can be taken as follows: I set can be taken as 1A, k set can be taken as 2, can be taken as 150°, can be taken as 210°, can be taken as -30°, can be taken as 30°.

[0216] S7.2. Fault location criterion for the system with arc suppression coil grounded at the neutral point

[0217] Generate the fault location criterion for the system with arc suppression coil grounded at the neutral point based on Feature 3 and Feature 4. Among them, the criterion for the faulty line needs to meet the following features:

[0218] a) The effective energy of the fault component of the phase current in the faulty phase > the effective energy of the fault component of the phase current in the non-faulty phase;

[0219] b) During the transient process after the fault occurs, there is an obvious phase change process between the fault component of the phase current in the faulty phase and the power frequency phase of the non-faulty phase (gradually transitioning from the initial reverse phase to the same phase or nearly the same phase);

[0220] c) During the transient process after the fault occurs, there may be a decaying DC component in the fault component of the phase current in the faulty phase.

[0221] Specifically, the mathematical expression of the fault criterion for the system with the neutral point grounded through an arc suppression coil is:

[0222] Criterion 1:

[0223] I max_RMS ≥I set

[0224] k1≥k set and k2≥k set

[0225]

[0226] Criterion 2:

[0227] I max_RMS ≥I set

[0228] k1≥k set and k2≥k set

[0229] D≥0,

[0230] where D is a threshold parameter for judging the faulty line, representing the difference value of the decaying DC component.

[0231] According to a preferred embodiment, in step S7.1, after a large number of simulation tests and data calculation statistics, the recommended setting parameters can be taken as follows: I set can be taken as 1A, k set can be taken as 2, in the calculation formula for the difference value of the decaying DC component in S6.2, k can be taken as 10, D min can be taken as 0.5A, can be taken as -30°, can be taken as 30°.

[0232] Example 1 is a high-resistance grounding fault of 2000Ω in a system with the neutral point ungrounded, and the grounded fault phase is phase C. Figure 3 (a) and Figure 3(b) are the three-phase phase current fault component waveforms, the effective frequency band characteristic waveforms of the phase current fault components, and the power frequency component characteristic waveforms of the phase current fault components on the faulty line and the healthy line respectively. From Figure 3 (a), it can be seen that the energy of the effective frequency band characteristic waveform of the faulty line varies greatly among the three phases. In the power frequency component characteristic waveform, it can be clearly seen that after the fault, the faulty phase and the non-faulty phase are continuously out of phase, and the non-faulty phases are basically in phase with each other after the fault. From Figure 3 (b), it can be seen that there is almost no difference in the energy of the effective frequency band characteristic waveform of the healthy line among the three phases. In the power frequency component characteristic waveform, it can be clearly seen that the faulty phase and the non-faulty phase are almost in phase throughout the entire fault process. Calculate the energy eigenvalue of the effective frequency band characteristic waveform and the curve of the change in the phase difference of the power frequency component. The characteristic parameters are calculated as shown in Table 1 below. According to the fault criterion of the ungrounded neutral system, accurately judge Figure 3 (a) as the faulty line, Figure 3 (b) as the healthy line.

[0233] Table 1 Calculation results of fault characteristic parameters for a 2000Ω high-resistance grounding fault

[0234] Line <![CDATA[k1]]> <![CDATA[k2]]> <![CDATA[I max_RMS > <![CDATA[φ z1 (x)]]> <![CDATA[φ z2 (x)]]> D Faulty Line 5.97 5.77 2.38 175～185 Basically in Phase - Healthy Line 1.02 1.02 0.39 Basically in Phase Basically in Phase -

[0235] Example 2 is an arc grounding fault in an ungrounded neutral system, and the grounded fault phase is phase C. Figure 4 (a) and Figure 4 (b) are the three-phase phase current fault component waveforms, the effective frequency band characteristic waveforms of the phase current fault components, and the power frequency component characteristic waveforms of the phase current fault components on the faulty line and the healthy line respectively. From Figure 4 (a), it can be seen that the energy of the effective frequency band characteristic waveform of the faulty line varies greatly among the three phases. In the power frequency component characteristic waveform, it can be clearly seen that after the fault, the faulty phase and the non-faulty phase are continuously out of phase, and the non-faulty phases are basically in phase with each other after the fault. From Figure 4 (b), it can be seen that there are obvious differences between the faulty phase and the non-faulty phase in the phase current fault component waveform of the healthy line, and it is easy to misjudge by conventional methods. However, there is almost no difference in the energy of the effective frequency band characteristic waveform among the three phases, and it can be clearly seen that the faulty phase and the non-faulty phase are almost in phase throughout the entire fault process. Calculate the energy eigenvalue of the effective frequency band characteristic waveform and the curve of the change in the phase difference of the power frequency component. The characteristic parameters are calculated as shown in Table 2 below. According to the fault criterion of the ungrounded neutral system, accurately judge Figure 4 (a) as the faulty line, Figure 4 (b) as the healthy line.

[0236] Table 2 Calculation results of fault characteristic parameters for an arc grounding fault

[0237] Line <![CDATA[k1]]> <![CDATA[k2]]> <![CDATA[I max_RMS > <![CDATA[φ z1 (x)]]> <![CDATA[φ z2 (x)]]> D Faulty Line 5.13 5.09 20.05 178～184 Basically in Phase - Healthy Line 1.14 1.17 4.11 Basically in Phase Basically in Phase -

[0238] Example 3 is a high-resistance grounding fault of 5000Ω for the neutral point grounded through an arc suppression coil system, and the grounded fault phase is phase B. Figure 5 (a) and Figure 5 (b) are respectively the waveforms of the three-phase phase current fault components, the characteristic waveforms of the effective frequency bands of the phase current fault components, and the characteristic waveforms of the power frequency components of the phase current fault components on the faulty line and the healthy line. From Figure 5 (a), it can be seen that there are significant differences in the energy of the effective frequency band characteristic waveforms among the three phases of the faulty line. In the characteristic waveforms of the power frequency components, it can be clearly seen that there is an obvious phase change process between the faulty phase and the non-faulty phases after the fault (a process from starting to be out of phase and gradually transitioning to being in phase or nearly in phase), and the phases of the non-faulty phases are basically in phase after the fault. From Figure 8 , it can be clearly seen that the curve process of the change in the phase difference of the power frequency components between the faulty phase and the non-faulty phases, from starting to be out of phase to gradually being in phase.

[0239] From Figure 5 (b), it can be seen that there is almost no difference in the energy of the effective frequency band characteristic waveforms among the three phases of the healthy line. In the characteristic waveforms of the power frequency components, it can be clearly seen that the faulty phase and the non-faulty phases are almost in phase throughout the entire fault process. Calculate the energy eigenvalue of the effective frequency band characteristic waveform and the curve of the change in the phase difference of the power frequency components. The characteristic parameters are calculated as shown in Table 3 below. According to the fault criterion 1 of the neutral point grounded through an arc suppression coil system, it is accurately judged that Figure 5 (a) is the faulty line, Figure 5 (b) is the healthy line.

[0240] Table 3 Calculation results of characteristic parameters for a 5000Ω high-resistance grounding fault

[0241] Line <![CDATA[k1]]> <![CDATA[k2]]> <![CDATA[I max_RMS > <![CDATA[φ z1 (x)]]> <![CDATA[φ z2 (x)]]> D Faulty Line 4.22 4.50 1.18 -160→-40 Basically in Phase -5.86 Healthy Line 1.17 1.19 0.5 Basically in Phase Basically in Phase -5.41

[0242] Example 4 is a high-resistance grounding fault of 1000Ω for the neutral point grounded through an arc suppression coil system, and the grounded fault phase is phase B. Figure 6 (a) and Figure 6 (b) are respectively the waveforms of the three-phase phase current fault components, the characteristic waveforms of the effective frequency bands of the phase current fault components, and the characteristic waveforms of the power frequency components of the phase current fault components on the faulty line and the healthy line. From Figure 6 (a), it can be seen that there are significant differences in the energy of the effective frequency band characteristic waveforms among the three phases of the faulty line. In the characteristic waveforms of the power frequency components, it can be clearly seen that there is an obvious phase change process between the faulty phase and the non-faulty phases after the fault (a process from starting to be out of phase and gradually transitioning to being in phase or nearly in phase).

[0243] From Figure 6As can be seen from (b), there is almost no difference in the energy of the characteristic waveforms of the effective frequency bands of the sound lines among the three phases. In the characteristic waveforms of the power frequency components, it can be clearly seen that after the fault, the fault phase and the non-fault phase are almost in phase throughout the entire fault process. The calculation of the characteristic parameters of the energy eigenvalue of the effective frequency band characteristic waveform and the change curve of the power frequency component phase difference is shown in Table 4 below. According to the fault criterion 1 of the system with arc suppression coil grounded through the neutral point, it can be accurately judged Figure 6 (a) is the fault line, Figure 6 (b) is the sound line.

[0244] Table 4 Calculation results of fault characteristic parameters for 1000Ω high-resistance grounding

[0245] Line <![CDATA[k1]]> <![CDATA[k2]]> <![CDATA[I max_RMS > <![CDATA[φ z1 (x)]]> <![CDATA[φ z2 (x)]]> D Faulty Line 3.01 3.31 5.09 -140→-30 Basically in Phase -11.2 Healthy Line 1.01 1.01 1.61 Basically in Phase Basically in Phase -11.5

[0246] Example 5 is an arc grounding fault in a system with arc suppression coil grounded through the neutral point, and the grounded fault phase is phase B. Figure 7 (a) and Figure 7 (b) respectively show the waveforms of the fault components of the three-phase phase currents, the characteristic waveforms of the effective frequency bands of the fault components of the phase currents, and the characteristic waveforms of the power frequency components of the fault components of the phase currents on the fault line and the sound line. As can be seen from Figure 7 (a), there are significant differences in the energy of the characteristic waveforms of the effective frequency bands among the three phases of the fault line. In the characteristic waveforms of the power frequency components, it can be clearly seen that there is an obvious phase change process between the fault phase and the non-fault phase after the fault (a process from starting to be out of phase and gradually transitioning to being in phase or nearly in phase), the non-fault phases are basically in phase after the fault, and there is an obvious decaying DC component in the fault phase;

[0247] From Figure 9 , it can be clearly seen that the process of the change curve of the phase difference of the power frequency components between the fault phase and the non-fault phase, from starting to be out of phase to gradually being in phase.

[0248] As can be seen from Figure 7 (b), the difference between the fault phase and the non-fault phase in the waveform of the fault component of the phase current of the sound line is very obvious, and it is easy to misjudge by the transient method. However, there is almost no difference in the energy of the characteristic waveforms of the effective frequency bands among the three phases, and it can be clearly seen that the fault phase and the non-fault phase are almost in phase throughout the entire fault process in the characteristic waveforms of the power frequency components. Calculate the energy eigenvalue of the effective frequency band characteristic waveform, the change curve of the power frequency component phase difference, and the eigenvalue of the difference value of the decaying DC component. The characteristic parameters are calculated as shown in Table 5 below. According to the fault criterion 1 of the system with arc suppression coil grounded through the neutral point or the fault criterion 2 of the system with arc suppression coil grounded through the neutral point, it can be accurately judged Figure 7 (a) is the fault line, Figure 7 (b) is the sound line.

[0249] Table 5 Calculation results of fault characteristic parameters for arc grounding fault

[0250] Line <![CDATA[k1]]> <![CDATA[k2]]> <![CDATA[I max_RMS > <![CDATA[φ z1 (x)]]> <![CDATA[φ z2 (x)]]> D Faulty Line 8.61 9.31 100.26 -168→-20 Basically in Phase 68 Healthy Line 1.15 1.02 15.54 Basically in Phase Basically in Phase -6.78

[0251] S8, Local tripping logic

[0252] Start timing with the fault starting point as the time, and continuously monitor the effective value I of the zero-sequence current signal 02 , when the following conditions are simultaneously satisfied, execute the local tripping protection action:

[0253] I 02 ≥I 0SET2

[0254] t2≥t SET2 ,

[0255] where, t2 is the time counted from the fault starting point, and t SET2 is the set time threshold, which can be set according to user requirements, and generally takes a value ≥1S.

[0256] S9, Post-acceleration action for permanent faults after automatic reclosing

[0257] After the local tripping protection action in step S8 is completed, if reclosing operation is not required, return to step S1 and wait for the three-phase current to be powered on for judgment; if reclosing needs to be enabled, wait for the delay time t3 after reclosing, where t3 meets the condition of avoiding inrush current, that is, t3≥t SET3 , t SET3 is the set time threshold, and when the time t3 reaches or exceeds this threshold, trigger the protection action.

[0258] Furthermore, continuously monitor the effective value I of the zero-sequence current signal 03 , when the following conditions are simultaneously satisfied, execute the post-acceleration tripping protection action after local reclosing:

[0259] I 03 ≥min(I 0SET2 , k3*I 0ave )

[0260] t3≥t SET3 ,

[0261] where, k3 is the reliability coefficient, generally taking a value of 0.9, and t SET3 generally takes a value ≥0.2S to avoid inrush current and different periods of three-phase closing. I 0ave is the average value of the zero-sequence current, that is, by monitoring the zero-sequence current for a period of time and calculating its average value. Specifically, I 0ave is the zero-sequence current at the first fault start judgment in step S3 before reclosing. Therefore, when reclosing on a permanent fault again, the fault quantity can be remembered by identifying the zero-sequence current to achieve fast protection action.

[0262] Further, after the post-acceleration tripping is completed, return to step S1 and wait for the three-phase current to be powered on for judgment.

[0263] Preferably, when the above conditions are not met, the return process, i.e., step S10, is executed.

[0264] S10. Return process

[0265] By tracking sampling with frequency, the zero-sequence current signal is monitored in real time and its effective value is calculated. When the following conditions are met, return to step S2 and the system is reset:

[0266] I0 < k4 * I 0bph

[0267] t4 ≥ t SET4 ,

[0268] wherein, I 04 is the effective value of the zero-sequence current signal, k4 is the reliability coefficient, generally taking a value of 1.1, I 0bph is the reference zero-sequence current value, which is a preset reference value for judging whether the zero-sequence current is normal, t4 is the time counted from the starting point, and t SET4 is the set time threshold, generally taking a value ≥ 1S.

[0269] It should be noted that the above specific embodiments are exemplary. Those skilled in the art can come up with various solutions inspired by the disclosure of the present invention, and these solutions also fall within the scope of the disclosure of the present invention and within the protection scope of the present invention.

Claims

1. A method for identifying single-phase grounding faults in a distribution network based on the characteristics of sudden changes in phase current, characterized in that, It includes the following steps: Collect three-phase current signals according to the set theoretical sampling frequency, and calculate their effective values. When specific conditions are met simultaneously, the power-on judgment of the three-phase current is completed; Perform real-time sampling and monitoring of the three-phase current and zero-sequence current at the sampling frequency that tracks the frequency, extract the differential current curve of the sudden change of the zero-sequence current, and judge the fault starting conditions for the differential current curve; Extract the fault component waveforms of the three-phase current starting from the fault starting point, and then perform low-pass filtering on these waveforms respectively to obtain the effective frequency band characteristic waveforms and power frequency component characteristic waveforms; Calculate the fault characteristic quantities based on the effective frequency band characteristic waveforms and power frequency component characteristic waveforms; According to the fault characteristic quantities, different single-phase ground fault location algorithms are executed according to different types of neutral grounding systems.

2. The recognition method according to claim 1, wherein The specific conditions include: RMSI A ≥I ON and RMSI B ≥I ON and RMSI C ≥I ON and t≥t ON , Among them, RMSI A , RMSI B , RMSI C represent the effective values of three-phase current signals. The calculation method of the effective value adopts the root mean square value calculation method of the instantaneous sampling value of the phase current. I oN is the power-on current setting value, t oN is the power-on delay time setting value. t is a time variable, representing the time starting from the moment when the effective values of the three-phase current signals simultaneously meet the power-on current setting value.

3. The recognition method according to claim 1, wherein The described fault starting conditions include detecting whether there is an effective value greater than a preset threshold I in each successive cycle of the zero-sequence current mutation differential current curve OSET1 , and the average value of the effective values of the zero-sequence current in multiple consecutive cycles is greater than another preset threshold I OSET2 .

4. The recognition method according to claim 1, wherein After extracting the fault component waveforms of the three-phase current, low-pass filters with two different cut-off frequencies are used to obtain the effective frequency band characteristic waveforms and power frequency component characteristic waveforms respectively.

5. The recognition method according to claim 1, wherein For an ungrounded neutral system, if the following fault criterion combining the energy eigenvalue of the effective frequency band characteristic waveform and the change curve characteristic of the phase difference of the power frequency component is satisfied, it is determined that a single-phase ground fault has occurred: I max_RMS ≥ I set and k1≥k set and k2≥k set and and Among them, I max_RMS is the maximum effective value current, I set is the set threshold current, k1 and k2 are two coefficients used to judge the effective energy difference of the current fault components between the faulty phase and the non-faulty phase, k set is the set threshold coefficient, is the curve of the phase difference change amount, and is the upper and lower limits of the set range of the curve of the phase difference change amount range, and is the upper and lower limits of the set range of the curve of the phase difference change amount range.

6. The recognition method according to claim 1, wherein For a system with a neutral point grounded through an arc suppression coil, if the following fault criterion combining the energy eigenvalue of the effective frequency band characteristic waveform and the change curve characteristic of the phase difference of the power frequency component is satisfied, it is determined that a single-phase ground fault has occurred: I max_RMS ≥ I set and k1≥k set and k2≥k set and And And 7. The recognition method according to claim 1, wherein For a system with a neutral point grounded through an arc suppression coil, if the following fault criterion combining the energy eigenvalue of the effective frequency band characteristic waveform and the difference value characteristic of the decaying DC component is satisfied, it is determined that a single-phase ground fault has occurred: I max_RMS ≥ I set and k1 ≥ k set and k2 ≥ k set and D≥0, where D is a threshold parameter for judging the fault line, representing the difference value of the decaying DC component.

8. The recognition method according to claim 1, wherein It also includes the following steps: realizing the sampling that tracks the frequency by controlling the PWM output through real-time calculation of the phase angle difference of the three-phase current. Among them, the sampling method that tracks the frequency is to dynamically adjust the PWM output frequency by calculating the phase angle difference between two adjacent full cycles, so as to ensure accurate full cycle sampling even when the power grid system frequency changes.

9. The recognition method according to claim 1, characterized in that It further includes the following steps: After performing low-pass filtering on the sudden change current differential current curve of zero-sequence current, find and identify the fault starting point, where a low-pass filter with a cut-off frequency of f s is used to process the sudden change current differential current curve of zero-sequence current, and the first-order difference quotient method is used to quickly find the fault starting point.

10. The recognition method according to claim 1, characterized in that Calculating the fault characteristic quantities includes calculating the energy eigenvalue of the effective frequency band characteristic waveform, the difference value of the decaying DC component, the instantaneous phase angle of the power frequency component characteristic waveform and its phase difference change curve. Among them, by calculating the energy eigenvalues k1 and k2 of the effective frequency band characteristic waveform, it is judged whether there is a significant energy difference between the fault phase and the non-fault phase; through the fast calculation method of the instantaneous phase angle based on the estimation of the peak and valley points, the instantaneous phase angle of the power frequency component characteristic waveform is calculated, and the phase difference change curve of the power frequency component characteristic waveform is calculated.

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