Method for identifying single-phase ground fault of distribution network based on phase current abrupt variable characteristics

Abstract

The application relates to a single-phase grounding fault identification method of a power distribution network based on phase current mutation characteristics, which comprises the following steps: collecting three-phase current signals according to a set theoretical sampling frequency, calculating the effective values, and completing the power-on judgment of the three-phase current when specific conditions are simultaneously met; performing real-time sampling monitoring on the zero-sequence current according to the frequency tracking sampling frequency, extracting a zero-sequence current mutation difference current curve, and performing a fault starting condition judgment on the difference current curve; extracting three-phase current fault component waveforms from the fault starting point, and then respectively performing low-pass filtering processing on the waveforms to obtain effective frequency band characteristic waveforms and power frequency component characteristic waveforms; calculating fault characteristic quantities, and executing different single-phase grounding fault positioning algorithms according to different types of neutral point grounding systems. The application realizes the single-phase grounding fault identification of the power distribution network through phase current and zero-sequence current signals, and adopts a technical principle based on clear physical laws.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power distribution network, and particularly relates to a method for identifying single-phase grounding fault of power distribution network based on phase current abruptness. BACKGROUND

[0002] Many switch cabinets, ring network cabinets and cable branch boxes in the power distribution network do not install zero sequence PT (or three-phase PT), so that the device cannot obtain zero sequence voltage signal. In recent years, researchers have tried to use only current signal for fault identification, to complete single-phase grounding fault identification by analyzing transient or steady-state process of phase current, to complete single-phase grounding fault positioning by using phase asymmetry method, and to effectively identify single-phase grounding fault as a research and application hotspot in recent years.

[0003] CN105914718A discloses a grounding fault protection method based on phase current abruptness, which obtains transient phase current abruptness and calculates inter-phase correlation coefficient to determine fault line. However, the correlation coefficient is calculated directly from the sampled value of transient phase current abruptness, which completely depends on actual sampling value, has low signal-to-noise ratio and poor anti-interference performance.

[0004] CN111999674A discloses a single-phase grounding fault detection and positioning method for power distribution line based on phase current change, which synthesizes zero sequence current from three-phase current, and positions fault by judging polarity and amplitude of three-phase current change. By synthesizing zero sequence current from self-production, the error of synthesized zero sequence current and extracted phase current fault component is significantly increased when the load current is large, and high-resistance grounding fault starting faces difficulties. Meanwhile, the maximum value of three-phase current change and polarity and size are found in two cycles after fault, and the polarity point is likely to be misjudged when arc grounding occurs.

[0005] CN113219300A discloses a single-phase grounding fault sensing method for power distribution network based on phase current transient and steady state, which respectively analyzes fault by using steady-state and transient characteristics, and positions result by using steady-state characteristics for resistance grounding fault and by using transient characteristics according to three-phase current abruptness direction for arc grounding fault. Purely relying on transient or steady-state characteristics for analysis often has blind area, and the abruptness direction is often difficult to accurately obtain, so that the overall reliability of the algorithm is reduced.

[0006] CN115792504A discloses a method and system for locating single-phase grounding faults in distribution networks based on phase current mutations. This invention calculates the effective values ​​of the three-phase current mutations according to the waveforms of the phase current mutations before and after the grounding fault, and sorts them; it calculates the ratio of the effective values ​​of the mutations, and locates the single-phase grounding fault in the distribution network line based on the relationship between the ratio and a set value. However, in actual arc grounding faults, relying solely on the effective value ratio calculation method often makes it difficult to set a set value, resulting in practical difficulties.

[0007] In recent years, the phase current method (phase asymmetry method) has been applied to single-phase grounding fault identification. However, when the fault coincides with a permanent fault, the fault characteristics are often mixed with inrush current and asynchronous phases of the three phases, leading to frequent failures or maloperations after reclosing. Therefore, this invention urgently needs a method for accurate and efficient identification of single-phase grounding faults in distribution networks.

[0008] 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

[0009] To address the shortcomings of existing technologies, this invention provides a method for identifying single-phase grounding faults in distribution networks based on the characteristics of phase current abrupt changes. This method aims to overcome the deficiencies of existing single-phase grounding fault identification techniques using phase current methods in distribution networks, which rely solely on calculating correlation coefficients, directions, and polarities from sampled values, or on single transient or steady-state characteristics for fault identification. This is particularly true for distribution network systems where the neutral point is grounded via an arc suppression coil, where the phase current method presents significant challenges in effective fault identification. The identification method of this invention is particularly applicable to distribution networks operating without zero-sequence voltage signals. It utilizes phase current and zero-sequence current signals, employing technical principles based on well-defined physical laws, to achieve single-phase grounding fault identification in distribution networks.

[0010] This invention discloses a method for identifying single-phase grounding faults in distribution networks based on the characteristics of phase current abrupt changes, 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, complete the three-phase current power-on judgment.

[0012] S2. Frequency tracking sampling is achieved by controlling the PWM output through real-time calculation of the phase angle difference of the three-phase current;

[0013] S3. Perform real-time sampling and monitoring of the three-phase current and zero-sequence current at the frequency tracking sampling frequency, extract the differential current curve of the zero-sequence current change, and determine the start-up condition based on the combined over-determined value of the differential current curve and the zero-sequence current curve.

[0014] S4. After performing low-pass filtering on the differential current curve of zero-sequence current mutation, locate and mark the fault start point.

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

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

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

[0018] S8. Implement 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 and continue monitoring the system status.

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

[0022] Preferably, the three-phase current and zero-sequence current are sampled and monitored in real time using frequency tracking. The zero-sequence current waveform within a specified time window (N cycles) is extracted periodically. Starting from the first cycle, the sampling points of each subsequent cycle of the N-cycle zero-sequence current waveform are subtracted from the corresponding sampling points of the previous cycle to extract the differential current curve of the zero-sequence current surge. The differential current curve is then used to determine fault initiation conditions. The fault initiation conditions include detecting whether there is an effective value greater than a preset threshold I in each cycle of the zero-sequence current surge differential current curve. 0SET1 Furthermore, the average effective value of the zero-sequence current for multiple consecutive cycles is greater than another preset threshold I. 0SET2 .

[0023] Preferably, the three-phase current fault component waveform is extracted by subtracting the corresponding sampling point of the previous cycle from each sampling point of the temporarily stored three-phase current waveform starting from the fault initiation point. Then, these waveforms are subjected to low-pass filtering with two different cutoff frequencies to obtain the effective frequency band characteristic waveform and the power frequency component characteristic waveform.

[0024] Preferably, fault characteristic quantities are calculated based on the effective frequency band characteristic waveform and the power frequency component characteristic waveform. These calculated fault characteristic quantities include the energy characteristic value of the effective frequency band characteristic waveform, the difference value of the attenuated DC component, and the instantaneous phase angle and phase difference change curve of the power frequency component characteristic waveform. The root mean square (RMS) value of the effective frequency band waveform one cycle after the fault initiation point is calculated, and the maximum value and its corresponding phase sequence are found as reference phases to identify the energy difference between the faulty and non-faulty phases. The attenuated DC component of the fault current in each phase is calculated using the power frequency component characteristic waveform, and the difference value of the attenuated DC component between the faulty and non-faulty phases is constructed to determine whether a significant difference in attenuated DC component exists. The instantaneous phase angle of the power frequency characteristic waveform after fault initiation is quickly calculated based on the peak-trough point estimation method. Based on the calculated instantaneous phase angle, the phase difference change curve of the power frequency component characteristic waveform between the faulty and non-faulty phases is constructed to provide basic judgment data for fault location.

[0025] Preferably, based on fault characteristic quantities, different single-phase grounding fault location algorithms are executed according to different types of neutral-point grounded systems. For neutral-point ungrounded systems, the fault location criteria must meet the following characteristics: a) the steady-state power frequency components of the phase current fault components of the fault phase and the non-faulty phase are different in magnitude and opposite in direction; b) the effective energy (steady-state power frequency and transient components) of the phase current fault components of the fault phase and the non-faulty phase differs greatly. For neutral-point grounded systems via arc suppression coils, the fault location criteria must meet the following characteristics: a) the effective energy of the phase current fault component of the faulty phase is greater than that of the non-faulty phase current fault component; b) during the transition process after the fault occurs, the phase current fault component exhibits a significant phase change process between the power frequency phases of the faulty phase and the non-faulty phase (gradually transitioning from opposite phases to in-phase or near-in-phase); c) during the transition process after the fault occurs, the phase current fault component of the faulty phase may have a decaying DC component.

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

[0027] 1. This invention uses a frequency-following sampling method based on phase difference calculation, which can extract the fault component of the phase current more accurately than the conventional fixed-frequency sampling method. Especially when the three-phase load current is large, it significantly improves the identification capability of high-resistance grounding faults.

[0028] 2. This invention uses a combination of zero-sequence current mutation differential current curve calculation of starting variables and zero-sequence current over-determined value starting to form a new fault starting condition, which has stronger sensitivity and anti-interference ability, can effectively avoid misjudgment caused by load fluctuation, and 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 phase difference change curve proposed in this invention have low computational complexity, simple algorithm, and clear physical concept. It can quickly calculate the instantaneous phase of non-standard sine waves during fault transients. Because the peak and trough signals are extracted instead of the zero-crossing signals for cycle number calibration, it can effectively accommodate the influence of attenuated DC components on the calculation results. Therefore, the algorithm has strong adaptability.

[0030] 4. The method for extracting attenuated DC components from the characteristic waveform of power frequency components proposed in this invention has stronger anti-interference properties because there is no interference from high-frequency components after low-pass filtering.

[0031] 5. When applied to neutral point ungrounded systems and neutral point grounded systems via arc suppression coils, this invention employs a unified low-pass filtering algorithm. The constructed effective frequency band characteristic waveform retains the power frequency characteristics of the fault and the high-frequency characteristics of the possible fault first-stage. The constructed power frequency component characteristic waveform retains the phase change characteristics of the power frequency component and the attenuation DC component characteristics. In this way, a unified feature quantity for low-resistance, high-resistance, and arc grounding fault identification algorithms is established, thereby improving the adaptability of the algorithm.

[0032] 6. The fault identification technology proposed in this invention, which combines the energy characteristic value of the effective frequency band characteristic waveform of the faulty phase and the non-faulty phase with the curve characteristics of the phase difference change of the power frequency component, is applied to the fault identification algorithm of the neutral point grounded by the arc suppression coil as criterion 1. At the same time, a fault identification technology combining the energy characteristic value of the effective frequency band characteristic waveform with the difference value of the attenuated DC component is added as criterion 2. The calculation is simple and the physical process is clear. It effectively avoids the problems of difficulty in setting values, easy malfunction, and poor reliability caused by the existing algorithm relying solely on the correlation coefficient method of transient or steady-state fault component waveform calculation. The algorithm is simple to implement and easy to realize.

[0033] 7. The fault identification technology proposed in this invention, which combines the energy characteristic value of the effective frequency band characteristic waveform of the faulty phase and the non-faulty phase with the curve characteristics of the phase difference change of the power frequency component, is applied to the fault identification algorithm of the neutral point ungrounded system. The calculation is simple and the physical process is clear.

[0034] 8. The present invention adopts the judgment condition for reclosing to a permanent fault after reclosing and the fault reset process, which can effectively avoid the failure to operate and false operation caused by inrush current or three-phase asynchronous closing when reclosing to a permanent fault in the existing technology. At the same time, it realizes fast protection operation by identifying the zero-sequence current memory fault quantity.

[0035] 9. This invention uses zero-sequence current and three-phase current, eliminating the need for a zero-sequence voltage signal. The algorithm is simple, reliable, and easy to integrate in the field. This algorithm can be easily embedded into existing power distribution terminals that lack zero-sequence voltage acquisition signals without hardware modifications. Simply adding this algorithm allows each power distribution terminal to independently perform single-phase grounding fault analysis, demonstrating its self-bootstrapping capability. Therefore, it has profound significance and broad application prospects. Attached Figure Description

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

[0037] Figure 2 This invention provides a graph showing the difference in abrupt changes between adjacent cycles calculated using frequency-following tracking sampling and fixed-frequency sampling.

[0038] Figure 3 The present invention provides a full current characteristic waveform diagram of the three-phase current fault component of the faulted line and the healthy line when a 2000-ohm high-resistance ground fault occurs in a neutral point ungrounded system.

[0039] Figure 4 This invention provides a full current characteristic waveform diagram of the three-phase current fault component of the faulted line and the intact line in a neutral point ungrounded system experiencing an arc grounding fault.

[0040] Figure 5 The present invention provides a full current characteristic waveform diagram of the three-phase current fault component of a faulted line and an intact line in a neutral point grounding system with a 5000-ohm high resistance grounding fault.

[0041] Figure 6 The present invention provides a waveform diagram of the total current characteristics of the three-phase current fault components in a neutral point grounding system with a 1000-ohm high-resistance grounding fault, for both the faulted and intact lines.

[0042] Figure 7 This invention provides a waveform diagram showing the full current characteristics of the three-phase current fault components in both the faulted and intact lines of a neutral point grounded system with an arc suppression coil.

[0043] Figure 8 The present invention provides the power frequency characteristic waveform of the fault component of the fault line phase current in a neutral point grounding system with a 5000-ohm high resistance grounding fault, as well as the curve of the change in the phase difference between the fault phase and the non-fault phase.

[0044] Figure 9 The present invention provides the power frequency characteristic waveform of the fault component of the fault line phase current in a neutral point grounded system with an arc suppression coil, as well as the curve of the change in the phase difference between the fault phase and the non-fault phase. Detailed Implementation

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

[0046] This invention discloses a method for identifying single-phase grounding faults in distribution networks based on the characteristics of phase current abrupt changes. To better understand the operational principles and underlying technical details of the identification method, 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 healthy lines and faulty lines show significant differences. For healthy lines, the three-phase current fault components consist of steady-state power frequency components and transient free oscillation components, and the magnitudes of the fault components in each phase are nearly identical, and their directions are in phase or nearly in phase. However, on faulty lines, the current fault components of the faulty phase not only contain the above components, but also generate additional current changes due to the presence of the fault, such as differences in steady-state power frequency components, opposite directions, phase change processes from opposite to in phase, and significant differences in effective energy. In addition, in a neutral point grounded system via an arc suppression coil, the faulty phase current may also exhibit obvious attenuated DC component characteristics, which helps to further distinguish faulty lines from healthy lines.

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

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

[0049]

[0050] In equations (1), (2), and (3) above, Δi kA , Δi kB , Δi kC i represents the phase current fault components of phases A, B, and C of a healthy circuit, respectively. kA ′、i kB ′、i kC To ensure the proper functioning of phases A, B, and C currents after a line fault, i kA i kB i kC To accurately measure the currents in phases A, B, and C before a line fault, c k To ensure the proper capacitance between each phase and ground of the line, U0 is the neutral point offset voltage of the system, and t is a time variable, representing the time from the moment the fault occurs.

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

[0052]

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

[0054] Preferably, for a healthy line in a neutral-point ungrounded system, the expression for the total current of the phase current fault component can be written as:

[0055]

[0056] In equation (7) above, i Ck "、i Ck ′ represent the steady-state power frequency component and the transient free oscillation component, respectively. ck It represents the amplitude of the current in each phase of a healthy circuit, where ω is the power frequency angular frequency. It is the initial phase angle, ω f ω is the angular frequency of the transient free oscillation component, σ is the transient attenuation coefficient, and t is a time variable, representing the time from the moment the fault occurs.

[0057] This indicates that the phase current fault component of a healthy line consists of a steady-state power frequency component and a transient free oscillation component, and the fault components of each phase are nearly identical in magnitude and in the same or nearly in the same direction.

[0058] Preferably, for a faulted line in a neutral-point ungrounded system, the expression for the total current of the fault component of the non-faulted phase current can be written as:

[0059]

[0060] In equation (8) above, i CJ "、i CJ ′ represent the steady-state power frequency component and the transient free oscillation component, respectively. cJ It represents the amplitude of the current in each phase of the faulty line, and ω is the power frequency angular frequency. It is the initial phase angle, ω fω is the angular frequency of the transient free oscillation component, σ is the transient attenuation coefficient, and t is a time variable, representing the time from the moment the fault occurs.

[0061] For a faulted line in a neutral-point ungrounded system, the expression for the total current of the fault phase current (fault component) can be written as:

[0062]

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

[0064] Equation (9) above shows that the phase current fault component consists 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 component of the fault phase current and the steady-state power frequency component of all non-fault phases in the network. The transient free oscillation component is the vector sum of the transient free oscillation component of the fault phase current and the transient free oscillation component of all non-fault phases in the network. Both the steady-state power frequency component and the transient free oscillation component play an anti-phase boosting role in the phase current fault component of the fault phase.

[0065] Preferably, when the distribution network is a neutral point ungrounded system, the phase current fault component characteristics may include the following two characteristics, wherein, characteristic 1 is the three-phase phase current fault component law of the neutral point ungrounded system - healthy line, and characteristic 2 is the three-phase phase current fault component law of the neutral point ungrounded system - faulty line.

[0066] Furthermore, the characteristics of feature 1 are as follows: 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 nearly identical, and their directions are in phase or nearly in phase; the effective energy (steady-state power frequency and transient components) of the phase current fault components of the fault phase and the non-fault phases are small.

[0067] Furthermore, characteristic 2 shows the following: the steady-state power frequency components of the phase current fault components in the faulty phase and the non-faulty phases are different in magnitude and opposite in direction; the steady-state power frequency components of the phase current fault components in the non-faulty phases are nearly in the same direction. The effective energy (steady-state power frequency and transient components) of the phase current fault components in the faulty phase and the non-faulty phases differs greatly.

[0068] Preferably, the criteria for judging faulty lines in a neutral-point ungrounded system can be obtained based on features 1 and 2, namely, the following features must be met:

[0069] ① The effective energy of the phase current fault component of the faulted phase is greater than the effective energy of the phase current fault component of the non-faulted phase.

[0070] ② The phase angles of the steady-state power frequency components of the phase current fault components of the fault phase and the non-fault phase are continuously opposite, while the phase angles of the steady-state power frequency components of the phase current fault components of the non-fault phase and the non-fault phase are nearly the same.

[0071] Preferably, when the neutral point is grounded through an arc suppression coil, the phase current fault component 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 connection of the neutral point arc suppression coil causes the current i flowing through the fault point to... f It not only includes the capacitance current to ground, but also adds the steady-state power frequency component i, which includes the arc suppression coil inductance. L "and the transient free oscillation component of the arc suppression coil inductance i" L The two parts are shown in equation (10) below:

[0073]

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

[0075] Preferably, for a distribution network with a neutral point grounded via an arc suppression coil, the expressions for the total current of the fault component of the phase current of the faulty line and the expressions for the total current of the fault component of the non-faulty phase current of the faulty line are the same as those in equations (7) and (8) above, while the expression for the total current of the fault component of the phase current of the faulty line-faulty phase will be increased by i. L "+i L ′, as shown in equation (11):

[0076]

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

[0078] Furthermore, in the above equation (11), when At that time, its When the value is at its maximum, there will be a decaying DC component; when At that time, its When the value is minimized, there will be no transient free oscillation component of the arc suppression coil inductance.

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

[0080] Furthermore, the characteristics of feature 3 are as follows: 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 nearly identical, and their directions are in phase or nearly in phase; the effective energy (steady-state power frequency and transient components) of the phase current fault components of the fault phase and the non-fault phases are small.

[0081] Furthermore, the pattern of characteristic 4 is as follows: Because the arc suppression coil generally operates in overcompensation mode, after full compensation, the steady-state power frequency component of the fault phase current in the total current expression of the fault current component of the fault line will eventually be in phase or nearly in phase with the non-fault steady-state power frequency component. However, due to the 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 of the fault occurrence. Therefore, the steady-state power frequency component of the fault phase current in the fault line will exhibit a significant phase change process during this period (from the moment of the fault, it gradually transitions from being out of phase to being in phase or nearly in phase after the arc suppression coil starts working). Further, the phase change process refers to the process during the fault occurrence where the phase angle of the steady-state power frequency component of the fault current component of the fault phase relative to the non-fault phase gradually transitions from being out of phase to being in phase or nearly in phase. This phenomenon is the significant phase change process. During the fault occurrence, the transient free oscillation component of the fault phase fault component is in ( When the phase current of the faulty phase is at a small angle (or at a very small angle), the fault component exhibits a decaying DC component characteristic, and its waveform clearly shows a unidirectional offset. The fault component of the phase current of the non-faulty phase shows almost no decaying DC component; this phenomenon is known as the decaying DC component phenomenon. During the transition process, the effective energy of the phase current fault components of the faulty and non-faulty phases differs significantly. The steady-state power frequency components of the phase current fault components of the non-faulty and non-faulty phases are nearly in the same direction.

[0082] Preferably, based on features 3 and 4, the criteria for judging fault lines in a neutral point grounded system via an arc suppression coil can be obtained, which requires the following features to be met:

[0083] ① The effective energy of the fault component of the phase current in the faulted phase is greater than the effective energy of the fault component of the phase current in the non-faulted phase.

[0084] ② During the transition process after the fault occurs, the phase current fault component will show a significant phase change process between the power frequency phase angles of the fault phase and the non-fault phase (from the initial phase reversal to the phase in or near phase in), while the power frequency phase angles of the phase current fault components of the non-fault phase are directly close to the same, without this phase change process.

[0085] ③During the transition process after a fault occurs, the fault component of the phase current of the faulted phase may have a decaying DC component.

[0086] Based on the above analytical principles, such as Figure 1 As shown, the present invention provides a method for identifying single-phase grounding faults in distribution networks based on the full-current characteristics of phase current mutations, the steps of which may include:

[0087] S1, Three-phase current power-on monitoring

[0088] Three-phase current signals are acquired according to the theoretical sampling frequency (fixed-frequency sampling) and the effective value is calculated. Three-phase current power-on monitoring is completed when the following specific conditions are met simultaneously:

[0089] RMSI A ≥I ON

[0090] RMSI B ≥I ON

[0091] RMSI C ≥I ON

[0092] t≥t ON ,

[0093] Among them, RMSI A RMSI B RMSI C This represents the effective value of the three-phase current signal. The effective value is calculated using the root mean square (RMS) method. ON For the current setpoint, t ON The power-on delay time is the set value, and t is a time variable, representing the time from the moment when the effective values ​​of the three-phase current signals simultaneously meet the power-on current set value.

[0094] According to a preferred embodiment, in step S1, I ON It can be approximately 2A or higher, tON A value of 5 seconds is generally acceptable.

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

[0096] Real-time calculation of the phase angle difference of the three-phase current controls the PWM output to achieve frequency tracking sampling.

[0097] Preferably, after completing the three-phase current power-on monitoring in step S1, the sampling frequency is f... s The three-phase current signals are collected periodically, and I is calculated respectively. A I B I C The phase of the three-phase current is calculated, and the phase angle difference between two adjacent full cycles is calculated:

[0098]

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

[0100] Furthermore, the average phase angle difference of the three-phase currents can be calculated using the following formula:

[0101]

[0102] Preferably, by Dynamic feedback control tracks and samples the PWM signal output, with a new sampling frequency f. s The control method is as follows:

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

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

[0105] This method is used to continuously update f s ′, to achieve frequency-based tracking sampling.

[0106] According to a preferred embodiment, in step S2, m can be 128 or 256 points as required by industry standards, in order to achieve f s =Sampling frequency of 6.4k or 12.8k.

[0107] According to a preferred embodiment, in step S2, the phase angle difference can be calculated every 15 cycles (0.3 seconds). Based on the above control method, frequency tracking can generally be completed within three steps (0.3 seconds × 3 = 0.9 seconds) of initial adjustment, fine adjustment, and micro-adjustment, achieving tracking at the sampling frequency f. s The sampling points are sampled using integer frequency sampling.

[0108] According to a preferred embodiment, in step S2, after completing the three-phase current frequency tracking sampling, the following technical indicators are met: within the rated current range (e.g., the distribution network load current is 600A), a stable 600A current is applied, and the error of the effective value change between the two whole cycles is calculated to be ≤0.25A, thereby ensuring the accurate extraction of the fault component when the minimum grounding current may be around 1A when a high-resistance grounding fault occurs in the distribution network.

[0109] like Figure 2 As shown in (a), frequency tracking sampling is used (the power grid fluctuates randomly in the range of 49.95Hz to 50.05Hz). When the three-phase current is 600A, the error of the change in the effective value of the calculated two whole cycles is ≤0.25A (peak-to-peak value ±0.5A).

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

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

[0112] from Figure 2 The comparison between (a) and (b) and (c) clearly shows that after adopting frequency-following tracking sampling, the calculation error of the effective value change is reduced by an order of magnitude compared with fixed-frequency sampling.

[0113] S3, Fault Startup Judgment

[0114] S3.1, Zero-sequence current is sampled according to frequency tracking frequency f.s Real-time sampling monitoring

[0115] S3.2 Extracting the differential current curve of zero-sequence current mutation

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

[0117] Furthermore, the zero-sequence current abrupt change differential current curve can be calculated as follows: Starting from the first cycle, subtract the corresponding sampling points of the previous cycle from the sampling points of the next cycle of the N-cycle zero-sequence current waveform i0(x). The constructed new N-cycle waveform is defined as the zero-sequence current abrupt change differential current curve Δi0(x), as shown below:

[0118] Δi0(x)=0x∈(0,m-1)

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

[0120] S3.3 Zero-sequence current mutation start-up judgment

[0121] For the zero-sequence current surge differential current curve Δi0(x), calculate the effective value of each cycle sequentially from beginning to end according to the integer cycle, and check whether there is an effective value greater than the preset threshold in each cycle of the zero-sequence current surge differential current curve, that is, the fault initiation condition 1 shown below:

[0122] ΔI 0i ≥I 0SET1 ,

[0123] Where, ΔI 0i This is the effective value of the i-th cycle of the differential current curve Δi0(x) used to calculate the sudden change in zero-sequence current. 0SET1 It is a fault start threshold set for fault start condition 1.

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

[0125] If fault initiation condition 1 is met, the effective value of the zero-sequence current for M consecutive cycles starting from the i-th cycle is calculated as follows, and the average value of the M effective values ​​of the zero-sequence current is calculated:

[0126]

[0127] Among them, I 0ave I is the average value of the effective value of the zero-sequence current over M consecutive cycles starting from the i-th cycle. 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, determine whether the average value satisfies fault initiation condition 2 as shown below:

[0129] I 0ave ≥I 0SET2 ,

[0130] Among them, I 0SET2 It is a fault start threshold set for fault start condition 2.

[0131] Furthermore, if fault initiation condition 2 is met, it indicates that a single-phase ground fault has occurred in the distribution network. At this time, the zero-sequence current and three-phase current within the time window specified in step S3.2 above are recorded and temporarily stored (typically, the first 4 and last 8 cycles of the index of the starting cycle are saved for subsequent fault analysis and judgment), and the single-phase ground fault detection method is started. If fault initiation condition 2 is not met, the return process is executed, i.e., 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 collecting the zero-sequence current through a three-phase current calculation and synthesis method. As a preferred example, we use the method of directly collecting the zero-sequence current through a zero-sequence current transformer to minimize the problems of uncontrollable and large zero-sequence current error caused by the three-phase current calculation and synthesis method.

[0133] According to a preferred embodiment, in step S3, the fault start judgment is performed synchronously with step S2 within a specified time window, which can be performed every 15 cycles.

[0134] According to a preferred embodiment, in step S3, I 0SET1 and I 0SET2 All values ​​can be 1A.

[0135] S4. Fault Start Point Locator

[0136] The differential current curve Δi0(x) of the zero-sequence current mutation is subjected to low-pass filtering (cutoff frequency is f). s Obtain the zero-sequence differential current low-pass filter curve Δi0(x)′, and use the second-order difference quotient method to find and mark the fault initiation point of the zero-sequence differential current low-pass filter curve Δi0(x)′:

[0137] Δi0(i)′=F{Δi0(i)*g(t)},

[0138] Where Δi0(i)′ is the zero-sequence current mutation differential value at the i-th sampling point after low-pass filtering, F is the filtering operation, 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, the low-pass filter can be an FIR filter with a cutoff frequency f. s The cutoff frequency can be selected based on the relevant references for lines of different lengths and references; for example, 600Hz can be selected.

[0140] S5. Extract the full current characteristic waveform of the three-phase current fault component.

[0141] S5.1 Extracting the three-phase current fault component waveform

[0142] Extracting the three-phase current waveform fault component waveform Δi two cycles after the fault initiation point A (i), Δi B (i), Δi C (i) where the extraction method is as follows: subtract the corresponding sampling point of the previous cycle from the first cycle sampling point of the temporarily stored three-phase current waveform starting from the fault initiation point, and subtract the corresponding sampling point of the first cycle from the second cycle sampling point starting from the fault initiation 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 sampling point corresponding to the fault initiation point, and its value ranges from 2 cycles.

[0148] S5.2 Extracting the effective frequency band characteristic waveform of the three-phase current fault component

[0149] The above two-cycle three-phase current fault component waveforms are subjected to low-pass filtering (cutoff frequency f).s1 The fault component waveform is subjected to mode 1 low-pass filtering (cutoff frequency f). s1 =600Hz), the effective frequency band characteristic waveform of the three-phase phase current fault component of the second cycle is obtained 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)′ represent the effective frequency band characteristic waveforms of the phase current fault components of phases A, B, and C in a 2-cycle wave, respectively. F is the filtering operation, g(t) is the filtering function, and low-pass filtering data processing is performed (cutoff frequency is f). s1 ).

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

[0155] The above two-cycle three-phase current fault component waveforms are subjected to low-pass filtering (cutoff frequency f). s2 The fault component waveform is subjected to mode 2 low-pass filtering (cutoff frequency f). s2 =100Hz), the characteristic waveforms of the power frequency component of the three-phase phase current fault component were obtained for 2 cycles, as shown below:

[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)″ represents the characteristic waveforms of the power frequency components of the phase current fault components of phases A, B, and C in a 2-cycle waveform. F is the filtering operation, g(t) is the filtering function, and low-pass filtering data processing is performed (cutoff frequency is f). s2 ).

[0160] According to a preferred embodiment, in step S5, "Mode 1 - Low-pass filter (cutoff frequency is f)" s1 =600Hz), the effective frequency band characteristic waveform of the three-phase current fault component of the two-cycle three-phase current fault component is obtained. It includes the power frequency component and the free transient component, and provides characteristic waveforms for the fault identification algorithm. The constructed effective frequency band characteristic waveform of the three-phase current fault component retains the power frequency fault characteristics and the possible high-frequency fault characteristics of the first capacity section.

[0161] According to a preferred embodiment, in step S5, "Mode 2 - Low-pass filter (cutoff frequency is f)" s2 =100Hz), "obtain the characteristic waveform of the three-phase phase current fault component power frequency component for 2 cycles" includes power frequency component and attenuated DC component information, providing characteristic waveforms for fault identification algorithms.

[0162] Preferably, after a fault occurs in a power distribution line, the fault component full current characteristic waveform is extracted according to step S5 above, and then the characteristics of the intact line and the faulty line are analyzed through subsequent steps.

[0163] S6. Calculate fault characteristic quantities

[0164] S6.1 Calculation of Energy Characteristic Values ​​of Effective Frequency Band Characteristic Waveforms

[0165] via Δi A (i)′、Δi B (i)′、Δi C (i)′ Effective frequency band characteristic waveform (one cycle starting after the fault initiation point), calculate the energy I of the effective frequency band characteristic waveform of the three-phase currents A, B, and C. a_RMS I b_RMS I c_RMS Furthermore, I a_RMS I b_RMS I c_RMS It can be obtained by calculating the root mean square (RMS) value, where the value is obtained by searching {I}. a_RMS I b_RMS I c_RMS The maximum value in} is recorded as I. max_RMS The phase sequence (A, B, or C) corresponding to its maximum value is identified as the reference phase.

[0166] If phase A is the faulty phase, k1 and k2 can be calculated as follows:

[0167]

[0168] If phase B is the faulty phase, k1 and k2 can be calculated as follows:

[0169]

[0170] If phase C is the faulty phase, k1 and k2 can be calculated as follows:

[0171]

[0172] By constructing the energy characteristic value of the effective frequency band characteristic waveform, the difference in fault energy between the faulty phase and the non-faulty phase can be calculated, which can provide basic judgment data for the effective frequency band characteristic waveform energy characteristic value algorithm based on the phase current fault component proposed in this invention.

[0173] S6.2 Extraction of Attenuated DC Component and Calculation of Eigenvalues

[0174] The characteristic waveform Δi of the phase current fault component and power frequency component. A (i)″、Δi B (i)″、Δi C (i) The attenuated DC components of the fault current in phases A, B, and C are quickly calculated using arithmetic and average methods, as shown in the following formula:

[0175]

[0176] Where m is the number of sampling points per cycle, used to calculate the attenuation of the DC component of the characteristic waveform of the power frequency component.

[0177] Furthermore, the difference value of the attenuated DC component is constructed according to the following calculation:

[0178] (1) If phase A is the faulty phase, then the difference in the attenuated DC component is:

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

[0180] (2) If phase B is the faulty phase, then the difference in the attenuated DC component is:

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

[0182] (3) If phase C is the faulty phase, then the difference in the attenuated DC component is:

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

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

[0185] By constructing the attenuation DC component difference value, it is possible to calculate whether there is a significant difference in the attenuation DC component between the faulty phase and the non-faulty phase, which can provide basic judgment data for the feature algorithm based on the attenuation DC component difference value proposed in this invention.

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

[0187] S6.3 Calculation of Instantaneous Phase Angle of Characteristic Waveform of Power Frequency Component

[0188] Power frequency component characteristic waveform Δi A (i)″、Δi B (i)″、Δi C (i)″ is a non-standard sinusoidal waveform. Furthermore, due to the significant phase change process, the presence of a decaying DC component, and the fact that the transition frequency is not 50Hz, resulting in a non-integer cycle sampling number, the traditional Fourier algorithm for calculating the phase angle is not feasible. Based on this, this invention proposes a rapid instantaneous phase angle calculation method based on peak and trough point estimation to quickly calculate the instantaneous phase angle of the power frequency characteristic waveform of the phase current fault component. Ultimately, the change curve of the phase difference between the power frequency component characteristic waveforms of the faulty phase and the non-faulty phase is calculated.

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

[0190] a) First, calculate the number of sampling points n between the peaks and troughs, i.e., n = x b -x a Then calculate the point x between the peak and trough. i phase angle

[0191]

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

[0193]

[0194] Based on the above rules, the instantaneous phase angles of the power frequency characteristic waveforms of the three-phase fault components A, B, and C are calculated. curve.

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

[0196] In step S6.3, the phase angle of the power frequency characteristic waveform of the three-phase phase current fault component is calculated. After the curve, the phase difference change curve of the characteristic waveform of the power frequency component can be calculated according to the following rules:

[0197] a) If phase A is the faulty phase, construct the phase difference change curve of the power frequency component characteristic waveform as follows:

[0198]

[0199] b) If phase B is the faulty phase, construct the phase difference change curve of the power frequency component characteristic waveform as follows:

[0200]

[0201] c) If phase C is the faulty phase, construct the phase difference change curve of the power frequency component characteristic waveform as follows:

[0202]

[0203] In the above formula, These are the curves showing the change in phase difference between the characteristic waveforms of the power frequency components of the faulty phase and the non-faulty phase, respectively.

[0204] Furthermore, the constructed power frequency characteristic waveform phase difference change curve This can provide basic judgment data for the feature value algorithm based on the phase difference change curve of the power frequency component characteristic waveform proposed in this invention.

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

[0206] S7.1 Fault Location Criteria for Neutral Point Ungrounded Systems

[0207] Based on features 1 and 2, a fault location criterion for a neutral-point ungrounded system is generated. The fault line criterion must satisfy the following features:

[0208] a) The phase current of the faulted phase and the non-faulted phase have different magnitudes and opposite directions in steady-state power frequency components.

[0209] b) The effective energy (steady-state power frequency and transient components) of the phase current of the faulty phase and the non-faulty phase differs greatly.

[0210] Specifically, the mathematical expression of the fault criterion for an ungrounded neutral point system is as follows:

[0211] I max_RMS ≥I set

[0212] k1≥k set And k2≥k set

[0213]

[0214] Among them, I max_RMS It is the maximum effective value current, I set The threshold current is set, and k1 and k2 are two coefficients used to determine the effective energy difference between the fault components of the current in the faulty phase and the non-faulty phase. set It is the set threshold coefficient. It is the curve of phase difference change. and It is the set phase difference change curve The upper and lower limits of the range, and It is the set phase difference change curve The upper and lower limits of the range.

[0215] According to a preferred embodiment, in step S7.1, after extensive simulation testing and data calculation and statistics, the recommended setpoint parameter can be selected as follows: I set One can choose 1A, k set Option 2 is acceptable. 150° is acceptable. 210° is acceptable. -30° is acceptable. 30° is acceptable.

[0216] S7.2 Fault Location Criteria for Neutral Point Grounded Through Arc Suppression Coil System

[0217] Based on features 3 and 4, a fault location criterion is generated for a neutral point grounded system via an arc suppression coil. The fault line criterion must satisfy the following features:

[0218] a) The effective energy of the phase current fault component of the faulted phase is greater than the effective energy of the phase current fault component of the non-faulted phase.

[0219] b) During the transition process after the fault occurs, the fault component of the phase current exhibits a significant phase change process between the power frequency phases of the faulted phase and the non-faulted phase (from the initial phase being out of phase to gradually transitioning to the same phase or close to the same phase).

[0220] c) During the transition process after a fault occurs, the fault component of the phase current of the faulted phase may have a decaying DC component.

[0221] Specifically, the mathematical expression of the fault criterion for a neutral point grounded system via an arc suppression coil is as follows:

[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 used to determine faulty lines, representing the difference in attenuation of the DC component.

[0231] According to a preferred embodiment, in step S7.1, after extensive simulation testing and data calculation and statistics, the recommended setpoint parameter can be selected as follows: I set One can choose 1A, k set A value of 2 can be chosen. In the formula for calculating the difference in attenuation DC components in S6.2, k can be taken as 10, and D... min 0.5A can be selected. -30° is acceptable. 30° is acceptable.

[0232] Example 1 is a neutral point ungrounded system with a ground fault through a 2000Ω high resistance, and the ground fault phase is phase C. Figure 3 (a) and Figure 3(b) shows the waveforms of the three-phase phase current fault components, 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 faulted and intact lines, respectively. Figure 3 (a) It can be seen that the energy differences among the three phases of the effective frequency band characteristic waveform of the faulty line are significant. In the power frequency component characteristic waveform, it is clearly visible that the faulty phase and the non-faulty phase are continuously out of phase after the fault, while the non-faulty phases are basically in phase after the fault. From... Figure 3 (b) It can be seen that the energy of the effective frequency band characteristic waveform of a healthy line is almost identical among the three phases. The power frequency component characteristic waveform clearly shows that the faulty phase and the non-faulty phase are almost in phase throughout the fault process. The characteristic values ​​of the effective frequency band characteristic waveform energy and the curves showing the change in the phase difference of the power frequency component are calculated. The characteristic parameters are shown in Table 1 below. Based on the fault criteria for a neutral-point ungrounded system, accurate judgment can be made. Figure 3 (a) is the faulty line. Figure 3 (b) To improve the line.

[0233] Table 1. Calculation results of characteristic parameters of grounding faults with a 2000Ω high resistance.

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

[0235] Example 2 is an arc grounding fault in a neutral ungrounded system, with the faulty phase being phase C. Figure 4 (a) and Figure 4 (b) shows the waveforms of the three-phase phase current fault components, 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 faulted and intact lines, respectively. Figure 4 (a) It can be seen that the energy differences among the three phases of the effective frequency band characteristic waveform of the faulty line are significant. In the power frequency component characteristic waveform, it is clearly visible that the faulty phase and the non-faulty phase are continuously out of phase after the fault, while the non-faulty phases are basically in phase after the fault. From... Figure 4 (b) It can be seen that the fault current waveform of a healthy line phase current shows significant differences between the faulty and non-faulty phases, making it easy to misjudge using conventional methods. However, the energy of the effective frequency band characteristic waveform is almost identical among the three phases. The power frequency component characteristic waveform clearly shows that the faulty and non-faulty phases are almost in phase throughout the fault process. The characteristic values ​​of the effective frequency band characteristic waveform energy and the curve of the power frequency component phase difference change are calculated. The characteristic parameters are shown in Table 2 below. Based on the fault criterion for a neutral-point ungrounded system, accurate judgment can be made. Figure 4 (a) is the faulty line. Figure 4 (b) To improve the line.

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

[0237] Line ​ <k2> I max_RMS ]] z1 (x)]]> ​ z2 (x)]]> ​ D Faulty line 5.13 5.09 20.05 178～184 Fundamental in phase - Healthy line 1.14 1.17 4.11 Fundamental in phase Fundamental in phase -

[0238] Example 3 is a neutral point grounding system with an arc suppression coil grounding fault via a 5000Ω high resistance phase, with phase B being the faulty phase. Figure 5 (a) and Figure 5 (b) shows the waveforms of the three-phase phase current fault components, 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 faulted and intact lines, respectively. Figure 5 (a) It can be seen that the energy differences among the three phases of the effective frequency band characteristic waveform of the faulty line are significant. The power frequency component characteristic waveform clearly shows a phase change process between the faulty and non-faulty phases after the fault (a gradual transition from initial out-of-phase to in-phase or near-in-phase). The non-faulty phases are basically in phase with each other after the fault. Figure 8 In the figure, it can be clearly seen that the curve of the change in the phase difference of the power frequency components between the faulty phase and the non-faulty phase changes from being out of phase to gradually being in phase.

[0239] Depend on Figure 5 (b) It can be seen that the energy of the effective frequency band characteristic waveform of a healthy line is almost identical among the three phases. The power frequency component characteristic waveform clearly shows that the faulty phase and the non-faulty phase are almost in phase throughout the fault process. The characteristic values ​​of the effective frequency band characteristic waveform energy and the curve of the change in the phase difference of the power frequency component are calculated. The characteristic parameters are shown in Table 3 below. Based on fault criterion 1 for neutral point grounded systems via arc suppression coils, accurate judgment can be made. Figure 5 (a) is the faulty line. Figure 5 (b) To improve the line.

[0240] Table 3. Calculation results of characteristic parameters of grounding faults with a 5000Ω high resistance.

[0241] Line ​ <k2> I max_RMS ]]> z1 (x)]]> ​ z2 (x)]]> ​ D Faulty line 4.22 4.50 1.18 -160→-40 Fundamental in phase -5.86 Healthy line 1.17 1.19 0.5 Fundamental in phase Fundamental in phase -5.41

[0242] Example 4 is a neutral point grounding system with an arc suppression coil grounding fault via a 1000Ω high resistance phase, with phase B being the faulty phase. Figure 6 (a) and Figure 6 (b) shows the waveforms of the three-phase phase current fault components, 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 faulted and intact lines, respectively. Figure 6 (a) It can be seen that the energy of the three phases of the effective frequency band characteristic waveform of the faulted line is very different. The power frequency component characteristic waveform clearly shows the obvious phase change process between the faulted phase and the non-faulted phase after the fault (from the initial phase reversal to the phase in or close to the phase in). The phases of the non-faulted phase and the non-faulted phase are basically in phase after the fault.

[0243] Depend on Figure 6(b) It can be seen that the energy of the effective frequency band characteristic waveform of a healthy line is almost identical among the three phases. The power frequency component characteristic waveform clearly shows that the faulty phase and the non-faulty phase are almost in phase throughout the fault process. The energy characteristic values ​​of the effective frequency band characteristic waveform and the characteristic parameters of the power frequency component phase difference change curve are calculated as shown in Table 4 below. Based on fault criterion 1 for neutral point grounded systems via arc suppression coils, accurate judgment can be made. Figure 6 (a) is the faulty line. Figure 6 (b) To improve the line.

[0244] Table 4. Calculation results of characteristic parameters of grounding faults with a 1000Ω high resistance.

[0245] Line ​ [ k2 ] I max_RMS ]] z1 (x)]]> ​ z2 (x)]]> ​ D Faulty line 3.01 3.31 5.09 -140→-30 Fundamental in phase -11.2 Healthy line 1.01 1.01 1.61 Fundamental in phase Fundamental in phase -11.5

[0246] Example 5 is an arc-grounded fault in a neutral point grounded system via an arc suppression coil, with the faulty phase being phase B. Figure 7 (a) and Figure 7 (b) shows the waveforms of the three-phase current fault components, 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 faulted and intact lines, respectively. Figure 7 (a) It can be seen that the energy of the three phases of the effective frequency band characteristic waveform of the faulted line is very different. In the power frequency component characteristic waveform, it can be clearly seen that there is a significant phase change process between the faulted phase and the non-faulted phase after the fault (from the initial phase reversal to the phase in phase or close to the phase in phase). The phases of the non-faulted phase and the non-faulted phase are basically in phase after the fault. The faulted phase has a significant attenuated DC component.

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

[0248] Depend on Figure 7 (b) It can be seen that the fault current waveform of a healthy line phase current shows a significant difference between the faulty and non-faulty phases, making the transient method prone to misjudgment. However, the energy of the effective frequency band characteristic waveform is almost identical among the three phases. The power frequency component characteristic waveform clearly shows that the faulty and non-faulty phases are almost in phase throughout the fault process. The characteristic values ​​of the effective frequency band characteristic waveform energy, the power frequency component phase difference change curve, and the attenuation DC component difference value are calculated. The characteristic parameters are shown in Table 5 below. Based on fault criterion 1 or fault criterion 2 for neutral point grounded systems via arc suppression coils, accurate judgment can be made. Figure 7 (a) is the faulty line. Figure 7 (b) To improve the line.

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

[0250] Line ​ <k2> I max_RMS ]] z1 (x)]]> ​ z2 (x)]]> ​ D Faulty line 8.61 9.31 100.26 -168→-20 Fundamental in phase 68 Healthy line 1.15 1.02 15.54 Fundamental in phase Fundamental in phase -6.78

[0251] S8, Local Trip Logic

[0252] Timing is started at the fault initiation point, and the effective value of the zero-sequence current signal I is monitored in real time. 02 The local trip protection action will be executed when the following conditions are met simultaneously:

[0253] I 02 ≥I 0SET2

[0254] t2≥t SET2 ,

[0255] Where t2 is the time starting from the fault initiation point, t SET2 This is a set time threshold, which can be set according to user requirements, and is generally ≥1 second.

[0256] S9, Accelerated action after permanent fault following automatic reclosing.

[0257] After the local trip protection action in step S8 is completed, if reclosing is not required, return to step S1 and wait for the three-phase current to be energized for judgment; if reclosing is required, wait for a delay time t3 after reclosing, where t3 satisfies the condition of avoiding the inrush current, i.e., t3 ≥ t SET3 , t SET3 The protection action is triggered when time t3 reaches or exceeds the set time threshold.

[0258] Furthermore, the effective value I of the zero-sequence current signal is monitored in real time. 03 When the following conditions are met simultaneously, the local reclosing and accelerated trip protection action will be executed:

[0259] I 03 ≥min(I 0SET2 ,k3*I 0ave )

[0260] t3≥t SET3 ,

[0261] Where k3 is the reliability coefficient, which is generally taken as 0.9, and t SET3 Generally, the value is taken to be ≥0.2S to avoid inrush current and asynchronous three-phase closing. 0ave It is the average value of the zero-sequence current, calculated by monitoring the zero-sequence current over a period of time. Specifically, I 0ave This refers to the zero-sequence current that is first determined during fault initiation in step S3 before reclosing. Therefore, when reclosing on a permanent fault, the zero-sequence current can be identified and the fault quantity memorized to achieve rapid protection action.

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

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

[0264] S10, Return Process

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

[0266] I0<k4*I 0bph

[0267] t4≥t SET4 ,

[0268] Among them, I 04 It is the effective value of the zero-sequence current signal, k4 is the reliability coefficient, which can generally be taken as 1.1, I 0bph It is the reference zero-sequence current value, a pre-set reference value used to determine whether the zero-sequence current is normal. t4 is the time from the starting point. SET4 It is a set time threshold, which is generally ≥1 second.

[0269] It should be noted that the above specific embodiments are exemplary, and those skilled in the art can devise various solutions inspired by the disclosure of this invention, and these solutions are all within the scope of the disclosure of this invention and fall within the protection scope of this invention.

Claims

1. A method for identifying single-phase earth fault of power distribution network based on phase current sudden change quantity characteristics, characterized in that, It comprises the following steps: According to the set theoretical sampling frequency, the three-phase current signal is collected, and the effective value is calculated, and when the specific conditions are met at the same time, the three-phase current power-on judgment is completed; The three-phase current and zero sequence current are monitored by real-time sampling of the frequency tracking sampling frequency, the zero sequence current mutation difference flow curve is extracted, and the fault starting condition judgment is performed on the difference flow curve; From the fault starting point, the three-phase current fault component waveform is extracted, and then low-pass filtering is performed on these waveforms to obtain effective frequency band characteristic waveform and power frequency component characteristic waveform; According to the effective frequency band characteristic waveform and the power frequency component characteristic waveform, the fault characteristic quantity is calculated, and the calculated fault characteristic quantity includes the effective frequency band characteristic waveform energy characteristic value, the attenuation DC component difference value and the power frequency component characteristic waveform instantaneous phase angle and the phase difference change quantity curve; According to the fault characteristic quantity, different single-phase grounding fault positioning algorithms are executed according to different types of neutral point grounding systems, Wherein, by calculating the root mean square value of the effective frequency band waveform of the fault starting point after 1 cycle, the maximum value and the corresponding phase sequence are found as the reference phase to construct the effective frequency band characteristic waveform energy characteristic value, and the energy difference between the fault phase and the non-fault phase is calculated; the attenuation DC component of each phase current fault component is calculated by using the power frequency component characteristic waveform, and the attenuation DC component difference between the fault phase and the non-fault phase is constructed, which is used to judge whether there is attenuation DC component difference; the instantaneous phase angle of the power frequency characteristic waveform after the fault starting is calculated based on the wave peak and wave trough point estimation method; according to the calculated instantaneous phase angle, the power frequency component characteristic waveform phase difference change quantity curve between the fault phase and the non-fault phase is constructed to provide the basic judgment data for fault positioning.

2. The identification method according to claim 1, characterized in that, The specific conditions include: RMSI A ≥I ON and RMSI B ≥I ON and RMSI C ≥I ON and t≥t ON , Wherein, RMSI A , RMSI B , RMSI C Indicates the three-phase current signal effective value, the calculation method of effective value adopts the root mean square value calculation method of phase current instantaneous sampling value, I ON Is the power-on current constant value, t ON Is the power-on delay time constant value, t is the time variable, which indicates the time from the moment when the three-phase current signal effective value meets the power-on current constant value.

3. The identification method according to claim 1, characterized in that, The fault starting condition comprises detecting whether there is a valid value greater than a preset threshold I 0SET1 in each of the successive cycles of the zero-sequence current abrupt change variable difference current curve, and the average value of the zero-sequence current valid values of successive multiple cycles is greater than another preset threshold I 0SET2 .

4. The identification method of claim 1, wherein, After extracting the three-phase current fault component waveform, two low-pass filters with different cutoff frequencies are used to obtain the effective frequency band characteristic waveform and the power frequency component characteristic waveform.

5. The identification method of claim 1, wherein, For the neutral point ungrounded system, if the following neutral point ungrounded system, effective frequency band characteristic waveform energy characteristic value and power frequency component phase difference change quantity curve characteristic combined fault criterion are met, it is determined that a single-phase grounding fault has occurred: I max_RMS ≥I set and k1≥k set and k2≥k set and and wherein I max_RMS is the maximum effective value current, I set is the set threshold current, k1, k2 are two coefficients for judging the effective energy difference of the fault component of the fault phase and the non-fault phase current, set is the set threshold coefficient, is the phase difference change amount curve, and is the set phase difference change amount curve upper and lower limits of the range, and is the set phase difference change amount curve upper and lower limits of the range.

6. The identification method of claim 1, wherein, For the neutral point grounded through arc suppression coil system, if the following neutral point grounded through arc suppression coil system, effective frequency band characteristic waveform energy characteristic value and power frequency component phase difference change quantity curve characteristic combined fault criterion are met, it is determined that a single-phase grounding fault has occurred: I max_RMS ≥I set and k1≥k set and k2≥k set and and and wherein I max_RMS is the maximum effective value current, I set is the set threshold current, k1, k2 are two coefficients for judging the effective energy difference of the fault component of the fault phase and the non-fault phase current, k set is the set threshold coefficient, is the phase difference change amount curve, and is the set phase difference change amount curve upper and lower limits of the range, and is the set phase difference change amount curve upper and lower limits of the range.

7. The identification method of claim 1, wherein, For the neutral point grounded through arc suppression coil system, if the following neutral point grounded through arc suppression coil system, effective frequency band characteristic waveform energy characteristic value and attenuation DC component difference value characteristic combined fault criterion are met, it is determined that a single-phase grounding 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, represents the difference value of the decaying DC component, I max_RMS is the maximum effective value current, I set is the set threshold current, k1, k2 are two coefficients for judging the effective energy difference of the fault component of the fault phase and the non-fault phase current, k set is the set threshold coefficient.

8. The identification method of claim 1, wherein, It also comprises the following steps: real-time calculation of three-phase current phase angle difference control PWM output to realize frequency tracking sampling, wherein the frequency tracking sampling method is to dynamically adjust the PWM output frequency by calculating the phase angle difference of adjacent two integral cycles, so as to ensure accurate integral cycle sampling even if the grid system frequency changes.

9. The identification method of claim 1, wherein, It also includes the following steps: the zero sequence current mutation variable difference flow curve is processed by a low-pass filter with a cutoff frequency of f s = 0.5 Hz, and a first-order difference quotient method is used to quickly find the fault starting point.

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