Small current grounding fault identification method and system, and readable storage medium

Abstract

The invention relates to a low-current grounding fault identification method and system and a readable storage medium. The method comprises the following steps: sampling a zero-sequence voltage and a zero-current of a distribution line and carrying out fault starting judgment; two-cycle sampling data of the zero-sequence voltage and the zero-current after starting are cached and are extended into a zero-sequence voltage array and a zero-sequence current array, wherein the number of dimensions of the zero-sequence voltage array and the zero-sequence current array is integer multiples of the number of two-cycle wave points; calculating a transient zero-sequence active power instantaneous value by using the extended data to form a transient zero-sequence active power instantaneous value array; performing low-pass filtering by using a Hamming window FIR low-pass digital filter; accumulating transient zero-sequence active power instantaneous values of other points except the first two cycles in the filtered transient zero-sequence active power instantaneous value array to obtain transient zero-sequence full-wave active energy integral values of all the points, and forming a active energy integral value array; and performing internal and external identification on the small current grounding fault according to the active energy integral value array. According to the invention, the accuracy of internal and external identification of the small-current grounding fault area is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of distribution network fault identification, and in particular to a method, system and readable storage medium for identifying a small current grounding fault. Background Art

[0002] 6-66 kV medium- and low-voltage power grids belong to distribution networks. Their neutral points are generally grounded indirectly, including ungrounded, grounded via resistors, and grounded via arc suppression coils. When a single-phase ground fault occurs in a distribution network, the short-circuit current can only form a small current loop through distributed capacitance and transition resistance, resulting in a distribution network also known as a low-current grounding system. A single-phase ground fault in a distribution network increases the voltage in the non-fault phase, which can easily develop into a multiphase fault, endangering the safe operation of the system. Therefore, the latest technical guidelines for distribution networks stipulate that single-phase ground faults in low-current grounding systems should be handled according to the principle of rapid and local fault isolation. This requires that distribution terminals be equipped with the ability to identify the fault zone inside and outside the low-current grounding fault zone. Due to the characteristics of low-current single-phase ground faults, such as weak current and unstable fault arcs, as well as the influence of factors such as neutral point grounding method, system distributed capacitance, fault closing angle, and transition resistance, identifying low-current ground faults has always been a difficult problem in distribution network fault detection.

[0003] Existing identification methods based on analog signals include steady-state feature method, transient feature method and multi-feature fusion algorithm. Transient feature method includes first half wave method, transient capacitive direction method, transient energy method, wavelet transform method, traveling wave method, parameter identification method, etc. Among them, transient energy method is theoretically not affected by neutral point grounding mode. It identifies based on the theory that transient energy of fault line is negative and transient energy of non-fault line is positive. However, currently all methods adopt Transient energy is calculated, where u0 and i0 represent the zero-sequence voltage and zero-sequence current, respectively. This formula does not fully consider the impact of the resistive component of the zero-sequence current, the initial fault phase, the double frequency component of the active power, and the energy integration period on the calculation result. This can easily lead to misjudgment when the resistive component of the zero-sequence current is small.

[0004] For example, the Chinese invention patent "Distributed Line Selection and Positioning Method and Coefficient" with publication number CN116520086A provides a ground fault location method based on the transient zero-sequence energy method and the three-phase current transient correlation method. Its transient zero-sequence energy is simply calculated by integrating the zero-sequence voltage and zero-sequence current, and does not take into account the possibility of positioning failure when the resistive component of the zero-sequence current accounts for a relatively small proportion.

[0005] There is also a Chinese invention patent with publication number CN110865278A, "Ground Fault Location Method Based on Transient Mutation Energy Capture Method", which accurately determines the fault location by comparing the transient zero voltage change and the direction of transient zero current at the fault point when a single-phase ground fault occurs. Its essence is the transient reactive energy method and is not suitable for neutral point over-compensated grounding systems via arc suppression coils. Summary of the Invention

[0006] The purpose of the present invention is to provide a small current grounding fault identification method, system and readable storage medium, which can solve the problem that when a single-phase grounding fault occurs in the distribution network, the resistive component of the zero-sequence current is very small, and the existing transient energy method is easily affected by the proportion of the zero-sequence current resistive component, the initial phase of the fault, the active power double frequency component and the energy integration period, thereby leading to misjudgment.

[0007] In order to achieve the above object, the present invention provides a method for identifying a small current ground fault, comprising the following steps:

[0008] Sample the zero-sequence voltage and zero-sequence current of the distribution line according to the set sampling frequency and make fault start judgment;

[0009] When a small current ground fault occurs, two-cycle sampling data of zero-sequence voltage and zero-sequence current after startup are cached and extended into zero-sequence voltage array and zero-sequence current array with a dimension that is an integer multiple of the number of two-cycle points;

[0010] Calculating the transient zero-sequence active power instantaneous value using the extended zero-sequence voltage array and the zero-sequence current array to form a transient zero-sequence active power instantaneous value array;

[0011] Performing low-pass filtering on the transient zero-sequence active power instantaneous value array using a Hamming window FIR low-pass digital filter to obtain a filtered transient zero-sequence active power instantaneous value array;

[0012] Accumulating the transient zero-sequence active power instantaneous values of other points in the filtered transient zero-sequence active power instantaneous value array except for the first two cycles to obtain the transient zero-sequence full-wave active energy integral value of each point, and forming an active energy integral value array;

[0013] The small current grounding fault is identified as being within or outside a zone according to the active energy integral value array.

[0014] Optionally, the set sampling frequency is between 3200 Hz and 12800 Hz.

[0015] Optionally, when performing fault startup judgment, the small current grounding fault startup moment is obtained according to the zero-sequence voltage sudden change startup criterion.

[0016] Optionally, when a small current grounding fault occurs, the transient sampling data of the zero-sequence voltage U0 and the zero-sequence current I0 after startup are cached for two cycles each to form an initial zero-sequence voltage array U0[] and an initial zero-sequence current array I0[] with an initial dimension of 2N, where N is the number of sampling points per cycle.

[0017] Optionally, the initial zero-sequence voltage array U0[] and the initial zero-sequence current array I0[] are extended into a zero-sequence voltage array and a zero-sequence current array with a dimension of M in the following manner:

[0018] M=2N*p,p≥2

[0019] U0[i+2N*l]=U0[i]

[0020] I0[i+2N*l]=I0[i]

[0021] i=0,1,…,2N-1; l=1,…,p

[0022] Where p is a multiple of the data length after extension.

[0023] Optionally, the extended zero-sequence voltage array and the zero-sequence current array are used to calculate the transient zero-sequence active power instantaneous value P0 to form a transient zero-sequence active power instantaneous value array P0[] with a dimension of M, and the formula is as follows:

[0024] P0[j]=U0[j]*I0[j]

[0025] j=0,1,…,M-1.

[0026] Optionally, a low-pass filtering is performed on the transient zero-sequence active power instantaneous value array P0[] using an N-order Hamming window FIR low-pass digital filter to filter out all components above 1 Hz, which include the double frequency component of the active power, to obtain a filtered transient zero-sequence active power instantaneous value array P′0[], wherein the order of the Hamming window FIR low-pass digital filter is the same as the number of sampling points per cycle;

[0027] The transient zero-sequence active power instantaneous value of each point between 2N and (M-1) in the filtered transient zero-sequence active power instantaneous value array P′0[] is accumulated to obtain the transient zero-sequence full-wave active energy integral value of each point, forming an M-2N dimensional active energy integral value array W0[].

[0028] Optionally, the small current grounding fault is identified as inside or outside the zone based on the active energy integral value array. If the transient zero-sequence full-wave active energy integral value of any point in the active energy integral value array is less than zero, it is judged as an inside-zone fault; otherwise, it is judged as an outside-zone fault.

[0029] Based on the same inventive concept, the present invention also provides a low-current grounding fault identification system, comprising:

[0030] The fault judgment module is used to sample the zero-sequence voltage and zero-sequence current of the distribution line at a set sampling frequency and perform fault start judgment;

[0031] The data extension module is used to cache two-cycle sampling data of zero-sequence voltage and zero-sequence current after startup and extend them into zero-sequence voltage array and zero-sequence current array with dimensions that are integer multiples of the number of two-cycle points when a small current grounding fault occurs;

[0032] An active power calculation module is used to calculate the instantaneous value of transient zero-sequence active power using the extended zero-sequence voltage array and the zero-sequence current array to form an instantaneous value array of transient zero-sequence active power;

[0033] A straightening low-pass filtering module is used to perform low-pass filtering on the transient zero-sequence active power instantaneous value array using a Hamming window FIR low-pass digital filter to obtain a filtered transient zero-sequence active power instantaneous value array;

[0034] An active energy extraction module is used to accumulate the transient zero-sequence active power instantaneous values of other points in the filtered transient zero-sequence active power instantaneous value array except the first two cycles to obtain the transient zero-sequence full-wave active energy integral value of each point, and form an active energy integral value array;

[0035] The internal and external fault identification module is used to identify the internal and external faults of the small current grounding fault according to the active energy integral value array.

[0036] Based on the same inventive concept, the present invention further provides a readable storage medium having a computer program stored thereon, which can implement the above-mentioned low-current grounding fault identification method when executed.

[0037] In the small current grounding fault identification method, system, and readable storage medium provided by the present invention, by sampling the zero-sequence voltage and zero-sequence current and extending the two-cycle signals after startup, the transient zero-sequence active power instantaneous value is calculated, and a Hamming window FIR low-pass digital filter is designed for low-pass filtering. Finally, the transient zero-sequence active power instantaneous values of other points in the filtered transient zero-sequence active power instantaneous value array except for the first two cycles are selectively accumulated to obtain an active energy integral value array composed of the transient zero-sequence full-wave active energy integral values of each point for in-zone and out-of-zone identification of small current grounding faults, thereby greatly improving the accuracy of small current grounding fault identification using the transient energy method, and having at least one of the following beneficial effects:

[0038] 1) The signal extension method is used to solve the endpoint effect caused by conventional digital filtering, and the time window of active energy integration can be lengthened when the resistive component of the zero-sequence current is small, which is conducive to amplifying the transient energy signal and improving the sensitivity of identifying inside and outside the small current grounding fault zone;

[0039] 2) A Hamming window FIR low-pass filter is designed to eliminate the double frequency component in the instantaneous value of active power, retaining only the DC component. This avoids the influence of all transient high-frequency components on the accurate value of zero-sequence active energy extraction and improves the reliability of identification inside and outside the small current grounding fault zone;

[0040] 3) When extracting the transient active energy by accumulating and integrating the instantaneous value of the filtered active power, the distortion of the filtered signal caused by the first convolution window length caused by the FIR filter endpoint effect and the initial phase of the zero-sequence voltage and zero-sequence current is avoided. At the same time, the superposition of the initial values of the transient active energy formed by different initial phases of the fault on the final calculation result is cleverly eliminated, thereby improving the selectivity of identifying inside and outside the small current grounding fault zone;

[0041] 4) The transient zero-sequence full-wave active energy algorithm proposed in this invention fundamentally eliminates the influence of the proportion of the zero-sequence current resistive component, the initial phase of the fault, the active power double frequency component and the energy integration period on the extraction accuracy, greatly improving the accuracy of identification inside and outside the small current grounding fault zone, and can adapt to the single-phase grounding fault identification with different neutral point grounding methods, different transition resistances and fault closing angles. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Those skilled in the art will appreciate that the accompanying drawings are provided for a better understanding of the present invention and do not constitute any limitation on the scope of the present invention.

[0043] Figure 1 A flowchart of a method for identifying a small current ground fault provided by an embodiment of the present invention;

[0044] Figure 2 A schematic diagram of the amplitude-frequency characteristics of an N-order Hamming window FIR low-pass filter provided in an embodiment of the present invention;

[0045] Figure 3 A graph showing transient fault active power and active energy within the conventional algorithm area provided by an embodiment of the present invention;

[0046] Figure 4 A graph showing the transient active power and active energy of a fault within the algorithm area of the present invention provided by an embodiment of the present invention;

[0047] Figure 5 A graph showing transient active power and active energy for out-of-area faults using a conventional algorithm according to an embodiment of the present invention;

[0048] Figure 6 A graph showing transient active power and active energy of faults outside the algorithm area provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0049] In order to make the purpose, advantages and features of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the drawings are in a very simplified form and use non-precise proportions, which are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention. In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, please refer to the accompanying drawings. It should be noted that the structures, proportions, sizes, etc. illustrated in the drawings of this specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the conditions for the implementation of the present invention. Any modification of the structure, change in the proportional relationship or adjustment of the size, under the condition that the effect produced by the present invention and the purpose that can be achieved are the same or similar, should still fall within the scope of the technical content disclosed by the present invention.

[0050] As used herein, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. As used herein, the term "or" is generally used in a sense including "and / or" unless the context clearly dictates otherwise.

[0051] Reference Figure 1 This embodiment provides a method for identifying a low-current ground fault, comprising the following steps:

[0052] S1. Sample the zero-sequence voltage and zero-sequence current of the distribution line at the set sampling frequency and make fault start judgment;

[0053] S2. When a small current ground fault occurs, two-cycle sampling data of zero-sequence voltage and zero-sequence current after startup are cached and extended into zero-sequence voltage array and zero-sequence current array with a dimension that is an integer multiple of the number of two-cycle points;

[0054] S3. Calculating the instantaneous value of transient zero-sequence active power using the zero-sequence voltage array and the zero-sequence current array to form an array of transient zero-sequence active power instantaneous values;

[0055] S4. Low-pass filtering the transient zero-sequence active power instantaneous value array using a Hamming window FIR low-pass digital filter to obtain a filtered transient zero-sequence active power instantaneous value array;

[0056] S5. Accumulating the transient zero-sequence active power instantaneous values of the points other than the first two cycles in the filtered transient zero-sequence active power instantaneous value array to obtain the transient zero-sequence full-wave active energy integral value of each point, and forming an active energy integral value array;

[0057] S6. Identify the small current grounding fault as being within or outside the zone according to the active energy integral value array.

[0058] This embodiment extracts the precise transient zero-sequence full-wave active energy after a small current grounding fault occurs by extending the fault transient signal, calculating the instantaneous value of the transient zero-sequence active power, performing FIR straightening low-pass digital filtering, and performing energy integration calculations over a specific time period. This is used for in-zone and out-of-zone fault identification, thereby improving the accuracy of small current grounding fault identification using the transient energy method.

[0059] First, execute S1 to sample the zero-sequence voltage and zero-sequence current of the distribution line at the set sampling frequency and perform fault start judgment.

[0060] Preferably, the set sampling frequency fs is between 3200Hz and 12800Hz. In this embodiment, the distribution terminal samples the zero-sequence voltage U0 and zero-current I0 of the distribution line at a sampling frequency of 6400Hz. When performing fault startup judgment, the startup time of the small current grounding fault is obtained according to the startup criterion of the zero-sequence voltage U0 sudden change. The startup criterion of the zero-sequence voltage U0 sudden change is as follows:

[0061] |U0[k]-U0[k-2N]|>U 0set

[0062] Among them, k is the current sampling point number, k-2N is the sampling point number two cycles ago, N=fs / 50 is the number of sampling points per cycle, U 0set It is the zero-sequence voltage starting threshold, and its value range is 5~50V.

[0063] Then, S2 is executed. When a low-current ground fault occurs, the two-cycle sampling data of the zero-sequence voltage and zero-sequence current after startup is cached and extended to form a zero-sequence voltage array and a zero-sequence current array with a dimension of M, where M is an integer multiple of the number of two-cycle points. The signal extension method is used to address the endpoint effect caused by conventional digital filtering and to extend the time window for active energy integration when the resistive component of the zero-sequence current is small. This helps amplify the transient energy signal and improves the sensitivity of identifying inside and outside the low-current ground fault zone.

[0064] In this embodiment, after a low-current ground fault occurs, two cycles of transient sampling data of the zero-sequence voltage U0 and zero-sequence current I0 after startup are cached to form an initial zero-sequence voltage array U0[] and an initial zero-sequence current array I0[] with an initial dimension of 2N. The initial zero-sequence voltage array U0[] and the initial zero-sequence current array I0[] are then extended to form a zero-sequence voltage array and a zero-sequence current array with a dimension of M in the following manner:

[0065] M=2N*p,p≥2

[0066] U0[i+2N*l]=U0[i]

[0067] I0[i+2N*l]=I0[i]

[0068] i=0,1,…,2N-1; l=1,…,p

[0069] Where p is a multiple of the data length after extension.

[0070] Then, S3 is executed to calculate the transient zero-sequence active power instantaneous value P0 using the extended zero-sequence voltage array and the zero-sequence current array to form a transient zero-sequence active power instantaneous value array P0[] with a dimension of M. The formula is as follows:

[0071] P0[j]=U0[j]*I0[j]

[0072] j=0,1,…,M-1

[0073] Then, S4 is executed to perform low-pass filtering on the transient zero-sequence active power instantaneous value array using a Hamming window FIR low-pass digital filter to obtain a filtered transient zero-sequence active power instantaneous value array P′0[]. In this embodiment, the transient zero-sequence active power instantaneous value array P0[] is low-pass filtered using an N-order Hamming window FIR low-pass digital filter to filter out all components above 1 Hz, which include the double frequency component of the active power. In this embodiment, by designing the Hamming window FIR low-pass filter, the double frequency component in the instantaneous value of the active power is eliminated, and only the DC component is retained, thereby avoiding the influence of all transient high-frequency components on the extraction of the precise value of the zero-sequence active energy, and improving the accuracy of identification inside and outside the small current grounding fault zone.

[0074] The transfer function hd[] of an N-order Hamming window FIR low-pass filter with a cutoff frequency of 1 Hz can be designed using Matlab tools. Then, the transfer function hd[] of the Hamming window FIR low-pass filter is used to perform straightening and low-pass filtering on the transient zero-sequence active power instantaneous value array P0[] to obtain the filtered transient zero-sequence active power instantaneous value array P′0[]. The calculation formula is P′0=P0*hd, where * represents the convolution operation of two one-dimensional arrays, that is, the conventional algorithm of the FIR digital filter, and the dimension of the output signal P′0 is equal to the dimension of the input signal P0.

[0075] The order of the Hamming window FIR low-pass digital filter is the same as the number of sampling points per cycle. In this embodiment, the number of sampling points per cycle is 128, so the order of the Hamming window FIR low-pass filter is 128.

[0076] Then, S5 is executed to accumulate the transient zero-sequence active power instantaneous values of other points in the filtered transient zero-sequence active power instantaneous value array except the first two cycles to obtain the transient zero-sequence full-wave active energy integral value of each point, thereby forming an active energy integral value array.

[0077] In this embodiment, the transient zero-sequence active power instantaneous value of each point between 2N and (M-1) in the filtered transient zero-sequence active power instantaneous value array P′0[] is accumulated to obtain the transient zero-sequence full-wave active energy integral value of each point, forming an M-2N-dimensional active energy integral value array W0[] as follows:

[0078] W0[0]=P0′[2N]

[0079] W0[j]=W0[j-1]+P0′[2N+j]

[0080] j=1,2,…,M-2N-1

[0081] It should be noted that when extracting the transient zero-sequence full-wave active energy by accumulating and integrating the instantaneous value of the filtered transient zero-sequence active power, the purpose of not accumulating the first two-wave data between 0 and (2N-1) of the array P′0[] is to avoid the distortion of the filtered signal of the first convolution window length caused by the endpoint effect of the FIR filter and the initial phase of the zero-sequence voltage U0 and the zero-flow current I0. At the same time, it cleverly eliminates the superposition of the initial values of the transient active energy formed by different initial phases of the fault to the final calculation result, thereby improving the accuracy of identification inside and outside the small current grounding fault zone.

[0082] Finally, S6 is executed to identify the small current grounding fault within or outside the zone based on the active energy integral value array W0[]. If the transient zero-sequence full-wave active energy integral value of any point in the active energy integral value array W0[] is less than zero, it is judged as an internal fault; otherwise, it is judged as an external fault.

[0083] In this embodiment, the small current grounding fault can be identified as inside or outside the zone according to the sign of the W0[j] value: when W0[j]<0 at any point, it is judged as an inside fault, otherwise it is judged as an outside fault.

[0084] The technical concept of the present invention is further illustrated below through a specific example.

[0085] First, the transient signals of zero-sequence voltage and zero-sequence current within two cycles after the occurrence of a small current grounding fault are constructed as follows:

[0086]

[0087] Wherein, f is the signal frequency, and f=50Hz during the test; f s is the sampling frequency, take fs =6400Hz; is the initial phase of the zero-sequence voltage U0, is the initial phase of the zero-sequence current I0; is the angle at which the zero-sequence voltage U0 leads the zero-sequence current I0 during internal and external faults. -0.005π represents that the resistive component of the zero-sequence current is very small; j = 0, 1, ..., 2N-1 indicates that the length of the transient signal of the two-cycle is 2N = 256.

[0088] The U0 and I0 signals are extended to 5 times 10 cycles according to the method proposed in the present invention to form U0[] and I0[] arrays respectively, with a dimension of M=2N*5=1280.

[0089] Then calculate the instantaneous value of transient zero-sequence active power according to the following formula:

[0090]

[0091] j=1,2,…,M-1

[0092] The array P0[] is written as a continuous signal:

[0093]

[0094] The theoretical value of active power It corresponds exactly to the first term on the right side of the above formula. The second term on the right side of the above formula is the double frequency component. Its existence has a certain influence on the integral value of the transient active energy. The analysis is as follows:

[0095] By integrating P0(t), we can get the transient zero-sequence full-wave active energy W0(t), which is expressed as follows:

[0096]

[0097] It can be seen that the integral value of the transient zero-sequence full-wave active energy at any moment includes the following three items:

[0098] Item 1 is the theoretical active energy value, which is the value that needs to be accurately extracted;

[0099] Item 2 It is a double frequency component and needs to be filtered out by low-pass filtering;

[0100] Item 3 is the initial value, when There is an initial value of energy.

[0101] when and When they are close, it means that the resistive component of the zero-sequence current is very small. The influence of the latter two parts on the integral value of the active energy cannot be ignored. The error of the transient zero-sequence full-wave active energy calculated directly according to the above formula is large, which can easily lead to misjudgment of faults inside and outside the small current grounding fault area.

[0102] In this example, we take Used to simulate faults within the area, when W0<0, due to At this time, the first negative value of the integral value of the active energy is close to the minimum, and the third positive value is close to the maximum. The conventional transient energy method is most likely to misjudge it as an out-of-zone fault; When it is used to simulate the fault outside the zone, W0>0, and at this time, The first positive value of the integral value of the active energy is close to the minimum, and the third negative value is close to the maximum. The conventional transient energy method is most likely to misjudge it as an internal fault.

[0103] The following conventional algorithms are used The method proposed by the present invention is used to calculate and extract the transient zero-sequence full-wave active energy of the fault simulation signals inside and outside the above-mentioned area to verify the superiority of the method of the present invention.

[0104] The amplitude-frequency characteristics of the N-order Hamming window low-pass digital filter with a cutoff frequency of 1 Hz designed according to the present invention are as follows: Figure 2 As shown, the filter has varying degrees of suppression on various frequency signals, except for DC, and in particular, almost completely filters out the 100 Hz double frequency component. The present invention uses the Hamming window function because both the Hamming window and the Hanning window have wider main lobes, smaller side lobes, and faster decay rates. Both are commonly used window functions, but the Hamming window has faster side lobe decay than the Hanning window, which means it can reduce spectral leakage.

[0105] When taking When simulating faults within the area, the time domain expressions of the zero-sequence voltage and zero-sequence current signals are:

[0106] u0=10*sin(2*pi*50*t+pi / 2)

[0107] i0=sin(2*pi*50*t-0.005*pi)

[0108] Among them, in Matlab, the constant pi = π, and the value range of t is 0~0.2s, which is used to simulate the extension of the two-cycle signal after the fault into a 10-cycle signal.

[0109] The instantaneous active power P0(t)=U0(t)I0(t) curve and instantaneous active energy calculated using conventional algorithms The curves are as follows Figure 3As shown in the figure, it can be seen that the P0 signal contains a double frequency component, where the bit line is close to 0, indicating that the active power at this time is very small; the W0 signal also contains a double frequency component, and its envelope shows a downward trend, indicating that the active energy value of this line is negative, but there is still a large part of the time period above the red line of W0=0. When the data window length of the energy integration in the transient process is short or non-integer cycle, or when the sampling signal error is large, it is easy to cause the extracted transient active energy value W0>0, thereby misjudging it as an out-of-zone fault.

[0110] By adopting the algorithm proposed by the present invention, the transient zero-sequence active power instantaneous value array P0[] is formed into the transient zero-sequence active power instantaneous value array P′0[] after being digitally filtered by the Hamming window FIR low-pass filter. The transient zero-sequence full-wave active energy W1 curve obtained by accumulating P′0[] from 0 and the transient zero-sequence full-wave active energy W2 curve obtained by accumulating P′0[] from point 2N are shown as follows: Figure 4 As shown. It can be seen that, except for the first cycle N point where the P′0 signal has a filter signal distortion phenomenon caused by the FIR filter endpoint effect and the initial phase of U0 and I0, all the double frequency components and the initial value of the transient active energy formed by the initial phase are completely filtered out in the subsequent period, and only the DC component representing the theoretical value of the active power is retained. The W1 signal always satisfies W1>0 due to the integration of the data of the distortion segment of the array P′0[]. The W2 signal avoids the data of the distortion segment of the array P′0[], so that the overall curve satisfies W2<0 and shows a linear downward trend with the integration time. The present invention uses the criterion of the transient zero-sequence full-wave active energy integral value W0[j]<0 at any point between 2N and (M-1) to ensure that it is always satisfied when a small current grounding fault occurs in the area under any circumstances, thereby improving the sensitivity, reliability and selectivity of the identification of small current grounding faults in the area.

[0111] When taking When simulating out-of-zone faults, the time domain expressions of the zero-sequence voltage and zero-sequence current signals are:

[0112] u0=10*sin(2*pi*50*t-pi / 2)

[0113] i0=sin(2*pi*50*t-0.005*pi)

[0114] Among them, in Matlab, the constant pi = π, and the value range of t is 0~0.2s, which is used to simulate the extension of the two-cycle signal after the fault into a 10-cycle signal.

[0115] The instantaneous active power P0(U)=U0(t)I0(t) curve and instantaneous active energy calculated using conventional algorithms The curves are as follows Figure 5As shown in the figure, it can be seen that the P0 signal contains a double frequency component, where the bit line is close to 0, indicating that the active power at this time is very small; the W0 signal also contains a double frequency component, and its envelope shows an upward trend, indicating that the active energy value of this line is positive, but there is still a large part of the time period below the red line of W0=0. When the data window length of the energy integration in the transient process is short or non-integer cycle, or when the sampling signal error is large, it is easy to cause the extracted transient active energy value W0<0, thereby misjudging it as an internal fault.

[0116] By adopting the algorithm proposed by the present invention, the transient zero-sequence active power instantaneous value array P0[] is formed into the transient zero-sequence active power instantaneous value array P′0[] after being digitally filtered by the Hamming window FIR low-pass filter. The transient active energy W1 curve obtained by accumulating P′0[] from 0 and the transient active energy W2 curve obtained by accumulating P′0[] from 2N are shown as follows: Figure 6 As shown, it can be seen that except for the first cycle N point where the P′0 signal has a filter signal distortion phenomenon caused by the FIR filter endpoint effect and the initial phase of U0 and I0, all the double frequency components and the initial value of the transient active energy formed by the initial phase are basically filtered out in the subsequent period, and only the DC component representing the theoretical value of the active power is retained. The W1 signal always satisfies W1<0 due to the addition of the integral of the distorted segment data of the array P′0[]. The W2 signal avoids the data of the distorted segment of the array P′[], so that the overall curve satisfies W2>0 and shows a linear upward trend with the integration time. The present invention uses the criterion of the transient zero-sequence full-wave active energy integral value W0[j]>0 at any point between 2N and (M-1) to ensure that it is always satisfied when a small current grounding fault occurs outside the zone under any circumstances, thereby improving the sensitivity, reliability and selectivity of the identification of small current grounding faults outside the zone.

[0117] Based on the same inventive concept, an embodiment of the present invention further provides a low-current grounding fault identification system, comprising:

[0118] The fault judgment module is used to sample the zero-sequence voltage and zero-sequence current of the distribution line at a set sampling frequency and perform fault start judgment;

[0119] The data extension module is used to cache two-cycle sampling data of zero-sequence voltage and zero-sequence current after startup and extend them into zero-sequence voltage array and zero-sequence current array with dimensions that are integer multiples of the number of two-cycle points when a small current grounding fault occurs;

[0120] An active power calculation module is used to calculate the instantaneous value of transient zero-sequence active power using the extended zero-sequence voltage array and the zero-sequence current array to form an instantaneous value array of transient zero-sequence active power;

[0121] A straightening low-pass filtering module is used to perform low-pass filtering on the transient zero-sequence active power instantaneous value array using a Hamming window FIR low-pass digital filter to obtain a filtered transient zero-sequence active power instantaneous value array;

[0122] An active energy extraction module is used to accumulate the transient zero-sequence active power instantaneous values of other points in the filtered transient zero-sequence active power instantaneous value array except the first two cycles to obtain the transient zero-sequence full-wave active energy integral value of each point, and form an active energy integral value array;

[0123] The internal and external fault identification module is used to identify the internal and external faults of the small current grounding fault according to the active energy integral value array.

[0124] Since the low-current grounding fault identification system provided by the embodiment of the present invention and the low-current grounding fault identification method described above belong to the same inventive concept, the low-current grounding fault identification system provided by the present invention has all the advantages of the low-current grounding fault identification method described above. Therefore, the beneficial effects of the low-current grounding fault identification system provided by the present invention will not be described one by one here.

[0125] Based on the same inventive concept, an embodiment of the present invention further provides a readable storage medium having a computer program stored thereon. When the computer program is executed, the method for identifying a small current grounding fault as described above can be implemented.

[0126] Readable storage medium can be the tangible device that can keep and store the instruction used by instruction execution device, for example, can be but not limited to electric storage device, magnetic storage device, optical storage device, electromagnetic storage device, semiconductor storage device or above-mentioned any suitable combination.The more specific example (non-exhaustive list) of readable storage medium comprises: portable computer disk, hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital versatile disk (DVD), memory stick, floppy disk, mechanical coding device, for example, punch card or the convex structure in the groove that stores instruction thereon and above-mentioned any suitable combination.Computer program described herein can be downloaded to various computing / processing equipment from readable storage medium, or is downloaded to external computer or external storage device by network, for example Internet, local area network, wide area network and / or wireless network.Network can comprise copper transmission cable, optical fiber transmission, wireless transmission, router, firewall, switch, gateway computer and / or edge server. The network adapter card or network interface in each computing / processing device receives the computer program from the network and forwards the computer program for storage in a readable storage medium in the various computing / processing devices. The computer program for performing the operations of the present invention can be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, state setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages such as Smalltalk, C++, etc., and conventional procedural programming languages such as "C" or similar programming languages. The computer program can be executed entirely on the user's computer, partially on the user's computer, as a separate software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In the case of a remote computer, the remote computer can be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or can be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, various aspects of the present invention are implemented by utilizing state information of a computer program to personalize an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA), which can execute computer-readable program instructions.

[0127] Various aspects of the present invention are described herein with reference to flowcharts and / or block diagrams of methods, systems, and computer program products according to embodiments of the present invention. It should be understood that each block of the flowcharts and / or block diagrams, as well as combinations of blocks in the flowcharts and / or block diagrams, can be implemented by computer programs. These computer programs can be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing device, thereby producing a machine such that when these programs are executed by the processor of the computer or other programmable data processing device, a device is generated that implements the functions / actions specified in one or more blocks in the flowcharts and / or block diagrams. These computer programs can also be stored in a readable storage medium that causes the computer, programmable data processing device, and / or other device to operate in a specific manner, such that the readable storage medium storing the computer program comprises an article of manufacture that includes instructions for implementing various aspects of the functions / actions specified in one or more blocks in the flowcharts and / or block diagrams.

[0128] The computer program may also be loaded onto a computer, other programmable data processing apparatus, or other device so that a series of operational steps are performed on the computer, other programmable data processing apparatus, or other device to produce a computer-implemented process, thereby causing the computer program executed on the computer, other programmable data processing apparatus, or other device to implement the functions / actions specified in one or more blocks in the flowchart and / or block diagram.

[0129] Since the readable storage medium provided in the embodiment of the present invention and the small current grounding fault identification method described above belong to the same inventive concept, the readable storage medium provided by the present invention has all the advantages of the small current grounding fault identification method described above. Therefore, the beneficial effects of the readable storage medium provided by the present invention will not be described in detail here.

[0130] In summary, the embodiments of the present invention provide a method, system, and readable storage medium for identifying a small current grounding fault. By sampling the zero-sequence voltage and zero-sequence current and extending the two-wave signals after startup, the instantaneous value of the transient zero-sequence active power is calculated. An N-order Hamming window FIR low-pass digital filter is then designed for low-pass filtering. Finally, the transient zero-sequence full-wave active energy integral value for a specific period is extracted through accumulation to form an active energy integral value array for identifying small current grounding faults within and outside the zone. This method greatly improves the accuracy of identifying small current grounding faults using the transient energy method. Its beneficial effects include:

[0131] 1) The signal extension method is used to solve the endpoint effect caused by conventional digital filtering, and the time window of active energy integration can be lengthened when the resistive component of the zero-sequence current is small, which is conducive to amplifying the transient energy signal and improving the sensitivity of identifying inside and outside the small current grounding fault zone;

[0132] 2) A Hamming window FIR low-pass filter is designed to eliminate the double frequency component in the instantaneous value of active power, retaining only the DC component. This avoids the influence of all transient high-frequency components on the accurate value of zero-sequence active energy extraction and improves the reliability of identification inside and outside the small current grounding fault zone;

[0133] 3) When extracting the transient active energy by accumulating and integrating the instantaneous value of the filtered active power, the distortion of the filtered signal caused by the first convolution window length caused by the FIR filter endpoint effect and the initial phase of the zero-sequence voltage and zero-sequence current is avoided. At the same time, the superposition of the initial values of the transient active energy formed by different initial phases of the fault on the final calculation result is cleverly eliminated, thereby improving the selectivity of identifying inside and outside the small current grounding fault zone;

[0134] 4) The transient zero-sequence full-wave active energy algorithm proposed in this invention fundamentally eliminates the influence of the proportion of the zero-sequence current resistive component, the initial phase of the fault, the active power double frequency component and the energy integration period on the extraction accuracy, greatly improving the accuracy of identification inside and outside the small current grounding fault zone, and can adapt to the single-phase grounding fault identification with different neutral point grounding methods, different transition resistances and fault closing angles.

[0135] Furthermore, it should be recognized that although the present invention has been disclosed above with reference to preferred embodiments, the above embodiments are not intended to limit the present invention. Any person skilled in the art can utilize the above disclosed technical content to make many possible changes and modifications to the technical solution of the present invention, or modify it into equivalent embodiments with equivalent variations, without departing from the scope of the technical solution of the present invention. Therefore, any simple modifications, equivalent variations, and modifications made to the above embodiments based on the technical essence of the present invention, without departing from the content of the technical solution of the present invention, shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for identifying a small current grounding fault, characterized in that: The following steps are involved: Sample the zero-sequence voltage and zero-sequence current of the distribution line according to the set sampling frequency and make fault start judgment; When a small current ground fault occurs, two-cycle sampling data of zero-sequence voltage and zero-sequence current after startup are cached and extended into zero-sequence voltage array and zero-sequence current array with a dimension that is an integer multiple of the number of two-cycle points; Calculating the transient zero-sequence active power instantaneous value using the extended zero-sequence voltage array and the zero-sequence current array to form a transient zero-sequence active power instantaneous value array; Performing low-pass filtering on the transient zero-sequence active power instantaneous value array using a Hamming window FIR low-pass digital filter to obtain a filtered transient zero-sequence active power instantaneous value array; Accumulating the transient zero-sequence active power instantaneous values of other points in the filtered transient zero-sequence active power instantaneous value array except for the first two cycles to obtain the transient zero-sequence full-wave active energy integral value of each point, and forming an active energy integral value array; The small current grounding fault is identified as being within or outside a zone according to the active energy integral value array.

2. The method for identifying a small current grounding fault according to claim 1, wherein: The set sampling frequency is between 3200 Hz and 12800 Hz.

3. The method for identifying a small current grounding fault according to claim 1, wherein: When making fault startup judgment, the startup time of the small current grounding fault is obtained according to the startup judgment criterion of the zero-sequence voltage mutation.

4. The method for identifying a small current grounding fault according to claim 1, wherein: When a small current grounding fault occurs, the transient sampling data of the zero-sequence voltage U0 and the zero-sequence current I0 after startup are cached for two cycles each, forming an initial zero-sequence voltage array U0[] and an initial zero-sequence current array I0[] with an initial dimension of 2N, where N is the number of sampling points per cycle.

5. The method for identifying a small current grounding fault according to claim 4, characterized in that: The initial zero-sequence voltage array U0[] and the initial zero-sequence current array I0[] are extended into a zero-sequence voltage array and a zero-sequence current array with a dimension of M in the following manner: M=2N*p,p≥2 U0[i+2N*l]=U0[i] I0[i+2N*l]=I0[i] i=0,1,…,2N-1; l=1,…,p Where p is a multiple of the data length after extension.

6. The method for identifying a small current grounding fault according to claim 5, characterized in that: The transient zero-sequence active power instantaneous value P0 is calculated using the extended zero-sequence voltage array and the zero-sequence current array to form a transient zero-sequence active power instantaneous value array P0[] with a dimension of M. The formula is as follows: P0[j]=U0[j]*I0[j] j=0,1,…,M-1.

7. The method for identifying a small current grounding fault according to claim 6, characterized in that: The transient zero-sequence active power instantaneous value array P0[] is low-pass filtered using an N-order Hamming window FIR low-pass digital filter to remove all components above 1 Hz, including the double frequency component of the active power, and obtain the filtered transient zero-sequence active power instantaneous value array P0 ′ [], wherein the order of the Hamming window FIR low-pass digital filter is the same as the number of sampling points per cycle; The instantaneous value array P0 of the transient zero-sequence active power after filtering ′ The instantaneous values of the transient zero-sequence active power of each point between 2N and (M-1) in [] are accumulated to obtain the transient zero-sequence full-wave active energy integral value of each point, forming an M-2N dimensional active energy integral value array W0[].

8. The method for identifying a small current grounding fault according to claim 1, wherein: The small current grounding fault is identified as being within the zone according to the active energy integral value array. If the transient zero-sequence full-wave active energy integral value of any point in the active energy integral value array is less than zero, it is determined to be an within-zone fault; otherwise, it is determined to be an out-of-zone fault.

9. A small current ground fault identification system, characterized in that: include: The fault judgment module is used to sample the zero-sequence voltage and zero-sequence current of the distribution line at a set sampling frequency and perform fault start judgment; The data extension module is used to cache two-cycle sampling data of zero-sequence voltage and zero-sequence current after startup and extend them into zero-sequence voltage array and zero-sequence current array with dimensions that are integer multiples of the number of two-cycle points when a small current grounding fault occurs; An active power calculation module is used to calculate the instantaneous value of transient zero-sequence active power using the extended zero-sequence voltage array and the zero-sequence current array to form an instantaneous value array of transient zero-sequence active power; A straightening low-pass filtering module is used to perform low-pass filtering on the transient zero-sequence active power instantaneous value array using a Hamming window FIR low-pass digital filter to obtain a filtered transient zero-sequence active power instantaneous value array; An active energy extraction module is used to accumulate the transient zero-sequence active power instantaneous values of other points in the filtered transient zero-sequence active power instantaneous value array except the first two cycles to obtain the transient zero-sequence full-wave active energy integral value of each point, and form an active energy integral value array; The internal and external fault identification module is used to identify the internal and external faults of the small current grounding fault according to the active energy integral value array.

10. A readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed, the method for identifying a small current grounding fault according to any one of claims 1 to 8 can be implemented.

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