Resonant grounding system single-phase grounding fault line selection method based on resonance peak ratio

Abstract

The invention relates to a resonance peak ratio-based single-phase earth fault line selection method for a resonance grounding system, and belongs to the technical field of power system relay protection. The method comprises the following steps of: calculating zero-sequence current data within one tenth of a fundamental frequency period after a fault starting moment of each line; performing Fourier transform on the zero-sequence current of each line to obtain a spectrogram; for each line, determining a resonance peak and a resonance peak in the spectrogram by using a local peak detection algorithm; calculating the harmonic peak coefficient of each line and the average slope of two maximum harmonic peaks before and after the boundary; the resonance peak value ratio of each line is obtained; and finally, determining a fault line by judging whether the resonance peak value ratio meets the criterion or not. The key point is to detect the significant difference between the harmonic peak coefficient after the fault and the average slope of the two maximum harmonic peaks, and the method has the advantages of strong adaptability, high line selection accuracy, easy engineering realization and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of relay protection in power systems, and particularly relates to a single-phase grounding fault line selection method for resonant grounding systems based on the resonant peak ratio. Background Art

[0002] With the continuous development of power system technology and the increasing scale of the distribution network, ensuring the stable operation of the distribution network is crucial for maintaining the power supply capacity of the power system. Relevant data shows that approximately 85% of power outages in the power system are caused by faults in the distribution network. During the operation of the distribution network, as a frequently occurring fault, the rapid and accurate detection and location of single-phase grounding faults are extremely important for improving power supply reliability and system stability.

[0003] In recent years, experts and scholars at home and abroad have carried out extensive and in-depth research on the problem of single-phase grounding fault line selection in resonant grounding systems. Relevant technologies and research results emerge in an endless stream, mainly divided into line selection methods based on steady-state components and line selection methods such as the first half-wave method, wavelet analysis method, and transient traveling wave method based on transient components. The line selection method based on steady-state components relies on the steady-state current signal after the fault occurs. However, in the case of low-resistance grounding faults, the steady-state current signal may be weak, resulting in inaccurate line selection. At the same time, different system operation modes may lead to different steady-state current distributions, thus affecting the line selection result. The first half-wave method uses the transient current signal within the first cycle after the fault occurs for line selection. However, since the first half-wave method relies on the initial transient signal after the fault occurs, these signals may contain a large amount of high-frequency noise, which in turn affects the accuracy of line selection. The wavelet analysis method judges the fault line by performing wavelet transform on the fault signal and extracting fault features. However, different wavelet basis functions may lead to different line selection results, and selecting the appropriate wavelet basis function is a challenge. The transient traveling wave method uses the transient traveling wave signal after the fault occurs for line selection. However, the transient traveling wave signal usually contains a large amount of high-frequency noise, which may interfere with the line selection result.

[0004] In view of this, the current fault line selection technology applied to resonant grounding systems still has many deficiencies and has not been fully perfected. The existing fault line selection methods are difficult to accurately identify the fault line, resulting in a low correct rate of selecting the fault line and unable to meet the actual application requirements of high efficiency and reliability. Especially when dealing with single-phase grounding faults in resonant grounding systems, the existing technical means still face great challenges, and the line selection effect needs to be further improved.

[0005] Therefore, it is urgent for those skilled in the art to develop a single-phase grounding fault line selection method for resonant grounding systems based on the resonant peak ratio. Summary of the Invention

[0006] The present invention aims to solve the defects and deficiencies in the fault line selection of the existing resonant grounding system, and proposes a single-phase grounding fault line selection method for the resonant grounding system based on the resonant peak ratio.

[0007] The single-phase grounding fault line selection method for the resonant grounding system based on the resonant peak ratio according to the present invention is realized through the following technical solutions:

[0008] Step 1: Monitor the zero-sequence voltage of the resonant grounding system in real time, and judge whether a single-phase grounding fault occurs in the system according to whether the zero-sequence voltage is greater than the setting value; if the zero-sequence voltage is greater than the setting value, it is determined that a single-phase grounding fault has occurred in the system, start the line selection program, and enter Step 2;

[0009] Step 2: Calculate the zero-sequence current data within one-tenth of a system fundamental frequency period after the fault starting moment of each line;

[0010] Step 3: Perform Fourier transform on the zero-sequence current of each line to obtain the corresponding frequency spectrum diagram;

[0011] Step 4: For each line, determine the resonant peak and resonant peak value in the frequency spectrum diagram by the local peak detection algorithm; take 2000 Hz as the boundary, find the largest resonant peak before the boundary, and find the largest resonant peak after the boundary;

[0012] Step 5: Calculate the resonant peak coefficient α through Equation (1);

[0013] α = εΔf (1)

[0014] In the formula, Δf is the set step parameter, and the value of Δf is 500 Hz; ε is the adjustment coefficient, and the value range of ε is 1 to 3;

[0015] Step 6: Calculate the average slopes P1 and P2 of the two largest resonant peaks before and after the boundary;

[0016] Step 7: Calculate the resonant peak ratio H of the line through Equation (3);

[0017]

[0018] For each line connected to the system bus, the resonant peak ratio of the line can be calculated through Equation (3);

[0019] Step 8: If the resonant peak ratio of the i-th line satisfies the criterion shown in Equation (4), it is determined that the line is a fault line; otherwise, it is determined that the line is a sound line;

[0020] H (i) <0.5min{H (k)} (4)

[0021] In the formula, H(i) , H (k) respectively represent the resonance peak ratios of the i-th and k-th lines, where i and k are line numbers, i, k = 1, 2…, n, i ≠ k, and n is the total number of lines connected to the system bus.

[0022] For the single-phase grounding fault line selection method of the resonant grounding system based on the resonance peak ratio as described above, preferably,

[0023] Specifically, step 4 is as follows: For each line, the resonance peaks and resonance peak values in the spectrogram are determined by the local peak detection algorithm; taking 2000 Hz as the boundary, find the maximum resonance peak before the boundary, record its abscissa as the frequency point f1, and record its resonance peak value as F1; find the maximum resonance peak after the boundary, record its abscissa as the frequency point f2, and record its resonance peak value as F2;

[0024] Specifically, step 6 is as follows: Calculate the average slopes P1 and P2 of the two maximum resonance peaks before and after the boundary through Equation (2);

[0025]

[0026] In the formula, S(f1), S(f1 - Δf), and S(f1 + Δf) are the ordinate values corresponding to the frequency points f1, f1 - Δf, and f1 + Δf in the abscissa of the spectrogram respectively; S(f2), S(f2 - Δf), and S(f2 + Δf) are the ordinate values corresponding to the frequency points f2, f2 - Δf, and f2 + Δf in the abscissa of the spectrogram respectively.

[0027] For the single-phase grounding fault line selection method of the resonant grounding system based on the resonance peak ratio as described above, preferably, the calculation method of the zero-sequence current data within one-tenth of a system fundamental frequency period after the starting moment of the line fault in step 2 is as follows:

[0028] 1) Extract the zero-sequence current sampling data between the moment t and the moment t + 0.1T, where t is the starting moment of the fault and T is the system fundamental frequency period;

[0029] 2) Calculate the zero-sequence current of each line through Equation (5):

[0030]

[0031] In the formula, I0 is the zero-sequence current of the line; I a , I b , I c are the zero-sequence current sampling data of phases a, b, and c of the line between the moment t and the moment t + 0.1T respectively.

[0032] The single-phase grounding fault line selection method for resonant grounding systems based on the resonant peak ratio as described above, preferably, the fast Fourier transform is used for the Fourier transform in step 3.

[0033] The single-phase grounding fault line selection method for resonant grounding systems based on the resonant peak ratio as described above, preferably, the method for determining the resonant peaks and resonant peak values in the spectrogram by the local peak detection algorithm in step 4 is specifically as follows:

[0034] 1) For any frequency point f on the abscissa of the spectrogram, its first-order difference S'(f) is obtained through Equation (6);

[0035]

[0036] In the formula, S(f + Δf) and S(f - Δf) are the ordinate values corresponding to the frequency points f + Δf and f - Δf on the abscissa of the spectrogram respectively; Δf is the set step parameter, and the value of Δf is 500 Hz;

[0037] 2) Its second-order difference S"(f) is obtained through Equation (7);

[0038] S”(f) = S(f + Δf) - 2S(f) + S(f - Δf) (7)

[0039] In the formula, S(f) is the ordinate value corresponding to the frequency point f on the abscissa of the spectrogram;

[0040] 3) Establish the local peak detection criterion shown in Equation (8):

[0041]

[0042] For any frequency point f on the abscissa of the spectrogram, if Equation (8) is satisfied, then this point in the spectrogram is a resonant peak, and S(f) is a resonant peak value in the spectrogram determined by the local peak detection algorithm.

[0043] The basic principle of the method of the present invention is as follows:

[0044] The basic principle of the present invention lies in using the difference in the zero-sequence current frequency distribution between the faulty line and the healthy line in the resonant grounding system for fault line selection. Specifically, when a single-phase grounding fault occurs in the system, the present invention measures and analyzes the zero-sequence current of each line, and identifies and compares its distribution characteristics at different resonant frequencies.

[0045] In the method of the present invention, for healthy lines, zero-sequence current is mainly distributed at the transient main resonance frequency and the secondary resonance frequency, and the amplitudes of these two frequency components are not very different. On the contrary, for faulty lines, zero-sequence current is mainly concentrated at the transient main resonance frequency, and the amplitude of the component at this frequency is significantly higher than that of the component at the secondary resonance frequency.

[0046] Through this comparative analysis, the present invention can accurately identify faulty lines and can work effectively even when the fault characteristics are not obvious. The key to this method lies in being able to detect the significant difference between the zero-sequence current at the main resonance frequency and the secondary resonance frequency, thereby achieving high-accuracy fault line selection.

[0047] Specifically: 1) The technology described in the present invention does not perform fault line selection based on the amplitude change in the time domain, but conducts spectral analysis (frequency domain) on zero-sequence current. The basic principle of the present invention is to use the difference in the zero-sequence current spectral distribution between faulty lines and healthy lines in a resonant grounding system for fault line selection. When a single-phase ground fault occurs in the system, first, the zero-sequence current of each line is Fourier-transformed to obtain its corresponding spectrogram; secondly, two local peaks are found before and after 2000 Hz in the spectrogram for spectral analysis, and their distribution characteristics at different resonance frequencies are identified and compared; finally, the faulty line is determined by constructing corresponding criteria, thereby achieving fault line selection. Therefore, the technology described in the present invention is significantly different from the fault line selection technology based on amplitude change in the time domain. 2) In addition, the technology described in the present invention simultaneously considers the average slope and resonance peak ratio at the local peak points in the spectrum (specifically including P1, P2, F1, F2 in the technology), and comprehensively constitutes criteria to achieve fault line selection. By such processing, the technology described in the present invention is more effective and reliable. If only the change in resonance peak or average slope is used to judge the fault, it may cause changes in fault characteristics due to changes in the system operation state, load switching, and other factors, making the fault characteristic difference between healthy lines and faulty lines not obvious enough, which is not conducive to better determining the faulty line. The technology described in the present invention analyzes the resonance peak ratio and the average slope of the resonance peak in each line, and can obtain significant criteria for distinguishing faulty lines and healthy lines, and can reliably select the faulty line even under different fault conditions, such as different fault initial phase angles, fault distances, and system operation states. 3) Based on the analysis of the zero-sequence admittance of healthy and faulty lines varying with the transmission line frequency, this patent obtains that there are differences in the resonance local peak ratio and the average slope of the resonance peak before and after 2000 Hz for the two types of lines; in order to avoid the problem that relying solely on the transient resonance frequency value for fault determination is easily affected by external disturbances or system nonlinear factors, by introducing the resonance peak ratio and the average slope to form a comprehensive criterion, it helps to improve the identification effect of faulty lines from multiple dimensions, thereby improving the accuracy and robustness of the line selection result.

[0048] The technical advantages of the present invention are as follows: 1) The method of the present invention can adapt to different fault conditions, such as different fault initial phase angles, fault distances, and system operating states, ensuring reliable selection of the fault line under various circumstances and having strong adaptability. 2) Based on the significant difference distribution of the zero-sequence current of the line at the characteristic frequency, the method of the present invention can accurately identify the fault line and can still work effectively even when the fault characteristics are not obvious, ensuring a high line selection accuracy. 3) Based on the in-depth analysis of the single-phase grounding fault mechanism, the method of the present invention ensures that its implementation method can be closely combined with the actual fault handling operations of the distribution network and is easy to implement in engineering. Brief Description of the Drawings

[0049] Figure 1 It is a flowchart of the implementation of the fault line selection method described in the present invention.

[0050] Figure 2 It is a schematic diagram of the simulation model of the embodiment of the present invention.

[0051] Figure 3 It is a zero-sequence current frequency spectrum diagram of Line 1 in the embodiment of the present invention.

[0052] Figure 4 It is a zero-sequence current frequency spectrum diagram of Line 2 in the embodiment of the present invention.

[0053] Figure 5 It is a zero-sequence current frequency spectrum diagram of Line 3 in the embodiment of the present invention.

[0054] Figure 6 It is a zero-sequence current frequency spectrum diagram of Line 4 in the embodiment of the present invention.

[0055] Figure 7 It is a schematic diagram of the resonance peak and resonance peak value of Line 1 in the embodiment of the present invention.

[0056] Figure 8 It is a schematic diagram of the resonance peak and resonance peak value of Line 2 in the embodiment of the present invention.

[0057] Figure 9 It is a schematic diagram of the resonance peak and resonance peak value of Line 3 in the embodiment of the present invention.

[0058] Figure 10 It is a schematic diagram of the resonance peak and resonance peak value of Line 4 in the embodiment of the present invention.

[0059] In the figure: 1. Power supply; 2. Transformer; 3. Arc suppression coil; 4. Phase C grounding fault. Detailed Embodiment

[0060] The single-phase grounding fault line selection method for a resonant grounding system based on the resonance peak ratio described in the present invention will be further described in detail in combination with the following embodiments.

[0061]

Embodiment

[0062] Build a simulation model of a resonant grounding system distribution network on MATLAB / Simulink software, as shown in the attached drawings Figure 2 The system is a 10kV distribution network with a sampling frequency of 50000Hz and a system frequency of 50Hz. The distribution network simulation model includes 4 distribution lines, all of which adopt distributed parameter lines. Among them, the length of line 1 is 4km; the length of line 2 is 5km; the length of line 3 is 4km; the length of line 4 is 6km. Set a phase C grounding fault 4 at the 3km position of line 2.

[0063] According to a single-phase grounding fault line selection method for a resonant grounding system based on the resonant peak ratio of the present invention, the line selection for the fault is implemented through the following steps:

[0064] Step 1: Monitor the zero-sequence voltage of the resonant grounding system in real time. When the monitored zero-sequence voltage is greater than the set value, it is determined that a single-phase grounding fault has occurred in the system, and the line selection program is started, and step 2 is entered.

[0065] Step 2: Calculate the zero-sequence current data within one-tenth of a system fundamental frequency period after the fault starting moment of each line through formula (5).

[0066]

[0067] In the formula, I0 is the zero-sequence current of the line; I a , I b , I c are the zero-sequence current sampling data of phases a, b, and c between the t moment and the t + 0.1T moment of the line respectively, where t is the fault starting moment and T is the system fundamental frequency period.

[0068] Step 3: Perform Fourier transform on the zero-sequence current of each line to obtain the corresponding spectrogram, as shown in the attached drawings Figure 3 , 4 , 5, 6.

[0069] Step 4: For each line, determine the resonant peak and resonant peak value in the spectrogram by the local peak detection algorithm; take 2000Hz as the boundary, find the largest resonant peak before the boundary, record its abscissa as the frequency point f1, and record its resonant peak value as F1; find the largest resonant peak after the boundary, record its abscissa as the frequency point f2, and record its resonant peak value as F2, as shown in the attached drawings Figure 7 , 8 , 9, 10.

[0070] The method for determining the resonant peak and resonant peak value in the spectrogram by the local peak detection algorithm in step 4 is specifically as follows:

[0071] 1) For any frequency point f on the abscissa of the spectrogram, calculate its first-order difference S'(f) through Equation (6);

[0072]

[0073] Where S(f + Δf) and S(f - Δf) are the ordinate values corresponding to the frequency points f + Δf and f - Δf on the abscissa of the spectrogram respectively; Δf is the set step parameter, and the value of Δf is 500 Hz;

[0074] 2) Calculate its second-order difference S"(f) through Equation (7);

[0075] S”(f) = S(f + Δf) - 2S(f) + S(f - Δf) (7)

[0076] Where S(f) is the ordinate value corresponding to the frequency point f on the abscissa of the spectrogram;

[0077] 3) Establish the local peak detection criterion shown in Equation (8):

[0078]

[0079] For any frequency point f on the abscissa of the spectrogram, if Equation (8) is satisfied, then this point in the spectrogram is a resonance peak, and S(f) is a resonance peak value in the spectrogram determined by the local peak detection algorithm.

[0080] Step 5: Calculate the resonance peak coefficient α through Equation (1);

[0081] α = εΔf (1)

[0082] Where Δf is the set step parameter, and the value of Δf is 500 Hz; ε is the adjustment coefficient, and the value range of ε is 1 to 3;

[0083] Let the adjustment coefficient ε take 2, and the resonance peak coefficient α of each line is obtained as 1000.

[0084] Step 6: Calculate the average slopes P1 and P2 of the maximum resonance peaks before and after 2000 Hz for each line through Equation (2);

[0085]

[0086] Where S(f1), S(f1 - Δf), and S(f1 + Δf) are the ordinate values corresponding to the frequency points f1, f1 - Δf, and f1 + Δf on the abscissa of the spectrogram respectively; S(f2), S(f2 - Δf), and S(f2 + Δf) are the ordinate values corresponding to the frequency points f2, f2 - Δf, and f2 + Δf on the abscissa of the spectrogram respectively.

[0087] The average slopes P1 and P2 of the maximum resonance peaks before and after 2000 Hz for each line are respectively:

[0088] Line 1: 0.0003246, 0.0001869; Line 2: 0.0009714, 0.0000884

[0089] Line 3: 0.0002942, 0.0001668; Line 4: 0.0004877, 0.0001932

[0090] Step 7: Substitute the above data into Equation (3);

[0091]

[0092] The resonance peak ratio H of each line is obtained as follows:

[0093] Line 1: 0.3414; Line 2: 0.0737; Line 3: 0.3066; Line 4: 0.2439

[0094] Step 8: Discriminate the resonance peak ratio of each line according to the criterion described in Equation (4);

[0095] H (i) <0.5min{H (k)} (4)

[0096] In the formula, H (i) 、H (k) respectively represent the resonance peak ratios of the i-th and k-th lines, where i and k are line numbers, i, k = 1, 2…, n, i ≠ k, and n is the total number of lines connected to the system bus.

[0097] It can be seen that the resonance peak ratio of the zero-sequence current on Line 2 satisfies the criterion described in Equation (4), so it can be determined that Line 2 is the faulty line, and the other lines are healthy lines, and the fault line selection is completed.

[0098] The fault line selection result in the above embodiment is consistent with the preset faulty line, verifying the correctness of the fault line selection method of the present invention.

[0099] The above embodiments are only application examples of the present invention, and are only used to more clearly illustrate the specific implementation manner of the method of the present invention. The protection scope of the present invention is not limited to the above embodiments, and the specific protection scope is subject to the claims. It should be noted that within the scope of the present technical field, based on the basic principles and spirit of the present invention, any simple modifications, refinements, simplifications, combinations, and substitutions of its forms or details shall fall within the protection scope of the present invention.

Claims

1. A single-phase grounding fault line selection method for a resonant grounding system based on the resonant peak ratio, characterized in that It includes the following steps: Step 1: Monitor the zero-sequence voltage of the resonant grounding system in real time, and judge whether a single-phase grounding fault occurs in the system according to whether the zero-sequence voltage is greater than the set value; if the zero-sequence voltage is greater than the set value, it is determined that a single-phase grounding fault has occurred in the system, start the line selection program, and enter Step 2; Step 2: Calculate the zero-sequence current data within one-tenth of a system fundamental frequency period after the fault starting moment of each line; Step 3: Perform Fourier transform on the zero-sequence current of each line to obtain the corresponding spectrogram; Step 4: For each line, determine the resonance peak and resonance peak value in the spectrogram by the local peak detection algorithm; taking 2000Hz as the boundary, find the largest resonance peak before the boundary, find the largest resonance peak after the boundary; Step 5: Calculate the resonance peak coefficient α through Equation (1); α = εΔf (1) In the formula, Δf is the set step parameter, and the value of Δf is 500Hz; ε is the adjustment coefficient, and the value range of ε is 1 to 3; Step 6: Calculate the average slopes P1 and P2 of the two largest resonance peaks before and after the boundary; Step 7: Calculate the resonance peak ratio H of the line through Equation (3); For each line connected to the system bus, the resonance peak ratio of the line can be calculated through Equation (3); Step 8: If the resonance peak ratio of the i-th line satisfies the criterion shown in Equation (4), it is determined that the line is a faulty line; otherwise, it is determined that the line is a sound line; H (i) <0.5 min {H (k)}} (4) where, H (i) , H (k) respectively represent the resonance peak ratios of the i-th and k-th lines, where i and k are line numbers, i, k = 1, 2…, n, i ≠ k, and n is the total number of lines connected to the system bus.

2. The single-phase grounding fault line selection method for a resonant grounding system based on the resonance peak ratio according to claim 1, characterized in that The specific content of Step 4 is: For each line, determine the resonance peak and resonance peak value in the spectrogram by the local peak detection algorithm; taking 2000Hz as the boundary, find the largest resonance peak before the boundary, record its abscissa as the frequency point f1, and record its resonance peak value as F1; find the largest resonance peak after the boundary, record its abscissa as the frequency point f2, and record its resonance peak value as F2; The specific content of Step 6 is: Calculate the average slopes P1 and P2 of the two largest resonance peaks before and after the boundary through Equation (2); In the formula, S(f1), S(f1 - Δf), S(f1 + Δf) are the values of the ordinates corresponding to the frequency points f1, f1 - Δf, f1 + Δf in the abscissa of the spectrogram respectively; S(f2), S(f2 - Δf), S(f2 + Δf) are the values of the ordinates corresponding to the frequency points f2, f2 - Δf, f2 + Δf in the abscissa of the spectrogram respectively.

3. The single-phase grounding fault line selection method for a resonant grounding system based on the resonant peak ratio according to claim 1, wherein The calculation method of the zero-sequence current data within one-tenth of a system fundamental frequency period after the fault starting moment of each line in Step 2 is as follows: 1) Extract the zero-sequence current sampling data between the moment t and the moment t + 0.1T, where t is the fault starting moment and T is the system fundamental frequency period; 2) Calculate the zero-sequence current of each line through Equation (5); Where, I0 is the zero-sequence current of the line; I a , I b , I c are the zero-sequence current sampling data of phases a, b, and c between the t-th moment and the (t + 0.1T)-th moment of the line respectively.

4. The single-phase grounding fault line selection method for a resonant grounding system based on the resonant peak ratio according to claim 1, characterized in that The Fourier transform in Step 3 adopts the fast Fourier transform.

5. The single-phase grounding fault line selection method for a resonant grounding system based on the resonant peak ratio according to claim 1, characterized in that The method for determining the resonance peak and resonance peak value in the spectrogram by the local peak detection algorithm in Step 4 is specifically as follows: 1) For any frequency point f in the abscissa of the spectrogram, calculate its first-order difference S'(f) through Equation (6); Wherein, S(f + Δf) and S(f - Δf) are respectively the values of the ordinates corresponding to the frequency points f + Δf and f - Δf in the abscissa of the spectrogram; Δf is the set step parameter, and the value of Δf is 500 Hz; 2) Calculate its second-order difference S''(f) through Equation (7); S''(f) = S(f + Δf) - 2S(f) + S(f - Δf) (7) Wherein, S(f) is the value of the ordinate corresponding to the frequency point f in the abscissa of the spectrogram; 3) Establish the local peak detection criterion shown in Equation (8): For any frequency point f in the abscissa of the spectrogram, if Equation (8) is satisfied, then this point in the spectrogram is a resonance peak, and S(f) is a resonance peak value in the spectrogram determined by the local peak detection algorithm.

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