Method for judging single-phase grounding fault of small-current grounding system

Abstract

The invention belongs to the technical field of relay protection of a power system, and particularly relates to a single-phase grounding fault judgment method for a small-current grounding system. The method comprises the following steps: step 1, collecting zero-sequence voltage of a bus and zero-sequence current of each interval on the bus, and calculating instantaneous value variation of the zero-sequence voltage in real time; 2, when a zero-sequence voltage variation starting criterion is met, considering that a system has a single-phase earth fault, and starting an earth fault judgment program; 3, sampling data of zero-sequence current and zero-sequence voltage of each interval at the starting moment are intercepted, wavelet packet decomposition is carried out on the sampling data, and energy of each frequency band after decomposition of each interval is solved; 4, selecting three characteristic frequency bands with maximum interval energy to perform wavelet packet weighted reconstruction; and step 5, fault discrimination. The method has the beneficial effects that transient data at a fault moment can be quickly and accurately obtained, weighted reconstruction is carried out on three characteristic frequency band waveforms with maximum energy at each interval, and the grounding interval is identified by using a phase comparison method.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of relay protection of power systems, and particularly relates to a single-phase grounding fault determination method for a small-current grounding system. BACKGROUND

[0002] In domestic 3-66kV power supply and distribution networks and nuclear power plant power systems, most of them adopt neutral point non-direct grounding mode, and are commonly called small-current grounding systems (NUGS) because the current flowing through the grounding point is small when a grounding fault occurs. The small-current grounding system includes a neutral point non-grounding system (NUS), a neutral point grounding system through an arc suppression coil (NES, also called a resonance grounding system) and a neutral point grounding system through a resistance (NRS).

[0003] The single-phase grounding fault occurs most frequently in the small-current grounding system. However, when the single-phase grounding fault occurs in the system, the grounding fault current is much smaller than the load current because it does not form a short-circuit loop, and the three-phase line voltage still maintains a symmetrical relationship, which does not affect the continuous power supply to the load, so it is not necessary to trip immediately. The regulation stipulates that it can continue to operate for 1-2 hours. However, due to the presence of the grounding point, the voltage to ground of the non-fault phase in the system rises to 1.732 times the original voltage, which threatens the insulation of the power grid and easily induces another grounding at the weak point of the power grid, thereby forming an inter-phase short circuit. With the increase of the system capacity and the total length of the line, the capacitive current becomes larger and larger, and the overvoltage multiple caused by arc grounding is very high.

[0004] Therefore, domestic and foreign scientific and technological workers have successively conducted researches in the aspects of wiring mode, overvoltage limiting measures and grounding protection, and hope to minimize the probability of single-phase grounding fault and quickly select the fault line to remove the fault as soon as possible. However, when the single-phase grounding fault occurs in the neutral point grounding system through an arc suppression coil, the phase and amplitude of the zero sequence current of the fault line and the non-fault line are relatively close, which leads to a high misjudgment rate of line selection. Most of the commonly used grounding fault determination methods utilize the zero sequence steady-state electrical quantity after the fault. Because the amplitude of the zero sequence steady-state electrical quantity is small when the single-phase grounding occurs, it is difficult to accurately measure, and therefore it is difficult to be practically applied. The injection method is limited by the capacity of the voltage transformer, and the injected signal is small. When there is a grounding with a transition resistance, the fault determination is prone to error. The transient method is currently applied more, but because the characteristic frequency band is not reasonably selected, it is difficult to obtain the waveform at the starting moment, which affects the line selection accuracy. SUMMARY

[0005] The application aims to provide a small current grounding system single-phase grounding fault determination method, which adopts a sudden variable quick start algorithm based on an instantaneous value floating threshold, performs weighted reconstruction on the waveforms of the three characteristic frequency bands with the maximum energy of each interval through a wavelet packet algorithm, and finally uses a phase comparison method to identify the grounding interval, thereby improving the reliability and rapidity of device action in the small current grounding system when a single-phase grounding fault occurs, and solving the problem of high misjudgment rate in the small current grounding system when a single-phase grounding fault occurs.

[0006] The technical scheme of the application is as follows: a small current grounding system single-phase grounding fault determination method, comprising the following steps:

[0007] Step 1: collecting the zero sequence voltage of the bus and the zero sequence current of each interval on the bus, and calculating the zero sequence voltage instantaneous value change amount in real time;

[0008] Step 2: when the zero sequence voltage change amount starting criterion is met, considering that the system is a single-phase grounding fault, starting the grounding fault determination program;

[0009] Step 3: intercepting the sampling data of the zero sequence current and the zero sequence voltage of each interval at the starting moment, performing wavelet packet decomposition on the sampling data, and calculating the energy of each frequency band after decomposition of each interval;

[0010] Step 4: selecting the three characteristic frequency bands with the maximum energy of each interval for wavelet packet weighted reconstruction;

[0011] Step 5: fault discrimination.

[0012] The zero sequence voltage instantaneous value change amount calculation method in step 1 is as follows:

[0013] (1)

[0014] Wherein, is the current sampling value of the zero sequence voltage, is the sampling value of the previous 192 points of the zero sequence voltage buffer.

[0015] The sudden variable quick detection algorithm based on the instantaneous value floating threshold is used in step 1 to calculate , as formula (2):

[0016] (2)

[0017] Wherein, is the current calculation value of the zero sequence voltage change amount, is the calculation value of the previous (192+n) points of the zero sequence voltage change amount.

[0018] When the time satisfying formula (2) is greater than 4 ms in step 2, it is considered that the system is a single-phase grounding fault, and the grounding fault determination program is started.

[0019] The step 3 comprises:

[0020]

[0021] Wherein, is the energy of the interval n zero sequence current in the frequency band m, is the basic wavelet of the interval n zero sequence current in the frequency band m, the three frequency bands with the largest energy of each interval are selected as the characteristic frequency bands, and the weighted coefficients of the three characteristic frequency bands of each interval are determined according to the proportion of each characteristic frequency band in the sum of the three characteristic frequency bands.

[0022] The step 4 comprises that the weighted coefficients of the three characteristic frequency bands with the largest energy are determined according to the proportion of the energy of each characteristic frequency band in the sum of the energy of the three characteristic frequency bands.

[0023] The step 5 comprises comparing the directions of the zero sequence currents of each interval after the wavelet packet weighted reconstruction, when the direction of an interval is opposite to the phase of other intervals, the interval is determined as the grounding interval, and when the directions of the zero sequence currents of each interval are consistent, the bus fault is determined.

[0024] The beneficial effects of the present application are that: the sudden variable quick start algorithm based on the instantaneous value floating threshold can quickly and accurately obtain the transient data at the fault time, then the transient data is decomposed through the wavelet packet algorithm, then the three characteristic frequency band waveforms with the largest energy of each interval are weighted and reconstructed, and finally the grounding interval is identified by using the phase comparison method. Through the accurate acquisition of the transient data at the fault time and the weighted reconstruction calculation of the three characteristic frequency band waveforms, the reliability and rapidity of the device action when the single-phase grounding fault occurs in the small current grounding system are improved, and the problem of high misjudgment rate when the single-phase grounding fault occurs in the small current grounding system is solved. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 It is a wiring schematic diagram realized in the grounding line selection device;

[0026] Figure 2 It is a small current grounding system single-phase grounding fault determination method flow chart provided by the present application. DETAILED DESCRIPTION

[0027] The present application will be further described in detail below in combination with the drawings and specific embodiments.

[0028] As Figure 2 shown, a small current grounding system single-phase grounding fault determination method comprises the following steps:

[0029] Step 1: collecting the zero sequence voltage of the bus and the zero sequence current of each interval on the bus, and calculating the instantaneous value change amount of the zero sequence voltage in real time;

[0030] The zero-sequence voltage instantaneous value change amount is calculated as follows:

[0031] (1)

[0032] wherein, is the current sampling value of the zero-sequence voltage, is the sampling value of the previous 192 points (i.e., the sampling value of one cycle) of the zero-sequence voltage buffered;

[0033] Step 2: When the zero-sequence voltage change amount starting criterion is met, the system single-phase ground fault is considered, and the ground fault determination procedure is started;

[0034] In step 1, The sudden change amount rapid detection algorithm based on the instantaneous value floating threshold is adopted, as formula (2):

[0035] (2)

[0036] wherein, is the current calculation value of the zero-sequence voltage change amount, is the calculation value of the previous (192+n) points of the zero-sequence voltage change amount; when the time satisfying formula (2) is greater than 4 ms, the system single-phase ground fault is considered, and the ground fault determination procedure is started;

[0037] Step 3: The sampling data of the zero-sequence current and the zero-sequence voltage of each interval at the starting moment are intercepted, the sampling data is wavelet packet decomposed, and the energy of each frequency band after decomposition of each interval is calculated:

[0038]

[0039] wherein, is the energy of the interval n zero-sequence current in the frequency band m, is the basic wavelet of the interval n zero-sequence current in the frequency band m, the three frequency bands with the largest energy of each interval are selected as the characteristic frequency bands, and the weighting coefficients of the three characteristic frequency bands of each interval are determined according to the proportion of each characteristic frequency band in the sum of the three characteristic frequency bands;

[0040] Step 4: The three characteristic frequency bands with the largest energy of each interval are selected for wavelet packet weighted reconstruction;

[0041] The weighting coefficients of the three characteristic frequency bands with the largest energy are determined according to the proportion of the energy of each characteristic frequency band in the sum of the energy of the three characteristic frequency bands; and the three frequency bands with the largest energy of each interval are selected as the characteristic frequency bands, and the weighting coefficients of the three characteristic frequency bands of each interval are determined according to the proportion of each characteristic frequency band in the sum of the three characteristic frequency bands; it is assumed that , , For 3 frequency bands for 3 maximum characteristic frequency bands with interval n, the weighting coefficients of the three frequency bands are:

[0042]

[0043]

[0044]

[0045] In the formula, k 1n is the weighting coefficient of the interval n frequency band 1, k mn is the weighting coefficient of the interval n frequency band m, and k nn is the weighting coefficient of the interval n frequency band n.

[0046] Step 5: Compare the directions of the zero sequence currents of each interval after wavelet packet weighting reconstruction. When the direction of one interval is opposite to the phase of the other intervals, it is determined that the interval is a grounding interval. When the directions of the zero sequence currents of each interval are consistent, it is determined that the bus fault.

[0047] Embodiment:

[0048] A method for determining single-phase grounding fault of a small current system, comprising the following steps:

[0049] Step 1: Collect the zero sequence voltage of the bus and the zero sequence current of each interval on the bus at a sampling rate of 9.6K, and calculate the instantaneous value change of the zero sequence voltage in real time.

[0050] The calculation method of the instantaneous value change of the zero sequence voltage is as follows:

[0051] (1)

[0052] Wherein, is the current sampling value of the zero sequence voltage, is the sampling value of the previous 192 points (i.e. the sampling value of one cycle) of the zero sequence voltage buffer

[0053] Step 2: When the zero sequence voltage change meets the starting criterion, it is considered that the system is single-phase grounding fault, and the grounding fault determination program is started. The sudden change quantity rapid detection algorithm based on the instantaneous value floating threshold is adopted, as shown in formula (2):

[0054] (2)

[0055] Wherein, is the current calculation value of the zero sequence voltage change, is the calculation value of the previous (192+n) points of the zero sequence voltage change; when the time meeting the formula (2) is greater than 4ms, it is considered that the system is single-phase grounding fault, and the grounding fault determination program is started.

[0056] Step 3: Take the data of the half cycle before starting and the half cycle after starting, and perform 8-layer wavelet packet decomposition on the data; and calculate the energy of each interval after decomposition in 32 frequency bands:

[0057]

[0058] wherein, is the energy of the zero sequence current of interval n in frequency band m, is the basic wavelet of the zero sequence current of interval n in frequency band m.

[0059] And select the three largest energy bands in each interval as the characteristic frequency bands, and the weighted coefficients of the three characteristic frequency bands in each interval are determined according to the proportion of each characteristic frequency band in the sum of the three characteristic frequency bands;

[0060] Suppose , , are the three largest characteristic frequency bands of interval n, then the weighted coefficients of the three frequency bands are:

[0061]

[0062]

[0063]

[0064] Step 4: According to the calculated weighted coefficients, perform wavelet packet weighted reconstruction on the data of the three characteristic frequency bands; obtain the waveform filtered by wavelet packet.

[0065] Step 5: Compare the directions of the zero sequence currents of each interval after wavelet packet reconstruction, when the direction of one interval is opposite to the phase of the other intervals, it is determined that the interval is a grounding interval, and when the directions of the zero sequence currents of each interval are consistent, it is determined that the bus is faulty.

[0066] The above only describes the preferred embodiments of the present application, and it should be noted that for ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for determining single-phase grounding faults in a low-current grounding system, characterized in that, It includes the following steps: Step 1: Collect the zero-sequence voltage of the bus and the zero-sequence current of each interval on the bus, and calculate the instantaneous change of the zero-sequence voltage in real time. Step 2: When the zero-sequence voltage change trigger criterion is met, the system is considered to have a single-phase ground fault, and the ground fault determination procedure is initiated. Step 3: Extract the sampling data of zero-sequence current and zero-sequence voltage at each interval during startup, perform wavelet packet decomposition on the sampling data, and calculate the energy of each frequency band after decomposition of each interval. Step 4: Select the three characteristic frequency bands with the largest energy in each interval for wavelet packet weighted reconstruction; Step 5: Fault diagnosis.

2. The method for determining single-phase grounding faults in a low-current grounding system as described in claim 1, characterized in that, The method for calculating the instantaneous change in zero-sequence voltage in step 1 is as follows: （1） in, This is the current sampled value of the zero-sequence voltage. These are the sampled values ​​of the first 192 points of the zero-sequence voltage buffer.

3. The method for determining single-phase grounding faults in a low-current grounding system as described in claim 2, characterized in that, Step 1 employs a fast mutation detection algorithm based on an instantaneous value floating threshold to calculate... For example, in formula (2): （2） in, This is the current calculated value of the zero-sequence voltage change. This is the calculated value at point (192+n) before the zero-sequence voltage change.

4. The method for determining single-phase grounding faults in a low-current grounding system as described in claim 3, characterized in that, In step 2, when the time for satisfying formula (2) is greater than 4ms, the system is considered to have a single-phase ground fault, and the ground fault determination program is started.

5. The method for determining single-phase grounding faults in a low-current grounding system as described in claim 1, characterized in that, Step 3 includes: in, Let n be the energy of the zero-sequence current at interval n in frequency band m. To determine the basic wavelet of zero-sequence current at interval n in frequency band m, the three frequency bands with the largest energy in each interval are selected as characteristic frequency bands. The weighting coefficients of the three characteristic frequency bands in each interval are determined according to the proportion of each characteristic frequency band to the sum of the three characteristic frequency bands.

6. The method for determining single-phase grounding faults in a low-current grounding system as described in claim 1, characterized in that: Step 4 involves determining the weighting coefficients of the three characteristic frequency bands with the highest energy based on the proportion of the energy of each characteristic frequency band to the sum of the energy of the three characteristic frequency bands.

7. The method for determining single-phase grounding faults in a low-current grounding system as described in claim 1, characterized in that: Step 5 includes comparing the direction of the zero-sequence current in each interval after wavelet packet weighted reconstruction. When the direction of one interval is opposite to the phase of the other intervals, the interval is determined to be a grounding interval. When the direction of the zero-sequence current in each interval is consistent, it is determined to be a bus fault.

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