A small primary current phasor measurement method suitable for PT inter-turn short circuit detection

Abstract

A primary small current phasor measurement method suitable for PT interturn short circuit detection uses a through-hole CT with a source conversion function to collect three-phase PT-to-ground current. The through-hole CT converts the small current signal into a voltage signal and transmits the signal to a protection device using a shielded grounding cable. The protection device uses a PT that matches the secondary of the through-hole CT to collect the voltage signal to obtain the secondary sampling instantaneous value of the three-phase PT-to-ground current of the protection device. The software of the protection device stores the original sampling instantaneous value to obtain a three-phase PT-to-ground current sampling sequence. The protection program processes the sampling points using an anti-bad point processing algorithm and then uses a bandpass filter to filter out interference signals to obtain a new sampling value sequence. Based on the new sampling value sequence, Fourier calculation is used to obtain the phasor of the three-phase PT-to-ground current that can be directly used for PT interturn short circuit discrimination.

Description

Technical Field

[0001] The present invention relates to the technical field of generator relay protection, and in particular to a primary small current phasor measurement method suitable for PT inter-turn short circuit detection. Background Art

[0002] PT turn-to-turn short-circuit detection is a crucial component of relay protection. For example, when a generator-side PT short-circuit occurs, a significant zero-sequence voltage will appear in the generator PT's open-ended secondary winding, posing a risk of false tripping of the stator ground fault protection. To prevent false tripping caused by a primary PT turn-to-turn short-circuit, PT turn-to-turn short-circuit detection is necessary to identify PT turn-to-turn faults in advance. To identify PT turn-to-turn faults, both the PT secondary voltage and the PT primary current to ground must be collected. Because the PT primary current to ground is very low during normal system operation and even when a small PT turn-to-turn short-circuit occurs, mA-level primary current acquisition is required. Furthermore, the secondary acquisition circuit must be effectively isolated from the primary cable. This low-current acquisition circuit is susceptible to interference in the strong magnetic field of the field. Therefore, employing appropriate hardware and software acquisition methods is crucial for PT turn-to-turn short-circuit detection.

[0003] Methods for detecting PT turn-to-turn short circuits are available in the prior art. For example, patent document CN 114325466B discloses a generator output transformer turn-to-turn short circuit self-detection system, comprising a low-frequency power supply, a current transformer, and a protection and analysis device. The two ends of the load resistor RL in the generator neutral-point grounding transformer secondary circuit are connected to the low-frequency power supply. The current transformer is installed in the grounding transformer secondary circuit and is used to measure the load current IL in the grounding transformer secondary circuit. The load voltage UL in the grounding transformer secondary circuit is measured via a selected tap. The protection and analysis device is used to analyze the measured load current IL and load voltage UL to determine whether a ground short circuit fault has occurred in the generator stator. Combining the generator stator ground fault protection with the generator output voltage transformer turn-to-turn short circuit provides 100% protection for the generator stator winding. The system's sensitivity is unaffected by factors such as the fault location and unit operating conditions, reducing the probability of protection misoperation while further eliminating misoperation caused by the generator output voltage transformer turn-to-turn short circuit.

[0004] The aforementioned interturn short-circuit detection requires sampling the PT primary and secondary currents to calculate the excitation current. While theoretically feasible, the actual primary PT current is very small and susceptible to interference. The invention does not address how to ensure accurate acquisition. Furthermore, the angle between the secondary phase voltage and the excitation current can also change when the stator is grounded, making it easy to misjudge an interturn short-circuit.

[0005] Patent document CN 117054954A discloses a method for monitoring the inter-turn short-circuit status of a voltage transformer on the generator output side. The method comprises the following steps: obtaining raw data from the voltage transformer; constructing a mathematical model based on the raw data, and simulating the voltage transformer using the mathematical model to obtain an output waveform of the voltage transformer's secondary current; performing an inter-turn short-circuit simulation based on the mathematical model to obtain a linear relationship between the secondary current and the number of short-circuited turns when an inter-turn short-circuit occurs; monitoring the secondary current of the voltage transformer in real time to obtain a real-time operating waveform, comparing the real-time operating waveform with the output waveform to determine whether the voltage transformer is short-circuited, and obtaining the number of short-circuited turns of the voltage transformer based on the linear relationship and the monitored secondary current. This method, through the constructed mathematical model, enables online monitoring of the PT insulation status.

[0006] It is not easy to obtain the accurate mathematical model of the actual on-site PT mentioned above. Moreover, from the secondary side current waveform diagram of the embodiment of the invention, after the inter-turn short circuit, the secondary current exceeds 100,000A, and the large secondary current is difficult to collect. The invention does not mention how to overcome this technical difficulty. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a primary small current phasor measurement method suitable for PT inter-turn short circuit detection, so as to realize accurate detection of PT inter-turn short circuit.

[0008] During the detection process, the signal strength of the small current signal is relatively weak, and the acquisition circuit is easily affected by electromagnetic interference, resulting in bad points and affecting the accuracy of the data. In order to improve the reliability and accuracy of the acquisition and solve the above technical problems, the technical solution adopted by the present invention is:

[0009] A method for measuring a small primary current phasor suitable for detecting a PT inter-turn short circuit comprises the following steps:

[0010] Step 1: Use a through-hole CT with active conversion function to collect three-phase PT-to-ground current. The conversion circuit in the through-hole CT converts the current small signal into a voltage signal that can be collected by conventional protection devices during local collection, and sends the signal to the protection device through the collection cable;

[0011] Step 2: The protection device uses the PT that matches the secondary of the through-hole CT to collect the voltage signal and obtain the secondary sampling instantaneous value of the three-phase PT to ground current of the protection device. i A ( t ), i B ( t ), i C ( t), the rated voltage of the protection device PT is m times the maximum effective output voltage of the through-hole CT;

[0012] Step 3: The protection device stores the original sampling value to obtain the three-phase PT ground current sampling sequence i A ( n ), i B ( n ), i C ( n ),in n Indicates the sampling point at the current moment, n -1 means the first 1 / f s The sampling point at the moment, f s is the sampling rate of the protection device;

[0013] Step 4: Since the voltage signal converted by the through-hole CT is still small when the PT is working normally, it is easily affected by external interference and produces bad pixels. The protection device performs anti-bad pixel processing and filtering on the sampling queue, including using an anti-bad pixel processing algorithm to process the sampling points;

[0014] Step 5: The protection device uses bandpass filtering to filter out interference signals. i A ( n ), i B ( n ), i C ( n ) channel is band-pass filtered, and the filtered sampling value of the current point is , M is the length of the filter data window. Bandpass filtering mainly filters out signals other than 50Hz. h ( k ) is the filter coefficient; after bandpass filtering, a new sampling value sequence is obtained I A ( n ), I B ( n ), I C ( n );

[0015] Step 6. Use Fourier calculation to obtain the phase quantities of the three-phase PT to ground currents 、 、 , which can be directly used to judge the short circuit between PT turns.

[0016] The anti-bad pixel processing in Step 4 above includes:

[0017] Step 4.1. Calculate the current change rate , x represents the phase, which can be A, B, or C phase. Represents the interval period of each sampling point, that is, 1 / f s If the absolute value of the current change rate of two consecutive points in a phase is greater than 1.5, and the signs of the secondary change rates are opposite, and the half-wave integral value is greater than 0.4, that is:

[0018] ;

[0019] Then the n-1th point of the phase is determined to be a bad point, where N represents the number of points per cycle under the power frequency;

[0020] Step 4.2: After determining that there are bad pixels, repair the bad pixels. The expression for repairing bad pixels is:

[0021] ;

[0022] Output sampling point quality mark at the same time q x ( n )=1, x represents the phase difference, n represents the current sampling point;

[0023] Step 4.3, calculate the quality mark sequence of the sampling points, within half a cycle, from q x ( n ), q x ( n -1)… q x ( n - N / 2) If the quality flags of two sampling points are 1, the PT inter-turn short circuit judgment is locked for 50ms.

[0024] In the above Step 5, in order to filter out the interference signal other than 50Hz, a high-order bandpass filter is designed with a center frequency of 50Hz, an upper cutoff frequency of 65Hz, and a lower cutoff frequency of 35Hz. There are 23 filter coefficients in total, and the coefficient serial number is k From 0 to 22, the filter coefficient h ( k )for:

[0025] -0.1095601674,-0.04851654475558,-0.05068769378425,-0.04629358396191,-0.03497767310127,-0.01737573563024,0.004773007126296,0.02896608614105,0.05230176960597,0.07166537816895,0.08451457097657,0.088898 78702655,0.08451457097657,0.07166537816895,0.05230176960597,0.02896608614105,0.004773007126296,-0.01737573563024,-0.03497767310127,-0.04629358396191,-0.05068769378425,-0.04851654475558,-0.1095601674.

[0026] In Step 6 above, to ensure calculation accuracy and take into account calculation efficiency, a sampling rate of 1200 Hz is used, and the power frequency electrical quantity has 24 points per cycle. In order to obtain the current phasor required for PT inter-turn short-circuit calculation, the full-wave Fourier algorithm is preferably used for phasor calculation, and the expression is:

[0027] ;

[0028] N is 24 per wave.

[0029] In the above Step 2, m is set to 2, and the maximum effective output voltage of the through-hole CT is 5V, that is, the rated voltage value of the protection device PT is 10V.

[0030] The present invention provides a primary small current phasor measurement method suitable for PT interturn short circuit detection. The method uses a through-hole CT with an active conversion function to collect three-phase PT-to-ground current. The protection device performs effective anti-bad point processing and filtering calculation on the secondary sampling instantaneous value of the three-phase PT-to-ground current. Finally, a high-precision full-wave Fourier algorithm is used to obtain the phasor of the three-phase PT-to-ground current that can be directly used for PT interturn short circuit judgment. This method avoids interference with small current collection caused by the strong magnetic environment on site and improves the reliability of PT-related protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be further described below with reference to the accompanying drawings and examples:

[0032] Figure 1 This is a circuit diagram of a primary small current phasor measurement for detecting a PT inter-turn short circuit;

[0033] Figure 2 This is a flow chart of the PT-to-ground primary small current phasor measurement process of the present invention. DETAILED DESCRIPTION

[0034] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0035] like Figure 1 and 2 As shown in , a primary small current phasor measurement method suitable for PT inter-turn short circuit detection includes:

[0036] A through-hole CT with source conversion function is used to collect the three-phase PT-to-ground current. The through-hole CT converts the current small signal into a voltage signal and uses a collection cable to send the signal to the protection device; the protection device uses a PT that matches the through-hole CT secondary to collect the voltage signal and obtain the secondary sampling instantaneous value of the three-phase PT-to-ground current of the protection device. i A ( t ), i B ( t ), i C ( t ); The protection device software stores the original sampling value and obtains the three-phase PT ground current sampling sequence i A ( n ), i B ( n ), i C ( n ),in n Indicates the sampling point at the current moment, n -1 means the first 1 / f s The sampling point at the moment, f s To protect the sampling rate of the device; the protection program performs anti-bad point processing and filtering on the sampling queue, including using the anti-bad point processing algorithm to process the sampling points, and then using bandpass filtering to filter out interference signals to obtain a new sampling value sequence I A ( n ), I B ( n ), I C ( n ); Finally, Fourier calculation is used to obtain the phasor of the three-phase PT to ground current 、 、 , which can be directly used to judge the short circuit between PT turns.

[0037] like Figure 1 As shown in the figure, the protection device provides power for the active through-hole CT. The active through-hole CT collects the three-phase PT to ground current and converts the current small signal into a voltage signal. The signal is then sent to the protection device using a shielded grounding cable. The protection device uses a PT that matches the secondary of the through-hole CT to collect the voltage signal.

[0038] The protection device uses a PT that matches the secondary of the through-hole CT. The PT rating is twice the maximum effective output of the through-hole CT, which is 10V.

[0039] Anti-bad pixel processing includes:

[0040] 1) Calculate the current change rate , x represents the phase, which can be A, B, or C phase. Represents the interval period of each sampling point, that is, 1 / f s , considering that the derivative of the sine wave is not greater than 1, if the absolute value of the current change rate of two consecutive points in a phase current is greater than 1.5, and the signs of the second change rate are opposite, and the half-wave integral value is greater than 0.4, that is , then the n-1th point of the phase is determined to be a bad point, where N represents the number of points per cycle under the power frequency;

[0041] 2) After determining that there is a bad pixel, the difference between the two points before and after the bad pixel is calculated to repair the bad pixel. The expression for repairing the bad pixel is: , and output the sampling point quality mark at the same time q x ( n )=1, x represents the phase difference, n represents the current sampling point;

[0042] 3) To prevent the bad points before and after the bad point from being bad points as well, and thus making it impossible to repair the bad point, the quality mark sequence of the sampling points is calculated. If the bad point is within half a cycle, i.e., from q x ( n ), q x ( n -1)… q x ( n - N / 2) If the quality flags of two sampling points are 1, the PT inter-turn short circuit judgment is locked for 50ms.

[0043] Bandpass filtering is used to filter out low-frequency and high-frequency interference signals. i A ( n ), i B( n ), i C ( n ) channel for bandpass filtering, x The filtered sampling value of the previous point is , h ( k ) is the filter coefficient, M is the filter data window length.

[0044] Bandpass filtering mainly filters out signals other than 50Hz. h ( k ) is the filter coefficient, k From 0 to 22, the filter coefficient h ( k )for:

[0045] -0.1095601674,-0.04851654475558,-0.05068769378425,-0.04629358396191,-0.03497767310127,-0.01737573563024,0.004773007126296,0.02896608614105,0.05230176960597,0.07166537816895,0.08451457097657,0.088898 78702655,0.08451457097657,0.07166537816895,0.05230176960597,0.02896608614105,0.004773007126296,-0.01737573563024,-0.03497767310127,-0.04629358396191,-0.05068769378425,-0.04851654475558,-0.1095601674.

[0046] Using Fourier calculation, the phasor of the three-phase PT to ground current is obtained 、 、 , the Fourier algorithm optimizes the full-wave Fourier calculation, and the expression is , N=24 per wave.

[0047] Calculate the phase of the three-phase PT to ground current 、 、 After that, the effective value of each phase can be calculated , the current change of each phase , x represents the phase difference; if the effective value of the current or the current change of a phase is greater than the set threshold value, it can be used as a starting element for PT inter-turn short circuit detection. Combined with the judgment of each phase PT, the PT inter-turn short circuit can be further identified.

[0048] The present invention mainly adopts active through-hole CT to convert mA-level small current signals to obtain voltage signals. At the same time, in order to avoid external interference when collecting voltage signals at a long distance, the voltage signal is designed to be below 10V, thereby improving the collection accuracy from the perspectives of filtering out signals other than 50Hz and detecting and repairing bad points.

Claims

1. A method for measuring a small primary current phasor suitable for detecting a PT inter-turn short circuit, characterized in that: The following steps are involved: Step 1: Use a through-hole CT with source conversion function to collect the three-phase PT-to-ground current. The through-hole CT converts the current signal into a voltage signal and sends the signal to the protection device through the acquisition cable; Step 2: The protection device uses the PT that matches the secondary of the through-hole CT to collect the voltage signal and obtain the secondary sampling instantaneous value of the three-phase PT to ground current of the protection device. i A ( t ), i B ( t ), i C ( t ), the rated voltage of the protection device PT is m times the maximum effective output voltage of the through-hole CT; Step 3: The protection device stores the original sampling value to obtain the three-phase PT ground current sampling sequence i A ( n ), i B ( n ), i C ( n ),in n Indicates the sampling point at the current moment, n -1 means the first 1 / f s The sampling point at the moment, f s is the sampling rate of the protection device; Step 4: The protection device performs anti-bad pixel processing and filtering on the sampling queue, including processing the sampling points using an anti-bad pixel processing algorithm; Step 5: The protection device uses bandpass filtering to filter out interference signals. i A ( n ), i B ( n ), i C ( n ) channel is band-pass filtered, and the filtered sampling value of the current point is , M is the filter data window length, h ( k ) is the filter coefficient; after bandpass filtering, a new sampling value sequence is obtained I A ( n ), I B ( n ), I C ( n ); Step 6. Use Fourier calculation to obtain the phase of the three-phase PT to ground current 、 、 , can be directly used for PT inter-turn short circuit judgment; The anti-bad pixel processing in Step 4 includes: Step 4.

1. Calculate the current change rate , x represents the phase, which can be A, B, or C phase. Represents the interval period of each sampling point, that is, 1 / f s If the absolute value of the current change rate of two consecutive points in a phase is greater than 1.5, and the signs of the secondary change rates are opposite, and the half-wave integral value is greater than 0.4, that is: ； Then the n-1th point of the phase is determined to be a bad point, where N represents the number of points per cycle under the power frequency; Step 4.2: After determining that there are bad pixels, repair the bad pixels. The expression for repairing bad pixels is: ； Output sampling point quality mark at the same time q x ( n )=1, x represents the phase difference, n represents the current sampling point; Step 4.3, calculate the quality mark sequence of the sampling points, within half a cycle, from q x ( n ), q x ( n -1)… q x ( n - N / 2) If the quality flags of two sampling points are 1, the PT inter-turn short circuit judgment is locked for 50ms.

2. A method for measuring a primary small current phasor suitable for detecting a PT turn-to-turn short circuit according to claim 1, characterized in that: In Step 5 k From 0 to 22, the filter coefficient h ( k )for: -0.1095601674,-0.04851654475558,-0.05068769378425,-0.04629358396191,-0.03497767310127,-0.01737573563024,0.004773007126296,0.02896608614105,0.05230176960597,0.07166537816895,0.08451457097657,0.08889878702655,0.08451457097657,0.07166537816895,0.05230176960597,0.02896608614105,0.004773007126296,-0.01737573563024,-0.03497767310127,-0.04629358396191,-0.05068769378425,-0.04851654475558,-0.1095601674。 3. A method for measuring a primary small current phasor suitable for detecting a PT turn-to-turn short circuit according to claim 2, characterized in that: In Step 6, the Fourier algorithm preferably uses full-wave Fourier calculation, and the expression is: ； N is 24 per wave.

4. A method for measuring a primary small current phasor suitable for detecting a PT turn-to-turn short circuit according to claim 3, characterized in that: In the aforementioned Step 2, m is set to 2, and the maximum effective output voltage of the through-hole CT is 5V, that is, the rated voltage value of the protection device PT is 10V.

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