A method for identifying inrush current based on waveform morphology

Abstract

The present invention relates to a method for identifying a surge current based on waveform morphology, and belongs to the field of power systems. The method comprises the following steps: S1: acquiring current data with one cycle as a data window; S2: setting a setting value that can distinguish a fault from a surge current by analyzing the distance value; S3: distinguishing the time difference between the peak and valley values ​​of the symmetrical waveform; S4: calculating the peak change of the surge current per cycle, and distinguishing the surge current by the peak change rate. Under the condition that the speed of relay protection is met, the waveform similarity comparison is performed with the current data of one cycle, and when the similarity is greater than the set threshold, it is comprehensively judged whether there is an interruption angle, thereby judging the internal fault current; then, the surge current and the excitation surge current are distinguished by the change trend of the surge current waveform, so as to ensure the correct action of the transformer relay protection and the normal operation of the power system.

Claims

1. A method for identifying a surge current based on waveform morphology, characterized by: The method comprises the following steps: S1: Obtain current data with one cycle as the data window, remove the amplitude characteristics of the sampling points, and compress the current sequence longitudinally based on the maximum current value within the cycle. The amplitude of the normalized current waveform sampling points is in the range of [-1, 1]. S2: The collected periodic internal fault current and the summed inrush current are normalized and used as the analysis objects. The shortest bending distance value of the dynamic time warping algorithm DTW is calculated for the standard sine wave. The setting value that can distinguish the fault from the summed inrush current is set by analyzing the distance value. S3: DTW stretches and twists the symmetrical and inrush current waveforms. If the time difference between the peak and valley values ​​of the symmetrical and inrush current cycles before the twist is smaller than the internal fault current, the peak-valley interval time difference of the symmetrical waveform is determined. S4: Calculate the peak change of the inrush current per cycle and use the peak change rate to identify the expected inrush current; when an unloaded transformer is connected to the power system, the differential current amplitude of the connected transformer or the operating transformer exceeds the minimum operating current, and the identification method judgment criteria are activated; Judgment D c >D set :Get one cycle of current data and calculate the shortest bending distance D with the standard sine wave c , when D c >D set When , calculate the peak-to-valley time difference S of the periodic waveform; otherwise calculate the peak change rate of the inrush current for three consecutive cycles; Judgment S>S set :When S>S set When , it is judged as internal fault current, otherwise it is judged as symmetrical inrush current; Calculate the peak value change rate of the inrush current for three consecutive cycles: when the peak value change rate of the inrush current for three consecutive cycles is >0, it is judged as the response inrush current, otherwise it is judged as the excitation inrush current.

2. The method for identifying a responsive inrush current based on waveform morphology according to claim 1, characterized in that: The dynamic time warping algorithm DTW uses elasticity measurement when comparing two time series data. Suppose there are two sequences A = {a1, a2, ..., a m} , where a1, a2…a m represents the continuous sampling points of sequence A; B={b1,b2,…,b n} , where b1, b2…b n Represents the continuous sampling points of sequence B, and uses an m*n matrix to characterize the DTW path; in a two-dimensional coordinate system, waveform A is on the horizontal axis and waveform B is on the vertical axis. The regular path will pass through the matrix; The DTW path is the shortest curved path S between waveforms A and B, which is represented by S = {s1, s2, ..., s k }, where s1,s2,…,s k Represents the continuous sampling points of the curved path S; the qth element of the S path is represented by S q =(i q ,j q ), where i q 、j q Respectively represent S q The horizontal and vertical coordinate points of the A and B waveforms are aligned. At the same time, the constraints satisfied by DTW path calculation are: (1) Boundary conditions: starting point S1 = (1, 1), end point S k =(m, n), where m and n represent the end point S k The horizontal and vertical coordinate points of the two waveforms match each other; (2) Continuity: The curved path S is continuous in S k-1 Move to S k When , the moving distance between two adjacent points on the horizontal and vertical coordinates must be less than or equal to 1, that is, i k -i k-1 ≤1,j k -j k-1 ≤1 condition, where i k -i k-1 ≤1 means that the distance between two adjacent points of the horizontal coordinate is less than or equal to 1 when S moves continuously. k -j k-1 ≤1 means that the distance between two adjacent points of the ordinate is less than or equal to 1 when S moves continuously; that is, in the matching process of waveforms A and B, matching with points of the adjacent time step cannot cross a certain point, ensuring that each coordinate of the A and B trajectories appears in a regular curved path; (3) Monotonicity: Adjacent steps must satisfy i k -i k-1 ≥0,j k -j k-1 The ≥0 condition indicates that the path S is monotonically increasing over time; DTW calculates the sum of the distances between similar points in two sequences to measure the similarity of the waveforms and finds the optimal path in dynamic regularization, which can be described as: γ(i,j)=d(a i ,b j )+min{γ(i-1,j-1),γ(i-1,j),γ(i,j-1)} (1) Among the regular paths that satisfy the above three constraints, the shortest curved path is selected. This path is the DTW distance between waveforms A and B, expressed as: Where d(S k ) represents the curved path S that satisfies the above conditions and moves from S1 to S k The distance moved, DTW(A,B) represents the shortest bending distance calculated; Normalize the path distance to get the standard DTW distance, which is expressed as: When using the DTW algorithm to determine waveform similarity, waveform A is used as the benchmark for the standard sine wave, and waveform B represents current signal data such as internal fault current, combined inrush current, and combined inrush current from superimposed faults. The standard DTW distance value is in the (0, 1) range, and the calculated standard D value is the similarity between each type of telecommunications signal and the standard sine wave. The closer the D value is to 0, the higher the similarity. The closer the D value is to 1, the lower the similarity is.

3. The method for identifying a responsive inrush current based on waveform morphology according to claim 2, characterized in that: In S3, the time difference between the peak and valley values ​​of the symmetrical waveform is determined as: When DTW analysis shows that Dc>Dset, the peak and valley of the current are taken, and the time difference L between the peak and valley values ​​of the symmetrical waveform is added. The S value is used to measure the proportion of the relative distance between the peak and valley values ​​in a cycle, that is, S=L / 200. The phase difference is set to a margin of 30°, that is, when the phase difference between the peak and valley values ​​of the cycle is less than 150°, it is a symmetrical inrush current. The judgment threshold is set to the ratio of the peak and valley phase difference to the complete cycle, that is, the judgment threshold S set It is 0.4167.

4. The method for identifying a responsive inrush current based on waveform morphology according to claim 3, characterized in that: In S4, the changes in the calculated inrush current and the magnetizing inrush current are expressed as follows: Where, f n is the cycle peak change rate; |I n | is the peak value of the inrush current in the nth cycle; |I n+1 | is the peak value of the inrush current in the n+1th cycle; When the inrush current transiently increases, f>0, and when it decays in a steady state, f<0; due to the continuous decay of the excitation inrush current, its cycle peak change rate is always f<0; in order to ensure the effectiveness of the judgment of the transient increase stage of the inrush current, the change rate of the peak value of three consecutive cycles after the inrush current appears is used for judgment.

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