Research On High Impedance Grounding Fault Detection Method Of Distribution Network

At present,the modes of neutral point of distribution networks generally are grounded with a arc-suppression coil.This grounding mode can significantly reduce the capacitor current at the fault point.However,due to the weak fault current and indefinite direction of the zero-sequence current,faulty feeder identification is very difficult.Due to the high transient impedence of the high-resistance grounding fault,the fault point is unstable,and conventional protection is difficult to operate.When a single-phase high impedance fault(HIF)for the resonant grounding system(RGS)happened,the fault electrical characteristics are more difficult to extract,thus the fault detection and faulty feeder identification are more difficult to achieve.In order to solve this problem,this paper studies the method of identifying and selecting highresistance ground faults in resonant grounding systems.A method for identifying high impedance faults in a resonant grounding system based on MFDFA(Multifractal Detrended Fluctuation Analysis)and multi-stage SVM(Support Vector Machine)is proposed.First,a zero-sequence transient equivalent model suitable for high-resistance grounding faults and low-resistance grounding faults is established,and the transient processes of the two faults underdamped and over-damped are analyzed in detail.Theoretical analysis and simulation results show that most high-impedance faults are under-damped and the zero sequence current of feeder will occur beat frequency phenomenon after fault.The selfsimilarity of the waveform is low,while the waveforms of the low-impedance fault under-damped and over-damped are of high self-similar.Then,the MFDFA algorithm is used to analyze the zero-sequence current of the arc-suppression coil after the fault.The fractal parameter curves of different fault states have obvious differences: H(q)of the multi-fractal characteristic parameter curve of the zero-sequence current of the arc-suppression coil under high resistance and under-damped state satisfies the arc tangent distribution;the fractal parameter H(q)in the low-resistance and underdamped state tends to be constant;the fractal parameter H(q)in the over-damped state changes slightly,but the variation range is small.Finally,the four eigenvalues of the fractal parameters are selected to form a eigenvector,and the eigenvectors are input to multi-stage SVM to realize the detection of a high-impedance grounding fault.The numerical simulation results of MATLAB and recorded data verify the accuracy and anti-interference of the method.A method for faulty feeder selection of high impedance fault based on zerosequence current characteristics is proposed.First,the zero-sequence equivalent model of the high-resistance grounding fault in the distribution network is established,and the impedance characteristics of the feeder and the amplitude and polarity relationships of the zero-sequence current in different frequency bands are analyzed.According to the minimum series resonance frequency of each feeder,and the parallel resonance frequency of the feeder and the arc suppression coil,high frequency bands and low frequency bands are defined.After the single-phase high-impedance grounding fault occurs in the resonant grounding system,the zero-sequence impedance of healthy feeder presents capacitance characteristic in both high and low frequency bands and the zero-sequence current is purely capacitive.However,the zero-sequence impedance of the faulty feeder is inductive at low frequency and the zero-sequence current contains the inductive component of the arc suppression coil,and its amplitude is greater than that of the high frequency.Then,the low frequency band is selected as the characteristic frequency band.The low-pass filter is used to extract the bus zero-sequence voltage and the signal components of the feeder zerosequence current in the low-frequency band.The difference between the product of zero sequence current and pure capacitive current cross of any two healthy feeders is 0.But the difference between the product of zero sequence current and pure capacitive current cross between the faulty feeder and healthy feeder is not 0.Therefore,the difference between the products of zero sequence current and pure capacitive current cross-multiplication of any two feeders is compared.The largest one is the faulty feeder.Finally,the single-phase high-resistance grounding faults under different working conditions were verified respectively.The MATLAB simulation results and recorded data verification results show that the method is accurate and adaptable.