Fault Line Detection And Location Of Single-phase Fault In Distribution Networks Based On Frequency Domain Features

Single-phase-to-earth faults occupy 80% of fault events in distribution network.In addition,two-phase faults and three-phase faults are mostly developed from single-phase-to-earth faults.Fault line detection and fault location is meaningful for eliminating fault rapidly and raising reliability of power system.However,medium voltage distribution network in China usually uses neutral ineffectively grounded system,whose currents of single-phase-to-earth faults are small.So,fault line detection and fault location is difficult.Nowadays,many researches are aimed to solve the problems.Unfortunately,as a result of complex grid structures and fault features,methods can not handle some special situations such as high resistance grounding,low fault closing angle grounding and faults of hybrid lines or feeders with branches.Although the fault features of steady states are not distinct to detect,the transient zero-sequence currents contain much fault information.Common fault characteristics is extracted by analyzing zero-sequence current frequency spectrums.Based on this,methods for fault line detection and fault location are proposed.The work content of this paper is as follows:(1)An improved method for traditional fault line detection methods based transient energy is proposed.Through derivation,the frequency band to calculate transient energy is obtained,which avoids errors caused by compensation of arc suppression coil and raise accuracy in the situation of high transition resistance grounding.Aiming to solve the problem that traditional methods can not distinguish bus faults and the longest feeder line faults effectively,an identification method for bus faults is proposed based on the feature that the amplitudes of the power frequency component of zero-sequence current in each feeder are in direct proportion to capacitance to earth.Simulation proves that the method of this paper can detect fault line correctly in different fault situations and has the ability to make accurate results when capacitance to earth has measuring errors.(2)A fault location method based on centroid frequency and BPNN(Back Propagation Neural Network)is proposed.The centroid frequency is used to describe the feature that low frequency components of zero-sequence currents increase when fault distance grows longer.The impact on centroid frequency from fault distance,fault closing angle and transition resistance is researched,the relation between centroid frequency and its factors is simulated by BPNN.What's more,an assisted criterion is proposed to confirm fault location for the situation that the centroid frequency-fault distance curve is not monotone.Results prove that the method has an obvious advantage over other methods based on frequency domain feature in the terms of hybrid lines and feeder lines with branches and is applied for complex structures in distribution network.(3)A fault location method based on parameter estimation is proposed,which is anti-transition resistance and only use single terminal information.This method uses the ratio of amplitudes of the same frequency of zero-sequence current spectrum between fault point upstream and fault line head end.The ratio is called as ?.The frequency characteristic of ? is simulated by equivalent circuit,and the best values of equivalent circuit parameters are get by parameter estimation.Then,build the BPNN to simulate the relationship between fault distance,fault closing angle and circuit parameters.At last,the objective function is built to solve fault distance.For the reason that ? is not influenced by transition resistance,the method avoid impacts of transition resistances and errors caused by the variation of transition resistances during transient process.Through selecting different equivalent circuit models to simulate ?,this method can be applied for hybrid lines and feeders with branches,further,high resistance grounding and low fault closing angle grounding.