Research On A New Method For High Impedance Faults Detection Under Different Grounding Methods In Medium Voltage Distribution Networks

High impedance faults are a common form of distribution network faults with weak fault characteristics,accounting for about 5%to 20%of medium voltage distribution network faults.If the unrecorded situation is considered,this number will be even higher.For some grounding materials with very high impedance,the grounding current amplitude is only a few amperes,accompanied mainly by arc generation,and traditional protection cannot guarantee reliability.The IEEE Relay Protection Committee reports that the success rate of detecting high impedance faults above 1kΩ is still less than 20%.If a faulty feeder is not detected in time,this could lead to a massive power outage or forest fire and the risk of personal electrocution.Therefore,it is important to study high impedance faults detection techniques.In this thesis,based on the analysis of the fault characteristics of the resonant grounding system,small resistance grounding system,and flexible grounding system of distribution network,high impedance faults detection method of the resonant grounding system,the fault detection method of small resistance grounding system and high impedance faults detection method of flexible grounding system are studied.(1)For resonant grounding system high impedance faults detection,the analysis of zerosequence impedance characteristics through a zero-sequence network is obtained that the amplitude of the healthy feeder current in the characteristic frequency band range is smaller than the faulty feeder.,and the phase of the healthy feeder current is opposite to the faulty feeder.Thus,the resulting features propose a method for detecting high impedance faults in resonant grounding system based on the empirical wavelet transform(EWT)and multi-criteria fusion.Specifically:Firstly,the EWT algorithm is used to effectively extract the current signal in the characteristic frequency band.Secondly,the amplitude,phase,and polarity of the original waveform are measured by constructing the characteristic energy ratio,characteristic interrelationship number,and characteristic integrated area for the current signal,respectively.Finally,the final voting method is used to achieve the high impedance faults detection based on the principle of "minority follows majority".In the radial resonant grounding system containing Distributed Generations(DGs),different types of high impedance conditions were verified,and all of them showed that the method has good accuracy and remains effective even under 5dB strong noise interference.In addition,the method can detect faults in 10kV true-type test data and field recording data.(2)For small resistance grounding system high impedance faults detection,the analysis of the zero-sequence equivalent network is obtained that the phase angle difference between the faulty feeder zero-sequence current and the healthy feeder zero-sequence current is 90.97°～119.29°,and the amplitude of the faulty feeder zero-sequence current is greater than that of the healthy feeder.Thus,the characteristics propose high impedance faults detection method for small resistance grounding system based on the integrated inner product value.Specifically:The integrated inner product value is constructed using the zero-sequence current and neutral current of each feeder,and the sign criterion and magnitude criterion are constructed for the integrated inner product value to achieve fault detection.The method is validated in a typical radial distribution network,modified IEEE-13 node,and modified IEEE-34 node,and also,considering the case of asynchronous sampling,missing data and,noise interference,it is still effective even under 5dB noise interference.The method has short calculation time and high accuracy compared with the composite power method,and the method still has high accuracy in filed recording data.In addition,based on the integrated inner product value,the method of detecting the same phase and simultaneous ground fault of two feeders in a small resistance grounding system is proposed.The method is verified in a typical radial distribution network,showing high accuracy.(3)For flexible grounding system high impedance faults detection,the analysis of the zerosequence equivalent network is obtained that the amplitude of the steady-state component of the faulty feeder current is significantly larger than that of the healthy feeder after the small resistance input,and at the same time,a significant reduction of the bus zero-sequence voltage occurs after the small resistance input.Thus,the characteristics propose a flexible grounding system for detecting high impedance faults based on product projection in different time periods.Specifically:Firstly,the inner product projection of each feeder’s zero-sequence current after small resistance input onto the bus zero-sequence voltage before small resistance input is constructed as criteria 1.Secondly,the inner product projection of each feeder’s zero-sequence current after small resistance input onto the neutral point zero-sequence current before small resistance input is constructed as criteria 2.Finally,when the results of the faulty feeder determined by criterion 1 and criterion 2 agree,thus,the faulty feeder is obtained.Tests in a typical radial distribution network show that the method has good stability under extreme conditions such as asynchronous sampling,missing data,arcing and cross-line two-point successive ground faults,even with 2dB noise interference,and comparison with detection methods for power,harmonics,and similarity shows that the method has high accuracy and the shortest judgment time.In addition,the method is suitable for the reverse connection of transformers.