Research On The Fault Location Method Of Small Current Neutral Grounding System Based On The Distribution Network Automation System

The main function of distribution network automation system is fault location and isolation.The fault location is difficult because of the low fault current and the unstable grounding arc in the small current neutral grounding system.Most current theories about fault location method of small current neutral grounding system are still in the research stage.Hence,the main work of this paper is to study the fault location method of small current neutral grounding system.Firstly,the transient characteristics of small current neutral grounding fault in distribution network are analyzed.By using Karrenbauer transformation,the three-phase system is transformed into a module system without electromagnetic coupling,and it is simplified to obtain a simplified model of small current ground fault.Through analyzing the model,the paper can obtain the distribution rules of the transient fault current in the distribution network.Because the upstream and the downstream at the fault position circuits are relatively independent,the transient module current of each side is determined by the respective circuit parameters.The results show that the upstream and the downstream at the fault position circuits are relatively independent,and the transient module current of each side is determined by the respective circuit parameters.So the resonant frequency and amplitude of zero-mode current on both sides of fault point are very different.Using Matlab/Simulink software,the simulation model of small current neutral grounding fault system is built,and the fault characteristics are verified.Secondly,a method of small current neutral grounding fault system based on cross approximate entropy is proposed.The similarity of the transient fault signals in each section of the feeder is measured by cross approximate entropy.There exists high similarity of transient zero mode current waveform which is obtained from the FTUs on both sides of the sound section and the mutual approximate entropy of zero mode current is close to 0.On the contrast,the transient zero mode current waveform of FTUs on both sides of fault section is not similar and its mutual approximate entropy is greater than 1.By using this rule,the fault current waveform of the feeder terminal can be used to locate the fault position of small current neutral grounding fault and the fault location realization process of the distribution network automation system is given.Finally,a fault location method based on SVD is proposed.Two similar signals can be constructed into Hankel matrix respectively.Two groups of singular vectors,which decompose by SVD,are similar and their inner product is closed to 1.It is by inner product of these vectors that the similarity of transient fault signals of different FTUs can be measured and the fault position of small current neutral grounding system can be located.Two methods do not need voltage transformer to obtain voltage signal and they are simple implementations and low investment costs.The simulation verifies the feasibility and effectiveness of the proposed method in the isolated neutral system and neutral grounded by arc suppression coil system of the module of small current neutral grounding system.