A Study On Location And Identification Method For Single-phase-to-ground Fault With Line Break In Distributiion Network

Single-phase-to-ground fault(SPGF)and single-phase-to-ground fault with line break(SPGF-LB)are common faults in distribution networks,and it is difficult to distinguish between SPGF and SPGF-LB by traditional centralized measurement systems.SPGF-LB is much more dangerous than SPGF because it can cause economic losses and safety accidents such as equipment burning,human and animal electric shock.In addition,distribution network systems in China are mostly small current grounding,which can keep running for hours after SPGF-LB and cause huge economic losses and injuries for human.Therefore,it is very important to find a way to locate and identify SPGF-LB.By current researches on location and identification method for SPGF-LB,it is founded that though location methods for faults have been widely used in distribution network,but these methods rely on steady-state or transient information,which is influenced by grid states.And there are few foreign related researches on the identification methods for SPGF-LB.Domestic researches are immature,which have insufficient considerations for fault conditions and is dependent on operation experience.To solve these problems,this paper proposed a location and identification method for SPGF-LB based on the studies of characteristics of SPGF-LB.The main research work and achievements are listed as follows:(1)A preliminary location method for fault section based on current-correlation is proposed.In this paper,the equivalent circuit model of fault section is established,and the difference of characteristics of three-phase currents between the fault section and the non-fault section are compared and analyzed as:the current-correlation between the source-side and load-side measurement points on the fault section is low.And in the non-fault section,the current-correlation is high.Based on this characteristic,a preliminary location for fault section can be achieved by comparing the current-correlations between each measurement point and its load-side measurement points.(2)A method based on wavelet transform for precise location for SPGF in distribution network is proposed.In this paper,the wavelet transform method is used to get the initial traveling wave from measuring points in the preliminary fault section to determine the polarity and arrival time of the initial traveling wave.And then the D-type traveling wave location method used to locate the fault point accurately.(3)The characteristics of the source-side and load-side voltage vectors of SPGF and SPGF-LB are analyzed.In this paper,the source-side and load-side equivalent circuit models with SPGF or SPGF-LB are established.The characteristics of source-side and load-side voltage vector of SPGF and SPGF-LB in conditions of different neutral grounding modes and different grounding resistances are calculated and analyzed carefully.The result shows that SPGF and SPGF-LB have almost same characteristics of voltage vectors,which makes it difficult to distinguish by source-side information.And at the load-side,there is a huge difference between characteristics of voltage vectors of SPGF and SPGF-LB,which can be used as the criterion for fault identification..(4)An identification method for SPGF-LB based on vector angle is proposed.Based on the result above,this paper proposes using vector angle |?| as the criterion for identification for SPGF-LB:if |?| ??2,the fault is a identified as SPGF-LB;and if|?|??1,,the fault is identified as SPGF;and if ?2<|?|<?1,the fault cannot be identified accurately.(5)Reasonable values of the identification thresholds ?1 and ?2 are proposed.In this paper,the factors influencing the accuracy of identification method are analyzed and calculated.Based on the result,reasonable values of the fault identification threshold are proposed as ?1-95° and ?2-85°.(6)The experiments to verify effectiveness of the fault location and identification method proposed is carried out.The simulation results based on PSCAD/EMTDC show that the fault location method for SPGF-LB can accurately locate faults at different locations with very small errors.And the field experimental results show that the identification method for SPGF-LB can keep extremely high identification accuracy in condition of different neutral grounding methods,different grounding resistances and different three-phase voltage unbalances.In general,the location and identification method for SPGF-LB which is proposed in this paper has solid theoretical foundation,wide applicability and high accuracy for fault location and identification.And it is promising to solve the problem of location and identification for SPGF-LB in distribution networks.