Research On Single-Phase To Ground Fault Location Method In Distribution Network Based On Phase Current Power Frequency Variation

The distribution network branches go deep into each load center,and the operating environment is complicated.The transmission line is easy to contact with trees or buildings,and form a high impedance ground fault.When a high impedance ground fault occurs,the fault characteristics are very weak due to the large fault resistance value.Even when a single-phase high impedance ground fault occurs in a low-resistance grounding system,the fault characteristics are still very weak.The traditional zero-sequence over-current protection is difficult to operate reliably when the fault resistance reaches about 100Ω.In addition,when a permanent ground fault occurs in the flexible grounding system,a small resistance is connected in parallel with the arc-suppression coil to ground,so that the system changes from a low-current grounding mode to a large-current grounding mode,thereby enhancing the fault characteristics.However,the flexible grounding system also uses the traditional zero-sequence overcurrent protection to remove the faulty line.When a high impedance ground fault occurs,the protection is difficult to operate.Therefore,this article has carried out the following research work for the fault location problem of high impedance ground fault in the distribution network:(1)Analyze the characteristics of power frequency current variation when a single-phase ground fault occurs in the distribution network.The symmetrical component method is used to decompose each sequence network when a single-phase grounding fault occurs in the distribution network,and the sequence current branch coefficient is combined to deduce the characteristics of the power frequency phase current variation.The power frequency phase current variation is greatly affected by the neutral grounding method and fault resistance;the simulation based on PSCAD /EMTDC verifies the correctness of the theoretical analysis.(2)Combining the topological structure characteristics of each sequence network,analyze the value rule of the positive sequence and zero sequence current branch coefficients at different measurement points in the low resistance grounded system;and then analyze the the characteristics of power frequency variation at different measurement points,including upstream of faults point,healthy line,downstream of fault point and branch of fault feeder.Based on this fault characteristics,a fault location method based on the power frequency phase current variation ratio is proposed;The simulation based on PSCAD/EMTDC platform verifies that the proposed method can reliably identify the fault section and is less affected by the fault resistance.(3)Analyze the characteristics of the power frequency phase current variation in a flexible grounding system with arc-suppression coil and low resistance at the neutral point when a single-phase grounding fault occurs.The analysis shows that the arc-suppression coil in the flexible grounding system weakens the difference of power frequency phase current variation between the faulty phase and the healthy phases.The fault section location method based on the power frequency phase current variation ratio has certain limitations in the flexible grounding system.For this reason,the power frequency phase current variation is reconstructed by using the power frequency phase current variations before and after the small resistance put in.Combining with the reconstructed power frequency phase current variation and the limitation of the fault location method based on the power frequency phase current variation ration in the flexible grounding system,a fault location method based on the comprehensive power frequency phase current variation ratio is proposed for the flexible grounding system;the simulation based on PSCAD/EMTDC platform verifies that the proposed method can be used in the flexible grounding system to reliably select the fault section,and can withstand high fault resistance.