Faulty Line Selection Method For Single-phase-to-ground Fault In Resonant Grounding System

In a resonant grounding system,the steady fault feature of a single-phase-to-ground fault is greatly compensated,and the transient fault feature varies under different conditions.Therefore,faulty line selection of single-phase-to-ground fault is always a major issue in the resonant grounding system.Among all types of grounding faults,the intermittent grounding fault may lead to a long-lasting unsteady state of electromagnetic oscillation,making it even more difficult to select the faulty line.Without prompt measures taken to determine and to remove the fault,severe phase-to-phase faults are likely to develop in a very short time,which will have a negative impact on the reliability of the distribution network.This thesis first presents the equivalent circuit of the single-phase-to-ground fault in a resonant grounding system,and then analyzes both the power-frequency characteristic and the high-frequency characteristic of transient zero-sequence current.Analysis shows that with the change of compensation state there is a significant difference in the changing trend between the power-frequency current of the faulty line and that of a non-fault line.The distribution characteristic of transient current in the selected frequency band may suffer from the influence of the high-frequency output current of Petersen coil.As to intermittent arcing fault,its time sequence and fault feature in different stages are studied based on the cybernetic arc model.To verify the analysis above,simulation cases are built with PSCAD/EMTDC of grounding faults in a resonant grounding distribution network.Based on the changing characteristic of power-frequency current,a new faulty line selection method for single-phase-to-ground fault in resonant grounding system is presented.Prony algorithm is used to analyze the phase change of each line of power-frequency current signals during the transient stage.By comparing the difference among all lines in the amount of phase change,the result of faulty line selection is obtained.Simulation results have verified that the proposed method is capable of selecting different types of single-phase-to-ground faults,including the cases with high fault resistance and with arcing fault.In the case of intermittent fault,the phase change in a certain amount of time during fault transient is used to select the faulty line.The accuracy of faulty line selection for intermittent grounding faults may be improved by making full use of both the transient and steady fault features.Based on the analysis of fault sequence,this thesis proposes dividing the fault into transient stage and steady stage according to the state of zero-sequence voltage.Then the transient component of selected frequency band and the steady component of zero sequence active power are calculated separately.Furthermore,the improved criterion using the magnitude of transient power is proved to be effective in reducing the impact of Petersen coil based on thyristor controlled reactor.Simulation results verify that the proposed method can improve the sensitivity and accuracy of faulty line selection for intermittent grounding faults.