High-impedance Fault Detection Technology Based On Transient Current Projection In A Resonant Grounding System

Resonant grounding system high-impedance fault(HIF),such as tree,lightning arrester incomplete breakdown,and conductor drop on the grass,road,land,ponds,etc.,occurred frequently in distribution networks.The system will run with faults for a long time,and this may result in phase to phase faults or short circuit faults,which expands the scope of the fault and fault properties,especially the wire fall the earth will have great personal safety hidden trouble.Therefore,for HIF,an effective HIF detection technology is in high demand.In a resonant grounding system,the characteristics of the electric parameters comparing with the low-impedance fault are less obvious and the fault point is more unstable.All of these cause great difficulties to the high impedance faults detection.The fault transient mechanism and characteristics of transient electric parameters of high impedance fault in a resonant grounding system is analyzed.The transient process is the parallel resonance between the Petersen coil and the system capacitance to ground,and the range of the resonance frequency is from zero to a little larger than the power-frequency.The its decay time constant is generally between 3ms and 600 ms.With the increasing of the fault resistance,the transient current magnitude becomes smaller.Different from the transient electric parameters characteristics of low-impedance fault,the magnitude of the transient current in high impedance fault of the faulty feeder is not the largest and the polarity is not opposite to the healthy feeder,the fault characteristics are not certain.In the projection component of the transient zero-sequence current with respect to the transient zero-sequence voltage,the magnitude of faulty feeder is the largest and the polarity is opposite to the healthy feeders.The fault characteristics are obvious.The transient zero-sequence currents of the healthy feeder are the feeder capacitance current to ground which is proportional to the derivative of the transient zero-sequence voltage,and approximately orthogonal to it,and the projection coefficient is a small negative value.The transient current of the faulty feeder consists of the feeder capacitance current to ground and the transient current at the fault point,the latter is equal to the quotient of the transient zero-sequence voltage and the transition resistance,and the projection coefficient is a large positive value.The transient zero-sequence currents of detecting points downstream fault point are the sum of capacitance current to ground of all downstream section,which is proportional to the derivative of the transient zero-sequence voltage,and approximately orthogonal to it,and the projection coefficient is a small negative value.The transient current of detecting points upstream fault point consists of the transient current at the fault point and the sum of capacitance current to ground of all downstream section,the former is proportional to the transient zero-sequence voltage's magnitude,the scale factor is the reciprocal of the equivalent resistance,and the projection coefficient is a large positive value.Therefore,the faulty feeder can be determined by the difference of the projection coefficients.The current at the fault point and the transition resistance can be calculated by the projection components.Similar to faulty feeder selection,faulty section can be determined by the difference of the projection coefficients.The feeder which the projection coefficient is positive and greater than the sum of the projection coefficients of all the other feeders is faulty feeder.Otherwise,there is a bus fault.If the difference of current projection magnitudes between two adjacent detecting points is larger than presetting number,this section could be located as fault section.Otherwise,the downstream of last detecting point is the fault section.By using the transient zero-sequence voltage as well as the transient current in the faulty feeder and one of the healthy feeder,the transient current at the fault point and the transition resistance can be calculated.The correctness of the fault characteristics,mechanism,principle is verified by MATLAB simulation.The feasibility of the faulty feeder selection and faulty section location methods are verified by simulation and field data.This paper improves the fault transient analysis and accuracy of faulty identification in high impedance faults.And the paper provides basis for high impedance fault compensation,and over-voltage protection technologies.