Distribution Network High Impedance Fault Resolving Technique Study Based On Transient State Projection Method

Neutral resonant grounded system(NES)is widely adopted for our county's low and medium voltage distribution network,by this kind of grounding when single phase to earth fault occurs the current at the grounding point is small.It could be deemed as high impedance fault(HIF)when fault resistance is bigger than three times of zero sequence impedance of the system,at this moment the voltage of system phase and phase position will change according to fault resistance to maintain the system running for a certain period.When the circuit running with fault for a long term,phase voltage will raise and damage the insulation over system equipment and cause fire at the fault point,especially when broken cable dropped on the ground fault with fault current contacting ground step voltage will be generated and life be threatened.So when single-phase earth fault occurred fault circuit should be identified at once,and to be got rid of.Comparing with metal grounding and low resistance grounding,HIF is more difficult to be resolved since its electric value change is not obvious.Over studying steady state equivalent circuit we learn impact of transient resistance on neutral voltage,fault phase voltage and current at the fault point;over studying HIF transient process we learn feature of bus bar zero-sequence voltage,difference between intact circuit and fault circuit,difference between amplitudes and phase angle of fault point upstream zero-phase-sequence current and that of down steam zero-phase-sequence current at moment of HIF,based on the above differences using a new algorithms to amplify difference between intact circuit and fault circuit,difference between fault point upstream and fault point downstream,to identify fault line and section.Based on the above method,according to the fact that line frequency reactive-power value is positive under overcompensation situation and at the moment of HIF,distinguished zero-phase-sequence current-transformer connected on circuit in counter way.By obtaining pure transient state voltage quantity and pure transient state voltage quantity of each testing point to eliminate impact of system unbalanced current and steady state current on fault feature.Based on the principle active power of intact circuit and each testing point at down stream of fault point are small,but active power is big on up stream of fault point,project pure transient current quantity of each testing point on that of bus-bar to amplify difference between amplitudes and polarization of intact circuit and that of fault circuit and difference between amplitudes and polarization of fault point upstream and that of fault point down stream.To identify line and section of HIF by calculating dynamic time warping distance of pure transient zero-sequence projected current.Built NES simulation model by ATP-EMTP software,and analyzed impact of various fact on transient state zero-sequence current.Lots of simulating trials were done and it was proved that algorithms adopted in this article could identify the fault circuit specifically,the fault section could be distinguished effectively according to circuit selecting result produced in this way,it have same performance even under electromagnet interference with intermittent defect.At last the distribution net work automatic system capable of tackling single-phase earth fault was adopted to try the transient state projection method on single HIF circuit and section identifying to verifying the actual recorded wave properties,again it proved the suitability and accuracy of mentioned method.