Optimization Of Fault Line Selection And Adaptive Tripping Technology Under Small Current Grounding Fault

The existing single-phase grounding fault line selection methods are based on radial lines,but the characteristics of grounding fault will change when power is supplied to the bus line,so the existing line selection method need to be re-examined.At the same time,the penetration rate of inverter-type and rotating distributed generation(DG)into the distribution network is getting higher and higher,whose influence on the transient characteristics and transient detection of single-phase grounding faults needs to be further clarified.The research into adaptive trip of grounding fault based on the optimization of line selection technology can take personal safety and power supply reliability into account all at once.In this thesis,with the establishment of the equivalent circuit on single-phase grounding fault in distribution network with loop,the distribution characteristics of the steady and transient zero-sequence current at the exits of the loop are deduced,and the adaptability of the existing line selection methods is analyzed.When grounding faults occur in the loop,the electrical quantity at both ends of the loop does not always appear as the characteristics of the fault line,which means the existing steady and transient line selection techniques may cause miss election.This thesis proposes an improved method for grounding fault line selection,which can identify the fault line by increasing the number of alternative lines and using the change of each remaining line’s zero-sequence admittance before and after the resection of an exit line in the loop,and which can also make it by taking the sum of the zero-sequence currents at both ends of the loop into the line selection comparison.The simulation and field data have verified the correctness of the fault characteristics and the effectiveness of the line selection methods.In this thesis,for DG distribution networks,the grounding fault equivalent circuit of distribution network with DG is established,the variation law of grounding fault transient characteristics under different DG types is studied as well as different access locations.Its impact on the performance of device and the principle of transient line selection and location method is analyzed.Because the access of DG does not change the faulty zero-mode network,the existing line selection and location method using the transient zero-mode signal can still be applicable in principle.The access of rotating DG will increase the main resonant frequency and amplitude of the fault transient current,and expand the distribution range of attenuation time,correspondingly,the sampling frequency and current measurement range of the grounding fault detecting device should be moderately increased.The simulation and field fault data have verified the correctness of the results.Under the optimization technology of single-phase grounding fault line selection,an adaptive tripping scheme for single-phase grounding fault based on analytic hierarchy process is proposed through analyzing the requirements of power supply reliability in different power supply areas,and the development of grounding faults under different line types,and the probability of flashover of weak points caused by overvoltage,and the degree of damage of different fault types.By real-time monitoring of the system fault state,the scheme uses the analytic hierarchy process to determine the weighting factors of the four indicator factors,which include power supply area,line type,over-voltage level and fault type.The optimal tripping scheme will be selected according to the comprehensive weight comparison,which adaptively determine whether tripping and when to trip,and the credibility of the solution is verified through case analysis.