Research On New Fault Line Selection Device For Single-phase Earth Fault In Small Current Grounding System

In China,the power grid of 110 KV and above generally adopts the neutral grounding effective grounding operation mode,and the 3~66KV adopts the neutral point non-effective grounding operation mode,also called the small current grounding mode.In the distribution network failure,the proportion of single-phase ground faults can account for more than 80% of the total number of faults.When a single-phase ground fault occurs in a small current grounding system,the magnitude of the fault current is small,the fault signal is unstable,and it is susceptible to interference from the site.The algorithm used in the existing line selection device is still based on the steady state method,and the fault information cannot be fully utilized,and the actual operation effect is not satisfactory.Therefore,the line selection problem of singlephase ground faults in small current grounding systems needs further study.This paper first studies the fault characteristics of single-phase ground faults in small current grounding systems,that provides a theoretical basis for the algorithm design of the new line selection device.The steady-state characteristics and transient characteristics of zero-sequence current in single-phase ground fault are analyzed.The model of typical small current grounding system is built in PSCAD simulation software to verify the variation of voltage and current after fault.At the same time,it lays the foundation for the proposal and verification of the later line selection algorithm.Because the small current ground fault condition is complex and variable,and the fault signal is weak,the single line selection algorithm has limitations.Based on the study of various line selection methods,this paper combines the advantages of transient and steady-state algorithms.The mathematical morphology mutation method using transient signals,the zero sequence current amplitude,the phase comparison method,the zero-sequence current residual current increment method and the active power component method using steady-state signals are simultaneously applied to the line selection device.The corresponding criteria for each algorithm is constructed to clearly and intuitively indicate the probability that each line is a faulty line.Comprehensively discriminate the results obtained by different line selection algorithms to achieve the final decision of the line selection results.Finally,the software and hardware structure of the line selection device are given.The line selection device uses the AM335 X as the CPU chip,which has low power consumption and low cost,and supports the Linux operating system.Data acquisition(A/D)function is realized by using AD7606+FPGA as sampling chip.The device adopts modular design,and each functional module is combined according to a certain logical relationship,which is easy to expand and modify.This device is suitable for neutral point ungrounded systems and arc suppression coil grounding systems.