Study On Single Phase-to-earth Fault Line Selection And Fault Point Location In Distribution Network Based On Direct Current Injection

In China, most middle voltage distribution power system (6-35kV) employs non- effectively earthed neutrals, which include isolated neutral networks and networks grounded with the neutral compensator coil. Single-phase to earth fault has a high failure rate in Middle voltage distribution networks. When single -phase to earth fault occurs, it doesn't form a short-circuit loop. The fault current is small and the three phase line to line voltages are still symmetric, so the load can be in normal operation for a period of time. But after the fault, the non-fault phase voltage would increase. Long time earthing operation will destroy the systematic safe operation as the result of two-point, moreover, multipoint earthing as well as the systematic over voltage caused by arc light. So single-phase to earth fault should be removed as soon as possible. In case of the fault can't be removed immediately, the fault line and fault path should be found as quickly as possible.Relay protection researchers all over the world have made much study and exploration on how to find single-phase earthed fault in non-effective earthed power system. Based on many literatures of China and foreign countries, reviews the developing history of the relay about the selection of single-phase earthed fault line in small current neutral grounding system all over the world, as well as principles and traits of various selecting criteria are analyzed in detail. This paper present the characteristics of single phase-to-earth fault in small current neutral grounding system. The steady and transient characteristic of ineffectively earthed distribution system are researched while single phase-to-earth fault occurs in it.This paper introduces a new criterion about the selection of fault line and the location of fault point in small current neutral grounded system-the direct current (DC) signal injection method. When single phase-to-earth fault occurs, a DC signal is injected to the fault line through the earthing transformer. The DC signal injected method identifies the feeder and locates the fault point by detecting the trace of the injected DC current signal. Demonstrates in theory the feasibility of the DC signal injection method is the paper's key point. The circuit model is built and the calculation of relevant parameters is analyzed. The characteristic formula used to calculate the critical fault resistance is deduced. Factors influencing the distribution system reliability are also analyzed. The DC signal injection method has improved the accuracy of line selection in single-phase to earth fault and increased the critical fault resistance。In this paper, the function, elements and simulation procedure of the power system transient simulation software PSCAD/EMTDC is introduced briefly. The model of the single-phase earthed fault in the distribution system is established using PSCAD/EMTDC software. From the simulation result, a conclusion can be drawn that the DC current in the fault line is distinctly different from that in the non-fault line. So it can be used for identifying single -phase to earth fault.In this paper, the key technologies of DC signal injection principles are introduced, including implementation of the DC signal generator, methods of detecting DC signal as well as realization of adaptive and adjustable resistor. This chapter mainly introduces the methods of detecting DC signal through wireless sensor technology as well as the implementation of adaptive and adjustable resistor using ordinary incandescent lamp.