Fault Type Identification In Isolated Neutral System

In isolated neutral distribution network,single line to ground fault and fundamental frequency ferroresonance may result in the rise of zero sequence voltage. While the initiate mode of traditional line selection device relying on the zero sequence voltage, the state of single line to ground fault and fundamental frequency ferroresonance can’t be distinguished,so it may result in the misjudgment of the grounding line selection and computer harmonic elimination device.For single line to ground fault which has many forms, single phase metal grounding and single phase arc grounding are two typical fault types. When single phase metal grounding occurs, phase-to-phase voltages keep balanced, which has no effects on power supply to load,so the system can operate 1 to2 hours uninterruptedly according to the related regulations; But when single phase arc grounding occurs in power system, the intermittent arc light can further develop into overvoltage which results in the breakdown of equipment insulation and causes damages to safe operation of power grid, so the overvoltage should should be eliminated without delay.When the three types of fault metioned above occur in distribution network, the accurate identification、the corresponding processing and the quick power restoration play an important part in safe operation and the reliability of power supply. So fault type identification is of a certain significance to theoretical significance and engineering application value.The occurrence mechanism and fault characteristics of single phase metal grounding、single phase arc grounding and fundamental frequency ferroresonance in isolated neutral distribution network are firstly analysed in the paper. Then the function expressions of voltage in three conditions are deducted respectively by the methods of nodal voltage method、the fundamental frequency theory and kirchhoff’s law. Based on the arc model and electromagnetic voltage transformer model established firstly, the simulation model of single phase metal grounding、single phase arc grounding and fundamental frequency ferroresonance are also established respectively in MATLAB. Because of the obvious differences betweenthree types of fault are the waveform distortion and the amplitude of the voltage whose vlotage is minimum, so the waveform distortion and the amplitude of voltage are chosen as the characteristic quantities. In the paper, the wavelet multi-resolution analysis is adopted as a method for original signal processing. Then the total harmonic distortion and wavelet energy are quantitatively calculated, which are defined as a criterion of fault type identification.Simulation verifies the criterion for fault type identification proposed is correct and reliable.