Fault Diagnosis And Solution For Open-neutral And Line-break Fault In Low Voltage System

Low-voltage Distribution Network(LVDN) often encounters the problems of open-neutral and line-break resulting from thefts or other external forces, which will pose threat to the operation of distribution line and user equipment. However, the two problems failed to attract much attention for a long time, due to the poor automation level and the fact that the faults are usually imperceptible.The faults of open-neutral and line-break in LVDN will lead to economic loss or personal injury. When open-neutral happens in LVDN, the potential of zero line in users’ side will shift, the shift will further result in overvoltage or undervoltage accidents, the accidents cause users’ device burned or not work. Asynchronous motor is the primary load in low voltage system. When line-break occurs in power supply system, the non-broken line would have to bear large current, thereby the quick accumulation of heat can damage the insulation or burn the windings. Therefore, it is quite necessary to detect open-neutral and line-break faults in low voltage system and take according measures to avoid possible accidents.According to the features of 400V three-phase four-wire LVDN whose neutral point is grounded directly, this dissertation analyzing the varying of electrical characteristic parameters that are caused by open-neutral and line-break. As a result, the dissertation concludes the adaptive zero-sequence differential voltage method to detect open-neutral fault, as for line-break fault detection, a method that based on amplitude and phase mutation of line negative sequence current is provided. Simulation shows that the criterion can effectively identify open-neutral and line-break fault. The criterion is simple and easy to extract the signal. It can quickly determine open-neutral or line-break and take according protection measures. On Matlab/Simulink platform, a typical 400V LVDN system is built, by testing the open-neutral and line-break fault, the detection sensitivity and correctness of the criterion are both verified, which provide a basis for fault protection device.The dissertation proposes different solution for different user load. In this dissertation, the way to select attention node and make a node distribution solution are discussed for cable lines, overhead lines, mixed type and other types of power-distribution lines.when open-neutral and line-break fault are identified correctly, meanwhile, the power supply of the fault line is timely removed, for better ensuring the safety of power distribution system, it is recommended to provide back-up power for important load where power loss is not allowed.