The Study Of Fault Detection Before Switching On

In power system, all closing actions are blind, in other words, we do not know whether there is a fault in the transmission line before the breaker switches on. If the transmission line with a fault is connected with the bus, the fault would be brought to the power system which is normally running, which will cause a series of serious effects. To avoid this kind of situations, the most effective way is that we detect the transmission line before it is connected to the bus and determine whether there is a fault in it. The breaker is forbidden to switch on if a fault is found in the detecting.This idea is partly implemented with the appearance of adaptive reclosure. After the adaptive reclosures adopted in extra high voltage transmission lines trip, it can automatically determine what kind of faults (transient faults or permanent faults) occurred and avoid reclosing with permanent faults. Because of the limitation of adaptive reclosure, it can not effectively solve all problems concerned with the fault detecting before switching on, for example, we can not detect faults before the breaker switching on in a transmission line after long-time power interruption.This paper proposes effective methods to detect most kinds of faults occurred in transmission lines which are in effectively neutral grounded system. In accordance with the character that there is an imbalance in three-phase loads in the transmission line after a ground fault occurs in effectively neutral grounded system, we propose a new method named zero sequence current detecting method to detect whether there is a ground fault in the transmission line. An adjustable pulse current is synchronously injected to there-phase wires and then we can determine whether there is a ground fault occurred in the transmission line according to the difference in there-phase pulse currents. Results of theoretical analysis, computer simulation and lab experiments have all confirmed the accuracy of the proposed zero phase current detecting method, which can effectively detect the ground faults in effectively neutral grounded system.According to the potential phase-to-phase faults in effectively neutral grounded system, this paper proposes a new method named grounding current detecting method which can determine whether there is a phase-to-phase fault in a transmission line. Before switching on, a pulse generated from a pulse generator is injected to a certain phase wire in the transmission line and the other two phase wires are connected with ground; we can determine whether there is a phase-to-phase fault according to detecting and identifying the respective currents of the two grounded wires. The grounding current detecting method bases on the character that there is a direct electrical relation via fault resistance between two wires in which a phase-to-phase fault occurred and there is only a couple effect via phase-to-phase reactance and phase-to-phase capacitance between two wires in which a phase-to-phase fault did not occur. Results of theoretical analysis, computer simulation and lab experiments have all confirmed the effectiveness of the proposed method.According to the fact that there are either ground faults or phase-to-phase faults in transmission lines, this paper proposes an integrated method which can detect both ground faults and phase-to-phase faults. No matter what kind of faults occurring, the integrated method can determine them before switching on.Lab test is the important method to check the valid of new theory. The lab test platform which is made up of controllable pulse generator, transient current pulse detector and simple distribution network lays a foundation for proving the valid of fault detecting method before switching on.