A Study On Single Phase High Impedance Protect Set

Generally, single phase grounding fault can cause both high phase current and zero sequence current. Proper grounding protection set can act reliably and cut off fault. However, some unique geographical characters, such as fire grounding, bamboo grounding and treetop grounding, can cause single phase high-impedance grounding fault frequently. Since the voltage drops down very little and the voltage among three phases keeps symmetric, this fault increases the difficulty of the check on this fault. For fire grounding, grounded impedance varies greatly because of air dissociation, and this causes protection set to startup and resume again and again. Therefore, the main protection of the fault line can't cut off the fault quickly and accurately, and may lead to overstep tripping of near lines' protection sets and eventually cause more serious faults on the power system. So, detecting and finding this fault in time is very important. It's very necessary to develop a new high-performance scheme, which involves both principle and hardware. Firstly, we analyze the transient and steady character of single phase high-impedance grounding fault. Secondly, based on wavelet analysis on checking mutational signal and direction high frequency protection theory, utilizing zero sequence current and module current, this paper provides a new scheme including fault starting element, fault phase selection element, directional element and main criterion. This scheme can bear high impedance and is immune to system surge and frequency warp, which gets over the shortcoming of the lack of sensitivity. When this fault is cutted off and the breaker is reclosed, this scheme can't start by mistake. The availability of this scheme is proved with MATLAB test. The single phase high-impedance grounding fault is settled properly. In order to meet with the rate of sampling and operation, we design a suit of hardware with high sampling and operation rate. This hardware can complete 120 million times float operation in one second. The rate of A/D conversion is up to 500KSPS and the precision is up to 16 bits.