Research On The Fault Detection In The Small Current Grounding Power System

That the neutral point is not grounded or grounded with arc suppression coil is defined as neutral point indirectly grounded. In the small current grounding power systems, when the single phase-to-ground fault occurs, because the amplitude of steady state current is small and intermittent arc ground, fault line selecting is very difficult. Several methods have been developed to solve the problem, but none of them can perform well under various fault conditions.To find an effective method, the strong transient components in the fault currents are considered to utilize. On the base of having introduced the fault characteristics in the steady state, we study the transition course when the single phase-to-ground fault occurs, and use the distributed parameter model of the line to analyse angle-frequency characteristics of the zero sequence network, then find the distributing rule if the zero sequence current. It is that in the frequency band from ω'L to ω'd(ω'L is trinal power frequency or higher frequency, ω'd is the minimum of all the heath lines' self resonance frequency), the impedeances of all the lines present capacitive; and the direction of the fault line's capacitive current is opposite to the heath lone's, if a fault occurs on the bus bar, they have the same direction; so it offers academic bases to find a new means for fault detection.In the paper, we utilize wavelet transform to analyze the transient zero sequence current and put forward a new criterion to select the fault line through many simulation experiments. The criterion is based on waveform recognition and the core of it is the principle that the minority is subordinate to the majority, i.e., provided in a certain high frequency band of the fault current the majority of sampled points conform to the fault characteristics, so according to the principle the fault line can be correctly detected. It is because that the fault signal is faintness and that all of the sampled points conform to the fault characteristics is difficult. And the new criterion isn't only limited in a frequency band to select the fault line. If it can't detect the fault line in the first frequency band, then it will continue the process in the next frequency band. Because of the characteristic it can be applied to different systems that have floating selection frequency bands. In the paper, aiming at different grounding resistances, different initial fault angles, different fault point, different fault phase and the variety of the load, large numbers of experiments have been done. The results show that the selection method can select the fault line accurately in each fault condition.A great deal of simulation experiments have proved that this method has high reliability; strong ability of anti-interference, and it needs not the on-site adjustment of the settings and is suitable to the distribution networks with different grounding modes. So it has a beautiful practicing future.