Based On Fault Transient Information In Small Current Grounding System Grounding Line Selection Method

In the neural indirectly grounded power systems, the single phase to earth fault occurs most frequently. To select the fault line and locate the fault point quickly is of great importance for isolating the fault line, as well as improving the operation reliability and reducing the economic loss. However, the current theories and apparatus cannot select the fault line accurately at site, so could hardly satisfy the requirement of fault line selection and the reliability of power supply.To overcome the problem of fault line selection, the thesis summarizes the present theories for fault line selection and points out their advantages and shortcomings. At the same time, the steady and transient features of single phase to earth fault in various neural indirectly grounded systems is analyzed. Thus provides theoretical basis for the research on new fault line selection , and points out the direction of utilizing transient information.In this method, wavelet packet with the advantage of time-frequency zoom is used to decompose each line' s post-fault transient current with proper bandwidth. In view of maximum energy, the characteristic band, in which the transient capacity current is most concentrative and the feature of fault is most explicit, is adaptively taken out for each line. Based on the principle that the fault line's phase of transient capacity current are different from normal line's, the fault-line will be found out by contrasting polarity of each line's wavelet packet decomposition coefficient in characteristic bands. To eliminate the dead zone of fault detection, which appears when the fault is occurring with phase voltage near to zero, there is a creative method proposed in this paper called wavelet injection method. After the fault appearing, a wavelet signal will be injected the fault phase .Then by comparing the continue wavelet transform coefficients , the fault line will be detected easily.A fault line selection principle is analyzed in detail in this paper. A lot of simulation are used to analyze transient earthing fault signals in a 10KV neutral indirectly grounded system. Simulation data verifies the sensitivity and reliability of the proposed method. This paper may be valuable for the application of the wavelet in the power system.