Research On Single-phase Grounding Fault Location Approach In Distribution Network Based On Time-frequency Energy Spectrum Analysis

To ensure the security and stability of economical power system operation in China, small current grounding system is mainly used in lo w voltage distribution network of China. The existing literatures have shown that the single-phase grounding fault is the most prone to various types of power system fault. When the line fault occurs, the system operation stay stable for a period of time, but it is easy to expanding the scope of the fault and reducing power supply reliability of the power system if the fault position may not be found in time. In many cases, it is desirable to have a protective relay operate as fast as possible in order to control damage to major equipment and preserve system stability. Therefore, locating the fault position quickly and accurately has extremely important significance to shorten the power recovery time, reduce the economic loss and improve security of the grid. Because the fault feature extracting is difficult, the problem of fault line selection and fault position location in distribution network has not been effectively solved. In this case, the paper proposes a procedure based on the time-frequency spectrum analysis of wavelet transform for the analysis of current transients due to line faults, and discusses its application to fault location in power distribution systems.Through detailed analysis of the single-phase-grounding fault transient characteristics based on the excellent time-frequency analysis ability of wavelet transform, a new fault line selection method based on the transient energ y of zero sequence current signals is presented in this paper. In this method, wavelet transform is utilized to decompose and reconstruct the transient zero sequence current and calculate the energy value of the reconstructed signal at each scale. The maximum energy band is selected as the characteristic energy band. By improving the energy calculation methods, an energy difference between the healthy line s is reduced, while difference between the fault line and the healthy line is expanded. It’s easy to calculate by the energy calculation method, which overcomes the influences caused by the signal reversed phase to fault line selection. Finally, a large number of simulation results demonstrate that the proposed method has a good adaptability to select the fault line correctly under different fault conditions.In view of the waveforms discrepancy of transient zero-sequence current between the adjacent sides of the fault position, combined with similarity theory in image processing, a fault section location method in distribution network based on the time-frequency spectrum similarity is presented. Firstly, we apply wavelet packet to decompose transient zero sequence current of each detection point on the fault line. Secondly, we calculate energy for each frequency band, which is divided on the equal time interval. Then we obtain wavelet precise spectrum matrix of each detection point. Afterwards, by further calculating the time-frequency energy spectrum similarity between the adjacent detection points, the fault area is selected as the region with the smallest time-frequency spectrum similarity. The simulation analysis shows that the proposed method has strong adaptability and high positioning accuracy which is not affected by condition of different grounding resistance or fault initial angle.