Research Of Fault Line Selection In The Low And Medium Voltage Distribution Network

In the power system of low voltage distribution network, most of the faults are single-phase ground fault, taking percentage of about 80 percent. The most difficult problem is the fault-line selection in the neutral point by the arc suppression coil grounded. The traditional way of line selection is the manual way method. Through the studies by the researchers at home and broad, new ways of line selection have been put forward, and line selection devices are put into operation. However, the effects are not ideal. The dropout rate is over 86 percent. When single-phase ground fault happens in the power system, the fault condition is complex, grounding current is small, and the problem of fault-line selection has not been completely solved. Reliable and practical distribution network is the insurance of improving the quality of electric energy, the reliability of power supply, and economical operation of power system.So,the problem of fault line selection in the distribution network has important research significance.First, for the method of wavelet analysis, this thesis introduces the basic principles of it.Meanwhile, it makes a detail introduction to the selection, sampling frequency, identification of decomposition scale, of the wavelet function. For solving the problem of fault-line under the circumstance of unsymmetrical power grid and the transition resistance being too large,taking the transient energy. This thesis makes an introduction to the basic principles of the line selection in the method of transient energy.Comparing two methods of the fault line selection.If the power grid unsymmetrical or transition resistance being too large,firstly used the method of transient energy to select fault line,then,using the wavelet analysis method to decomposite and reconstruct the zero sequence current of fault line.Making sure the time of occuring fault more exact.In combination with two methods of fault line selection.Then, in the simulation platform of MATLAB/Simulink, for the constructed distribution network of 10 k V, the system of neutral point non-ground and neutral point by the arc suppression coil grounded.Through the references and the actual situation of distribution network, to adjust the parameters of system model. when single-phase ground fault happens in the system of neutral point non-ground or neutral point by the arc suppression coil grounded,the steady and transient state characteristics of the two systems are analyzed. Under different condition, this paper through a lot of experiments to testify the methods of wavelet analysis and transient energy.Finally, analysis voltage waveform of each line and decomposition and reconstruction of the zero sequence current waveform, to analysis the fault characteristics when single-phase ground fault happens in the system. The correctness of the wavelet analysis method can be verified,and making sure the time of occuring fault more exact.For the fault line selection method of transient energy,it is suitable for single phase ground fault system that neutral point in any grounding way.The paper take the example of the single-phase ground fault in thesystem of neutral point by the arc suppression coil grounded, through simulation, lots of calculation, data statistics and analysis, the fault line can be found.The correctness of the method can be verified. The problem of the fault-line selection of the grounding system of small current under the circumstanced of unsymmetrical power grid and transition resistance being too large is solved.