Study On Adaptive Earthing Protection Algorithm Base On Transient Feature

In our country, non-effectively earthed neutral way which also called small current grounding system is widely used in 3kV-66kV medium voltage distribution networks. Failures happened in this system are mostly single-phase ground fault. As the social development and the advancement of technology, people asked higher requirements on the stability and quality of power supply. As a result, electricity sectors must find the exact point of failure in the short time as possible as they can when a failure happened, remove the fault line and resume normal power supply. Although domestic and foreign researchers have done a lot of works on earth fault line selection, made a lot of method to select fault line and have a far-reaching impact .but view from the result of selecting device operation, the rate of selecting the correct line is low.so it is necessary to continue study methods of fault line selection.The model which a lot of researchers useing of is always only aerial lines or only cables. It have a few lines which is so long.So the model is different of actual power .In reality, transmission lines of distribution network is often line - cable Mashup .Some is short and often shorter then 10Km. Therefore, this thesis build simulation model basing on a 10KV substation system parameters of Manchurian.This model have seven lines and every line is shore,have aerial line、cable and line - cable Mashup.As synthesizing a large number of documents and their analysis and a lots of simulation,the thesis conclude that:the result using Mayr arc model is more consistent to theoretical analysis then using pure resistance.So study line failure with Mayr arc model. Although the transient state process of single-phase fault is short, its characteristic quantities have more fault information then the steady state. Wavelet packet can decompose low and high frequency parts of a abrupt change signal and select the appropriate frequency band adaptively according to the characteristics of the analyzed signal.it have good time-frequency localization characteristics.So it can be used in analyze the transient characteristics.In special frequency band, failure phenomenon is obvious. The zero-mode current amplitude of fault line is greater then all non-fault lines and the zero-mode current phase of fault line is opposite with all non-fault lines.so it can be as the theoretical basis of fault line selection. Line selection method is that: in special frequency band, decompose the characteristic quantity of the top three lines with db5 wavelet packet which amplitude is large, select the node which have maximum energy and compare amplitude and phase.The line is fault line which zero-mode current amplitude is largest and zero-mode current phase of first half-wave is opposite with other line and zero-mode voltage of system.The thesis test this selection method can select fault line accurately and has some practical value by several typical examples of fault simulation...