Research On Detecting Single Phase-to-ground Fault For Distribution Network Based On Wavelet Packet Transform And ARMA Models

With the rapid development of power system, in order to ensure the reliability of power supply system, many medium and low voltage distribution networks in the world use neutral point un-earth system and resonant grounded system. Facing to more and more requirements, such as demands of power quality and environmental influences, it was very important to send alarm information to control center and select the exact fault line when a single phase-to-ground fault happens. Though there are always line selection and alarm equipments consists in many power systems, they offen run with very low efficeincy of line selection. To the characteristics of single phase-to-ground fault of distribution network, this thesis proposed a method of detecting single phase-to-ground fault for distribution network based on wavelet packer transform and ARMA models, by summarizing and analyzing the methods used before and the characteristics of transient quantity of electricity in the fault line. And the simulation results verified the proposed method by the software of MATLAB (Simulink).Based on the analysis of the transient and steady characteristics of quantity of electricity, this thesis proposed an idea of reconstructing the zero-sequence-current and method of line-selecting during falt occures according to its data characteristics. Since the wavelet packer can do the decomposition and denoising for both the low frequency signal part and high frequency signal part at the same time, db8basis function of wavelet was chose to do the denoising pretreatment. And the signal’s outliers is analyzed and the zero-sequence current signal is obtained by time series analysing ARMA model, and the corresponding transient response curve was also obtained. According to the different amplitude and phase of refitting zero-sequence current as the main criterion for line-selecting, and its different amplitude of transient response as auxiliary criteria for the line-selecting to complete fault line selection function. Finally, this thesis constructed a10kv distribution network simulation model through the simulation software of MATLAB(Simulink). For the different fault resistance and fault location, this thesis carried out a lot of simulations and analysis through the Wavelet Toolsbox and the System Identification Toolbox. The results of simulations verified the validity of the line-selecting method. And we can also get the conclusion that the proposed method has great ability of anti-interference with different fault positions or fault resistences. The method is suitable to nertral ungrounded system and neutral earth with an arc suppression coil system, it’s important to theory significance and practical value.