The Study Of The Single-phase Fault Line Selection Of The Small Current Grounding System Based On Wavelet Analysis

Neutral non-grounded or Petersen-coil grounded systems,namely neutral un-effectual grounded system(NUGS),are widely applied to power distribution networks in China.Although the single phase-to-ground fault happens most frequently in distribution network,the phase of zero sequence current on fault line is weak,specially for the neutral resonant grounded system,and the remnant current of the grounded point on fault line is more weaker,due to the small fault current and unobvious fault features after grounding.The problem of fault line detection has been studied for years and still has not been solved satisfactorily,which has seriously blocked the reliability of power supply and the development of synthesis automatization level.So advanced wavelet theory is applied to solving the technique problem of fault line selection for single phase-to-ground fault in NUGS in this thesis,which assumes huge theoretical importance and significant practical value.Firstly,single phase to ground fault course of small current grounding system is deeply researched into,with many high frequency component features discovered in transient fault course.Wavelet analysis is localized both in time domain and frequency domain,and can cut any signal into different frequency components,thus making it a suited tool for analyzing unsteady signal and transient signal.Theories and concepts about wavelet analysis are given an in-depth analysis,which provides a theoretical basis for the extraction of fault features.Making use of wavelet's superior ability to dispose of the transient singular and faint signal,the paper constructs a fault line detection criterion based on the maximum value irregularity examination of the wavelet's transformation mold.The fault line is detected in terms of the features of magnitude and polarity of wavelet coefficient of transient zero-sequence current in certain scale,thus the proposed method overcomes the limitation of steady and other transient approaches.Secondly,a lot of simulations are used to analyze transient earthed fault signals in35 kV voltage and three lines connected to bus of Petersen-coil grounded systems by MATLAB.The analytic results show that the method presented by this paper can help spot fault line quickly and has higher reliability and accuracy.Finally,on the design of the line selection device,combined with the particularity of the protection of line selection,the technical characteristics of the line selection device as well as the functions and function requirements of the line selection device in the modern distribution network are analyzed.On this basis,a single-phase fault line selection device for the distribution network is designed which employs embedded system and bus way.