Research On UHF Characteristics Of Partial Discharges In Transformers

Insulation monitoring of power transformers is important for thepower system’s safety. Partial discharge (PD) is a crucial indicator ofinsulating deterioration. Methods to detect partial discharge are dividedinto electricity detection and non-electricity detection. Ultra-HighFrequency (UHF) method belongs to the former. It uses UHF antennasinstalled on the wall of the transformer or in the valves to detect UHFelectro-magnetic waves radiated from discharging sources. Such UHFsignals have abundant information which is useful for solving problemssuch as whether, when, where, what kind of, how intense PD happens.UHF method, is widely used in condition monitoring recently for the sakeof its wider spectrum and higher sensitivity. Further analysis of UHFsignals could provide time domain information with parameters like phasesbeing researched. They could also tell frequency domain information withFourier Transform. Using Wavelet Analysis and Multi-Resolution Analysiscould tell time-frequency domain characteristics of partial discharges.Experimental studies on five different types of partial dischargesoccurred in oil-insulated devices using Ultra-High Frequency sensors todetect PD signals and using wavelet analysis to process the acquired UHFsignals are shown. After got proper wavelets for Continuous WaveletTransform (CWT) and Multi-Resolution Analysis (MRA), a thresholdselecting method adapting to various noise circumstances when de-noisingUHF signals using wavelet transform is proposed in this paper. The threshold method was based on numerical fitting of standard deviationsand manually set thresholds for brush-fire samples from a large scale.Horizontal comparisons between different geometries and verticalcomparisons between different intensities of partial discharges in oil wereanother part and the mathematical tool was wavelet analysis.Multi-Resolution Analysis (MRA) results show these significantdifferences above. Such differences can be used to recognize the dischargepattern and to judge the intensity.Particularly attention was pay to needle-plate discharge geometry,which was often used by researchers focusing on mechanism ofpre-breakdown phenomena in liquid dielectrics. The results of needle-platemodel show good agreement with electronic theory of liquidpre-breakdown mechanism.