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The partial discharge development processes of needle-plate defect on oil-paper insulation under combined AC and DC voltage are studied in this paper.Firstly,several parameters of partial discharge processes were obtained by experiment,including mean PD frequency and amplitude which reflect the statistic distribution features of PD processes,as well as mean equivalent time and frequency which reflect the characteristics of discharge pulse waveforms.By analyzing the time-variation trend of these parameters,the PD development processes was divided into four stages,including initial,developing,steady and pre-breakdown stage.Secondly,in order to further study the characteristics of those four discharge stages,several kinds of spectrograms were constructed and characteristic variables were extracted from the spectrograms based on wavelet transform.At last,fuzzy clustering analysis was used to classify these characteristic variables into different groups,which represent features of different discharge stages.Once the database of samples was constructed,unknown samples could be identified into certain discharge stages.