Partition and recognition of partial discharge development stages in oil-pressboard insulation with needle-plate electrodes under combined AC-DC voltage stress

Partition and Recognition of the partial discharge (PD) development stages in oil-pressboard insulation under AC-DC combined voltage stress are the bases for analysis of the insulation faults in converter transformers. In this paper, behavior of different characteristic parameters of the PD activity recorded in the oil-pressboard insulation with needle-plate electrodes is investigated. Beside the known characteristic parameters like amplitude of apparent charge in pC, frequency of occurrence of PD pulse, PD pattern (PRPD-pattern), additional parameters were defined to describe the PD development processes under AC-DC combined voltage stress. Additional parameters are: pulse equivalent time, pulse equivalent frequency and equivalent time-frequency pattern (ETF-pattern). According to the time-variation trends of these characteristic parameters the PD development process can be divided into five stages: initial stage (IS), transition stage (TS), developing stage (DS), steady stage (SS), and pre-breakdown stage (PS). In these five stages the PRPD pattern and ETF pattern have different shapes and characteristics. For the analysis of PRPD pattern and ETF pattern the wavelet moment invariants are extracted. Based on these new features the PD development stages can be recognized using the least square support vector machine (LS-SVM) with the particle swarm optimization (PSO). The recognition rate is about 94%. Finally the possible physical effects of the identified PD development stages in the oil pressboards insulation under AC-DC combined voltage stress are also discussed in this paper.