The objective of this paper is the interpretation of distributions and phase-resolved patterns of partial discharges apparent charge and energy. Results of the measurements performed have been obtained in long-term tests at increased electrical and thermal stresses. This issue is of vital cognitive importance for the assessment of material degradation mechanisms; it is also very important while diagnosing the high voltage insulating systems. Test and measurements were carried out using a thermosetting insulation system in high voltage machines in which PD occurred at an increased temperature and increased working intensity of the electrical field was applied in investigations has been used as the object of tests and measurements. After these tests the decrease of the PD pulses magnitude and the increase of the number of small pulses were observed in the PD charge distributions. This means the transition of partial discharges to a high repetition form-multiply tiny discharges. In this case, it is also observed in the PD pulses energy distribution, the presence of the extremum in the range of small values of energy. This effect provides new information about the degradation process.
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