Development Rules Of Degradation Induced By Partial Discharge On Oil-paper Insulation In Transformer

Partial discharge on oil-paper insulation in power transformer can be broadly divied into three categories:creepage discharge on pressboard or paper, internal discharge in pressboard or paper and corona discharge in oil. Among which the creepage damaging, followed by the internal discharge, the dangers of corona discharge in oil is relatively small. This article focuses on the first two types of discharges.Tests based on the inter-turn partial discharge model are conducted to study the development rules of oil-paper insulation material defects generated by partial discharge in power transformers and its possible correlation with the characteristics of partial discharge. Constant voltage application was adopted in this series of tests. High voltage is applied on the inter-turn electrode specimens for different time in different groups of tests. Real-time detection is conducted to record conventional impulse current signals from partial discharge. After tests, insulation tapes can be stripped from inter-turn electrodes lay by layer. By this way, the statistical result about the development rules of carbon channels which generated by accelerated degradation between turns. The results show that the development of carbon channels which generated by accelerated degradation under a high constant voltage can be divided into three basic stages:growth, stagnation, and breakdown. The pulse repetition rate and the summation of pulse amplitude per second can reflect the insulating state of the inter-turn oil impregnated paper system. Based on these results, the severity of partial discharge on oil-paper insulation can be assessed accurately.Creepage discharge is likely the most dangerous failure mode that typically results in catastrophic failures at operating conditions. The phenomenon occurs in the oil-barrier insulation in power transformer. None of the existing experimental research on creepage discharge reveals the possible correlation between insulation defects induced by creepage discharge and the characteristics of partial discharge. Using point-to-plate electrode configurations under AC voltage, this paper presents test phenomenon of creepage discharge between layers of oil-immersed pressboard. Development rules of insulation defects known as white marks are also studied. Real-time detection is conducted to record conventional impulse current signal during tests. According to the analysis of partial discharge pulse signals, the outlines and statistical parameters of q-cp distribution show mutations around inception of white marks, and that an indication of inception of white marks can be obtained. The pulse repetition rate and the summation of pulse amplitude per second can reflect the development of white marks.