Internal Partial Discharge and Material Degradation

Partial discharge (PD) pulses are considered to be generated after application of overvoltage across a cavity. Their magnitude is proportional to the n-th power of overvoltage. It decreases with an increase in cavity space and moisture content. Very long time lags for PD inception, such as even a day, was confirmed. This phenomenon is associated with PD´s statistical time lag in the number of free electrons in the gas (O2, N2). Oxygen and nitrogen exhibit their distinctive features. Nitrogen and moisture generate swarming micro PD pulses, while oxygen causes large PD pulses. Nitrogen PD tends to produce local degradation, i.e., pits, while oxygen PD contributed to uniform erosion. Oxygen is presumed to be almost all consumed in a closed cavity by PD at its early stage, leaving nitrogen, moisture and other gases behind to facilitate swarming micro PD to take place. Pits are followed by treeing. A new measure, i.e., the skewness of the profile of a PD pulse train can be used to check whether insulation is treed or not. A new CIGRE electrode is highly recommended to assess the resistance of insulating materials against internal PD.