The influence of the stress time duration on the partial discharge behavior in cast resin insulation

It is well known that thermal and electrical stresses have a strong influence on the degradation of insulation, but the effect of the stress duration is not yet well analysed. On this account thermal, electrical, and a combination of thermo-electrical stresses were applied to a number of cast-resin samples, and their effects were evaluated via partial discharge measurements. Stress duration as well as time without stress was varied. This information will be used in a statistical method to recognise the influence of the duration of the applied stress on the partial discharge activity of every sample. The statistical method is the analysis of variance (ANOVA), which is useful when the frequency distribution of the measurements is a so-called normal distribution. All measurements were carried out following this distribution and thus the reliability of such tests is only dependent on the linear behaviour of the following variables: partial discharge inception and extinction voltage, number of discharge impulses, sum of the charges of all impulses, and average charge per impulse. The other variables are the stress duration and the time without stress. In order to recognise variations which only correspond to the same effect, tests must be ordered in groups with the same characteristics. Tests must also be performed in continuous and repetitive cycles under stress in order to avoid undesirable effects. The results of these experiments may form the basis for the identification of a relationship between one or more partial discharge parameters and the stress duration