Weibull statistical analysis of size effects on the impulse breakdown strength in transformer oil

Transformer oil has been used to electrically insulate a variety of electrical equipment including circuit breakers, cables and power transformers. Its insulation properties and breakdown characteristics under impulse overvoltage is the key factor to determine the safety of system. The insulation configurations of power transformers and reactors are affected by the electrode surface area and stressed oil volume. Therefore, it's of great importance to establish proper mathematical models to cope with the scale effects in transformer oil under impulse voltage. In order to study the scale effects in transformer oil, an impulse voltage generator with adjustable pulse width was set up to generate a steep front voltage with a relative long duration impulse voltage (0.12/50μs). The electrical breakdown characteristics of transformer oil under variable electrode area and interelectrode distance have been studied using Weibull distribution. It turns out that Weibull distribution function of three-parameter fits better than two-parameter. The empirical formulas of the dependence of the average electric strength on different stressed electrode areas and interelectrode distances are expressed as E= CAeff
^-β
(d=0.5mm) and E=Bd
^-ζ
(D=30mm) with C=1.819, β=0.286, B=0.823, ξ=0.196 according to experimental results. By comparison between the calculated A
_eff
and the pit area on the electrode surface after discharge, it is shown that the Aeff obtained from using 3-parameter Weibull distribution's shape parameter m is tally with the actual pit area.