Statistical Analysis of Electrical and Mechanical Breakdown Stress for Insulation Performance in High Voltage Power Transformer

Original scientific paper In electrical power system, variety of solid, liquid and gaseous materials are used for insulation target to protect the incipient failure inside the high voltage power transformers. The insulation is practically ended when the insulation system has become fragile enough to flourish cracks under the electrical and mechanical stresses to which it is subjected. The electrical and mechanical stresses are induced by short circuit currents, thermal expansion and contraction of the conductors and vibration. The dielectric strength of insulation is not vitally diminished by brittleness alone, but electrical breakdown will rapidly follow the improvement of ensuing cracks. The purpose of this paper is to briefly cover the methods favourable for determining the insulating value of the insulation on high voltage power transformer windings and bushings. Many failures of insulation are caused by the entrance of moisture. Insulating materials used on high voltage power transformer windings have a high affinity for moisture from the surrounding atmosphere or oil. Effects of the stressed oil volume and stressing time on the breakdown stress of high voltage power transformer insulation were experimentally investigated. The factors influencing the effects and the results obtained based on them were statistically analysed and systematized by the Weibull distribution method. A Four-dimensional volume theory allowing good estimation of breakdown stress in terms of specific probable values has been established.