The effect of barriers on electrical treeing

Presents the initial results of an experiment to evaluate the factors determining the life of simple and multi-component systems. The addition of another material as a `barrier´ between the electrodes has been noted to produce anomalous results by altering the properties. Kako et al. (1978) studied the short term breakdown of epoxy-mica composites using McKeown electrodes. They found the breakdown field of the composite to be lower than that of either mica or epoxy resin. It was concluded that micro-cracks at the mica-epoxy interface provided a trigger for electrical breakdown processes. A point-plane electrode system was used with a loose needle to provide the initial defect and hence reduce the initiation period. The needle-plane configuration mimics the effect of a contaminant by concentrating the electrical stress. An electrode gap of 3 mm was used and the diameter of the samples ranged from 8 to 50 mm. The initial tests were performed using simple epoxy-resin samples. When a treeing pattern had been established the samples were adapted to include barriers. Due to the compressed laminate structure of epoxy-mica machine insulation analysis of the breakdown mechanism is difficult. Consequently an expanded model of the interface between the mica paper and the epoxy-resin was used to obtain a clearer picture. Both muscovite and phlogopite mica paper were used