Thermal effect on AC breakdown characteristic of transformer oil under inhomogeneous fields

This paper deals mainly with the impact of temperature and the effect of field nonuniformity on the breakdown characteristic of transformer oil under AC and impulse voltage stress. The degree of field nonuniformity, i.e. E_max/E_avg, in this experiment was varied in the range between 1.1 to 5. Under AC stress, the breakdown strength of transformer oil was increased with increasing temperature in the range between 30 and 80°C. However, under slightly nonuniform fields, the breakdown strength was only slightly increased with temperature, while it was significantly increased with temperature under highly nonuniform fields. The impulse breakdown strength seemed to vary with both temperature and field nonuniformities. Under slightly nonuniform fields, the impulse breakdown strength was increased by 10 to 20% with increasing oil temperature, while showing no such increase in breakdown strength with temperature under highly nonuniform fields