Temperature dependence of the pre‐breakdown and breakdown phenomena in natural esters under AC stress.

Natural ester dielectric fluids are being used as alternatives for mineral insulating oils in oil‐filled power apparatuses. Although partial discharges (PDs) are recognised as one of the main causes of degradation of dielectrics used in power equipment, their characteristic voltages appear to be temperature‐dependent. In this contribution, the pre‐breakdown and breakdown behaviour of two natural ester fluids, extracted from canola oil (NE1) and soybean oil (NE2) is reported. To assess the effectiveness of the temperature dependence, −5, 0, and + 20°C have been identified as test temperatures. It was found that the different pre‐breakdown and breakdown characteristic voltages increase with increasing temperature for both types of esters. The delay time is always longer for canola oil when compared to soybean oil regardless of the test temperature. The analysis of the average propagation velocity of NE1 and NE2 indicates that PDs propagate faster in NE2 than in NE1. The field strength at the tip of the needle electrode during the inception of the streamers leading to breakdown indicates that streamers in both oils are propagating in the slow mode. It is found that the temperature has a significant influence on the PDs' parameters for both fluids. [ABSTRACT FROM AUTHOR]