Effect of Moisture Content on Temperature Characteristics of Dielectric Breakdown Strength of Biodegradable Ester Oils

Abstract

The authors showed in previous report that the relative moisture content of biodegradable ester oil increases with increasing temperature, which affects the AC breakdown voltage (BDV). This paper presents the dependence of BDV on moisture content and temperature under conditions of high moisture content in biodegradable ester oil (EO). AC BDV of the three biodegradable ester oils, PFAE (monoester), FR3 (triester), and MIDEL (tetra-ester), with different ester linkages, was measured at different oil temperatures and relative water contents using sphere electrodes. Results showed that BDV decreased with increasing moisture content (MC) in all kinds of the ester oils. The BDV of PFAE and FR3 decreased from room temperature to 50 °C, but then increased in the temperature range from 50 °C to 80 °C. The decrease and increase in insulation properties are attributed to an increase in water content in the oil and to the viscosity decrease resulting in suppression of streamer discharge propagation, respectively. On the other hand, BDV of tetra-ester continued to decrease with increasing temperature. This result can be attributed to the larger number of ester bonds, unlike monoesters and tri-ester. The ester linkages in the material are a weak point in the insulating properties, which can easily lead to a decrease in BDV at all the measuring temperatures.