Effect of Electric Field on Conductivity of Palm Fatty Acid Ester

Biodegradable ester oils have emerged as an environmentally sustainable alternative to traditional mineral oils in various applications. New uses, particularly in converter transformers, energy storage systems, and HVDC transformers, are under continuous investigation. The electrical conductivity of insulating materials is a key parameter for these novel applications; however, limited research has been conducted on the conductivity of biodegradable ester oils in comparison to mineral oil. This study examines the influence of electric field strength on the conductivity σ of palm fatty acid esters (PFAE) under DC voltage application ranging from 10 to 3000 seconds, and compares the results with those of mineral oil. The experimental results indicated that σ of PFAE decreases over time under electric fields ranging from 100 V/mm to 1 kV/mm, but increases with time under fields exceeding 1 kV/mm. Notably, the conductivity in long-term measurements was lower than in short-term measurements. When comparing the conductivities of PFAE, mineral oil, and synthetic esters, PFAE exhibited the highest conductivity across both short- and long-term measurements and various electric field strengths. This difference is attributed to the lower kinematic viscosity of PFAE compared to mineral oils and synthetic esters. The significant effect of kinematic viscosity on the conductivity was observed among the ester oils, with the ratio of conductivities closely matching the ratio of their kinematic viscosities. Furthermore, a comparison between ester oil and mineral oil revealed a more than 100-fold difference in conductivity, primarily due to the effect of ionic carrier conduction in the presence or absence of polar groups. Additionally, the higher charge density in ester-based insulating oils compared to mineral oils was identified as a contributing factor to this disparity.