Effect of thermal aging on the dielectric properties of mixture between mineral oil and natural ester

Power transformer is a very important equipment in a power system. In general, power transformer contains liquid insulation and solid insulations to withstand stresses during operation. Transformer oils and kraft paper are commonly used. The lifetime of a power transformer is very dependent on the thermal aging of the insulations. Therefore, the investigation of thermal aging of the insulation is very important. This paper reports the investigation results on the thermal aging of composite insulation between mineral oil-ester mixture and kraft paper at 120°C and 150°C for 20days, 40days and 60days. The liquid insulation were mixtures between mineral oil and palm oil ester with composition of 100% mineral, 75%mineral(25%ester), 50%mineral(50%ester), 25%mineral(75%ester), and 100%Ester. Copper wire that has been layered with kraft paper is put into the liquid insulation samples. Parameters measured are breakdown voltage, resistivity, permittivity, loss factor, color scale, viscosity, and water content. The effects of the aging temperatures as well as the duration of aging on the dielectric properties are discussed. The effect of composition between mineral oil and ester from palm oil on the dielectric properties are also discussed.