Effect of water content on dielectric properties and partial discharge characteristics of silicone oil

Insulation is a very important in high voltage equipments installed in an electric power system. Since long time ago, mineral oil is being used as high voltage insulation and coolant for high voltage equipments like transformer. Silicone oil is an alternative oil as a substitute for mineral oil. It has superior dielectric properties such as high breakdown voltage, low dielectric losses and high long term stability. In general water absorption in dielectric material may reduce the performances. In tropical environment, where humidity is high, water absorption may take place with high intensity. Therefore, it is important to be investigated. The purpose of this study is to determine the influence of water content on dielectric properties and partial discharge properties of silicone oil. The sample used was silicone oil with viscosity of 100 cSt and water content varied from 25–250 ppm. Dielectric properties like breakdown voltage, dielectric constant, dielectric losses factor, resistivity were investigated. Partial discharge under needle-plane electrode and AC voltage was also tested. The experimental results indicated that for water content below 50 ppm (20%), the value of breakdown voltage tends to a constant at 80 kV. When the water content increased the breakdown voltage decreased significantly to as low as 10 kV. Tan δ increases significantly for water content higher than 50 ppm, while reduction of resistivity also observed for water content of more than 50 ppm. The saturated water content of silicone oil is 250 ppm thus drastic change of dielectric properties took place above 20% water content. Increase in water content also reduced the partial discharge (PD) inception voltage and increase the magnitude of partial discharges.