Dielectric characteristics of rape-seed oil

This paper deals with the investigation of fundamental dielectric characteristics of rape-seed oil considered to be a substitute of mineral oil in power transformers. The 50 Hz breakdown voltage is comparable to normal mineral oil, e.g. more than 50 kV/2.5 mm. Furthermore, the permittivity ε_R and the dielectric loss factor tan δ for various temperatures and frequencies were measured. Using a 50 Hz voltage at a temperature of 90°C new rape-seed oil has an ε_R of 3,18 and a tan δ of 0.015. These data demonstrate that rape-seed oil is usable as an insulating liquid. In a power transformer the insulating oil has also a cooling function. Therefore, the oil-viscosity is an important property. The dynamic viscosity of rape-seed oil is about 70 mm²/s at 40°C, that means it is much higher than of mineral oil, and at temperatures under -5°C rape-seed oil even becomes solid. So, additional substances must be used to regulate, i.d. to guarantee, the viscosity in these ranges. Because it is vegetable-based, the chemical structures of rape-seed oil are very different from those of mineral-based oil