Rape-seed oil-a substitute for mineral oil?

This paper presents the results obtained from the study on the basic characteristics of rape-seed oil considered to be a substitute of mineral oil in power transformers. The results show obviously that rape-seed oil is usable as an insulating liquid. Furthermore, results are presented concerning the breakdown behaviour in uniform (plane-plane geometry) and nonuniform (rod-plane geometry) fields under impulse conditions with gaps up to 40 mm and voltages up to 650 kV. A sufficiently high 50 Hz breakdown voltage is the most important precondition for a medium to be used as an insulator. The dielectric strength of rape-seed oil exceeds more than 50 kV/2, 5 mm and consequently fulfils the requirements of the standards. The measurement of the permittivity and tan δ at 90°C yields to 3, 18 and 0.015 resp. and is comparable to the values of standard mineral oil. However, there are still some problems to be overcome. Beside the insulating task, a liquid dielectric has also a cooling function in power apparatus. Since the dynamic viscosity of rape-seed oil (about 70 mm²/s at 40°C) is higher than that of mineral oil, rape-seed oil has a lower ability for heat conduction. A further problem is the solidification of rape-seed oil below -5°C