Selection of a suitable vegetable oil for high voltage insulation applications

Many items of high voltage plant employ a liquid both as a dielectric and a coolant. Currently these systems use a mineral oil, however, this suffers from the drawback of being potentially toxic and hence leakages and eventual disposal can be serious issues. To overcome this problem, an increasing trend in the UK is to backfill existing paper/oil cable systems with dodecylbenzene (DDB). This fluid possesses the advantages of improved gas absorption, good dielectric properties and biodegradability; nevertheless it is still derived from crude oil, a nonrenewable resource. Vegetable oils offer the added advantage of being renewable although many types are available with very different properties. In order to select a suitable vegetable oil for high voltage applications, a standardised ageing and testing regime is required. In this paper, a wide range of vegetable oils were subjected to controlled laboratory ageing and the resulting aged oils were characterised by a number of analytical techniques. The results from these tests were then used to rank the different oils, and to select the most ageing resistant oil