Dielectric Properties of Mineral and Vegetable Oils Mixtures for Power Transformers

Abstract

In the last 20 years the vegetable oils have been widely used in power transformers isolation systems. These dielectric liquids that are obtained from natural renewable resources have important advantages over conventional mineral oil, including high biodegradability, low flammability, and very convenient dielectric properties. It is found that an effective method to increase the safe operation of power transformers initially filled with conventional mineral oil is to refill this equipment with vegetable oils. In certain cases, depending on climate condition, the load and design of the power transformers their refilling can be carried out with mixtures of mineral and vegetable oils. This paper presents an experimental study concerning the main dielectric properties of mineral and vegetable oil mixtures. It has been found that the accelerated thermal ageing of these mixtures causes important variation of electrical resistivity and real and imaginary parts of complex permittivity.