Influence of ageing on conduction and breakdown in rape-seed and mineral oils

The purpose of this work is to evaluate the chemical stability of a bio-degradable insulating oil composed by a mixture of esters based on rape-seed oil. The measurements are done in comparison with a commercial mineral oil. Both oils are subjected to accelerated ageing conditions at 120degC under air (open beaker method). Dielectric properties (dissipation factor, permittivity, conductivity, AC breakdown voltage) and chemical parameters (acidity, water content) are monitored during the ageing process. In the rape-seed based oil, ageing has a negligible influence on the breakdown voltage, whereas conductivity and acid content show a large increase.