Thermal and electrical ageing of silicone oil

Silcone oil provides an excellent self healing liquid insulation system for high voltage applications, combining good dielectric properties with the ability to operate at very high temperatures. As such, it lends itself to applications outside the reach of conventional hydrocarbon based oils; however, its ageing behaviour is relatively poorly understood. In these investigations, a model silicone oil has been subjected to both thermal and electrical ageing. The resulting aged oils have been studied by UV/Vis, infrared and dielectric spectroscopy techniques supplemented by measurements of viscosity. Thermal ageing results in no visible changes to the oils whereas electrical ageing causes the oil to become clouded. Under the conditions used in these experiments, electrical ageing results in a far greater change to the dielectric properties than thermal ageing. Increased viscosity and oxidation can accompany thermal ageing which was not observed after electrical ageing. The two ageing routes therefore produce quite different effects