Silicone oil provides an excellent self healing liquid insulation system for high voltage applications and has minimal environmental impact. Unlike conventional hydrocarbon oils, it is capable of withstanding extremely high temperatures, such as those found in small transformers, and hence lends itself to more specialised applications outside of the reach of conventional oil/paper or polymeric insulation systems. These oils have also been used in terminations for many years with little understanding of their degradation or ageing behaviour and one noticeable problem is that these oils can become cloudy/black after relatively few years in service. The current project, in collaboration with National Grid, aims to quantify the effects of ageing on a range of silicone oils, to establish the pertinent chemistry and to identify suitable diagnostic tests to allow service engineers to make an informed decision of whether to replace or recondition plant.
The initial phase of the project concerned thermal and electrical ageing of a number of "model oil systems". A range of diagnostic indicators have been identified which provide indication of ageing in the field such as colour changes, oxidation, increased dielectric loss, formation of precipitates and increased viscosity. Whilst silicone oils are certainly capable of withstanding very high temperatures with very little degradation of thier dielectric properties, electrical ageing (even at moderate discharge energies) caused the oils to be rapidly blackened with significant deterioration in properties. It was clear from this work that electrical, rather than thermal ageing, is the dominant ageing mechanism under the conditions found in a typical termination and should therefore provide the focus for further work.
Construction of a new experimental apparatus to enable the study of electrical ageing on a local level (i.e. near the electrodes) is currently underway. Understanding the mechanism of ageing will provide pointers for improved designs to mitigate internal discharge activity and recommendations for more suitable oils or additives.