Analysis of ageing characteristics of solid insulation in mixed insulating fluids

The practical stability of solid insulation used in the transformer depends on liquid insulation, where periodic assessment of solid and liquid insulation is mandatory. The usage of mineral oil in the transformer system cause rapid deterioration of solid insulation and which in turn weakens the strength of liquid insulation. Instead of traditional mineral oil, combinations of natural esters and mineral oil have been formed and these mixed natural ester insulating fluid samples are compared against the commercial mineral oil. The ageing tendency of Kraft paper and Pressboard are evaluated by treating the test pieces with samples of mixed insulating fluid groups `A´, `B´, `C´ and `D´ by accelerated thermal ageing test. Samples and test piece (Kraft paper and Pressboard) were subjected to acidity and water content test before and after accelerated thermal ageing test as per IEC and IEEE standards. By visual observation, it is agreed that the solid insulation in the 100% composition of natural esters samples ages at a much slower rate than sample of 100% mineral oil. Also the ageing rate increases rapidly with the increasing proportion of mineral oil in the samples.