Assessment of the status of insulation during thermal stress accelerated experiments on transformer prototypes

The status of insulation in power equipment at any given point in time can be assessed by monitoring properties which are sensitive to the amount of aging. This procedure, called the diagnostic or non-destructive testing, has been in vogue for some time now. The diagnosis, as the name suggests, enables one to disseminate the condition of the insulation by making terminal or enclosure-centric measurements and as applied to transformers, a minimally invasive test procedure involving the examination of the oil phase, for arriving at information on the degradation of the cellulose insulation on the winding is indicated. An examination of the tracks provided by the by-products of chemical dissociation of cellulose, in solution with oil is recognized as a possible approach to condition monitoring. To this end, carefully controlled laboratory aging experiments have been performed on eight, nominally identical and scaled-down (pro-rated) models of single phase transformer units, incorporating all the essential features of actual power transformers under normal operating electric stress and accelerated thermal stress. Acquisition, analysis and validation of experimental data including dissolved gas analysis (DGA), examination of oil phase for the presence of Furans, measurement of the degree of polymerization (DP) of the paper insulation, electrical and certain chemical tests have all been performed to evolve an experimental data-base for assessing condition of insulation.