Studies of the condition of insulation in aged power transformers. 1. Insulation condition and remnant life assessments for in-service units

The authors describe practical insulation life assessment techniques developed within the Queensland Electricity Commission for the estimation of remaining useful life of its large power transformers. Novel aspects of the technique are: (i) the use of a database of oil test results for statistical analysis of several aging indicators, to derive an `insulation age´ for the transformer; (ii) the extraction of paper samples from in-situ transformers for analysis of their degree of polymerization (DP); and (iii) the application of appropriate life models to typical power transformers to determine remaining life on a probabilistic basis. These analyses are supported by studies on dismantled large power transformers at the Commission´s workshop, to determine the DP profile through typical transformers. From this, relationships are estimated between the DP at hot spots in the transformer and that obtained at convenient sampling locations