Abstract
The condition assessment of the 190 MVA, OFWF, step-up generator transformer was performed by the mapping of degree of polymerization (DP). The transformer was taken out of service after 36 years and the active part, (transformer core and windings), was removed from the tank for close examination due to the major thermal fault at the transformer core. The transformer insulation consisted of two different materials, i.e. winding insulation was the thermally upgraded paper and spacer-pressboard system was Kraft paper. The temperature mapping is also shown, where the temperature estimation is based on the paper aging kinetics, transformer loading and insulation operating history. New equation for the relative aging rate is given considering all insulation conditions providing possibility of counting transformer loss-of-life more accurately. Copyright © 2010 John Wiley & Sons, Ltd.