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Insulation within an oil-impregnated insulation consists of mainly organic materials including mineral oil, cellulose paper and pressboard. Temperature is a major cause of degradation of the cellulose insulation. Increased temperature accelerates chemical change in the insulation thus reducing its lifetime. In addition to normal temperature operation, transformers are sometimes subject to sudden thermal overload and the effect of this on the insulation has not been fully investigated. In particular the partial discharge (PD) activity has not been well documented. This paper investigates the thermal dependence of PD activity in new and aged paper insulation, using temperatures up to 145Â°C. At these temperatures there is substantial gas bubble formation and these bubbles have a significant impact on the PD activity. In addition, the level of PD activity is also affected directly by the temperature itself. The paper provides results of PD monitoring at a range of temperatures with and without bubble formation and discusses these effects on the interpretation of PD levels obtained in either separate source or on-line PD tests of transformers. The physical effect of ageing on oil-impregnated insulation is also considered in this paper.