Determining the ageing parameters of cellulosic insulation in a transformer

The original properties of solid (cellulose) insulation in a transformer deteriorate under thermal stress. Decomposition products (gases and furanic compounds) are released, which can be detected in the oil. The degree of polymerisation (DP) of the paper decreases and with it the tensile strength (TS). In order to predict the life expectancy of paper insulation under working conditions, it is necessary to understand the processes of degradation, and measure parameters of ageing that can be used to calculate the life of the different paper types. This paper presents results for various insulating papers aged under accelerated ageing conditions in the laboratory. A new method of relating tensile strength and degree of polymerisation to ageing time is compared to traditional mathematical formulae derived for the ageing of pressboard, previously reported by Moser and Dahinden (1987).