THE KINETICS AND MECHANISMS OF DEGRADATION OF CELLULOSIC INSULATION IN POWER TRANSFORMERS

Cellulose based paper is widely used as electrical insulation in large electrical power transformers and cables. Its deterioration under the combined effects of thermal, oxidative and hydrolytic degradation determines the ultimate life of the insulation system, although other factors may cause it to fail earlier. The use of chemical indicators of degradation, such as 2-furaldehyde, is increasingly being used to assess the condition of the paper and, in time, their cumulative concentrations in the oil will be used to aid estimation of the remaining life of the plant. The activation energy of degradation of cellulose in the temperature range 100-200-degrees-C is 111 +/- 6 kJ/mole and is independent of the reaction conditions. The degradation rate is significantly influenced by the pre-exponential factor, which increases with increasingly aggressive environments, such as the presence of oxygen and water. Water is a product of degradation and there is some evidence that it auto-accelerates the reaction in oil. The mechanisms of formation of products such as furaldehyde from glucose have been investigated by radiolabelling, but more complex mechanisms may exist for formation from cellulose.