The physico-chemical basis of ageing in polymeric insulation

Insulation systems are expected to operate without failure for decades. Manufacturers assess the ability of their product to meet these requirements by means of accelerated tests. A pragmatic approach is usually adopted. Accelerated tests are used to obtain the failure statistics and determine a functional relationship between the applied voltage (field) and the characteristic lifetime. Extrapolation to service stresses can then be used to predict both the characteristic lifetime and reliability after a given period of ageing. Because a number of different breakdown mechanisms may occur, each with an initiation and formative phase, it is not obvious that the mechanism dominant under accelerated conditions remains dominant at service stresses. In order to establish confidence in the predictions it is therefore necessary to have a quantitative description of the behaviour expected for the various possible mechanisms. If this can be carried through, then identification of the mechanism and determination of the values of its appropriate parameters can be used to underpin reliability predictions. It is the purpose here to evaluate the general features involved in ageing theories so as to determine their life-functions, and assess whether it is possible to establish the operative mechanism from experimental data