Modelling of multifactor ageing processes occurring in solid insulation

A discussion is presented of some of the most recent models describing insulation life under multiple thermal-electrical stress. It is shown that the description of the whole life (breakdown time) at a given thermo-electrical stress can be obtained by combining suitable physical models valid for the different stages of aging. For example, the combination (according to an additive law) of certain models provides electrical life lines showing linear behavior, downward curvature and upward curvature with tendency to threshold (S-shape) in semilog plot. Thus, these lines are able to describe almost any behaviour observed for insulating materials and systems aged under thermo-electrical stress. It is concluded that a general phenomenological model, associated with a probabilistic approach, can provide the basis for the characterization and design on any insulating material or system.