Simulation of electro-thermal ageing and breakdown in polymeric insulation under high frequency trapezoidal-wave pulses

A quantitative ageing model expressed in terms of two generic parameters, one describing the ability of local regions to resist degradation and the other describing their capacity to concentrate energy from the electric field, has been used to simulate the electro-thermal ageing of Polyethylene Terephthalate (PET) in pulsed trapezoidal-wave electric fields. Allowance has been made for the dielectric heating produced by the alternate-polarity voltage pulses. It was found that for 90% of the lifetime the degradation was that of isolated regions. Only when the regions started to group to produce a local field in excess of a critical value did the degradation accelerate to failure in the form of a single track. The critical field is associated with the effective onset of irreversibility in the production of degradation, such that a local equilibrium state in the presence of the field was no longer possible regardless of the nature of the surrounding material. The temperature increases were found to be small and had very little influence upon degradation formation, though they had a minor influence on the initiation of runaway failure.