The measurement of space charge and its effect on the breakdown strength of solid polymeric insulation

The research presented in this thesis investigated how the charge trapping in polymeric insulation is affected by the morphology of the polymer and by its impurity content. The effects these charge distributions have on the short-term electric strength of the polymer were also studied.A laser induced pressure pulse technique (LIPP) was used for the measurement of the internal electric stress and associated bulk charge distribution within both electrically aged and unaged polyethylene and polypropylene samples. Material characterisation techniques were used to relate these measured bulk charge distributions to the morphologies of the polymers, and impulse breakdown tests were used to investigate the relationship between the changes in the trapped charge distribution and changes in the electronic breakdown strength of the polymers.The effects of an antioxidant additive on the morphology of low density polyethylene and its space charge distribution under dc electric stress were investigated. It was shown that the inclusion of the antioxidant dramatically changed the morphology of the polymer, moreover accelerated dc electrical ageing and relaxing of the polymer produced remarkably different space charge distributions for each of the morphologies. The effect on the space charge distributions of altering the morphology of a polymer without the addition of impurities was also investigated. In this case polypropylene was used as its morphology could be modified relatively easily through additional thermal treatment. However, in this study the results of dc electrical ageing the samples produced the same trapped charge distribution regardless of the sample morphology. (DX 183,. 710)