Aging mechanisms of polymeric materials under DC electrical stress: A new approach and similarities to mechanical aging

The main fundamental mechanisms proposed for the aging of polymeric insulation to breakdown at the DC design field are reviewed. A correlation between the action of static mechanical stress affecting the whole insulation volume, and DC electric-mechanical stress providing local strain large enough to bring the polymer to plastic deformation is hypothesized. With the corollary that local plastic deformation leads to the formation of cavities, or crazes, of size large enough to trigger highly energetic phenomena and bring about insulation failure, an inter-relationship between mechanical and electrical life models can be obtained and supported by calculation and fitting to experimental results. The influence of space charge is discussed and it is argued that it would play a central role in DC ageing even if its presence is not a precondition.