Bringing an insulation to failure: The role of space charge

Considering the title of this paper, one might think that everything has been said, shown, proven and discussed already on ageing and failure of electrical insulating materials, based on at least half century of research work and field feedbacks. However, perhaps there is still room to collect ideas and speculate on apparently proven or brand new concepts, such as the interrelation between ageing mechanisms and physically measurable quantities as space charge and conduction current. The latter is, indeed, the scope of this paper. On one hand we will revisit the concepts of space charge accumulation in insulating (polymeric) materials, considering also the last achievements which deal with ultra-fast charge pulses (solitons), on the other hand we will discuss how space charge can contribute directly or indirectly, through local electric field magnification or alteration, to increase ageing rate or even trigger more energetic, thus faster, degradation processes (from partial discharges to thermal instability breakdown). The goal is, at the very end, to show how much such basic investigations can have a fundamental impact on insulation design and electrical apparatus long-term performance/reliability.