High Voltage Insulation Failure Mechanisms

There are many statistics dealing with failure causes of high voltage rotating machines. While these statistics prove the relevance of the insulation as failure cause, they give little information on the real failure mechanism. This paper gives results of a research project which was launched to investigate degradation and final breakdown of the mica insulation in rotating machines. Final failure of the insulation is usually caused by electric breakdown through a tree channel. Formation and propagation of trees in the mica insulation therefore reflect the ageing process of the insulation. Dominating ageing factors of the mica insulation are thermal and mechanical stress. Manufacturing defects of the mica insulation - such as delaminations, cracks, voids and wrinkled or damaged mica layers - accelerate the formation and propagation of trees. Good workmanship and proper selection and application of the insulation materials are important factors for the lifetime of high voltage rotating machines.