The short and long term electric strength of polyethylene insulation

The mechanisms which lead to the long term deterioration of electric strength in polyethylene insulated high voltage cables are diverse. The degree to which each contributes to the degradation of the material and the consequential reduction in its insulating properties is dependent on many factors. This research investigated the long term degradation of polyethylene insulation and the factors which contribute to reduced working life. The short term electric strength of crosslinked polyethylene was also examined using a.c. stress and single impulses. These methods were used to explore the characteristics of thin slices microtomed from cable insulation. The a.c. electric strength was examined and was found to decrease as temperature increased. The electric strength of similar samples which had been treated in a vacuum was also examined. This treatment had the effect of raising the electric strength of the material. Large single impulses were used to measure the electric strength of polyethylene insulation. Impulses with rise times ranging from 0.25μs to 300μs were used at temperatures between 50oC and 110oC. It was found that the electric strength determined by this method was again dependent on temperature, and also on the rise time of the impulse. Additive free high density polyethylene was used for the investigation into long term degradation. Thus ageing was not affected by the presence of additives and crosslinking by-products. Moulded samples were electrically stressed at 40kV/mm for up to 14 months. The short term electric strength was measured prior to and after the period of ageing, using the single impulse method. This was found to decrease from approximately 370kV/mm to about 300kV/mm after 10,000 hours. Infrared spectroscopy was used to examine structural changes in the material, but was not able to determine the cause of the reduction in electric strength. The effect of oxidation on the electric strength of the material was examined. The polyethylene powder was thermally oxidised in air before thin films were moulded. The degree of oxidation was quantified using infrared spectroscopy and the electric strength was measured using the single impulse method. With a carbonyl concentration estimated to be 4 carbonyl groups per 104 carbon atoms, the electric strength was found to be reduced by about 10%.