Space charge formation and conductivity characteristics of PE and oil impregnated paper under a temperature gradient

Polyethylene (PE) and oil impregnated paper are widely used as insulation materials in high voltage power equipment however, at high voltages the space charge inside the insulating material can distort the electric field distribution and thus greatly influence aging and degradation. Moreover, there is usually a temperature gradient across the insulation when the equipment is in service, and this can also influence the space charge accumulation. In order to study the effect of temperature gradient systematically a PEA system suitable for measurement under a temperature gradient has been developed and appropriate corrections for the measurement error introduced by the temperature gradient implemented. Separate determinations of the space charge distributions of PE and oil-impregnated paper at different temperatures and voltages have been made. The results for both materials showed some similar characteristics. Obvious heterocharge accumulates at the low temperature side and the higher the temperature difference between the two electrodes or the higher the applied voltage, the more the heterocharge accumulates. As a result the electric field is greatly distorted and the maximum field always appears at the low temperature side. The effects of temperature gradient on the space charge distribution are discussed in detail, and the differences between the two materials are identified. The conductivity of both materials has also been investigated, and the space charge induced by the conductivity gradient produced by a temperature gradient in both materials has also been evaluated. The results show that in oil-impregnated paper the space charge induced by a conductivity gradient plays a great part in space charge formation, while in LDPE the conductivity gradient contributes much less to the space charge. These differences are discussed.