Space charge behavior under temperature gradient and its effects on material aging

Abstract

When a power apparatus is put into operation, temperature difference always exists in its insulation. As to HVDC insulation system, this temperature gradient may play an important role on the space charge behavior and field distribution. This report summarized our attempts to understand the effects of temperature gradient on space charge and material lifetime. Modified pulsed electro-acoustic (PEA) measurement systems and corresponding waveform recovery algorithms applicable to temperature gradient for film samples and coaxial cables were developed. Space charge behaviors under different temperature gradients of film samples, like LDPE, oil-paper and epoxy resin, as well as of coaxial cable were compared. Moreover, a bipolar charge transport model was used to elucidate these phenomena and study the mechanism of space charge behavior under temperature gradient. It was concluded that conductivity gradient caused by temperature gradient across the bulk and the different injection and extraction behavior of charges due to temperature difference of two electrodes brought up the different experimental results. Furthermore, step voltage test was carried out to study the aging law of XLPE. The obvious increase of aging exponent under temperature gradient was attributed to the hetero-charge accumulation, which increased rapidly with electrical field and thus greatly enhanced the field distortion and recombination density in the adjacent of low-temperature electrode.