Influence of natural ester on frequency dielectric response of impregnated insulation pressboard

It is important to investigate the dielectric behaviour of natural ester-paper insulation, as the number of transformers choosing natural ester as their insulation oil is increasing. A comparative study of the pressboard with different conditions immersed in natural ester and mineral oil was conducted. X-Y model simulation was used to indicate the difference between the frequency-domain spectroscopy characteristics of transformer main insulation system using natural ester-pressboard insulation (NEPI) system and mineral oil-pressboard insulation (MOPI) system, which have the same moisture content or experience the same ageing period. Compared with the MOPI with low moisture content, the NEPI with the same low moisture content has higher ε_r and tanδ values in the lower frequency region at various temperatures; although no significant differences exist in the ε_r and tanδ curves of these two kinds of oil-impregnated pressboard, when its moisture content is high. The X-Y model simulation results also show that the dielectric loss difference between the NEPI system and the MOPI system with low moisture content is more obvious at higher temperature. Besides, the pressboard aged in natural ester has higher ε_r and tand values than that aged in mineral oil after ageing for the same time. X-Y model simulation results show that the tan/spl delta values in the lower and middle frequency regions could be used as the ´fingerprint´ of the ageing condition of the NEPI system and the mineral-oil pressboard insulation system.