Investigation into Impact of Electric Field on Long-term Conductivity of Synthetic Ester

Synthetic ester has emerged as an environmentally friendly alternative to mineral oil for power transformers. It is also being considered as a candidate liquid insulation for HVDC converter transformers. Conductivity is a key material property for electrical design of converter transformer due to the potential DC stresses in operation. Compared to mineral oil, there has been little research on the long-term conductivity of esters. Therefore, this paper first investigates the impact of the electric field on the long-term conductivity of a synthetic ester. The experimental results indicate that in general the conductivity of the synthetic ester is about 100 times higher than that of conventional mineral oil due to higher impurity levels. The long-term conductivity of the synthetic ester exhibits a non-linear trend against the electric field, with an initial decreasing trend in low electric field region and then an increasing trend from 1 to $10 \mathrm{kV} / \mathrm{mm}$. A computational model has been utilized to investigate the reasons behind the non-linear behaviour. Simulation results indicated the feature of the long-term conductivity is dominated by the corresponding space charge accumulation and field distortion phenomena.