Effect of Dissolved Water in Ester Insulating Oils on the Discharge Characteristics Under AC and Lightning Impulse Voltages

Ester-based insulating liquids are expected not only to protect the environment considering carbon neutrality, but also to be economical by extending the service life of electrical equipment with composite insulation systems. Because esters have extremely high saturated water content compared to mineral oil, this study clarified the effect of dissolved water in esters on discharges in uniform and non-uniform fields under AC and lightning impulse voltages, respectively. As a result, it was found that the breakdown voltage of AC voltage decreased with RS (dissolved water content/saturated water content), and the dispersion among the measured data increased with or without stirring when RS exceeded 30%, regardless of the liquid types. In contrast, the lightning impulse voltage was found to be less affected by the dissolved water contents in the liquids and more affected by the applied polarity, that is, the space charge was found to have a larger effect on the discharge. Furthermore, the discharge mechanisms with AC and impulse voltages for dissolved water in the liquids were proposed.