Breakdown Characteristics of Mineral Oil in Uniform to Non-Uniform Electric Field Under AC Voltage Using Bare Electrodes

It is imperative that insulation designers understand breakdown voltage (BDV). BDV of transformer oil are generally understood to follow stressed oil volume (SOV) theory. However, estimating more than 90% stressed oil volume in non-uniform electric fields in a transformer is extremely time consuming. In this study, a prediction formula that can easily estimate the BDV of mineral oil is proposed. The breakdown characteristics of mineral oil were investigated in a non-uniform electric field under AC voltage application. Experimental results were obtained with those several bare electrodes and oil gap lengths, hemisphere - plane electrodes, rod - plane electrodes, and edge - plane electrodes, respectively. As a result, it was derived that a prediction formula represented by
$\sqrt{U}E_{\max}=39.2D^{-0.28}$
. It was also confirmed that experimental results matched to the prediction curve and could easily estimate the breakdown voltage.