Transient charge injection from blocking electrodes in transformer oil on the reversal of voltage polarity

In high resistivity oil, the electric field distribution is well described by a model based on charge carrier generation through the dissociation of a species of ion pairs in the liquid bulk. As liquid conductivity increases, the close agreement between model and experimental results weakens. The aim of this work was to determine whether or not there exists a violation of the model assumption that the generated charge carriers are neutralized on arriving at metallic electrodes and to consider the consequences of such a violation. This work has demonstrated that stainless steel electrodes are indeed blocking as charges previously collected on an electrode surface are injected into the liquid on polarity reversal of the applied voltage. Although the model assumption is invalid, the violation cannot totally account for the additional space charge distortion. It is increased liquid resistivity, reflecting the electrostatic cleaning of the oil, that results in a closer fit between model and experiment.