Impact of Electrode Material on Transient Current and Conduction Mechanisms in Oil Impregnated Pressboard Insulation

Abstract

The conduction behavior in oil-impregnated pressboard insulation has been extensively investigated under various conditions. However, few studies have addressed the potential influence of the electrode’s metallic nature and the fibrous structure of the pressboard material on the conduction process at the metal/pressboard interface. In this work, conduction currents are recorded using the polarization-depolarization current (PDC) technique, and a comparative analysis is performed on three metal/pressboard/metal (M/P/M) systems under different DC voltages. The results show an effect of the metal electrode on the transient current features. The conduction current is relatively high and shows slight variations depending on the type of metal electrode, following the same trend as the work function. This high current can be attributed to the moisture content of the pressboard, which may introduce ionic species (such as H+ and OH ${}^{-}$ generated by water dissociation within the cellulose structure of the pressboard) and induce charge accumulation at interfaces under an applied electric field. It is also highlighted that an increase in oil content in the M/P/M configuration with a steel electrode leads to space charge effects. Moreover, experimental data align with the thermionic emission mechanism at the M/P interface and the bulk-limited ionic conduction. This helps in estimating the height of the energy barrier at the M/P interface, which is found to be around 1 electron-Volt. In addition, the impact of both the metal electrode and oil content (in the impregnated pressboard) on the energy barrier height at the M/P contact is also observed.