Time and Space Transition of DC Electric Field Distributions in Oil-Pressboard Composite Insulation in AC/DC Converter Transformer

It is necessary to enhance an HVDC electrical insulation performance for further introduction of DC electric power systems. In AC/DC converter transformers under DC application, the electric field distributions are distorted due to the deposited charge on pressboard (PB). However, DC electric field stress in oil-PB composite insulation has not yet been clarified in detail. In this paper, we investigated the time and space dependent characteristics of DC electric field stress in oil from DC-on (initial state) to DC steady-state in oil-PB composite insulation systems. As a result, we quantitatively clarified that DC electric field stress strongly depends on the elapsed time and the location in the electric field space in oil. In particular, it was found that the most severe electric field stress in oil for the electrical insulation performance may emerge during the time transition process between initial DC-on to DC steady-state, using an insulated conductor lead and an actual AC/DC converter transformer model.