HVDC electrical insulation performance in oil/pressboard composite insulation system based on Kerr electro-optic field measurement and electric field analysis

For the development of HVDC electrical insulation performance in liquid dielectric systems such as oil-pressboard (PB) composite systems in DC converter transformers, a Kerr electro-optic electric field measurement (Kerr-EOM) technique has been well established and will be considered as the key technique to be introduced. In this paper, the Kerr-EOM was experimentally applied to identify the electric field stress distribution and clarify charging activities in oil/PB composite insulation systems, under HVDC and DC polarity reversal (DC-PR) conditions. The physical mechanisms of charge movement and the amount of charge accumulation in liquid/solid insulation systems are also thoroughly investigated and discussed. Moreover, the breakdown discharge initiation by accumulated charges on the PB was observed and discussed together with the field measurement results. Secondly, by combining the Kerr-EOM with the electric field analysis using the Finite Element Method (FEM), the enhancement of the electrical insulation performance for HVDC applications was discussed and the critical testing voltage conditions in DC converter transformers were addressed. This paper is based on the Hans Tropper Memorial Lecture given at the IEEE DEIS, International Conference on Dielectric Liquids (ICDL) in 2017 in Manchester, UK.