Charge Conduction Modeling in Synthetic Ester and Kerr effect Measurements of the Electric Field

The presence of DC voltages in the electricity supply chain is growing due to the increasing use of static conversion. In this context, emerging technologies are often required to comply with dielectric withstand requirements against DC voltages. Synthetic ester (SE) is considered in liquid-immersed technologies as an alternative to mineral oil, thanks to the fire safety and biodegradability. However, for SE and DC voltages, the experimental validation of the electric field models used during design is very limited. In this work, the electric field between two electrodes insulated with SE and cellulose presspaper is analyzed by the quasi-Ohmic ($q \mathrm{Ohm}$) conduction model, and the Planck-Nernst-Poisson (PNP) charge transport model. A criterion for assessing the applicability of the qOhm model is proposed. This criterion and the aforementioned models are validated experimentally at various voltages, by measuring the electric field in the liquid using the Kerr effect. This reveals that the field in the liquid is not negligible. Within the ohmic conduction regime, the qOhm and the PNP models provide equivalent results, in agreement with measurements. Furthermore, the PNP model provides more accurate results when conduction is non-Ohmic. This work adds to the understanding of dielectric stress with DC voltages for SE-based insulation systems.