Green Insulation System for Transformers in MVDC and HVDC Energy Networks

The feasibility of using natural ester liquids as insulation materials for transformers in MVDC and HVDC networks is investigated. A comparative study with mineral oilbased insulation system is presented. The comparison includes the dielectric characteristics such as the permittivity and electrical conductivity of the liquids and their impregnated pressboard. The characterizations have been made at 30°C and 70°C using Novocontrol Dielectric Spectroscopy and Polarization and Depolarization Current measurements. The AC and DC electric field stress on the insulation materials is studied using a finite element method-based simulation model. It can be observed that the AC stress in the pressboard is higher for ester liquid than for mineral oil. However, the DC field stress on pressboard is lower for ester liquid than on mineral oil. The study shows that ester and ester impregnated pressboards are suitable substitutes for mineral oil system.