Insulation systems for HVDC transformers: Present configurations, trends, challenges, solutions and open points

Examples of present configurations of insulation systems for windings and bushings in HVDC transformers are presented. Active trends are higher operating voltage (up to 1100 kV DC), more polarity reversals (PR) due to trading of energy and an increasing use of voltage sourced converters. There are several challenges to reliability of these units. Main ones are the uncertainty about oil electrical conductivity and the effectiveness of DC and PR tests. CIGRE Joint Working Group A2/D1.41 “HVDC transformer insulation - Oil conductivity” is preparing recommendations to address both issues. There is still considerable work to do in the characterization of insulating materials, in simulation of dielectric transients with ion drift for realistic insulation systems, in investigation of insulation behavior under both DC and AC stress and in monitoring oil conductivity over HVDC transformers life cycle.