Dielectric Losses in Dry-Type Insulation of Medium-Voltage Power Electronic Converters

Newly available medium-voltage (MV) silicon carbide (SiC) transistors are setting new limits for the design space of MV converters. Unprecedented blocking voltages, higher switching frequencies, higher commutation speeds, lower losses, and high temperature operation can be reached, which, however, create new challenges for the electrical insulation. In particular, the dielectric losses can become significant for MV converters operated at higher switching frequencies. Moreover, the evaluation of the dielectric losses is a key element for assessing the insulation stress and for insulation diagnostics. Therefore, this paper analyzes the modeling and the computation of dielectric losses with pulsewidth modulated (PWM) voltages. After a review of the dielectric loss mechanisms occurring in polymeric insulation materials, scalable analytical expressions are proposed for the losses produced by PWM voltages. Afterward, the dielectric losses of a medium-frequency (MF) transformer, employed in a 25 kW MV dc-dc converter (7 kV-400 V) operated at 48 kHz, are analyzed and measured in detail. With epoxy resin, the insulation losses represent a significant share, i.e., up to 17%, of the total transformer losses. As shown, the transformer performance can be significantly improved with silicone elastomer insulation. Finally, design guidelines are provided for the selection of insulation materials.