Analytical modeling of a medium-voltage and high-frequency resonant coaxial-type power transformer for a solid state transformer application

This paper introduces a unique medium-voltage and high-frequency resonant coaxial-type power transformer (RCT) for a power-distribution level Solid State Transformer (SST) application [1]. The dc-dc stage of the SST application requires a compact high performance resonant transformer that operates under high electric stress and frequency. The RCT discussed in this work brings the virtues of the coaxial-type transformer, which has been use in radio frequency applications, to medium-voltage power conversion applications. The high frequency parasitic effects, which are negligible at 60Hz, become a significant concern in a SST operating above kHz range. Hence, the parasitics of the transformer need to be accurately predicted and controlled for quality power control and safety. The RCT not only minimizes the parasitics of the transformer but also integrates a resonant tank comprised of a series inductance and shunt capacitance by utilizing the stray magnetic and electric flux within the transformer. The limited space in compact size is efficiently used and materials can be optimized. The unique analytic design method of an RCT is introduced and the equivalent circuit model is developed in this paper. The specific design is based on the high performance transformer for a dual-active bridge (DAB) converter, which is one of the most popular topologies in bidirectional power conversion, in the dc-dc stage of the SST application being developed at the FREEDM System Center and its concept is verified with FEM analysis and experiments.