Foil-winding design for medium-frequency medium-voltage transformers

Abstract

In this paper, the challenges tied to the design of foil windings for medium-frequency medium-voltage transformers for solid-state transformer applications, are addressed. Besides the core losses, winding losses represent the most important design challenge of any transformer, limiting its efficiency and power-processing capability. This is especially true for transformers operating at medium frequency, where additional winding losses are unavoidable and cooling surfaces are decreased due to frequency scaling. While litz-wire windings can help to overcome this challenge to some extent, foil windings remain a technology of choice in many industrial applications due to their low cost, easy manufacturability, good fill factor and high temperature class. A detailed case study of optimally designed foil-winding medium-frequency transformers, for the relevant range of standardized medium voltage levels, and considering state-of-the-art materials and technologies, is presented in this paper. This is done using a custom developed design optimizer that employs 2D finite-element simulations for accurate modeling of AC effects within the foil windings. The analysis of the mentioned results provides valuable insights, highlighting the influence of the selected insulation technology and a strong correlation between the rated insulation voltage and the additional frequency-dependent foil-winding losses.