Auxiliary power supply for medium-voltage modular multilevel converters

Design considerations for a modularized auxiliary power supply suitable for variable voltage modular multilevel converters, in terms of electrical performance and isolation against high voltages, are presented. In the presented auxiliary supply, an LLC resonant converter along with several individual floated voltage sources containing high-frequency and high-voltage-isolation transformers are employed. The design process of the individual floating voltage sources is based on circuit and FEM simulations. Partial discharge measurements are also shown for the proposed high-voltage transformer design. Last but not least, electrical measurements performed on a down-scaled laboratory prototype reveal the stable and proper operation of the system at various operating points.