Output Voltage Stability of Series Connected Transformers for Isolated Auxiliary Supplies in Modular Medium Voltage Converter Systems

A common way of implementing MV insulated gate and/or auxiliary power supplies for modular converters is using series connected transformers in an LLC resonant converter topology. The LLC converter can result in output voltage instabilities for differing constant power loads at the outputs when operated at resonance frequency. To balance the output voltages, a new balancing concept based on a coupling between the individual transformers is proposed, which guarantees constant output voltages even for highly unbalanced power consumptions at the outputs. Based on the coupling, a concept for a modular MV isolated auxiliary supply for modular multilevel converters (MMCs) is presented. Simulation and measurement results for the presented concept, as well as a comparison with alternative output voltage balancing solutions are provided.