Optimisation of the HVDC auto transformer by using hybrid MMC modulation

Abstract

Over the recent years, the modular multilevel converter has become the state-of-the-art technology for high-voltage (HV) alternating current (AC) to direct current (DC) converters and also upcoming HV DC to DC converters. Especially modular multilevel converters (MMCs) with half-bridge (HB) sub-modules (SMs) are already widely established. Due to the increasing requirements, such as a DC fault ride through and active DC fault current suppression, also MMCs with full-bridge (FB) SMs become more and more attractive. However, the higher losses of the FB SMs require further optimisation to achieve full market acceptance. With the new hybrid modulation technique, combining the advantages of HB and FB SMs, more efficient DC fault resilient converters can be designed. In this study, a new hybrid modulation technique is investigated and implemented for the new HV DC-DC auto transformer, to create a highly efficient, DC fault resilient HV DC-DC converter. A theoretical loss calculation method for hybrid MMCs is developed. The theoretical analysis is validated by means of a full-scale simulation model.