A New Cell Bypass Arrangement and Control for Modular Multilevel Converters based on Thyristor Forced Commutation Circuit

Abstract: HVDC transmission lines are a competitive and in some cases are proven to bea superior choice compared to AC transmission applications. Suitable convertershave been developed for that matter where Modulär Multilevel Converters (MMC)are highly preferred due to their low losses, no filtering requirementsand direct andfast control of AC and DC side. However, the overall eciency of the converteris higher than of a six pulse voltage source converter, it is still lower than the linecommutated converter type.In this master thesis an attempt to decrease the conduction losses of the MMC isinvestigated. A new cell structure design used in MMC is proposed along with itsassociated control strategy. The main idea is to divert the current at steady stateoperation through thyristors, which have lower conduction resistance than IGBTsthat are used in MMC topologies, at time intervals where the capacitor is bypassedfrom the cell. This new cell commutation is tested initially in the lab and thenthe whole structure operation is validated on a 3 phase MMC PSCAD model. Theresults from the lab confirmed the commutation of the new cell and the results fromthe 3 phase model showed that the new cell structure does not disturb the normaloperation of the MMC. A rough loss comparison that have been conducted betweenthe new cell structure and a half bridge that is used in a typical MMC, showed thatthe first one was less efficient. For that reason a generalized concept is introducedwhich promise higher efficiency than of the proposed concept.