Charge balancing for advanced MMC-Double-Submodules with ultra-low loss

Reduction of capacitor size and further reduction of power loss are important for many future application fields of Modular Multilevel Converter (MMC). In addition, fully electronic current limitation and cut off - both at AC- and DC-side - and a wide operating range of DC-voltage are valuable at the system level. Submodules with bipolar terminal voltage are known to be helpful for achieving these objectives. Unfortunately, power losses increase, considerably, when applying conventional Full-Bridge submodules. Very few submodule topologies are existing, which enable bipolar terminal voltage combined with reduced power loss and capacitor size. In the past, mainly the “double submodule approach” has led to essential improvements concerning these issues [1], [2], [3]. The Semi-Full-Bridge is one of the most promising topologies, following this approach [4], but internal capacitor balancing and power loss are not fully satisfying [5], [6]. An advanced submodule topology and a suitable control concept are presented.