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.

I. Introduction

Progress of high power voltage source converters (VSC) remains important for many future application fields, in particular: Grid infrastructure, better exploitation of solar and wind power resources, multi-terminal HVDC- and MVDC-networks, medium voltage drives and others. Modular Multilevel Converters (MMC) have been introduced successfully into these fields, because of several essential advantages compared to conventional VSC or multilevel VSC [7], [8], [9], [10], [11]. With respect to performance and extended functionality of the converters, a superordinate system level view is necessary, when looking at these future applications: Passive components “around” the converter, like harmonic filters, transformers, mechanical switch gears and protection devices offer very low potential for improvements, in future. The scope of protection must be extended (beyond the semiconductors) to the system level. Looking into the converter, the size and cost of the submodule capacitors have remained unsatisfying in many applications of MMC. Obviously, this specific drawback is originating from the basic concept of distributed energy storage. Therefore, progress - concerning this point - will not come for free. New submodule topologies with higher expense for the semiconductors and improved control concepts are the main options [12], [13], [14]. This paper will focus on the first point.