Functional modeling and Energetic Macroscopic Representation of Modular Multilevel Converters

A key feature of Modular Multilevel Converters (MMC) is the continuous and independent nature of the branch currents. Therefore, a decoupled control of the line and partial bus currents is possible, unlike for classical topologies. To this end, several control strategies have been proposed in the literature. Unfortunately, some of these strategies appear to be rather complex and are prone to conflicting control loops. This paper uses the formalism of the Energetic Macroscopic Representation (EMR) in order to develop functional representations of the MMC phase legs, which reveals to be very helpful in the understanding of the intrinsic converter behavior as well as its control structures. Both coupled and non-coupled branch inductors are taken into account in the developments. In a second step, this paper shows how such representations can be used to deduce simple and effective control strategies through elementary inversion rules. As an example, a typical decoupled current control strategy is developed and its effectiveness is verified by simulations.