Averaged models and control of modular multilevel converters

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The research project deals with the control design for modular multilevel converters which are used for instance in medium-voltage drives or for high-voltage dc transmission applications. More specifically, the problem of symmetrizing the energies between the arms is to be tackled while the output currents must meet the requirements of the load. This renders the system underactuated which means that the dimension of the input is too low in order to control all variables. However, considering the averaged behavior of the system quantities on a slower timescale offers the possibility to introduce more input variables. This step, often made implicitly and without deriving a dynamic model for the averaged system, is to be backed by singular perturbation methods closing the existing scientific gap. As a side effect, it is desired to show that the known implementations are compatible with the averaging framework to be developed. It is expected that characteristic properties of the original system are maintained by a formal application of the modelling scheme such that these properties can be taken care of in the controller design. Furthermore, the averaging formalism will introduce more input variables than required which offers the possibility to share the control action among them in order to meet additional goals such as the consideration of limitations. Since the modelling scheme will be attractive for other (underactuated) arm-based topologies as well, it is desired to generalize the findings appropriately. The results of the modelling and control design will be verified experimentally using an already existing testbench.