Modular Multilevel Converter With Series and Parallel Module Connectivity: Topology and Control

This paper introduces a novel modular multilevel series/parallel converter that allows switching modules dynamically not only in series, as in the traditional modular multilevel converter (M2C), but also in parallel. As in M2C, the semiconductor voltages do not exceed the module capacitor voltage for any module state. While the new topology is a generalization of M2C and could, therefore, be operated identically to it, the additional states provide degrees of freedom that the controller can dynamically employ to achieve several advantages. Whereas in M2C many modules are bypassed if the instantaneous converter voltage is lower than the system´s peak voltage, the parallel connectivity enables these modules to contribute to the current load, thus reducing conduction losses. In addition, the parallel configuration of modules can be used for balancing the modules´ state of charge (SOC). The parallelization losses are moderate or negligible, dependent on the switching rate. Since the parallel connection of capacitors can ensure balancing, it enables stable operation of a multilevel converter without the need for monitoring the module SOCs. While such economical control hardware may be appropriate for low-power systems, we also present more sophisticated control that uses the additional degrees of freedom to minimize losses. Finally, we point to further extensions of the circuit topology to multipole module connectivity that could enable additional functionality and applications.