Control of switching frequency for modular multilevel converters by a variable hysteresis band modulation

The average switching frequency of the Modular Multilevel Converter (MMC) is a characteristic system parameter, which affects the losses of the converter system. Several investigations were done to reduce the switching frequency by applying different modulation strategies. Up to now the switching frequency is always an indirect and random outcome of the modulator and it is not directly controlled. Based on the known techniques for two-level converters a method is proposed to control the switching frequency in steady state operation of a MMC by varying the hysteresis band of the modulator. This allows design calculations with a well-defined average switching frequency, because the control guarantees the desired switching frequency in steady state operation. Simulation results from PSCAD/EMTDC show the proper work of the control loop in a HVDC transmission system model.