Low frequency operation of Modular Multilevel Converters with embedded battery cells for traction drives

Traditional Modular Multilevel Converters (MMCs) need a special care to be taken for starting and for operation at low motor speeds in traction drives. Indeed, the low-frequency current may cause a significant unbalance between the converter sub-module capacitor voltages and then disturb the output waveforms. MMCs with the battery cells integrated inside the sub-modules are an alternative configuration where the traditional battery management system of battery electric vehicles is replaced by the control of the converter, which individually balances all the cells. The paper evaluates the low frequency operation of the MMCs with embedded battery cells when used as traction drives for battery electric vehicles by means of numerical simulations obtained from Matlab/ Simulink and eexperimental results from a down-scaled prototype. Results confirm that MMCs with embedded battery cells do not present any operational issue in traction drives even at low speed.