A new state of charge control of modular multilevel converters with supercapacitors for traction drives

Modular multilevel converters with storage devices integrated in the submodules are emerging topologies for traction drives. This paper proposes a new state-of-charge control technique to optimally utilise supercapacitor cells that are connected to the sub-module capacitors by means of boost DC/DC converters. The proposed strategy ensures that the charging/discharging trajectory of all the supercapacitor cells during a charging or discharging cycle arrive at their final state-of-charge values at the same time, irrespective of their initial balancing. With this control, the energy stored in the super-capacitor cells is appropriately supplied/absorbed to/from the motor and, at the same time, it eliminates extra control loops for the state-of-charge balancing control. The proposed technique modulates the output voltage of the sub-module DC-DC converter according to its instantaneous state-of-charge of supercapacitor cells. The key challenge regarding the voltage based control of the modular multilevel converter is discussed in detailed. The advantages and drawbacks of this technique compared to the existing balancing technique are documented. At last, the validity of the proposed technique is presented with the help of numerical simulations in Matlab/Simulink.