A Control Scheme to Reduce the Current Load of Integrated Batteries in Cascaded Multilevel Converters

While battery-integrated modular multilevel converters are a promising alternative to conventional inverters in applications such as automobiles, they often cause significant second-harmonic pulsating currents in the batteries, increasing their degradation rate and the system losses. In this paper, we propose a control scheme for the Modular Multilevel Series Parallel Converter (MMSPC) that reduces the pulsating current seen by the batteries. A harmonic voltage is injected into the controller output voltage, allowing the converter to significantly reduce the low frequency current pulsations by switching modules in parallel more often. This is combined with a simple balancing algorithm to determine the upcoming switching states. We have shown that-without the need of additional hardware-the second-harmonic pulsation can be reduced by over 40 % using voltage injection, by taking advantage of the fact that the MMSPC can parallelize battery-modules when the output voltage is low. This may allow battery degradation to be reduced and the range of an electric vehicle with an MMSPC-system to be increased.