Experimental Evaluation of Submodule Losses in Battery-Integrated MMCs with NLM and PSPWM

Electric vehicle (EV) battery packs contain several parallel and series-connected cells and variations in leakage currents and cell characteristics result in heterogeneous discharge rates among cells, thus limiting the total energy delivery of the pack. Battery-integrated modular multilevel converters (BI-MMCs) increase the controllability of cells thereby improving the energy utilization of the battery pack. Design optimization for BI-MMC with phase-shifted modulation (PSPWM) showed that submodule (SM) DC-link capacitors designed to bypass the switching frequency components result in minimum total losses. However, this requires a large DC-link capacitor bank, which increases the system cost. An alternative modulation technique, nearest level modulation (NLM), characterized by low semiconductor switching frequency, is often preferred for MMCs with many SMs. The first contribution is an experimental loss comparison in an SM of a BI-MMC with PSPWM and NLM. The second contribution is investigating the impact of the size of DC-link capacitors on battery and capacitor losses for NLM. The experiments showed that the battery and capacitor losses are independent of the DC-link capacitor size when using NLM. Furthermore, NLM has lower total losses but higher battery losses than PSPWM. A single-phase 4-SM BI-MMC is used as the experimental platform for the comparison.