A critical component of a battery electric vehicle (BEV) is the battery pack, which has many series- and parallel-connected electrochemical cells. The total power, energy delivered, and lifetime of the battery pack are limited by the weakest cell in the pack. Battery-integrated modular multilevel converters (BI-MMC) can overcome this limitation by increasing cell-level control. BI-MMCs have several series-connected DC-to-AC converters with a battery module having a few series- and parallel-connected cells called submodules (SM). The research in this thesis focuses on the design, modulation, and control of BI-MMCs.

The efficiency and adaptability of five basic BI-MMC topologies with half-bridge and full-bridge SMs across three main system configurations are presented. Full-bridge topologies offer high efficiency, some even higher than the state-of- the-art SiC two-level inverter. However, adapting them to BEVs requires significant architectural modifications to the BEV’s electrical system. The half-bridge topologies require fewer architectural modifications for adaption into the BEVs. However, they have lower efficiency and require a larger number of SMs, which increases the cost. The efficiency is increased with six-phase system configurations but at the cost of more SMs than three-phase system configurations. Another aspect of adaptability is the DC charging capabilities of BI-MMCs. The maximum DC charging power of the BI-MMCs with the same SM semiconductor losses as during traction is derived, and results show that most BI-MMCs have a maximum DC charging power of about 1MW.

Key design parameters that affect the efficiency and cost of BI-MMCs are identified. They are the number of series-connected cells in an SM, SM DC-link capacitor energy, and MOSFET switching frequency. BI-MMCs with five to seven series-connected cells per SM have the highest efficiency, at an average power of 100 kW and considering phase-shifted carrier-based modulation. Selecting the MOSFET switching frequency close to the resonant frequency of the SM DC-link capacitors and the SM battery modules decreases the total efficiency. Increasing or decreasing the MOSFET switching frequency increases the efficiency but affects the loss distribution between the SM DC-link capacitors and the SM battery modules.

BI-MMCs with nearest level modulation (NLM) have higher efficiencies than phase-shifted carrier-based modulation and the SiC two-level inverter. However, using NLM with low-frequency sort-and-select inter-SM balancing methods (sNLM) results in an uneven distribution of battery losses among the SMs, which may impact the thermal design. Using NLM with cyclic submodule duty cycle rotation at the fundamental frequency gives higher efficiencies than sNLM and an even distribution of battery losses among the SMs.

Reconstruction of converter reference signals with a higher sample frequency at the submodule level can be used to adapt distributed control architecture to BI-MMCs. The advantage is the low communication burden between the central and the SM control units. Furthermore, the accuracy of the SM battery currents (over one fundamental period) is improved, and the output distortion is low.