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.

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.

The potential of additional steering possibilities (like rear-wheel or all-wheel steering) is analyzed for critical situations to investigate possible safety improvements. For this purpose, a dynamic optimization problem is formulated to find the best possible maneuver. The optimization criterion is to maximize the entry speed into a constant radius -curve. The optimization problem is solved for different steering topologies, and the results quantify the increase in maximum entry speed, highlighting the potential for safety improvements. Further, the optimal steering strategies are determined, and they show interesting characteristics like initial diagonal driving or, in other cases, a transition from initial out-of-phase steering to in-phase steering.

Safety, reliability, and durability are targets of all engineering systems, including Li-ion batteries in electric vehicles. This paper focuses on sensor setup exploration for a battery-integrated modular multilevel converter (BI-MMC) that can be part of a solution to sustainable electrification of vehicles. BI-MMC contains switches to convert DC to AC to drive an electric machine. The various configurations of switches result in different operation modes, which in turn, pose great challenges for diagnostics. The study explores diverse sensor arrangements and system configurations for detecting and isolating faults in modular battery packs. Configurations involving a minimum of two modules integrated into the pack are essential to successfully isolate all faults. The findings indicate that the default sensor setup is insufficient for achieving complete fault isolability. Additionally, the investigation also demonstrates that current sensors in the submodules do not contribute significantly to fault isolability. Further, the results on switch positions show that the system configuration has a significant impact on fault isolability. A combination of appropriate sensor data and system configuration is important in achieving optimal diagnosability, which is a paramount objective in ensuring system safety.

Electric vehicles are rapidly developing in response to the need for increasing sustainable energy sources. The range and lifetime of an electric vehicle are limited by the battery pack. A pack comprises modules with several parallel and/or series-connected cells. Differences in leakage currents and cell in-homogeneities cause individual cell voltage and state-of-charge distribution among the cells to be non-homogeneous. As a result, over time, some cells discharge faster than other cells, thus limiting the total energy delivered by the pack. In order to maximize the energy delivered by the pack, individual cell control is desirable. As a solution, battery-integrated modular multi-level converter (BI-MMC) topologies are proposed, presented, and evaluated. BI-MMC topology consists of either one or two arms per phase, and each arm comprises several cascaded stages of DC–AC converters and is commonly referred to as submodules. BI-MMCs provide increased controllability and potential improvement in the lifetime of the battery pack. Furthermore, BI-MMCs have low output total harmonic distortion, further improving the powertrain efficiency.

The first contribution is the design and evaluation of 3-phase and 6-phase BI-MMCs; comparisons are made against a conventional 2-level inverter for a 40-ton 400 kW commercial vehicle. The evaluation considers the total number of submodules, energy rating of the DC-link capacitors, battery losses, capacitor losses, and semiconductor losses. The evaluation showed that the BI-MMCs have lower semiconductor losses than the conventional 2-level inverter. However, the BI-MMCs have higher capacitor and battery losses. The second contribution is the investigation of the impact that the number of series connected cells per submodule has on the total losses of the BI-MMC. The study showed that 5- to 6-series connected cells have the lowest losses. The third contribution is the design principles for optimization of the DC-link capacitors and the MOSFET switching frequency; this is supported by experimental validation for the loss distribution within a submodule. The fourth contribution is a methodology for determining the battery impedance using the full-load converter current. In a conventional battery pack, the battery is connected directly to the fast charger’s DC supply. However, in a BI-MMC, the battery and the inverter are integrated, potentially increasing the DC charging capabilities because higher voltages can be achieved during charging than during operation. The fifth contribution is thus the derivation and investigation of the maximum DC charging power of BI-MMCs assuming the same submodule semiconductor losses during traction. The analysis showed that most BI-MMCs have a maximum DC charging power of about 1MW.

The increase in the average global temperature is a consequence of high greenhouse gas emissions. Therefore, using alternative energy carriers that can replace fossil fuels, especially for automotive applications, is of high importance. Introducing more electronics into an automotive battery pack provides more precise control and increases the available energy from the pack. Battery-integrated modular multilevel converters (BI-MMCs) have high efficiency, improved controllability, and better fault isolation capability. However, integrating the battery and inverter influences the maximum DC charging power. Therefore, the DC charging capabilities of 5 3-phase BI-MMCs for a 40-ton commercial vehicle designed for a maximum tractive power of 400 kW was investigated. Two continuous DC charging scenarios are considered for two cases: the first considers the total number of submodules during traction, and the second increases the total number of submodules to ensure a maximum DC charging voltage of 1250 V. The investigation shows that both DC charging scenarios have similar maximum power between 1 and 3 MW. Altering the number of submodules increases the maximum DC charging power at the cost of increased losses.