Modular Battery Systems for Electric Vehicles based on Multilevel Inverter Topologies - Opportunities and Challenges

Doctoral thesis, 2021

Modular battery systems based on multilevel inverter (MLI) topologies can possibly overcome some shortcomings of two-level inverters when used for vehicle propulsion. The results presented in this thesis aim to point out the advantages and disadvantages, as well as the technical challenges, of modular vehicle battery systems based on MLIs in comparison to a conventional, two-level IGBT inverter drivetrain. The considered key aspects for this comparative investigation are the drive cycle efficiency, the inverter cost, the fault tolerance capability of the drivetrain and the conducted electromagnetic emissions. Extensive experiments have been performed to support the results and conclusions.

In this work, it is shown that the simulated drive cycle efficiency of different low-voltage-MOSFET-based, cascaded seven-level inverter types is improved in comparison to a similarly rated, two-level IGBT inverter drivetrain. For example, the simulated WLTP drive cycle efficiency of a cascaded double-H-bridge (CDHB) inverter drivetrain in comparison to a two-level IGBT inverter, when used in a small passenger car, is increased from 94.24% to 95.04%, considering the inverter and the ohmic battery losses. In contrast, the obtained efficiency of a similar rated seven-level cascaded H-bridge (CHB) drivetrain is almost equal to that of the two-level inverter drivetrain, but with the help of a hybrid modulation technique, utilizing fundamental selective harmonic elimination at lower speeds, it could be improved to 94.85%. In addition, the CDHB and CHB inverters’ cost, in comparison to the two-level inverter, is reduced from 342€ to 202€ and 121€, respectively.

Furthermore, based on a simple three-level inverter with a dual battery pack, it is shown that MLIs inherently allow for a fault tolerant operation. It is explained how the drivetrain of a neutral point clamped (NPC) inverter can be operated under a fault condition, so that the vehicle can drive with a limited maximum power to the next service station, referred to as limp home mode. Especially, the detection and localization of open circuit faults has been investigated and verified through simulations and experiments.

Moreover, it is explained how to measure the conducted emissions of an NPC inverter with a dual battery pack according to the governing standard, CISPR 25, because the additional neutral point connection forms a peculiar three-wire DC source. To separate the measured noise spectra into CM, line-DM and phase-DMquantities, two hardware separators based on HF transformers are developed and utilized. It is shown that the CM noise is dominant. Furthermore, the CM noise is reduced by 3dB to 6dB when operating the inverter with three-level instead of two-level modulation.