title: Battery Power Electronics Integration for Multiphase EV Drivetrains
Creators: name: Khan, MU
Publisher: name: Liverpool John Moores University
Year: 2025

description: The battery electric vehicle (EV) is the most prominent alternative to traditional fossil fuel powered cars. Most modern EVs use three-phase machines for propulsion supplied by a standard IGBT-based two-level voltage source converter connected to a single large battery pack. The two-level inverter is well-known to suffer from several disadvantages. Moreover, recent research has indicated that most driving takes place at low speeds and the two-level converter has been shown to be inefficient in such operating conditions. Multilevel converters have been suggested for use in EV drivetrains due to their inherent benefits compared to the two-level converters. Specifically, the cascaded H-bridge (CHB) converter retains well-known benefits of multilevel converter topologies while offering unique benefits such as simplicity, modularity and redundancy, low device count, fault tolerance, reliability, and its suitability for applications where independent dc sources are readily available. This makes it particularly suitable for traction applications, especially in EV drivetrains with a distributed battery pack. Likewise, multiphase electric machines are prime candidates for propulsion in electric transportation. They offer improved reliability, natural fault tolerance, lower current per phase, and high power density. They have also been shown to be used as current filters in on-board fast and slow charging of EV batteries with minimal hardware reconfiguration leading to weight, space, and cost savings. Therefore, this thesis examines an alternative EV drivetrain based on the combination of these two technologies to form a multilevel multiphase drive where a battery-supplied CHB multilevel inverter is used in conjunction with a six-phase induction machine. Initially, simulation and experimental results are presented for the three-phase multilevel CHB inverter, with equal dc-source voltages, connected to an RL load under different carrier-based modulation methods and min-max based offset voltage injections for the whole modulation index range. Issue of unequal dc-source and switching device utilisation in level-shifted PWM is addressed through the appropriate rotation of the reference waveforms. The operation of the three-phase CHB inverter is also investigated under the inevitable imbalance of the dc-source voltages. Improvement in the performance of a previously suggested method is made leading to better harmonic performance of resulting currents as well as achieving enhancement in dc-source energy management. The operation of the drive is verified with the open-loop control of a three-phase induction machine. High performance control of the multilevel multiphase drive is presented with experimental validation in equal dc-source voltage conditions. The previously presented method for the operation of the CHB inverter in unequal dc-source voltage conditions is further extended to a six-phase multiphase drive with multiple neutral points. The operation of the multilevel multiphase drive is verified in simulation with high performance speed control in unequal dc-source voltage conditions. The drive benefits from modularity, inherent fault tolerance, low THD at low switching frequencies, fast dynamic performance, and the possibility to be used in both propulsion and battery charging modes with minimal hardware reconfiguration. Additionally, smaller per phase current would require smaller sized cables as well as facilitating the use of lower rated semiconductor devices. Likewise, the suggested multilevel multiphase converter requires much smaller dc-link capacitors, which translates to savings in terms of cost and space.