Grimm, Ferdinand; (2023) A Modular Multilevel Inverter for Automotive Drivetrains with Transformer-Based Submodule Voltage Balancing. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This dissertation studies an AC-link-based drivetrain concept for battery-electric vehicles. At the heart of the drivetrain lies a Three-phase modular multilevel in- verter topology that employs a novel submodule voltage balancing scheme. Pre- vious multilevel inverter topologies required the usage of redundant switches and large capacitors to balance their submodule voltages, resulting in lower power den- sity and complicated topologies and control. In the proposed topology, this problem is solved with a balancing magnetic field, interfaced through a multi-active bridge. This topology allows to remove all redundant switching states and thus simplifies the modulation while increasing the power density. A special focus was laid on developing methods that are easy to upscale to larger numbers of modules. To investigate the detailed behaviour of the drivetrain, a sys- tem model of the topology was created and simplifications were subsequently de- rived, including linearization, averaging and model order reduction. Several methods to reduce the losses of the device were presented throughout the project. The switching losses of the multi-active bridge were minimized by the de- sign of a specialized gate driving circuit, employing a combination of soft-switching and adiabatic switching. Guidelines for the design and operation of the gate driver circuit were subsequently discussed. To verify the findings, a Three-phase prototype was constructed within the project based on a planar transformer with custom-made ferrites. To reduce the power density and increase the efficiency of the prototype different techniques including interleaving techniques, generalized from two-winding transformers, input signal coding, and a 3D assembly tailored to the prototype were used.

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