Voltage source inverters (VSIs), electrical devices that convert DC power into AC power, have seen significant development and widespread adoption in various power conversion scenarios, including residential, commercial, and industrial systems. They provide the interaction between energy storage units, DC-link, AC loads, or the power grid in different kinds of power systems. Besides, they also play a crucial role in motor drives, bringing variable frequency capability for adjustable speed operation. However, due to the limitations of the semiconductor power devices and the actual operation conditions, conventional three-phase star-connected VSIs cannot meet the growing demands of high performance and high reliability. To fulfill these requirements, more degrees of freedom on leg-current stress, DC-link voltage utilization, fault tolerance capability, zero-sequence loop, etc. can be added to VSIs by topology improvements, such as the connection of the neutral point, the addition of inverter legs, and the adjustment of winding connection sequence (WCS). In this thesis, such more-degree-of-freedom VSIs (MDOF-VSIs) are discussed and investigated, including their features, modulation strategies, and applications.

Firstly, this thesis reviews the existing works about multi-phase VSIs. Their main features and modulation strategies are summarized, which provides the basic support and reference for the following analysis and summary of MDOF-VSIs.

Secondly, the relationship between different MDOF-VSIs is revealed. It shows that all these MDOF-VSIs can be classified into two categories, namely, connected-neutral topology and open-neutral topology. Besides, the key features of MDOF-VSIs are investigated, including the current feature and DC-link voltage utilization analysis. Based on the phasor diagram, the conjunction of the phase currents and leg currents is studied, which is further applied to reduce the leg-current stress by adjusting the WCS. Meanwhile, DC-link voltage utilization of MDOF-VSIs and the corresponding effect of WCS are investigated by various analytical methods, illustrating the feasible region and linear modulation region in multi-dimensional space. Not only do these analyses demonstrate the theoretical limits of MDOF-VSIs for modulation strategy development, but also provide general guidelines for actual application design.

Thirdly, the modulation strategies for MDOF-VSIs are developed and optimized gradually. For the simple implementation, carrier-based pulse-width modulation based on leg potential is proposed at first, but it cannot fully utilize the dc-link voltage. Thus, multi-dimensional space vector modulation is developed to fully extract the output capacity of MDOF-VSIs. Further, based on online voltage vector determination, the advantages of these two modulation strategies are combined, realizing simple implementation and full DC-link voltage utilization simultaneously.

Finally, all the feature analyses and modulation strategies for MDOF-VSIs are experimentally verified, and some application samples of MDOF-VSIs are presented, including the combination of three-phase series-end winding VSI and advanced model predictive control, and half-open-end winding VSI fed asymmetrical six-phase motor drive for general performance improvement. On the advantages of more degrees of freedom, these MDOF-VSIs can provide single function upgrade or general system optimization, and they also bring a novel perspective for AC drives.