Advanced control and modulation strategies for high-performance multilevel power converters

This thesis reports the outcomes of three years of doctoral research undertaken during the XXXVIII cycle of the PhD Program in Electrical and Information Engineering at the Polytechnic University of Bari. Conducted at the Power Electronics Laboratory of the same institution, the work investigates and develops advanced modulation strategies for multilevel power converters, with the primary aim of improving efficiency, reliability, and grid harmonic performance in both drive and grid-connected applications. As part of this work, a six-month research stay has been undertaken at Eidgenössische Technische Hochschule (ETH), Zurich, Switzerland. Beginning with an in-depth analysis of conventional Pulse Width Modulation techniques for multilevel converters, particularly the Cascaded H-Bridge topology, the work develops optimized phase-shift Pulse Width Modulation methods to minimize harmonic distortion while maintaining low switching losses. Analytical derivations are complemented by extensive experimental validation carried out on the dedicated setups of the Power Electronics Laboratory at the Polytechnic University of Bari, which also reveal the inherent limitations of conventional PWM, especially its inability to independently control pulses within each control period. Building on these findings, the research introduces a novel Pulse Position Modulation scheme that generates gate signals as a sum of individually timed pulses optimized for specific harmonic objectives, thereby increasing the modulation degrees of freedom and enabling more precise harmonic cancellation with reduced switching losses and fast dynamic performance. The proposed method is first validated on a full-bridge converter and then applied to two-level inverters to suppress common-mode voltage, reduce leakage currents, and enhance the operational lifetime of photovoltaic panels and motor bearings. Pulse Position Modulation is subsequently extended to multilevel Cascaded H-Bridge converters to mitigate harmonics under unbalanced conditions, with experimental results on laboratory prototypes demonstrating superior performance compared to traditional modulation strategies. The thesis further proposes an innovative interleaved Cascaded H-Bridge topology designed to increase voltage levels while reducing the number of semiconductor devices, supported by a dedicated modulation strategy whose experimental validation confirms improvements in scalability, harmonic performance, and overall efficiency. Finally, an enhanced modulation and control approach is applied to the Flying Capacitor Buck Converter, leveraging the flying capacitor as an energy buffer for active damping and improved DC system stability. This last contribution, developed during a six-month visiting period at ETH Zurich, shows how advanced modulation and control strategies can reduce the need for input filters and additional damping circuits, thereby consolidating the role of innovative control strategies in optimizing converter performance. This thesis ultimately demonstrates that advanced and experimentally validated modulation strategies are not simply auxiliary control tools but constitute the core enabling technology that drives the evolution of high-performance, reliable and sustainable next-generation power converters.