Development of a Test Bench for Multilevel Cascaded H-Bridge Converter with Self-Balancing Level Doubling Network

This Master degree project was developed during an exchange program, established between the University of Bologna and the Technical University of Munich (TUM). The research activity was conducted at the Institute of Energy Conversion Technology (TUM department) in collobaration with Prof. Dr.-Ing. Hans-Georg Herzog and his research team. A symmetric 3-Phase Cascaded H-bridge Multilevel Inverter (CHBMLI), that is available in the TUM university laboratory, is reconfigured to operate as proposed using a Level Doubling Network (LDN). The LDN takes the form of a 3-phase half-bridge inverter that shares a common DC bus connected to a floating capacitor. This configuration allows almost to double number of output voltage levels. The LDN concept has inherent self-balancing capability that guarantees to maintain voltage across the LDN capacitor at nearly constant value and without any closed-loop control, while it does not consume or supply any power, apart from losses in the circuit. The proposed topology preserves the merit of CHBMLI modular structure, improving overall inverter’s reliability with reduced number of switching devices and required isolated DC sources compare with standard CHBMLI topology. Therefore, it significantly improves power quality, allows to reduce average device switching frequency, while minimizing cost and size of the power filter. Operation of the circuit is extensively verified by simulation in MATLAB/Simulink framework and experiments, performed on a grid-connected 3-phase five level laboratory prototype, specifically built as a part of the current Master Thesis. This work is a first step towards studying the proposed topology. Nevertheless, it provides a baseline for future analyses of the architecture and its possible variations.