Theoretical and Experimental Analysis of Output Power Quality in Single-Phase Cascaded H-Bridge Multilevel Inverters

In this thesis simple closed-form asymptotic solutions for estimating the output power quality in single-phase cascaded H-bridge multilevel inverters are presented for staircase modulation technique and pulse-width modulation (PWM) technique. The analysis is carried out in the time domain considering the whole harmonic content and being used for an arbitrary inverter level count. In case of the staircase modulation technique, the voltage and current ripple normalized means square (NMS) expressions are obtained in time domain considering the fundamental period. Voltage and current total harmonic distortions (THDs) as a function of the corresponding NMS values are defined as constrained optimization ones. Optimizing the voltage and current THDs determines the voltage and current optimal switching angles over the modulation index range. The current THD is understood as voltage frequency weighted THD that assumes a pure inductive load, but it is practically accurate for inductively dominant RL-loads. The same approach for estimating the current quality is given for a grid-connected inverter. In the case of the PWM technique, the voltage and current THDs are estimated supposing that the ratio between switching and fundamental frequencies is (infinitely) large (asymptotic assumption). The voltage and current ripple normalized mean square (NMS) values are obtained in time domain by double integration of their normalized squared ripples over the switching and fundamental periods. They present piecewise continuously differentiable analytical solutions with only elementary functions and can be understood as the time-domain equivalent of the frequency-domain double Fourier transformation. The direct relation between the voltage and current NMS values and their qualities is presented. Considering the same approach, the current THD evaluation in case of a grid-connected system is presented. Besides analytical developments, simulation and experimental verifications for three-level (one H-bridge), five-level (two cascaded H-bridges) and seven-level (three cascaded H-bridges) single-phase inverters are analysed, presented and compared in details.