Analysis and Minimization of THD by 7L- Neutral Point Clamped Converter and Its Modulation Strategy

This thesis aims to extend the knowledge about the performance of multilevel inverter through harmonic analysis. Large electric drives and utility applications require advanced power electronics converter to meet the high power demands. As a result, multilevel power converter structure has been introduced as an alternative in high power and medium voltage situations. A multilevel converter not only achieves high power ratings, but also improves the performance of the whole system in terms of harmonics, dv/dt stresses, and stresses in the bearings of a motor. Several multilevel converter topologies have been developed; i) diode clamped, ii) flying capacitors, and cascaded or H-bridge. Referring to the literature reviews, the cascaded multilevel inverter (CMI) with separated DC sources is clearly the most feasible topology for use as a power converter for medium & high power applications due to their modularization and extensibility. The H-bridge inverter eliminates the excessively large number of (i) bulky transformers required by conventional multilevel inverters, (ii) clamping diodes required by multilevel diode-clamped inverters, , and (iii) flying capacitors required by multilevel flying-capacitor inverter. As a preliminary study the thesis examined and compared the most common multilevel topologies found in the published literature. Starting from the essential requirements, the different approaches to the construction of multilevel inverter are explained and compared. In particular, aspects of total harmonic distortion (THD) and modulation which are required or desirable for multilevel converters are discussed. Sine-triangle carrier modulation is identified as the most promising technique to pursue for both technical and pedagogical reasons. Since multilevel inverter is considered to be suitable for medium & high power applications, the thesis examined & compared the harmonic analysis of 7-level multilevel inverter through analysis and simulation.