Design and Development of Novel Multilevel Inverters With Common Leg Configuration by Employing Transformers

Abstract

Multilevel inverters (MLIs) are becoming increasingly popular for medium and low-power DC to AC energy conversion applications due to their ability to ensure high power quality. Compared to conventional two-level inverters, MLIs offer several advantages, including low dv/dt with low total harmonic distortion (THD), decreased common-mode voltage, the capacity to work at low switching frequencies, and the ability to handle higher voltage levels with devices of lower voltage rating. Therefore, they look attractive in numerous applications, such as marine propulsion, mixers,electric locomotives, renewable power generation, flexible AC transmission systems (FACTS), and high-voltage DC transmission (HVDC) systems.There is a high interest in electric power generation from renewable sources, and using MLIs can reduce filter size or even eliminate the need for filters entirely in such applications. However, the primary challenge in implementing multilevel configurations is the increased number of power devices and circuit intricacies, which can improve overall control complexity and cost. Thus, an MLI must have reduced circuit complexity and enhanced reliability to qualify as a functional power processing unit. Cascaded H-bridge (CHB), neutral point clamped (NPC), and flying capacitor (FC) converters are among the earliest well-established topologies since the inception of MLIs. The CHB multilevel inverter (CMLI) gained attention for generating higher voltage levels with the minimum number of switching components compared with former MLIs like NPC and FC. However, a wide variety of functions and features are available in CMLIs. CMLIs have their drawbacks, like using separate DC sources for each H-bridge. In fact, the provision of separate DC sources in the power electronics system has significant drawbacks. Further, to avoid this situation, transformer-based multilevel inverter (TMLI) configurations are introduced. The finite merit of employing transformers with cascaded Hbrides is utilizing a single DC source for the entire network and inherent galvanic isolation between source and load. Additionally, the number of levels can be increased drastically with asymmetrical arrangements in the same system. Since then, numerous derivatives and modifications of theseTMLI topologies have been proposed. This research is motivated by the need to generate a large number of voltage levels with a minimal number of devices. This thesis mainly deals with the design and development of multilevel converter topologies with the reduced number of power devices and realtime implementation of the converters. Firstly, a 7L TMLI, which employed low-frequency transformers to boost output voltage with increased levels and provide isolation between the load and supply, was presented for high-power applications. The performance of the proposed MLI is investigated with an SPWM technique. The proposed topology is designed and verified for a 7-level PWM output waveform with only two bridges. Furthermore, a comparison in terms of component count is deliberated to highlight the potential merits of the proposed TMLI. All these features recommend the proposed configuration towards active filters, var compensators, and grid-connected applications.Further, two new 9L TMLI configurations are proposed: 9L TMLI and 9L RTMLI with galvanic isolation and less component count. A hybrid switching technique is developed for the presented circuits. Simple structure, voltage boosting, and easy control are the additional benefits of the proposed arrangements. Output voltage waveforms have proved a gain of two for nine-level constructions. Further, the performance of the proposed circuits is validated experimentally with a hybrid switching method at different modulation indices and load conditions. Later, two novel TMLI configurations are proposed: 9L TMI and 17L TMI with a single transformer and less component count. Further, a novel improved switching strategy has been introduced for the proposed topology. After that, experimental and simulation results were provided to verify the performance of the proposed switching technique for the proposed TMLI. In fact, the proposed technique can be adopted in place of the former switching technique for improving the harmonic profile of the output voltage waveform.Lastly, the proposed TMLI is designed for shunt active power filter application.In this regard, a seven-level common arm TMLI is utilized.