Investigation on Cascade Multilevel inverter for Medium and High-Power Applications

Abstract

It is hard to connect a single power semiconductor switch directly to medium voltage grids (2.3, 3.3, 4.16, or 6.9 kV). For these reasons, a new family of multilevel inverters has emerged as the solution for working with higher voltage levels. Multilevel inverters have received more attention in industrial application, such as motor drives, static VAR compensators and renewable energy systems, etc. Primarily multilevel inverters are known to have output voltages with more than two levels. As a result, the inverter output voltages have reduced harmonic distortions and high quality of waveforms. Additionally, the devices are confined to fraction of dc-link voltage. These characteristics make multilevel inverter to adopt for high-power and high-voltage applications. A good number of multilevel inverter topologies have been proposed during the last two decades. Contemporary research has engaged novel converter topologies and unique modulation schemes. Moreover, four major multilevel inverter structures have been reported in the literature these are as follows: cascaded H-bridges inverter (CHB) with separate dc sources, diode clamped (neutral-clamped), and flying capacitors (capacitor clamped), P2 Multilevel inverters. Although different multilevel inverter exists, Cascade Multilevel Inverter (CMI) is one of the productive topology from multilevel family. In reality, on comparing with other multilevel based topologies, CMI feature a high modularity degree because each inverter can be seen as a module with similar circuit topology, control structure, and modulation. Therefore, in the case of a fault in one of these modules, it is possible to replace it quickly and easily. Moreover, with an appropriated control strategy, it is possible to bypass the faulty module without stopping the load, bringing an almost continuous overall availability. All this features make CMI an outstanding power converter. However, one of the greatest limitations of CMI is utilization of separate DC source for each H-Bridge cell. This not only increases cost but also affects the reliability of the system. This is the key motivation for this dissertation. In the present work, we have investigated different CMI based topologies with separate and single DC sources and finally proposed a new CMI based configuration with single dc source by using three-phase transformers. The proposed CMI based inverter presented in this thesis is well defined with logical and mathematical approach. Additionally
to illustrate the merits, it is compared with traditional multilevel inverters. The feasibility of proposed inverter is demonstrated with different illustrations and confirmed by experimental results. The proposed CMI is well suited for grid / photovoltaic and FACTS systems. To elevate the application of proposed CMI a shunt active power filter (APF) design is demonstrated. In this case, the goal is to inject, in parallel with the load, compensation current to get a sinusoidal source current. The proposed APF is verified through Matlabsimulation. Finally, Opal-RT verifications are performed to verify the final design.