Design and Development of Capacitor-Based Multilevel Inverters

Abstract

As a result of enormous research and development in multilevel inverters(MLIs), their presence in industries over a wide-ranging application is noticeable. With the advent of high-power semiconductors in the last two decades, conventional voltage source inverters (VSIs) are replaced by MLIs. Output voltage boosting property along with curtailment in the circuit voltage stress and component count are considered as the essential topological features for the new MLI circuits. At present, electric vehicles and renewable power generation are subjects of high interest. In such applications, a secondary circuit such as front- or back-end boosting stage is incorporated into the converter to meet the requirements. In such cases, using MLIs with boosting ability is more logical and reduces the intermediate boosting stage or even eliminates them. Furthermore, most of the available high-power rotating machines require variable speeds and special control algorithms. A power conversion stage using semiconductor switches is required in these renewable energy systems and industrial applications.Therefore, a deep understanding of the design of high-power converters is required for researchers and industrial engineers.Ever since the inception of MLIs, cascaded H-bridge (CHB), neutral point clamped (NPC), and flying capacitor (FC) converters are among the earliest topologies deemed to be well-established. Though these converter circuits have their remarkable peculiarities, they suffer from a control complexity,higher components. The Diode-clamped inverters require more power diodes. Also, the capacitor voltage balancing in diode-,capacitorclamped MLIs is a severe problem that requires sophisticated modulation techniques and external balance circuitry. Since then, many derivatives and refinements to these classic topologies have been proposed. In recentyears, the evolution of MLIs is tending to reduce the number of components, especially dc sources based on the combination of capacitors. In these topologies, dc sources are replaced with switched capacitors (SCs)to reduce the number of dc sources and the cost of the converter. SCs are used as alternative DC supply to boost the output voltage. Due to the self-voltage balancing property of SCs, an uncomplicated sensor-less control scheme can be employed. SC-based converters are the basic alternative solution that does not need any transformer to boost the voltage. This research work’s motivation stems from the demand to generate substantial voltage levels while keeping the circuit reliability as high as possible.Therefore, three different topologies with voltage boosting property are proposed in this thesis by taking advantage of the switched-capacitor multilevel inverter (SC-MLI) configurations. The offered solutions exhibit considerable topological improvements with reduced and control complexity.This thesis mainly deals with the design and development of capacitor based multilevel converter topologies with the reduced number of powerdevices and real-time implementation of the converters. Firstly, a DClink capacitor based fault tolerant multilevel inverter (FTNIT) with a reduced part count is proposed. Besides, With the tiniest changes in the switching combinations, the provided inverter topology can sustain system faults caused by the failure of the source and/or switching devices. Subsequently,When compared to standard nine-level inverters, it features fewer switching devices. The results are observed and validated with a hardware platform while the suggested system is simulated in a MATLAB/Simulink environment under standard and malfunctioning settings.Secondly, a capacitor based inductor less nine level inverter topology(CBILNIT) with reduced part count is proposed. Besides, a simple control approach to regulate the flying capacitor (FC) voltage is elobarated. Here described a simple logic gate-based pulse-width modulation technique thatensures capacitor power balancing. The proposed inverter operation and capability are validated by experimental results derived from a laboratory prototype. Finally, by contrasting the new and standard inverter topologies,the virtues of the suggested architecture by the number of devices and price of the equipment are highlighted.Furtherly, a capacitor-based boost multilevel inverter (CB-MLI) topology as it found suitable for EV and HEV applications is proposed. Theself balancing property of the capacitors makes the topology one of its kind. A constant carrier PWM based control strategy is utilized to switch the IGBTs. Testing results from hardware setup confirm the proposed capacitor-based CB-MLI topology operating modes and potentiality. Finally, by highlighting the proposed and existing MLI circuits, the benefits of the recommended configuration are outlined by component count and total cost.Lastly, a novel switched-capacitor based nine-level inverter (SC-NLI) structure with a new optimal control switching technique for Electric Vehicle (EV) applications is proposed. It comprises ten switches with one dcvoltage source and two floating capacitors and boosts the output voltage with a multiplier of four. The proposed structure’s circuit description, modes of operation, proper component selection, and a new optimal switching scheme are presented. And also, a discussion about the comparativeanalysis of suggested topology with currently developed MLI structures is presented. For theoretical synthesis validation, simulation results are presented. In addition, experimental tests are conducted under various load conditions from the built-in hardware prototype to evaluate the proposed SC-NLI structure. All the developed circuits are simulated in MATLAB/Simulink for standalone operations. All the topologies are tested experimentally in the laboratory at scaled-down ratings. Further, quantitative and generic cost comparisons are conducted among the state-of-art capacitor-based MLIs to highlight the superiority of the proposed configurations.