Investigation and Control of Hybrid Multilevel Inverter Topologies with Reduced Part Count

Abstract

Multilevel inverters (MLIs) have become a preferred choice for low- and medium-power dc to ac energy conversion applications to ensure high power quality. The MLIs exhibit several advantages over the conventional two-level inverter which include reduced dv=dt, lesser electromagnetic interference, and capability to handle higher voltage levels with devices of lower voltage rating. These features have enabled them to gain popularity in a variety of industrial applications like locomotives, mixers, marine propulsion, reactive power compensation, renewable energy power conversion, to name a few. At present, electric power generation from renewables is being one of the subjects of high-interest; use of MLIs leads to reduced filter size, or even eliminate completely the filter requirements in such applications. The primary challenge in employing the multilevel configurations is the increased number of power devices and the circuit intricacies which adds on to the overall control complexity resulting in a higher cost. Thus, reduced circuit complexity and improved reliability are the desirable characteristics for an MLI to be qualified as an applicable power processing unit. Ever since the inception of MLIs, cascaded H-bridge (CHB), neutral point clamped (NPC) and flying capacitor converters are among the earliest topologies that are deemed to be well-established. Each of them has advantages and disadvantages. An NPC MLI requires additional clamping diodes for its extension whereas, CHB MLI and flying capacitor MLI needs many isolated dc sources to generate a multistep output and multiple capacitors respectively. Since then, many derivatives and refinements to these classic topologies have been proposed. The motivation for this research work stems out from the demand to generate a substantial number of voltage levels while keeping the circuit reliability as high as possible. Therefore, by taking advantage of the basic MLI configurations, a few schemes emanating as a result of combining two or more MLIs in part or fully, referred to as hybrid MLIs are proposed in this thesis for gridconnected renewables. The offered solutions exhibit considerable topological improvements with reduced control complexity. iFirst, efforts are made to derive an innovative power circuit that generates nine-level (9L) voltage waveform from the three-level (3L) T-type NPC converter which is widely regarded as a highly efficient solution for lowpower applications. It is achieved in two ways; in one way, by using a 3L floating capacitor H-bridge (FCHB) and a two-level (2L) converter leg in conjunction with the 3L-TNPC MLI whereas, in the other, by replacing the 3L-FCHB with a 2L switched-capacitor unit. In the latter case the number of controlled switches are reduced to 8 from 10. Further, the control complexity is reduced by applying a sensorless voltage control which is devoid of voltage and current sensor(s). Second, an active NPC (ANPC) MLI suitable for medium-power applications is considered. Two completely new 9L topologies are derived advancing the ANPC converter. A hybrid topology based on 3L-ANPC MLI is the first power circuit built by connecting it with 3L-FCHB and also employs a 2L converter leg. The second hybrid MLI serves as an upgrade of the industrial standard 5L-ANPC MLI to 9L while requiring minimum structural disruption and modification which includes the addition of the 2L converter leg to the 5L-ANPC MLI. A simple logic-form-equations (LFE)-based closed-loop floating capacitor (FC) voltage balancing scheme is implemented by using the redundant switching states. Third, a new hybrid stacked multi-cell (SMC) MLI is proposed. The power circuit is formed as a cascade connection of 5L-SMC and 3L-FCHB. This way the required number of capacitors are reduced to 3 from 8 as compared to the conventional extension of SMC MLI resulting in an increased reliability. The LFE-based voltage balancing scheme achieves the capacitors voltage regulation. Last, the reliability of the proposed topologies is systematically evaluated and are compared. It is seen that each of the topologies qualify as a promising solution for multilevel voltage generation. All the developed schemes are simulated in MATLAB/Simulink for gridconnected and stand-alone modes. The topologies are tested experimentally using dSPACE 1104 controller. Results validating the operability of the proposed topologies and the developed control schemes are p