Reduced Power Components Based Asymmetric Multilevel Inverter With Elimination of Lower Order Harmonics

Conventional drive converters suffer from several power component counts, a complex circuit design, large switching losses, and other problems. Motivated by these issues, a reduced power components-based asymmetric hybrid inverter is presented for a medium voltage drive. A new structure is designed with a series connection of (1:1) dc-source-based level generator unit with polarity converter H-bridge, which is cascaded to a conventional H-bridge, whose dc source magnitude is double. An eleven-step inverter is configured with a ten-power switches and three dc sources. An extension of proposed inverter to higher voltage levels can be easily achieved by cascading several level-generator and H-bridge units. Due to utilization of selective harmonic elimination (SHE), most power switches are turned on and off at a low and fundamental frequency, which significantly reduces losses and enhances converter efficiency. A further benefit of SHE is that it provides precise control over harmonic spectrum of a specific voltage and/or current waveform produced by a power converter. Nonlinear equation of selected harmonic is solved optimally using particle swarm optimization (PSO) in this study. Apart from this, a 54-pulse rectifier is designed to maintain dc-link voltage and to feed input power to this eleven-level inverter. Apart from this, presented drive ends eleven level dc-ac and grid side fifty-four pulse ac-dc converter, are used to mitigate lower order harmonics at motor terminal currents/ voltages and grid currents. Drive design details, operational modes and fundamental switching scheme are described in detail. This topology is validated using tests on a down-scale prototype, and test results are presented in detail.

I. Introduction

Fundamental switching frequency-controlled power converters, used for a variety of medium-voltage and high-power applications, have witnessed a rise in popularity in recent years. Performance characteristics of power conversion in industrial applications largely depend on selection of power converter and switching technique. A conventional multilevel inverter uses voltage levels from several dc sources [1]. These dc sources can be coupled or isolated depending on circuit topologies. Principle idea of multilevel dc to ac-conversion is to synthesize a high-quality ac voltage with several isolated dc sources and an array of power semiconductor switches. Due to capability of transforming power with high power quality, neutral-point clamped, and flying-capacitor (FC) based multilevel converters have attained more popularity for medium-voltage (MV) electric power applications in early 1990’s [2]. Contrarily, these configurations of multilevel power converters have challenges regarding need for several clamping diodes/capacitors, cost of a capacitor bank, and voltage imbalance, especially for generating higher voltage levels. Researchers, therefore, continue to focus on reducing issues through various circuit configurations and control algorithms [3], [4]. However, these control algorithms are very complex and require a huge number of sensing elements, which is not cost-effective [5]. Hence, later, various medium voltage motor drive manufacturer industries (Delta Group, Robicon Group, Hitachi, Yaskawa, AMTEC, ABB, and Schneider Electric) have manufactured cascaded H-bridge multilevel configurations for high-level inverter. Main problem with this kind of multilevel power converter is that to increase voltage steps, multiple switches and separate dc voltage sources/ dc -link capacitors connected to individual H-bridge cells are still needed.