A Reduced Switch-Count Power Cell Configuration for Medium Voltage Cascaded Cell Motor Drives

Abstract

Multilevel inverters (MLI) are the foundation of motor drives in medium and high-voltage applications, allowing low voltage devices in high power applications. The Cascaded H-Bridge (CHB) is a multilevel inverter with significant popularity and unique advantages due to its modular structure. The modularity of the CHB allows a scalable design that accommodates various power ratings with minimal changes. In a typical CHB, a six-pulse diode rectifier and an H-Bridge inverter provide power from the grid to the load. As a diode rectifier only allow unidirectional power flow, the typical CHB is confined to only receiving power from the grid. The unidirectional nature of the typical CHB can propose energy loss if used in regenerative applications. A regenerative CHB with two-level voltage source inverters rather than diode rectifiers has been introduced to address the unidirectional power flow. However, this solution proposes a significant increase in the system's complexity, cost, and size. As the demand for efficient energy increases, the research on more suitable-sized and economical regenerative CHBs has gained substantial attention. In this thesis work, two new reduced switch-count regenerative CHB power cells are proposed. Both proposed power cells reduce the number of switches per cell by four. The challenges that arise are addressed by introducing new control and carrier phase-shifting techniques. The switch-count reduction aims to reduce the complexity, cost, and size of the conventional regenerative CHB. The theoretical analysis and simulation studies demonstrate the potential of the new reduced switch-count regenerative power cells and its capability to replace the conventional regenerative CHB power cell. Experimental validation of one of the proposed power cells is conducted with a single-cell configuration. The experimental result for conventional regenerative, three-phase reduced switch-count and the new proposed power cells are presented to compare their operation. The future work of this thesis includes experimental validation of the new proposed power cell in a 7-level regenerative CHB drive and operation of the system with a regenerative motor load.