This paper presents a simplified qualitative com parison of previously presented cascaded multilevel converter topologies for ac-ac conversion with particular emphasis on motor drive applications. Performance criteria such as the pulsation of the stored energy in the cell capacitors and the total required semiconductor rating are derived by analytical methods. The main conclusion is that the back-to-back connected modular multilevel converter operates best at synchronous speed, whereas the modular matrix converter and Hexverter are better suited for low-frequency output. However, by injecting circulating currents in the phase arms the operating region can be extended for all of the studied topologies.
I. INTRODUC TIONThe market for medium voltage (MV) motor drives is expanding rapidly, propelled by demand from industry for improved process control. The energy savings that can be archived by variable-frequency operation also contributes sig nificantly to this growth. Also within renewable energy ap plications such as grid integration of large wind turbines, the interest for MV drives is rapidly increasing. The conversion function that should be fulfilled is ac-to-ac conversion where both ac voltages lie in the range 2.3 kV through 6.9 kV On the motor side variable frequency and variable voltage operation is normally required. Bidirectional power flow is sometimes desired such that regenerative operation is possible. Reliability is generally of high importance since a converter failure may halt an entire industrial process rapidly incurring costs of the same magnitude as the cost of the drive System. In terms of topologies the field is currently dominated by three-level neutral-poi nt-clamped (NPC) converters [1 ] and topologies derived from the NPC, such as the five-level active NPC (ANPC). These are all characterized by the fact that there is a single common dc link taking up the full dc voltage. Generally, this dc link is split by a number of series-connected dc capacitors whereby the ditlerent voltage levels are created.Within high-voltage applications, such as VSC-based high voltage direct current-transmission (HVDC) and flexible ac transmission Systems (FACTS), a dramatic shift has taken place over the last ten years towards cell-based topologies in which cascaded strings of converter cells act as controllable voltage sources. A crucial question at this point is whether cell-based topologies will also gain a more prominent role for motor drive applications.The first cell-based converter to be proposed for motor drives was based on having chains of cells where each of them is fed power from a dedicated transformer winding [2]. Depending on the design of the cells, the converter can have capability for bidirectional power flow. Medium voltage drives operating according to this principle have been available in the market for more than fifteen years, however with a limited market share. The fact that a specialized transformer, with many secondary windings, is needed, is likely to increase the cost of this convert...