Investigation and loss comparison of 6.6kV 5-level converters

This paper investigates some 6.6kV 5-level converter topologies, including 5-level neutral point clamped converter (NPC-5L), 5-level flying capacitor converter (FC-5L), 5-level active neutral point clamped converter (ANPC-5L) and 5-level hybrid converter, on the basis of state-of-the-art 4.5kV insulated gate bipolar transistors (IGBT). Semiconductor loss and capacitor loss distribution are compared under same equivalent switching frequency, with In-Phase Disposition (IPD) modulation method, which ensures a similar complexity of control implementation. In order to evaluate the performance and characteristics of these topologies, a 5-level as well as a 7-level cascaded H-bridge converter (CHB) with multiple isolated DC sources are also considered. Results show that except for hybrid type, all other 5-level converters have similar conduction loss, which is mainly determined by the number of switching devices in the current path. Switching loss in ANPC-5L is higher than NPC-5L and FC-5L due to its extra switching during zero crossing periods. On the other hand, capacitors loss in FC-5L is more significant than other topologies. By comparing the loss distribution with that in CHB, it can be known that there are still some margins in topology design for improvement to reduce the converter switching loss and capacitors loss, if high efficiency is the target.