Neutral-point-clamped and T-type multilevel inverters

This chapter introduces neutral-point-clamped (NPC) and T-type multilevel inverters. The three-level NPC multilevel inverter has found wide applications in high-power medium-voltage (MV) drives. The main features of the NPC inverter include reduced dv/d and total harmonic distortion (THD) in its ac output voltages in comparison to the two-level inverter. T-type inverters are the new generation of multilevel inverters offering better efficiency than the NPC inverters when they are operated within the medium switching frequency range (6-30kHz). The improvement in the efficiency is due to the small conduction and switching losses of the T-type topology. Another advantage of the T-type inverters is that, unlike the NPC inverters, no clamping diodes are required for clamping the neutral point to positive or negative dc voltages. In T-type inverters, clamping is achieved by using an active bidirectional switching device connected between the midpoint of each phase leg and midpoint of series connected dc-link capacitors. The operating principles, switching states, switch fault analysis, influence of switching states on the dc capacitor voltages, and modulation of the NPC and T-type inverter topologies are discussed in the following sections.