Level shifted SPWM of a seven level cascaded multilevel inverter for STATCOM applications

Multilevel inverters had gain popularity over the years in industrial applications such as motor drives, static var compensators and renewable energy systems. Multilevel inverters have received this attention as it promises fewer disturbances and are capable to function at low switching frequency as compared to two level inverters. Out of many multilevel inverter topologies proposed, Cascaded Multilevel Inverter (CMI) features a high modularity degree because each inverter can be seen as a module with similar circuit topology, control structure and modulation. With large number of advantages, cascaded multilevel inverter has one major limitation of voltage unbalance across the dc capacitors. This paper explores basic structure and operating principle of cascaded multilevel inverter. Common modulation techniques are summarized. This paper also presents Phase Disposition (PD) and Phase Opposition Disposition (POD) SPWM techniques along with module voltage balancing algorithm such as sorting strategy for a cascaded multilevel inverter. The THD of PD and POD SPWM techniques are compared. For PD-SPWM technique, it is observed that the output voltage of inverter contains even harmonic components because of half wave unsymmetry in phase switching function.