Abstract: In the recent years, multilevel inverters (MLI) are highly being used for medium voltage and high power applications due to their various advantages such as low voltage stress on the power switches, low electromagnetic interferences (EMI), low dv/dt ratio to supply lower harmonic contents in the output voltage and current. Multilevel inverters have become more popular over the years in electric high power application with the promise of very low disturbances and the possibility to function at lower switching frequencies than ordinary two-level inverters. This paper presents different topologies, emphasizing mainly on seven level inverters. The different topologies compared are the diode-clamped inverter (neutral-point clamped), capacitor clamped (flying capacitor), and cascaded multi-cell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. Finally a seven level inverter topology with lesser number of switches is discussed and compared along with the other seven level inverter topologies. Simulation studies of diode clamped seven level inverter, cascade seven level inverter and seven level inverter with reduced number of switches are done. The seven level inverter with reduced number of switch with PWM (Pulse Width Modulation) switching is also simulated. It is observed that the pulse generation using PWM switching leads to further reduction of THD.
Keywords: Multilevel Converter, Multilevel inverter (MLI), Power converters, Total Harmonic Distortion.