Lalit Mohan, S. K. Agarwal, Dharam Vir
Abstract: Multilevel inverters have been widely applied in industries. A family of optimal pulse width modulation (PWM) methods for multilevel inverters, such as step modulation, can generate output voltage with less harmonic distortion than popular modulation strategies, such as the carrier-based sinusoidal PWM or the space vector PWM. Multilevel converters are become very popular in medium and high power applications due to their ability to meet the increasing demand of power ratings and power quality associated with reduced harmonic distortion and lower EMI. We can stress the importance of their capability of increasing the output voltage magnitude and reducing the output voltage and current harmonic content, the switching frequency and the voltage supported by each power semiconductors. By synthesizing the ac output voltage from several levels of voltages, staircase waveforms are produced, which approach the sinusoidal waveform with low harmonic distortion. Multilevel converter enables the ac voltage to be increased without a transformer. In addition, the cancellation of low frequency harmonics from the ac voltages at different levels means that the size of the ac inductances can be reduced. These new types of converters are suitable for high voltage and high power application due to their ability to synthesize waveforms with better harmonic spectrum. Amongst these topologies, the multilevel cascaded inverter was introduced in Static Var compensation and drive systems. In this paper we work on three level, five level (converter) and the harmonics analysis of different level inverters are compared .Also work have been done on PWM based inverter to reduced the harmonics further .A selective harmonics distortion elimination method is also discussed in which we can eliminate the required harmonics .This is actually the combination of conventional multilevel inverter and PWM based inverter.
Keywords: Multilevel inverter, Multi level converter, Pulse width modulation, PWM, Total Harmonic Distortion, THD