INVESTIGATION ON PWM TECHNIQUES FOR HYBRID MULTILEVEL INVERTER TOPOLOGIES

Abstract

An Inverter is basically a converter that converts DC-AC power. The word “inverter” in the context of power electronics denotes a class of power conversion circuits that operates from a dc voltage source or a dc current source and converts it into ac voltage or current. Even though input to an inverter circuit is a dc source, it not uncommon to have this dc derived from an ac source such as utility ac supply. Thus, for example, the primary source of input power may be utility ac voltage supply that is converted to dc by an ac to dc converter and then inverted back to ac using an inverter. Here, the final output may be of a different frequency and magnitude than the input ac of the utility supply. Typical Applications such as Un-interruptible Power Supply (UPS), Industrial (induction motor) drives, Traction, HVDC. A multilevel power converter structure has been introduced as an alternative in high power and medium voltage situation. The concept of multilevel converters has been introduced since 1975. The term multilevel began with the three-level converter. Subsequently, several multilevel converter topologies have been developed. However, the elementary concept of a multilevel converter to achieve higher power is to use a series of power semiconductor switches with several lower voltage dc sources to perform the power conversion by synthesizing a staircase voltage waveform. Capacitors, batteries, and renewable energy voltage sources can be used as the multiple dc voltage sources. Multilevel inverters were basically developed to reach higher voltage operation, before being restricted by semiconductor limitations, the extra switches and dc sources (supplied by dc-link capacitors) could be used to generate different voltage levels, enabling the generation of stepped waveform with less harmonic distortion, reducing dv/dt and common-mode voltages. These characteristics have made them popular for high-power medium-voltage applications but the large number of semiconductor switches in these inverters, result in a reduction both of the reliability and efficiency of the drive. The importance of multilevel inverters has been increased since last few decades. These new types of inverters are suitable for high voltage and high power application due to their ability to iv synthesize waveforms with better harmonic spectrum and with less Total Harmonic Distortion (THD).