Simulation Of Multilevel Inverter With Six Switches

Many industrial applications demand an inverter for its faithful and smooth operation. Research is going on to make the inverter more efficient. As the industry demand is to get more power with reduced harmonics, a Multilevel Inverter (MLI) is a viable solution. In an MLI output voltage is derived from multiple dc voltage sources connected at the input side. The higher the number of levels for the inverter, the lesser will be the harmonics present in the output voltage but the number of switch and associated auxiliary requirements will increase. MATLAB simulation was performed with a five-level cascaded H Bridge Multilevel Inverter (CHB-MLI) with eight switches. Later to improve the performance and efficiency of the inverter the count of switches was reduced to six and simulations were performed on the modified version. The control technique used for pulse generation is Phase disposition pulse width modulation(PD-PWM. Simulation of both inverters using MATLAB Simulink was performed and comparison of THD results are presented.

I. Introduction

Many industrial applications demand a smooth controllable ac power supply. Hence inverter is an important part of many application. Research is going on to improve the quality of power supplied by the inverter by increasing the number of steps in the output voltage of the inverter. Two level inverter is a primary level inverter that contains higher harmonics in the output voltage. Also in this inverter voltage stress on the switches will be more [1]. Hence more than two levels in the output voltage, lead to what are called multilevel inverters. Instead of the square waveform, the multilevel inverter (MLI) output voltage will be having a stepped waveform. Hence as the number of steps in output voltage is increased, more approximate it becomes to sinusoidal waveform [2]. But in proportion to increase in voltage levels, count of switches and the complexity of switching control circuit also will increase. In multilevel inverters, more than one dc source are connected at the input side. The dc voltage at the input is usually obtained from renewable energy sources, fuel cells, capacitor voltage source etc. The block diagram of the operation of a multilevel inverter is shown in Fig1. The gating pulses required for controlling the switches of the inverter are obtained by comparing high frequency triangular wave and a modulating wave. The switches of the inverter are operated with these pulse width modulated (PWM) pulses to obtain the required AC waveform at the inverter output terminal. To filter out the harmonic present in the output waveform they are screened through a filter circuit before connecting to any load. Hence filter circuit further reduce the THD present in the inverter output waveform. Multilevel inverters find a place in vast applications of renewable energy sources [3].