Contents I. Introduction
In recent years, people were looking for renewable energies to produce electricity because fossil fuels are limited and harmful to the environment. Most of the renewable and alternative energy sources are mainly photovoltaic and wind turbines which are based on DC power. But most of our household and industrial load needs AC power. So, in order to get AC power from DC power, a converter is required. Inverters are the type of power converters that converts DC electric power to AC electric power. Achieving high efficiency is the most crucial part of designing an inverter. Total Harmonic Distortion (THD) is the primary parameter that is used to determine the quality of AC electrical power. THD represents the amount of distortion generated by all the harmonic components [1]. This parameter is used to measure the deviation of periodic waveforms containing harmonics of perfect sine wave. According to IEEE STD 519-1992 THD should be less than 5% for any power systems [2]. THD can be reduced by using multiple strategies, which include different modulation techniques, different types of efficient inverter topologies, a single unit or cascaded multilevel inverters, etc. The idea of a multilevel inverter is basically based on the amplitude modulation topology. Here, the instantaneous voltage of a modulating sinusoidal signal is measured, and based on that voltage, different switches are operated to produce different levels of output. A 9-level inverter with phase disposition PWM technique was published and this model achieved 10.29% THD [3]. This THD level is exceed IEEE Standard. Then, a 15-level asymmetric multilevel inverter topology was published where the topology was tested with different PWM techniques in MATLAB / SIMULINK. The proposed topology archived THD 9.42 %, 9.15 %, 9.14 %, 9.03 % and 8.98 % applying PODPWM, Hybrid PWM, Phase Shifted PWM, APODPWM, and PDPWM PWM modulation techniques respectively [4]. Still, this THD level is not in the range of standard. To reduce the THD level need to increase the level but then a greater number of switches will be required. But more switch means more complex switching algorithm, switching loss, and complex circuitry. So, A 31-level Inverter with an Optimal Number of Switches was proposed and this topology achieved 2.83% of THD in MATLAB / SIMULINK platform [5]. Another 11-level inverter topology with FACTS capability for distributed energy systems shows voltage THD and current THD around 2.7 and 2.12 % respectively [6]. Though the THD is very low with a smaller number of switches but a low pass passive filter was used. Recently a Single-Phase 63-Level Modular Multilevel Inverter topology was published to Drive Induction Motor for Solar PV Applications and achieved 2. 10% THD in MATLAB / SIMULINK platform [7]. Another work carried out recently, a new 63-level Reduce Switch Asymmetrical Cascaded H-Bridge Multilevel Inverter topology which achieved 1.02% THD considering 1st 27 harmonics in MATLAB / SIMULINK platform [8]. Recently, a novel multilevel topology is proposed which has 3-dc source and generates 15 levels. It has high THD around 6.26% which makes it unusable in grid integrations [12].
