International Journal of Engineering Research ISSN:2319-6890 (online),2347-5013(print) Volume No.6, Issue No.6, pp : 336-340 1 June 2017 DOI : 10.5958/2319-6890.2017.00032.0 Page 336 A Review of Multilevel Inverters with Reduce THD for Solar Application Deepa Raghuwanshi, Santosh Kumar Department of Electrical & Electronics Engineering deeparaghuwanshi26@gmail.com, santosh_en@reddiffmail.com Abstract— Multilevel inverters have become more popular over the years in electric high power application with the promise of less disturbances and the possibility to function at lower switching frequencies than ordinary two-level inverters. This report presents information about several multilevel inverter topologies, such as the Neutral-Point Clamped Inverter and the Cascaded Multicell Inverter. These multilevel inverters will also be compared with two-level inverters in simulations to investigate the advantages of using multilevel inverters. Modulation strategies, component comparison and solutions to the multilevel voltage source balancing problem will also be presented in this work. Keywords— Multilevel Inverter (MLI), Total Harmonic Distortion (THD), Photo Voltaic (PV), DC-DC Converter, DC to AC I. Introduction Conventional two-level inverters, seen in Figure 1, are mostly used today to generate an AC voltage from a DC voltage. This may not always be a problem but for some applications there may be a need for low distortion in the output voltage. Instead several voltage levels are added to each other to create a smoother stepped waveform, see Figure 2, with lower dv dt and lower harmonic distortions. With more voltage levels in the inverter the waveform it creates becomes smoother, but with many levels the design becomes more complicated, with more components and a more complicated controller for the inverter is needed. A three-level inverter design is similar to that of a conventional two-level inverter but there are twice as many valves in each phase-leg. In between the upper and lower two valves there are diodes, called clamping diodes [1], connected to the neutral midpoint in between two capacitors. Together with another phase-leg an output line-to-line voltage with even more levels can be obtained. To create the zero voltage the two switches closest to the midpoint are switched on and the clamping diodes hold the voltage to zero with the neutral point. Now, if more valve pairs, clamping diodes and capacitors are added the inverter can generate even more voltage levels. There are also different kinds of topologies of multilevel inverters that can generate a stepped voltage waveform and that are suitable for different applications. By designing multilevel circuits in different ways, topologies with different properties have been developed, some of which will be looked upon in this report. Figure 1: One phase leg of a two-level inverter and a two-level waveform without PWM Figure 2: PWM voltage output, reference wave in dashed blue As the population goes on increasing, the demand of electricity also goes on increasing. To fulfill this demand of electricity huge power has to be generated by the generating stations. Due to the usage of fossil fuels to generate the power, the environment is getting highly polluted because of the emission of carbon dioxide gases. It also leads to global warming, acid rain which causes huge losses to humans and even damage plants. Hence for survival of mankind and nature, there is a need to generate power from renewable energy sources, which is eco-friendly and pollution free. In this renewable energy sources, solar energy can be used to generate power by utilizing PV cells, which converts sunlight directly in to electricity. PV cells generate DC voltage. By using an inverter this DC is converted in to AC and used for AC applications. For this conversion purpose initially conventional inverters were used, but this conventional type of inverters had many limitations like high THD in output voltage, EMI problems, switching stress was high, and not suitable for applications which require huge power. As a result of research on inverters, MLI were invented. This MLI will produce AC output in many levels, which helps in reducing THD. When there is an increase in output levels the THD decreases. Most widely used MLI is H-bridge type of inverter. MLI can be used with lower switching frequencies due to which switching losses