A New Switching Pattern for Multilevel Inverter Based on Selective Harmonic Elimination Using Genetic Algorithm

Abstract

Abstract – In this paper, a new switching pattern is presented for multilevel inverters. With changing off-angel of each switch, the on time interval of all switches will approximately be equal and then the lifetime of inverter will increase, also using this method can reduce electrical stress on switches in higher levels of inverter. Switching angels as for desired modulation index are calculated using genetic algorithm whereas selective harmonics are controlled within the allowable range. The computed angels are simulated in Matlab/Simulink for respective circuits to validate the results. Keywords : Multilevel Inverter, Harmonic Elimination, THD, Genetic Algorithm 1. Introduction providing lower levels of voltage,. Also some switches are The main concepts of multilevel inverters, introduced about 3 decades ago [1-2], entails performing power conversion in multiple voltage steps to obtain improved power quality, lower switching losses, better electromagnetic compatibility, higher voltage capability, eliminating the harmonics from the output voltage, and minimizing the THD [3]. Several topologies for multilevel inverter have been proposed over these years, the most popular ones are the diode-clamped [4-5], flying capacitor [6-7] and the cascaded H -bridge [8-11] structures. Theoretically, multilevel inverters can synthesize an infinite output-voltage level. By increasing the number of levels in the inverter, the output voltage has more step generation, a staircase waveform, which has a reduced harmonic distortion [12]. However, a large number of levels increase the number of switching devices, diodes, and other passive elements. Cascade H-Bridge multilevel inverter is common structure used in construction of multilevel inverter. In this structure several H-bridge single phase inverter is connected series and provide the desired voltage level. One of the essential problems in these inverters is failing of switches during inverter’s operation time which leads to a failure in a whole inverter. This failure is due to two reasons: One or more switches in bridges providing lower levels of voltage are defective due to longer operation time rather than other switches, these switches are in bridges defective because of working on higher switching frequency rather than other switches, these switches are in bridges providing higher levels of voltage,. Also switching loss at the second group switches is higher than other switches, it reduces the efficiency of the circuit.In this work to overcome these problems a new switching method is proposed. In this method on time of all switches in all bridges of Multilevel inverter is approximately equal. This method is quite similar to common multilevel inverter however there is the difference in each switch’s offangle.There have been many attempts at finding an optimal switching sequence for both voltage, and current. The most common method is either pulse width modulation (PWM) or space vector modulation (SVM). While attempts at PWM and SVM have success, it is hard to claim that any have found the absolute best switching sequence solution [13-15].This problem leads to non-linear equations and direct solution of non-linear equations can lead to discontinuity at certain modulation indices. The prevalent methods to solve this problem utilize intelligent methods. In this paper GA is used to solve this problem. The optimal switching angles are generated, selective harmonic are eliminated and consequently THD of output voltage is minimized. To validate the results, the computed switching angles are used in simulated circuit [12].