A Simplified Solution to Determine Switching Angles for Effective Control of Total Harmonic Distortion in Multilevel Inverters

Multilevel inverters (MLIs) are utilized to generate stepped output voltages to overcome the limitations of conventional inverters that are known to generate square waves containing harmonics. Predefined switching of MLIs enables stepped output voltage generation with relatively lower THD. The fact that the THD is less in the stepped output voltage compared to that from the conventional inverter makes application of MLI very attractive for high voltage applications. However generation of stepped wave without proper selection of switching instants and durations will not ensure low THD. So THD minimization involves selection of proper switching sequence and precise control of switching duration. One of the well-known methods to determine the switching intervals' durations is the Selective Harmonic Elimination (SHE) wherein the angles at which the steps are to be generated are obtained by solving a set of nonlinear equations. The angles which are also called as notches are calculated by solving SHE equations such that a particular harmonic or pair of harmonics is/are eliminated while achieving the specified fundamental voltage [2]. These SHE equations are nonlinear transcendental in nature and require complex numerical techniques. Many methods have been presented in literature where they make use of iterative procedures to solve the transcendental equations which are computationally complex. In the present investigation, a simple Excel based calculator is developed with user friendly interface to derive the switching angles from the set of non linear equations so as to reduce THD. The Excel based tool is compared with the Newton Raphson (NR) method by determining the angles of switching for a five sourced Cascaded H Bridge MLI (CHBMLI). The applet help researchers to generate the angles for any number of cascaded MLI operating at any modulation index without the need of commercial software package and without having to specify initial guesses. It is envisaged that the applet saves considerable amount of computational time of researchers who need not spend time in solving the equations.