Harmonic Reduction Technique for a Cascade Multilevel Inverter

In this paper, an optimization technique is proposed to compute switching angles at fundamental frequency switching scheme by solving non linear transcendental equations (known as selective harmonic elimination equations), thereby eliminating certain predominating lower order harmonics, and simultaneously, control over magnitude of output voltage of a multilevel inverter is achieved. As these equations are nonlinear transcendental in nature, there may exist simple, multiple or even no solution for a particular value of a modulation index. The proposed scheme is implemented in such a way that all possible solutions are obtained without knowing proper initial guess at the solutions. Moreover, this technique is suitable for higher level of multilevel inverters where other existing methods fail to compute the switching angles due to more computational burden. For the values of modulation indices where multiple solutions exist, the solutions which produce least THD in the output voltage is chosen. A significant decrease in THD is obtained by considering multiple solution sets instead of taking a single set of solution. The computational results are shown graphically for better understanding and to prove the effectiveness of the method. An experimental 11-level cascade multilevel inverter is employed to validate the computational results.