THD Analysis for Different Levels of Cascade Multilevel Inverters for Industrial Applications

Cascade Multilevel Inverters are very popular and have many applications in electric utility and for industrial drives. When these inverters are used for industrial drive directly, the THD contents in output voltage of inverters is very significant index as the performance of drive depends very much on the quality of voltage applied to drive. In this article, the THD contents of 7, 11 and 15 level cascade multilevel inverters have been analysed. The THD depends on the switching angles for different units of multilevel inverters, therefore, the switching angles are calculated first by using N-R method where certain number of harmonic components has been eliminated. Using calculated switching angles, THD analysis is carried out analytically as well as using MATLAB simulation (both results are in close agreement). As per IEEE-519 standard, the output voltage produced should satisfy the limit on THD. It has been found that the fifteen level CMLI satisfies this limit while seven and eleven level CMLI violates this limit.