THD minimization in variable input cascaded H-bridge multi-level inverters via state table

This paper proposes a method for achieving the optimum Total Harmonic Distortion (THD) of the output voltage in the cascaded multilevel inverter in case that dc cells' voltages vary during operation. It considers switching angles and voltage steps as optimization variables in order to obtain the minimum THD for the 5, 7, 9, 11, and 13-level cases. The proposed method studies +20% and -20% variations in the optimum dc voltage levels in 7-level cascaded inverters. It obtains new optimum switching angles according to cells' voltages by building a state table of the possible output vectors and applying the new switching angles from a lookup table by means of a microcontroller. Furthermore, it examines fault conditions in the optimized voltages leading a drop of 50% and 80% for each cell. By building the fault state table, the proposed method results in voltage THD reduction for each state after re-optimizing the switching angles. Also, it compares the output voltage THD before and after applying the new switching angle values from a lookup table. Up to 12% THD reduction during fault conditions is achieved by applying the proposed optimization method. The analytical results are verified experimentally.