Application of OMTHD on the line voltage of cascaded multi-level inverters with adjustable DC sources

In this paper, the Optimal Minimization of the Total Harmonic Distortion (OMTHD) technique is improved to calculate minimum phase THD and approximate line to line THD versus variable Modulation Indices (M). In Developed single-phase OMTHD technique, the Fitness function is proposed to minimize phase THD, and fundamental component as a constraint is separately added to Fitness function to adjust the fundamental component of the phase voltage at the desired value. In developed three-phase approximate OMTHD technique, the Fitness function is proposed to minimize approximate line to line THD, and fundamental component as a constraint is separately added to Fitness function to adjust the fundamental component of the line to line voltage at the desired value. In this paper, Genetic algorithm is applied to minimize Fitness functions. Most control methods used in cascaded multi-level inverters are based on the assumption that the DC sources all have the constant and equal value whereas employing adjustable DC sources if it is possible can improve the total harmonic distortion (THD) in both the developed single-phase and three-phase approximate OMTHD techniques. First, the structure of cascaded multi-level inverter is briefly explained. Then, the switching algorithm for the inverter based on developed single-phase OMTHD technique and developed three-phase approximate OMTHD technique are proposed. Afterwards, the proposed techniques are applied to the multi-level inverter with adjustable DC sources and constant DC sources. Ultimately, an accurate comparison of THD between multi-level inverter with constant and adjustable DC sources is performed. Comparisons show that employing adjustable DC sources which can be controlled results in substantial improvement in the harmonic minimization both in single-phase OMTHD technique and three-phase approximate OMTHD technique.