MINIMIZING THE THD OF 27-LEVEL CMLIS FOR MANY APPLICATONS

Abstract

: The 27-level cascaded multilevel inverter (CMLI) has recently many applications in electrical power engineering, such as for control of ac motor drives and for interfacing renewable energy sources with the smart grid. This is due to its simplicity of construction as a trinary asymmetric CMLI, that consists of only three H-bridges with unequal dc sources of values E, 3E and 9E., and at the time cane provide a nearly sinusoidal output voltage. Minimizing the exact total harmonic distortion (THDE) of the output voltage of the 27-level CMLI is an important issue, since this will improve the performance by decreasing the losses and increasing the efficiency. An approach using a mixed integer nonlinear programming (MINLP) model is introduced to determine the switching angles of the power switches of the 27-level CMLI that minimize the THDE for both single phase and three phase CMLIs. Formulas for the THDE are included in the model for both single phase and three phase cases. The results show very low values of %THDE and of the upper limit of the amplitude of any undesired harmonic relative to the amplitude of the main harmonic (% V Hmax ) in both cases over wide voltage ranges, which agree with the IEEE standards 519-1992 for voltage distortion limits till 161 kv .