A new concept in control of cascaded H-Bridge multi-level inverters is proposed in this paper. According to this concept, switching angles are considered to be independent from the fundamental voltage. A polynomial term is presented to show the relation between switching angles and DC voltages. Based on this concept, Total Harmonic Distortion (THD) calculations are updated and proved to be independent from the fundamental voltage. Thus, once calculated for minimum THD, the switching pattern can be used for any required level of output voltage. To examine the effectiveness of the proposed method, it is applied in control of an eleven level inverter. The simulation results are demonstrated and verified through experiments with a setup controlled by Xilinx SPARTAN3 family FPGA (XC3S400-PQG208).
In this paper, total harmonic distortion (THD) minimization problem for cascaded H-Bridge multilevel inverters (CHB-MLIs) with unequal DC sources is studied, which the DC voltage levels of CHB-MLI is considered to be dependent on switching angles. Two forms of variations are proposed for DC voltage, considering corresponding switching angles. A simplified THD formulation, independent from the DC voltage is presented. Both Homotopy method and Genetic Algorithm is applied for THD minimization using Selective Harmonic Elimination PWM (SHEPWM). The results show less THD results using GA. The simulation results are demonstrated by experiments on a seven-level inverter controlled by Xilinx SPARTAN3 FPGA (XC3S400-PQG208). The results show that switching angles for minimum THD can be considered constant for desired fundamental voltages.
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