THD Minimization Applied Directly on the Line-to-Line Voltage of Multilevel Inverters

In this paper, total harmonic distortion (THD) minimization of the output voltage of multilevel inverters is discussed. An efficient approach in reducing the harmonic contents of the inverter's output voltage is THD minimization. In multilevel inverters with a fundamental frequency switching strategy (each switch turning on and off once per output cycle), the switching angles can be selected so that the output THD is minimized (such as the so-called optimal-minimization-of-THD strategy). To obtain the optimum switching angles, an optimization algorithm is applied to the output-voltage THD. In three-phase multilevel inverters, the optimization algorithm is commonly applied to the phase voltage of the inverter. This results in the minimum THD in phase voltage but not necessarily in the line-to-line minimum THD, whereas in three-phase applications, the line-voltage harmonics are of the main concern from the load point of view. In this paper, using the genetic algorithm, a THD minimization process is directly applied to the line-to-line voltage of the inverter. This paper is based on a seven-level inverter. To verify the simulation results, a seven-level-cascaded-H-bridge-inverter-based hardware prototype, including an ATMEGA32 AVR microcontroller, has been implemented. Both simulation and experimental results indicate superiority of this approach over the commonly used phase-voltage THD minimization approach.