IMPROVING THE HARMONIC COMPOSITION OF OUTPUT VOLTAGE IN MULTILEVEL INVERTERS UNDER AN OPTIMUM MODE OF AMPLITUDE MODULATION

One of the most important parameters of multilevel inverters is the sinusoidal output voltage. There are many different modulation algorithms, which make it possible to obtain different indicators of the sinusoidal output voltage and different content of the higher harmonics. This paper reports a universal modulation algorithm, which makes it possible to obtain the shape of the output voltage of a multilevel inverter at any number of stages, optimized for the content of the higher harmonics, namely a minimum of the coefficient of harmonic distortions. The proposed algorithm enables obtaining the lowest possible THD for any level voltage. The advantage of the proposed algorithm compared to similar optimization algorithms is ensuring smaller harmonic distortions, as well as its relative simplicity. The reported algorithm is based on the amplitude modulation of the sine signal with a 25 % modulation relative to the highest discreteness. The analytical expressions have been given that make it possible to determine the time (angle) of enabling each level of the output voltage to form a minimum of the root mean square value of higher harmonics. To confirm the optimum analytic point, the MATLAB/Simulink programming environment was employed to design a series of multi-level voltage inverters, which form the five-, seven-, nine-, and eleven-level shapes of output voltage. The current study has shown that the optimum points for all shapes of multilevel voltages are achieved at the same coefficient of amplitude modulation. It has been demonstrated that the proposed modulation algorithm could also be used to control the amplitude and frequency of the output voltage in a multilevel inverter. The paper gives a control characteristic of the output voltage of a multilevel inverter at the pulse amplitude modulatio