Digital-Scalar PWM Approaches Applied to Four-Leg Voltage-Source Inverters

In this paper, we present two modulation schemes for generating unbalanced voltages by means of four-leg voltage-source inverters. The first modulation is an extended scheme of a digital-scalar pulsewidth modulation previously proposed for controlling three-leg inverters. The second one is based on the decomposition of the voltage reference signals into their symmetrical components. The schemes are compared theoretically by simulations and experimental results. As a consequence, we find out a total equivalence between the two modulations despite their different conceptions. The fundamentals of both techniques and a mathematical proof of their equivalence are unfolded. Also, the performances of the techniques with respect to the harmonic distortion and power switching loss are discussed. The results show that the techniques are effective.