Hybrid Modulation Technique for the Neutral-Point-Clamped Converter

This paper presents a hybrid modulation technique for the three-level neutral-point-clamped converter. A modulation strategy that was presented previously is able to remove completely the low-frequency voltage oscillations that appear on the neutral point for some operation conditions. However, such a modulation strategy has an important drawback; it increases significantly switching losses on the converter. The hybrid modulation proposed here combines such a modulation with sinusoidal PWM. The main characteristic of this hybrid modulation is the capacity to reduce switching losses at the cost of having some low-frequency voltage oscillations on the neutral point. The amplitude of these oscillations can be controlled thanks to the degree of mixture between the two strategies. Performance of the hybrid modulation is analyzed in this paper and compared with the original strategies. Switching losses are evaluated by simulation, and some experimental results show how the hybrid modulation performs with balancing the dc-link capacitors.