Tracking method for the hybrid modulation to minimize neutral-point voltage oscillations in the three-level converter

The hybrid modulation is a carrier-based PWM technique that can be applied to the neutral-point-clamped converter. It makes use of two modulation signals per phase. The hybrid modulation is based on combining two modulation strategies; double-signal PWM and sinusoidal PWM. The main characteristic of the 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 by varying the combination of the two strategies. The tracking method proposed in this paper defines the intervals in which sinusoidal PWM should be used to achieve the maximum reduction of the voltage oscillation amplitudes. Consequently, the low-frequency neutral-point voltage oscillation is further minimized. Some simulation results are presented to show good performance of the proposed strategy and are compared with the nontracking method.