Extension of the Nearest-Three Virtual-Space-Vector PWM to the Four-Level Diode-Clamped dc-ac Converter

Several pulsewidth modulation strategies have been proposed for the three-level three-phase diode-clamped dc-ac converter. Among them, the nearest-three virtual-space-vector (NTV²) pulsewidth modulation (PWM) guarantees the dc-link capacitor voltage balance under any operating condition, provided that the addition of the three phase currents equals zero. This paper extends this modulation concept to the four level converter. The new virtual vectors are presented and practical modulation solutions are defined. Conventional nearest-three space vector (NTV) PWM cannot comprehensively achieve balanced and stable dc-link voltages. The proposed modulation solutions enable the practical use of the four-level converter since they guarantee the dc-link capacitor voltage balance for any operating condition and load, provided that the addition of three- phase currents equals zero. Simulation and experimental results prove the goodness of the presented approach.