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This paper presents the definition of the virtual vectors for the general case of an n-level two-leg diode-clamped converter, suggests guidelines for designing virtual-vector pulsewidth modulation (PWM) strategies, and provides the mathematical expressions of the leg duty-ratio waveforms corresponding to this family of PWMs. Modulations defined upon these vectors guarantee the dc-link capacitor voltage balance in every switching cycle under any type of load and enable the use of these topologies with passive front ends. The performance of these multilevel converters operated with the proposed PWMs is compared to the performance of alternative designs through analysis, simulation, and experiments. [ABSTRACT FROM AUTHOR]

Abstract-In the previous literature, the introduction of the virtual-space-vector (VV) concept for the three-level, three-leg neutral-point-clamped converter has led to the definition of pulsewidth modulation (PWM) strategies, guaranteeing a dc-link capacitor voltage balance in every switching cycle under any type of load, with the only requirement being that the addition of the three phase currents equals zero. This paper presents the definition of the VVs for the general case of an n-level converter, suggests guidelines for designing VV PWM strategies, and provides the expressions of the leg duty-ratio waveforms corresponding to this family of PWMs for an easy implementation. Modulations defined upon these vectors enable the use of diode-clamped topologies with passive front-ends. The performance of these converters operated with the proposed PWMs is compared to the performance of alternative designs through analysis, simulation, and experiments. [ABSTRACT FROM AUTHOR]

In the previous literature, it has been reported that it is not possible to guarantee the balance of the dc-link capacitor voltages of multilevel three-phase diode-clamped dc-ac converters with passive front-ends for high modulation indices, especially for more than three levels. This paper proposes a novel closed-loop control approach capable of guaranteeing such balance for all ope]rating conditions of the converters without the need for additional hardware. Three different phase duty-ratio perturbation schemes are proposed. They are compared through simulation for the case of a four-level three-phase diode-clamped dc-ac converter operated with a virtual-vector-based modulation. The most simple and effective perturbation scheme, only requiring the sensing of all dc-link capacitor voltages, is tested experimentally in the same four-level converter. The results demonstrate the feasibility of guaranteeing the dc-link capacitor voltage balance for all converter operating conditions. [ABSTRACT FROM AUTHOR]