Voltage Balancing Control of Diode-Clamped Multilevel Converters with Passive Front-Ends

In the previous literature, it has been reported that it is not possible to guarantee the balance of the dc-link capacitor voltages of three-phase diode-clamped multilevel 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 operating 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.