Dual T-Type Four-Level Converter

This letter proposes a three-phase dual T-type four-level converter topology that consists of two T-type networks per phase leg. The proposed converter has only six power switches per phase leg without any clamping diodes or flying capacitors. Besides, it is a flexible topology as there are several bidirectional switch alternatives that can be used for the proposed converter. As for the common severe voltage imbalance issue of the proposed topology and other four-level converters with three series-connected capacitors, the voltage imbalance mechanism is first revealed, and then a variable-carrier based voltage-balance control method is proposed. It aims at controlling the middle capacitor voltage at 1/3 of the full dc-link voltage and the upper and lower capacitor voltages are automatically balanced. Experimental results have been carried out to verify the proposed topology and the proposed variable-carrier based voltage-balance control method.