Abstract

A control scheme for a hybrid multilevel converter which is capable of blocking DC fault in HVDC systems is developed. Converter uses two Asymmetric Alternate Arm Converter (A3C) operating in interleaved fashion on DC network. Each A3C unit of this dual A3C topology is equipped with Full-bridge submodules (FB-SM) only in lower arms. Upper arms use conventional thyristor valves. Control scheme enables soft commutation of thyristor valves, maintains arm energy balance and is suitable for operation during severe system disturbances, such as three-phase faults and DC pole-to-pole fault. As required by grid standards, dual A3C provides adequate reactive power support to the grid during AC faults. Simulations, using detailed switching model, confirm that dual A3C is able to ride-through severe system faults and rapidly recover normal operation once fault clears.