Zero-current switching control of the alternate arm HVdc converter station with an extended overlap period

The alternate arm converter (AAC) is a promising candidate for HVdc converter stations due to its reduced number of devices and capability of blocking the dc fault current. This paper is focused on modeling and control of the AAC with an extended overlap period. First, a generic mathematical model considering all the switching state combinations of the AAC is developed, which governs the characteristic of the time-varying, three-phase imbalanced circuit parameters of the AAC. Based on the developed model, an ac-side current control strategy with a feed-forward compensation method to overcome the time-varying nature of the circuit parameters is proposed. Then, by introducing the so-called 'double-overlap period,' a synthetic circulating current strategy is proposed, which enables both dc current active filtering and zero-current switching of the director switches. The validity of the model and effectiveness of the proposed strategies are confirmed by simulation studies on an HVdc system.