Extended ZCS Region of AAC-HVDC Systems through On-Load Tap Changer Coordination

Arm inductances and the operation of two arms of a phase leg in an alternate arm converter (AAC) can lead to hard-switching of the director switches (DSs) during steady-state operation, increasing voltage stresses across the DSs. The DS voltage stress also depends on the magnitude of the interrupted arm current, which, in turn, depends on the output active/reactive power. Hard-switching of the DSs limits the operating region of the AAC. This article characterizes the zero-current switching (ZCS) capability of the AAC and proposes extended ZCS operation in steady state for HVDC applications. This is achieved by coordinating the AAC operating point and the transformer on-load tap changer (OLTC). The coordinated operation utilizes the tap positions of the HVDC transformer effectively to extend the ZCS operating region with no added effort on circulating current control. Results based on an AAC-HVDC transmission system demonstrate the effective coordination of the transformer OLTC and the converter operating point to achieve extended ZCS performance.