The impact of control design on dynamic behaviour of multi-terminal VSC-HVDC (MTDC) system under AC grid fault conditions

This paper presents the impacts of selected key factors including control designs and converter operating point on the dynamic responses of multi-terminal VSC-HVDC to ac system faults. Within the framework of ac fault studies, transient performance issues associated with dc voltage control, the phase locked loop, and reactive power control are addressed. Fundamental limitations imposed on the behaviours of converter power and current control by inverter operations are identified. An adaptive current limit is introduced to maintain the dc grid voltage and converter currents within acceptable levels, and to improve the quality of the system responses to ac system faults. Simulations are performed employing a four-terminal HVDC test system.