Droop Control for a Multi-Line Current Flow Controller in Meshed Multi-Terminal HVDC Grid Under Large DC Disturbances

The security, stability, and reliability of power supply are essential and must be guaranteed during operation of multi-terminal HVDC (MTDC) grids, particularly under disturbances, such as a line or converter loss, on the dc grid side. In this paper, a multi-line current flow controller (CFC) is expanded from the existing two-line CFC for meshed MTDC grids with two main contributions: a droop control scheme for a multi-line CFC, which is capable of coordinating dc line current/power sharing under large dc disturbances is proposed and a selection method for the droop gains is proposed. The multi-line CFC with the droop control scheme proposed can achieve full controllability of the currents among multiple dc lines and the security and continuity of power supply of the unfaulty dc lines can be guaranteed. The stability and dynamic performance of the droop controller can both be satisfied with the proposed selection method. The effectiveness of the proposed scheme and selection method is justified by RTDS simulations. The proposed scheme is simple and easy to implement. The proposed dynamic simulation-based selection method has the flexibility suiting both online and offline calculation tools and the consideration of system nonlinear characteristics and control limits/constraints.