Pole balancing and thermal management in multi-terminal HVDC grids using single H-bridge based current flow controllers

Current flow controllers (CFCs) are power electronics based devices that may remove some technical barriers preventing multi-terminal HVDC (MTDC) grid deployment. In this paper, an inter-line CFC topology is investigated. The single H-bridge CFC (1B-CFC) alters the grid power flow by transferring power between neighboring dc lines. The operation and control of a 1B-CFC under a single modulation scheme is presented. A control strategy has been proposed to provide pole balancing support during imbalance conditions. Small-scale prototypes have been developed to demonstrate the functionality and operational range of the device. To this end, an experimental MTDC grid test-rig has been employed. It is shown that a 1B-CFC could be used to limit the dc line current and, additionally, it could be employed to enable asymmetrical tapping of dc lines. For completeness, the performance of the device has been experimentally validated under line overloading, pole imbalance conditions, and a pole-to-pole dc fault.