Branch Current Control on a Superconducting DC Grid

The challenges of transferring large levels of renewable generation output from remote locations to load centers through congested ac transmission networks has renewed interest in high-voltage dc (HVDC) transmission. Mesh-connected HVDC grids are under consideration to maximize utilization of available transmission accessible corridors. High-temperature superconducting (HTS) cables are candidates for future implementations of future dc grids with their high current capabilities and low losses. The branch current flows in a conventional HVDC grid are determined by voltage differences between the ends of a line, making it difficult to regulate currents in different paths. This becomes even more difficult in a superconducting grid where the steady-state voltage is equal across the system, and line current flows only change during temporary voltage differences. If a line needs to be removed from service for maintenance it is difficult to bring the current in that line to zero without de-energizing the entire system. In addition, once the line is re-energized it will not carry any load current until a voltage difference occurs. This paper will explore options to control individual branch currents on an HTS mesh by exploring controllable circuit configurations without the need to add series elements. A control scheme will be developed for a dc grid with a controllable configuration.