Hierarchical Control of Multiterminal DC Grids for Large-Scale Renewable Energy Integration

In this paper, a hierarchical control framework is proposed for multiterminal dc (MTDC) grids with large-scale renewable power generations. In the primary control, dc voltage droop control is adopted for the multiple voltage source converters (VSCs) connected to ac main grids. A novel optimal control strategy with enhanced flexibility and compatibility has been developed in the secondary control, which can simultaneously address issues including dc grid loss minimization, power sharing accuracy among droop-based VSCs, and influence of possible power disturbances during the optimization intervals on dc voltages security. After thorough theoretical analysis and study on an tested five-terminal dc grid, a cosimulation platform based on PSCAD/EMTDC and MATLAB has been setup to verify the proposed hierarchical control, with the dynamics of main circuits of MTDC dc grid and primary controllers simulated in PSCAD, and the novel secondary optimal control strategy implemented in MATLAB.