Offshore wind farms integration and frequency support control utilizing hybrid multi-terminal HVDC transmission

This paper proposed a hybrid multi-terminal HVDC (MTDC) system to integrate several distributed offshore wind farms to onshore power grids. And a comprehensive frequency support algorithm for onshore grid was designed. The configuration and operation principles of a typical 4 terminals system are described. A system control strategies are proposed for both wind farm side VSCs (WFVSCs) and grid side LCCs (GSLCCs). To obtain smooth operation, the WFVSCs are controlled as a voltage source that automatically absorbs power generated by the wind farm and establish a stable AC voltage for wind turbines. DC voltage-current droop control is designed for GSLCCs to maintain the DC voltage and distribute power between two onshore girds. A utility black start-up process without auxiliary power supply was designed and tested. The simulation results in PSCAD/EMTDC were presented to demonstrate the system performance during black starting-up, converter disconnection and frequency variation of one onshore power grid.