Offshore Wind Farm Integration and Frequency Support Control Utilizing Hybrid Multiterminal HVDC Transmission

This paper proposed a hybrid multiterminal-high-voltage-direct-current system to integrate several distributed offshore wind farms to onshore power grids. A comprehensive frequency support algorithm for the onshore grid was designed based on this system. The configuration and operating principles of a typical four-terminal system are described. System control strategies are proposed for both wind-farm-side voltage-source converters (WFVSCs) and grid-side line-commutated converters (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 grids. A utility black start-up process without auxiliary power supply was designed and tested. The simulation results in Power System Computer Aided Design/ Electromagnetic Transients including DC are presented to demonstrate the system performance during black starting-up, converter disconnection, and frequency variation of one onshore power grid.