Fast frequency response from offshore multi-terminal VSC-HVDC schemes

This paper analyzes the frequency support characteristics of multiterminal voltage source converter HVdc (VSC–HVdc) (MTDC) schemes using the energy transferred from wind turbine rotating mass and other ac systems. An alternative coordinated control (ACC) scheme that gives priority to a frequency versus active power droop fitted to onshore VSCs is proposed to: transfer wind turbine recovery power to undisturbed ac grids, and allow correct control operation of MTDC systems during multiple power imbalances on different ac grids. The fast frequency response capability of MTDC systems equipped with the proposed ACC scheme is compared against a coordinated control scheme, which uses a frequency versus dc voltage droop. The frequency control schemes are demonstrated on an experimental test rig, which represents a three-terminal HVdc system. Also, the MTDC frequency support capability when wind farms do not provide extra power is tested using a four-terminal HVdc system.