Main grid frequency support strategy for VSC-HVDC connected wind farms with variable speed wind turbines

A wind farm (WF) connected by voltage source converter-based HVDC (VSC-HVDC) results in a completely `inertia-less´ in offshore AC grid . AC frequency is determined by control of the VSC-HVDC on the WF side, independently of the main grid frequency. Moreover, the rotational speed of a variable speed wind turbine is independent of the AC frequency of the offshore AC grid. These two aforementioned features result in `inertia-less´ system, that is absence of mechanical energy storage to accomodate for abrupt changes in generation and load. An `inertia-less´ system does not have a load-frequency response similar to traditional AC grid with inertia, hence making it difficult for the wind turbines to contribute to frequency support of the main grid. This paper develops an artificial coupling of the wind farm grid and the main grid frequencies, for main grid frequency support by the wind farm. Artificial frequency coupling is achieved by use of droop controllers for the DC voltage control. A case of study, using PSCAD, demonstrates the effectiveness of artificial frequency coupling for the main grid frequency support.