Adjustable Wind Farm Frequency Support Through Multi-Terminal HVDC Grids

In the future power systems, a large number of off-shore wind farms will be connected to the AC grids through high voltage DC (HVDC) and multi-terminal DC (MTDC) grids. As wind power penetration level increases, complex grid codes and regulationswill be imposed onwind turbines for frequency support. To follow any grid code and requirement for frequency support, two important features should be included in the wind turbine frequency support: i) It should be able to adjust the maximum additional power that the wind turbine temporarily provides for frequency support; ii) It should be capable of adjusting the time interval in which the wind turbine provides additional temporary power. The first feature is mainly important for reducing rate of change of frequency (RoCoF) and improving the frequency nadir while the second one is mainly important for fast frequency recovery from its nadir and improving the second frequency drop. This paper indicates that the conventional method cannot offer both of the two aforementioned features. To address this issue, two approaches are proposed for frequency support by wind turbines. The first one uses P - omega(r) and P - f(WF) droops in each wind turbine controller, where P, omega(r), and f(WF) represent the wind turbine power, rotor speed, and wind farm frequency. The second method employs P - (omega) over dot(r) and P - f(WF) droops in each wind turbine controller. Performance and effectiveness of the proposed methods are evaluated by time-domain simulation studies on an MTDC grid in the PSCAD/EMTDC software environment.