Coordinated Wind Farm Inertia Support Through Multi-Terminal DC Grids

This paper proposes a wind farm inertia support method, which coordinates all the participating wind farms interconnected through a Multi-Terminal DC (MTDC) grid. Under normal conditions, all the wind turbines in each wind farm operate by the Maximum Power Point Tracking (MPPT) controller. Subsequent to any significant frequency disturbance in an AC grid, its corresponding MTDC converter station switches its droop from P - V-dc to P - f. The DC voltage of one of the MTDC grid buses is communicated to all the wind farm converter stations. Frequency of each wind farm converter station is related to the DC voltage through a linear V-dc - f(WF) droop. Any disturbance in an onshore AC grid causes a DC voltage deviation in the MTDC grid and consequently, frequency deviation of the wind farm converter stations. Subsequent to any disturbance, each wind turbine of any wind farm translates the frequency deviation to a temporary power setpoint, which is added to the power setpoint of its MPPT controller. To implement the proposed method, this paper proposes an additional P - omega(r) droop to ensure that the post-disturbance power deviations of the wind farms at any time after the disturbance follow a pre-specified ratio. Accordingly, the share of each wind farm in the total inertia support is predetermined by the MTDC operator. Furthermore, the proposed approach enables the MTDC operator to adjust the speed of response of the wind farms to any disturbance in any AC grid. The performance and effectiveness of the proposed frequency support method are evaluated by time-domain simulation studies on a 5-bus MTDC grid in the PSCAD/EMTDC software environment.