Analysis and Impacts of Implementing Droop Control in DFIG-Based Wind Turbines on Microgrid/Weak-Grid Stability

Wind energy is going to be a significant part of electric energy generation in the very near future. However, in addition to its intermittent nature that could lead to major difficulties for power system reliability and stability, the conventional control applied to wind turbines and their generators, usually doubly-fed induction generators (DFIGs), does not allow them to participate in frequency regulation, whether short or long term. Moreover, the use of wind generators for autonomous frequency regulation is becoming an essential objective in power grids with reduced inertia and isolated microgrid operation. While droop-control is suggested by many researchers to solve these problems, detailed analysis of droop-controlled DFIG units in microgrids has not been reported. To fill-out this gap, this paper presents torque- and power-droop implementations in DFIG-based units by some simple modifications in the conventional control and then, by means of small-signal modeling and eigen-value studies, shows how both techniques influence frequency stability. Sensitivity studies, with respect to the presence of turbine- and inverter-based generators in microgrids; and impacts of pitch-angle controller, wind speed variation and isolated mode operation with only wind-generators, are conducted. Time-domain simulation is utilized to verify the analytical results.