Optimal individual operation of DFIG-driven wind turbines for reactive power support from large WPPs under all wind conditions

Voltage control in transmission network requires sufficient quantity and flexibility of reactive power resources, which can be effectively provided by wind power plants based on doubly-fed induction generators. This paper presents the mathematical formulation of an optimization problem in form of mixed-integer quadratically constrained programming developed to determine both the available reactive power operating range and the optimal redistribution of reactive power set points across the turbines under the current wind conditions. Low-wind periods limit the technical capabilities of wind power plants compared to standard operating conditions, reducing their capacity and reliability in providing ancillary services. This paper proposes a strategy to operate doubly-fed induction generators in motoring mode, rather than shutting down, during periods of low or no wind, thereby enabling them to continue providing reactive power reserves at the same level as during active generation. Since motoring operation requires additional energy consumption, an optimal dispatch of reactive power among turbines is introduced to achieve the most efficient internal operating state of the wind power plant. A case study based on real-world data from the wind power plant Čibuk in Serbia demonstrates that motoring operation ensures a more stable and uninterrupted reactive power capability for additional 15% of the time. Moreover, motoring operation enables the reference reactive set point at point of common coupling to be met more efficiently by reducing internal energy losses.