Wind Turbine Fatigue Load Analysis for Derating Control Strategies Aimed at Grid Balancing

With the exponential growth of wind energy worldwide, wind power plant owners are eager to participate in grid balancing, in order to compete with traditional power plant owners. In this way, they take part in frequency control which notably amounts to reacting to grid faults by an adequate injection of power. The latter calls for power reserve. For wind farms, this is achieved by derating methods which consist in extracting from the wind less power than the available power, in order to be able to increase the produced power when needed. The objective of this paper is to compare two wind turbine (WT) derating methods in terms of lifetime damage equivalent load (DEL). The first strategy is based on a modification of the generator torque set point to achieve derating, while keeping the standard pitch angle control. The second strategy achieves derating by pitch control while keeping the standard tip speed ratio set point. Both strategies were simulated in OpenFAST under various wind conditions covering all the WT operating regions. Focusing on having a constant power reserve, the simulation results on a single WT show that the pitch-based strategy is the best, among the two considered ones, when it comes to derating while reducing lifetime DEL.