A New Control Strategy for Participation of Wind Power Plants with a Doubly-fed Induction Generator in Frequency-control Ancillary Services

In past, electric power systems have experienced major changes by shifting from conventional energy sources to 'nonconventional' renewable energy sources. The biggest increase in production of renewable energy in Europe was caused by wind energy systems, with their generation capacity of 65 GW in the EU-27 at the end of 2008 [1]. The above mentioned increase has given rise to new concerns about power system stability, since wind generation is considered as an intermittent source which by its nature does not participate in frequency control. Nevertheless, wind farms (WF) have often been asked to provide some sort of ancillary services, such as reactive-power control and frequency/active-power control. This paper focuses on the provision of primary frequency control by wind generators. Wind generators can provide frequency control only by operating at a de-loaded optimum-power extraction curve, where the active power provided by each wind turbine increases or decreases during system frequency changes. The control strategy can be achieved by a combination of pitch control on one side and speed control at the wind generator on the other. Regarding participation in frequency control, different types of operation can be considered. The first strategy is to achieve a de-loaded optimum power only by pitch control and at the same time controlling speed of the wind generator at its optimum. In this case, a decrease in the pitch angle enables the increase in the active power provided. The second control strategy is to obtain a power reserve by controlling the rotor speed of the wind turbine. In this case, a decrease in the rotor speed enables the increase in the active power provided and at the same time restoration of a substantial amount of a kinetic energy stored in spinning inertia. This paper proposes an optimized control strategy for the active power delivered by a doubly fed induction generator (DFIG). Wind turbines are supposed to operate over a de-loaded maximum-power extraction curve in the control approach proposed in the paper. The optimized control strategy is defined at the maximum spinning inertia achieved by the wind turbine. The definition of the wind turbine operating points, such as the rotor speed and pitch angle, is extracted from an optimization algorithm that uses the differential evolution method. The optimization problem is described by the objective function (9), operational bounds (11) and (12) speed and pitch controller and equality constraint (10). Results from Figure 9 show that the optimized inertial response from the DFIG wind generators can be obtained using the proposed method. An optimal control strategy of the wind turbine can be achieved at rotor speeds, Figure 7, and pitch angles, Figure 8, which were determined at three power-reserve factors. The control strategy described in the paper was compared with the strategy proposed by other authors, Figure 10. The results show a higher inertial storage at the same proportion of the de-loaded maximum power.