# Model Based Open-Loop Wind Farm Control Using Active Power for Power Increase and Load Reduction
## 2 Methodology
### Wind Turbine Controller  
The wind turbine controller in the wind turbine model is based on a comprehensive controller including the MPPT (Maximum Power Point Tracking) torque control for the below rated wind speed region, collective pitch PI control for the above rated wind speed region, and a driver-train damper to reduce the in-plain torsional mode. The MPPT torque control is for power maximization and the collective pitch control is for power regulation to the rated power. For a given generator speed (related to the wind speed), the blade pitch angle and the generator torque must be a constant value to achieve the power maximum or the power regulation. When the wind speed is lower than the rated wind speed, the pitch angle is kept to be the fine pitch known as the pitch angle where the aerodynamic conversion efficiency of the rotor is maximized. At the same time, the generator torque that can achieve the maximum power is commanded from the wind turbine controller to the generator. When the wind speed is higher than the rated wind speed, the generator torque is kept to be the rated torque, and the blade pitch angle is PI controlled to target the rated generator speed [21]. As the result, the rated power is maintained.  
Figure 3: Difference between wind farm simulation tool and wind farm model on wind farm controller: (**a**) structure of dynamic wind farm simulation tool; and (**b**) structure of simplified wind farm model in wind farm controller.  
For this study, the wind turbine controller was needed to receive a command from a higher level wind farm controller, and a DPPT (demanded power point tracking) control algorithm was constructed and implemented to the basic wind turbine controller. When the power command from the wind farm controller is larger than the available power of the wind turbine, the wind turbine behaves like an ordinary wind turbine with the torque control mode. Thus, if the wind speed is lower than the rated wind speed, the pitch angle is fixed to be the fine pitch and the generator torque command that produces the maximum \(C_{P}\) is sent. If the wind speed is higher than the rated wind speed, the mode switch is turned on to make the wind turbine operate in an ordinary pitch control mode. In this mode, the rated generator toque command and the pitch command to achieve the rated rotational speed are sent from the controller [21].  
However, if the power command from the wind farm controller to the wind turbine controller is lower than the available power of the wind turbine, the DPPT control is turned on. The wind turbine controller then sends the generator torque command, which is equivalent to the torque command to achieve the same power output in the ordinary torque control mode. This generator torque is lower than the generator torque needed to achieve the maximum efficiency in the ordinary torque command mode at the same wind speed, and therefore the rotational speed of the generator increases. Now the generator speed, which is equivalent to the generator speed to achieve the same power in the ordinary torque control mode, is targeted in the collective pitch controller. Therefore, in the end, the generator power, which is the generator torque multiplied by the generator speed, is controlled to be the power command from the wind farm controller. The structure of the control algorithm imbedded into the wind turbine controller of the simulation tool without affecting its conventional torque and pitch control logic is shown in Figure 4[21].