Generic Internal Wind Turbine Controller

The implemented control system is based on a conventional variable-speed, variable blade-pitch-to feather configuration wind turbine and consists of two basic control systems: a generator torque controller and a full span collective blade-pitch controller. The two control systems are designed to work independently. The objective of generator-torque controller is to maximize power capture below the rated operation point. The goal of the blade-pitch controller is to regulate generator speed above the rated operation point.

Both the generator torque and blade pitch controllers use the generator speed as the sole feedback input. A recursive, single-pole low-pass filter exponential smoothing to reduce the high frequency excitation of the control systems is provided. The filter time constant \((TC=1/(2 \pi f_C))\) is given as input in the Controller Data section. A typical value for \(f_C\) is one-quarter of the blades first edgewise natural frequency (in Hz). Control actions are carried out at time steps corresponding to the input Sample Interval.

The generator torque is computed as a tabulated function of the filtered generator speed, incorporating five control regions:
1, 1 1/2, 2, 2 1/2 and 3 as illustrated below. Region 1 is a control region before cut-in wind speed, where the generator torque is zero and no power is extracted. Instead, the wind is used to accelerate the rotor for start-up.

Region 2 is a control region for optimizing power capture. Here, the generator torque is proportional to the square of the filtered generator speed to maintain a constant (optimal) tip-speed ratio.

In region 3, the generator torque or the generator power is held constant, depending on the choice of Power Extraction. In case of constant power, the generator torque is inversely proportional to the filtered generator speed.

In SIMA, the boundaries of these control regions are given in terms of the generator speed in rad/s (TransitionalSpeed15, 20, 25, and 30 refer to the lower bounds of regions 1 1/2, 2, 2 1/2, and 3, respectively). The constant of proportionality in region 2 is given as Region 2 Torque. The generator torque controller also behaves as though in Region 3 if the blade pitch is above a given minimum value. Note that all torque values are given with respect to the high-speed shaft.

In region 3, the collective blade pitch angle commands are computed using a gain-scheduled proportional-integral (PI) control on the speed error between the filtered generator speed and the rated generator speed. The constants of proportionality are given as Kp and Ki, while the gain scheduling can be defined as a function of the blade pitch angle (and is applied to both constants). A default table is provided.