Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 469

of systems have been implemented to control the loads and the power output.
Most of these systems comprise of a combination of variable or fi xed rotational
speed and pitch angle. Currently, the most widely implemented system [4] is the
variable speed, variable pitch machine. These machines maintain an optimal tip
speed ratio l below rated wind speeds and the blades are pitched to regulate power
above rated wind speeds. Usually pitching to feather is implemented. The control-
ler is based on the rotational speed and the generator torque signals. The system is
mainly in place to control the quality of the power output and the mean loading of
the turbine, but it can also alleviate transient loading. Above rated wind speeds, an
increase in loading will lead to an increase in generator torque and the controller
will react to this increase by pitching the blades to feather. Pitching can be imple-
mented with all blades being pitched collectively or with an actuator pitching each
blade. If each blade is driven by its own pitch actuator, two control systems can
be distinguished. In one case there exists one controller and the blades are pitched
with a 120° phase difference. This so-called cyclic pitch control is used to miti-
gate 1P loads due to wind shear and yaw misalignment. With true individual pitch
control (IPC) each blade is really controlled separately, based on local fl ow or load
measurements. See Fig. 2 for the control loop of a variable speed, variable pitch
turbine above rated wind speeds.
Here

stands for rotational speed, T for torque, b for the pitch angle and q for
the scheduling parameter that is tailored as the operating point of the turbine
changes. The subscript g indicates that it concerns the generator and c that it is a
control signal. However, numerous other concepts exist [5]. These include con-
cepts where the loading above rated wind speeds is controlled by active or passive
stall. Also passive control systems such as the flexhat have been developed
where a part of the blade is passively pitched as the rotational speed increases
because it reacts to centrifugal forces.
Some concepts have been proposed with the goal of mitigating root bending
moment fl uctuations, such as teetering hinges in the hub and (active) cyclic pitch
control. Also, a passive concept, called bent-twist coupling, exists. In this concept
the UD laminate in the main bending load carrying spar caps is placed under an
angle to the blade's longitudinal axis. This way, as the blade bends, it will twist [6].

Ω

Figure 2: Variable speed, variable pitch control scheme for above rated wind
speeds (fi gure adapted from [5]).