Structural Dynamic Considerations in Wind Turbine Design - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 590

Rotor Controls
The fundamental purpose of wind turbine rotor control elements is, of course, the
adjustment of steady and cyclic air loadings that would otherwise occur in response to
spatial and temporal variations of wind speed. One school of conceptual design, motivated
by an urge for simplicity as discussed earlier, has chosen to operate the rotor near
aerodynamic stall. In this stall control or passive control concept, load increases are limited
by the maximum lift coefficient that the airfoil generates at the advent of stall. This has led
to the design of new airfoils for wind turbines with lower maximum lift coefficients and a
smoother transition to stalled behavior (called soft stall ) than available aircraft airfoils ( e.g. ,
see Table 6-1).
Another design-concept school chooses to operate blades at lower lift coefficients ( i.e. ,
lower angles of attack), in order to keep away from stall and increase the rotor coefficient
of performance , C P , because of the higher lift-to-drag ratios in the low-lift regime. Control
of aerodynamic loading must then be accomplished by control of the blade's angle of
attack, which leads to pitch control of either the full length of the blade or at least the
outboard highly-active portion of the blade span. Although pitch-controlled rotors are in
service that have quite successfully achieved control of structural loads, they are generally
criticized for their complexity. This complexity and the sophisticated design involved in
getting the desired sharpness of load control led to the selection of stall control for the most
of the wind turbines in service at the end of the 1980s. However, the majority of HAWTs
installed in the '90s and later have variable-pitch rotors.
A third approach to aerodynamic load control of a HAWT is that of yawing the rotor
out of alignment with the wind. This approach has been used for centuries without much
attention to the cyclic load aspects. As explained by Drees [1977] blade bending flexibility
in a hingeless rotor can provide substantial alleviation of the cyclic loads that would
otherwise arise during yawed operation. Hohenemser [1981] demonstrated in the early-
1980s that a teeter-hinged rotor can be forced by the control system to yaw at very high
rates without creating load problems in either the rotor or the nacelle. The structural
feasibility of high-rate yaw control of a teetered rotor was also demonstrated in the mid-
1980s by field tests on the WTS-4 4-MW HAWT [Stoltze 1985]. These demonstrations led
to the selection of yaw control and fixed blade pitch for the Gamma 60 HAWT, which
started operating in 1992.
Figure 10-2 illustrates the relationships between wind speed, rotor speed, yaw angle,
turbine torque, and power output that exist in a HAWT with broad-range variable speed and
yaw control of peak power, using the Gamma 60 operating map as an example. Where
broad-range variable speed is adopted as the basic operating mode of a HAWT, aero-
dynamic torque control is required only to limit rotor speed in high winds and large gusts,
and to control speed in the event that electrical load is lost [DiValentin et al. 1986]. Under
all other operating conditions, the rotor speed follows the wind speed, while changes in
rotor momentum store and then give back most of the energy associated with large-scale
turbulence. Aerodynamic torque control can be accomplished either by blade pitch or by
yawing the rotor. In either event, aerodynamic control is confined to managing overspeed
and is not active over most of the operating envelope.
It is significant to note that while the yaw control concept has the advantage of a
simple fixed-pitch rotor, it also applies true stall avoidance principles. Aerodynamic loads
are adjusted by combining reduction of effective disk area with reduction in the mean
effective angle of attack. Meanwhile, cyclic air loadings that would otherwise result from
yawed operation are essentially eliminated by the teetering action of the rotor. Control of
the air loading on the rotor is achieved from outside the rotor system, without the need of
blade pitch change and without stalling the active airfoils.