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for attenuating loads caused by asymmetric wind variations across the rotor, demonstrating
the advantages of performing multiple control objectives with a single MIMO control loop.
This controller performs speed regulation, damping of the tower's fore-aft and side-to-side
modes, damping of the first drive train torsion mode and mitigation of asymmetric wind
variations across the rotor.
Compared to simulated cyclic loads in the CART system with a standard PID controller,
this state-space controller achieved the following load reductions:
· 
Low-speed shaft torque ...................................28 percent reduction
· 
Tower side-side bending moment ...................76 percent reduction
· 
Tower fore-aft bending moment .....................20 percent reduction
· 
Blade flap bending moment ............................16 percent reduction
Another control design for attenuating tower motions is presented by Malcom and Hansen
[2002].
Control Methods to Accommodate Nonlinearity Effects
The nonlinear behavior of a wind turbine can make control design difficult. For exam-
ple, rotor aerodynamic behavior is highly nonlinear. In pitch control, the control input gains
are usually the partial derivative of the rotor aerodynamic torque with respect to blade pitch
angle. These input gains are not constant but vary with wind speed, rotor speed, and pitch
angle. Figure 14-4 shows the variation of rotor aerodynamic torque with blade pitch angle
for the CART. A linear controller designed for a turbine at one operating point (such as the
control design point shown in this figure) may give poor results for other operating points.
400
400
(
(
Torque
pitch
Torque
pitch
)
)
(
(
)
)
300
300
14m/s
16m/s
18m/s
20m/s
22m/s
14m/s
16m/s
18m/s
20m/s
22m/s
200
200
100
100
Control De sign
Point
Control De sign
Point
0
0
-100
-100
-10
-10
-5
-5
0
0
5
5
10
10
15
15
20
20
Pitch (deg)
Pitch (deg)
Figure 14-4. Variation of control input gains with pitch angle and wind speed. Gains
are the slopes of the torque vs. pitch angle curves.
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