Environmental Engineering Reference
In-Depth Information
14-3, we see that generator or rotor speed is measured and passed to the pitch controller.
The goal is to use PID pitch control to regulate turbine speed in the presence of wind-speed
disturbances. The expression for the blade pitch command is:
DW(
t
)
dt
+
K
D
D W(
t
)
ò
Dq(
t
) =
K
p
DW(
t
) +
K
I
(14-2)
where
Dq = commanded blade pitch change (rad)
DW = generator or rotor rotational speed error relative to set-point (rad)
K
P
= proportional feedback gain (s)
K
I
= integral feedback gain
= derivative feedback gain (s
2
)
K
D
The goal in Region 3 control design is to determine values for the three gains that give the
desired tracking of rotor speed to the desired set-point and maintain closed-loop stability.
Methods for choosing these gains are further given by Hansen
et al
[2005] and Wright and
Fingersh [2008].
To verify satisfactory control performance with these gains, the control designer usually
simulates the closed-loop turbine response before implementing and testing such a control-
ler in the field. Some refinements to Equation (14-2) to improve performance include gain
scheduling, including anti-windup, and introducing filters to prevent excitation of high fre-
quency modes [Wright and Fingersh 2008].
An additional control design goal is to mitigate structural dynamic loads. One way is
to design controls that actively dampen the motions of turbine components. In commercial
turbines, an additional generator torque control loop in Region 2 is often used to actively
damp the drive train torsion mode of the turbine, as illustrated in Figure 14-3. In Region 3,
classical control design methods have been used to design controllers to add damping to the
tower's first fore-aft mode with blade pitch changes [Bossanyi 2000]. The pitch control to
actively damp this tower motion is usually implemented by adding a single-input single-
output (SISO) control loop to the basic Region 3 speed control loop just discussed.
Other studies with classical control include the use of independent pitch control to miti-
gate asymmetric wind variations across the swept area of the rotor [Bossanyi 2003]. In this
approach, two separate SISO control loops were used to mitigate the tilt- and yaw- oriented
loads in the fixed frame of reference with independent pitch control of each blade. This
work was extended with alternative sensors to measure the asymmetric loading on the rotor
[Bossanyi 2004]. Good results were obtained when suitable sensors were used.
In these various applications, the use of classical controls to address more than one
control objective is not straightforward. Often, multiple control loops are used, which adds
complexity to the control design and the dynamic behavior of the wind turbine system. If
these complex controls are not designed with great care, the control loops interfere with each
other and destabilize the turbine. The potential for instability increases as turbines become
larger and more flexible, and the degree of coupling between turbine components increases.
Design of Advanced Wind Turbine Controllers
As already mentioned, wind turbines are flexible systems acted on by stochastic wind
disturbances and time-varying gravitational, centrifugal, and gyroscopic loads. While the
simple classical controllers discussed previously can give good speed regulation performance
in Region 3, other control objectives may not be met. These include accounting for stochastic
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