Environmental Engineering Reference
In-Depth Information
T
g
¼
P
rated
gX
g
ð
11
:
10
Þ
where P
rated
is the rated output power of the generator, and g is the generator
efficiency.
11.2.4 Pitch System
The pitch system adjusts the angle of the blade by rotating it. In a three-bladed
wind turbine, three identical pitch systems are used. These systems could be
electrical or hydraulic, and regardless of the system nature they share common
components, system response, and fault symptoms. The common components
include the actuator, the controller, and the sensors. The pitch system can be
satisfactory modeled as a second-order system [
16
] with a time delay t
d
and a
reference signal b
ref
such as
b
ð
t
Þ¼
2fx
n
b
ð
t
Þ
x
n
ð
b
ð
t
Þ
b
ref
ð
t
t
d
Þ
Þ
ð
11
:
11
Þ
where t
d
is the time delay related to the communication between the turbine
controller and the pitch system controller, b
ðÞ
is the pitch angle, b
ref
ðÞ
is the
reference pitch angle, x
n
and f are the natural frequency and damping ratio of
the pitch system model, respectively. The block diagram that illustrates the pitch
system model along with the constraints on the pitch slow rate and range is shown
in Fig.
11.6
. The constraints are added to represent the limitations of the actual
pitch actuator.
The GSPI controller was originally designed [
5
] without considering the pitch
system model in the turbine. As the considered pitch system model has higher
dynamics than the turbine dynamics, the pitch system does not affect the controller
design. Figure
11.7
shows the bode magnitude of the transfer function from the
collective pitch angle to the generator speed for the RWT model with and without
pitch system model. The transfer function is obtained from linearizing the non-
linear model of the turbine implemented in FAST (All DOFs in FAST are enabled
such as tower fore-aft and side-side modes, blade flap-wise and edge-wise modes,
Fig. 11.6 Block diagram of pitch actuator including slow rate and range limitations. s is Laplace
complex variable
