Environmental Engineering Reference
In-Depth Information
then k
opt
can be obtained as follows:
k
opt
¼
2pR
nS
ð
7
:
20
Þ
Hence, wind turbines must be properly controlled to operate at their optimal
wind tip speed ratio in order to extract as much wind power as possible. It should
be noted that k
opt
is determined empirically by the wind turbine manufacturer since
it is clear that all the parameters in Eq. (
7.20
) are dependent on the wind turbine
structure. Consequently, the dependence of k
opt
on S causes a serious challenge in
wind turbine control since S is highly dependent on the blade design. Hence, with
turbine ageing any deformation in the blade structure causes permanent uncer-
tainty in the value of k
opt
.
7.5 Investigation of the Effects of Some Faults Scenarios
As stated in Sect. 1.4, the controller optimises the power captured by controlling
the rotor rotational speed by varying the reference generator torque T
gr
so that the
wind turbine rotor speed x
r
follows the optimal rotor speed given by:
x
ropt
¼
k
opt
v
R
ð
7
:
21
Þ
where x
ropt
and k
opt
are the optimal rotor speed and the optimal tip speed ratio.
Moreover, the controller must be designed to have fault tolerance capability against
probable fault scenarios that degrade the required closed-loop performance.
The following faults are considered and the proposed control strategies need to
tolerate the fault effects so that good tracking performance to x
ropt
can be maintained.
• Rotor speed sensor scaling fault: the sensor scaling fault (decreasing or
increasing) drive the turbine away from the optimal operation. It is very clear that
the controller is designed to provide good tracking of x
ropt
(i.e.
e
t
¼
x
r
measured
x
opt
0). However, due to the scale factor fault the controller
now tries to force the faulty measurement to follow x
ropt
(i.e. if the scale faults
are ±10 % then 1
:
1
x
r
x
opt
0or0
:
9
x
r
x
opt
0) causing a deceler-
ating or accelerating of the actual rotor speed and hence causing the wind turbine
to operate away from the optimal value x
opt
. Additionally, more sever sensor
scale faults can affect the structure of the wind turbine or guide the wind turbine to
the cut-off region. For example, severe scale-down sensor faults cause the turbine
to rotate faster according to the available wind speed. Hence, the fast rotation
scenario means that the blade passes through the turbulence component of the
previous blade before re-establishing the undisturbed wind speed. This induces
excessive vibration of the overall structure of the wind turbine. On the other hand,
