Environmental Engineering Reference
In-Depth Information
Fig. 4.4 Wind turbine
operating regions
P N
Wind speed
1
2
3
V min
V N
V max
controlling the generator torque. Above rated wind speed (region 3), the objective
is to regulate the rotational speed and power at their rated values in order to protect
the wind turbine from high mechanical loads and excessive electrical currents. In
this region, the rotational speed is regulated acting on the pitch angle, whereas the
generator torque is kept constant at its rated value. Between these two regions,
there is a transition zone (region 2), in which the objectives and control structure
exhibit significant changes.
The operating locus of the wind turbine corresponding to the power-wind
strategy of Fig. 4.4 can be conveniently represented on a torque-rotational speed-
pitch angle space. This graph is shown in Fig. 4.5 . In this figure, the projection of
this 3D curve onto the torque-rotational speed plane is also depicted. The three
operating regions can be easily identified.
• In region 1, the objective is energy capture maximization. Both the tip-speed
ratio and the pitch angle should be maintained as close as possible to their
optimum values: C P ð k o ; b o Þ¼ C P max . To this end, the generator torque is
conventionally chosen to follow a quadratical relationship with the rotational
speed, i.e.,
!
pqR 5
2k o
X g ¼ k t
X g :
T g ¼
C Pmax
ð 4 : 7 Þ
• The transition region 2 commonly comprises two subregions. Once the rota-
tional speed reaches the lower limit X lim , the to rq ue increases proportionally
until the rated value T N . Beyond the upper limit X lim , the torque is maintained
constant at the rated value. The objective in this region is to decouple as much
as possible the control laws for regions 1 and 3.
• In region 3, the generator torque is kept constant at the rated value whereas the
pitch angle is used to regulate the rotational speed.
In the literature, several control schemes covering the entire operating range
have been proposed. Basically, two approaches can be identified among the
 
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