Environmental Engineering Reference
In-Depth Information
5.2 General Control Concepts for Wind Turbines
Wind turbines with generator variable speed regulation based on blade pitch angle
control have been commonly used over the last few years by wind turbine manu-
factures. The wind turbine control system is divided into two layers: the wind farm
supervisory control, which generates external electric power demand set-points for
each wind turbine, and the wind turbine supervisory control for each individual wind
turbine. Furthermore, the wind turbine supervisory control is also divided into four
operating states: startup, shutdown, park and power production. The control strategy
in the electric power production zone is determined by a curve where the generator
speed and the generator torque values are carefully related [
7
-
9
]. Figure
5.1
shows
this curve for the 5 MW wind turbine explained in Sect.
5.2.1
. The power pro-
duction zone is defined by the curve ABCD to be more time working at the optimum
power coefficient value. The vertical sections AB and CD are implemented with
generator torque controllers to regulate the generator speed at the references existing
in the points A and C respectively. Between B and C points, the control zone is called
below rated and it is implemented using a look up table control to work with the
optimum power coefficient value while the pitch angle is fixed at the fine pitch angle,
which is usually zero. However, in the D zone, the generator speed is regulated with
a collective blade pitch angle control and the generator torque is kept at the nominal
value. The transition between the generator torque control in the zone CD (transition
zone) and the collective pitch control in zone D, called above rated, has to be soft to
improve the controller performance [
7
,
10
]. Sometimes, the rotor rotational fre-
quencies 1P,2P and 3P are equal to other wind turbine structural modes in the
tower, blades or drive train. If this coincidence exists, these modes can be dan-
gerously excited. In [
9
,
11
,
12
], a strategy to avoid this coincidence is proposed,
where the below rated zone is divided into five sub-zones: BE and GC to work in the
power coefficient optimum value, EF and GH to regulate the generator speed outside
the forbidden speed zone EG with a generator torque control.
Figure
5.2
shows the generator speed and electric power signals for the 5 MW
wind turbine model in the different zones at the power production state. Also, the
collective pitch angle and generator torque control signals are shown in next
figures and they vary according to the wind operating point.
As it is mentioned in the introduction, the continuous increase of the size of
wind turbines has led to new challenges in the design of wind turbine control
systems beyond the main objective of electric power production. Today's control
strategies trend towards being robust, multivariable and multi-objective in order
to fulfill the numerous control design specifications considering the hardly
coupling effects existing in a wind turbine non-linear system. One of the most
important specifications is to mitigate loads in the turbine structural components.
In spite of the coupling existing in wind turbines, classical control strategies for
wind turbines in the power production zone uncouples the control problem into
different Single Input Single Output (SISO) control loops to make easier the
control system design:
