Performance and Operation Management - Wind Energy Systems for Electric Power Generation - page 134

a)

Fig. 6.12 Control scheme of a system for constant speed operation

Figure 6.12 is related to a constant speed WES with an induction generator
directly feeding into the grid. The rotational speed is prescribed by the grid fre-
quency with only small deviations. The system control provides reference values
P ref of power and n ref of rotational speed. The power control is cascaded with the
control of the blade angle

and its setting velocity. The speed controller is used
to control the run-up procedure and to limit the power for wind velocities above
rated value by acting on the pitch mechanism. Input values of the turbine model
are wind speed, rotational speed and pitch angle; output is the turbine torque. The
generators model inputs are grid frequency and voltage, together with speed n ;
output is the generator torque. Torques and rotational speed define the mechani-
cal power values. Turbine and generator torques add up to the acceleration torque
which is zero during steady state. The accelerating torque acts on the mechani-
cal drive train model which in simplified form is an integrator, with the speed as
output.
Figure 6.13 extends the block diagram to a variable speed system, the example as-
suming a fully-fed concept. While speed controller 1 limits the turbine power, speed
controller 2 acts on the converter, supplying the set values for operation with e.g.
optimum power. A reactive power (or power factor) control is usual but not shown
in the figure. Variations of the figure are due when the generator is a doubly-fed
induction machine, or when a synchronous generator requires additionally control
of the excitation winding current.
A monograph on wind turbine control with special view on a proposed variant of
gain scheduling techniques is in [Bian07].

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