Environmental Engineering Reference
In-Depth Information
foreseeable that in the future, the overall control scenario of a fully-optimized wind turbine
will give more (and different) attention to the control of thrust loads.
Power Trains and Power Extraction
If the wind turbine rotor is stiffly coupled to an essentially constant-speed electrical
generator, the control of fluctuations in torque and power is made difficult, requiring high
rates of aerodynamic load control. Furthermore, the action of controlling torque transients
will necessarily spill energy that might otherwise be captured. Reducing the stiffness of the
power train, either mechanically or by adopting a
constant-torque control
on the generator,
permits a great reduction in the required rate of aerodynamic load control. Constant torque
control allows the rotor to operate at variable speed, so that the rotor inertia becomes both
a source and a sink for energy transients, torque loads can be held within limits, and less
energy is spilled.
If a constant electrical torque generator is adopted, the shaft stiffness felt by the turbine
rotor is effectively reduced to zero, and the need to control transient aerodynamic torques
is eliminated. Any method that reduces the
torsional impedance
of the power train also has
the effect of decoupling the structural dynamic behavior of the rotor from that of the power
train. This can simplify the simulation of the complete dynamic system and the analysis
of internal rotor loads. However, to get these energy and load benefits, power train impe-
dance must be reduced far below that obtained from the
slip
of an induction generator of
reasonable efficiency. Therefore, merely substituting an induction generator for a synchro-
nous generator leaves the system with essentially the same dynamic response.
A power train can be softened mechanically by introducing torsional springs with
relatively low stiffness, as shown in Figure 10-3. Here the input shaft of the planetary
gearbox of the WTS-4 HAWT is connected to the forward end of the turbine shaft and the
gearbox case is connected to the bedplate through swing-links and springs. The latter are
composed of stacks of
Belleville washers.
When the power train is softened by springs in
this way, it becomes very beneficial to provide damping of the lowest-frequency torsional
vibration mode wherein the turbine rotor oscillates like a torsional pendulum. The required
amount of damping is not effectively available from pitch control when the rotor is
operating near its peak of performance, since any change in pitch lowers torque even when
an increase in torque is called for. Damping can be readily obtained, however, by adding
damping devices to the torsional springs in the power train, as indicated in Figure 10-3.
If low power-train impedance is obtained electrically from generator torque control,
analogous damping can also be obtained easily from the same electrical torque control. In
that event, neither mechanical softening nor mechanical damping of the power train are
necessary or beneficial.
As discussed above, control systems are very interactive with power train impedance
characteristics. Design and development progress can only occur when the interaction of
the controls with the system response dynamics is rigorously examined and adjusted. To
repeat a previous point, there is a fortunate synergism when the soft-system structural
design philosophy is applied to the power train. Not only are transient loads reduced and
required control rates eased, but there is a decoupling of rotor torsional dynamics from that
of the nacelle and its equipment in ways that permit simplification of analysis.
Tower Design
The advantages of the compliant structural design philosophy apply very clearly in
tower design. There was an early traditional approach to HAWT tower design which
sought to keep the lowest
system natural frequency
,
in which the masses of the rotor and
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