Environmental Engineering Reference
In-Depth Information
in resonance within the operating regime.
In such cases the control system can be
programmed to “step over” the resonant rotor speeds [DiValentin 1985].
Prediction of Design Loads
It is most useful to determine and evaluate blade and hub loads as time histories, as
they would be seen by an observer in a
rotating coordinate system
attached to a HAWT or
VAWT rotor. For the nacelle, tower, and cable structures, time histories in a
fixed
coordinate system
attached to the ground are easiest to comprehend and apply to design.
For the power train, either a rotating or a fixed coordinate system (or both systems) can be
suitable. A simulation which analyzes the system in the
time domain
will make the needed
computations involved in coupling the rotating and fixed systems. By suitable post-
processing of the simulation output, the loads transferred between the rotating and fixed
systems (
e.g.
,
at the apex of a coned HAWT rotor) can thus be obtained as time histories
in both the rotating and fixed coordinates. Generally, the rotor designer will think and work
in the rotating system, and others will think in the fixed systems as they perform analyses
of internal loads, stresses, and fatigue lives.
The prime method of load computation should be to run steady-state or transient
response cases on the system dynamic simulation and then adapt the time-history output to
obtain
maximum, minimum, mean
(
i.e.
(maximum + minimum)/2), and
cyclic
(
i.e.
(maximum - minimum)/2) loads that occur. Included must be the essential aspects of
control action and all the elements of aeroelastic behavior that could influence loads. If
there are potential stability problems, these will show up as poor damping during transients,
which can usually be corrected without resorting to the added complexity of a
frequency-
domain analysis.
The latter is usually of little value in a loads analysis.
Mean Operating Loads
Simulation runs at steady wind speeds (with rotor-induced retardation effects) give
ready access to the mean operating loads upon which cyclic loads can later by superimposed
for fatigue life prediction. It should be noted that mean load analysis is required in the
simulation model because of the complex counterbalancing of centrifugal and thrust loads.
Limit Loads
Establishing design limit loads is a matter of identifying sources of
one-time
or
infrequent events
that might do structural or mechanical damage to the system. Major gusts
(without retardation effects), dropped electrical load events, and control failure events can
be examined in the system simulation to identify which cause limit loads. Often, the
resulting limit load will not cause damage in a structure that is designed for long fatigue
life (
i.e.
fatigue, not limit loading, is the
design driver
)
.
Moreover, if damage is indicated
from a one-time event, steps can often be taken to reduce or control the limit load so that
it does not remain the design driver.
Fatigue Loads
As discussed earlier, the sources of design-driving cyclic loads are longer-term changes
in wind speed, large-scale temporal turbulence, small-scale spatial turbulence (within the
swept area of the rotor), gravity, wind shear, and tower shadow. In theory, all of these can
be readily modeled for input to the simulation. Determinate cyclic loads (like those from
Search WWH ::
Custom Search