The mechanical effects of short-circuit currents in - Montefiore

Only the first eigenfrequencies can be compared
in Figure 3.14, which shows an excellent
conformity between the calculated and the
measured values. The very good conformity of
the calculated spring constants and first
eigenfreqencies with the measured values proves
the validity of the FE model used for the
simulation of the portals.
Hence, all static and dynamic analyses can be
performed on this model.
Figure 3.14 Comparison of calculated and measured 1 st
eigenfrequencies
Experience has shown that, although the shortcircuit
movements of stranded conductors seem
to be chaotic, the relevant effects of symmetric
arrangements and excitation are in fact also
symmetrical. The measuring points could,
therefore, be arranged to a half of the plane of
symmetry as shown in Figure 3.15 on crossarm,
tower and bottom end of tower, i.e. interface to
foundation.
In order to be able to validate and compare the
measured, respectively calculated mechanical
stresses at the different points of the structure as
illustrated in Figure 3.15, the Equivalent Static
Load ESL and the ESL-factors are introduced.
Since the short-circuit forces Fpi, Ft and Ff
contain the static pre-load the ESL-factor is to be
defined as the quotient of the Equivalent Static
Load divided by the short-circuit load including
the static pre-load.
40
Figure 3.15 Strain gauge measuring points in the South portal
Since it is possible to separate the static pre-load and
the dynamic part of Fpi, Fb Ff, one can, also define
another type of ESL related to the dynamic portion
alone. Since the static pre-Ioad is a particularly
important parameter for the magnitude of Fpi, this
other definition is more suited and practical for the
dynamic portion of the swing-out and the conductorfall
maximum. In the present context the index d -for
dynamic -is employed in ESLd and ESLd Factor.
The evaluation of the present tests does not
explicitly distinguish between swing-out and
conductor-fall ESL, the relevant short-circuit test
maxima being swing-out (Index t), and the
conductor drop- ping tests delivering ESLf anyway.
Because of the single conductors there is no pinch
effect, i.e. no Fpi.
Figure 3.16 and Figure 3.17 display the comparison
of the measured and calculated relative static strain
for the South/North portals and the Mid portal.
Generally the comparison shows a very good
agreement between calculated and measured values
with the exception of one diagonal in the bottom
girder of the crossarm, for which particular anomaly
a reason is being looked for .
Figure 3.16 Comparison calculated and measured relative static
strain South portal