The mechanical effects of short-circuit currents in - Montefiore

Zug-/Druck-Kraft
in kN
Z eit in s
Figure 3.64 Time characteristic of the
tensile/compressive force in the
interphase-spacer
The frequency of ca. 2 Hz is identical with the one in
Figure 3.63b. The values of amplitudes are proportional
to the magnitude of the electromagnetic force acting on
the phase spacer. The substantially higher frequency
harmonics of ca. 40 – 80 Hz arise due to the bracing
between conductor and phase spacer. Frequency and
amplitude of the harmonics depend on the rigidity of the
phase spacer / conductor clamping system. For the case
in question, the use of conductor spirals would cause
damping of the harmonic amplitudes. However, this
effect cannot be said to be universal, i.e. in other
arrangements it could just as well result in a gain.
Therefore, the use of conductor spirals is not a sensible
course of action without prior investigation.
Despite the higher harmonic components, the
''
compressive forces always (even when I k 2 = 40 kA)
remained below the permitted Euler buckling force (16
kN). Their proportion of the total loading on the round
bar is low.
As Figure 3.62d shows, the interphase-spacer cambers
vertically downwards when the span drops. The
potential energy that builds up during the first 500 ms is
converted to energy of deformation (bending) and
brings about the first maximum of the reference stress at
0.92 s (Fig. 7).
150.
100.
Spannung in
N/mm²
50.
0.
.0 .5 1.0 1.5 2.0 2.5 3 .0 3.5
Z eit in s
Figure 3.65 Time characteristic of the van Mises stress at the midpoint
of the interphase-spacer
The nature of the coupling between conductor and
interphase-spacer is irrelevant here. The results with
free horizontal or horizontal and vertical rotational
motion of the coupling were identical, which was to be
expected in view of the low stiffness of the conductor.
Therefore, the use of horizontal or even articulated
conductor clamps is unnecessary.
65
In practice, the interphase-spacers are not fitted
precisely perpendicular to the conductors. Calculations
have been performed in which, for the sake of example,
the interphase-spacer has been shifted on one side by
0.2 m along the axis of the conductor. However, the
resulting horizontal bending moments only caused a
slight increase in the loading on the interphase-spacer.
When the interphase-spacer was fitted correctly, such
shifts are considerably less than the calculated value so
that the stress acting in the horizontal plane is of
secondary importance.
Thus, the major load on the interphase-spacer results
from the dropping motion.
Summarising the investigation one can say that by
fitting a interphase-spacer at the mid-point of the
conductor span it is possible to halve the conductor
deflection. The static and dynamic conductor tensile
forces are little affected in the process.
The coupling between the interphase-spacer and the
conductor has no effect on the behavior of the strained
conductor or on the mechanical load exerted on the
spacer. Couplings with horizontal or articulated joints
are unnecessary.
Similarly, the conductor spirals that are widely used in
the world of overhead-lines are also unnecessary at the
coupling points. Depending on the rigidity of the overall
structure, their use can either increase or decrease the
tensile/compressive loading on the interphase-spacer.
The compressive loading on the interphase-spacer is of
the same order of magnitude as the electromagnetic
' '
force F ⋅ l ⋅( F as defined in equation 15, IEC 60865-1)
and, in the case of the alternatives examined, always
remained less than the maximum permitted value of
buckling load.
The major load on the interphase-spacer results from the
bending moments caused by the dropping motion.
The fitting of interphase-spacers at the mid-point of the
span enabled the short-circuit strength o to be increased
from 21 kA to over 46 kA (at F t = 38 kN).
3.7.3. A new simplified method for spacer
compression evaluation.
As discussed already in [Ref 54] the well known
Manuzio simple formula [Ref 53] neglect some
important effects, mainly:
• Tension changes in the subconductor during contact
period, so called pinch effect. This effect can be
quite large, as the pinch can be several times the
initial tension (CIGRE brochure 105 [Ref 1]). For
example, in our tests presented in this document,
the pinch is two times initial value (third case),
even higher in [Ref 54]. It is quite clear that the
spacer compression, which is directly related to the
pinch (both pinch and spacer compression are
quasi-simultaneous) is also affected by the pinch.
• Asymmetry of the short-circuit current. With
increasing short-circuit current, time to contact and
to maximum pinch and spacer compression
becomes smaller and smaller (lower than 0.1 s in