The mechanical effects of short-circuit currents in - Montefiore

moment when the distance between the conductors is
minimum at mid-span.
The highest magnitude of the force during the second
short-circuit is obtained slightly after the moment when
the distance between the conductors at mid-span
exceeds the distance in normal position. The drop or
tensile forces during the short-circuit vary very slightly
with the moment of reclosure. Figure 3.57 below
indicates, for the 68 m span, the drop force variation
versus the reclosing interval (Ti) :
Figure 3.57 Drop force (Newton) versus no supply time (ms).
S.M.= Simplified Method
ASTER= EDF software
TESTS=EDF tests
Ff1= no reclosure
Ff2=with reclosure
The worst case occurs when the reclosing interval is in
the following range :
(3.23) Tk1 + Ti < 0,25 T°
and
(3.24) 0,75 T° < Tk1 + Ti < 1,25 T°
where :
(3.25)
T
0
0,
8.
bc
2π
g
=
2
π ⎛ δ M ⎞
1−
⎜ ⎟
64 ⎝ 90 ⎠
is the period of the free motion (without current) and δM
is the maximum swing-out angle in degree.
The maximum of Ff2 occurs when the reclosing interval
is between 0,75 T° and T°, and the maximum of Ft2
occurs when the reclosing interval is between T° and
1,25 T°.
f) Calculation approach
Without bundle
To calculate the maximum tensile force after an autorecloser,
IEC 60865 standard can be used with a
clearance time equal Tk = Tk1 + Tk2. If the influence of
reclosing interval has to be studied, a simplified method
[Ref 100,] can be used in the more simple cases (Annex
8.4 & 8.5).
2
62
With bundle
To date pinch effect after auto-reclosing cannot be
studied by a simplified method. But after a fast
reclosure, pinch effect can have an influence if the pinch
effect in IEC 60865 conditions are not satisfied.
3.7.2. Interphase-Spacer
Short circuit power of AIS is a value which is subject to
a permanent increase. Unfortunately this leads to an
increase of mechanical effects too. Thus investigations
have to be carried out to prove that short-circuit
withstand is sufficient. Usually this will be done with
[Ref 2] which covers short-circuit effects of cable spans
up to 60 m. During such calculation it was discovered
that, in the case of span lengths with more than about
35 m and short-circuit currents in excess of 20 kA, the
usual clearances mainly in the 110kV-level are not
sufficient. Given that in overhead-lines interphasespacers
are in common use to avoid wind induced
effects, it is obvious they can be used for AIS busbars
too.
The subject of the investigation was the 40 m span
without droppers as described in case 4 in Vol 2 of this
brochure. Test/calculation parameters may be taken
from there. The line-to-line initial symmetrical shortcircuit
current was 28.5 kA and the duration of shortcircuit
current was 103 ms. The period of investigation
chosen was 2 to 3 s. No damping effects were taken into
account and the portals were considered by their spring
constant.
In a first step calculation was established without
interphase spacer. The results of the finite-element
calculations are shown in Figs. 1 and 2. Fig. 1
4. 40.
Seilzugkraft
in kN
2. 20.
20.7 kN
0.
.0 .5 1.0 1.5 2.0 2.5 3.0 3.5
Zeit in s
Figure 3.58 Conductor tensile force: comparison between test results
and calculation results without interphase-spacer
(thin = test; thick = calculation)
shows the movement at the mid-point of the conductor
with a maximum deflection of 1.12 m which is in good
accordance with test results and calculation of IEC
60865-1 (1.27m). In Fig. 2, where the conductor tensile
force is plotted against time, the thin curve shows the
test results and the thick curve the calculation results of
case 4. The comparison shows good agreement between
the time characteristic and the amplitude, which gave
22.0 kN