The mechanical effects of short-circuit currents in - Montefiore

Section 3.4 deals with the effects of close and
wide bundling. Typical oscillograms are shown
and results of advanced calculations are given.
Design loads suggested in IEC/EN 60865-1 take
the maximum instantaneous values of Ft, Ff, Fpi
as static load for design purposes. The aim of
Section 3.5 is to define the limits in which such
design recommendations are valid and in which
some correcting factors including dynamics of
structure must be taken into account. Also new
design loads are evaluated.
In Section 3.5.4, special problems are discussed.
Interphase spacers allow to reduce the horizontal
movement of the spans. The extension of
IEC/EN 60865-1 on three-phase automatic
reclosing, spacer compression and jumpers are
described and it shows how to handle spans with
springs.
3.2. TYPICAL OSCILLOGRAMS AND
CALCULATIONS
3.2.1. Typical oscillograms of forces in the bus
Below are typical oscillograms for the shortcircuit
mechanical effects in the buses: forces in
the supporting structure of slack conductors and
tensile forces in strained conductors. A detailed
description of the test structures and further
informations are given in Volume two of this
brochure and Volume two of [Ref 1] 2 .
Typical forces in droppers are reproduced in
Section 3.3.
The first effect is caused by the short-circuit
forces between the phase conductors. In the
upper oscillogram of Figure 3.1, the span of a
slack conductor rotates several times and leads to
nearly equal maxima during swing out and when
is passes its initial static position.
In the lower oscillogram of Figure 3.1 and Figure
3.2 to Figure 3.4, mainly the two tensile force
maxima caused by the movement of the span can
be clearly pointed out; the first one due to swing
out and the second one due to falling down. The
basic frequency content of the time scale is the
swing frequency of the busbars, with its sag
according to the equation in section 1.3.3 of [Ref
1].
2 In the following, the numbers of test cases
presented in Volume two of [Ref 1] are marked
by *; test cases presented in Volume two of this
brochure are not marked.
37
9
kN
3
0
-3
9
kN
3
0
-3
0
tk 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 s 1,8
Figure 3.1 Single slack conductor:equivalent dynamic force at
the clamp representing the stress at the bottom of the
insulator (15-m-span); case 2, Figure 3.1 of the
volume 2.
Figure 3.2 Single strained conductor (40-m-span); case *6,
Figure 6.6
Figure 3.3 Single strain conductor (40-m-span); case *8,
Figure 8.5
Figure 3.4 Single strain conductor (102-m-span); case *11,
Figure 11.11
The pinch is another effect due to bundle collapse
and the presence of spacers which impede
conductors to come together near their location. In
the subspan between two adjacent spacers, the subconductors
can clash or can only reduce their
distance. It depends on the ratio between the subspan
length to sub-conductor distance ls/as and subconductor
distance to sub-conductor diameter as/ds
as discussed in section 4 of [Ref 1]. The parameter j
according to equation (*49) in [Ref 1, Ref 2, Ref 3]
indicates clashing or not clashing. Figure 3.5 to
Figure 3.11 shows the tensile forces for different
subspan characteristics
(3.1)
ls as
π
λ = sin
a d n
s
If there is a close bundling which means that the
bundle has a large ratio ls/as, obtained by a very
s