The mechanical effects of short-circuit currents in - Montefiore

movement of the dropper (swing out and falling down)
were much more a swing with limited falling down.
We must consider different maxima, the maximum of
which will be Fds. Simplified methods are given in
paragraph 3.3.4.
b) Droppers with flexible upper ends (configurations 3
to 6)
Droppers in or near the middle of the span as case B in
Figure 1.1 have flexible upper ends. The short-circuit
current can flow through the whole span and the
dropper is without current, current path B in Figure
3.22, or through half the span and then through the
dropper, current path C. With path B the dropper
behaviour is only affected by the movement of the main
bus, whereas with path C additional loads due to the
electromagnetic forces between the droppers arise and
due to pinch effect in bundled conductors. Current path
A is the reference arrangement without dropper.
Figure 3.22 Current paths
A without dropper
B, C with dropper
Tests with droppers in midspan are described in test
cases *6, *7, 4 and 5 and also in [Ref 39].
The first point to develop is to determine if the dropper
will be stretched or not. Advanced method is used to
study the structure behaviour. Simplified method can be
used to evaluate the horizontal movement of the busbars
and then to calculate the maximum distance from
the attachment points (bus and support) and compare it
to the dropper length as derived in section 3.3.4a). If
dropper length is unknown you will always assume a
stretch.
Configuration 3: non stretched single conductor.
This case has not to be considered for design if the
current path is B, see Figure 3.23; other loading will be
more important (wind speed). With current path C, a
significant force at the lower end of the dropper has to
be taken into consideration, Figure 3.24 due to the
forces induced in the droppers by the currents in the
phases. In the main bus, maximum horizontal
displacement and tensile forces caused by swing-out
and fall of span are similar to those known from spans
without droppers, but lower values with current path C
than with current path B, see Figure 3.34.
43
Figure 3.23 Droppers not stretched, current path B: horizontal and
vertical forces at the top of the insulator; case 5, of the
volume 2 Figure 7.1
Figure 3.24 Droppers not stretched, current path C: horizontal and
vertical forces at the top of the insulator; case 5, of the
volume 2 Figure 7.2
Configuration 4: non stretched bundle conductor.
With current path B this case needs no consideration in
design. With current path C, the pinch effect is in
addition to the effects between phases in case 3. The
pinch effect can be evaluated following IEC/EN 60865-
1 and then transferred to filter for ESL, see paragraph
6.3. The main bus behaviour is the same as in
configuration 3.
It should be noticed, that a non stretched dropper also
has an influence on the movement of the main bus
which is shown in Figure 3.34.
Configuration 5: stretched single conductor.
This is an important configuration observed in data
reference tests (tests *6, *7, 4 and 5 and [Ref 39]).
Some typical dropper stretch are studied in annex 8.1.
Applied load on structures can overpass 20 kN. The
load is a typical impulse load, maxima occur during the
stretching of the dropper, due to the movement of the
main busbar. The impulse come back with a frequency
connected to the main busbar swing frequency. Impulse
duration is about 0,1 s and depends on the dropper
length. The swing-out of the busbar is hindered, the
swing-out tensile force is nearly the same as that
without dropper, but the appearance of a fall of span
depends on the dropper length, see Figure 3.35. In
addition with current path C, the forces at the lower
ends of the droppers are to be minded.