The mechanical effects of short-circuit currents in - Montefiore

3.3. THE DROPPER BEHAVIOUR
3.3.1. Introduction
Droppers are suspended conductors between the main
bus bars or switch-bays and apparatus, insulator support
or bushing. These are the typical situations B and E in
Figure 1.1. The dropper stress is generally very low in
initial conditions, i.e. less than 1 N/mm 2 , but can be
stretched during short-circuit mechanical effects. Either
due to main bus or switch-bay cable oscillations (in
such a case there is no need to have any current in it) or
simply due to electrodynamic forces if the short-circuit
current is going through it or in combination of both.
In addition to stretch, droppers are also submitted to
tensile force during swing-out, drop force and pinch
effect; (in the case of bundle configuration only). Due to
the very unleveled structure and very low tension in it,
dropper behaviour is quite different from the main bus
and corresponding effects cannot be evaluated by actual
IEC/EN 60865-1.
Droppers, generally more than one per span, and very
sensibly influences the main bus mechanical behaviour
limiting by, for example, the falling down effects. Their
position in the bus, moreover to practical electrical
aspects for design, can be chosen to limit the shortcircuit
effects in the main bus. But the consequence on
the dropper and corresponding loads applied at the
bottom of the dropper on apparatus must be carefully
evaluated.
Figure 3.20 details all kind of dropper effects used in
open-air substations and classifies six different
configurations.
dropper
fixed upper ends flexible upper ends
effects
between
phases
effects
between
sub-conductors
non
stretched
single bundle single bundle single
stretched
bundle
1 2 3 4 5 6
Figure 3.20 Case study for dropper configuration
With configurations 1 and 2 the current is always
flowing through the droppers. Configurations 3 to 5
occur when short-circuit current is going through the
droppers. When there is no current in the dropper, the
dangerous situation is limited to configurations 5 and 6
with stretched droppers.
In this chapter all these configurations will be evaluated
either by simplified, if possible, or advanced computation
method, which can be used in all
configurations. Experimental confrontations with
advanced methods are given in section 3.3.3, with
42
simplified method according to IEC/EN 60865-1 in
annex 8.3.
The final result will be to obtain the forces Ft and Ff in
the bus during swing-out and fall of the bus and its
maximum horizontal displacement as well as the load
Fds in the lower clamp of the dropper and its
corresponding ESL.
3.3.2. Typical oscillograms
a) Droppers with fixed upper ends (configurations 1 and
2)
A dropper can have a fixed upper end as connection E
in Figure 1.1. When connection B is fastened near the
insulator string, the upper end will move slightly due to
the movement of the span and can be treated as (nearly)
fixed. Both correspond to configurations 1 and 2 in
Figure 3.20.
In configurations *17 for single conductors and *18 for
bundled conductors, oscillograms for droppers with
fixed upper ends are given. Figure 3.21 shows an
example.
Figure 3.21 Oscillograms of horizontal components x and y of the
load applied to supporting insulator at the bottom side.
Case *17, p. 79, Figure 17.2
The droppers have no stretching because the two end
points are more or less fixed. Generally the
electrodynamic load can be directly seen on the force. If
the dropper length permits large movement, same
behaviour as a main bus can be seen, swing out and
falling down can occur, but the force value is much
lower due to initial very limited value. Some local
effects i.e. right angle at the top can induce some peaks
just after short-circuit inception.
The test configuration can considerably affect the
results. Especially for voltage level around 123 kV; that
is because basic frequency of apparatus are close to 50
or 60 Hz. This is true for test configurations *17 and
*18.
Bending stiffness of the conductors usually ignored has
also significant influence on the loads applied at the
fixation on the insulator at the bottom. The clamp on the
top of insulator support was very rigid so that general