The mechanical effects of short-circuit currents in - Montefiore

3.5. LOAD ON PORTALS, APPARATUS TERMINALS
AND EQUIVALENT STATIC LOAD
3.5.1. Introduction
These loads may be summarized as follows:
1) Ft, Ff typical low frequency, quasi-steady loads, see
chapter 3.5.4 for ESL
2) Fpi loads due to pinch effect, typical impulse load.
See chapter 3.5.3 For ESL
3) Fds loads due to dropper stretch, typical impulse
load. See chapter 3.5.2 for ESL
Typical apparatus have a range of frequencies mainly
dependent of voltage level, following the table given in
[Table 1.1 in the introduction]
As a global overview, let’s keep in mind that :
- typical insulator support may have very wide range
of their first eigenfrequency and may cover all the
range between 4 and 60 Hz. Lower voltage
corresponds to higher frequency.
- typical portal structure have very low
eigenfrequencies, in the range 1 to 6 Hz. Voltage
correlation is very low.
- typical apparatus (isolator, circuit breaker,
measuring transformer, surge arrester) have very
low eigenfrequencies due to their high mass located
on the top of it. Range 1 to 6 Hz. The higher the
voltage, the lower the frequency.
Design load on these structures have to be deduced from
instantaneous top load applied by the cable connections
(Ft, Ff, Fpi and Fds). The difference between these loads
and design loads is given by ESL factor given in this
chapter. ESL factor depends on the structural dynamics
as explained in appendix 1.
Such ESL factor is only of interest if using simplified
method to evaluate Ft, Ff and Fpi for example to
calculate the design load for a supporting structure at
the lower end of a dropper or if the forces Ft, Ff and Fpi
on the main conductors of a horizontal strain bus
connected to portals (case A in Figure 1.1) are estimated
according to IEC60 865. In contrast, IEC 60865 gives
the ESL for the design of a slack bus connection (case C
in Figure 1.1), see Annex 8.3.1 and Part I in volume 2.
The design load can obviously be evaluated directly by
advanced method. But such evaluation can be
cumbersome, need software, and give only accurate
results if you are using accurate data's.
In the suggested simplified method using ESL factor,
some conservative approach has been taken in the
synthetic curves given in this chapter.
It is recommended not to add any safety factor over the
calculated values.
3.5.2. Maximum instantaneous load and equivalent
static load for dropper stretch
The definition of ESL factor is explained in annex 8.1.
58
There are two different curves to be used for design and
concerning ESL factor.
The first one in Figure 3.50 concerns the dropper stretch
(in span dropper, single conductor or close bundle), this
is an envelop curve obtained from many tests results
and simulations. The dropper stretch load shape is a
typical triangular excitation, the duration of which is
depending on its overlength compared to the distance
between its two extremities. But because of its upper
fixation to cable structure, the duration of the stretch,
even if short, is generally in the range0.05 to 0.2 s.. It
means that it is a quasi-static load (ESL factor = 1) for
frequencies higher than about 50 Hz , but higher than
one in the range 5-30 Hz and lower than one for lower
frequencies. This is quantified on Figure 3.50.
Figure 3.50 ESL factor for typical dropper stretch, envelop curve.
Abscissa is the first eigenfrequency of apparatus or
bushing or insulator support. The abscissa are not at all
distributed linearly.
You just need the maximum stretch force in the cable
near the clamp (evaluated by advanced method or
simplified method developed in this brochure; no
simplified method for in span droppers is given in this
brochure). Then following the basic first frequency of
your apparatus, you deduce the design load by
multiplying the maximum stretch force by the
coefficient obtained from the curve. The obtained load
must be compared to cantilever breaking load of
corresponding apparatus.
3.5.3. Maximum instantaneous load and equivalent
static load for bundle pinch.
The second one in Figure 3.51 concerns bundle pinch
and is valid for all bundle configuration, this is an
envelop curve of many tested cases.