The mechanical effects of short-circuit currents in - Montefiore

The numerical resolution of these equations leads to the
evaluation of the compressive force and of the minimum
length of the sub-conductor, which is still free.
The use of such method, combined with evaluation of
pinch give access to the following results (Figure 3.67)
on 28 known tests (case 8 of volume 2 of this brochure).
What is clear is that Manuzio formula is correct for low
spacer compression (the first dots are coming from
Manuzio tests) but deviate up to 25% for other cases.
The new method , described in this paper remains in the
range of measurement precision. One dot is clearly out
of the range, probably connected to measurement error.
The main problem of Manuzio formula is coming from
the systematic under-evaluation of spacer compression,
which cannot be accepted for spacer design.
Most of the validation cases are in the range of overhead
line practice (30, 40, 60 and 120 meters), more results
would be needed to extend our proposal to substations
range, especially if we have no contact between
subconductors. Nevertheless, we can point out the good
correlation for 10 m subspan length.
Of course more complex situations, like asymmetric
bundle, very wide bundle, and other range than those
used for validations can be evaluated by complex
advanced methods, as used in [Ref 54]). But in most
practical cases, this method can be used for evaluating
calculated values (kN)
20
18
16
14
12
10
8
6
4
2
0
spacer compression
(measurement vs calculation)
0 5 10 15 20
measured values (kN)
67
spacer compression, then replacing short-circuit tests by
equivalent mechanical test in a classical lab.
When designing a spacer or a spacer damper for
overhead line, not only the short-circuit level is
important, but also the initial value of the tension in the
conductors (as well as subconductor spacing,
subconductor diameter and subspan length). The same
short-circuit on the same structure may have very
different effects depending on short-circuit time
inception (driving asymmetry of the current).
Important data to be used for IEC evaluation of Fpi
The use of this method will need a data in relation with
tower stiffness. The proposed method to evaluate spacer
compression need a heuristic value of 100 N/mm for
each support instead of actual stiffness, used in other
part of IEC60865 (see annex 8.3) to evaluate Fpi for
example.
This relatively low value compared to actual tower data
is in relation with a “dynamic stiffness” (which should
include the dynamic behaviour of the support and
anchoring insulator chain in such a dynamic excitation,
more close to an impulse).
+10%
-10%
-25%
new method
"Manuzio"
Figure 3.67Comparison of spacer compression between tests and simplified method