The mechanical effects of short-circuit currents in - Montefiore

Figure 8.2
What is ESL in this case :
Assume that F(t) is the tension in the cable
connecting the equipment to another one or to another
flexible connection. F(t) could result from the
following effects 1) from pinch effect (if the
connection is a bundle), 2) from swing out or falling
down loads (horizontal connection between two
equipment), 3) from dropper stretch (if the equipment
is connected to another level with flexible
connection).
In the case shown, effect of F(t) and bending moment
at the bottom of the equipment is completely
different, it means that the inertial forces have clearly
some action. The design value is the maximum
bending moment at the bottom. We could have had
the same M max by applying ESL instead of F(t).at the
top of the equipments.
The situation could have been more complex if M max
had not occurred at the bottom of the apparatus.
How to apply the simplified method
The problem is to define the cantilever strength
needed for such equipment.
Figure 8.3 Laborelec 150 kV test structure for two-phase fault (Belgium).
We have extracted from CIGRE brochure N’105,
second part, the case 5 tested in Belgium, for which
we have access to both tension in the cable and
114
1) evaluate Ft, Ff and Fpi by IEC simplified method
2) search first eigenfrequency of the equipment (if
not known, use curve given in chapter 1),
3) evaluate ESL using synthetic curve given in
chapter 3 (one curve for interphase effects,
another curve for pinch effect), for example
ESL1 = max(Ft-Fst, Ff-Fst) x ESL
factor(interphase effect) and ESL2 = (Fpi-Fst) x
ESL factor (pinch). Then consider max (ESL1,
ESL2) = ESL
4) In case of initial static loading (Fst), just add Fst to
ESL and compare total value to cantilever
strength of the equipment.
Remark : If you have access to advanced calculation
method, you simply evaluate the dynamic response
and locate maximum dynamic bending moment in the
support, that bending is converted in equivalent top
static load, which is compared to the cantilever
breaking load. No need of ESL theory in that case.
b) On support structures
bending moment at the bottom of the support
structures.