The mechanical effects of short-circuit currents in - Montefiore

structures. Advanced simulations are recommended in
such cases.
4.4. SUPPORTING STRUCTURES
One of the recommendations in [Ref 1] is to calculate
static loads of supporting structures first and then to
proceed with short-circuit effects. Although it is
common to consider short-circuit load in two lines only
while having in the third line the static load (at threephase
short-circuits Fpi is assumed to be at it’s
maximum in two lines at the same time only) shortcircuit
levels nowadays lead to the situation that shortcircuit
is the governing load if other severe exceptional
loads as extreme wind or earthquake are absent. The
new awareness’ of resonance effects in steel structures
which are expressed by ESL-factors up to 1.4 (see
chapters 3.5 and 3.5.4) are intensifying this effect.
Economical solution may be obtained to this day if
calculation models of Eurocode 3 are put opposite
precise determination of short-circuit load. Whereas for
static (normal) loads a linear material model still is
recommended, the (exceptional) short-circuit load may
be regarded in a non-linear material model using
plastic zones, and the plastic hinge theory as [Ref 2]
does for rigid busbars right from the start.
4.5. FOUNDATIONS
Forces which affect structures of AIS are transmitted to
the soil by foundations usually designed in accordance
with current standards of civil engineering, which
solely recommend static calculation methods. As long
as those forces are in a vertical direction, sufficient
soil withstanding pressure is the only design criterion.
Having horizontal forces like short-circuit current
forces, and provided that soil conditions are in a
normal range, the proof of stability gets to be the most
important one. Due to the dynamic character of shortcircuit
force and the static calculation methods; ESL is
the governing factor and not the maximum value of
short-circuit current force. This ESL respectively ESLfactor
are not checked for foundation until now but
contrary to chapter 3.6 where ESL factors are given
for steel structures, ESL factors for foundations are
assumed to be much smaller than one because
foundations are heavy and inert structures. Thus shortcircuit
current forces are of minor importance for
foundation design especially if other horizontal forces
as cable pull and wind or earthquake have to be
considered. In other words we means that the dynamic
mechanical effects of short-circuit on foundation can
be neglected.
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4.6. SAFETY FACTORS AND LOAD COMBINATIONS
In a substation external forces apply on the structure
and cause internal stresses and forces. The design for
mechanical strength requires not only the exact
knowledge of the forces and their effects, but also
analyses regarding the statistical probability of several
events coinciding with regard of the rigidity and
ductility characteristics of the elements. Under the
aspects of optimum technical and economic efficiency,
therefore, it is necessary to lay down requirements
which assure the reliability of equipment throughout
their whole service life and prevent any danger to life
or limb. While taking into account knowledge gained
from the building of switchgear it is also possible to
refer to existing civil engineering standards for this
purpose, such as IEC Publications, European
Standards, National Standards, etc. In the following,
the safety factors and the load cases are defined and the
assumed loads and permitted stresses in substations are
described.
According to [Ref 18], an action is defined as
− a force (load) applied to the structure (direct
action)
− an imposed deformation (indirect action); e. g.
temperature effects or settlements.
These actions are classified
a) by their variation in time:
− permanent actions
− variable actions
− accidental actions
b) by their spatial variation.
The design value Fd of an action is generally defined
as
(4.1) F d = γ F F k
where γF is the partial safety factor for the action
considered, taking into account of, for example, the
possibility of unfavourable deviations of the actions,
the possibility of inaccurate modelling of actions,
uncertainties in the assessment of effects of actions and
uncertainties in the assessment of the limit state
considered. Fk is the characteristic value of the
applying actions. Characteristic values, in general,
correspond to a fractile in the assumed statistical
distribution. If the necessary basic data is lacking, it is
also possible to use deterministic limit values for Fk.
In the same way as the effect, it is also possible to
specify a fractile for the materials that will be subject
to stress. A partial safety factor γM is added to the
characteristic value Xk to ensure that any chance
deviations in the resistance of the material or the
geometrical dimensions are safely taken into account.
Characteristic values are specified by relevant