The mechanical effects of short-circuit currents in - Montefiore

4. GUIDELINES FOR DESIGN AND
UPRATING
4.1. INTRODUCTION
Design guidelines for arrangements, mechanical loads
and electrical clearances are provided in [Ref 1]
together with detailed recommendation for rigid bus
arrangements and for flexible bus (eq: strained and
slack connection), droppers and jumpers subjected to
mechanical loads due to short-circuit currents. In
addition, suggestions for design criteria of support
structures can be found there. The object of this
chapter is to amend [Ref 1] recommendations due to
perceptions presented in chapters 2 and 3. Affected are
the design of droppers and the design of support
structures – mainly portals – where the implementation
of ESL adds new elements. Additional information will
be given for limitation of magnitude and duration of
short-circuit current and the treatment of foundations.
Last but not least recommendations for load cases and
safety factors are given (based on an international
inquiry) gave a wide range of possibilities, not always
meeting an economical solution.
4.2. LIMITATION OF MAGNITUDE AND DURATION
OF SHORT-CIRCUIT CURRENT
As mentioned in [Ref 1] many parameters influence
magnitude and duration of short-circuit current and
subsequently mechanical stress of substation
equipment.
Limiting the magnitude of short-circuit current may be
achieved by increasing the equivalent impedance of the
network. But for this modifications to the network
design or operation are necessary which obviously are
difficult to perform in existing networks. Nevertheless
the use of D.C. links, which separate networks for
short-circuit or the choice of uprated voltage level are
measures to decrease short-circuit current. Others
methods, though expensive, are to increase
transformer reactance or to decrease line numbers
(increase of the cross section of conductors) which
leads to a higher reactive energy consumption but as
well to a less degree of network stability. Opening of a
ring network could be an efficient alternative since it
allows electrical separation of some parts of the
network. Frequently adopted for HV networks this
method of action is also used for EHV installations
even if it decreases network safety.
Another factor which may influence the magnitude and
duration of short-circuit current is the performance of
the protection system. As an example the mechanical
stress of a flexible busbar may be substantially
influenced by a reduction of the clearing time. As a
matter of fact, diversity and complexity of existing
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installations do not allow adoption of a general
method. Each case must be studied separately in
collaboration with departments having expertise in this
domain. A comprehensive analysis is required to check
the operational coherence of the systems in order to
secure a satisfactory supply quality. But, if measures to
decrease mechanical effects are considered
improvement of protection systems should be regarded
as well.
4.3. FLEXIBLE CONDUCTORS
In [Ref 2] a simplified calculation method for flexible
conductors is given. However it is limited and has been
verified for the case of horizontal spans only. Whereas
in long strained spans the vertical extension is more or
less uncommon its a rule in slack connections of
different devices. Nevertheless formulas of [Ref 2]
may be used for both strained and slack conductor
spans up to the point where ∆h/l =1/4 (l length of the
span; ∆h vertical extension). If ∆h/l ratio is greater than
¼, the physical model of [Ref 2] cease to be valid and
recommendations and formulas of chapter 3.3 have to
be used.
On the evidence of eq. 3.13 there, the swing out of
droppers doesn’t depend furthermore on the magnitude
of short-circuit current. Only lc/K and K itself are
parameters in this equation which points out that
dropper geometry and the length of cable are of
essential influence. The flexibility of fixing points, if
they are definitely fixed, has just as much influence. In
the same way as a small change of the arc length in a
horizontal span leads to respectable change of its sag, a
slight flexibility of the upper fixing point (e.g. at the
end of a long span) leads to remarkable increase of the
swing out value calculated according eq. 3.13 with
corresponding effects to clearance.
Other design criteria for droppers are their length and
their position in the span. As described in chapter 3.3
the swing out of the span itself is influenced by this.
Consequently the forces on the anchor points of the
span vary. Tests at EDF showed that a decrease of 30%
of these forces is possible (see Figure 3.34, Figure
3.35). A simplified calculation method is not yet
available but advanced methods allow today safe
prediction of these phenomena.
Savings in the same order were found in the same test
series of EDF through the use of interphase-spacer in
long strained spans. This was not confirmed in
examinations described in chapter 3.7.2. Thus in these
cases as well, advanced calculation is necessary to
meet the effects of important parameters e.g. the spring
constant of the system.
The combined effect of wind pressure and short-circuit
on the dynamic loading is very complex on flexible