The mechanical effects of short-circuit currents in - Montefiore

3. FLEXIBLE BUSBARS
3.1. INTRODUCTION
In the past, extensive tests had been carried out
to study the behaviour of arrangements with
flexible conductors in substations and to evaluate
methods for design purposes. At first simplified
methods for calculation by hand had been
developed for typical cases and later on
analytical and numerical methods or a
combination of both on computers. Nowadays
two different methods are used depending on the
task, the situation and the arrangement:
simplified and advanced methods.
Simplified methods allow calculations by hand or
personal computers [Ref 7] with analytical
equations and figures. They are helpful for
typical design cases with single or bundled
conductors. Only general input data are
necessary and the results are maximum values of
tensile forces and displacements. The procedures
are adjusted to practical requirements and
contain simplifications with safety margins.
Parameter sensitive investigations can be done in
a very short time.
Advanced methods use finite element or finite
difference modelling and powerful software is
available for workstations and personal
computers. They can be applied to any structural
configuration with single and bundled
conductors. Detailed modelling and basic
structural data are necessary. The computation of
the dynamic response of the complete structure
including their non-linear behaviour is possible
and accurate results can be obtained. The results
are limited only by the degree of detail in the
modelling and the availability of reliable basic
structural data. The calculation of
eigenfrequencies, time histories of forces,
moments and deformations allow to study the
system behaviour, to detect and improve weak
points or to ascertain the short-circuit strength
even for complex cases. Advanced methods will
always be very helpful to better simulate actual
loading, to limit safety factor used for inaccurate
evaluation, to evaluate special cases, actual
geometry, etc. Also the range of validity of
simplified methods can be investigated.
In the International Standard IEC 60865-1 [Ref
2], which is identical with the European Standard
EN 60865-1 [Ref 3], a simplified method is
stated for the calculation of maximum values of
the
– short-circuit tensile force Ft during or at the
end of the short-circuit current flow,
– drop force Ff when the span falls down from
its highest point of movement,
36
– pinch force Fpi in the case of bundled conductors,
when the sub-conductors clash or reduce their
distance without clashing,
– horizontal displacement bh during the swing out
of the span.
The physical background of the method, the
assumptions and the derivation of the equations are
given in detail in sections 4.2 and 4.3 of [Ref 1]. The
good agreement of the calculation according to IEC
60865-1 with test results is shown in Annex 8.3 IEC
60865-2 [Ref 35] gives calculation examples.
Figure 1.1 shows seven different arrangements of
flexible conductors which are predominant in
substations; they are described in detail in clause 1.4
of [Ref 1]. IEC/EN 60865-1 [Ref 2, Ref 3] is
applicable to the following horizontal spans in sideby-side
configuration:
– Strain bus connected by insulator chains to steel
structures (case A) without droppers in the span
(case B).
– Strain bus, when the droppers are close to the
insulator chains. The droppers may be neglected
in the calculation.
– Connection between components or horizontal
slack bus connected to support insulators (case
C).
– Inclining span (case G).
In the standard, the span length is restricted to l ≤ 60
m. However, the calculation is in good agreement
with test results, even for strain bus with l > 60 m,
see Annex 8.3.
As far as new developments concerned are concern,
compared to IEC/EN 60865-1 [Ref 2, Ref 3], and
CIGRE/IEC Brochure [Ref 1], there are some more
steps to evaluate for understanding of the
phenomena and getting new rules for design:
– the effects caused by droppers,
– the bundle pinch effect including spacer
compression,
– the definition of equivalent static design loads
taking into account structural dynamics
– and special problems.
First, Section 3.2 gives a survey of typical
oscillograms of forces in substations and the
application
foundations.
of advanced methods including
Section 3.3 deals with the behaviour of the droppers.
It exemplifies by typical oscillograms and gives the
results of advanced calculations. Calculations for a
strain bus with dropper in the span (case A and B in
Figure 1.1) as well as vertical droppers (cases E, F)
cannot be actually done by applying IEC/EN 60865-
1. Section 3.3 shows how to extend the method
based on IEC/EN 60865-1 and new procedures are
evaluated.