The mechanical effects of short-circuit currents in - Montefiore

1. INTRODUCTION
1.1. GENERAL PRESENTATION OF THE BOOK
CONTENT
This brochure is a companion to CIGRE thematic
publication 105 published in 1996 [Ref 101] on the
same subject.
Publication 105 can be considered as a very useful
tool for defining the problem of short-circuit
mechanical effects in substations with rigid or
flexible busbars. The content has been transposed
to IEC recommendation 60865 [Ref 2].
This book is divided in several parts. Some of one
being considered as a definitive proposal (like rigid
conductors), others are completely new approach
(like equivalent static load in flexible conductors),
considered as enough mature to be included in IEC
and others are proposal for future trends
(probabilistic approach).
In the past only the static behaviour of the support
has been taken into account. Recent development
concerns a more comprehensive approach of bundle
pinch effect, dropper stretch and other features and
their true impact on design as dynamic loading.
The new approach for flexible busbars is closely
related to the dynamics of structures. The reader,
not expert in that field, will find in annex 1 some
basics.
This brochure is focused on dynamic effects and
corresponding methods to define actual design
loads for substations. A very good tentative has
already been described in brochure 105 [Ref 101]
as far as it concerns rigid busbars (see also chapter
2 hereafter). However, flexible busbars had not
been evaluated in a similar way. The existing
methods define maximal tensile load in the
conductor but there are no detailed methods to
define corresponding design loads. In a sense these
loads have to be considered as acting in a quasisteady
regime, which is not true in most of the
actual cases.
Figure 1.1 Flexible bus configuration for calculation and tests
A) horizontal strain bus connected by insulator
chains to steel structures
B) vertical dropper between strained bus and
apparatus
C) horizontal connection between components
D) jumper connecting two strained conductor
sections
E,F) end-span droppers (classical or spring loaded).
5
The Figure 1.1 details typical flexible busbars
layout. Only case A has been evaluated in the
former brochure. And even in the treated cases, the
evaluation was restricted to estimate tension in the
main bus (A on the Figure 1.1) in case of single and
bundle conductors. Maximum tension in the cable
is not necessary a design load for the supporting
structure.
Annex 8.3 details what is possible to evaluate using
former brochure 105.
Moreover, to evaluate correctly the design load, we
will also consider new cases, like dropper (case B
(in-span dropper), E (end-span dropper), D
(jumper) ).
The relationship between forces and design loads is
strongly related to dynamic behaviour of structures.
As power engineers are not generally familiar with
such problems, some detailed informations will be
given.
The content of the brochure is divided in 6
chapters, one annex, references and a detailed new
data base on short-circuit tests (new cases
supplement brochure 105, volume 2).
Chapter 2 deals with rigid busbars. In the 105
brochure [Ref 101], the electromagnetic forces on
the conductors are discussed and their dynamic
effects on the arrangements shown. In this paper,
first the simplified method stated in the IEC
Publication 60865-1 [Ref 2] and the European
Standard EN 60865-1 [Ref 3] is described and it is
shown that a permitted use of plastic deformation
allows a higher loading. In the second part, the
calculation of forces and stresses in special
configurations is derived such as associated phase
structures busbar on a common support, parallel
busbars.
Chapter 3 deals with flexible bus systems.
Chapter 4 is giving extra recommendations (see
also brochure 105) for design and uprating.
Chapter 5 is giving a new trend in the design
approach, taking into account probabilistic aspects.
There is not yet a consensus of using such methods
mainly because of the lack of data's needed to use
them. However, they can bring substantial benefit
in substation design and such approaches have
merit to be presented and this brochure is
addressing them.
In power plants and substations, dc auxiliary
systems are used to supply motors, measuring
devices and other kind of loads. Power converters,
storage batteries, smoothing capacitors and motors
can cause high short-circuit currents which lead to
high mechanical and thermal stresses. Therefore in
chapter 6, the variety of short-circuit currents
occurring in dc configurations are presented and