The mechanical effects of short-circuit currents in - Montefiore

2. RIGID BUSBARS
2.1. INTRODUCTION
Rigid busbars are used at all voltage and current levels
and distinguish themselves in need of less space in
contrast to busbars with flexible conductors. Conductors
used usually have tubular or rectangular profile and are
mounted on supporting structures mainly composed of
insulators and steel structures.
In the brochure No 105 [Ref 1], the electromagnetic
forces on the conductors in one plane are derived and
discussed, and their dynamic effects on the
arrangements are shown. This section continues the
presentation on rigid busbars and is divided in two parts.
In sub-section 2.2, the simplified method stated in the
IEC Publication 60865-1 [Ref 2] and the European
Standard EN 60865-1 [Ref 3] is described and shown
that a permitted use of plastic deformation allows a
higher loading. In sub-section 2.3, special
configurations are regarded such as parallel busbars,
associated phase structures on a common support, the
influence of two busbars.
2.2. SHORT-CIRCUIT STRENGTH ACCORDING TO
IEC 60865-1 AND EN 60865-1
Advanced methods allow to evaluate time histories of
stresses, forces and displacements caused by shortcircuit
currents with high accuracy in any structure of
substations with rigid conductors. But this needs many
detail data for input. Very often busbars are built with
conductors side by side in one plane. For this purposes,
simplified methods are very useful. The short-circuit
strength of three-phase systems and single-phase twoline
systems can be established with the IEC Publication
60865-1 [Ref 2] and the European standard EN 60865-1
[Ref 3]; both have an identical wording.
The method used with rigid busbars had been described
first in 1955 by W. Lehmann [Ref 4] and in 1961 it had
been taken over in the first edition of the German
standard VDE 0103 [Ref 5]. New investigations had
been dropped in the following editions. In 1986, the first
IEC Publication 865 [Ref 6] had been published and in
1993 the actual standard IEC/EN 60865-1 [Ref 2, Ref 3]
was approved.
This method stated in the standard determines the shortcircuit
stresses and forces with the rules of statics and
the dynamic of the system is taken into account by
‘dynamic factors’. The permission to restrict stresses not
only to elasticity of the material but also to allow plastic
deformations makes possible a better utilisation of the
material and therefore a more economical design.
The derivation of the method and the background are
written in many publications and internal reports in
10
which, however, only aspects of the method are given.
A complete presentation has not been available until
now. To explain the background of the standard it is
necessary to look at the totality of all substations with
different types of conductors, supports and so on. That
means all voltage levels have to be regarded: High
voltage HV as well as medium voltage MV and low
voltage LV.
2.2.1. Method
The electromagnetic forces on the conductors caused by
the short-circuit currents in the conductors act on
arrangements, which are called supporting framework
made from elastic beams. The time dependent current
forces induce oscillations, and there are maximum
values of effects in certain locations at certain times:
maximum deflection, maximum stress in the conductor,
maximum forces on sub-structures. If the last two
mentioned force are known, the short-circuit strength of
an arrangement can be usually assessed with sufficient
accuracy. Normally only the values of the maxima are
needed and not the location or time.
Therefore IEC/EN 60865-1 calculates maximum values
for the conductor stress including its strength and the
forces on the substructures; the maximum deflection is
not of interest. The method is adjusted to practical
requirements and contains simplifications with safety
margins.
The calculations are done in two steps according to the
rules of statics:
- The maximum of the short-circuit current force
acts as a static force F on the conductors and
from this the relevant quantities are calculated.
- The dynamic response of the structure due to
the time dependent excitation is described by
the factors :
response caused by dynamic load
(2.1) V =
response caused by static load
- They are different for conductor stresses
and forces on the substructures. With these
factors the static calculated quantities are
multiplied to get the actual dynamic
response of the structure.
These steps can also be read as follows: The static force
F is multiplied with the dynamic factor V and this force
V⋅F acts as an equivalent static load ESL on the
arrangement and gives the relevant quantities for design.
Remains be proven whether the maximum permissible
stresses are exceeded or not.
There has been a great effort to get the factors V: Tests
with actual arrangements, analytical and numerical
calculations were required.