1.1 Introduction
Copper had long been identified as the ideal
conductor of electricity, not only for power transmission and distribution but
also for connected power equipment such as generators, transformers, motors,
and switchgear. The metal was so much associated with the industry that any
resistive loss in a conductor was termed copper-loss. International Annealed Copper Standard (IACS)
established in 1914 had pegged the conductivity of pure Copper as 100% and the
conductivity of all other metals & alloys is still expressed as a
percentage of this standard conductivity.
Interestingly, even today, many consumers are prepared to pay a premium
to install Copper conductors for certain applications because of its perceived
higher reliability.
With the increase in demand for power, it became
imperative to search for an alternative material for the conductor to meet the
demand and offer cost benefits. The
volatility in the price of Copper hastened the need to find a substitute. Aluminium, an element that is abundantly
available on the planet, had also been identified as a good conductor of
electricity. In time, driven by cost
considerations, Aluminium, with a conductivity of 55% to 62% of Copper was
widely accepted as a replacement in overhead conductors, cables, switchgear
panels, and Busbar Systems.
The ascension of Aluminium to the position of a
conducting material did not happen overnight.
Industry and consumers had been reluctant to use Aluminium as a
conductor, based on their experience and feedback from the building
industry. There were failures due to
overheating, which in some cases, resulted in the outbreak of fire. These occurrences may have even been due to
poor connections at the devices.
Aluminium conductors were soft and yielded to contact pressures
resulting in high contact resistance and consequent development of hot
spots. Inadequate sizing and improper
installation of rising mains with Aluminium conductors in high rise building
also contributed to the material getting a bad name.
Many switchgear components were made compact
with terminals, suitable for receiving Copper conductors. When Aluminium conductors were terminated for
the same ampacity, hot spots developed in some cases. The problem has now been overcome. Many components are now designed to receive
Aluminium conductors or provide with adapter terminal plates to overcome this
situation.
The markets, however, could not ignore the
relative costs of Copper and Aluminium. Countries,
where Copper was imported but had large deposits of Aluminium, actively
encouraged the use of Aluminium by imposing tariffs. Though the pure form of Aluminium did suffer
from many shortcomings as a replacement for Copper, thanks to continuous
research in the metallurgy and alloy composition, heat treatment, development
of Aluminium welding and plating processes, Aluminium has been established for
many years now as a viable alternative to Copper for several applications.
Over the years, markets have divided the zone of
usage of Copper and Aluminium as a conductor material in various products. The overlap is not significant.
Presently the conductor & enclosure material
for an isolated phase bus is entirely made of Aluminium. As the magnitude of current increases, as in
the case of generator connections and high current open bus installed in Aluminium
smelters, Magnesium extraction plants, and similar process industries,
Aluminium has a distinct overall techno-commercial advantage over Copper. Most of the switchyard rigid conductors are
fabricated with Aluminium alloy extruded tubes as they are eminently suitable
for the application. In Chlor-alkali and
metal refining plants, Copper busbars are used inside the process plant, where
a corrosive atmosphere exists because of the presence of Chlorine, and
Aluminium busbars are used in outdoor areas such as transformer to rectifier
connections. (In the past, due to the
acute shortage of Copper and Aluminium (and its alloys), Silver was used as
conductor material. This practice has
now been discarded.)
Copper clad Aluminium (CCA) has been introduced
into the market by several manufacturers, worldwide, for use as busbar material. It uses the skin-effect to its advantage. The thickness of Copper cladding can be
customized to meet the specific requirement.
Manufacturers claim that, in general, it is 30 to 35% less expensive
when carrying a current of 86 to 88 % of the Copper conductor of the same size. Coppre clad Aluminium conductors are economical
only for AC power distribution.
Titanium, Zirconium, and stainless steel clad
Copper busbars are used for special purpose applications to provide resistance
to corrosion. They find application in
electrolysis & electrowinning plants.
Silver & Gold are still used as conductors
in printed circuit boards and connectors in the electronics industry.
Steel has been used as a return conductor in
metro rails and some electric overhead travelling (EOT) cranes. Galvanized steel is still extensively used as
an earth conductor in buildings, plants, and switchyards. It has also been used as a down conductor for
lightning protection in various buildings, plants, and industrial complexes.
Discussions in this book will be limited to the
use of Copper and Aluminium conductors in Busbar Systems.
Copper or Aluminium conductors installed in Busbar
Systems will meet the industry requirements if they are optimally
designed, professionally installed, and maintained. If operated within the mechanical,
electrical, thermal, and environmental limiting parameters, no conductor metal
can claim superiority over the other. It
is for the designers to study their properties and ensure proper selection of
the conductor material. Project
procurement and running costs of the Busbar System that meets the
specification requirements shall decide the material of the conductor. If a client has a specific preference for a
given conductor material (for whatever reason) and is willing to, perhaps, pay
a premium, the choice is his.
The selection of conductor material is dependent
upon application, ambient environment, and system parameters among other
project-specific requirements. Some of
the properties that need to be considered before selecting the material are
detailed below:
1.1.1 Resistivity / Conductivity
Losses are proportional to the resistance in
transmission & distribution. Lesser
resistance will also result in a lower voltage drop in a DC distribution. (In the AC distribution, the other major
contributing factor to the voltage drop is the reactance that depends upon the
spacing between conductors and the system frequency).
Lesser power loss also translates to a lower
running cost, which may be capitalized by the project authorities while
evaluating a bid. Lesser power loss also
implies lower heat generation. Many
electrical product designs are limited by operating temperature either because
of safety, withstand capability of the insulation, or the softening temperature
of the conductor.
1.1.2 Mechanical
Strength
A conductor and its support system need to have
sufficient, mechanical strength to withstand electro-dynamic forces arising due
to high short circuit currents. In
outdoor exposed conductors, as in switchyards, it must also withstand the load
due to snow, ice, wind, and seismic activity.
1.1.3 Fatigue
Strength
A conductor will be subjected to cyclic stress
at the joints, due to variation in ambient temperature & load and vibration
& oscillation of the conductor due to the wind. Wind can cause low amplitude high-frequency
oscillation called aeolian vibration.
Wind can also cause a high amplitude low-frequency oscillation called galloping
in strung conductors. A conductor
material with higher fatigue strength will have a longer life. Fatigue strength is expressed in terms of the
number of cycles.
1.1.4 Creep Resistance
To make a good bolted joint, it is necessary to
keep the two mating surfaces under constant pressure. Creep is a phenomenon wherein a metal suffers
a permanent-set even when the stress is within the elastic limit of the
metal. A metal with high creep strength
will offer better resistance to permanent set.
The creep, if left uncompensated, can result in the joint becoming
loose, generating a hot spot, and may result in failure.
1.1.5 Surface Hardness
High surface hardness results in higher fibre
stress withstand capability. This is of
vital importance when the conductor is subjected to large forces between two
support. A higher surface hardness will
facilitate a larger span. Higher surface
hardness will also reduce creep.
1.1.6
Good Dimensional Stability
Conductors, when extruded, must be straight,
flat, and devoid of twists.
Manufacturing tolerances are defined in the Standards. This is of significance when long sections of
conductors are installed in a factory-built assembly. This depends on the process of extrusion
(fabrication) of the Aluminium & Copper conductors. Individual section length in Busbar
Systems of up to 12 meters (40’) is factory built for assembly at the
site to reduce the installation cost and time.
The jointing method should not induce stress along the conductor length
or at its support locations. This is
very critical when conductors are specified, hard.
1.1.7 Resistance to Corrosion
The gradual disintegration of conductor material
takes place due to corrosion during its life span. Corrosion are of different types depending
upon the atmospheric conditions, material, and method of connection, and
several other factors. Corrosion can be
eliminated or minimized by many processes that include painting, coating,
sleeving, plating, galvanizing, and cathodic protection. It is necessary to understand the mechanism
of corrosion in specific circumstances, to take preventive action.
Corrosion normally
commences on the exposed surface and gradually bites through the material. Corrosion can also be initiated from crevices
and spread to the surface.
Corrosion is a chemical
or an electrochemical phenomenon that occurs between the metallic component and
the surrounding atmosphere. A very brief
description of different types of corrosion, relevant to the subject, is
explained without going into the chemistry of the mechanism.
1.1.7.1 Oxidation
Both Copper and Aluminium conductors will
oxidize when exposed to the atmosphere.
Oxidation of Copper, though comparatively slow,
nevertheless forms an oxide layer. The
coat is not impermeable and does not prevent further oxidation. Consequently, the oxidation process will
continue until the entire metal gets oxidized and disintegrates. Copper oxide
is soft and flakes and therefore susceptible to contact pressure. An interesting aspect of Copper oxide is that
it is semiconducting and therefore, a joint made with poor surface preparation,
might initially show a decrease in contact resistance with time as oxide
spreads to a large area.
Oxidation of Aluminium is much faster than that
of Copper. However, once the oxidation takes place and a thin oxide layer/film of
the order of 10 nanometers (4 x
Should there be an arc on the conductor,
Aluminium will melt earlier than Copper because of its lower melting
point. This will result in oxidation of
the molten Aluminium that will generate more heat due to the exothermic
reaction, resulting in a run-away process.
1.1.7.2 Chemical
Corrosion
Copper and Aluminium conductors will react in a
toxic environment when exposed to oxides of Sulphur
1.1.7.3 Galvanic Corrosion
When two dissimilar metals are immersed in an
ionized salt solution, they form a Galvanic cell. Corrosion will take place when there is a
conducting path for the current to flow.
This form of corrosion is the most onerous and leads to fast
degradation/disintegration of the metal.
The speed of corrosion depends, among other parameters, the
electro-potential difference between the two metals.
Anodic electro-potential is defined relative to
a noble metal Gold and is termed as Anodic Index. Electro-potentials of the metals, relevant to
Busbar
Systems, are detailed in
Table – 1.1.
Anodes and cathodes are relative terms. In the presence of an electrolyte, the anode
will get dissolved and get deposited on the cathode. It is suggested that two different metals may
be placed in direct contact if the anodic index difference between the two does
not exceed 0.1 Volts. A relatively large
area of the anode reduces the current density and decelerates corrosion.
(Galvanic corrosion can also occur when large
metallic objects are buried in the soil.
The rate of corrosion, among other parameters, will depend upon the soil
resistivity. Under such circumstances,
objects can be protected by placing sacrificial anodes at strategic
locations. Additionally, the object can
be raised to a predetermined potential to nullify the current and the process
is called the cathodic protection.)
1.1.7.4 Crevice
Corrosion
Crevice corrosion takes place even between like
metals due to the variation in the Oxygen concentration exposed to an
electrolyte. The formation of a local
cell results in a corrosive attack in the Oxygen-starved area. At the deep end of the crevice, the
concentration of Oxygen will be less than at the exposed end. Corrosion can be expected at the remote
end. Such crevices can be a natural
defect during the manufacture of conductor or man-made during manufacture and assembly.
1.1.7.5 Pitting
Pitting is a phenomenon wherein, on the surface
of a metal, a small area becomes anodic, a vast area remains cathodic and a
galvanic reaction takes place. This
results in the formation of holes on the surface.
1.1.8
Thermal Conductivity
Good electrical conductivity is normally
associated with good thermal conductivity.
A typical length of a conductor may have several connections in series
such as bolted & welded joints, laminates, interfacing equipment
(transformers & switchgear) terminals, generating different quantum of heat
at these locations. Higher thermal
conductivity will smooth out the temperature rise along the length of the
conductor and reduce the hot spot temperature.
Certain types of Busbar Systems, such as the
sandwich bus rely upon conduction for heat evacuation across the
conductor. For such applications, the
thermal conductivity of the conductor is an important factor.
1.1.9 Painting, Coating, and Plating
The busbar material should permit the above
processes to be carried out on it, with ease.
Painting, coating & plating of conductors requires interaction with
chemicals and sometimes at elevated temperatures. Painting, coating & plating must result
in good adhesion and must not react with the conductor material.
1.1.10 Weldability
This is a very important requirement for a
busbar. Welded joints are the most
reliable joints in a Busbar System. This operation is most critical in the
manufacture & installation of isolated phase bus in power plants,
gas-insulated bus in substations & transmission lines, and high current
open bus in metal extraction plants.
Welding in the factory is a more controlled operation, often carried out
by robots or manually using jigs, fixtures, and manipulators. Welding at the site is more challenging.
1.1.11 Workability
Many conductors are fabricated out of sheets and
extruded sections. The fabrication
process involves operations such as rolling, bending, shearing, forming,
drilling, punching, and machining. Such
operations are carried out on larger current-carrying conductors. Several conductor sections & accessories
are standardized and are mass-produced in machining centers.
1.1.12 Low
Weight
A light-weight conductor is easier to
install. It makes the task of assembling
at works and at the site, that much easier.
The lighter weight will also reduce the cost of supply and installation
of the support structure.
1.1.13 Low
Procurement Cost
The market will encourage a metal for the
conductor as long as it is cost-effective and delivers the intended performance
without any compromise. With a
specification that is well laid out to meet the project requirement,
procurement, and running costs of the product with a given conductor material
well established, the cost-effective option will remain (and rightly so) the
criteria for the selection of the conductor material.
How each of these properties is considered in
the design of different types of Busbar Systems and processes, will
be discussed in the succeeding chapters.
Copper conductors used in the electrical
industry are pure with very little impurity content. Copper is not alloyed with other elements for
use as an electrical conductor.
Mechanical properties are enhanced by heat treatment and cold working.
Over the years, significant developments in the
alloying techniques and heat treatment process have brought out a wide range of
Aluminium alloys for different applications.
Pure Aluminium has a conductivity of 65% IACS and commercially available
pure Aluminium has a conductivity of 61% IACS.
Aluminium alloy with vastly improved mechanical properties with
marginally less conductivity of 56% is used as a conductor for most
applications. (Alloys have been
developed with very high mechanical strength and conductivity as low as
30%. These are used in the manufacture
of the support structure.)
Copper is drawn and Aluminium is extruded to
different shapes for varied applications.
Complicated profiles are sliced and assembled in the switchgear
industry. Hollow profiles have been used for water-cooled conductors. Profiles with fins have been used for better
heat dissipation. The most commonly used
profiles for conductors are flats, channels, and tubes. Channels can be formed or extruded depending
upon the application. Aluminium sheets
are rolled into cylindrical conductors for use in an isolated phase bus.
Continued..........
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