4.3
Jointing
There are different methods of jointing of
current-carrying conductors in Busbar Systems. The most commonly used method in the order of
usage is welding, bolting, and clamping.
Riveting & brazing have almost become obsolete with the advancement
in welding technology. In an enclosed
bus system, it is suggested that wherever it is possible to make a welded
joint, it should be opted for, since each bolted or clamped joint, apart from
being expensive, would require access for maintenance. (Welding of an Aluminium conductor is far
easier than welding of a Copper conductor.)
A busbar joint is efficient if the resistance
across it is less than or equal to the resistance of an equivalent length of
the busbar, without the joint. (In isolated
phase bus, one of the methods of establishing these criteria is to compare the
voltage drop across a welded section and a non-welded section of equal length,
with an injected current of approximately 1000 A. This is done for both conductor and enclosure.)
In an isolated phase bus, the most common method
of jointing
of the enclosure as well as the conductor is by welding. Bolting is resorted to only at the
termination ends. In the non-segregated and segregated
phase bus, the method of jointing conductors of adjacent sections and
termination is by bolting. In the sandwich
bus, the method of jointing conductors of adjacent sections is by clamping
while bolting is carried out at terminating ends.
Outdoor switchyards use tubular conductors. Welding and clamping are extensively
employed. The welded area must be
dressed to ensure that it does not contribute to corona discharge. Many, outdoor equipment, have Copper
terminals and may be required to form connections to an Aluminium busbar. Bolting and clamping can form an effective
Copper to Aluminium, joint.
Clamps are also used for holding the parallel
conductors of the same phase. This is
different from the clamps used for current transfer and act as current
equalizers. Such clamps are necessary to
limit the stresses and deflections between the conductors of the same phase
during short circuit forces. Such clamps
are also used on bundled conductors in transmission lines and are referred to
as spacers.
4.3.1 Welding
4.3.2 Bolting
Where it is not possible to carry out welding,
bolting is the next preferred method of busbar jointing. These joints have been in the industry,
giving satisfactory performances, in onerous service conditions, for a
sufficient length of time to give confidence to engineers. An efficient joint must have a low contact
resistance, maintain its integrity during its life of the joint by containing
the effects of corrosion, fretting, thermal cycling, differential expansion,
vibration, and fatigue.
The joint resistance Rj can be split into two components
Rj = Rs + Ri
Where Rs
4.3.2.1 Resistance due to Streamline Effect – Spreading Resistance (Rs)
It has been established theoretically and
observed practically that in an overlap, the current density is maximum at
either end of the joint. Consider two
bars of similar dimensions having a width w and thickness t overlapping to a
distance of L. Let
Joint Resistance (Figure – 4.9)
The graph above details the streamline effect on the
joint. It is not difficult to conclude
that the value will reach 0.5 asymptotically at either end. Increasing the length of overlap beyond a
certain ratio does not yield any significant benefit in terms of reduction in
streamline resistance.
4.3.2.2 Interface Resistance (Ri)
The interface resistance depends upon the
condition of the mating surface, the effective area of contact, and the pressure.
The mating surfaces need to be flat. It is obvious that any surface, however flat
it may be machined, will have peaks and valleys when observed under a microscope. A bar that is cast needs to be machined while
a bar that is drawn or extruded need not be subjected to any further surface
finishing. The mating surfaces must have
a uniform gap. When the width of the bar
is large and the surface hard, it may be necessary to cut grooves along the
length of the bar up to the length of the overlap, to ensure that mating
surfaces are parallel.
When a bolted joint is made, most of the current
flows in the area below the washer. It
has been observed that interface resistance decreases with an increase in the
effective contact area and applied pressure.
It is quite obvious that the pressure under the area of the washer will
be more than the average pressure under the overlap. It is, therefore, evident that the pressure
should not be increased beyond the elastic value of the metal.
If the metal creeps, it is quite possible that
it will develop hot spots during thermal cycling. Due to the difference in the coefficient of
linear expansion between the Steel fixings and the busbar (Copper or
Aluminium), the contact pressure will increase with the increase in temperature. It is, therefore, recommended to use the Belleville
washers to limit and maintain the pressure on the mating surface at a safe
value, at ambient temperature. Softer
conditions of alloy materials are more susceptible to joint deterioration.
Every hole punched or drilled to accommodate a
bolt, reduces the overlap area.
Therefore, for every bolted joint, there should be an optimum bolting
schedule (number of bolts, size, and pattern) that would provide the most
optimal solution.
It is necessary to ensure that the mating
surfaces remain in prime condition for the rest of their life. Aluminium and Copper will oxidize, when
exposed to the atmosphere, at different rates.
These will deteriorate the parent metal and increase the joint
resistance. To prevent oxidation, it is
recommended to apply a thin layer of petroleum jelly or a suitable compound. A
compound with a higher softening temperature is preferable as it will not
change its state when the busbars operate at high temperatures.
For Copper busbars, the joint resistance will
also depend upon the hardness
For Aluminium busbars, the joint resistance will
depend upon the alloy composition and temper.
Aluminium, with a conductivity of 61% (grade 1350) is soft and
susceptible to creep.
Interphase resistance between Busbars with Increasing and Decreasing pressure
(Figure - 4.11)
(The graphical representations in Figures 4.10 and 4.11 are indicative.)
4.3.2.3 Fretting Resistance (Rf)
A phenomenon of surface damage at the interface
of a joint, subject to vibration and relative movement, is called
fretting. It is established that a
vibration that causes a movement of 100 nanometers relative movement, can
result in fretting.
The relative movement of busbars at the joint
can occur due to thermal expansion/contraction due to load and ambient
temperature variations.
It is understood that the contact resistance can
be neglected if the film layer is less than 0.1 micron. The electrons can cover this distance without
loss of energy by tunneling. If the
layer is thick, the electrons lose energy and offer resistance to current
flow. This is known as a fretting resistance
(
Another process to maintain the integrity of the
mating surface is to plate the ends with Tin, Nickel, or Silver. The plated surfaces themselves tend to
oxidize albeit at a lower rate. It is
advisable to cover the plated surface, immediately after plating, with the
recommended covering material. A
protective compound still needs to be applied at the mating surfaces, before
making a joint. (Refer Section 4.4)
4.3.2.4 Bolting Schedule
There are no universal standards for the bolting
schedule for busbar joints. Most
manufacturers follow the recommendations furnished in the handbook, produced by
the association of Copper and Aluminium mills.
Many have in-house standards that have been generated and tested over a
period. Most manufacturers follow any one of the two
distinct practices detailed as under:
Bolting schedule (Figure – 4.12)
The choice of the Bolting Schedule will depend
upon the relative magnitude of the resistance to diversion (Rs
4.3.2.5 Joint Efficiency
For a bolted joint having ‘n’ holes with diameter ‘d’, it can be established that the total Joint Resistance
The joint efficiency depends upon several
factors and can exceed 1.0. An ideal
joint is one where the efficiency is close to 1.0 when the joint temperature is
the same as that of the busbar. (Since
the surface area of the joint will be more than that of the busbar, it will
offer more cooling area for convection and radiation.)
Bi-metallic strips, if introduced between a
Copper to an Aluminium joint to prevent Galvanic corrosion, will increase the
interface resistance.
4.3.3 Clamping
Clamping of conductors is extensively used in
transmission lines and outdoor switchyards.
In the transmission lines, the conductors are received in drums. The conductors are joined by Parallel Groove (PG)
clamps and tap-off with T clamps. The
material is mostly Aluminium, to ensure compatibility with the material of the
conductors. When each phase has more
than one conductor, spacer clamps are used to maintain the intra-phase
spacing. It also serves as a damper to
prevent galloping and aeolian vibrations, helps withstand forces due to short
circuit, and maintain electrical parameters.
In the switchyards, all the conductor joints and terminations are made
with clamps & connectors. Clamps
& connectors manufacturing is a very specialized and mature industry. The clamps and connectors installed in a
switchyard are voltage specific, as they are tested for corona discharge.
Clamping has the advantage that the ends of the
busbars do not need processing for jointing.
This allows a certain amount of flexibility that is needed in
Switchyards where tolerances are large.
When the clamp is of a different material from
that of the conductor, it is susceptible to the formation of a Galvanic cell.
Clamping is also universally adopted in the sandwich
bus
& low voltage cast resin bus, for jointing the conductors of the
adjacent sections.
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