2. BUS DUCTS
INTRODUCTION
Bus ducts or bus bars are used to carry very high current between
the generator and associated transformers, in the power stations. In
the power stations the generator voltages vary from 12 KV to 24
KV whereas auxiliary supply voltages are 3.3 KV, 6.6 KV, or 12
KV.
Bus bars are thick metal connectors, usually of aluminium, which
carry large current up to 10 KA for 200/250 MW sets and around
20 KA for 500 MW set. The conductors are metal enclosed,
usually aluminium to provide safety and reliable operation.
Considering the cost of copper and aluminium, the later has been
found to be an economical choice as conducting
Resistance-Temp coefficient per centigrade degree
0.00403 0.00381
Density gm. per cc 2.95 8.9
Relative density 1 3.29
material for higher current bus ducts. Aluminium has been used for
the enclosure, as well as to reduce magnetic losses.
The enclosures and conductors are provided welded joints as for as
possible to give reduced maintenance. Bolted joints are provided
where opening is necessary during operation.
Aluminium is being used increasingly as busbar material, in power
stations, distribution boards, switchgear, etc. one of the main
2
3. reasons for this is that it is cheaper to use than copper and making
allowance for the light metal’s.
TYPES OF BUSDUCTS
1. Isolated phase busduct
2. segregated busduct
ISOLATED PHASE BUSDUCT
INTRODUCTION:
Busduct forms the electrical link between generator , transformers
and associated equipments such as LAVT cubicle , UAT cubicle ,
NG cubicle etc. It is an assembly of busbars with associated joints
,connections and supporting insulators within a grounded metal
enclosure.
In Isolated Phase Bus duct each phase conductor is enclosed by an
individual metal housing and separated from adjacent phase
conductor housing by an air space. In the continuous I.P.Busduct
various sections are so interconnected that low resistance path for
the induced circulating current is provided from one phase
enclosure to other phase enclosure.
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4. DESIGN ASPECTS AND CRITERIA
The design of bus duct is governed by following Typical values
for parameters 3 X 210 MW Tenughat Proposal (ICB) is given
below:
a) Rated Continuous Current for main bus - 1000
Amps
b) Rated Continuous Current for Tap-off bus - 1600 / 800
Amps
c) Rated Voltage - 15.75 KV
d)
e) Short time Current for main bus -78 KA rms for 3Sec.
225 KAP
f) Short time Current for Tap-off bus - 150 KA rms for 3
sec/ 420 KAP
g) Temperature rise allowed for bus enclosure - 20 deg. C
Over 50 deg.C
g) Temperature rise allowed for bus Conductor - 40 deg.(Plain
boltejoint)
Over 50deg.C
55 deg. C(Silver
plated joint)
The bus duct is designed to meet the above requirements and
following are the design out puts :
4
5. I ) Bus bar :
Main bus Tap-off
a) Material and grade Al. Alloy Al.
Alloy
Grade 19501 Grade 63401
b) Shape Round Square
c) Size (Dia. & thickness) 450 O/D , 15 Tk.
152.4A/F,8.1Tk.
Box channel cond.
II ) Bus Enclosure :
a) Material and grade Al. Alloy Al. Alloy
Grade 19500 Grade
31000
b) Shape Round Round
c) Size (Dia. & thickness) 1000 O/D,6.35 Tk.
780 O/D4.78Tk.
III) Phase to Phase spacing 1250 mm 1000
mm
5
6. IV) Phase to earth clearance (min.) 220 mm 220
mm
V) Type of cooling Air Natraul
VI) Degree of Protection Air and Water tightness
tests as per Appendix ‘F’
of IS :8084
CONSTRUCTIONAL ASPECTS
The busduct enclosure is made of aluminium alloy sheet and
supplied in length upto 6-7 meters. It is further reinforced with
aluminium channel rings at intervals which are also used for
enclosure and insulator mounting . Sealed openings are provided in
the busduct run near insulator for inspection and maintenance.The
three phase enclosures are interconnected effectively at the ends to
permit flow of current. Different sections of each phase are
generally connected together by aluminium make up pieces at site.
ACCESSORIES :
RUBBER BELLOWS :
It is provided at Bus duct terminations and in the run of bus duct if
route length is more than 30 to 35 mtr. , to take care of machine
vibrations , alignment and expansion / contration due to
temperature variations. Further , it insulates the termination
6
7. equipments connecting to bus duct and thus do not let the bus
enclosure currents to flow in the connecting equipments.
FLEXIBLES :
Copper flexibles are provided at bus duct terminations i.e at
Generator end , Generator transformer end , LAVT Cubicle , NG
Cubicle and other connecting equipment end. Aluminium flexibles
are provided in the run of bus duct to take care of alignment ,
variation in bus bar lengths due to temperature variations.
SEAL OFF BUSHINGS :
Epoxy Seal off bushings are provided at Power house wall and at
Generator termination end to restrict the propogation of fire in case
of accident. For air pressurised job’s , Seal off bushing is also
provided at Generator transformer end to form a close loop for
compressed air.
AIR PRESSURISATION EQUIPMENT. :
To avoid the ingress of dust , moistures etc. inside the bus duct , air
at a pressure slightly above the atmospheric pressure 25 to 40 mm
WC is flown in the busduct with the help of Air Pressurisation
Equipment.
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8. HOT AIR BLOWER :
After prolonged shutdown , to improve the IR value of Bus duct ,
Hot air is blown inside the bus duct with the help of Hot Air
Blower.
Isolated Phase Bus-Duct
8
10. SEGREGATED PHASE BUSDUCT
INTRODUCTION:
Busduct forms the electrical link between generator , transformers
and associated equipments such as LAVT cubicle , UAT cubicle ,
NG cubicle etc. It is an assembly of busbars with associated joints
,connections and supporting insulators within a grounded metal
enclosure.
In Segregated Phase Bus duct all the three phase conductors are
housed in a metal enclosure with segregation between the phases
by means of a metallic / insulating barriers. This arrangement
reduces possibility of phase faults and also helps in reducing
temperature rise.
DESIGN ASPECTS AND CRITERIA
The design of bus duct is governed by following parameters .
Typical values for 250 MW Power Plant is given below:
h) Rated Continuous Current for main bus - I) 2000
Amps
- II) 2500 Amps
- III) 4000 Amps
i) Rated Voltage - 6.6 KV
10
11. j) Short time Current for - 40 KA rms for 1
Sec./102 KAP
k) Temperature rise allowed for bus enclosure - 20 deg.C
Over 50 deg. C
l) Temperature rise allowed for bus Conductor - 40 deg.C
Over 50 deg. C
The bus duct is designed to meet the above requirements and
following are the
design out puts :
I ) Bus bar :
2000A / 2500 A 4000 A
a) Material and grade Al. Alloy Al Alloy
Grade 63401 Grade63401
b) Shape Square Square
c) Size 2X127 A/F,8.01 tk.
2x177.8A/F,9.98 tk
Box channel cond. bcc
11
12. II ) Bus Enclosure :
a) Material and grade Al. Alloy Al. Alloy
Grade 31000 Grade 31000
b) Shape Rectangular
Rectangular
c) Size 450 X1350 ,3.15 tk.
600 x1600 ,3.15 tk
III) Phase to Phase spacing 450 mm 540 mm
IV) Phase to earth clearance (min.) 90 mm 90mm
V) Type of cooling Air Natural
VI) Degree of Protection Air and Water tightness
Tests as per
Appendix ‘F’ of IS : 8084
CONSTRUCTIONAL ASPECTS
The bus duct enclosure is made of aluminium alloy sheet and
supplied in length upto 3.72 meters. Insulating barriers of 2 mm
thick Aluminium sheet provide complete phase segregation inside
the enclosure. The Aluminium sheet is welded on a frame work
made up of Aluminium Angles.Bolted type inspection covers
provide access to the conductor joints and insulators. Neoprene
bonded cork gaskets are provided between the inspection covers
12
13. and the enclosures in order to achieve fully weather proof duct and
air tight construction.The adjacent enclosures are connected
together by means of bolted type flange to flange joints.
Space heaters are provided to maintain IR value inside the bus
duct.
ACESSOCRIES
RUBBER BELLOWS :
It is provided at Bus duct terminations and in the run of bus duct if
route length is more than 30 to 35 mtr. , to take care of machine
vibrations , alignment and expansion / contration due to
temperature variations. Further , it insulates the termination
equipments connecting to bus duct and thus do not let the bus
enclosure currents to flow in the connecting equipments.
FLEXIBLES :
Copper flexibles are provided at bus duct terminations. Aluminium
flexibles are provided in the run of bus duct to take care of
alignment , variation in bus bar lengths due to temperature
variations.
SEAL OFF BUSHINGS :
Epoxy Seal off bushings are provided at Power house wall to
restrict the propogation of fire in case of accident.
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16. TYPES OF BUSBAR.
The three main types of busbar are as follows
Flat shaped.
Channel shaped.
Tubular shaped.
The most commonly used busbars are rectangular in shape. Flat
conductors are easy to store, handle and erect. Whereas channel
busbars are however mechanically much stronger and electrically
more efficient because of larger effective cooling surfaces. This
construction also gives the structural advantage of a box girder
section, and can be used for unusually long spans or to withstand
high short circuit forces.
DESIGN CONSIDERATION
Busbar installation must be designed to operate within set
temperature limits and to withstand mechanical forces. A straight
substitution of aluminium for copper will not result in the most
economical use of material and designs for aluminium busbar
should be specially developed.
Very frequently the type, and sometimes the size, will be dictated
by the items served by the busbar; for example with switches and
circuit breakers, ease of connections will dictate the section
thickness and the space allowed may influence the choice of
conductor shape. In other cases, the designer may have a free hand,
unhampered by existing designs.
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17. TEMPERATURE RISE
In the majority of busbar installations the rating is established on
the basis of temperature rise. Connections are usually short and
power losses and voltage drop are not significant.
The operating temperature of a busbar must be limited to a level at
which there will be no long term deterioration of the conductor, the
joints or the equipment connected to the busbar. In normal practice
the temperature rise must not exceed 50◦ c on an ambient having a
peak value of 40◦ c and an average value of 35◦ c, giving a
maximum operating temperature of 85◦ c.
This allows an adequate margin of mechanical strength and can be
operated on a continuous basis at temperature of upto 110◦ c
without loss of strength.
Particular attention is given to factors such as joint design and
thermal expansion. The temperature of busbar will rise until the
heat dissipated is equal to the heat generated. This has direct
bearing on current carrying capacity of the conductor, by virtue of
the effect of temperature on resistance.
VOLTAGE-DROP&TEMPERATURE
RISE
The voltage-drop on a busbar can often be ignored, but cases occur
in both D.c and A.c systems where it is the criterion in the design.
In a D.c system, the voltage-drop is due solely to resistance. And
this can be reduced only by the use of larger total cross-section,
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18. either by increasing the number of conductors or by using larger
ones.
In A.c systems, voltage-drop is due mainly to reactance which can
be minimized by making the busbar spacing small
For the same energy loss an aluminium bar will carry 78.2% of the
full load current of a copper bar of the same physical dimensions.
POWER LOSS
Although a busbar is at times defined as a conductor having
negligible loss, there are many cases, particularly where the load
factor is high, where losses are very important. If the busbar is
designed to have the full permissible temperature rise, the loses
may be too great, and it may be more economical to spend more
money on the busbar to get higher efficiency.
The money value of the losses must take into account not only the
cost of units consumed over the years, but also the maximum
demand charge for the KW loss at full-load.
CLEARANCES
The recommended clearances are given between busbar and busbar
connections.
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19. FORCES ACTING ON BUSBARS
WEIGHT
The lightness of aluminium bus bar is of great assistance in
erection. It also confers a substantial advantage in the design of
rising main busbar where each busbar must be supported at its
upper end by a suspension insulator.
WIND AND ICE
In the case of outdoor busbar, the forces to be considered include
wind and ice. Allowances must be made for ice forming to a radial
thickness of 9.5 mm. The weight of ice is 913 Kg./cu. m. The wind
loading is to be taken as 39 Kg/sq m. on the ice covered conductor.
EXPANSION FORCES
Busbars changes temperature with load much more rapidly than its
support and hence relative movement between the two must occur.
If a bar is anchored in one place only, and allowed to slide
elsewhere, this expansion can probably be absorbed at corners in
the run. A further reason for employing expansion joints is to
ensure that deflections due to short-circuit forces do not cause
longitudinal forces that can be stress the material and damage the
insulators.
ELECTRO-MAGNETIC FORCES
In many cases, the electro-magnetic forces will be appreciably
larger than all others, running perhaps to a thousand Kgms per
meter run, or even more under the most severe short-circuit
conditions.
19
20. The mechanical forces and the temperature rise due to the very
high currents are often the limiting factors in the busbar design.
BUSDUCTS WELDING
Base Metal Preparation
Prior to weld Al, the base metal, must be cleaned to remove any
aluminium oxide and hydrocarbon contamination from oils or
cutting solvents.
Requirements of Aluminium Welding
Since the melting point of oxide (approx. 3700F) is greater than the
base metal (approx.1200F), therefore leaving any oxide on the
surface of the base metal will inhibit penetration of the filler metal
into the metal work piece. To remove the oxide use of stainless
steel brush or solvent or etching solution could be made..
Preheating
It helps avoid weld cracking. Placing tack welds in the beginning
and end of the area to be welded will aid in the preheating effort.
Travel Speed
Aluminium welding needs to be performed hot and fast. If speed is
to low the welder risks excessive bum through particularly on thin
gauge sheet.
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21. Filler Wire
Filler wire must be selected so that it has a melting temperature
similar to the base metal. The larger the wire diameter the easier it
feeds. Filler with high alloy content than the base should be easier
as filler remains plastic after the base hardens, relieving stresses by
yielding until solidification.
The Push Technique
With aluminium pushing the gun away from the weld puddle rather
than pulling it will result in better cleaning action, reduced weld
contamination, and improved shielding gas coverage.
Shielding Gas
It should have good cleaning action and penetration profile. Argon
is the most common shielding gas preferred in view of economy.
When welding aluminium argon gas of 99.997% purity is required
for radiographic welding.
Convex Shaped Weld
Till aluminium welding crater cracking causes most failures the
risk of cracking is greatest with concave craters, since the surface
of craters contracts and tears as it cools. Therefore craters should
be built up to convex or mould shape. As the weld cools, convex
shape of crater compensates for contraction forces.
Installation-photo-:
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23. BUSBARS PROTECTION
Busbars are vital parts of a power system and so a fault should be
cleared as fast as possible. A busbar must have its own protection
although their high degrees of reliability bearing in mind the risk
of unnecessary trips, so the protection should be dependable,
selective and should be stable for external faults, called through
faults.
The most common fault is phase to ground, which usually results
from human error.
There are many types of relaying principles used in busbar. A
special attention should be made to current transformer selection
since measuring errors need to be considered.
The proposed protection employs a protection technique based on
polarities of transient current waves for identification of the faults
internal and external to the busbar. In the technique, the polarities
of transient currents can be extracted reliably using a wavelet
transform modulus maximum. To improve the reliability of the
distributed busbar protection system, message exchange based on
"protection signal bus" is presented and applied for the
implementation of distributed bus protection. Using this method
the comparison and exchange of polarity information among the
protection units can be completed reliably. .
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