Bus Duct Design

BUS DUCTS

INTRODUCTIONBus 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 conductingResistance-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 reasons for this is that it is cheaper to use than copper and making allowance for the light metal’s.

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TYPES OF BUSDUCTS

1. Isolated phase busduct2. 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.

DESIGN ASPECTS AND CRITERIA

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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

Ampsb) Rated Continuous Current for Tap-off bus - 1600 / 800

Ampsc) Rated Voltage - 15.75 KVd)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 :

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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. AlloyGrade 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

IV) Phase to earth clearance (min.) 220 mm 220 mm

V) Type of cooling Air Natraul

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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 equipments connecting to bus duct and thus do not let the bus enclosure currents to flow in the connecting equipments.

FLEXIBLES :

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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.

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.

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Isolated Phase Bus-Duct

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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

j) Short time Current for - 40 KA rms for 1

10

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

II ) Bus Enclosure :

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a) Material and grade Al. Alloy Al. AlloyGrade 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 and the enclosures in order to achieve fully weather proof duct and

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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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Segregated Phase Bus-Duct

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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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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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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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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.

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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 SpeedAluminium 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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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 weldcontamination, 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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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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