Bus duct fabrication

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Bus duct fabrication

  1. 1. Bus Duct Design
  2. 2. BUS DUCTSINTRODUCTIONBus ducts or bus bars are used to carry very high current betweenthe generator and associated transformers, in the power stations. Inthe power stations the generator voltages vary from 12 KV to 24KV whereas auxiliary supply voltages are 3.3 KV, 6.6 KV, or 12KV.Bus bars are thick metal connectors, usually of aluminium, whichcarry large current up to 10 KA for 200/250 MW sets and around20 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 beenfound to be an economical choice as conductingResistance-Temp coefficient per centigrade degree 0.00403 0.00381Density gm. per cc 2.95 8.9Relative density 1 3.29material for higher current bus ducts. Aluminium has been used forthe enclosure, as well as to reduce magnetic losses.The enclosures and conductors are provided welded joints as for aspossible to give reduced maintenance. Bolted joints are providedwhere opening is necessary during operation.Aluminium is being used increasingly as busbar material, in powerstations, distribution boards, switchgear, etc. one of the main 2
  3. 3. reasons for this is that it is cheaper to use than copper and makingallowance for the light metal’s.TYPES OF BUSDUCTS 1. Isolated phase busduct 2. segregated busductISOLATED PHASE BUSDUCTINTRODUCTION:Busduct forms the electrical link between generator , transformersand 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 metalenclosure.In Isolated Phase Bus duct each phase conductor is enclosed by anindividual metal housing and separated from adjacent phaseconductor housing by an air space. In the continuous I.P.Busductvarious sections are so interconnected that low resistance path forthe induced circulating current is provided from one phaseenclosure to other phase enclosure. 3
  4. 4. DESIGN ASPECTS AND CRITERIAThe design of bus duct is governed by following Typical valuesfor parameters 3 X 210 MW Tenughat Proposal (ICB) is givenbelow: 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 andfollowing are the design out puts : 4
  5. 5. I ) Bus bar : Main bus Tap-offa) Material and grade Al. Alloy Al.Alloy Grade 19501 Grade 63401b) Shape Round Squarec) 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 Grade31000b) Shape Round Roundc) Size (Dia. & thickness) 1000 O/D,6.35 Tk. 780 O/D4.78Tk.III) Phase to Phase spacing 1250 mm 1000mm 5
  6. 6. IV) Phase to earth clearance (min.) 220 mm 220mmV) Type of cooling Air NatraulVI) Degree of Protection Air and Water tightnesstests as per Appendix ‘F’ of IS :8084CONSTRUCTIONAL ASPECTSThe busduct enclosure is made of aluminium alloy sheet andsupplied in length upto 6-7 meters. It is further reinforced withaluminium channel rings at intervals which are also used forenclosure and insulator mounting . Sealed openings are provided inthe busduct run near insulator for inspection and maintenance.Thethree phase enclosures are interconnected effectively at the ends topermit flow of current. Different sections of each phase aregenerally 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 ifroute length is more than 30 to 35 mtr. , to take care of machinevibrations , alignment and expansion / contration due totemperature variations. Further , it insulates the termination 6
  7. 7. equipments connecting to bus duct and thus do not let the busenclosure currents to flow in the connecting equipments.FLEXIBLES :Copper flexibles are provided at bus duct terminations i.e atGenerator end , Generator transformer end , LAVT Cubicle , NGCubicle and other connecting equipment end. Aluminium flexiblesare 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 atGenerator termination end to restrict the propogation of fire in caseof accident. For air pressurised job’s , Seal off bushing is alsoprovided at Generator transformer end to form a close loop forcompressed air.AIR PRESSURISATION EQUIPMENT. :To avoid the ingress of dust , moistures etc. inside the bus duct , airat a pressure slightly above the atmospheric pressure 25 to 40 mmWC is flown in the busduct with the help of Air PressurisationEquipment. 7
  8. 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 AirBlower. Isolated Phase Bus-Duct 8
  9. 9. 9
  10. 10. SEGREGATED PHASE BUSDUCTINTRODUCTION:Busduct forms the electrical link between generator , transformersand 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 metalenclosure.In Segregated Phase Bus duct all the three phase conductors arehoused in a metal enclosure with segregation between the phasesby means of a metallic / insulating barriers. This arrangementreduces possibility of phase faults and also helps in reducingtemperature rise.DESIGN ASPECTS AND CRITERIAThe 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. 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. CThe bus duct is designed to meet the above requirements andfollowing are the design out puts :I ) Bus bar : 2000A / 2500 A 4000 Aa) Material and grade Al. Alloy Al Alloy Grade 63401 Grade63401b) Shape Square Squarec) Size 2X127 A/F,8.01 tk. 2x177.8A/F,9.98 tk Box channel cond. bcc 11
  12. 12. II ) Bus Enclosure :a) Material and grade Al. Alloy Al. Alloy Grade 31000 Grade 31000b) Shape Rectangular Rectangularc) Size 450 X1350 ,3.15 tk. 600 x1600 ,3.15 tkIII) Phase to Phase spacing 450 mm 540 mmIV) Phase to earth clearance (min.) 90 mm 90mmV) Type of cooling Air NaturalVI) Degree of Protection Air and Water tightness Tests as per Appendix ‘F’ of IS : 8084CONSTRUCTIONAL ASPECTSThe bus duct enclosure is made of aluminium alloy sheet andsupplied in length upto 3.72 meters. Insulating barriers of 2 mmthick Aluminium sheet provide complete phase segregation insidethe enclosure. The Aluminium sheet is welded on a frame workmade up of Aluminium Angles.Bolted type inspection coversprovide access to the conductor joints and insulators. Neoprenebonded cork gaskets are provided between the inspection covers 12
  13. 13. and the enclosures in order to achieve fully weather proof duct andair tight construction.The adjacent enclosures are connectedtogether by means of bolted type flange to flange joints.Space heaters are provided to maintain IR value inside the busduct.ACESSOCRIESRUBBER BELLOWS : It is provided at Bus duct terminations and in the run of bus duct ifroute length is more than 30 to 35 mtr. , to take care of machinevibrations , alignment and expansion / contration due totemperature variations. Further , it insulates the terminationequipments connecting to bus duct and thus do not let the busenclosure currents to flow in the connecting equipments.FLEXIBLES :Copper flexibles are provided at bus duct terminations. Aluminiumflexibles are provided in the run of bus duct to take care ofalignment , variation in bus bar lengths due to temperaturevariations.SEAL OFF BUSHINGS :Epoxy Seal off bushings are provided at Power house wall torestrict the propogation of fire in case of accident. 13
  14. 14. Segregated Phase Bus-Duct 14
  15. 15. 15
  16. 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. Flatconductors are easy to store, handle and erect. Whereas channelbusbars are however mechanically much stronger and electricallymore efficient because of larger effective cooling surfaces. Thisconstruction also gives the structural advantage of a box girdersection, and can be used for unusually long spans or to withstandhigh short circuit forces.DESIGN CONSIDERATIONBusbar installation must be designed to operate within settemperature limits and to withstand mechanical forces. A straightsubstitution of aluminium for copper will not result in the mosteconomical use of material and designs for aluminium busbarshould be specially developed.Very frequently the type, and sometimes the size, will be dictatedby the items served by the busbar; for example with switches andcircuit breakers, ease of connections will dictate the sectionthickness and the space allowed may influence the choice ofconductor shape. In other cases, the designer may have a free hand,unhampered by existing designs. 16
  17. 17. TEMPERATURE RISEIn the majority of busbar installations the rating is established onthe basis of temperature rise. Connections are usually short andpower losses and voltage drop are not significant.The operating temperature of a busbar must be limited to a level atwhich there will be no long term deterioration of the conductor, thejoints or the equipment connected to the busbar. In normal practicethe temperature rise must not exceed 50◦ c on an ambient having apeak value of 40◦ c and an average value of 35◦ c, giving amaximum operating temperature of 85◦ c.This allows an adequate margin of mechanical strength and can beoperated on a continuous basis at temperature of upto 110◦ cwithout loss of strength.Particular attention is given to factors such as joint design andthermal expansion. The temperature of busbar will rise until theheat dissipated is equal to the heat generated. This has directbearing on current carrying capacity of the conductor, by virtue ofthe effect of temperature on resistance.VOLTAGE-DROP&TEMPERATURERISEThe voltage-drop on a busbar can often be ignored, but cases occurin 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. Andthis can be reduced only by the use of larger total cross-section, 17
  18. 18. either by increasing the number of conductors or by using largerones.In A.c systems, voltage-drop is due mainly to reactance which canbe minimized by making the busbar spacing smallFor the same energy loss an aluminium bar will carry 78.2% of thefull load current of a copper bar of the same physical dimensions.POWER LOSSAlthough a busbar is at times defined as a conductor havingnegligible loss, there are many cases, particularly where the loadfactor is high, where losses are very important. If the busbar isdesigned to have the full permissible temperature rise, the losesmay be too great, and it may be more economical to spend moremoney on the busbar to get higher efficiency.The money value of the losses must take into account not only thecost of units consumed over the years, but also the maximumdemand charge for the KW loss at full-load.CLEARANCESThe recommended clearances are given between busbar and busbarconnections. 18
  19. 19. FORCES ACTING ON BUSBARSWEIGHTThe lightness of aluminium bus bar is of great assistance inerection. It also confers a substantial advantage in the design ofrising main busbar where each busbar must be supported at itsupper end by a suspension insulator.WIND AND ICEIn the case of outdoor busbar, the forces to be considered includewind and ice. Allowances must be made for ice forming to a radialthickness of 9.5 mm. The weight of ice is 913 Kg./cu. m. The windloading is to be taken as 39 Kg/sq m. on the ice covered conductor.EXPANSION FORCESBusbars changes temperature with load much more rapidly than itssupport and hence relative movement between the two must occur.If a bar is anchored in one place only, and allowed to slideelsewhere, this expansion can probably be absorbed at corners inthe run. A further reason for employing expansion joints is toensure that deflections due to short-circuit forces do not causelongitudinal forces that can be stress the material and damage theinsulators.ELECTRO-MAGNETIC FORCESIn many cases, the electro-magnetic forces will be appreciablylarger than all others, running perhaps to a thousand Kgms permeter run, or even more under the most severe short-circuitconditions. 19
  20. 20. The mechanical forces and the temperature rise due to the veryhigh currents are often the limiting factors in the busbar design.BUSDUCTS WELDINGBase Metal PreparationPrior to weld Al, the base metal, must be cleaned to remove anyaluminium oxide and hydrocarbon contamination from oils orcutting solvents.Requirements of Aluminium WeldingSince the melting point of oxide (approx. 3700F) is greater than thebase metal (approx.1200F), therefore leaving any oxide on thesurface of the base metal will inhibit penetration of the filler metalinto the metal work piece. To remove the oxide use of stainlesssteel brush or solvent or etching solution could be made..PreheatingIt helps avoid weld cracking. Placing tack welds in the beginningand 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 isto low the welder risks excessive bum through particularly on thingauge sheet. 20
  21. 21. Filler WireFiller wire must be selected so that it has a melting temperaturesimilar to the base metal. The larger the wire diameter the easier itfeeds. Filler with high alloy content than the base should be easieras filler remains plastic after the base hardens, relieving stresses byyielding until solidification.The Push TechniqueWith aluminium pushing the gun away from the weld puddle ratherthan pulling it will result in better cleaning action, reduced weldcontamination, and improved shielding gas coverage.Shielding GasIt should have good cleaning action and penetration profile. Argonis the most common shielding gas preferred in view of economy.When welding aluminium argon gas of 99.997% purity is requiredfor radiographic welding.Convex Shaped WeldTill aluminium welding crater cracking causes most failures therisk of cracking is greatest with concave craters, since the surfaceof craters contracts and tears as it cools. Therefore craters shouldbe built up to convex or mould shape. As the weld cools, convexshape of crater compensates for contraction forces.Installation-photo-: 21
  22. 22. 22
  23. 23. BUSBARS PROTECTIONBusbars are vital parts of a power system and so a fault should becleared as fast as possible. A busbar must have its own protectionalthough their high degrees of reliability bearing in mind the riskof unnecessary trips, so the protection should be dependable,selective and should be stable for external faults, called throughfaults.The most common fault is phase to ground, which usually resultsfrom human error.There are many types of relaying principles used in busbar. Aspecial attention should be made to current transformer selectionsince measuring errors need to be considered.The proposed protection employs a protection technique based onpolarities of transient current waves for identification of the faultsinternal and external to the busbar. In the technique, the polaritiesof transient currents can be extracted reliably using a wavelettransform modulus maximum. To improve the reliability of thedistributed busbar protection system, message exchange based on"protection signal bus" is presented and applied for theimplementation of distributed bus protection. Using this methodthe comparison and exchange of polarity information among theprotection units can be completed reliably. . 23
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