Part V. Busbar Systems

Transcription

1 Prt V Busr Systems

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3 8/983 8 Crrying power through metlenclosed us systems Contents 8.1 Introduction 8/ Types of metl-enclosed us systems 8/ A non-segregted phse us system 8/ A segregted phse us system 8/ An isolted phse us (IPB) system 8/ A rising mins (verticl us system) 8/ An overhed us (horizontl us system) 8/ Non-conventionl, compct nd low loss us systems 8/ Design prmeters nd service conditions for metl-enclosed us system 8/ Design prmeters 8/ Short-circuit effects 8/ Therml effects 8/ Electrodynmic effects 8/ Service conditions 8/ Amient temperture 8/ Altitude 8/ Atmospheric conditions 8/ Excessive virtions nd seismic effects 8/ Other design considertions 8/ Size of enclosure 8/ Voltge drop 8/ Skin nd proximity effects on current-crrying conductor 8/ Skin effect 8/ Skin effect nlysis 8/ Determining the skin effect 8/ Proximity effect 8/ Proximity effect in terms of usr rectnce 8/ Voltge unlnce s consequence of the proximity effect 8/ Derting due to the proximity effect 8/ Minimizing the proximity effect 8/ Energy sving 8/ Smple clcultions for designing 0 A non-isolted phse luminium usr system 8/105 Relevnt Stndrds 8/1036 List of formule used 8/1037 Further Reding 8/1037

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5 Crrying power through metl-enclosed us systems 8/ Introduction In power-generting sttion power is crried from the genertor to the power trnsformer, to the unit uxiliry trnsformer (UAT) or to the unit uxiliry switchger s illustrted in Figure 13.1 through solid conductors (HV us systems). This is due to lrge cpcity of the genertors (up to 1000 MW). The trnsmission of such lrge mounts of power over long distnces is then through overhed lines or underground cles. Similrly, for distriution system of 3.3, 6.6 or 11 kv nd even higher such s 33 or 66 kv, feeding lrge commercil or industril lods, the distriution of power on the LV side (Figure 8.1) my e through cles or solid conductors (LV us systems), depending upon the size of the trnsformer. The HV side of the trnsformer my lso e connected through cles or the HV us system s illustrted. For moderte rtings on LV system, sy, up to 600/ 800 A, cles re preferred, while for higher rtings (1000 A nd ove), the prctice is to opt for solid conductors (LV us systems), on the grounds of cost, ppernce, sfety, ese of hndling nd mintennce. For lrger rtings, more cles in prllel my ecome unwieldy nd difficult to mintin nd present prolems in locting fults. The usr conductors my e of luminium or copper. The use of copper my e more pproprite t corrosive res Cle or us duct Bus duct * 11 kv/0.415 kv trnsformer LV reker LV power control centre (PCC) * 11 kv reker for isoltion nd protection of trnsformer nd interconnecting cles Figure 8.1 Appliction of us system (such s humid, sline or chemiclly ggressive loctions). In humid nd corrosive conditions, luminium erodes fster thn copper. These solid or hollow conductors connect the supply side to the receiving end nd re clled us ducts. They my e of the open type, such s to feed very high current t very low voltge. A smelter unit is one such ppliction. But normlly they re housed in sheet metl enclosure, Figures 8.() nd 8.33(). Our min concern here will e deling with lrge to very lrge currents, rther thn voltges. Currents re more difficult to hndle thn voltges due to mutul induction etween the conductors nd etween the conductor nd the enclosure. Here we riefly discuss the types of metl-enclosed us systems nd their design prmeters, to select the correct size nd type of luminium or copper sections nd the us enclosure for the required ppliction, current rting nd voltge system. More pplictions, illustrtions re provided for luminium conductors rther thn copper, s they re more commonly used on grounds of cost, ut dequte dt nd tles re provided to design copper usr system lso. 8. Types of metl-enclosed us systems A us system cn e one of the following types, depending upon its ppliction: Non-segregted Segregted Isolted phse Rising mins (verticl us systems) Overhed us (horizontl us system) Non-conventionl us systems (1) Compct nd sndwich type () Prtilly isolted phse us (PIPB) type (3) Gs (SF 6 ) insulted usrs (GIB) 8..1 A non-segregted phse us system In this construction ll the us phses re housed in one metllic enclosure, with dequte spcings etween them nd the enclosure ut without ny rriers etween the phses (Figure 8.()). Appliction Being simple nd economicl, it is the most widely used construction for ll types of LV systems. The ltest trend now is to go in for compct us systems where possile, in view of their inherent dvntges noted lter. Nominl current rtings The preferred current rtings my follow series R-10 of IEC nd s discussed in Section (4). They my increse to 6000 A or so, depending upon the ppliction like when required to connect lrge LV lterntor or the LV side of lrge trnsformer to its switchger. The preferred short-time rtings my e one of those indicted in Tle 13.7.

6 8/986 Electricl Power Engineering Reference & Applictions Hndook Flnge Enclosure Busrs Busr supporting insultors Figure 8.() Low rting LV us duct R Y B N Non-mgnetic or mgnetic enclosure Brriers (sme metl s the enclosure) Clmps (sme metl s the enclosure) Figure 8.() A segregted phse us system 8.. A segregted phse us system In this construction ll the phses re housed in one metllic enclosure s erlier, ut with metllic rrier etween ech phse, s illustrted in Figure 8.(). The metllic rriers provide the required mgnetic shielding nd isolte the usrs mgneticlly from ech other, like n isolted phse us system (IPB). The metllic rriers trnsform the enclosure into somewht like Frdy Cges (Section 3.18). For more detils see Section 31.. The enclosure cn e of MS or luminium nd the rriers lso of the sme metl s the enclosure to provide uniform distriution of field. The purpose of providing metllic rrier is not only to shroud the phses ginst short-circuits ut lso reduce the effect of proximity of one phse on the other y rresting the mgnetic field produced y the current crrying conductors within the enclosure itself. It now opertes like n enclosure with n interleving rrngement (Section 8.8.4) lncing the fields produced y the conductors to gret extent nd llowing only moderte field in the spce, s in n IPB system (Section 31.). The enclosure losses with such n rrngement my fll in the rnge of 6065% of conductors in cse of m.s. (mild steel) nd 3035% in cse of luminium enclosures for ll voltge systems kv nd current rtings ove 3000 A nd up to 6000 A or so. Only luminium enclosures should e preferred to minimize losses nd enclosure heting. The effect of proximity is now lmost nullified s lso n imlnce in the phse rectnces. An unlnce in the rectnce is otherwise responsile for voltge unlnce etween the three phses s discussed in Section 8.8. nd enhnce the electrodynmic forces tht my led to phse-to-phse fult t higher rted currents. Applictions They re generlly used for higher rtings, 3000 A nd ove, on ll voltge systems. They re, however, preferred on n HV rther thn n LV system, such s etween unit uxiliry trnsformer (UAT) nd its switchgers nd sttion trnsformer nd its switchgers s in powergenerting sttion nd shown in Figure 13.1, for resons of sfety nd lso cost of n isolted phse us system (IPB) (Chpter 31) over segregted system. With the vilility of compct nd prtilly isolted

7 Crrying power through metl-enclosed us systems 8/987 us systems s discussed lter, this type of us system cn e esily replced with the non-conventionl us systems. Note For such rtings, enclosure of non-mgnetic mteril lone is recommended due to high iron losses in mgnetic mteril. Nominl current rtings These will depend upon the ppliction. The preferred rtings my follow series R-10 of IEC 60059, s descried in Section (4). They my increse to 6000 A or so depending upon the ppliction An isolted phse us (IPB) system (for very lrge rtings A nd ove) The design criteri nd construction detils of this system re totlly different from those of non-isolted phse us system discussed ove. This type of enclosure is delt seprtely in Chpter 31. Rigid coupling stright through joint To upper floors SMC/DMC supports Outgoing tp-off plug-in ox O/G distriution ord (DB) to cter for ech floor Glsswool or epoxy fireproof rrier Floor thickness Floor sl Mounting clmps Thrust pd nd stoppers of ruer or similr mteril Rising mins Bus conductor Flexile coupling expnsion joint I/C plug-in ox to feed the us Incoming switch fuse MCCB ox High riser wll on which the us system runs () Front view with cover removed Cle ox Note: A ground us (not shown) shll run through the length of the enclosure. Figure 8.3 Incoming cle Side view () Rising on wll Rising mins mounting rrngement GF

8 8/988 Electricl Power Engineering Reference & Applictions Hndook 8..4 A rising mins (verticl us system) For power distriution in multi-storey uilding This is nother form of us system nd is used in verticl formtion to supply individul floors of highrise uilding (Figures 8.3() nd ()). This is much neter rrngement thn using cles nd running numerous lengths of them to ech floor which my not only e unwieldy ut lso more cumersome to terminte nd to locte fults. Such system is the norml prctice to distriute power in high-riser. It rises from the ottom of the uilding nd runs to the top floor. To sve on cost, the rtings my e in decresing order fter every three or four floors, s fter every floor the lod of tht floor will e reduced. The rting cn e grouped for three or four floors together, depending upon the totl lod nd the numer of floors. A smller rting of, sy, A need not e further stepped for it my not e of ny economic enefit. Specil fetures of rising mins 1 They re mnufctured in smll stndrd lengths, sy, m, nd re then joined together t site to fit into the lyout. Wherever the rising mins cross through floor of the uilding, fireproof rriers re provided s shown in Figure 8.3() to contin the spred of fire to other floors. 3 On ech floor opening is provided in the rising mins to receive plug-in ox (Figures 8.3() nd (c)) to tp-off the outgoing connections nd to meet the lod requirement of tht floor. The plug-in ox cn normlly e plugged in or withdrwn from the live us without requiring shutdown. 4 To tke up the verticl dynmic lod of usrs nd to prevent them from sliding down, two sets of thrust pds re generlly provided on the usrs in ech stndrd length of the rising mins, s illustrted in Figure 8.3(). 5 Flexile expnsion joints of luminium or copper re essentil fter every three or four stndrd lengths (sy, fter every m) to sor the expnsion of usrs on lod. Usully compct nd energy efficient us risers tht re light weight nd esy to mnoeuvre re preferred for such pplictions (Figure 8.4(f1)) An overhed us (horizontl us system) (Figures 8.4, nd c) Unlike high riser, now the overhed us system runs horizontlly, elow the ceiling t convenient height, s shown in Figure 8.4(c) to distriute power to light nd smll lod points. Lrge tool room or mchine shops re instlltions tht would otherwise require distriution system, for short distnces, to meet the needs of vrious lod points nd mke power distriution unwieldy nd cumersome. Moreover, it would lso men running mny cles under the floor to feed ech lod point. In n overhed usr system, the power cn e tpped from ny numer of points to supply the lod points just elow it through plug-in ox similr to tht used on rising mins. The floor cn now e left free from cles nd trenches. Here lso it is preferred to use compct nd energy efficient us system. DB with MCCB Figure 8.3(c) DB with HRC fuses Rising mins 8..6 Non-conventionl compct nd low loss us systems Cles re too compct (Appendix 16). If we cn crete similr conditions in us system lso, so s to e le to plce the conductors together, we cn chieve similr compctness in usrs lso. This technique is effectively nd meticulously developed nd utilized y some mnufcturers y providing dequte insultion to the current crrying conductors nd mking it possile to plce them together to produce very compct sizes of usrs for LV nd HV systems. The concept ehind these us systems hs revolutionized the power trnsfer technique through us systems. Plcing the usrs together reduces the inductnce of the usrs X, impednce (Z), voltge drop (I.Z) nd so lso the mgnetizing losses to very gret extent. Lesser the spcing etween the phse usrs lesser is the X. Figure 8.4 nd Tles 8.0(1) nd 8.0() elucidte this. Since there eing little scope for the movement of usrs higher EM forces re of no consequence like in cles.

9 Crrying power through metl-enclosed us systems 8/989 Plugged in position Overhed usr system in mchine shop Withdrwn position Figure 8.4() Plug-in tp-off ox Instlltion of overhed us system with tp-off oxes in lrge ssemly shop Figure 8.4(c) Figure 8.4() An overhed us system shown with tp off oxes (Courtesy: GE Power)

10 8/990 Electricl Power Engineering Reference & Applictions Hndook These systems therefore sve spce nd energy. The importnt feture of such us systems is their insultion technique. The het trnsfer now is usully through conduction rther thn convection. In conventionl us system the het dissiption is through conductor to ir, ir to metllic enclosure nd enclosure to surroundings. And ir eing poor medium of het trnsfer y nturl convection. A conventionl us system therefore clls for lrger cross-section of conductors nd the enclosure to dissipte the het generted, more so in lrger rtings 000A nd ove (see Ex 8.1, Figures 8.33 nd 8.34). We cn therefore cll them s energy efficient nd spcesver us systems nd preferred choice for ll pplictions for power trnsmission requiring us system. Below we provide rief detils of these us systems: (1) Compct nd sndwich type us systems To chieve good insultion the usrs my e epoxy or polyester insulted using vcuum or other effective process. Epoxy hs dielectric strength of out 3540 kv/mm, wheres polyester, het resistnt hlogen free insultion hs it of the order of 100 kv/mm nd more. Both re used extensively for LV nd HV sndwich usr systems. The coting in cse of polyester is usully in the form of thin film. With thinner insultion the het dissiption is efficient nd so lso the metl utiliztion nd the us system is even more compct. PVC hving low dielectric strength of the order of 18 kv/mm (Tle A16.) is suitle only for LV systems ut is usully not preferred for the following resons, Wrpping skin tight PVC sleeve over usrs is not sfe s it my er cuts nd crcks while sliding over the usrs. A perfect insultion s noted, is prerequisite for sfe opertion of sndwich usrs over long periods. Using thicker sleeve my defet the utility of het trnsfer y conduction, the min concept ehind such us systems. It my lso trp ir to form ir pockets restricting het dissiption. However, if it is possile to provide PVC coting thn sleeving over the usrs PVC my lso provide n cceptle insultion nd het dissiption system. Similrly is coted the inside of the enclosure so tht they ll (conductors nd enclosure) cn e plced touching. Het trnsfer y conduction mkes it n efficient het trnsfer system. Figure 8.4(d) shows few views of such us systems in LV nd HV nd few ends. When the usrs re plced touching with ech other they re termed s sndwiched nd when tp-off provision is mde, such s for rising mins or n over-hed us wys nd spce is left etween the phse conductors to receive the plug-in contcts, they re termed s compct us-systems. These usrs re usully mnufctured up to 6000 A or so in LV nd HV systems. They offer good sustitute to cles in lrger rtings. For generl reference Tles 8.0(1) nd 8.0() furnish the LV Bus Systems LV ccessories 1kV/1A Cst resin HV us system Figure 8.4(d) Sndwiched nd compct us systems nd their ccessories (Courtesy: Myduct Technology SDN.BHD.)

11 Crrying power through metl-enclosed us systems 8/991 Tle 8.0(1) LV compct us-ducts electricl chrcteristics for copper conductor (frequency : Hz) Concentrted No. of Busr Impednce micro-ohm Line voltge drop in milli-volt per meter t rted current nd t vrious power fctors rted current rs per size per meter t 95 C Amp phse mm R X Z Tle 8.0() LV compct us-ducts electricl chrcteristics for copper conductor (frequency : 60 Hz) Concentrted No. of Busr Impednce micro-ohm Line to line voltge drop in milli-volt per meter t rted current nd t vrious power fctors rted current rs per size per meter t 95 C Amp phse mm R X Z Source : Myduct Technology SDN. BHD Note R nd X would vry with the size nd configurtion of usrs nd their grde nd hence from mnufcturer to mnufcturer. The tles provided here re only for generl reference. For exct detils one my contct the mnufcturer.

12 8/99 Electricl Power Engineering Reference & Applictions Hndook technicl dt with copper conductors for LV us ducts of prticulr mnufcturer for nd 60 Hz systems. The usrs so insulted re sndwiched together. They cn e 3 1 / or 4 conductors. They cn lso e five conductor-3 phses, twice the size protective neutrl for clen grounding for electronic circuits (Section ) nd sme size of neutrl s the phses for power equipment grounding nd to hndle lrge hrmonics. All these conductors re encpsulted in n epoxy or polyester insulted metllic enclosure (usully non-mgnetic). To ugment het dissiption some mnufcturers even provide fins in their enclosures, prticulrly in lrge rtings to provide etter het sink. There eing no intentionl ir gp. The conductors nd the enclosure thus trnsform into compct enclosure. Since the usrs re lmost touching (seprted y thin epoxy or polyester insultion) nd hve little scope for movement, the proximity effect in terms of electromgnetic forces hs little influence on the us system. Becuse of this these us systems re suitle for power systems hving high fult levels. Since the proximity effect in terms of X is only little compred to conventionl usrs, the system does not cll for specil enclosure tretment to dissipte excessive mgnetic het or phse trnsposition or insertion of n inductor in the middle phse to mke up for the lost inductnce (Section 8.8.), to void voltge unlnce. Nevertheless in lrger rtings (3000 A nd ove) when the length of the us is more, it is dvisle to provide phse trnsposition chmers t resonle intervls (sy, m or so) to lnce us inductnces (for whtever little my exist), minimize conductor losses in the middle phse shring higher current, s well s voltge unlnce. Busrs so produced therefore help in mintining voltge lnce in the three phses unlike in conventionl us system. It is esy to provide tp-off joints s required in such system like in rising mins or horizontl uswys. Jointing nd termintion kits re lso supplied y the mnufcturer to fcilitte esy jointing nd termintion. A few such kits like T-joint, phse trnsposition chmer nd expnsion joint re shown in Figure 8.4(e). Busrs in flts, tues or chnnels in ox form cn lso e used depending upon the current rting. It is however usul to use flt rs, eing simpler to use nd cn meet most current requirements on n LV or HV systems. To further mitigte the skin nd proximity effects in lrge rtings using two usrs or more, the mnufcturer cn choose more efficient configurtion s shown in Figure To mintin current crrying cpcity over long yers of opertions it is impertive to void surfce oxidtion. Since the usrs re totlly encpsulted nd seled from tmosphere providing direct insultion coting on its surfces (surfces must e free from oxidtion) is quite sfe nd only the exposed portions (terminls) e silver plted for mking connections. Nevertheless, some mnufcturers dopt to tin plting the whole us lengths (except the end T-joint Phse trnsposition chmer Comintion elow Expnsion joint Figure 8.4(e) Views of T-joint, phse trnsposition chmer, elow nd expnsion joint for sndwiched us system (Courtesy: Myduct Technology SDN.BHD.)

13 Crrying power through metl-enclosed us systems 8/993 Ground clmp G R Y B N Copper us Configurtion of usrs Plug-in contcts Self interlocking clmp Plug-in outlet Guide port Guide clmp GRYBN Note: Similr rrngement for horizontl uswys Figure 8.4(f1) View of compct rising mins with 5 uses (ground, RYB nd N) nd plug-in oxes (Courtesy: Myduct Technology SDN.BHD.) terminls for resons noted elow) to sfegurd ginst possile pin holes or wek insultion res left out during the process of insultion, lso ginst ccidentl rupture of insultion during ssemly. Such miniture exposures to tmosphere my cuse trcking over period of time nd render the usrs vulnerle to grdul oxidtion consequently withering of its current crrying cpcity. Limittion of tin plting t the end terminls Tin lso mkes good conductor of electricity nd costs only one tenth tht of silver plting s rough estimte ut clls for the following sfegurds, The plting procedure clls for utmost cre. It hs high contct resistnce nd clls for high derting of the terminls nd hence the entire us system (up to 0%) Arcing ets wy tin coting. It is therefore not suitle for drw-out contcts tht my wither the coting due to their movements nd silver plting lone is recommended. At properly mde stright-through joints, tin coted surfces my e good s proper contcts cuse no rcing. But it is too theoreticl in view of geing nd loosening of hrdwres with time tht my render the joint vulnerle to filure. Usully therefore t stright-through joints most mnufcturers dopt to silver plting only. For fixed usrs, however, like the stright lengths tin plting is quite suitle nd economicl. Aout 810 microns of tin coting is found dequte for fixed usrs. Therefore electricl grde silver plting t joints 56 microns nd t drw-out contcts 810 microns nd tin plting 810 microns t the fixed lengths is usul prctice of most mnufcturers. Silver coted portions tht re exposed to tmosphere ut re non-functionl re usully flshed with just 1 micron of silver coting. Busrs so seled cn e operted t tempertures higher thn 90ºC (see Section 8.5.1). It is however dvisle to choose higher cross-sectionl re of usrs to keep the het loss low (loss R 1/A (A re of cross-section)) s mesure to sving energy. Rising mins nd overhed uswys cn e insulted with IP55 or IP66 enclosures to mke them fire retrdnt or self-extinguishing like fire retrdnt cles. Figure 8.4(f1) shows typicl rising mins nd Figure 8.4(f) shows when it is instlled s overhed us system. Epoxy or polyester encpsulted system cn withstnd forceful wter jets during fire fighting opertions nd cn even e sumerged in wter. These usr systems re like stndrd products for mnufcturer nd re not required to e custom-uilt for every ppliction except for vritions in mient conditions or specil site requirement like contminted, humid or hzrdous loctions. Strightthrough joints, ends (elows), T-joints, flnges, trnsposition chmers, flexile nd expnsion joints re stndrdized nd re mnufctured s stndrd products. These usrs cn e ordered s per site pln nd esily ssemled t site. Mny leding mnufcturers even stock the stndrd lengths in different rtings with their ccessories for redy deliveries like ny other stndrd product. Only short lengths or end connections need e mnufctured s per the site pln t the lst moment.

14 8/994 Electricl Power Engineering Reference & Applictions Hndook Joint connector ssemly Grounding jw for plug-in unit Moulded plug-in socket Elow nd tee Fittings Flnged end Figure 8.4(f) Plug-in usr trunking in horizontl formtion (800A00A) (Courtesy: Schneider Electric) They conform to the sme test requirements s other us systems nd lso withstnd fire retrdnt, humidity nd geing tests. The compct us systems re usully fire resistnt for hrs s per ISO 834. () Prtilly isolted phse us systems (PIPBs) (for HV nd MV systems) For switchyrds, lrge susttions 15 MVA nd ove, medium-sized to lrge generting sttions or lrge industries nd cptive power genertions where ny of the ove conventionl us systems my pose limittion either ecuse of their ulk (lrge conductor spcings in segregted us system) or cost (s for IPBs) or ecuse of their rigidity tht the PIPBs, s evolved y some mnufcturers, cn provide n esy lterntive. The usul method tht one cn choose to interconnect switchger ssemly with trnsformer, trnsformer with swithchyrd or n over-hed line with trnsformer or switchger cn e one of the following, (i) XLPE cles XLPE cles is n idel method ut these my not e esily ville in short lengths, s they re usully produced in lengths of 1000 m or so unless the project is lredy using these cles. When only short lengths re required for interconnections, vilility of these cles my pose limittion nd PIPBs cn provide redy nswer. Moreover, XLPE cles my not e suitle for lrge rtings ecuse multiple runs of them my render them unwieldy nd cumersome to hndle nd terminte. (ii) GIB (gs insulted us system) In GIS susttions to interconnect the switchger with the trnsformer through GIB is n esy wy nd is usully prctised. GIBs cn e produced compct nd esily fricted y GIS mnufcturers ccording to the site requirements. To provide GIB etween trnsformer nd switchyrd however, is usully not prcticle for ovious resons nd XLPE cles or PIPBs lone cn serve the purpose. (iii) In n SF 6 ir insulted susttion one cn use XLPE cles if ville in short lengths or opt for PIPB system. Since, XLPE cles my not lwys e possile s noted ove unless they re eing lredy used t the sme site for trnsmission or distriution purposes, PIPBs provide n esy nswer. (iv) PIPBs The sic purpose of this system is sfety nd security. For inter-connecting trnsformer nd switchyrd usully re conductors re used. In seismic res for sfety nd integrity of the system it is dvisle to dpt for enclosed conductors nd PIPBs provide redy solution. Another dvntge of PIPBs is tht they re custom-uilt nd cn e supplied s per site requirement with uilt-in jointing rrngement. It is therefore esy nd fst to instl nd mke end termintions of such us system t site. A few more points in fvour of PIPBs compred to XLPE cles nd other us systems re noted elow, These us systems re esy to hndle, instl, ly nd terminte nd cn e used s cles of much higher rtings. Here lso, the sic concept is tht of cles

15 Crrying power through metl-enclosed us systems 8/995 PIPB system PIPB system (g1) Connecting on the 4 kv/0a pnel side (g) Connecting on the 4 kv/0a trnsformer side Figure 8.4(g) Appliction of prtilly isolted phse us systems (PIPBs) similr to cles (Courtesy: MGC Technologie AG) PIPB system Figure 8.4(h) Lyout of 7.5 kv/1a PIPB t switchyrd (Courtesy: MGC Technologie AG) (Appendix 16) where XLPE cles re produced up to 5 kv in extremely compct sizes. Becuse of PIPB flexiility, trnsformer nd switchger cn e plced t different loctions s convenient thn in the sme room if tht e constrint. Figures 8.4(g1 nd g) illustrte the ppliction of PIPBs similr to cles in susttion while Figure 8.4(h) shows its use in switchyrd. PIPBs therefore my e preferred choice for inter-connecting switchgers to trnsformers nd trnsformers to switchgers for n industry or lrge instlltion or switchyrd for further distriution. This system is usully mnufctured up to 45 kv nd 8000 A or so in single phse configurtion like n isolted phse system (IPB) nd hence the nme PIPB. For ccurte detils one my consult the mnufcturer. Technicl dt of few voltge systems nd corresponding current rtings of prticulr mnufcturer re furnished in Tles 8.0(3) nd 8.0(4), for luminium nd copper conductors respectively for generl reference. The min conductor tht cn e of luminium or copper is vcuum epoxy resin cst, is compct nd encpsulted within igger dimeter tue preferly of luminium, CrNi steel or polymide, like n IPB, mking the enclosure somewht Frdy Cge (Section 3.18). Now it serves dul purpose role of shield for the spce field nd lso s ground conductor similr to XLPE cles. Since mgnetic forces re low nd uses flexile, they too re cple to withstnd lrge fult currents. The outer insultion is protected ginst sudden shocks nd humidity through protection tue tht lso mkes it resistnt to moisture ingress. The touch voltge, surfce to ground is mintined within sfe limit of V (Section.9.6). All this mkes it n IP65 enclosure nd since the usrs re seled they cn e sfely operted up to much higher temperture s noted in Section It is however preferred to operte them t lower tempertures s noted lredy, to sve on het loss nd energy. Insulting system etween the conductor nd its metllic shielding, nd etween the metllic shield nd the outer sheth is the most importnt feture of such usrs. This insultion is usully epoxy resin tht mkes the whole conductor nd its shield s solid mss to enle n effective nd efficient het trnsfer through conduction rther thn convection. Ech individul usr is cpcitive grded*. *Cpcitive grding It is mesure of controlling electric field distriution long the surfce of the conductor insultion during trnsient condition, smoothing the distriution of surge voltges nd sving the us insultion system (mjor insultion re, Section ) from the rriving surges. Now lso the sme theory of trvelling wves pply s discussed in Section Cpcitive grding shrouds the us insultion nd is chieved y providing dielectric rriers usully t the ends (terminls) of the us system tht re more vulnerle to the rriving surges. Dielectric rriers cn e provided through non-liner resistor (SiC or ZnO) (Sections nd 18.1.) or through cpcitive grding foils (Figure 5.1). In cse of cpcitive grding, metllic foils re inserted during the usr insulting process. The foils form series of cpcitors etween the current crrying conductor nd the ground nd re designed to grde the electric field t the terminls to optimize the insulting system during trnsient condition.

16 8/996 Electricl Power Engineering Reference & Applictions Hndook Tle 8.0(3) Technicl dt nd dimensions with conductor in luminium for prtilly isolted phse us systems (PlPBs) Rted voltge Power frequency withstnd voltge, Hz, 1 minute, dry kv kv kv A mm mm kg/m mm pf/m 1/ / Dry lightning impulse voltge, 1./ ms 75/ Rted current Dimeter of the conductor / 110/ /40 70/40 100/ /40 100/ / Dimeter of the protection tue Weight per single phse Stndrd end rdius Cpcitnce Cpcitnce Tle 8.0(4) Technicl dt nd dimensions with conductor in copper for prtilly isolted phse us systems (PlPBs) Rted voltge Power frequency withstnd voltge, Hz, 1 minute, dry Courtesy: MGC Technologie Dry lightning impulse voltge, 1./ ms Rted current Dimeter of the conductor Dimeter of the protection tue Weight per single phse Stndrd end rdius kv kv kv A mm mm kg/m mm pf/m 1/17.5 8/38 75/ / / / / / / / / / / / / /

17 Crrying power through metl-enclosed us systems 8/997 The outer sheth my e of PVC or synthetic mteril to protect the metllic shield nd the conductor nd the insultion system from mechnicl dmge. And if pper is used s insulting medium, prevent it from the ingress of tmospheric moisture. The conductor nd its metllic shield re mde of tuulr section for ese of construction nd to lso extend flexiility in mnoeuvring the usrs t ends, joints nd termintions. It is esy to crry nd clmp the us lengths on structures or hng them through the ceiling. Figure 8.4(i) shows PIPB on rcks. Up to 3000 A there will e little field in the spce, most of it eing sored y the metllic shield itself (encpsulted ground conductor). Nevertheless for lrger rtings it is desirle to tke extr precution t the joints nd termintions to provide dequte onding to void loclized hot spots or even smll field in the spce. Jointing nd termintion kits re supplied y the mnufcturer to ridge this requirement nd provide n lmost continuous single conductor us system. Due to low X on djcent phses this system lso, like the sndwich us system, provides n lmost lnced voltge system nd clls for no-phse trnsposition. However, where necessry (like for long routes nd very lrge current systems) the uses eing flexile cn e esily trnsposed t suitle intervls. These usrs re usully produced in HV nd MV systems eing costly in LV. Where XLPE cles hve limittion PIPBs provide the solution. They conform to the sme test requirements s other us systems nd lso withstnd fire retrdnt, humidity nd geing tests. (3) Gs insulted usrs (GIB) For SF 6 insulted usrs see Section Design prmeters nd service conditions for metl-enclosed us system Design prmeters A us system would e designed to fulfil the following prmeters. Figure 8.4(i) PIPB on horizontl rcks (Courtesy: MGC Technologie AG) Rting A us system, like switchger ssemly, would e ssigned the following rtings: Rted voltge: the sme s tht ssigned to the ssocited switchger (Section (1)) Rted frequency: the sme s tht ssigned to the ssocited switchger (Section ()) Rted insultion level (i) Power frequency voltge withstnd see Section 3.3. (ii) Impulse voltge withstnd test for ll LV nd HV us systems see Section Continuous mximum rting (CMR) nd permissile temperture rise: this is the mximum r.m.s. current tht the us system cn crry continuously without exceeding temperture rise limits, s shown in Tle 3.3. The preferred current rtings of the us system would follow series R-10 of IEC 60059, s shown in Section (4). Energy conservtion Like for cles Section A 16.9, it is suggestive to choose for slightly higher cross-section for us sections for min us s well s links (where convenient) to conserve on energy losses. It is lso suggestive tht mnufcturers nd equipment suppliers, who del with energy efficient products or technologies, provide repyment schedule s customry, to their users to encourge them use energy efficient products nd technologies nd enle them tke more prgmtic decision while mking the purchses. Rted short-time current rting: this is the sme s for the system to which it is connected, nd s ssigned to the ssocited switchger (Section (5)). The effects of short-circuit on n electricl system re discussed elow. Rted momentry pek vlue of the fult current: the sme s ssigned to the ssocited switchger s in Tles or 8.1. See lso Section (7). Durtion of fult: the sme s ssigned to the ssocited switchger (Section (6)). 8.4 Short-circuit effects (To determine the minimum size of currentcrrying conductors nd decide on the mounting rrngement) A short-circuit results in n excessive current due to low impednce of the fulty circuit etween the source of supply nd the fult. This excessive current cuses excessive het ( µ Isc R ) in the current-crrying conductors nd genertes electromgnetic effects (electric field) nd electrodynmic forces of ttrction nd repulsion due to d.c. component (symmetry) etween the conductors nd their mounting structure. These forces my e ssumed s distriuted uniformly over the length of conductors nd cuse shering forces due to the cntilever effect s well s compressive nd tensile stresses on the mounting structure. The effect of short-circuit

18 8/998 Electricl Power Engineering Reference & Applictions Hndook Tle 8.1 C-37/0C Momentry pek current rtings (symmetricl) for switchger nd metl-enclosed us systems, sed on ANSI- *Nominl voltge Rted current (I r ) Non-segregted phse system Segregted phse system Isolted phse system (V r ) kv(r.m.s.) A ka ka ka to to to to to 190 To mtch with the rting of the connected interrupting device to to 190 *For new voltge systems s per IEC 60038, see Introduction. (i)these vlues re sed for system, pertining to series II nd frequency of 60 Hz. (ii) For systems pertining to Series I nd frequency of Hz, vlues furnished in Section (4), would pply. The pek vlue is function of fult level Section (7), Tle Which in turn, is function of size nd impednce of the feeding source, such s trnsformer or genertor, Section (5), Tle The vlues prescried in the ove tle re thus sed on these prmeters. therefore requires these two very vitl fctors (therml effects nd electrodynmic forces) to e tken into ccount while designing the size of the current-crrying conductors nd their mounting structure. The ltter will include mechnicl supports, type of insultors nd type of hrdwre, esides the longitudinl distnce etween the supports nd the gp etween phse-to-phse conductors. The electrodynmic forces my exist for only three or four cycles (Section (7)), ut the mechnicl system must e designed for these forces. On the other hnd, the min current-crrying system is designed for the symmetricl fult current, I sc (Tle 13.7) for one or three* seconds ccording to the system design. For more detils refer to Section The fult level, which is function of the size of the feeding trnsformer, is generlly considered to lst for only one second, s discussed in Section (5), unless the system requirements re more stringent. This durtion of one second on fult my cuse such temperture rise (not the electrodynmic forces), tht unless dequte cre is tken in selecting the size of the current-crrying conductors, they my melt or soften to vulnerle level efore the fult is interrupted y the protective devices. This philosophy, however, is not pplicle for circuits protected through current-limiting devices. See note elow. Note When the circuit is protected through HRC fuses or uilt-in shortcircuit releses of current limiting interrupting device the cut-off time my e extremely low, of the order of less thn one qurter of cycle, i.e. < second (for Hz system) (Section ) depending upon the size nd the chrcteristics of the fuses or the interrupting device nd the intensity of the fult current. Any level of fult for such system would e of little consequence, s the interrupting device would isolte the circuit long efore the fult current reches its first pek. This is when the fult is downstrem of the protective device. Refer to Exmple 8.1 elow. Exmple 8.1 Since the heting effect µ I sc t therefore heting effect of ka fult current for second µ 0.005, compred to the heting effect of n equivlent fult current I sc for 1 second, i.e. µ I 1 sc or I sc = i.e I sc = or 3.5 ka only Thus to design system protected through HRC fuses or current limiting device for higher fult level thn necessry will only led to overprotection nd the extr cost of the current-crrying system, switching equipment nd power cles. An individul device or component nd its connecting links in such cses my therefore e designed for size commensurte to its current rting. See lso Section (Figure 13.9). Below we discuss the therml effects nd the electrodynmic forces which my develop during fult to decide on the correct size of the conductor nd its supporting system Therml effects With norml interrupting devices the fult current would lst for only few cycles (mximum up to one or three* seconds, depending upon the system design). This time is too short to llow het dissiption from the conductor through rdition or convection. The totl het generted on fult will thus e sored y the conductor itself. The size of the conductor therefore should e such tht its temperture rise during fult will mintin its end temperture elow the level where the metl of the conductor will strt to soften. Aluminium, the most widely used metl for power cles, overhed trnsmission nd distriution lines or the LV nd HV switchger ssemlies nd us duct pplictions, strts softening t temperture of round C. As rule of thum, on fult sfe temperture rise of 100 C ove the llowle end temperture of 85 C or 90 C of the conductor during norml service, i.e. up to C during fult condition, is considered sfe nd tken s the sis to determine the size of the conductor luminium or copper. *See Section 13.5()

19 Crrying power through metl-enclosed us systems 8/999 The welded portion, such s t the flexile joints*, should lso e sfe up to this temperture. Welding of edges is essentil to sel off flexile ends to prevent them from moisture condenstion, oxidtion nd erosion of metl. Tin or led solder strts softening t round this temperture nd should not e used for this purpose. For joints other thn flexiles it is dvisle to use oxycetylene gs welding or rzing for copper nd tungsten inert gs (TIG) or metl inert gs (MIG) welding for luminium joints. Note In cse of copper lso, the end temperture is considered s 185 C only. Although this metl cn sustin much higher temperture thn this, without ny dverse chnge in its mechnicl properties, merely s considertion to Tle 3.3, nd to sfegurd other components, insultions nd welded prts etc., used in the sme circuit. To determine the minimum size of conductor for required fult level, I sc, to ccount for the therml effects one cn use the following formul to determine the minimum size of conductor for ny fult level: sc qt = k I Ê ˆ (1 + µ 0 q) t (8.1) 100 Ë A where q t = temperture rise (in C) I sc = symmetricl fult current r.m.s. (in Amps) A = cross-sectionl re of the conductor (in mm ) µ 0 = temperture coefficient of resistnce t 0 C/ C, which s in Tle 30.1 is for pure luminium nd for luminium lloys nd for pure copper q = operting temperture of the conductor t which the fult occurs (in C) k = for luminium nd 0.5 for copper t = durtion of fult (in seconds) Exmple 8. Determine the minimum conductor size for fult level of ka for one second for n luminium conductor. Assuming the temperture rise to e 100 C nd the initil temperture of the conductor t the instnt of the fult 85 C then 100 = or 100 = or A = Ê ( ) 1 Ë Á ˆ A 000 Ê Ë Á ˆ A mm for pure luminium or mm for lloys of luminium (ssuming 0 = ) The stndrd size of luminium flt nerest to this is.8 *Welding of flexile joints should preferly e crried out with high-injection pressing (welding y press heting), eliminting the use of welding rods. mm 1.7 mm or ('' 1 / '' ) or ny other equivlent flt size (Tles 30.4 or 30.5). This formul is lso drwn in the form of curves s shown in Figure 8.5, Isc t (I sc in ka) versus finl temperture. A From these curves the minimum conductor size cn e esily found for ny fult level, for oth luminium nd copper conductors nd for ny desired end temperture. As in the ove cse 100 = sc Ê I ˆ t 100 Ë A I 4 sc or t = 10 A = (I sc is in ka) Generlizing, Isc t = for n operting temperture A t 85 C nd end temperture on (8.) fult t 185 C Therefore, for the sme prmeters s in Exmple 8. A = mm A smll difference, if ny, etween this nd tht clculted ove my e due to pproximtion nd interpoltion only. This minimum conductor size will tke ccount of the heting effects during the fult, irrespective of the current rting of the conductor. This much conductor size is essentil for this fult level even for very low current rtings. However, the required conductor size my e more thn this lso, depending upon the continuous current it hs to crry, s discussed lter. Exmple 8.3 If the conductor is of copper then, ssuming the sme prmeters, 100 = Ê 0.5 or A = 000 Á Ë 100 = 416 mm Ê 000 ( ) 1 Ë Á ˆ A Copper is two thirds the size of luminium for the sme prmeters. The melting point of copper t lmost 1083 C (Tle 30.1) is pproximtely 1.5 times tht of luminium t 660 C. These melting points re lso locted on the nomogrms in Figure 8.6. Refer to nomogrms () nd () for luminium nd (c) for copper conductors. The sme re cn lso e otined from the copper curves of Figure 8.5. Assuming the sme end temperture t 185 C, then corresponding to the operting curve of 85 C, Isc A t = 0.1 (8.3) nd for the sme prmeters s in Exmple 8.3, A 1 = 0.1 or A mm ˆ

20 8/1000 Electricl Power Engineering Reference & Applictions Hndook 400 Initil temperture 85 C 70 C 40 C 0 C 85 C 70 C 40 C 0 C Finl temperture ( C) Aluminium Copper I sc Symmetricl rms vlue of fult current in ka A Cross-sectionl re of conductor in sq mm. t Durtion of fult in seconds Figure I sc A t Determining the minimum size of conductor for required fult level Almost the sme size is lso determined through the use of nomogrms drwn on susidiry nomogrm (c) nd the min nomogrm (d). Nomogrms Figure 8.6()(d) hve lso een drwn sed on Eqution (8.1). From these nomogrms the minimum conductor size cn e extrpolted tht would e necessry to sustin given fult level for prticulr durtion. The results of these nomogrms re lso the sme s those from the erlier two methods except for the pproximtion nd the interpoltion. Exmple 8.4 Assume the sme prmeters s in Exmple 8.. Procedure Locte the initil temperture (85 C) nd the finl temperture (185 C) on the susidiry nomogrm (). Drw stright line etween these points to otin the heting function H. Trnsfer the vlue of the heting function H to the H scle on the min nomogrm (d). Locte time t s one second on the T scle. Locte the current to e crried, I sc, s 000 A on the I sc scle. Drw stright line through the points on the T nd I sc scles to intersect the turning xis X. Drw stright line through the points on the H scle nd on the turning xis X. The point where the line intersects on the A scle will determine the conductor re required. In our cse it is 1 squre inch or 645 mm Electrodynmic effects (pplicle in cse of conventionl usr systems) The short-circuit current is generlly symmetricl nd contins d.c. component, I dc, s discussed in Section (7). The d.c. component, lthough lsts for only three or four cycles, cretes su-trnsient condition nd cuses excessive electrodynmic forces etween the current-crrying conductors. The mounting structure, usr supports nd the fsteners re sujected to these forces. This force is gretest t the instnt of fult initition nd is represented y the first mjor loop of the fult current, s noted in Tle Although this force is only momentry, it my cuse permnent dmge to these components nd must e considered when designing the current-crrying system nd its mounting structure. The mximum force in flt usrs my e expressed y

21 Crrying power through metl-enclosed us systems 8/ Initil temperture ( C ) Melting point (liquid) Melting point 660 (solid) Finl temperture ( C) Heting function unit of 10 6 mp.sec.cm 4 Initil temperture ( C) Melting point (liquid) Melting point 660 (solid) Finl temperture ( C) Heting function unit of 10 6 mp.sec.cm () Susidiry nomogrm for electrolytic grde luminium INDAL CISM. 0 H-Scle 0 0 () Susidiry nomogrm for electrolytic grde luminium INDAL D S WP. 0 H-Scle Initil temperture ( C) Melting point (liquid) Melting point (solid) Finl temperture ( C) Heting function unit of 10 6 mp.sec.cm H-Scle (c) Susidiry nomogrm for 100% IACS copper. Figure 8.6 Use of nomogrms