Part V. Busbar Systems

Prt V Busr Systems

8/983 8 Crrying power through metlenclosed us systems Contents 8.1 Introduction 8/985 8. Types of metl-enclosed us systems 8/985 8..1 A non-segregted phse us system 8/985 8.. A segregted phse us system 8/986 8..3 An isolted phse us (IPB) system 8/987 8..4 A rising mins (verticl us system) 8/988 8..5 An overhed us (horizontl us system) 8/988 8..6 Non-conventionl, compct nd low loss us systems 8/988 8.3 Design prmeters nd service conditions for metl-enclosed us system 8/997 8.3.1 Design prmeters 8/997 8.4 Short-circuit effects 8/997 8.4.1 Therml effects 8/998 8.4. Electrodynmic effects 8/1000 8.5 Service conditions 8/1004 8.5.1 Amient temperture 8/1004 8.5. Altitude 8/1006 8.5.3 Atmospheric conditions 8/1006 8.5.4 Excessive virtions nd seismic effects 8/1006 8.6 Other design considertions 8/1007 8.6.1 Size of enclosure 8/1007 8.6. Voltge drop 8/1007 8.6.3 Skin nd proximity effects on current-crrying conductor 8/1008 8.7 Skin effect 8/1008 8.7.1 Skin effect nlysis 8/1009 8.7. Determining the skin effect 8/1010 8.8 Proximity effect 8/1013 8.8.1 Proximity effect in terms of usr rectnce 8/1014 8.8. Voltge unlnce s consequence of the proximity effect 8/1016 8.8.3 Derting due to the proximity effect 8/101 8.8.4 Minimizing the proximity effect 8/10 8.8.5 Energy sving 8/105 8.9 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

Crrying power through metl-enclosed us systems 8/985 8.1 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 60059 nd s discussed in Section 13.4.1(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.

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 3.311 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

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 13.4.1(4). They my increse to 6000 A or so depending upon the ppliction. 8..3 An isolted phse us (IPB) system (for very lrge rtings 10 000 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

8/988 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, 00400 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, 0.453.0 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 7.510 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)). 8..5 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.

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)

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.)

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 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0. 0.1 0.0 400 1 6 5 157.75 96.31 184.83 109.9 17.45 17.47 14.16 118.96 11.43 104.87 96.44 87.4 77.3 66.73 600 1 6 40 96.6 85.48 19.01 100.41 19.10 133.63 133.73 131.31 17.14 11.58 114.86 107.1 98.43 88.83 800 1 6 69.64 5.40 74.13 96. 10.0 98.31 9.68 86.05 78.73 70.86 6.5 53.78 44.67 35.0 1000 1 6 75.75 19.38 54.3 87.90 93.74 90.46 85. 79.60 73.0 65.9 58.39.47 4.19 33.57 1 1 6 100 37.89 14.84 40.69 8.03 87.84 84.91 80.37 74.9 68.84 6.6 55.6 47.89 40.17 3.13 17 1 6 15 30.4 11.86 3.48 91.66 98.16 94.90 89.84 83.76 76.96 69.61 61.79 53.55 44.94 35.95 000 1 6 1 5.15 9.80 7.00 87.1 93.1 90.07 85.3 79.43 7.96 65.96 58.5.69 4.49 33.95 0 1 6 00 18.95 7.57 0.41 8.06 88.14 85.31 80.85 75.46 69.4 6.87 55.89 48.53 40.8 3.78 3 6 15 15.1 6.05 16.9 78.57 84.41 81.7 77.45 7.9 66.51 60.4 53.56 46.5 39.14 31.44 4000 6 1 1.58 5.00 13.54 87.16 93.54 90.51 85.75 80.01 73.58 66.61 59.19 51.37 43.18 34.64 40 3 6 15 10.08 4.15 10.90 78.57 84.41 8.6 78.10 73.0 67.30 61.07 54.43 47.41 40.04 3.35 50 3 6 1 8.38 3.43 9.05 7.57 78.6 75.88 7.01 67.31 6.01 56.5.11 43.6 36.81 9.71 6300 3 6 00 7.5.87 7.80 75.34 80.81 78.17 74.40 69.07 63. 57.47 51.06 44.9 3.7.98 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 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0. 0.1 0.0 400 1 6 5 157.75 115.57 195.55 109.9 133.6 135.48 133.69 19.63 13.99 117.10 109.17 100.31 90.60 80.07 600 1 6 40 96.6 10.58 140.9 100.41 136.84 144.9 146.4 145.53 14.53 137.87 131.8 14.53 116.11 106.60 800 1 6 69.64 30.48 76.0 96. 105.6 10.54 97.71 91.69 84.83 77.31 69.4 60.68 51.67 4.3 1000 1 6 75.75 3.6 55.83 87.90 96.67 94.49 90.30 84.97 78.84 7.09 64.80 57.05 48.88 40.9 1 1 6 100 37.89 17.81 41.87 8.03 90.64 88.76 84.96 80.07 71.41 68.15 61.39 54.19 46.57 38.56 17 1 6 15 30.4 14.3 33.4 91.66 10.13 99.1 94.97 89. 83.18 76.0 68.64 60.59 5.08 43.13 000 1 6 1 5.15 11.76 7.78 87.1 96.17 94.14 90.08 84.86 78.84 7.19 65.00 57.34 49.5 40.74 0 1 6 00 18.95 9.08 1.01 8.06 90.99 89.4 85.5 80.69 75.08 68.86 6.1 54.93 47.33 39.3 3 6 15 15.1 7.6 16.77 78.57 87.15 85.49 81.94 77.3 71.95 66.00 59.56 5.68 45.39 37.7 4000 6 1 1.58 6.00 13.94 87.16 96.56 94.67 90.70 85.55 79.58 7.96 65.80 58.16.08 41.57 40 3 6 15 10.08 4.98 11.4 78.57 87.63 86.14 8.7 78.19 7.90 67.00 60.60 53.74 46.48 38.81 50 3 6 1 8.38 4.1 9.34 7.57 80.87 79.47 76.8 7.09 67.19 61.73 55.81 49.47 4.76 35.68 6300 3 6 00 7.5 3.13 7.90 75.34 81.99 79.79 75.97 71.3 65.84 59.95 53.63 46.94 3.99 3.5 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.

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 6.13.3) 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 8.14. 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.)

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.

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

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 65130 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 8.5.1. 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 17.10.1) from the rriving surges. Now lso the sme theory of trvelling wves pply s discussed in Section 17.8. 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 18.1.1 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.

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 1/17.5 4 36 5 7.5 13 8/38 70 105 140 30 Dry lightning impulse voltge, 1./ ms 75/95 15 170 35 5 Rted current 1 1600 000 0 31 1000 1 1600 000 0 31 800 1 1600 0 31 1100 000 900 1 1600 0 800 1 000 0 Dimeter of the conductor 36 45 55 80/ 110/80 30 40 70/40 70/40 100/70 5 36 45 70/40 100/70 36 60 30 40 80/ 30 55 55 70 Dimeter of the protection tue 55 67 80 106 146 55 67 80 106 106 146 55 67 80 106 146 80 106 80 106 106 146 106 146 146 146 Weight per single phse 4.1 6. 9 1 18.6 3.7 5.7 8.5 15.7 13.1 19.5 3.4 5.4 8 15.7 19.5 7. 14.3 6.8 1. 13.1 0.8 4 Stndrd end rdius 180 400 5 180 180 400 400 5 180 400 5 400 400 400 5 5 5 5 Cpcitnce Cpcitnce 190 1400 1515 410 410 640 80 930 1005 1440 105 45 595 655 1005 1300 370 300 90 410 555 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 75/95 1 3 55 8.8 180 845 1600 40 67 16.1 1405 000 80 0.5 876 0 70/ 106 1.8 400 1005 31 80/ 106 30.7 400 410 4000 110/80 146 46.4 5 410 4 15 1 3 55 8.8 180 845 1600 40 67 13.6 80 000 80 0.5 930 0 70/ 106 1.8 400 1005 31 80/ 146 30.7 5 4000 110/80 146 46.4 5 410 36 70 170 1000 5 55 6.5 180 45 1 3 67 10.1 590 1600 40 80 15.1 55 000 106 5 400 845 0 70/ 106 1.8 400 1005 31 80/ 146 39.8 5 1133 5 105 1 3 80 11.7 33 000 106 5 400 406 0 70/ 146 31 5 536 31 80/ 146 39.8 5 555 7.5 140 35 1 3 80 11.7 33 1600 106 5 400 406 0 70/ 146 31 5 536 31 80/ 146 39.8 5 555 13 30 5 1 45 146 34 5 000 146 5 5 0 70/ 146 31 5

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 19.10. 8.3 Design prmeters nd service conditions for metl-enclosed us system 8.3.1 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 13.4.1(1)) Rted frequency: the sme s tht ssigned to the ssocited switchger (Section 13.4.1()) 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 3.3.3 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 13.4.1(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 13.4.1(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 13.11 or 8.1. See lso Section 13.4.1(7). Durtion of fult: the sme s ssigned to the ssocited switchger (Section 13.4.1(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

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 0.6 1600 75 0.6 3000 100 0.6 4000 to 6000 1 4.1613.8 100 to 3000 19 to 78 14.4 100 to 0 000 60 to 190 To mtch with the rting of the connected interrupting device 334.5 100 to 0 000 58 60 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 13.4.1(4), would pply. The pek vlue is function of fult level Section 13.4.1(7), Tle 13.11. Which in turn, is function of size nd impednce of the feeding source, such s trnsformer or genertor, Section 13.4.1(5), Tle 13.7. 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 13.4.1(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 13.5. 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 13.4.1(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. < 0.005 second (for Hz system) (Section 13.5.1) 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 0.005 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 = 0.005 i.e I sc = 0.005 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 13.5.1 (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. 8.4.1 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 18000 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 185190 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()

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 0.00403 for pure luminium nd 0.00363 for luminium lloys nd 0.00393 for pure copper q = operting temperture of the conductor t which the fult occurs (in C) k = 1.166 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 = 1.166 100 or 100 = 1.166 100 or A = 000 100 000 Ê (1 + 0.00403 85) 1 Ë Á ˆ A 000 Ê 1.3455 Ë Á ˆ A 1.166 1.3455 65.6 mm for pure luminium or 617.6 mm for lloys of luminium (ssuming 0 = 0.00363) 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 = 1.166 sc Ê I ˆ 1.3455 t 100 Ë A I 4 sc or t = 10 A 6 1.166 1.3455 10 = 0.0799 (I sc is in ka) Generlizing, Isc t = 0.0799 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 = 1 65.8 mm 0.0799 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 = 0.5 100 Ê 0.5 or A = 000 Á Ë 100 = 416 mm Ê 000 (1 + 0.00393 85) 1 Ë Á ˆ A 1.33405 100 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 416.7 mm ˆ

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 380 360 340 Finl temperture ( C) 30 300 80 60 40 0 00 185 180 160 140 10 100 80 60 40 0 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 0 0.0155 0.031 0.0465 0.06 Figure 8.5 0.0775 0.0799 0.093 I sc A 0.1085 0.100 0.1 t 0.1395 0.155 0.1705 0.186 0.015 0.17 0.35 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. 8.4. 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 13.4.1(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 13.11. 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

Crrying power through metl-enclosed us systems 8/1001 3 3 Initil temperture ( C ) 1 100 Melting point (liquid) Melting point 660 (solid) 600 0 400 300 00 100 Finl temperture ( C) 300 00 1 100 Heting function unit of 10 6 mp.sec.cm 4 Initil temperture ( C) 1 100 85 Melting point (liquid) Melting point 660 (solid) 600 0 400 300 00 185 100 Finl temperture ( C) 300 00 1 100 60 Heting function unit of 10 6 mp.sec.cm 4 0 0 () 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 1000 00 900 Initil temperture ( C) 1 100 85 Melting point (liquid) Melting point (solid) 1083 1000 900 800 700 600 0 400 300 00 185 100 Finl temperture ( C) 800 700 600 0 400 300 00 1 100 Heting function unit of 10 6 mp.sec.cm 4 0 0 0 H-Scle (c) Susidiry nomogrm for 100% IACS copper. Figure 8.6 Use of nomogrms

8/100 Electricl Power Engineering Reference & Applictions Hndook 1000 900 800 700 600 0 400 300 00 For copper For luminium 100 90 80 70 60 40 30 0 10 5 10 0 30 40 100 00 415 400 300 0 645 1000 000 3000 4000 00 10000 0000 30000 40000 000 60000 0.001 0.005 0.01 Cross-sectionl re (sq. in) 0.05 0.10 0. 1.00 5 10 100 10 100 0 1000 00 10000 Current (mps rms) For luminium or copper for fult of ka for 1 sec,000 100,000 0,000 1000,000 5,000,000 10,000,000 100 10 5 1.0 0.5 Heting function (unit of 10 6 mp. sec.cm 4 ) Cross-sectionl re (sq mm) Time (sec.) H scle A scle X xis I sc scle T scle 0.1 0.05 0.01 Figure 8.6(d) Min nomogrm 1 (From Fig. c) 60 (From Fig. )

Crrying power through metl-enclosed us systems 8/1003 16 Isc 4 Fm = k 10 N/m (8.4) S where F m = estimted mximum dynmic force tht my develop in single- or three-phse system on fult. This will vry with the numer of current-crrying conductors nd their configurtion ut for ese of ppliction nd for revity only the mximum force tht will develop in ny configurtion is considered in the ove eqution. It will mke only mrginl difference to the clcultions, ut it will e on the sfe side. For more detils refer to the Further Reding t the end of the chpter. I sc =r.m.s. vlue of the symmetricl fult current in mperes Fctor of symmetry = s in Tle 13.11, representing the momentry pek vlue of the fult current. This fctor is considered in the numericl fctor 16 used in the ove eqution. k = spce fctor, which is 1 for circulr conductors. For rectngulr conductors it cn e found from the spce fctor grph (Figure 8.7) corresponding to S + 1.4 1.3 = 1. 1.1 = 5 = = 1 1.0 0.96 0.93 0.9 0.87 0.8 Spce fctor k 0.77 0.7 0.6 0.5 = 0.4 = 0.5 = 0.15 = 0.5 S 0.4 = 0.1875 0.3 = 0.1 S 0. 0.1 = 0 Figure 8.7 0 0. 0.4 0.6 0.8 1.0 0.358 0.47 0.6 0.84 0.958 0.87 1. 1.4 1.6 1.8.0 S + Spce fctor for rectngulr conductors (Source: The Copper Development Assocition)

8/1004 Electricl Power Engineering Reference & Applictions Hndook where S = centre spcing etween two phses in mm (Figure 8.8) = spce occupied y the conductors of one phse in mm, nd = width of the conductors in mm. The fctor k decreses with the increse in spcing S. For ppliction of the ove eqution, refer to Exmple 8.1. 8.5 Service conditions The performnce of us system cn e ffected y the following service conditions: 1 Amient temperture Altitude 3 Atmospheric conditions nd 4 Excessive virtions nd seismic effects 8.5.1 Amient temperture The rtings s provided in Tles 30. (, nd c), 30.4 nd 30.5 nd others refer to n mient temperture, with pek of 40 C nd n verge of 35 C over period of 4 hours. The end temperture for luminium is considered sfe t 8590 C, t which the metl does not degenerte (oxidise) or chnge its properties (mechnicl strength) over long period of opertion. Figure 8.9 shows the effect of higher operting tempertures on the mechnicl strength of luminium metl. The oxidtion nd mechnicl strength re two vitl fctors tht need e orne in mind when selecting usr size to ensure its dequcy during long hours of continuous opertion. Tle 8. lists the permissile operting tempertures of the vrious prts of us system. For higher mient tempertures, current cpcity should e suitly reduced to mintin the sme end temperture during continuous opertion. Refer to Tles 8.3() nd (), recommending the derting fctors for higher mient temperture or lower temperture rise for the sme end temperture of 85 C or 90 C respectively. For intermedite mient tempertures, see Figure 8.10. Operting tempertures of us conductors Aluminium nd copper conductors re susceptile to R Y B N Figure 8.8 proximity S S > or whichever is more. Plcement of usrs to minimize the effect of Stress kgf/mm 30 5 0 15 10 5 0.1 1 100 10 000 Heting period (seconds) Tle 8. Type of us connection Operting temperture of us system Bus conductor with plin connection joints 70 C Bus conductor with silver plted or welded contct surfces 105 C Enclosure Accessile prt 80 C Non-ccessile prt 110 C Termintion t cles with plin connections 70 C Termintion t cles with silver-surfced or equivlent connections 85 C 65 C, 85 C 15 C 1 C 175 C 00 C Figure 8.9 Curves showing the tensile strength of Indl DSWP t higher tempertures Mximum temperture limit s in IEEE-C-37-0.1 Or s specified y the user. Note For tempertures ove 100 C it is recommended to use epoxy insultors/supports, which cn continuously operte up to 15 C. FRP (fireglss reinforced plstic) insultors/supports my not withstnd 15 C. oxidtion nd corrosion s noted in Section 9.. But t elevted tempertures ove 8590ºC this phenomenon ecomes rpid nd my endnger the joint. The oxides of luminium (Al O) nd copper (CuO) re poor conductors of electricity nd dversely ffect us conductors, prticulrly t joints reducing their currenttrnsfer cpcity over time. This my led to their overheting, even n eventul filure. Universl prctice therefore, is to restrict the operting temperture of the us conductors luminium or copper to 8590ºC for ll rtings, t lest in the medium rnge sy, up to 300 A. The joints s such, however good they re mde, mke only micro contcts surfce to surfce nd contin ir

Crrying power through metl-enclosed us systems 8/1005 Tle 8.3 Derting fctors on ccount of higher mient temperture or restricted temperture rise () Operting temperture 85 C () Operting temperture 90 C Amient Permissile r Derting fctor Amient Permissile r Derting fctor temperture C temperture rise C temperture C temperture rise C 30 55 1.05 35 55 1.05 35 1.0 40 1.0 40 45 0.945 45 45 0.945 45 40 0.88 40 0.88 35 0.815 55 35 0.815 55 30 0.75 60 30 0.75 Notes 1 These dt re drwn in the shpe of grph in Figure 8.10. Intermedite vlues cn e otined y interpoltion. Derting fctor 1.5 1.05 1.0.945.88.815.75 1..5 30 35 40 45 55 35 40 45 55 60 Amient temperture ( C) 1 For mximum operting temperture 85 C For mximum operting temperture 90 C Figure 8.10 Derting fctors for different mient nd mximum operting tempertures pockets (s estlished y experiments). Outer contct surfces of the joints re noticed to e more prone to this feture which my led to quicker erosion of metl nd filure of joints. In chemiclly ggrvted, humid or sline loctions this phenomenon ccelertes yet more rpidly nd renders the joints more vulnerle to filures. At higher tempertures the erosion further ccelertes ecuse of rething ction s result of expnsions nd contrctions of usrs with the vritions in lod. There is differentil movement within the joint lso ecuse of different therml expnsions of usrs nd MS olts. A good joint cple of mintining dequte contct pressure over long periods nd proper gresing my prevent the erosion. But this is too theoreticl ecuse the regulr heting nd cooling of usrs my lso cuse grdul loosening of grip (contct pressure). To overcome ll this, seling of joints s noted elow is considered good technique to mke perfect joint. With proper seling the operting temperture of the us conductors cn lso e rised up to 10515 C. Using etter techniques of mking joints, the trend is chnging towrds ccepting higher operting tempertures. But it is not in keeping with the spirit of energy sving tht one is oliged to exercise (Section 1.19). It is therefore suggestive tht the mnufcturers use higher cross-sections of usrs to contin the het losses (I R) s low s possile nd furnish repyment schedule to the user, showing recovery of the initil higher cost of equipment in short period y sving on losses nd ensuring recurring svings therefter, sving environment in the process. Seling of joints Silver or silver oxide (Ag O) is good conductor of electricity nd so lso the welded joints. Silver plting* nd welding** re two different methods nd oth cn sel the inside surfces from the tmosphere nd cn lso prevent the contct surfces from oxidtion. If the joints re silver plted or welded, the us system cn lso e mde suitle to operte t higher tempertures. In luminium conductors, for instnce, they cn e operted up to n optimum temperture of 15 C, until luminium egins to lose its mechnicl strength (Figure 8.9). Similrly, copper conductors cn operte t still higher tempertures. The entire us system cn now e operted t much higher tempertures thn given in Tle 8.. However, opertion t such high tempertures my impose mny other constrints, such s high temperture in the vicinity which my endnger the operting personnel. It my even ecome source of fire hzrd. Such high temperture my lso dmge components mounted inside the enclosure, which my not e le to sustin such high tempertures. It my lso cuse limittions on gskets nd other hrdwre of the us system to operte t such high tempertures continuously. Accordingly, the mximum operting temperture of us system luminium or copper with silver-plted or welded joints is lso permitted up to 105 C only. The enclosure temperture is still restricted to 80 C, or up to 110 C t loctions tht re sfe nd inccessile to humn ody (Tle 8.). * The procedure of silver plting the joints is mentioned in Section 9..6. ** The welding is usully crried out y tungsten inert gs (TIG) or metl inert gs (MIG).

8/1006 Electricl Power Engineering Reference & Applictions Hndook 8.5. Altitude The stndrd ltitude for metl enclosed us system will remin the sme s for switchger ssemly (Section 13.4.). Higher ltitudes would require similr dertings in dielectric strength nd the current rtings s for switchger ssemly (Tle 13.1). To chieve the sme level of dielectric strength, the insultion of the us system my e improved y incresing the clernces nd creepge distnces to ground nd etween phses, s noted in Tles 8.4 nd 8.5. To chieve the sme vlue of continuous current, the size of the current-crrying conductors my e incresed sufficient to tke cre of the derting. Note It is lso possile to derive lmost the sme vlue of derting y reducing the llowle temperture increse y 1% for every 300 m rise in ltitude ove the prescried level. Clernces nd creepge distnces Clernce : It is the shortest distnce in ir etween two conductive prts Creepge : It is the shortest distnce long the surfce of n insulting mteril etween two conductive prts. The clernces nd creepge distnces for enclosed indoortype ir-insulted usrs, s suggested y BS 159, re given in Tles 8.4 nd 8.5 respectively. These vlues re considered for n ltitude of up to 000 m for LV nd 1000 m for HV systems. For higher ltitudes to chieve the sme level of dielectric strength, the vlues of clernces nd creepge distnces, my e incresed y t lest 1% for every 100 m rise in ltitude. 8.5.3 Atmospheric conditions The sme conditions would pply s for switchger ssemly (Section 13.4.). Unlike controlger or switchger ssemly, us system my e required to e prtly locted outdoors. This is true for most instlltions, s the switchyrd is normlly locted outdoors s is the feeding trnsformer, while, the switchgers re locted indoors, to which the us system is connected. In such conditions, it is importnt tht dequte cre is tken to construct the us enclosure to wether the outdoor conditions such s y providing cnopy on the top nd specil pint tretment on the outdoor prt. It is lso recommended to sel off the indoor from the outdoor prt to prevent the effect of rinwter, dust nd temperture nd other wether conditions on the indoor prt. This cn e chieved y providing sel-off ushings, one on ech phse nd neutrl, wherever the us enclosure psses through wll. The ushings my e of SMC/DMC/FRP or porcelin for LV nd epoxy compound for HV systems. They my e fitted t the crossovers so tht the indoor us is seled off from the outdoor one. The us conductors will pss through the ushings. The HV us conductors my e moulded with the epoxy ushings, s illustrted in Figure 8.11(), similr to r primry CTs (Figure 15.14) to mke the joint irtight. In LV simpler method is found y providing glss wool in the prt tht psses through the wll s illustrted in Figure 8.11(). 8.5.4 Excessive virtions nd seismic effects These will require more roust enclosure, similr to switchger ssemly. For detils refer to Section 13.4.. Tle 8.4 Clernces for enclosed, indoor ir-insulted usrs Rted voltge Minimum clernce Minimum clernce to ground in ir etween phses in ir kv (r.m.s.) mm mm Up to 0.415 16 19 0.6 19 19 3.3 51 51 6.6 64 89 11 76 17 15 10 165 140 41 33 356 Tle 8.5 Creepge distnces for enclosed indoor ir insulted usrs s in BS 159 Rted Minimum creepge Minimum creepge voltge distnce to ground distnce etween phses kv (r.m.s.) in ir mm in ir Up to 0.415 19 0.6 5 3.3 51 6.6 89 11 17 15 15 03 33 305 Ô Ô Minimum % more Ô Ô Ô Notes 1 The ove figures re only indictive, nd my e considered s minimum for us system tht is dry nd free from dust or ny contmintion, which my influence nd reduce the effective creepge over time. These creepges my e incresed for dmp, dirty or contminted loctions. For clrifiction nd more detils refer to BS 159. Common to oth tles 1 The ove clernces nd creepge distnces re for ltitudes of up to 000 m for LV nd 1000 m for HV systems. For higher ltitudes thn this, these distnces should e incresed y t lest 1%, for every 100 m rise in ltitude. 3 Voltges higher thn ove, re usully not pplicle in cse of conventionl us systems. Seemingly in this form ove 33 kv it my not e prcticl to use them ecuse of their rigidity. With the vilility of gs insultion (Section 19.10), however, there is no limittion in mnufcturing them for ny voltge system. They cn e fricted to ny shpe nd size to suit the site requirements prticulrly where GIS re instlled. 4 For higher voltge requirements such s for lrge susttions nd switchyrds or lrge industries, prticulrly t loctions not using gs insulted switchger (GIS) XLPE cles or prtilly isolted phse us system (PIPB) (Section 8..6) will e etter option s they cn fcilitte esy ending nd mnoeuvring.

Crrying power through metl-enclosed us systems 8/1007 R Y B Wll Glsswool Enclosure section Busrs R Phse segregtion (In cse of segregted us system) Section XX Outdoor Indoor Y B Sloping top X Sel-OFF ushing mounting plte Sel-OFF ushing (Epoxy) Inspection chmer N Epoxy compound or sleeving FRP or SMC/DMC supports Inspection chmer Busr Insultor () For LV systems () For HV systems X Elevtion Silicgel rether (typicl) Figure 8.11 Wll frme ssemly with sel-off ushing 8.6 Other design considertions Size of enclosure (not relevnt for non-conventionl us systems) Voltge drop Skin nd proximity effects 8.6.1 Size of enclosure The enclosure of the us system provides the cooling surfce for het dissiption. Its size hs n importnt ering on the temperture rise of conductors nd consequently their current-crrying cpcity. The enclosure effect nd the ventilting conditions of the surroundings in which the enclosure is to e instlled should thus e considered when designing us system. The rtio of the re of the current-crrying conductors to the re of the enclosure will provide the sis to determine the het dissiption effect. Tle 8.6 suggests the pproximte dissiption fctors tht cn e considered s likely dertings for us system under different conditions. See lso Exmple 8.1. Note For usr mounting configurtions see Section 13.6. 8.6. Voltge drop The voltge drop cross us system should e s low s possile nd generlly within 1% of the rted voltge. This criterion will generlly e pplicle to high current LV system. On HV nd low LV current-crrying systems, this drop my e quite low. The length of us system, in most of pplictions, my not e long enough to cuse high voltge drop, IZ, to e tken into considertion. It my e the connection from the incoming trnsformer to the min receiving switchger or the length of usrs of the min switchger ssemly itself. Applictions requiring extr-long current-crrying conductors, however, my hve lrge impednce nd cuse high voltge drops, of the order of 35% nd even more. When so, they my ffect the stility of the system s well s the performnce of the connected lod. This is illustrted in Exmple 8.9. To scertin the voltge drop in such cses it is essentil to determine the ctul vlues of the conductor s own resistnce, rectnce nd the impednce under ctul

8/1008 Electricl Power Engineering Reference & Applictions Hndook operting conditions. It my e noted tht rectnce is the min cuse of high voltge drop. Skin nd proximity effects ply vitl role in ffecting the resistnce nd rectnce of such systems. We discuss these spects riefly elow. 8.6.3 Skin nd proximity effects on currentcrrying conductor In d.c. system the current distriution through the crosssection of current-crrying conductor is uniform s it consists of only the resistnce. In n.c. system the inductive effect cused y the induced-electric field cuses skin nd proximity effects. These effects ply complex role in determining the current distriution through the crosssection of conductor. In n.c. system, the inductnce of conductor vries with the depth of the conductor due to the skin effect. This inductnce is further ffected y the presence of nother current-crrying conductor in the vicinity (the proximity effect). Thus, the impednce nd the current distriution (density) through the cross-section of the conductor vry. Both these fctors on n.c. system tend to increse the effective resistnce nd the impednce of the conductor, nd cuse higher Ic Rc loss, nd higher voltge drop I c Z, nd reduce its current-crrying cpcity. An.c. system is thus more complex thn d.c. system nd requires fr more cre when designing it for prticulr requirement. While these phenomen my e of little relevnce for low-current system, they ssume significnce t higher currents nd form n essentil prmeter to design high current-crrying system sy, 000 A nd ove. These phenomen re discussed riefly elow. Tle 8.6 Het dissiption fctor Enclosure Cross-sectionl re Derting of usrs cross- fctor sectionl re of enclosure 1 Outdoors < 1% 0.95 5% 0.90 10% 0.85 Indoors, where the < 1% 0.85 enclosure is in well- 5% 0.75 ventilted room 10% 0.65 3 Indoors, where the < 1% 0.65 enclosure is poorly 5% 0.60 ventilted nd the room 10% 0. temperture is high Notes 1 Intermedite vlues cn e otined y interpoltion. These dertings re ment only for non-mgnetic enclosures, where the het generted is only through induced electric currents (I R) nd hence low. There re no hysteresis or eddy current losses. 3 For MS enclosures, which will hve oth hysteresis loss (µ B 1.6 ) nd eddy current loss (µb ), higher derting fctor must e considered. The susequent text will clrify this spect. 8.7 Skin effect A current-crrying conductor produces n electric field round it which induces ck e.m.f. nd cuses n inductive effect. This e.m.f. is produced in the conductor y its own electric field cutting the conductor. It is more dense t the centre nd ecomes less t the surfce. The conductor thus hs higher inductnce t the centre thn t the surfce, nd cuses n uneven distriution of current through its own cross-section. The current tends to concentrte t the outer surfce of the conductor, i.e. its skin, shres more current thn the other prts of the conductor nd reduces with depth. It is lowest t the nucleus. For more thn one conductor per phse ll the conductors together my e considered s forming lrge conductor for the purpose of nlysing the skin effect. Now the ulk of the current will e shred y the end conductors nd only prtly y the middle conductors. Figure 8.1 demonstrtes n pproximte shring of current nd the het generted in one, two, three nd four flt sections per phse, plced in verticl disposition, in n lmost isolted plne, where they hve no or only negligile effect of proximity. Note These current shrings re only indictive. The ctul current shring will depend upon the thickness, the width nd the configurtion of the conductors. Refer to Tles 30.(, nd c), 30.4 nd 30.5. The phenomenon of uneven distriution of current within the sme conductor due to the inductive effect is known s the skin effect nd results in n incresed effective resistnce of the conductor. The rtio of.c. to d.c. resistnce, R c /R dc, is the mesure of the skin effect nd is known s the skin effect rtio. Figure 8.13() illustrtes the skin effect for vrious types nd sizes of luminium nd copper in flt sections. For esy reference, the skin effects in isolted hollow round nd chnnel conductors (in ox form) re lso shown in Figures 8.13() nd (c) respectively. Tles 30.7, 30.8 nd 30.9 for rectngulr, tuulr nd chnnel sections respectively, give the d.c. resistnce nd the rectnce vlues etween two luminium conductors of smll nd medium current rtings of ny two djcent phses, with centre-to-centre spcing of 305 mm or more, when the proximity effect is considered lmost negligile in these rtings. Since the skin effect results in n increse in the effective resistnce of the usr system it directly influences the heting nd the voltge drop of the conductor nd indirectly reduces its current-crrying cpcity. If R c is the resistnce s result of this effect then the het generted = I c R c where I c is the permissile current-crrying cpcity of the conductor on n.c. system to keep the sme heting effect s on d.c. system then the reduction in the current rting due to the skin effect cn e deduced y equting the two hets, i.e. I R = I R c c dc dc

Crrying power through metl-enclosed us systems 8/1009 305 mm 305 mm 1.5 4 = 8 16 40 R Y B N () 100 100 100 () 100% 100% 100% () () 305 mm 305 mm R Y B N ttt ttt ttt t 305 mm 305 mm 1:1 1:1 1:1 Skin effect rtio R c /R dc 1.45 1.4 1.33 1.3 1.75 1. 1.18 1.13 1.1 Interpolted for =.6 Interpolted for =.9 () () t t t t t 36 8 36 13 7.8 13 R Y B N 305 mm t t t t t 36 8 36 13 7.8 13 305 mm t t t t t 36 8 36 13 7.8 13 ttt R Y B N Cross-sectionl re of usrs (sq.cm) 1.055 1.0 6.45 10 1.9 19.35 0 5.8 30 38.71 40 10 30 40 60 70 80 90 100 11010 f/r ( R = W/1000 m) dc dc () () Note 30 0 0 30 Figure 8.1 phse 9 4 4 9 30 0 0 30 () Current shring % () Het generted (I R ) rtio 30 0 0 30 Ech phse is considered in isoltion not influenced y proximity effect Skin effect in different us sections of the sme Rdc or Ic = Idc (8.5) Rc The skin effect cn e minimized y employing different configurtions nd rrngement of usrs, s discussed lter nd illustrted in Figure 8.14. It cn lso e minimized y selecting hollow round or hollow rectngulr (chnnels in ox form) conductors, nd thus concentrting the mximum current in the nnulus nd optimizing metl utiliztion. For current rtings of luminium in round nd chnnel sections, refer to Tles 30.8 nd 30.9, respectively nd for copper Tles 30. (, nd c) for flt, tue nd chnnel sections, respectively. Exmple 8.5 If there is rise of 5% in the effective resistnce of the usrs due to the skin effect, then the.c. rting will e 9 4 4 9 9 4 4 9 = 60 1 Indl D SWP t 85 C Indl CISM t 85 C 3 Indl D SWP t 0 C 4 Indl CISM t 0 C R dc 1.05 R dc = 0.976 or 97.6% tht of the d.c. rting. 8.7.1 Skin effect nlysis 1 3 4 5 Copper t 85 C 6 Copper t 0 C Note 1. The lower cross-sectionl re curves relte to f = Hz. For other frequencies fr / dc must e clculted y multiplying these vlues y f /. Smll vrition is possile while drwing these curves () Flt usrs Figure 8.13() Skin effect in isolted usrs stnding on edges. (neglecting the proximity effect) (Source: Indin Aluminium Co. sed on Alcn of Cnd) When numer of flt rs re used in prllel their 6 5

8/1010 Electricl Power Engineering Reference & Applictions Hndook.0 t d = 0. 0.45 0.40 0.35 0.30 0.5 0.0 1.9 1.8 t t d = 0.15 Skin effect rtio R c /R dc 1.7 1.6 1.5 1.4 1.3 1. 1.1 1.0 1.0 0 10 d t d = 0.1 t d = 0.10 t d = 0.08 t d = 0.06 t d = 0.04 t d = 0.01 10 40 60 80 100 10 140 fr / ( R = /1000 m) dc dc W 1 3 4 5 6 Indl D SWP t 85 C Indl CISM t 85 C Indl D SWP t 0 C Indl CISM t 0 C Copper t 85 C Copper t 0 C Cross-sectionl re of usrs (sq.cm) 0 7.4 30 40 60 1 3 4 5 6 Note 1. The lower curves pply for f = Hz only. For other frequencies fr / dc must e clculted f y multiplying these vlues y. Smll vrition is possile while drwing these curves Figure 8.13() Skin effect in isolted tuulr conductors effective current-crrying cpcity is the result of the cumultive effect of the restricted het dissiption nd the incresed content of the skin effect. A stge my rise when further ddition of ny more rs my not pprecily increse the overll current-crrying cpcity of such system. Referring to Tles 30.4 nd 30.5, we cn oserve wide vrition in the current-crrying cpcity of conductor when it is dded to n existing system of one, two or three conductors per phse, depending upon the thickness nd width of the conductors. Thinner sections of shorter widths provide etter metl utiliztion, compred to thicker section nd lrger widths. Use of rs up to four sections per phse is quite common for higher current systems (0 A300 A). For still higher current rtings, use of more thn four rs in prllel is not dvisle due to n extremely low utiliztion of metl, prticulrly in lrger sections. While lrger sections would e impertive for such lrge rtings, their own rting would fll to low of 1418% of their norml current cpcity. (See Tle 30.5 for lrger sections, providing current rtings up to six rs in prllel.) In such cses it is dvisle to rrnge the rs in ny other convenient configurtion thn in prllel, s illustrted in Figure 8.14 or to use tuulr or chnnel sections which form into hollow conductors nd the current flows through their nnulus (skin) optimizing the metl utiliztion. 8.7. Determining the skin effect As result of the electric field round the conductors the frequency of the system hs very significnt ering on the skin effect. The vrious curves s estlished through experiments nd, s reproduced in Figures 8.13 (), () nd (c) respectively for rectngulr, tuulr nd chnnel conductors, re thus drwn on the f/ Rdc sis. At Hz, the vlue of the skin effect, R c /R dc, cn e red directly from these curves, s the curves for different cross-sectionl res nd conductivity, t Hz, hve lso een drwn in the lower prt of the figure. At 60 Hz the skin effect rtio cn e red corresponding 60 to or 1.095 1 Rdc Rdc One will notice tht lower the rting or conductor crosssection, lower is the effect of frequency on skin effect rtio (R c /R dc ). Accordingly, in smller sections nerly the sme rtings of conductors cn e ssumed t 60 Hz s for Hz system without much error. But s the rting or the cross-section of the conductor increses so increses

Crrying power through metl-enclosed us systems 8/1011 Skin effect rtio R c /R dc Cross-sectionl re of usrs (sq.cm).0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1. 1.1 1.05 1.0 0 0 33.9 40 60 80 100 10 140 t Solid rs 1 Indl D SWP t 85 C Indl CISM t 85 C 3 Indl D SWP t 0 C 4 Indl CISM t 0 C 5 Copper t 85 C 6 Copper t 0 C t = 0 60 80 100 10 140 160 180 1 3 4 Note 1. The lower curves pply for f = Hz only. For other frequencies, fr / dc must e clculted y multiplying these vlues y f /. Smll vrition is possile while drwing these curves the skin effect rtio. For lrge rtings sy 000 A nd ove compred to rtings t Hz, the rtings t 60 Hz my e reduced y roughly.55% s rough estimte. For ccurte clcultion one my interpolte the curves nd determine the skin effect rtio more ccurtely. CDA hs mentioned the sme rtings for nd 60 Hz systems, Tles 30.(),(), nd (c). However, if the rtings re estlished for 60 Hz, for lrger cross-sections the rtings t Hz my e enhnced y.55% without much error. Similrly, tles tht mention rtings t Hz, Tles 30.7, 30.8, 30.9, the rtings t 60 Hz my e reduced roughly y.55% (depending upon crosssection) to e on the sfe side. 0. 0.30 (c) Chnnels in ox form Figure 8.13(c) 0.0 0.15 0.10 0.08 0.06 0.04 0.0 0.01 t = 6 5 100% 118% 15% Rtings up to 300 A re normlly required for distriution purposes such s for inter-connecting distriution trnsformer to PCC, or lrge PCC to nother lrge PCC in susttion. Common prctice for mking such connections is to use rectngulr cross-sections, which re esy to hndle, mnoeuvre nd mke joints, compred to chnnel or tuulr section. Chnnel nd tuulr sections require specil tools nd skilled workers, prticulrly when ending or mking joints nd end termintions. However, suitle fittings nd fixtures, some of which re shown in Figure 8.15 () nd (), re lso provided y leding mnufcturers s stndrd prctice to fcilitte such connections. The welding of such joints will require specil welding equipment nd dequte inhouse testing fcilities to check the qulity of weld. It is, however, recommended to use such sections, for rtings 300 A nd ove, for etter utiliztion of ctive metl compred to flt sections. We riefly del with ll such sections s follows. (i) Rectngulr sections 18% Exmple 8.6 Consider section of 101.6 mm 6.35 mm of grde EIE-M s in Figure 8.16. From Tle 30.7 for its equivlent grde CIS-M (i) R dc = 44.55 mw/m t 0 C or 44.55 1000 10 6 W /1000 m i.e. 0.0445 W/1000 m Are of cross-section = 101.6 6.35 10 cm = 6.4516 cm 151% 157% 185% 1 3 4 5 6 7 Figure 8.14 Rtio of.c. current rtings for different configurtions of usrs of the sme cross-sectionl re (Source: The Copper Development Assocition, U.K.) Since the operting temperture should e considered to e 85 C, R dc t 85 C = R dc0 [1 + 0 (q q 1 )] (8.6) where 0 = temperture coefficient of resistnce for CIS-M grde of luminium from Tle 30.1,

8/101 Electricl Power Engineering Reference & Applictions Hndook R Y B N S = 100 100 100 = 101.6 = 6.35 = 6.35 = 101.60 S = 100.00 C L Figure 8.16 Illustrtion of Exmple 8.6 = 0.0445 (1 + 0.6195) Tue reducer () For tuulr section Tue for flt connection = 0.056 W/1000 m Now refer to Figure 8.13() to otin the skin effect rtio R c /R dc. Consider the cross-sectionl curves for EIE-M grde of flt usrs t n operting temperture of 85 C for cross-sectionl re of 6.45 cm nd determine the R c /R dc rtio on the skin effect curve hving / =101.6/6.35 = 16. \ R c R dc = 1.055 i.e. n increse of lmost 5.5%, due to the skin effect lone nd R c = 1.055 0.056 90 Horizontl splice pltes. Bolting rrngement will vry with the size of chnnel = 0.059 W/1000 m (ii) Skin effect for more thn one conductor per phse In such cses, the group of usrs in ech phse my e considered to e one lrge conductor nd outside dimensions nd s illustrted in Figure 8.8 mesured for ll clcultions. Exmple 8.7 Consider four-conductor system of section 101.6 mm 6.35 mm in ech phse (Figure 8.17()) of grde EIE-M for crrying current of 000 A. Figure 8.15 Spcer clmp () For chnnel section in ox form Typicl fittings for different usr sections = 0.00403 per C t 0 C R dc0 = d.c. resistnce t 0 C q = Operting temperture = 85 C q 1 = Since the vlue of R dc is ville t 0 C therefore, q 1 = 0 C. R dc t 85 C = 0.0445 [1 + 0.00403 (85 0)] R dc : As clculted ove for one section of us = 0.056 ohm/1000 m per conductor of four us-sections in prllel Skin effect rtio R c /R dc from the grph of Figure 8.13(), t n operting temperture of 85 C for cross-sectionl re of 5.8 cm (4 101.6 0.635) for n EIE-M grde of luminium hving = 101.60.9 44.45 R c /R dc 1.33 \ R c for the phse = 1 4 1.33 0.056 = 18.6 10 3 W/1000 m

Crrying power through metl-enclosed us systems 8/1013 6.35 S 6.35 44.45 44.45 19.05 () Uniform current distriution in n isolted conductor or d.c. conductor All dimensions in mm = 44.45 = 101.60 S = 184.45 (It is recommended to e min. 300) Attrctive force Figure 8.17() Illustrtion of Exmple 8.7 6.35 S 6.35 () Distortion in current distriution when the currents re in the sme direction, like 3f system Electric field Repulsive force 44.45 300 44.45 (iii) Busr configurtions (To improve het dissiption nd minimize skin effect) The usrs my e rrnged in different configurtions s shown in Figure 8.14 to improve het dissiption nd reduce the skin effect s well s the proximity effect. The improvement in the rtings is indictive of the cooling nd skin effects with different configurtions. When numer of rs re used in prllel, ech r shields the djcent r nd reduces its het dissiption. Moreover, together they form lrge conductor nd due to the skin effect the current will tend to concentrte t the outer surfces only. It will cuse inner surfces to shre smller nd the outer surfces the lrger currents. In configurtions other thn prllel rs, n ttempt is mde to improve het dissiption nd reduce the skin effect. It is ovious tht most of the conductors re now sufficiently independent of the others nd cn crry higher currents. 8.8 Proximity effect 19.05 Figure 8.17() Minimizing the effect of proximity in thicker sections (Section 8.8.4) If there is more thn one current-crrying conductor other thn of the sme phse, plced djcent to ech other, so tht the electric field produced y one cn link the other, mutul induction will tke plce. The mgnitude of this will depend upon the mount of current nd the spcing etween the two. This tends to further distort the selfresistnce of the conductor over nd ove the distortion lredy cused y the skin effect nd so lso the current distriution through its cross-section. Figure 8.18(), () nd (c) illustrte digrmmticlly distortion of current (c) Distortion in current distriution when the currents re in opposite directions, like 1f system Note 1. or Direction of current in conductor looking from top. Current coming out Current going in.. Direction of electric field y Cork-Screw rule. Figure 8.18 Current distriution in round conductors, illustrting the effect of proximity flow in round conductor nd lso the mechnicl forces exerted on the conductors, due to this distorted current distriution. There is lwys force etween two currentcrrying conductors plced djcent to ech other, whether it is d.c. or n.c. system. The proximity effect, however, will exist only in n.c. system due to mutul induction etween the two current-crrying conductors. It my e less pronounced in low current systems, sy, 1600 A or less, nd ll HV systems, where the spcings etween the phses re considerly more, except their effect on the enclosure, which is discussed in Chpter 31 on isolted phse us systems. If the second conductor crries current in the sme direction, such s in three-phse system (Figure 8.18()) the current will flow in the remote prts of the two conductors. If the current flows in the opposite direction, s in single-phse system (Figure 8.18(c)) the current will flow in the djcent prts of the two conductors. The displcement of current nd the forces (Eqution (8.4)) on the conductors re two different effects. The effect of current displcement is to increse the effective resistnce nd the impednce of the conductor on one side, s illustrted in Figure 8.18() nd (c) nd cuse distortion in its heting pttern. This will led the vrious conductors of prticulr phse to operte t different tempertures nd dd to I R losses. The rting of ll c c

8/1014 Electricl Power Engineering Reference & Applictions Hndook the conductors of one phse must therefore e determined y the hottest conductor. The distortion of current will lso distort the het produced. The re hving high current density will produce higher het. The proximity effect thus lso cuses derting in the current-crrying cpcity of conductor. In generl, the proximity effect is directly proportionl to the mgnitude of the current nd inversely to the spcing etween the two conductors. The smller the phse spcing, the greter will e the effect of proximity s well s the derting nd the greter will e the forces developed etween the djcent conductors (Eqution (8.4)). But the rectnce of the two phses is directly proportionl to the spcing. Rectnce is the min cuse of n excessive voltge drop (IZ). The smller the spcing, the lower will e the rectnce, due to the proximity effect nd vice vers. While the requirement of lower rectnce will require less spcing nd will men higher forces, demnding stronger usr supports nd mounting structure, requiring lower effect on current-crrying cpcity would require lrger spcing etween the phses, which would result in higher rectnce nd consequently higher voltge drop. But high rectnce would help to reduce the level of fult current, I sc nd lso forces, F m, etween the conductors. A compromise is therefore struck to meet oth needs nd otin more lnced system or other methods dopted, s discussed in Section 8.8.4, to reduce the skin nd proximity effects. 8.8.1 Proximity effect in terms of usr rectnce Self-rectnce, X, of the conductors plys significnt role in trnsmitting the power through us system from one end to the other. For long us systems, it must e scertined t the design stge whether the voltge drop in the totl us length on ccount of this will fll within the permissile limits, prticulrly for higher rtings (000 A nd ove), esides the current-crrying cpcity. A higher rectnce will men higher drop. For smller rtings nd shorter lengths, s well s HV systems, this drop would e too low s percentge of the rted voltge, to e tken into ccount. For higher rtings, however, it my ssume greter significnce nd precutionry mesures my ecome necessry to restrict it within permissile limits. To determine X, proximity effect curves hve een estlished for different us systems y conducting ctul tests on the metl nd re ville for ll sections, configurtions nd spcings of usrs. We hve reproduced few of them for luminium usrs for Hz system (for 60 Hz system, X 60 = X 60/ or 1. X ), for rectngulr sections s in Figure 8.19(), tuulr sections s in Figure 8.19() nd chnnel sections in ox form s in Figure 8.19(c). A rief procedure to determine the rectnces with the help of these curves is given elow. 00 X, rectnce t Hz (mw/m) 1 100 = 1 = = 5 = 1 Phse R Phse Y Phse B Phse R S S Phse Y S S s s Phse B = 1 5 Different configurtions of usrs = 0 0 0.5 1 1.5.5 3 3.5 4 4.5 5 5.5 6 6.5 Conductor spcing = S Note Semi perimeter + 1. For 3f systems red rectnce ginst Æ È Í Î S 1.6 +. The rectnce vries with the rtio nd therefore there my e numer of possile usr comintions nd the corresponding curves for different. However, only few curves hve een drwn for the likely minimum nd the mximum vlues of. Since the vrition is not lrge therefore y interpoltion the more pertinent vlue of rectnce cn e determined from these curves. 3. Smll vrition is possile while drwing these curves. Figure 8.19() Rectnce of rectngulr usrs t Hz on ccount of proximity effect

Crrying power through metl-enclosed us systems 8/1015 Note Similr procedure would pply for copper usrs lso. CDA hs provided simplified formule to determine the sme (for more detils one my see references 6, 7, 8 nd 15 of Further Reding in Chpter 31). Redymde curves s for luminium re not redily ville, ut the proximity curves of luminium cn e used to evlute Rectnce t Hz (mw/m) 80 60 40 0 00 180 160 140 10 100 80 60 40 0 r 1 r 0 0 0 40 60 80 100 10 Conductor spcing Se etween centres (cm) D s = 0.5 D s = 1.0 D s = 1.5 D s =.0 D s =.5 D s = 3.0 Figure 8.19() Rectnce of tuulr usrs for singlephse or three-phse systems t Hz pproximte vlues of X for copper usrs lso, for the purpose of design work without much error. Where ccurte vlues re imminent one my use the formule provided y CDA. Leding mnufcturers who hve stndrdized some of these products such s overhed uswys, rising mins, compct us systems nd prtilly isolted phse us (PIPB) system usully determine or work out these detils y ctul lortory experiments nd provide the sme s stndrd in their ctlogues such s shown in Tles 8.01 nd 8.0 or furnish them on demnd. For smller rtings up to 000 A or so, proximity effect is not of much concern s noted ove, unless the us length is too lrge nd it my e essentil t the design stge to determine the voltge drop. Single section X for different sections of luminium nd copper usrs re provided s stndrd y the mnufcturers of extruded sections. Some such dt for different sections in flts, tues nd chnnels re provided in Tles 30.(, nd c) for copper nd Tles, 30.7, 30.8 nd 30.9 for luminium. In sence of proximity curves the rectnce cn still e determined for different configurtions from the ville dt y pproximtion rther thn ttempting to work out the sme the onerous wy using the formule provided y CDA. For clrity see clcultion of Exmple 8.1 nd explntion () under Tle 8.8. The vrition in X suggests tht it vries with the configurtion of usrs nd ssumes very high vlue for more numer of usrs in prllel. It ecomes low with efficient use of metl. Like dpting to interleving or compct system it cn e chieved nerly up to the rectnce of one section divided y the numer of prllel pths. The X in compct us systems (Tle 3.0(1) nd 30.0()) corroortes this. Rectngulr sections (Figure 8.19()) The rectnce is drwn s function of Centre spcing ( S ) Semi-perimeter ( + ) At lower spcings this vlue will e influenced y the width () nd the thickness () of the conductor. At lower spcings, therefore, proximity curves re different for different rtios of / wheres for lrger spcings they pproch the sme curve. 0 Rectnce t Hz (mw/m) 00 180 160 140 10 108 100 80 60 t 76 mm chnnels 10 mm chnnels 17 mm chnnels chnnels 15 mm chnnels chnnels 178 mm mm 03 54 chnnels chnnels 305 mm mm 40 0 0 0 10 0 30 40 60 70 80 90 100 110 10 38.05 Conductor spcing Se etween centres (cm) Figure 8.19(c) Rectnce of chnnel usrs, two chnnels per phse in ox form, single-phse or three-phse, t Hz

8/1016 Electricl Power Engineering Reference & Applictions Hndook When more thn one section is used together, to mke lrger rtings, ll the sections of one phse my e considered to e one lrge section. The dimensions nd of the whole section re now considered s one conductor, s illustrted in Figure 8.8. The sic grphs represent singe phse system. The single phse rectnce is twice the rectnce so otined. For three-phse system the configurtion of the three phses with respect to ech other will ply significnt role nd the liner centre spcing S hs to e modified to n effective or geometric men spcing S e, where S e = (S S S c ) 1/3 (8.7) For configurtion () of Figure 8.0 S = S = S S c =S \ \ Se = S () 1/3 = 1.6 S nd for configurtion () of Figure 8.0 S = S = S c = S \ S e = S For ny configurtion, the effective spcing, S e, my thus e clculted. Tuulr sections (Figure 8.19()) For determining S e in solid or hollow round sections it is essentil to first determine the self geometric men distnce, D s, of the conductors which vries with the thickness t (nnulus) of the conductor. D s pproches its outer rdius, r 1, in n infinitely thin conductor nd to 0.778r 1 in solid r. This vrition, in the form of D s /r 1 is drwn in Figure 8.1, s function of r /r 1. For very thin conductors, when r r 1, r /r 1 = 1, D s /r 1 will lso pproch to unity nd D s r 1. For solid conductors, when r = 0, r /r 1 = 0, D s /r 1 ecomes 0.778 nd D s = 0.778 r 1 etc. After hving otined the vlue of D s, vlue of S e is determined s discussed ove. The rectnce of the conductors cn then e otined from the grphs of Figure 8.19() drwn for S e versus X, for vrying thicknesses of round conductors. Here lso the sic grph will represent single-phse system nd the single-phse 1.00 0.967 0.95 D s /r 1 0.90 0.85 r 1 r 0.80 0.778 0.75 0.1 0. 0.3 0.4 0.5 0.6 r 0.7 0.8 0.9 1.0 0.907 r1 Figure 8.1 Grph to determine D s of tuulr us section rectnce will e twice the rectnce so otined. Refer to Exmple 8.8. Chnnel sections (Figure 8.19(c)) These should normlly e used in ox form for etter metl utiliztion, esy mounting nd uniformity of conductors. The method of determining the rectnce for single- nd three-phse systems is the sme s for rectngulr sections (Figure 8.0()). From the proximity curves it my e noted tht X rises with S. While higher centre spcing would reduce the effect of proximity on the current-crrying conductors nd which is so much desired, it will increse X, which would men lower p.f. for the power eing trnsferred (through the usrs) nd higher voltge drop. In LV systems the spcings cn e djusted only mrginlly to reduce X, s lower spcing would men higher electrodynmic force, F m (Eqution (8.4)) nd greter proximity effects, requiring higher usr dertings. A compromise my therefore e drwn to economize on oth. In HV systems, however, which require lrger spcing, no such compromise would generlly e possile nd they will normlly hve high content of X. But in HV systems, voltge drop plys n insignificnt role in view of lower voltge drop s percentge of the system voltge. 8.8. Voltge unlnce s consequence of the proximity effect The proximity effect does not end here. It still hs some fr-reching consequences in terms of unequl voltge R R Y B S c S r 1 r t = (r 1 r ) S S S c B S N Y () Rectngulr sections () Rectngulr sections (c) Tuulr sections Figure 8.0 Influence of conductor configurtion on liner spcing S

Crrying power through metl-enclosed us systems 8/1017 drops in different phses t the sme time. This is more so on lrge LV current-crrying, non-isolted us systems of 000 A nd ove, resulting in n unlnce in the supply voltge, s discussed elow. A three-phse system hs three current-crrying conductors in close proximity. While the conductors of phses R nd B will hve n lmost identicl impednce, with the sme skin nd the proximity effects, the conductor of phse Y is under the cumultive effect of electric fields of the other two phses, which would offset their proximity effects (see Figure 8.). The conductor of phse Y therefore would crry no distortion eyond the distortion lredy cused y the skin effect (R c /R dc ). The result of this would e tht in lnced three-phse system the three phses will ssume different impednces nd cuse n unlnce in the current distriution. The Y phse hving smller impednce, would shre more current compred to the R nd B phses nd cuse smller voltge drop. Such n effect my not e s pronounced in lower rtings nd shorter lengths of current-crrying conductors, s much on higher currents, depending upon the spcing etween the phses nd the length of the system. Consider feeding line from trnsformer to power switchger through us duct. The voltge ville t the distriution end of this feeding line my e unequl nd tend to cuse voltge unlnce. Depending upon the rted current nd length of the feeding line, it my even cuse voltge unlnce eyond permissile limits (Section 1.(v)) nd render the system unstle nd in some cses even unsuitle for n industril ppliction. For lrger current systems, 000 A nd ove nd lengths of over m, correct nlysis for such n effect must e mde nd corrective mesures tken to equlize the voltge nd current distriution in ll the three phses. Where dequte precutions re not tken t the design stge through phse interleving or trnsposition techniques, s discussed lter, the prolem cn still e solved y mking up for the lost inductnce in the Y phse y introducing n externl inductnce of n pproprite vlue in this phse. It is possile to do this y introducing n ir core sturle type rector (Section 7.3) into this phse, s illustrted in Figure 8.3. This inductor will compenste for the deficient inductnce nd equlize the impednces in ll three phses, thus mking the system lnced nd stle. We illustrte R Y B + + + Influence of electric fields of conductors R nd B is offset in 3-f system. The proximity effect in phse Y therefore gets nullified Figure 8. Influence of proximity R Y B N riefly lter procedure to determine the size of sturle rector core, when required, to meet such need. Exmple 8.8 Consider Exmple 8.6 to determine the content of proximity; (i) For rectnce X on ccount of the proximity effect, use Figure 8.16 nd the grph of Figure 8.4: nd 1.6 S = 1.6 100 + (101.6 + 6.35) then from the grph = 1.6 Supports 100 1.167 107.95 = 6.35 101.6 = 0.065 X = 106 mw/m or 106 10 6 1000 W/1000 m i.e. 0.106 W/1000 m nd (ii) Impednce Z = R + X c = (0.059 + 0.106 ) W/1000 m = 0.1 W/1000 m Rector Pln of 3f nd N us system Figure 8.3 Blncing of phse currents in lrge threephse system y introducing rector in the middle phse (iii) Voltge drop If we consider the verge current-crrying cpcity of this section s 1000 A, fter norml dertings (without derting 135 A from Tle 30.4), then the voltge drop during norml running, sy, for 1 m length of this section of usr = 1000 A 0.1 1 1000 volts = 18.0 V which is round 4.3% of 415 V system. Such high voltge drop, lthough less thn 5% nd normlly permissile, my not e dvisle in this cse, since in ddition to this drop there 4 1 1 mm

8/1018 Electricl Power Engineering Reference & Applictions Hndook 00 X, rectnce t Hz (mw/m) 1 15 11 110 106 100 90 74 64 0.4 0.73 = 1 5 = 0 = 1 1.167 1.6 = = 5 = 1 1.59 Phse R Phse Y Phse B Phse R Conductor spcing = 1.6 S (for 3 f systems) Semi perimeter + Note Lesser the spcing S etween the phse conductors lesser is the rectnce X of the conductors. Figure 8.4 Rectnce of rectngulr usrs t Hz on ccount of proximity effect S S Different configurtions of usrs S S Phse Y Phse B 0 0.5.5 3 3.5 4 4.5 5 5.5 6 6.5 0.981 1.3441.5 S S my e further voltge drops in the connecting cles, resulting in higher drop thn 5% up to the connected lod. It is lso possile tht the voltge t the receiving end itself is lredy little less thn rted, due to drops in the upper network. This exmple is considered only to emphsize the significnce of voltge drops in current-crrying conductors, prticulrly when the system lod is high nd the end distriution is distnt from the receiving end. In norml prctice, however, considertion of voltge drop in us system my not e of much significnce, due to the generlly short lengths of the us ducts, which my not e more thn 3040 m in most of instlltions, irrespective of the size of the feeding trnsformer. However, if such sitution rises, s in this prticulr instnce, one my reduce the content of X y reducing S if permissile, or consider the next higher cross-section of usrs. This size of us section, in this prticulr instnce, my e considered for current rting up to 800 A. Exmple 8.9 Consider Exmple 8.7 to determine the effect of proximity: (i) For the configurtion of Figure 8.17() = 44.45 101.60 = 0.4375 1.6 S = 1.6 184.45 1.59 + 44.45 + 101.60 then X, due to the proximity effect from the grph of Figure 8.4, 15 mw/m or 0.15 W/1000 m per phse nd R c = 18.6 10 3 W/1000 m (ii) impednce, Z = 0.0186 + 0.15 = 0.16 W/1000 m per phse nd (iii) voltge drop, considering length of usrs s 40 m nd current rting s 000 A, = 000 0.16 40 1000 = 10.08 V which is.43% for 415 V system The us system is therefore suitle to crry 000 A up to length of 40 m. Beyond this the voltge drop my ecome higher thn permissile nd the us rting my cll for derting. Note The rting for this section considered here s 000 A, is hypotheticl nd must e checked for the vrious design prmeters s discussed lredy, in Sections 8.5 nd 8.6, nd nlysed in Exmple 8.1. Use of sturle rector (choke) to lnce lrge unlnced power distriution system Determining the size of rector Consider three-phse us system s shown in Figure 8.7. If X s nd X p re the inductive rectnces of ech phse on ccount of skin nd proximity effects respectively, then the impednces of ech of the three phses cn e expressed s ZR = R + Xs + Xp = Z ZY = R + Xs = Z Xp (since the Y phse will hve no proximity effect)

Crrying power through metl-enclosed us systems 8/1019 nd ZB = R + Xs + Xp = Z Therefore inductive rectnce equl to X p must e introduced into the Y phse to equlize the rectnce distriution nd mke the system lnced (Figure 8.5). If I r, I y nd I re the currents in the three phses nd V ph the phse voltge then I r = I = I r (sy) nd nd I I r y Vph V = or Z = Z I Vph = ( Z X ) = Ê V Á Ë I ph r V ph p ˆ Xp ph r (Bsiclly these re ll phsor quntities ut for ese of illustrtion solute vlues re considered) or V I y I I X = ph r p = Ir Vph ( V I X ) r ph r p V I ph y I I X = V r p ph r V I ph y determined erlier, to ccount for the proximity effect my e considered s the lost rectnce in the Y phse nd for which the rector my e designed for this phse. Designing rector The self-inductnce f Z c L = henry I r where L = self-inductnce of the choke in henry (H) f =flux produced in Weer (w) I r =rted current in Amps Z c = numer of turns in the choke = 1 (since it will e used like r primry, s shown in Figure 8.7) Totl reluctnce R 1 of the mgnetic circuit of the choke = MMF = f L Ir since f = Z \ R 1 c Zc I f Zc Ir = Zc or L I r r Z L The totl reluctnce of n iron choke (Figure 8.6) cn lso e expressed y R 1 = R ir + R core where R ir = reluctnce of the ir gp c or Xp = V ph Ê 1 1 Á Ë Ir Iy ˆ = p f L (8.8) The vlues of I r nd I y must e known to determine the vlue of the rector, X p. Otherwise the rectnce, X, s nd = g ml 0 A R core = reluctnce of the iron pth k = m m o r l g A R I r X p X s R V l l g R R X s X s X p I y1 X p B Y Figure 8.5 Distriution of inductive rectnce nd impednce of ech phse in three-phse system V l Iy I Rector mgnetic circuit (length = k ) l g Vrile ir gp, sy, 1.5 mm to 5.0 mm Figure 8.6 A typicl rector core Stmpings of 0.0.5 mm thick CRGO silicon steel lmintions

8/100 Electricl Power Engineering Reference & Applictions Hndook g k g \ R1 = l l + mo A mo mr A (8.9) where l g = length of the ir gp in metres (Figure 8.6) m o = permeility* of ir (free spce) = 4 p 10 7 H/m m r = reltive permeility of the silicon steel used for the lmintions in H/m A = re of cross-section of core in squre metres k = totl length of the mgnetic circuit in metres or R 1 k g = l [1 1/ mr] m m A m A o r o or ( R1 mo mr A k ) = l g ( mr 1) m m A m m A o r R A k or l g = ( 1 mo mr ) ( mr 1) where m m r = m o nd m is design prmeter for the silicon steel nd = B/H. where B = flux density in w/m nd H = mgnetic field strength in A/m It my e oserved tht the vlue of permeility is function of the flux density eing ttined to energize the mgnetic circuit. It is therefore not constnt prmeter nd is mesured t prticulr flux density. Exmple 8.10 Consider us duct hving rted current of 4000 A nd n unlnced current in the middle phse of 4400 A. Determine the size of the rector to chieve lnced voltge system. Solution If the line voltge = 440 V then V ph = 440/ 3 = 54 V \ X p = 54 Ê 1 4000 1 Ë Á ˆ 4400 = 54 400 W 4000 4400 = 0.00577 = p L (for Hz system) or L = 0.00577 3. 14 H = 18.38 10 6 H L \ Totl flux f = Z I r c *Permeility defines the mgnetic property of mteril, nd is mesure of how esily it cn e mgnetized. The greter the permeility of mteril, the esier it is to mgnetize. o r 6 i.e. f = 18.38 10 4000 w 1 = 73.5 10 3 w Therefore re of cross-section of the core A = f B = 73.5 10 1.1 3 (ssuming B = 1.1 w/m ) = 66.84 10 3 squre metres. If we ssume the stcking fctor of the core lmintes to e 0.9, the gross cross-sectionl re of the core A = 66.84 10 0.9 3 or 74.7 10 3 squre metres or 7470 mm Assuming the width of the lmintes to e 1 mm nd the depth of the core s d (Figure 8.7), \ Are of cross-section of the choke = 1 d = 7470 74 70 or d = 1 = 495 (sy, 0 mm) \ A = 1 0 = 75 000 mm or 0.075 m The size of the opening will depend upon the width nd height required y the current-crrying conductors. Sy, for six numers 15.4 mm 6.35 mm usrs, s shown in Figure 8.7, n opening of 170 mm mm will e dequte. To determine the vlue of m, the sturtion or the BH curve of the silicon steel eing used must e ville. R 1 5 400 1 Mgnetic field Y 30 1 170 Min mm Length = k 1 Min mm Air gp l g = mm 6.35 B Figure 8.7 Design of rector for power distriution us system. Illustrting Exmple 8.10 15.4 6.35 Depth d = 0

Crrying power through metl-enclosed us systems 8/101 Assuming norml flux density for such core to e 1.1 w/ m (see lso Section 1.9) nd mking use of norml BH curve s shown in Figure 8.8, the corresponding vlue of H for vlue of B s 1.1 w/m cn e red s 00 A/m, \ m = B = 1.1 = 0.0055 H/m H 00 nd m \ m r m = m o = 0.0055 4 p 10 r 7 = 4378 H/m length of mgnetic circuit k = (30 + 400) + l g nd R 1 70 mm or 1.44 m Z = L = 1 H 6 18.38 10 = 5.44 10 4 H \ Air gp Flux density (B)W/m c R m m A k ( 1) 1 o r l g = mr 1.1 4 7 5.44 10 4 p 10 4378 0.075 1.44 = (4378 1) =.44 1.44 m 4377 0.4 mm Figure 8.8 sheets 00 Mgnetic field strength H (A/m) A typicl mgnetic sturtion (BH) curve for CRGO nd weight of the choke W = Volume specific grvity where Volume = 1440 1 0 mm 3 Assuming the specific grvity of the lmintes to e 8.5 g/cm 3 \ W = 1440 1 0 8.5 1000 1000 kg = 918 kg 8.8.3 Derting due to the proximity effect We will discuss this spect in two prts, one for the nonisolted us systems nd the other for the phse-isolted us systems s in Chpter 31. Proximity effect on non-isolted us systems Drwing inferences from the literture ville on the suject (see the Further Reding t the end of the chpter), sed on lortory tests, prcticl experience nd the field dt ville, dertings for different configurtions re shown in Tle 8.7 which should e sufficient to ccount for the likely proximity effects. Proximity effect on the enclosure The electric field produced y the current-crrying conductors of ech phse lso links the metllic us enclosure, its mounting supports, nd structures existing in the vicinity, prllel nd round the xis of the currentcrrying conductors. It cuses induced (prsitic) currents in such structures nd leds to the following: Resistnce losses (I R) nd Mgnetic losses. Mgnetic losses will constitute the following: Eddy current losses (µ B, Section 1.6..A-iv) nd Hysteresis losses (µ B 1.6, Section 1.6..A-iv) The electrodynmic forces etween the enclosure nd the conductors will e smll ecuse the enclosure, which is non-continuous, will crry much less current thn the min conductors. They therefore need not e considered seprtely, s the metllic structure will hve sufficient strength to er them. In non-mgnetic enclosure, such s luminium or stinless steel, there will e only resistnce losses. In mgnetic enclosure, such s mild steel (MS), there will lso e hysteresis nd eddy current losses in ddition to resistnce losses. All these losses pper s het in the enclosure nd the metllic structures in the vicinity. At higher currents sy, 000 A nd ove, this phenomenon, prticulrly with MS enclosures, my ssume such lrge proportions tht the enclosure, insted of providing het-dissipting surfce to the het generted y the current-crrying conductors inside, my dd to their het. Depending upon the current rting nd the configurtion of the usrs, the mteril of the enclosure should e

8/10 Electricl Power Engineering Reference & Applictions Hndook Tle 8.7 Approximte dertings due to proximity effect for different configurtions of us systems Current rting Centre spcing S Approx. derting (1) Flt usr S (I) LV systems 1 Smller rtings up to 1600 A Norml spcings 5% 0003000 A (i) S 4 5% (ii) S 15% 3 Lrger rtings up to round 60 A* S 4 15% (II) For HV systems 0003000 A Generlly S 4 5% () For chnnel sections For chnnels in ox form S 3 18% S 4 11% S 5 5% S S 6 1% *Applictions: (i) Required for medium sized turo-lterntors, up to 5 MVA used for cptive power genertion in process plnt, such s sugr mill, mostly utilizing its own surplus or wste gses/fuel nd stem. (ii) Smll gs nd hydroelectric power-generting sttions Chnnels in ox form in smller sections > s shown in Tle 30.9. chosen to minimize these effects s fr s possile. It is possile to do this y dopting one or more of the following methods. Since the spcing in n HV system is lredy lrge, n HV system is generlly not ffected y the proximity effects. The following discussion therefore reltes primrily to n LV system. 8.8.4 Minimizing the proximity effect Following re some conspicuous methods to chieve this: 1 Mintining greter spcing (S) etween the phses This cn e done y providing dequte clernces (sy, 300 mm) etween the conductors nd the inside of the enclosure. Tle 30.5 nd ll the other tles in Chpter 30 re sed on the fct tht the proximity effect is lmost negligile t 300 mm from the centre of the currentcrrying conductors nd the conductor is suject to its self-inductnce only. But in thicker sections, or where numer of smller sections re used together to form phse, the current will concentrte t the outer surfces only (skin) rther thn the nucleus. Therefore, to chieve n lmost zero-proximity effect condition it is desirle to provide spce of 300 mm nd more etween the extreme outer surfces, rther thn etween the centres, s shown in Figure 8.17(). The condition of S > 4 (Tle 8.7) will lso e lmost stisfied y doing so. For still higher currents, this distnce must e incresed further or segregted construction dopted (Section 8..). But to keep the phse conductors completely out of the inductive effect of the other phses my require very lrge enclosures, prticulrly t higher rtings (ove 300 A or so), which my not e prcticl. Below we descrie improvised us systems to limit the rectnce nd hence the voltge drop nd otin n inductively lnced system to chieve lnced voltge nd equl lod shring y the three phses t the fr end. Phse interleving This is highly efficient nd more prcticl method for lrge rtings nd offers very high metl utiliztion of the conductors. It provides n lmost lnced nd low rectnce system. Ech phse, consisting of numer of conductors, is split into two or more groups nd ech group of conductors is then rerrnged into three or three nd hlf phses, ccording to the system requirement, s illustrted in Figures 8.9() nd (). It is, however, suggested, to limit the numer of groups to only two for considertions of size nd the cost of enclosure (for open us systems, however, such s for lrge smelters, electroplting nd rectifier plnts, there my not rise such limittion). The two groups would meet the design requirements in most cses. Therefore if four or more flts re used per phse it is not lwys necessry tht s mny groups e rrnged, unless the current rting of the system is too lrge to effectively reduce the rectnce of the entire system. In four conductors, for instnce, two groups, ech with two conductors per phse, cn e rrnged s

Crrying power through metl-enclosed us systems 8/103 Electric field R Y B N R Y B R Y B N 1st group nd group () Interleving of conductors per phse 300 mm preferly R Y B N R Y B N R Y B N 1st group nd group () Interleving of 4 conductors per phse Figure 8.9 Minimizing the effects of skin nd proximity through phse interleving shown in Figure 8.9() to chieve low rectnce system s result of the smller spcings etween the split phses, on the one hnd, nd two prllel pths, on the other. The two prllel pths will further reduce the totl rectnce to one hlf. The field produced y ech split phse would now ecome hlf (f µ I) nd fll out of the inductive region of the other. The rrngement would thus provide system with low proximity effect. Since the conductors in ech phse re now rrnged s close s di-electriclly possile, the electrodynmic forces on ech group in the event of fult on ccount of the spcing would e high s result of the smller spcing etween the phse conductors (F m µ 1/S Eqution (8.4)). But the overll forces would ecome much less compred to the conventionl rrngement ecuse of two or more prllel current pths, ech crrying reduced mount of current, depending upon the numer of prllel pths so formed. For two prllel pths, for instnce, sc sc F m µ ( I /) or µ I /. Generlizing, F m µ, 1/n if n is the numer of prllel pths. Moreover, the mounting supports will lso ecome stronger thn efore, ecuse there re s mny mounting supports s the numer of prllel pths, ech shring the totl force eqully. The method of interleving will therefore require no extr reinforcement of the usr supports or the mounting structures. (See lso Exmple 8.11.) As result of the low rectnce otined the rrngement will provide somewht inductively lnced system. The rectnce of the conductors cn e clculted on n individul group sis nd then hlved when the conductors re split into two hlves, or reduced y the numer of prllel pths rrnged. The rrngement will lso minimize the skin effect to very gret extent, s the current of ech phse is now shred y two or more independent circuits, ech of thinner section thn composite phse. It cn e considered n improvised version of rrngement in Figure 8.14. The thinner sections (smller nucleus) will provide etter nd more uniform shring of current y ll the conductors. The current rting my now e determined y multiplying the individul current rting of ech split phse y the numer of prllel circuits. As the spce etween the two groups of the sme phse will now e lrge, 300 mm or more, they will hve nil or only negligile influence of skin effect mong themselves. For instnce, us system with 4 15.4 6.35 mm conductors my e rrnged into two groups of two conductors ech, ccording to Figure 8.9(). Then the improved rting of this system s in Tle 30.4 will e = 860 = 570 A s ginst 440 A for ll conductors put together s in Figure 8.33() or 1.18 440, i.e. 03 A when rrnged s in rrngement of Figure 8.14. Thus, in phse interleving, there will e etter utiliztion of conductor cpcity y 570/03 or >14% over rrngement in Figure 8.14. Exmple 8.11 Consider Exmple 8.7 gin, using four sections of 101.6 6.35 mm Al conductors, now interleved s shown in Figure 8.30. To determine the improved rectnce nd resistnce of this rrngement we cn proceed s follows. Proximity effect: = 19.05 mm S = 94.05 mm = 19.05 101.6 = 0.1875 nd spce fctor, 1.6 S = 1.6 94.05 = 0.98 + 19.05 + 101.6 X from grph of Figure 8.4 = 90 mw/m nd for two prllel circuits = 90/ = 45 mw/m or 0.045 W/1000 m R 6.35 6.35 B 75 Y N R 85 S = 19.05 mm = 101.60 mm S = 94.05 mm (Depending upon the current rting, it would e dvisle to keep it minimum 300 mm, y incresing the gp etween the split phses.) Figure 8.30 Illustrting Exmple 8.11 B Y

8/104 Electricl Power Engineering Reference & Applictions Hndook s ginst 15 mw/m with the conventionl rrngement clculted in Exmple 8.9. Skin effect Are of cross-section per split phse = 101.6 6.35 = 1.9 cm = 101.6 19.05 = 5.33. \ R c from the grph in Figure 8.13() y interpoltion for R dc n EIE-M grde of luminium 1.13 R dc = 0.056 W/1000 m per conductor = 0.056/4 W/1000 m for 4 conductors \ R c = 1.13 0.056 4 = 0.0158 W/1000 m \ Impednce Z = 0.0158 + 0.045 = 0.0477 W/1000 m Voltge drop Accordingly the revised voltge drop for 40 m of us length = 000 0.0477 40 1000 = 3.8 V which is even less thn 1% for 415 V us system. Electrodynmic forces For system fult level of ka mximum forces on ech group, I sc Fm = k 16 10 4 N/m S (i) For conventionl rrngement (Figure 8.17()) k for spce fctor of S = 184 45 44.45, i.e. 0.958 + 44.45 + 101.6 corresponding to of 44.45, i.e. 0.4375 101.6 from the grph in Figure 8.7 y interpoltion k 0.96 \ F m = 0.96 16 000 184.45 10 = 0 819 N/m (ii) For the improvised interleving rrngement in Figure 8.30: 94.05 19.05 k for spce fctor of, i.e. 0.6 from the 19.05 + 101.6 grph in Figure 8.7 corresponding to of 19.05, i.e. 0.1875 0.87 101.6 \ F m on ech set of supports 0.87 16 Ê 000 Ë Á ˆ = 94.05 10 4 4 = 9.4 N/m \ F m on oth supports = 9.4 = 18 0.8 N/m. This is less thn the force developed with the conventionl rrngement in Figure 8.17(). 3 Phse trnsposition In this rrngement the three-phse conductors re evenly trnsposed in length of usrs y interchnging their physicl loction so tht ech phse is under n equl inductive effect (proximity effect) produced y the other two phses. The rrngement is illustrted in Figure 8.31. This cn e performed y rrnging stright length of us into three equl sections (or in multiples of three), s shown. If x is the rectnce of phses R nd B, nd y tht of phse Y in the first section, then phses B nd Y will hve rectnce x nd R rectnce y in the second section. In the third section, phses Y nd R will hve rectnce of x ech nd phse B will hve rectnce y. Hence, the rectnce of ech phse, t the end of the three lengths, will e lnced t (x + y), cusing equl lod shring nd n equl voltge drop in ll three phses. This rrngement would thus mke the system lmost lnced inductively y ech phse hving equl exposure to the inductive fields produced y the other two phses. Due to inductive lncing, the trnsposition equlizes the rectnces in ech phse nd improves the current shring y ll the three phses, esides n equl voltge drop through the length of the us. However, there my not e n pprecile improvement in the proximity effect etween ech section (it is not required lso), unless the trnspositions re incresed infinitely, s in the cse of strnded three-phse cle which hs continuously twisted conductors nd represents n idel trnsposition. In ddition, there is no chnge in the skin effect. This rrngement therefore hs the purpose primrily of chieving n inductively lnced system nd hence lnced shring of lod nd equl phse voltges t the fr end. Phse trnsposition oxes re mde seprtely nd instlled t resonle distnces to provide n inductively lnced system. This technique is found very useful in deling with inductive interferences in communiction lines (Section 3.5.(D)). Similr rrngement is prctised in EHV high rting XLPE cles lso. See Figure A16.6. R Y B Figure 8.31 trnsposition x x x y y y x x x l/3 l/3 l/3 X R = x + y X Y = x + y X B = x + y where X R, X Y nd X B re the rectnces of ech phse. Blncing of rectnces through phse Y B R

Crrying power through metl-enclosed us systems 8/105 4 Chnging the configurtion of usrs By rrnging the usrs into few more configurtions long the lines discussed ove it is possile to reduce the proximity effect to gret extent. Some of these configurtions re illustrted in Figure 8.14. See lso Exmple 8.1, illustrting the mrked improvement in the cpcity utiliztion of the usrs y using different configurtions. 5 Busr enclosure Non-mgnetic enclosure. The proximity effect cn lso e minimized y using non-mgnetic enclosure of luminium or stinless steel. In mgnetic mterils the field in the enclosure is produced in the form of smll mgnetic loops. Its effect cnnot e mitigted y reking the electricl pth lone, s illustrted in Figure 8.3. Its effect cn e diminished only y replcing few prts of the mgnetic enclosure itself, such s its top or ottom covers or oth, with nonmgnetic mteril. It is possile to chieve n economicl nd low-loss enclosure y replcing only its top nd ottom covers with non-mgnetic mteril. The covers constitute the lrger prt of the surfce re of the enclosure. By providing dequte louvres in the enclosure s shown in Figure 8.33() or y using forced-ir drught through the length of the enclosure. 6 Using non-conventionl us systems (Section 8..6) 8.8.5 Energy sving In tody s energy scenrio nd glol wrming energy sving y ll het generting equipment nd devices is desirle even mndtory s discussed in Section 1.19. In usr systems lso, like for cles, it is recommended to choose t lest the next higher usr size to reduce resistnce losses (I R) (higher the cross-section lower the conductor resistnce nd the het generted). So lso y using thinner cross-sections nd lrge surfce usrs. MS cover Breking of electricl pths MS cover Mgnetic loops Ech loop cuses mgnetic loss () Breking of electricl pths do not diminish the mgnetic field neither the losses Aluminium top nd ottom covers MS side covers Figure 8.3 () An economicl nd low loss enclosure Mgnetic field in mgnetic mteril 8.9 Smple clcultions for designing 0 A nonisolted phse luminium usr system Exmple 8.1 Design prmeters Supply system three-phse four-wire 415 V ± 10%, Hz ± 3% Fult level 45 ka Durtion of fult 1 second Continuous current rting 0 A Amient temperture C Mximum permissile operting temperture 85 C Permissile finl temperture t the end of the fult 185 C (A) Rectngulr sections (i) Minimum size of usrs for short-circuit conditions The minimum size of usrs for n operting temperture of 85 C nd finl temperture of 185 C cn e scertined from the curves of Figure 8.5, suggesting I A or A = t = 0.0799 45 0.0799 1 = 563. sq. mm. Mximum temperture rise of the usrs t the rted current = 85 = 35 C Assume the temperture of the usrs t the time of fult = 85 C nd rectngulr flts of electrolytic grde E-91E or its equivlent. Busrs chosen for ech phse four (15.4 mm 6.35 mm) which re more thn the minimum size required

8/106 Electricl Power Engineering Reference & Applictions Hndook to ccount for the therml effects during short-circuit condition, for neutrl two (15.4 mm 6.35 mm) Size of usr enclosure 870 mm 380 mm (Figure 8.33()). Mteril of enclosure luminium Busr configurtion s in Figure 8.33() Busr support mde of SMC or DMC (Section 13.6.1(iii)). Distnce etween two usr supports: 400 mm (Figure 8.33()) 380 10 6.35 6.35 Insultors (All dimensions in mm.) 870 S = 10 10 10 10 = 44.45 Busrs Insultor supporting chnnel MS or luminium enclosure Figure 8.33() Busr rrngement nd enclosure size for us duct of Exmple 8.1 113.8 = 15.4 113.8 (ii) Dt ville Aluminium usrs, from Tles 30.4 nd 30.6 Current rting = 440 A Electricl conductivity = 1 Minimum tensile strength = 0 kgf/cm (Tle 30.1) Minimum cross-reking or yield strength = 16 kgf/cm (Tle 30.1) Busr supports: from Tle 13.14 Mechnicl properties DMC SMC in kgf/cm in kgf/cm Minimum tensile or shering 0 0900 strength Minimum compressive 1001800 1600000 strength Minimum cross-reking or 700100 14001800 flexurl strength (ending strength) Hrdwre Mechnicl properties High tensile (HT) Ordinry MS fsteners s in (mild steel) ISO 4014 fsteners s nd ISO 898, in ISO 4016, grde 8.8 grde 4.6 Minimum tensile strength 8000 kgf/cm 4000 kgf/cm Minimum cross-reking or yield strength 4400 kgf/cm 00 kgf/cm Ground terminl 1 5 8 9 3 380 10 10 4 4 10 870 6 10 7 10 00 400 400 400 00 1600 Top view (without cover) Legend 1. M.S. ngle 4. Bottom nd top covers mm M.S. or 3 mm Al. 7. Gsket. Al. us 4 15.4 6.35 mm 5. Ground us 6 mm Al 8. Louvres 3. Side frmes mm M.S. or 3 mm Al. 6. Insultors 9. Metllic spcers 8 3 Side view (All dimensions in mm) Figure 8.33() Generl rrngement of typicl running section of the us duct of Figure 8.33()

Crrying power through metl-enclosed us systems 8/107 (iii) Deriving the ctul current rting Applicle dertings: Due to higher mient temperture For C s in Tle 8.3 nd Figure 8.10 = 0.815 Due to ltitude Nil, since the instlltion of the equipment is ssumed to e within 000 m ove the men se level Due to grde of usrs For E-91E or its equivlent, s in Tle 30.6 = 1.0 Due to size of enclosure nd environmentl conditions of loction The Enclosure is of non-mgnetic mteril, therefore it will e devoid of hysteresis nd eddy current losses. Het dissiption fctor = Cross-sectionl re of ctive luminium Are of enclosure (3 4 15.4 6.35) + ( 15.4 6.35) i.e. 380 870 = 14 15.4 6.35 380 870 = 0.041 or 4% As in Tle 8.6, y simple interpoltion, for condition of loction ginst seril numer, derting = 0.77 Due to skin effect Nil, s it is lredy considered in Tles 30., 30.4 nd 30.5, while estlishing the sic rtings of the usrs. Due to proximity effect Approximtely 0%, since S <. It is recommended to hve the centre spcing S t lest 15.4, i.e. 305 mm. If the width of the enclosure poses limittion, more pproprite configurtion such s in Figure 8.34 or the technique of interleving s in Figure 8.35 my e dopted to chieve etter utiliztion of the ctive metl. In our clcultions we hve considered ll these lterntives for etter clrity. Due to voltge vrition We hve lredy discussed the impct of voltge vrition on n industril drive in Section 1.6.(A-iii). The impct of this on us system my not e the sme. A us my hve to supply lighting, heting nd other resistive or inductive lods. All such lods except electric motors will perform low t lower voltges nd hence drw lower current. Generlly, we cn ssume tht the loding on us, s enhnced y the industril drives, will e lmost offset y the decrese of loding y the other lods if we ssume industril drives to e up to % of the totl connected lod. Usully therefore no derting will e necessry for lower voltge, except for lrge instlltions where the drives my constitute the ulk of the lod. Considertion of voltge vrition will therefore depend = 15.4 300 R Y S = 99.05 80 B 6.35 = 19.05 6.35 N R Y B Figure 8.35 For Exmple 8.1 (All dimensions in mm) 680 90 S = 180 S = 180 140 90 R Y B N 60 15.4 Supports of SMC/DMC or of non-mgnetic mteril luminium or stinless steel etc. 16 = 430.8 5.8 6.35 6.35 6.35 = 19.05 15.4 60 (All Dimensions in mm) Note: Horizontl distnce etween usr supports = 400 mm (sme s in Figure 8.33()) Figure 8.34 Illustrtion of Exmple 8.1

8/108 Electricl Power Engineering Reference & Applictions Hndook upon the type of instlltion. In our clcultions, however, we re ignoring the impct of this. Frequency vrition At higher frequencies up to 3% skin nd proximity effects would e slightly higher, ut cn e ignored s their impct will e only mrginl. Considering ll these dertings the totl derting for the configurtion of Figure 8.33() or () will e = 0.815 1 1 0.77 0.8 = 0. Bsic current rting of 4 15.4 mm 6.35 mm luminium usrs per phse s in Tle 30.4 = 440 A \ Effective rting fter considering ll possile dertings = 440 0.5 = 10 A These sections of usrs re not dequte for the required current rting of 0 A. The rting of the us system cn, however, e improved y lmost 0% nd mke it suitle for the required rting y providing the usrs nd the inside of the enclosure with non-metllic, mtt finish lck pint. If voltge vrition is lso to e considered, then this us my not e suitle for the required duty even fter pinting. (iv) Voltge drop R dc for E-91E grde of conductor, from Tle 30.7 = 3.38 mw/m per conductor t 0 C \ R dc t n operting temperture of 85 C = R dc0 [1 + 0 (q q 1 )] = 3.38 [1 + 0.00363 (85 0)] = 3.38 (1 + 0.36) or R dc = 40.0 mw/m per conductor nd for the phse = 40.0 or 10.005 10 3 W/1000 m 4 Are of cross-section per phse = 4 15.4 6.35 mm = 38.71 cm For this re of cross-section, the skin effect rtio R c /R dc from Figure 8.13() for luminium grde E-91E t 85 C, hving / = 15.4/44.45 3.43 mesures lmost 1.45 y pproximting the interpoltion, \ \ R c /R dc = 1.45 i.e. n increse of lmost 4.5%, due to the skin effect lone. \ Rc = 10.005 10 3 1.45 = 14.57 10 3 W/1000 m per phse Proximity effect Mesure rectnce X from Figure 8.4 for 1.6 S = 1.6 10 + 44.45 + 15.4 i.e. 1.344, s in the curve, corresponding to /, s 44.45 i.e. 0.9, 15.4 X = 110 mw/m per phse or 110 10 3 W/1000 m per phse. Impednce, Z = (14.57 + 110 ) 10 3 = 0.111 W/1000 m per phse For m length of us duct this impednce will cuse voltge drop of 0 0.111 = 13.9 V 1000 which is 3.3% of the rted voltge nd is therefore cceptle. For higher us lengths, however, which my e rre, while the current rting of the system selected will e suitle the voltge drop my exceed the recommended limits. In this cse it will e dvisle to dopt the lterntive configurtion of Figure 8.34 or the technique of interleving (Figure 8.35). A comprison is drwn for these configurtions s in Tle 8.8, which revels tht oth lterntives significntly improve the performnce of the sme us section. (v) Effect of proximity on the centre phse Y For length of m the voltge drop in phse Y, due to R c, nd ssuming the content of X 0 = 0 14.57 10 3 1000 = 1.78 V \ Receiving side voltge of phses R nd B = 415 3 13.9 = 5.7 V nd of phse Y = 415 3 1.78 = 37.8 V The imlnce for this length nd rting of us system is not sustntil, yet if we ssume tht lnced supply source is desirle, then we must mke up the lost inductnce in phse Y y inserting rector into this phse, s discussed in Section 8.8. of n equl vlue of X, i.e. 3 X = 110 10 1000 = 0.0055 W (vi) Clcultion for short-circuit effects Electrodynmic forces These cn e determined from Eqution (8.4), Isc k 4 Fm = 16 10 N/m S where I sc =r.m.s. vlue of fult current in Amperes = 45 000 A k = spce fctor for rectngulr conductors, determined from the curves of Figure 8.7, corresponding to S, i.e. 10 44.45 + 44.45 + 15.4 or 0.84 corresponding to the curve for / = 44.45/15.4 or 0.9

Crrying power through metl-enclosed us systems 8/109 Tle 8.8 Clcultion of electrodynmic forces Description Arrngement s in Arrngement s in Interleving s in Figure 8.33() Figure 8.34 Figure 8.35 Bsic current rting without 440 A 440 1.57 860 (860 A is the rting derting = 6657 A (Fig. 8.14) of one split phse Tle 30.4) = 570 A Effective current rting 0.5 440 0.5 6657 0.5 570 considering pproximtely = 10 A = 338 A = 860 A the sme dertings Proximity effect On per split phse sis 1.6 S + 44.45 15.4 = 0.9 19.05 430.8 = 0.044 19.05 15.4 = 0.15 1.6 10 44.45 + 15.4 = 1.344 1.6 180 19.05 + 430.8 = 0.4 1.6 99.05 19.05 + 15.4 = 0.73 Approx. X from Figure 8.4 110 mw/m 64 mw/m 74 mw/m per circuit. Since there re two prllel circuits, \ Comined X = 37 mw/m Skin effect Are of cross-section 4 15.4 6.35 4 15.4 6.35 15.4 6.35 = 38.71 cm = 38.71 cm = 19.35 cm (per circuit) R Approx. R c dc from Figure 8.13() 15.4 3.43 44.45 430.8 19.05 =.6 15.4 19.05 = 8 1.45 1.3 1.18 R dc t 85 C s clculted in 40.0 mw/m 40.0 mw/m 40.0 mw/m step iv per conductor per conductor per conductor \ R c = 1.45 40.0 = 1.3 40.0 = 1.18 40.0 (for oth circuits) 4 4 4 = 14.57 mw/m = 13.01 mw/m = 11.81 mw/m Impednce Z 110 + 14.57 64 + 13.01 37 + 11.81 = 111 mw/m = 65.31 mw/m = 38.84 mw/m Voltge drop 0 111 10 6 0 65.31 10 6 0 38.84 10 6 = 13.9 V = 8.16 V = 4.85 V s % of system voltge 13.9 100 3.3% 415 8.16 415 100 = < % 4.85 415 100 1.1% Electrodynmic forces S + 10 44.45 44.45 + 15.4 = 0.84 180 19.05 0.358 19.05 + 430.8 99.05 19.05 19.05 + 15.4 = 0.47 44.45 15.4 = 0.9 19.05 430.8 = 0.044 19.05 15.4 = 0.15 k from Figure 8.7 0.93 0.7 0.77 (considering curve for / = 0.1) F m in N/m 400) 16 (45 000) 0.93 4 16 c(400) 16 Ê 0.77 0.7 10 4 Ë Á ˆ 10 4 10 10 180 99.05 = 14,348.6 = 1 600 = 1593.64 In kgf/m 1463.1 185 184 (Contd.)

8/1030 Electricl Power Engineering Reference & Applictions Hndook Tle 8.8 (Contd.) Description Arrngement s in Arrngement s in Interleving s in Figure 8.33() Figure 8.34 Figure 8.35 Forces on ech set of usr insultors nd mounting = 1463.1 0.4 = 185 0.4 = 184 0.4 fsteners, when 400 mm prt = 585.4 kgf = 514 kgf = 513.6 kgf These rrngements of us systems re suitle for higher lod demnds lso t 10% voltge if it is n industril instlltion where most of the lods re industril drives. It is noticele tht s the configurtion of usrs improves, the X improves (decreses) nd so improves the metl utiliztion. Interleving hs worked out the est with lest X nerly equl to the rectnce of single section (t more thn 300 mm spcing) divided y numer of sections i.e., 134.51 or 33.6 mw/m (Tle 30.7). c 4 We hve considered ech phse composed of four us sections to clculte F m, to e on the sfe side. In fct the current of ech phse is split into two circuits in this rrngement similr to the rrngement of interleving nd hence the ctul F m will lso e only hlf tht considered ove. Assuming the curve for / = 0.5 with little error, k = 0.93 As in Figure 8.33() S = centre spcing etween two phses = 10 mm = spce occupied y the conductors of one phse = 44.45 mm = width of the usrs = 15.4 mm \ 16 (45 000) 0.93 4 F m = 10 N/m 10 14 348.6 N/m Q 1 N/m = 1 9.807 kgf/m \ F m 1463.1 kgf/m Since the usr supports re ssumed to e t distnce of 400 mm, \ force on ech section of usrs, insultors nd the mounting fsteners = 1463.1 0.4 = 585.4 kg We hve drwn comprison of these forces for the other usr configurtions in Tle 8.8 for more clrity. Since Figure 8.34 is found to e etter rrngement, we hve considered forces s in this rrngement only in ll our susequent clcultions. (vii) Mechnicl suitility of usrs nd their supporting system Below we nlyse the dequcy nd the suitility of usrs, fsteners nd the insultors supporting the usrs, to withstnd the ove forces cting differently t different loctions. Bending stresses on the usrs Bending stress t section x F l x = m 1 M N = kg/cm (8.10) where F m = mximum electrodynmic forces cting on ech support, in the event of fult, s clculted in Tle 8.8 for Figure 8.34 = 514 kgf l = centre distnce etween two usr supports = 40 cm M = sectionl modulus of ech usr t section x x = 1 in cm 3 6 where for 15.4 mm 6.35 mm usr section = 6.35 mm = 15.4 mm \ M = 1 6.35 15.4 6 1000 cm = 4.58 cm 3 (sme s indicted in Tle 30.7) N = numer of usrs per phse = 4 \ Bending stress = 514 40 1 4.58 4 kg/cm 17.43 kg/cm To clculte the sectionl modulus (or moment of resistnce) of the four us sections in prllel we hve multiplied the sectionl modulus of one us y 4. This is simple method when the usrs of ech phse re in the sme plne nd eqully spced s in Figure 8.33() with no dditionl spcers etween them to hold them together. But when other configurtions re dopted s shown in Figure 8.34, this concept my not hold true. In other configurtions, however, the sectionl modulus will only rise nd reduce the ending stress on the usrs. The method dopted to clculte the sectionl modulus is therefore simple nd on the sfe side. However, to clculte the sectionl modulus more ccurtely or to derive it for ny other section of the support thn considered here reference my e mde to textook on the strength of mterils or mchine hndook. Note The sectionl modulus, when required, cn e incresed y providing spcers etween the stright lengths of us sections, s shown in Figure 8.33(). The spcers, when provided, cn mke the usrs more rigid nd dd to their ending strength due to higher sectionl modulus. At joints these spcers occur utomticlly in the form of overlpping of usrs or fishpltes (Figures 9.4 nd 9.5). The spcers prevent the us lengths from getting deflected towrds ech other. Inference The minimum shering strength of luminium is 16 kg/cm (Tle 30.1) which is much lrger thn the ctul force to which the usrs will e suject, in the event of fult. They re thus more thn dequte in cross-section nd numers. Other thn ending stress, there is no significnt tensile or shering force cting on the usrs. 3 x y y x = 15.4 = 6.35

Crrying power through metl-enclosed us systems 8/1031 Suitility of fsteners On fsteners lso, other thn cross-reking or shering stress, there is no significnt compressive or ending force. Fsteners for usr supports As in Figure 8.34, ech phse of four luminium sections is supported on four two-wy insultors. Ech insultor is mounted on M8 size of olts (dimeter of olt shnk, 8 mm). \ \ Totl numer of olts = 4 = 8 of size M8 But it is possile tht the first pek of the force, F m, my ct either downwrds or upwrds. In which cse only four fsteners would e shring the force t time. Stress re of four olts = 4 p 8 sq.mm 4 =.01 cm Cross-reking strength of (i) Ordinry MS fsteners s in ISO 4016 of grde 4.6, = 00 kg/cm (minimum) \ Totl force they cn withstnd considering fctor of sfety s 100% =.01 00 = 11 kg (ii) High tensile fsteners s in ISO 4014 nd ISO 898 of grde 8.8, = 4400 kg/cm (minimum) \ Totl force they cn withstnd.01 4400 = 44 kg In this prticulr instnce, due to the lrge numer of fsteners, even n ordinry type of MS fstener will e suitle. Notes 1 The ove sitution my not lwys e true prticulrly when the current rting is low, sy up to 600 A nd the system fult level is still high. In this cse much less crosssection of luminium would e used, nd the numer of supports nd fsteners would lso e less. Then the fsteners will lso e of smller cross-sections. In such cses s the mgnitude of electrodynmic forces remin the sme, the suitility of fsteners ecomes more relevnt. See illustrtion of Exmple 8.6 elow. For usr joints, however, use of high tensile fsteners lone is recommended with view to ensuring dequte contct pressure per unit re over long periods of opertion, s discussed in Section 9. nd noted in Tle 9.1, which n ordinry fstener my not e le to mintin over long periods. Illustrtion of Exmple 8.6 for short-circuit forces For more clrity on the suject, in the light of note 1 ove we hve lso worked out the erlier Exmple 8.6, for shortcircuit conditions nd then nlysed the ove detils for this rrngement (Tle 8.9). Assuming nd I sc = 45 ka S = 100 mm (Figure 8.16) k = 0.9 from grph of Figure 8.7 for S = 100 6.35 + 6.35 + 101.6 0.87 nd / = 6.35/101.6, i.e. 0.065 Choosing the curve for / = 0.1. A slight interpoltion in this curve will determine k s 0.9 for / s 0.065 16 (45 000) 0.9 = 100 \ Fm 4 10 N/m = 9 160 N/m or 973.4 kg/m Assuming the distnce etween ech usr support of the sme phse to e 400 mm then the force on ech section of usrs, insultors nd the fsteners. = 973.4 0.4 kg = 1189.36 kg Suitility of usrs; for the given prmeters: F m = 1189.36 kg = 40 cm M = 1 6.35 101.6 3 cm 6 1000 10.95 cm 3 nd N = 1 \ Bending stress = 1189.36 40 (from Eqution 8.10) 1 10.95 1 = 36.89 kg/cm which is much less thn the cross-reking strength of luminium. Therefore size of usrs is ok.the nlysis of forces nd suitility of insultors is crried out in Tle 8.9. Fsteners for usr joints For usr joints we hve considered 8 numers olts of size M-10 (dimeter of olt shnk, 10 mm), s in Figure 9.4. As the size of these fsteners is greter thn tht of the usr supports, their suitility is not determined seprtely. Suitility of insultors The usr support is the most vulnerle component in current-crrying system. It hs to withstnd ll kinds of stresses developed y the usrs on fult. From Figure 8.36 we hve identified the following likely vulnerle loctions in support tht my yield on the occurrence of fult; Finger etween the two usrs, section, which my sher off from its roots. At the olt mounting holes section yy. At the wedges mrked with htching. But the insultor is more vulnerle t, thn t the wedges, since the sher re t is only 0.6 1.5 cm compred to 1.98 1.5 cm t the wedges. Hence, for revity, we nlyse possiilities (i) nd (ii) only. In Tle 8.9 we hve evluted the cross-reking, shering nd ending (flexurl) stresses, tht my ct t such loctions, to estlish the suitility of the supports used. Suitility of fsteners As in Figure 8.36, lthough ech phse will e supported on two insultors nd ech insultor mounted on M8 size of fsteners, it is possile tht t the instnt of fult, the forces re cting either upwrds or downwrds. Therefore, ssuming forces to e cting only on two fsteners, t the instnt of fult nd ssuming fctor of sfety s 100%.

8/103 Electricl Power Engineering Reference & Applictions Hndook Tle 8.9 Checking suitility of insultors Exmple 8.1 Exmple 8.6 Figures 8.34 nd 8.36 Figures 8.16 nd 8.37 1 Cross-reking stress X t section F m = 514 kgf/cm F m = 1189.36 kgf/cm Fm X = 1.5 A B = 1.3 cm = 1.3 cm A = 1.5 cm A = 1.5 cm B = 0.6 cm B =.4 cm \ X = 1.5 514 1.3 \ X = 1.5 1189.36 1.3 1.5 0.6 1.5.4 = 1856 kgf/cm = 308 kgf/cm Shred y 4 fingers 4 wedges Fctor of sfety 100% 100% \ Minimum cross-reking stress the = 1856 = 308 4 4 supports should e le to withstnd = 98 kgf/cm = 154 kgf/cm Inference Only SMC supports must e used DMC supports cn e used Shering stress S s t section A = 0.6 1.5 cm A =.4 1.5 cm Fm S s = \ S = 514 s \ S = 1189.36 A 0.6 1.5 s.4 1.5 (A = cross-sectionl re of section ) = 571 kgf/cm = 354 kgf/cm Shred y 4 fingers 4 wedges Fctor of sfety 100% 100% \ Min. shering stress the = 571 4 = 354 4 supports should e le to withstnd = 85.5 kgf/cm = 177 kgf/cm Inference DMC supports my e used DMC supports cn e used ut limiting fctor is cross-reking stress hence only SMC supports must e used 3 Bending (cntilever) L = 3.9 cm L = 3.9 cm or flexurl stress, B s t = 1.0 cm = 1.0 cm Fm L Section y y = M = 6 0.85 = 5. 0.85 = 4.3 cm = 3.5 cm where M = 1 \ M = 1 6 6 1.0 (4.3) \ M = 1 6 1.0 3.5 = 3.08 cm 3 =.04 cm 3 nd B s = 514 3.9 nd B 3.08 s = 1189.36 3.9.04 = 6.8 kgf/cm = 73.8 kgf/cm Shred y 4 supports supports Fctor of sfety 100% 100% \ Min. ending stress the supports = 6.8 4 = 73.8 my hve to withstnd = 35.4 kgf/cm = 73.8 kgf/cm

Crrying power through metl-enclosed us systems 8/1033 Tle 8.9 (Contd.) Exmple 8.1 Exmple 8.6 Figures 8.34 nd 8.36 Figures 8.16 nd 8.37 Inference DMC supports my e cceptle SMC supports lone must e ut limiting fctor is cross- used with modified design, reking stress t to withstnd higher ending loction. Hence only SMC stress or the design of the supports must e used us system itself e modified s mentioned elow under note 3. Conclusion In this prticulr instnce, section In this cse, section y y t the t the fingers is more olt mounting holes is more vulnerle. If the supports vulnerle. If the supports fil, they my fil from here. fil, they my fil from here. Notes 1 Fctor of sfety It is possile tht during the fult only one of the insultors is suject to the trnsitory first pek of the fult, s there my e slight mislignment etween the insultors, symmetry in the usrs, n imperfect olt fixing nd their fstening, or comintion of such fctors. To e on the sfe side it is dvisle to consider ech support nd its fsteners to e suitle to withstnd the forces y themselves. We hve ssumed fctor of sfety of 100% in ll the ove clcultions to ccount for this. F m for ll prticulr fult levels remins the sme, irrespective of the current rting of the system (Eqution (8.4)). A low current system employs fewer nd smller usrs nd is supported on less nd smller supports nd hrdwre. Such system therefore my hve to withstnd much higher stresses thn higher current-crrying system nd is more likely to yield to such stresses unless dequte mesures re tken while selecting the size of supporting hrdwre or spcing etween the djcent horizontl supports. 3 When component supporting the current-crrying system is likely to e suject to severe stresses, nd its own stressering cpcity my e mrginl s in Exmple 8.6, it is dvisle to tke higher size of such component in its thickness or dimeter or etter qulity mteril. It is lso possile to mitigte the stresses y reducing the distnce etween the two mounting supports of the sme phse. In the ove cse we hve ssumed this s 400 mm. If the centre spcing S etween the two phses cn e rised conveniently, it cn lso reduce the severity of F m. = 10 R Y B = 60 14 3 14 8.5f = 10 13 Bus length 65 Y 65 7. B = 6 7. 19.8 19.8 Y L = 39 65 A = 15 X F m F m X l = 13 A = 15 6.3 Busrs 6.3 l = 400 10 S = 10 Side view Elevtion l = 13 (All dimensions in mm) Wedges Y 7. 6 7. Y Detil of ech support Figure 8.36 Mounting rrngement of usrs for Exmple 8.1

8/1034 Electricl Power Engineering Reference & Applictions Hndook Y 65 = 5 6 13.4.4 7. (B) Y L = 39 8.5f l = 13 l = 13 = 10 A = 15 Electricl conductivity for grde E-91E = 1.0 Derting for n mient of C = 0.815 Derting for ltitude = 1.0 Derting for size of enclosure; Are of ech chnnel = 1695 mm (Tle 30.9) \ Totl re of ctive mteril = 7 1695 mm Are of enclosure = 115 4 mm \ enclosure fctor = 7 1695 115 4 101.6 6.35 Al section =.34% Derting s in Tle 8.6 for item 0.8 Derting due to skin effect included in the sic rting of chnnels t 5440 A (Tle 30.9) Derting due to proximity = minimum 0.80 \ Totl derting = 1 0.815 1 0.8 1 0.80 8.5f Figure 8.37 Exmple 8.6 S = 100 (All dimensions in mm) Mounting rrngement of usrs for = 0.516 \ Actul current rting of chnnels = 0.516 5440 = 837.5 A which is good for the required rting. The conductors hve enough mrgin for higher lod demnds which my rise due to voltge drop of up to 10% of the rted voltge if most of the lods re industril drives. Cross-reking strength of these fsteners = 1 p 8 00 kg (for ordinry MS fsteners) 4 100 = 1105.8 kg which is mrginl compred to F m of 1189.36 kg. It is dvisle to mke use of high tensile fsteners only. Suitility of insultors We give in Tle 8.9 comprison of different types of forces tht the insultors my hve to encounter during fult condition t different loctions. (B) Chnnel sections Consider chnnels in ox form for the sme requirements nd operting conditions s for rectngulr sections. Choose two chnnels for the phses nd one for the neutrl of size 17 mm s in Tle 30.9 nd let them e rrnged s shown in Figure 8.38. = 17 R 17 t = 8 85.5 175 Y 17 175 S = 30 S = 30 85.5 B N 17 175 48 115 Aluminium frme (All dimensions in mm) Figure 8.38 Configurtion of chnnels in ox form for Exmple 8.1() 4 Voltge drop Skin effect d.c. resistnce for one chnnel (Tle 30.9) = 18.41 mw/m t 0 C R dc t 85 C nd for the ox = = 18.41 1.36* per chnnel *(refer to step (iv) ove) 18.41 1.36 = 11.38 10 3 W/1000 m Active re per phse = 1695 mm = 33.9 cm Skin effect rtio from Figure 8.13(c) for t = 8 17 = 0.06 Rc = 1.04 ut considered minimum 1.05 Rdc \ R c = 1.05 11.38 10 3 = 0.01195 W/1000 m Proximity effect From Figure 8.19(c) conductor effective spcing S = 3 S S S e AB BC CA S AB = S BC = 30 mm nd S CA = S AB = 604 mm \ 3 S e = 30 30 604 = 380.5 mm

Crrying power through metl-enclosed us systems 8/1035 nd X from grph of 17 mm chnnel = 108 mw/m = 0.108 W/1000 m per phse Impednce Z = 0.01195 + 0.108 = 0.108 W/1000 m Voltge drop for m length in phses R nd B = 0 0.108 1000 = 13.5 V which is 3.5% for system voltge of 415 V. The system chosen is good up to length of m. Beyond this the voltge drop my exceed the desirle limits nd require either lrger chnnels or reduced centre spcing S. Reduced spcing S is possile in this instnce due to the sufficient mrgin ville in the rting of the chnnel section chosen. (viii) Effect of proximity on the centre phse Y Let us ssume length of m. The voltge drop in phse Y, ecuse of R c, ssuming content of X = 0 = 0 0.01195 1000 = 1.49 V \ Supply-side voltges of phses R nd B = 415 3 13.5 = 6 V nd of phse Y = 415 1.49 = 38 V 3 which will provide resonly lnced system. To mke it more lnced rector cn e inserted into the middle phse long similr lines to those clculted in Exmple 8.10 for rectnce of 1000 W 3 X = 0.108 = 5.40 10 For the rest of the clcultions for mechnicl suitility of the usr system the procedure for rectngulr sections cn e followed. (C) Tuulr sections Now consider tuulr section for the sme requirements. Choose tue of size 5 (stndrd pipe) from Tle 30.8 hving the following dimensions, For phses Outside dimeter (OD) = 141.30 mm = r 1 Inside dimeter (ID) = 18.0 mm = r A = 74 mm Nominl current rting = 35 A For neutrl choose tue of size 3", hving OD = 88.90 mm nd ID = 77.9 mm A = 1439 mm Arrnge them s in Figure 8.39. Likely dertings: 75 70 S AB = 300 S BC = 300 300 45 75 Figure 8.39 sections For grde of usrs, for E-91E = 1.0 For mient temperture = 0.815 For ltitude = 1.0 For enclosure fctor = 3 74 + 1 1439 4 1165 = 1.83% \ derting 0.83 (Tle 8.6) For proximity 0.9 (such sections, ecuse of their shpe, would hve low proximity effect s noted lter) \ Totl derting = 1 0.815 1 0.83 0.9 = 0.609 \ Actul current rting = 0.609 35 = 16 A The usrs nd the inside of the enclosure my e pinted with mtt finish lck pint to mke this section suitle for = 1. 16 = 594 A If voltge vrition is lso to e considered, then one my hve to choose the next higher size. As in Tle 30.8, one cn choose n extr-hevy pipe of 5 size. Voltge drop For skin effect: d.c. resistnce R dc0 = 11.3 mw/m from Tle 30.8 \ \ R dc85 = 11.3 1.36 (refer to step (iv) ove) = 13.97 mw/m Active re per phse = 74 mm \ skin effect rtio from Figure 8.13() for R R t d c dc = 6.55 È = 0.046 t = 141.3 18. = 6.55 141.3 Í Î 1 R Y B N 18. 141.30 S CA = 600 6.55 1165 Illustrting Exmple 8.1(c), with tuulr us \ R c85 R dc85 = 13.97 mw/m = 0.01397 W/1000 m 5.5 77.9 88.9 4 (All dimensions in mm)

8/1036 Electricl Power Engineering Reference & Applictions Hndook For proximity effect: From Figure 8.1 r r1 = 18. = 0.907 Corresponding to this D s /r 1 = 0.967 141.3 \ D s = 0.967 141.3 = 68.3 mm nd S e = 3 SAB SBC SCA 3 = 300 300 600 (refer to Figure 8.39) = 1.6 300 = 378 mm Corresponding to this, the rectnce from Figure 8.19() cn e determined y extrpoltion. We hve ssumed it to e 0.06 W/1000 m per phse. Impednce Z = 0.01397 + 0.06 = 0.0616W/1000 m Voltge drop for m length in phses R nd B = 0 0.0616 1000 = 7.7 V which is only 1.86% for system voltge of 415 V, nd is stisfctory. Effect of proximity on the centre phse Y The voltge drop in this phse, ssuming X = 0 = 0 0.01397 1000 = 1.75 V which is only 0.4% of the system voltge. This us system will thus provide ner-lnced system. For the rest of the clcultions for the mechnicl suitility of usr system the procedure for rectngulr sections cn e followed. Note Since sic properties of copper re similr to tht of luminium, the design prmeters nd service condition considertions in selecting the size of conductor nd enclosure for copper usrs will lso remin sme s for luminium usrs. Relevnt Stndrds IEC Title IS BS ISO 60059/1999 60439-1/004 60439-/000 Stndrd current rtings (sed on Renld Series R-10 of ISO-3). Low voltge switchger nd controlger ssemlies. Requirements for type-tested nd prtilly type tested ssemlies. Low voltge switchger nd controlger ssemlies. Prticulr requirements for usr trunking systems. Hexgon hed olts. Product grde C. Hexgon hed screws. Product grde C. Hexgon nuts. Product grde C. Hexgon hed olts. Product grdes A nd B. Hexgon hed screws. Product grdes A nd B. Hexgon nuts, style 1. Product grdes A nd B. Hexgon thin nuts (chmfered). Product grdes A nd B. Hexgon thin nuts. Product grde B. Mechnicl properties of fsteners. Criteri for erthquke resistnt design of structures. Generl provisions nd uildings. Plin wshers. Interconnecting usrs for.c. voltge ove 1kV up to nd including 36 kv. Fire resistnce tests Elements of uilding construction. 1076-1 to 3/000 863-1/1998 863-/1998 1363-1/00 1363-/00 1363-3/1998 1364-1/00 1364-/00 1364-3/00 1364-4/003 1364-5/199 1367-0/001 1893-1/00 016/001 8084/00 BS 045/198 BS EN 60439-1/1999 BS EN 60439-/000 BS EN 4016/199 BS EN 4018/199 BS EN 4034/199 BS EN 4014/199 BS EN 4017/199 BS EN 403/199 BS EN 4035/199 BS EN 4036/199 BS EN 0898-,7 DD ENV 1998 (1 to 5) BS 159/199 3/1973 4016/001 4018/1999 4034/1999 4014/1999 4017/1999 403/1999 4035/1999 4036/1999 898-1,,7 834/000 Relevnt US Stndrds ANSI/NEMA nd IEEE ANSI/IEEE-C37.0.1/00 ANSI/IEEE-C37.3/199 Stndrd for metl enclosed LV power circuit rekers. Metl enclosed us nd Guide for clculting losses in isolted phse us. Notes 1 In the tle of relevnt Stndrds while the ltest editions of the Stndrds re provided, it is possile tht revised editions hve ecome ville or some of them re even withdrwn. With the dvnces in technology nd/or its ppliction, the upgrding of Stndrds is continuous process y different Stndrds orgniztions. It is therefore dvisle tht for more uthentic references, one my consult the relevnt orgniztions for the ltest version of Stndrd. Some of the BS or IS Stndrds mentioned ginst IEC my not e identicl. 3 The yer noted ginst ech Stndrd my lso refer to the yer it ws lst reffirmed nd not necessrily the yer of puliction.

Crrying power through metl-enclosed us systems 8/1037 List of formule used Short-circuit effects (1) Therml effects sc qt = k I Ê ˆ (1 + 0q) t 100 Ë A (8.1) q t = temperture rise in C I sc = symmetricl fult current r.m.s. in Amp A = cross-sectionl re of the conductor in mm 0 = temperture coefficient of resistnce t 0 C/ C q = operting temperture of the conductor t which the fult occurs in C k = 1.166 for luminium nd 0.5 for copper t = durtion of fult in seconds sc or I t = 0.0799 for luminium for n operting A temperture t 85 C nd end temperture t 185 C (8.) Isc t = 0.1 for copper for n operting temperture t 85 C nd end temperture t A 185 C (8.3) () Electrodynmic effects 16 Isc 4 Fm = k 10 N/m (8.4) S F m = mximum dynmic force tht my develop on fult I sc =r.m.s. vlue of the symmetricl fult current in Amps k = spce fctor S = centre spcing etween two phses in mm Skin effect Effect on current-crrying cpcity Rdc Ic = Idc (8.5) Rc I c = permissile current cpcity of the system R dc = d.c. resistnce R c =.c. resistnce I dc = d.c. current Conductor resistnce t higher temperture R dc t 85 C = R dc0 [1 + 0 (q q 1 )] (8.6) 0 = temperture coefficient of resistnce t 0 C per C R dc0 = d.c. resistnce t 0 C q = operting temperture = 85 C q 1 = since the vlue of R dc is ville t 0 C therefore, q 1 = 0 C Proximity effect in terms of usr rectnce S e = (S S S c ) 1/3 (8.7) S e = effective or geometric men spcing S, S, nd S c = spcing etween conductors Use of sturle rector to lnce lrge unlnced power distriution system To determine size Ê Xp = V 1 ph 1 ˆ Á = f L Ë Ir I p (8.8) y X p = lost rectnce of Y phse L = inductnce of X p I r = current in R or B phse I y = current in Y phse Reluctnce of the mgnetic pth g k g Rl = l l + mo A mo mr A (8.9) l g = length of the ir gp in metres m o = permeility of ir (free spce) = 4p 10 7 H/m m r = reltive permeility of the silicon steel used for the lmintes in H/m A = re of cross-section of core in squre metres k = totl length of the mgnetic circuit in metres Clculting stresses on fult Bending stress on usrs t section Fm l x x = = kg/cm (8.10) 1 M N F m =mximum electrodynmic forces cting on ech support in the event of fult l = centre distnce etween two usr supports in cm M = sectionl modulus of ech usr t section x x = 1 6 in cm 3 N = numer of usrs per phse Further Reding 1 ERDA, Study on fesiility of upgrding the operting temperture of Al usrs without plting. Golding, E.W., Electricl Mesurements nd Mesuring Instruments. 3 Lynthll, R.T., The J & P Switchger Book. Butterworth, London. 4 Thoms, A.G. nd Rt, P.J.H., Aluminium Busr. Hutchinson Scientific nd Technicl for Alcn Industries Ltd. 5 Copper for Busr, (ville in FPS nd MKS systems), Pu. No., Copper Development Assocition, U.K.