Document D7gyvy4QBQpB97EmZM1EyK2M

2 utility feed Jr i The primary substation takes in electrical energy In bulk, trims its voltage to proper level and dis penses it. Transformation is often eliminated where utility voltage is below 15-kv ceiling sot by NEC for most installations. But it may be in older even then, if big motors or other loads operate best at some voltage other than utility's. Threo features stand out in any study ol primary substations: lo cation. reliability, and protection. Location. i( ideal, would be right In plant load center. But practical angles and utility requirements usually put It at edge ol plant property. Modem equipment en closures make outdoor buildingless Installation the rule rather them the exception. Additional cost of outdoor gear must be balanced against building costs. Some strive lor outdoor location of utility . .-- ---, t- -- f<uni p switchgear indoors. Wherecorroslve fumes, may settle on high-voltags ir laiors, study Indoor design, Oi least put substation on wind* side of source and outline pr_^ \em to oquipmenV manufacturer. Reliability of your Bystem can't ex ceed that ol utility supply. & check Its adequacy. Ask utility ^ IKp expectancy based on post jggfritricQ ih your area. UnderjsjAfld coble generally faros bef* overhead, buf faults often jKjjj longer (o find and lix. Faults BSgforerhead lines are often (em&&my; can bo cleared readily, Spyoteeiioa starts with pinning S&pwn available short-circuit kva %* utility con supply. Chock if Jffifpojectod utility interconnections Increase (his in the future. Work out with your utility the details on primary substation ownership, number of Tie down b<n(e point* early. Set number and arrangement of leads by joint agreement. Dscitlon hinges an: site oi toad, need (or duplicate service, volt age, (tansmUslon-ieodet capacity and nearness. Incoming (eed may vary from 1.4 kv ungrounded to 118 kv with neutral grounded solidly or through resistance. Type el feed and high-voltage swilcfilng hangs on supply voltage, service con tinuity, toad sire, shorl-circuh-currsni available, utility relaying, number and sixo ol slepdown transformers, flexibility needed (or iuture load changes. Utility leed may be a simple lap on a main transmission line, or Uns be looped into primary subnotion. #* ing, in eiiect. a 2-circuit teed. More lsdi may be brought In, (rota separate bus section* ol utility substation or separate substation*. Whether automatic or qk. ual ihrowover is used depends on con tinuity roqulred. Short-circuit duty re mains relatively constant regardless al number of lines brought in, it ihif'n ( not paralleled. Who should e*ra the primary substa tion? There's no universal maim. Where a choice Is eiisred. many pros and cons can only bt settled by as Modern packaged substation designs, with single 3-phase transformers, cost leu.. Today's packaged unit substation usu ally proves a better bet than stations assembled on the spot. Equipment cost may run higher but. with proper plan ning. inslalied cost shows savings and you have e smart, well-coordinated unit. Primary unit substation* are made up of one or -more transformer* mechan ically and electrically coordinated with one or more switchgear assemblies. In coming section provides lor hlgh-volloge circuits, which may or may not have Individual breakers. Transformer Is usu ally clMlIlod (sse pp 96, 97). Outgoing section handles plant primary feeders with either breakers or load-lntemrptor witches and power tuses Ipp 104.105). Weigh advantages at translormsr km cooling. Some engineers peg transform er site an existing load, then provtd* for taior fan cooling. How much extra capacity this gives depends on unit site. The peak fs 67% for 2-stage cooling In transformers over 10,000 kva (p 66). Single-phase vs 1-pbose transformer question is now pretty well settled. Re liability ol today's units makes the .single 3-phase transformer the logiad and economical choice. Some engineers UM ieel there are cases, in targe sub- Today'6 critical toads generally dictate need for load-ratio control or regulators to Motors, lamps, heating equipment are all designed to operate be| at rated Tallage. OtaUlbuttan systems atm to pro vide this but the best systom sudors unavoidable fluctuations In supply volt age and voltage drop. Today's industrial plant alms to hold substantially constant bus voltagewithin 3 to 8%. it sometimes is done with load-ratio control, generally built Into the primary transformer to change laps under load. There are usually 16 slops above and 16 below rated secon dary voltage. In some cases, U may prove economical to regulate specific loads instead of voltage (or entire plant. Tbit decision depends an voltage limit* ol supply and required regulation oi th specific toads. Figure cost of load-ratio conlrol at roughly 63.73 per kva of transformer e` pacily tor loads In the 10.008-kva rang*engineers now give more thought to selecting voltage control when desigh' Ing the primary substation, rather than later at greater cost. Many foot it is fo** tilled in nearly every Imtallailon whet* voltages above 13 kv ate stepped doA to tbe 2.4 to 13.8 range. Control can be set to hold voltage constant or to rai** it slightly as load Increases, la compen sate (or drop in plant system. II TOOAYl CKOtICAt HACTtCC FOWtl ieeders required, voltage \ f(0M0fo R__a_l_e_s..a..renlotaftnenmmOfore ' fgfuvW* where user owns substation. But Initial *' ond maintenance may etiet this. Many targe automotive plants fgi mi where possible, (a buy at utlllBriton voltage, usually 13.8 kv. Other farfuxtrialx angle for ovnstshlp because fesy want some control of operating re- UabSity. But remember, II you own, you sed rfianpewer versed in high voltage. ttrJteye cbm to iolt isn't a rule, but if* baud on good practice today. Note that ItS |r is omilmo* economical even to HUMS kva, but 13.8 opens the road to Plant safety needs good equipment grounding Equipment grounding is Important in primary substation design as well as throughout entire distribution system. It means con necting to ground all noncurrent-catrying metal parts: conduit, raceways, transformer coses, switchgear enclosures. Prime object is to limit voltage botween these metal ports and earth to a sate value under all operating conditions. Three fame plant expansion, components are used; Grounding electrode Is a conductor .than `'assembled'' stations embedded in the earth. Ground bus is a network that lies solidly to grounding electrodes. Grounding conductor connects Italian*, whore three tingle-phase unit* . ooy be best choice. They ' compare availability problem when replacing a (age 3-phase uni) apolnst continuing operation with two single* open'deha. Fbr everfiead structures, look into use ol ahifnfnum. especially where corrosive atmospheres may cut through galvan ised steel. When slsol coaling is gone, painting must start. This usually colts ter killing all or part oi substation and terms )us! onother maintenance head* eeh*. In a lew industries aluminum may nor stand upr some chemical and petro leum plants reported trouble. equipment and wiring enclosures to (he ground bus. Ground bus should surround building or structure. Ground ing electrodes should connect to bus a( two or more points. With a double path (o earth, a single broken tie won't open the connection. Size ol ground bus Is based on how much current may How through it and for how long. For mechanical strength, aside from current-carrying capacity, keep ground bus No. 2 awg, or larger. Modem practice may dictate coble sizes to one mUtipn cm lor a sound equi-pofonliaJ grounding scheme. Local grounding connections for lightning arresters are made by driving rods Into earth near arresters. In addition, tie light ning-arrester conductor to common station ground bus. Where it's exposed to possible mechanical injury, ground bus should be protected, generally by conduit. Connect both ends hold down voltage swings Shot's answer where supply ie below 18 kv and carried directly to secondary unit substations? Load-rotfo control in each secondary substalfort Is generally uneconomical and often impractical.The solution is either srep voltage regulator* or Induction voltage regulator* In the primary supply. Their standard regu lating range l* )0%, Can you save by of conduit to ground bus to avoid inductive eifect. New portable welding device oUefeo promising way of joining grounding electrodes, bus and conductors (Powes, March, 1953, pp 123-12S). Basic elements are powdered metal, firing charge, small crucible. Result la a hlgh-curtent-capadty Joint. General Pointers: Equipment frames are considered grounded if they're bolted or welded to properly grounded structural steel. Rigid conduit systems by themselves aren't usually con sidered good grounding connections, but NEC accepts at 609 v. booking two Induction regulator* opendeha? Ooliarwisa, yes, but epen-deho connection creates unbalanced voltage tan may oilset heating (n fully loaded pr less. A grounding connection run inside conduit or raceway through which power Hows must be Insulated, equal In size to largest line conductor but generally not larger than No. 4. polyphase induction motors. Best prac tice avoids open-delta in iavar oi straight 3-phase regulation. POOM* 1*33 rooAT-s ntciwCM rtAcnci 13