Document mB15N2wpd837LRxj3qqaxqKBO

2/a components: unit substations Secondary unit substations, placed dose to load centers, take primary distribution voltage, transform it to lower secondary value (usually 480 v today) and, through a system of circuit breakers, start it on its way to nearby load areas. Standard factoryassembled units offer (1) fast delivery, skipping draiting-room time needed with ' specials'' (2) top in terchangeability and flexibility when iuture changes are necessary (3) Quick replacement of any elements damaged during equipment life (4) lower cost than Itald-assembled substations. Strive lor unllormity in your subslaUona. q-JB loads so substations oi equivalent leva can be lull Aim for Interchangeability of transformer* switchgear. Then one spare is spare for all. -M Future Growth. Sizable load additions usually qB lor new substations. But small new loads can oIvotI be tacked on existing units, provided spore capo^a was incorporated when originally selected. In cnodl ern units, switchgear is so designed that compfoijn now leeder sections may be added with ease. DurUujI design stage, check that enough lloor space Isavajjg Single-ended unit substations satisfy the needs of most plant! Mfto take additional wilchgear oi low-voltage kr('substations. Since breakers (n the same infer- doss ore tolwchangeable, you can increase &ker size without structuraJ change, fffcgtation Sixe. For each plant Ihere ts generally an jglnjuin *lt gi^ng lowest cost per kva. With many. Sunits, m&re prtmory cable is required, boosting ^tolled cost- But smH QniU reduce the amount oi ondary coble. On the other hand, when subgtlonsaro large, swltchgearand secondary-feeder- &hle costs climb. Location. First determine where the substation would lit best in an ideal situation -- indoors or out. Modem practice includes the plant roof os well as space alongside the building in the term "outdoors". An other out-ol-the-way spot is in balcony areas. Such location offers savings in valuable floor area and in additional costs ol outdoor roof equipment. Unit substations are available In double-ended designs as well as the familiar single-ended typo. Weigh well the economics of double-ended designs before ruling out straight singles across the board. ifoiible-ended units may prove best betin concentrated load areas Incoming Mellon between primary feeder and transformer may have drcult breaker, load-interrupter twitch. oINuto cutout, selector switch, or a solid connection. Where ewltchee selected Interrupt trans former magnetising current only, a keyedinterlock l# usually used, tied in with main secondary breaker, toad-interrupter switches ore avail able with or without power fuses for transformer protection. Switch and lute combination Is economical, effective. With two incoming lines, a 3-positton selector switch Is used. Make sure there's enough space to throw switch without hazard. tttf Transformer can bo oil-tilled, askarel, vented or seated dry type tpp 68. 67). Askorel offers high impulse strength, can be put indoors or out. Sealed dry type combines some advantages ol liquid unite (sealed against outside dlri) yet needs no maintenance other than oc casional inspection, external cleaning. Secondary ewltchgear now makes wide use ol drawout breakers. Molded-case breakers can be used with substations up to about 300 kva, have widest use where operalion U Infrequent and drawout feature Is not necessary (lighting substations, lor example). Breakers moy be manually ox electrically operated. Control centers moy be used whore you want to centrqltte oc mater starters, lighting panels end associated controls In one spot. Provide spare breaker position when you want secondary-selective layout. Bun cable between this point and bus of nearby substation- Interrupting capacity ol secondary switchgear is important in substation siting To arrive at most economical substation site lor any plant, study the factors that directly atleet installed cost: length and site ol both primary and secondary cables, primary- voltage, heavy spot loads, required interrupting capacity lor secondary switchgear, primary shortcircuit protection. Curve, below, was pre pared on a basis ol 480-v secondary distribution, after considering above fac tors in a typical plant. Curve changes if 208 v is used; economical size then (ails around 300 kva. Substation Costs, For small units, costs climb mainly because of higher ironstermer cost In smaller kva ratings. At othor end oi scale, casts climb with big ger units mainly because ol greater available short-circuit currents, calling lor more expensive secondary switch gear. And slnee larger subs usually servo larger plants, secondary cable cast mounts last with stalion site. With 13.8 kv on primary feeders, aim for 4000 to 7500 kva per leeder. Then 750-. IQOO- and 1500-kva units can usual ly be applied without individual primary overcurrent protection INEC. paragraph 45i2b). Such protection would be need ed with 500-kva substations and some primary voltage and foods. If primary voltages lait between 2.4 and 4.16 kv, most economical primary loader loadings are betwoen 1000 3000 kva per feeder. In this range, small er substations can be used without the added primary protection. Meaty spot foods often lustily law* substations. Typical ate lumacss. oven*, large weldors -- these might call fw 2000-2500-kva subs at 480 v. Interrupting Rating, There are three com mon shorl-clrcull-roiing brockets in tbs low-voltage range: 15,000, 25,000 i4 50,000 amp, ail rms asymmetrical- As suming a 480-v secondary, largest sub generally used within 15,000 amp is KB kva, II 25,000-amp figure isn't to be ex ceeded, 750 kva would be top practical limit Where substations larger than 750 kva ore used, secondary switch?*0 generally carries a 50,000-amp interrupt' Ing rating. This last requirement tan* often dictates two 750-lva substation* rather lhan a single 1500-kva unit. 94 TODAY'* IIECUICM MaCTICI sown jfcboWs-ended substations go hand In units. Main secondary breakers are nec abend with secondary-selective arvd net- essarily large to carry full transformer jevork arrangements. II either ol these ^feokap* looks beet far your plant, you 9 fan nved to weigh single against jjdouble units. With doublet, maintenance ;aml testing ol transformers can be done load current. Whero toads are heavily concentrated In one area and Ihe choice Is between two economically sited smaller sup* - stations and one tingle large unit, without strrlce Interruption. But they east mere, mainly becauso o( the need double-ended design may prove bost. Spare Transformer. Some power engi lor two main secondary breakers and a neers buy a spare transformer as a re Ue breaker, not necessary with single placement lor many simllar-elte units throughout the plant, and stick to singleended substations, (n an emergency, spare can be put to work quickly. There are times when each trans former of a doubto-ondod unit must be sized to carry total substation load so no load need be dropped t| a leeder or single transformer (alls. More typically, load diversity and fudlcioue emergency operation me counted on to hold normal rating for both transformers. Secondary unit substation location affects costs and efficiency of distribution system. building roof_ This location proves popular where primary distribution Is overhead and but duct it used. Then short secondary runs drop direct to bus duct. Root location often sidesteps high ambient temperatures, dirt and lumet In plant: U alto His well where load areas change frequently. Remote tripping and dosing of breakers is usually needed. E-ilffMl-IHrIJi Steel structure withexpanded-molal or sheet-steel floor is ptacod high enough so spacs below is usable, often as a washroom. Balconies keep substations out ol noriso) i/ailic and. in food plants, out ol range ol splash Irotn equipment hosing. Substations are also suspended In roof trusses, just abovo lowest structural members. Check plant layout lor low-value areas indoors at contort ol load. Consider, too, pulling substation immediately outside. And where buildings are long and narrow, study areas between them II cable rune con be kepi short. Avoid spotting all equipment at a single trans former bank: slick to load centers and short secondary runs. OCTOItl 1933 rooArs etecTNOU machci