Document pBVv8QX04b79886bJrkgj1a0k

systems: basic considerations like leprosy, high voltage was held at a distance in yesterday's plants. Typically, utility feed camo into a large primary substation outside the plant proper. The substation housed three large singlephase translormers and a maze ol secondary break ers feeding the many loads throughout the plant. In event o( a transformer fault, use of three single phase units allowed cutting out a faulty one and connecting the two remaining translormers opendelta. In somo cases, a spare single-phase unit was kept lor Ju9t such emergencies. High voltage has now moved indoors. More depend able equipment, greater degree ol safety enclosure, better engineering know-how made this possible. Main feature of this evolution is the shilling of the point where voltage is lowered to secondary level. Instead ol being done at one point outside the plant, it s done at many unit aubstation* n load centers In the plant. There's still a prime? substation but, unless utility voltage is over f$t? it's now mainly a switching point. Cables from Uw primary substation still run throughout the plm,. but there are fewer o( them and they're small^ because they operate at higher voltage. But there's more to modem distribution than mot. ing high voltage indoors. This was a major step but for top results it must bo carried to its logical capelusion by choice of proper circuit arrangement. Available systoms. some II in number, range Iroat simple radial to more complex network hookups. have their place, but the survey on which this re. port is based shows that three of these find widest use. by far. So discussion here is limited to simp| radial, selective-radial and network systems. Today's load-center distribution system shows lower cost than,. Regardless ol (he reliability built Into production equipment, entire operation ot yout plant depends on uninterrupted upply of eleclfc power lo Ihe many molori, lamp*, furnace*. welders, etc. Thus, speaking broadly, function ol your electrical dislrlbutlon system Is lo re ceive power at one or more bulk-supply points and deliver It dependably to every electrical device In the plant. Comparative Costa. Unlike many olhtr advanced methods, carrying high volt age to heart ol the plant actually ools less than distributing at tow voltage. To drivs (his point home, let'* consider (he case Illustrated by Ihe relative-cost chart, upper right on lacing page. Figure* ate based on a 250,000-sq-U tingle-door plant. Old mslhod el supply would have o ono-Hne diagram like thol at tell ol Illustration. A single large substation would be located outside Ihe building, and long low-volioge loaders would string through the plant to serve Ihe loads. Wilh a 13.2-kv Incoming line, this powor syaltm, right through to final plug-in bus duct, would cost about $100,000 installed. Lead-Center Saving*. A modern foodcenler system using packaged second ary unll substations (see diagram, p 891. would cost about $83,000 lor the same plant. This fs a saving of 117.000. or (7%. Secondary substation system has about 3000-kva capacity to allow tor diversity, but these substations cost only aboul as much as a 2S00-kva outdoor substation assembled on Ihe job. Major saving comes in cable cost, as chart shows. This brings out the im parlance of looking at the over-all sys tem In any aconomlc study. A change In one pari may have a significant bear ing on cost of another part. Here, high voltage on feedort makes (hem smaller. In this ease, two primary leedets were selected lor the load-center system. This further Increases system selectivity com pared with old-eiyle layout. Here, by selectivity v mean being able to main tain pari of ihe load In an emergency such as a primary cable or transformer failure. Although sketch doesn't shew il, use el two primary (seders requires more dreulMniomipling equipment In spile of this, switchgear cost runs about the same tor either system. Then, while transformer costs are greater in the load-center system, sec ondary switchgear cost is materially re duced. since only IS.OOO-omp interruptIng-rating feeder breaker* <ln cascade) are neodod for smalt 7S0-kva secondary substations. With old low-voltage dis tribution scheme, SO.QOO-amp feeder breakers (in cascade) would be needed lor the large 2300-kva substation. Additional Soring*. Average load-center system shows lower losses than older low-voltage distribution. As plant grows, siiro savings result from load-center systemVgrealer flexibility in handling new food* In existing buildings, or expansion. We can Iflusirote this by supposing that the plant we'vo been talking about Is extended so as lo need about 1200 kra more. II we add this transformer capacity (o the old-style tow-voltage sys tem, *hort-ctrcuit currents on all prevf. ously installed loader breaker* are in creased. Possible shorl.circuit duty would be Increased so much that break er* would be (nadequalo. and thore'd be an immodlaie expansion bottleneck. Cosy Expansion. Contrast foregoing wilh expansion ol a load-cenler aysiem. Here we merely extend the two high-voltage feeder* and add two new unll sub stations. Short-circuit values aren't Iq. created at any existing unit substation*. But suppose wo had put eitra 1104*. former capacity In the old-siyl* to*, voltage systom. in anticipation of load growth. Loadcentor system still pro*** less costly lo expand, because Us copadIy can be built up In small units 0* load grows. Thus powsr-system and plant growth are closely coordinated. Wilh the older system, a large capital investment must bo made initially In a big single substation. Secondary lesden are addod as needed until Until ol sub station capacity is reached. Then anothersubstation Is built at great expense. II load doesn't grow, investment t* watted. In applying load<emet methods, how ever. initial investment can be lower because you need only Install capacity lor present foods plus a small reserve. Handling Large Loads. In Ihe following lour pages well folk aboul circuit ar rangements now In use and toms of th* thinking behind plant decisions on adop tion ol one or another system. But beiors we gel to fhal, there are some general thoughts which apply regardless of ths specific system used. In particular ths question ol handling large foods. Typical would bo large motor*, say above 200 hp. Such units can olten be served more economically of voltages above 600, Thu* they're out ol range ol secondary side of regular secondary unll substations. U primary voltage f suit able, they may be served directlyDirect from Primary. For example, motor* above 200 hp may (eed directly from o. 2400-v primary systom. Likewise, motors above, say, 2S0 hp, can be sorved from 4160-v systems and those over $000 hp from 6900-v. Motors above 2000 hp may be supplied at 11.0 to 13.8 kv. Id TODAY'S ClECTtlCAl MACTtCi fOWII p- _____ low-voltage layout, and flexibility makes its expansion easier BUI you'll still run into coses where foods that can't be led prop tdr from either primary or secondary. There or* two possible answer*. (I, say, job hav* one or two motors too large far secondary voltage {480-v systems) and toe small for primary voltage, best bet Is >0 feed such motors Uam primary though separate tranaformerx. ymetwm*i* at Lead. Let's look at an rarnpf--an 8C0-hp motor In a plant with f3.6-kv primary. Most economical --",,w *wsbablr be to serve the answer would probably be to serve the motor from the 13.8-kv system through a stepdown transformer with a second ary voltage ol the order oi 2300 v. On the other hand, lei's soy you have concentrated groups of large motors and an olactrfc furnace. You'd normally lead them direct at primary voftage but you can't becauso It's too hlgh-fet'* Wt 13.8, 30, or even 69 kv U It's a very large plant. With exception of 13.8, these volt ages would bo loo high to apply to any motors, and 13.8 Ils* too high for most. Medium-Voltag-e- "Su`bs-t>a*ti,o1n.,. TThhe* aannsswweerr In this case lies in an adaptation ol the load-center idea. Supply pig loads or groups of largo motors from a separate unit substation haring a secondary volt age in tho range between 3.4 and 13.8 kv. A typlcoi case b found in paper mills, whsre primary voltage Is often 13.6 kv. Here large motors are generally sup plied at 2.4 kv through a 13.8/2.4-kv sub station. Smaller motor* operate ol 460 v. t/ie bcic element, of a modern load-center .y.lem Here are Today, switch- gear Is oiten the major equipment Item . Ut ths primary substation where incom ing feeder voltage is less than IS kv. Cir cuit breakers are the usual interrupting aeon*. operated through induction re lays and cl's. But load-lntettupler twitches and high-voltage power fuses say be used. Either way, packaging It lbs theme, rather than field assembly ol lbs separate components. -- For connecting primary and secondary substations, cabl* in conduit U mooting tough com petition since odvent ol armored coble. This new version ol low-vofloge armored cable licks cost ol conduit Installation and cable putting. It offers increased mechanical protection. Simple mechantcal methods of splicing and terminating have been developed lo eliminate the I need far easily lead wiping of Joints. ifkkIIMBVBwww Heart of modorn system is the secondary unU substation, (l has three parts: Incoming ac-line sec tion, transformer, secondary switching section, tncomlng line may have break ers. function boxes, interrupter switches. Transformer may bo oil, atkarel, ven tilated or seated dry type. Secondary section may have drqwout air circuit brocket*, molded-case breakers or some form ol motor control Incorporated. - .tibt f -W-TTfr^ <yi ;1 i'V.lV;.:' i.ttii-/..; ^ fp Y--7---------------' ' 73# Vr* TOOAY'i IllCTIICM MACHtt JusJ