Document jy47xM2kOa64VjbvKbov48B6Q

Vc.| systems: grounding There are three camps on this mat ter ol neutral grounding. First, and growing, are those all (or It. Sec ond, there's a group against it. They cite belter service continuity they fee) is inherent in an un grounded system. Third are engi neers who have lived with an ungrounded system but are real ising the advantages of ground ing. On a modernization program Ihoy'll stick to ungrounded neu trals but will leave the door open tor a later switch by specifying Y-connected transformers. Sound engineering plus solid Held experience backs today's swing to neutral grounding. Most popular method grounds neutral of a Y-connected transformer or generator. Few ground corner or center of phase on 3-phase delta. In the ungrounded oamp there arc those who diligently search out each ground as it occurs. They aim to keep the system completely free of accidental grounds. Others still take the shortsighted ap proach ol keeping the system oper ating with one phase accidentally grounded. They depend on over current protection to clear the cir cuit if another phase grounds; in effect, a phase-to-phase short. If you ground neutvtiU: (I) Ground at each veltag* level-, only at cure*. (2) UoT# no part ungrounded. Ground each major but section It you can separate them. (3) Neutral grounding needs three overcurrent device*, one In each phase (4) Most systems need only, two overload relays tar motor running protection whether or not if* neutral grounded. Some need three. Check this. (SI U primary volt-, age is belweon 2.4 and 19.0 kv, consider resistance grounding. Grounded neutrals boost reliability, safely, equipment protection On Y-grounded tow-voltage sysloms, as on the secondary side o( load-center unit substations, tine-lo-neutral calculated voltage is maximum met either in norma) operation or with a ground on one phase, in ungrounded system*, two ungrounded phases ate at line-to-line potential measured to third phase when it accidentally grounds. And ilrti fault doesn't automatically dear. II lault occurs on another phase, there's a phase-lo-phase short. Operating Hotard*. When dillerenl phases ol an ungrounded eystem accidentally ground In near-by machines, a tire hazard may result. Current path between grounds can include greasy shavings or other materia) that could be ignited by localized heating or sparks. Impedance ol path is high, so current flow may be enough to start a fire but won't blow fuses or trip breakers. On ungrounded systems, "sputtering** grounds can cause overvoltages. Circuit formed between system capacitance (mainly cables) and system inductive roactanc* build* .up voltage bolween phases and ground to soveral limes normal line-to-llne. in cases-on record such sputtering grounds led to multiple motor faults unlit located and cleared. Overvoltages can stem Irom sources other than sputtering grounds -- lightning, contort with higher system voltages, resonance In serioa Inductlve-capaciiiv* circuits (POWER. Aug. 1952. pp 90-92). But a grounded neutral will hold phase-to-neutral voltage constant. Equipment Protection. Grounded systems oflsr better equipment protection. Relays will de lect grounds and clear circuit before arcing develops into phase-to-phaso fault*. On primaiy voltage* of, say, 2.4 to 13.8 kv, resistance grounding is oilon used lo hold down burning at ground point, reduce explosive ellects. // critical loads Justify ungrounded operation, here's hou> you make the best of i With so many arguments in iavor oi grounded neutrals, how do today's ungrounded systems manage? Two things are needod: (1) 'opnolch maintenance to keep circuit insulations in good shape (2) some practical method ol quickly locating grounds betore they develop into phase-to-phase fault*. That's fust about the way mony Urst-rate power engineer* handle the problem. They fully realise disadvantages ol an ungrounded eyetem. But they believe that, because ol criti cal nature oi their load*, demand lor contin uous service Justifies an ungrounded system. Lamp-Detectors, Where a simple lamp-detecior it used on an ungrounded systom It indicates only that a ground does or does not exist. It there is a ground it doesn't tell where. The only available way to (ind which branch circuit is causing tho trouble Is to trip the circuit breaker* one alter another until the iamp-detclor shows the lault is cleared. This procedure, ol course, isn't practical In a modern plant, tn Mou ol it w* now have trouble-snilltng Instruments that track down point ol fault on an ungrounded eystem with out shutting oil power. See sketch, loli. Impulse-Detectors. A modern ground-detector Impulse unit clamps to ungrounded phases at the main bus. A signal current is super imposed on tho grounded line all the way down to the trouble point. Then, with a sen sitive induction-type impulse receiver, the signal is traced, even through mela!-ciad switchgear and conduit on the grounded line. The portable detector unit will even pick up an impulse as small as hail an ampere. So it readily indicates the point at which .signal leaves the power line and enlers ground -- the lault point. For details ot this ground detection procedure, see Power, May 1952. pp 132. )33. ft rooArs (licriiCAi siactice POWII system*: voltage selection I Coliqgefl continue io climb. Power engineers and 'aief* equipment come closer to agreeing on fg^- voltages, so we move toward fewer levels, greeting Voltages. Rant safety and local codes enter gi/t three.main factors are: load size, distance and ^yjpment ratings. Bigger loads and longer distances jjcint to higher voltages. And. of course, voltage rat ings cf using equipment must match feeds. '.power engineers no longer consider 4.16 or 13.8 kv too dangerous to have around. Greater safely of to day's equipment, both in Insulation and mechanical enclosure, mokes high voltage nothing to fear. Primary Voltage. Here higher levels mean lower cost, greater flexibility and expansion margin. Savings in crease as plants spread out and loads got bigger. On the secondary side, apparent advantages of higher voltage point to making it as high as possible, up to limit oi 600 v. Actually. 460 v Is becoming the byword lor new load areas because ol ready avail ability ol utilization equipment for it. tgigher voltage levels offer economies, flexibility for expansion ich cl many voltages available for primary distribution -- 2.4. 4.16. 19.9 and tr - has its place. But today's thinkpoints to two: 4.16 kv lor plant* `^`liavtnfl o supply transformer and/ar gtneroUng capacity ot 10,000 kva, or ^K'less. and US kv where there's a supply ^Dveonilormor and/or generating capacity $-*1 26.000 kva, or more. So lor rang* bej; 'tween, either ol two voltages may be )V. Bunt economical. In choosing, don't for- 9t your plant will probably grow. rV. While 4.16 kv might possibly bo slightly f` lets expensive now, U plant should re- 'quire sizable expansion 13.9 kv would y' be more economical. ( 14 er 4.16 kv? With all (actors Included ."switchgear, motors, starter* -- costs v` gtnerally Iavor 4.16 kv. In addition, there's greater allowance for expansion at 4.16. In small plant* (say 2000 kvo) can bo several dollars por kva lor 4.16 kv. Feeder costs usually dtp as voltage rises. Data tor a modern armored highcable prlmary-leedor layout points to a saving ol about 62.00 per circuit kvo per 1000 (I. lor 4.16 over 2.4 kv. Operating at 13.0 oltor* an additional saving over 4.16 ol about SO.SO per circuil kva per 1000 it. whero individual circuit loads can be kept in the 3000-kva range. Cost oi ter minating may dotraet from economy of 13.8 kv il average longlh ol runs is low and there are many small loads or sub stations per leeder. But more kva can be carried at higher voltage: fewer lines may be needed. At 4.16 kv, switchgear for a given in terrupting capacity costs lose than at 2.4. And in a 4.f6-kv system, U a 1200 amp breaker Is neoded. a moro expen sive 2000-amp job would be required in a 2.4-kv system. clad breaker lor 2.4 kv is ISO mva max imum Interrupting rating. For 4.16 kv, maximum is 250 mva. This allows a much larger system without synchro nising buses, curront-ilmiling reactors, or other means ol cutting (Quit current. Secondary Voltages. Today's practice puts spotlight on 460 v; forward-looking power onginoar* seldom iind good rea son for selecting 240 v. Costwis*. 240>v systems run higher than 480: heavier current means larger cable, breaker*. tt used to be argued that 240 v ts re quired for area* with heav^ electric fur nace loads. Today most large lurnaces can operate directly Irom 480 v. Even though a slzablo portion oi load i* at 240 v lor small lurnaces, it's seldom economical to adopt 240 for general dis tribution. Select 460 and step It down to 240 lor specialised loads. Another point What about expansion? Largest metal- tecsedery unit tvbOntiore 4.16 or 13.6 kv In modernizing, present voltages needn't limit you, higher ones can be worked in Electrical modernization plans may involve: (I) replacement of some unoale or obsolete pari of the system, with iittfo or no ezpanslon ol capacity, or (2) expansion to boast capacity *er growing loads, in either case, biggest gain can be made by changing to a load-center system and higher voltage. Best Approach. Start by laying out a complete Ideal powor system of type and voltago desired. Plan it as if you wero building tho enllro plant new. Then see what modifications of the plan are noeded to use serviceable existing equipment. From then on. evory step should be based on tho Ideal plan, too olfon. power engineers spoil good intentions by letting the existing setup influence luture plans too greatly. This Is especially bad if the old system isn't right lor plant conditions. Higher Voltage. Always cheek this when expanding er modernizing an existing system. Converting a low-voltage eystem to a higher voltage is a simple procedure. As sketch shows, it merely requires addition ol a bus-tie translormer between the buses. Then expansion can be to higher vollago as il lower voltage eystem didn't exist. Old low-votlQge and new high-voltage system can be tied in several other ways. Mony plant* using a 2.4-kv system are facing expansion, yet want to hold onto 2.4 already Installed. This can be done by working in a 13.0-kv system to sorve new loads and also areas ot 2.4 kv through appropriate transformers. OCIOUt 1913 TOOAY'S CIEC1RICAI StACTlCt distribution