Document gax62M40jY0kvoMLZbeDKBJjq

yb circuit interruption: breaker! 1 Mainstay in most protection plans, loaays drv breaker keeps pace with growing demands lor trie reliablo protection, minimum downtime on lauita.' Industrial-plani breakers, on secondary o(' pr^c substation IrartBlormer and in secondary substatioS' use air rather than oil In contact-breaking chamber' In modern switchgear, individual breakers (also m'? tera and instruments) are removable so malntenan^ can be done with equipment deenergized. J1 The lew cycles needed lor breakers to trip alio* protective relay to close contacts, breaker trip coil move plunger releasing breaker operating mech& rtism, and opening contacts to interrupt current. Short-circuit current is maximum during lirst lev cycles because o( dc component and peak eonWbu# lion irom motors on line. Thus, there are two base* lor short-circult-current ratings ol circuit breaks*: (!) momentary rating--ability to stand mechanical* stresses which are maximum during early cycles (2flj interrupting* r--a--t-in- vg -- ability to ubrieauki uciirrccuuiitt ssaotleellyy..^ ^aap.between simple air-break switches and breakers, modem load-interrupter switches BijSfej, whem main need is switching. Where last Setting of breakers isn't required, where possible ^St^phasing car) be tolerated, combination interS@fcstfffeh and fuse serves as economical allernai-'\ot more comp/efe service given by breakers. i^indusinal primary substations, typical units are ifS^efto interrupt 600 amp ol load plus magnetizing Siring to interrupt station load means switch. r^flSfcon be done without mechanical and electrical I^WiHocking with secondary breakers. Some engi* specify combination interrupter switches and Swpeive'r fuses to give short-circuit protection lor pri* P^suxiy feeders and secondary transformers. Fuses are jjftujually high-voltage boric-acid type. y^-Sirifchlng confers in large industrial plants, where is distributed over a series of high-voltage uitffieclers to unit subs, use interrupter switches as an ^ilconomicai means of switching under load. how circuit breakers are now being applied in primary and*} IfVfsiIcondary unit substations, and on incoming utility-powerfeeders On the preceding page we strewed the importance ol ade quate interrupting capacity in today * protective equipment. Now let'* look ot some oi the practices being followed by power engineers in applying this equipment. Primary Substation. II a main transformer broader {in unit's secondary leads) Is used in the simple-radial system, over current relaying eon be applied to trip thto breaker os backup protection (or plant primary-feeder breakers. Main )ob ol this breaker, from system-protection standpoint, is to detect and isolate faults on primary substation bus. Relaying lor breakors, howover, must bo coordinated with other protective devices in system. Thermal ovorload protection for primary-subslatlon transformer can also control this breaker. With modern unti-substoiion construction, however, It is now general practice to eliminate tho transformer breaker. This can bo justified by the compactness and extreme reliability ol unit-type equipment, which makes possibility ot a primarysubstation-bus loult remote. In large primary substations, pos sible use ol bus and transformer differential protection should be considered. Additional insurance provided must Justify premium paid. In some special casee, differential schemes combining transformer and bus protection con be devlsod with a saving in cost ol protective lectures. If the utility sub-transmission system supplying plant pri mary substation consists ol a single radial line, no protection Is ordinarily provided at substation high-voltage terminals, with possible exception ol a high-voltage (use In combination with a switch. In casos where other independent loads are supplied over the same single radial line, however, tho utility When a high-voltage breaker Is used In the primary sub- i station, there Is little Justification for using a breaker in trarw. *] former secondary leads. Protection normally supplied by -; secondary breaker can be provided by high-voltage breaker, j making one less step (n roioy coordination. ; Beceadery Substations, In simple-radial systems, secondary sub* are protected in one ol two ways. A main breaker can be used between stepdown transformer and secondary bus. ' It so, breaker permits not more than 2.5 times normal rats ol . current to the substation transformer. More usually, trip setting of this main broaker is closer to rated current of transformer, common trip setting being I.2S limos rated current. In the second case, the main breaker is omitted, but sum ol trip settings of low-voltage feeder breakers is less than IS times ratod transformer current Transformer thermal over- current protection can be supplied by some manufacturers. NEC sets limits within which individual overcurrent protec tion on primary side ol secondary-substation transformers can be omitted (19S3 edition, SoctJon 45J2). Basically, the Code permits leaving out individual primary overeurrent protection when Ihere is adequate secondary overload protection (or the . transformer. Also, when primary-leader relaying can' be said to detect faults in low-voltage side of the transformer. If you interpret the Code literally, It permits connecting only 5 Idem p ossible at various voltages ,,* RVtlcol transformers to the Sams primary ieodsr without use of schemes havo been worked up to provide selective (ripping c-Vindividuel overcurrent protection on the primary side of the ^ lifanilointere, when i/ansformer impodanco it 6% or Jess, ft ot such devlcas. For example: A definite time delay ts olten introduced Into ih* opening of a !ow-volrog breaker at a ;V*;trenformer impedance is between 6*4 and 10%. it is permit- high fault-current level to give branch-ctreuit device enough Alibis to connect only three transformers of like site to the limo lo open fl fault Is in branch. It was specifically this problem that resulted in recent CV'tome primary feeder. tary While the code requirement is generally accepted by power f $ engineer# today as being approximately correct at 6*4 and development oi moro accurate adjustable soriss-trip alemenls lor tow-voltage edr breaker*. Since their entry into breaker ,10% iranslormer impodance, the regulation is unnecessarily lield, it i* usually easier lo coordinate characteristics. ^`restrictive lor other valuos of Impedance. In many eases today, j/.-the oversimplilication ol tho Code rule Imposes an economic Tripping characteristic* ol a breaker necessarily form a band rather than a line, it's defined by a lower ampere-time value below which the breaker will reset end not trip, and an r,-'* pfiKtfty art the induatria)-p)ani distribution system. krsaksr Coordination. Protection at load points must coordi- , *,' with breakers at supply ond or law-voltage side ol seei * ondary substation. This coordination between protective elaiv->r-Denti depends mojnfy on the (ow-voltage-foader breaker* upper value representing maximum time required lor breaker to open circuit (or a given flow of current. Usually, If a ratio ol 2:1 exists between tripping-current value ol larger breaker end smeller, the smaller trip* before largor breaker I* un J'.. canyIng current to supply several branch leoders. Even though ampere-time characteristics of two dsvices have dissimilar shapes, coordination is generally obtainable because of dif. latched. Network Protection. In a secondary network system, main substation breaker i* the network protector. This automati cally opens only on revorse power How. To get backup protec Isrenees in normal current ratings. ' Whsn breakers operate up near point oi maximum interrupt ing duty, opening times ol both low-vollage-feoder and branchdrcuh protection may be so nearly instantaneous that coor dination is impossible, it Is lor this reason thal various tion for network protector In case it falls to open, fuses are usually incorporated as part ol the network protoctor. Throe (uses generally provide overeurrent protection against cur rents greator than 2.S times Iranslormer rated current. Network relay that trip* protector is an induction-type 3- phase directional power relay. An overvoltage roclostng fea ture Is generally incorporated in modern network relays to This is modem mefalctod switchgear: initiate automatic recloeing ol proleclor when proper voltage may call ior an automatic switching devico, a circuit breaker, ot primary-substation high-veltage terminals. This protects incoming line against outage caused by faults in tho indus trial-plan! system. Such a breaker Is generally relayed by overcurrent units, coordinated so that it doesn't trip for faults that should be clearod by primary-leader breakers or other protective devices in the lf\dulrtal-plont system. The high-voltage breaker can provide backup protection.lor iaults on the plant system, and where transformer or bus dif ferential protection Is used, the breaker can be controlled by the differential relays. ftsfl-dreult energy 3-f*8* kvt disrt-circcil eip pee phase 15,000V 2500 v 480 v S3 v 25,000 1000 SJOO 31,000 70,000 50,000 2000 11,500 63,000 140,000 100,000 (000 23.000 125,000 280,000 Hoft The stove era attained values diauiSni inpedsic* fiat miy be met M (he frptaf pint ryitor oa teconfofai of unit ubitilons i { .< i j '! An assembly of main units and auxiliary compart ments with: (l) all parts enclosed in grounded metal (2) grounded meiai barrier* lo isolate secondary con trols and wiring Irom high-voltage primary device*, and primary units Irom each other 0) removabletypo circuit breaker having sell-coupling primary and secondary disconnecting contacts and a mech anism for moving il physically between connected and disconnected positions (4) interlocks lor sale operation, proper sequence. supply is restored to secondary substation. Secondary-Selective Systems. Coordination ol Iripping characleristlcs ol low-vollage feeder breakort and those in tianslormer leads is exactly the same as problem met in simple radials. Exception may came when all load Is carried by one translormer, as would be the case with a singla-endod sec ondary unit substotion. When substation (s operated with ono translormer. an added breaker is olten placed between main primary breaker and fow-yoifage feeder breakers, Then if bus-tie trips automatically on overcurront you avoid difficulty ol getting selective opera tion of main, bus-tio. and Jow-rofloge-leeder breakers. TODAYS fliCTBICAl PIACTIC* sown tooay's eiecmcAi mactic*