Document YVrD6zRxjMnd0oVGa46x5M88

LECTURE ON A5KAREL AND DRY TYPE TRANSFORMERS . BY Hr.Earl W. Tipton Fallow Member of Institute of Eloctfteai fi Electronic Engineers and National Society of Professional Engineers U.S.A. with 45 ycar% cxporionco in dusion and munufacturintf of power and distribution transformers with WESTXNGI101SH ELECTRIC CORPORATION ,U.S.A. . at the Confortnce roots of Faculty cf Engineering . GiUXALOMGKORN UMIVERSIYT l.T** FT*"' *7*.?. 7.975 2.00 p4~3..*"!L MONS 045641 ABKAREL AMD DRY TYPS TRAnSFORMERS A. ASKftREt, TRMMMHEM ' In the Into 1910'* and a<^l?10'* the demand for eloctrie power In office end public buildings was Increasing very rapidly. The aaim waa true to an oven groatar daqrco fer power in factories. To provide such power transfomers of larger sixes had to ho provided 2c ns dwsirabln to locate them innido the building or factory lnordor to roduco tho lan?tAof secondary bus runs for tho purpose of reducing power lossas and improving regulation. To do to with oil filled transformTM, in which the oil may couse an oxploslon and a flro, for safoty reasons, the transfoxsicr must be located In a fireproof concroto vault which It expensive to provide , . - To provide a solution to this problon transformer manufacturer! In The Unites States began development programs in the period from 1910 to 1934. The firot solution to become eenmorelally available wan a transformer using a non-flaanable liquid instead of oil. This liquid, callod by the tradu nano Pyranol, had no flash ,, point end would not burn end was developed by the central Bloctrie Co. its basic fluid used consisted of chemicals called aerochlors, and triohl^rcb^nsinf . xa order to be eoepotitive other manufacture adeveloped similar liquids. ` The one with which X am - familiar is that of the weetimgheuse Electrio <brp. which Is called Znerteen. as e close ` nrninanmsMs insulating liquids aro called Asharele. i' * JUkarel transformers at one tine were used ouito extensively in the United States, perhap for 9 to 10 percent of the production In the range from 100 to 1500 KVA. for voltages up to as KV. There is still a considerable activity in these sites for network end power center transformers. Itovmr, dry typo transformers designed for MC temperature rise ee moaaurod by resistance, have rool.iced the sokarcl transformer for most epplications. Askerel network traneforswjnhavo to eeno oxtont resisted tho ehargo ,;vcr to dry type construction bcoiuni of the mace limitation on network applications. JVC* oil insulated tr&nsiornurs tho ashesr) hy) o>\, and csscnti.il 1/ only otto, advantage^ nskarcl will not burp and thv* tronsforrr-r cm w used withnue a fireproof concrete vault incilc buildings end factories. Askorcl hoo good pow>:r frequency dielectric strength*. Thn dioloctric constant of eat'-arul is A. 4 a* emnnrrd to 1.2 for transformer oil which QivuJ It art "dvantage over oil in insulation barriers mdc up of sheets of preesboard separated by duets. Xn a uniform dielectric field the etreee in dielectrics in scries Inversely .as the diwlvrtric constant of the materials. For an.oil and presnhoard barrier tho stress in volte per Inch is twice as great in the oil as It ie in the prcsebcard. with proar.bnord end Osfcarcl the stress in nm>.ly <>,n m>m'. In tv* aalaral as In the preanhonrd. hekard can be used In transformers to 34.5 rv.caxirun. A typical ask.-.rcl line properties as follows 1 ^ Pout point -UC . ' ` Flash point non* .` Viscosity it J7.fC 2 aaybolt H1cm.1l cnpacity 0.251 calories pru gran per/(: Resistivity ?.0-ohm/em* * Uialvctrl'' ->V: >' " c:> 4 4Vf.w,yu)v> MONS 045642 Boat conductivity 30 0300 * : ' <jrn-caioLle*/0C/in Density 3,57 kg/liter type of electrodes separation inchae diso to plane turn to turn with P*P tape .25 .SO 1.00 2.00 .02 .04 .08 16 Dielectric trongth-KV fOcycl e'imieT oil aekarel FuiPwovc impulse oil eskarol 35 52 73 107 . 16.25 27.0 44.5 74.0 40 67.5 55 116 17. S 29.3 so.o 133 150 190 215 280 280 <10 340 (.83) 39 36 (.92) 65 54 (.63) 100 80 (.74) 177 " * Disadvantages of AskarsIs 1. Coat * The per unit cost of aakarcl was s to 7 timoe that of oil whan It was first introduced X do not hava a comparative coat for tho present time which would ahow tha effect of the rqoent soaring ooat of potxolaum produets . Tha coat of aekarel transfomara is further increased by tha naeaasity for duplleata Systems for storing, {Hearing, pumping and processing oil and askarsl. coat of aakaral la further incroaaad by tho double peas * wolds required for all tank and radiator seams, 3 Power Taotor ' * " Aakarol has a such higher power factor than, transformer oil. Xnertatn for owwgla hoe % power footer of .05 as ooavarad to .001 for good transformer ll.flshn.rol La sactrisly laniitiva to oontaainanta and tha power factor can ohly bo maintained at an acceptable value by keeping all Mtariala in the core end coil structure end the interior surfaces of tha tank and coolers clean. Ordinary varnishes^, gums* glue of paint can nut he ueod. 3. Chemically Active Aakaral la ehanieally very active and raacta with varnishes, paints, m<* trwinf rm.r oil,with ineroasaea In power factor and decreased dielectric ntrnngth.All tank wOds must ho double peas wolds to prevent leaks. Practically it in neeeeaary to dntermino hy toet just what material# are oempatable with tho particular font of aekarvl which ie is proposed to use. Initially some grades of prcbpard were found to contaminate the . aakarol liquid and for a tioo two grades of preosboerd wnro stocked, one for oil *n.l one for aakarol transformers, -'iftar 'Same years of uao a Hot of compatible materials aaa be accumulated. 4. Ourmatltie ' A minor d-laadventago nf aokarol ie that many people ,\re -ill.-mic ** *nd it my eause a skin rash. In such case the use of a protoctiv* ointment or r^^ar gloves can give relief. Host peoplo are not affoc'.ad. f. Spooific gravity .. dot a particular disadvantage is the fact that aaMr> 1 la hoavlur titan water. <*ny wator which enters tho transformer or which may be formed by hooting ami aging <f cellulose insulation will ilsu to the top surface of the aakarol. Coupling vaiui-s uat^l t^ obtain eesplee of tha usk?rol to tost for wator content must be located in the try HONS 045643 ' of tho aakartl. Zt it undosirablo that tho water Doing in the outtoat port of the Mkoral tands to flow down through tho coollag tubes of radiators and than risao again through tho coils whoro it can bo absorbed by tho calluloso insulation, . a. Dooooposltion by an Cloctric Arc A (tost sorious dofoet of askarol Is that short circuits or internoi faults which eauso arcing in the liquid produce wator vapor arvl hydrogen chloride {liCU which in . oosiblnatlon form hydrochloric acid. After a fault in an asknrel transformer if the core and colls aro not Immediately rceavod fro* tha tank and elcaned with a solvent such ae bontone tho eoro and coils will be ruined. Steel parts such as ths core and ths Inside of tho tank andrad^Ator2orrodo rapidly following ah arcing fault, for a minor fault, such ss^V&Pn^hich produces intermittent arcing for a ceneiderabla period of timo without a conplote broak down, tha production of hydrogen chloride stay badly damege ths transformer before tho trouble le discovered, raulty electrical contacts at totted joints or even faulty braces and proesure contacts can produce SO called Stowing ares ever long periods of time without conploto breakdown. To at laaat partially protoet against thssa sffocts a chemical "GETTER".or'*icavangor 15 usually added as a oo^onant of askarol. Ths scavenger reacts with end neutralises the HCL until it le used up. This is helpful when the stewing aro does not continue for too long a time. T, Impulse strength 4 Although aakarels have high dielectric strength against power frequencies the ispulao 'strength against lightning or switching surges is considerably lower than that of Oil.. For a S centimeter cap between a flat disc and a ground plane tho lnpmlee strength of aakarel'ia only S3 pereont of that for oil. *'. V -The tjrn of turn' **-ong*h betv*n row t\?ed elaetsicil T;:'!r.rt-r* sith 3 tillisstsrs of paper turn to tufn has an impulse strength in atkarel of only 74 pereont of that in oil. When atkarel transformers are designed for basic lmpuleo lewis higher than ISO KVA. tha insulation clearances must be those of the next higher basic impulse level. So far as X know aakarel it not used for transformers over 300 basic lavelU4.SKV.class). S. Explosions Askar*! liquids v))) iot bum nor support combustion, but. when subjec ted to high temperature or to an olactrie are gaseous decomposition products aro produced, for tho case of a high energy internal fault enough gas can bo producod to rupture tho transformer tank. These sennet, however, be a secondary explosion duo to ignition of tho deeesp< sltion products such es enn occur with oil transformers. Xu the Unltod States ask.ircl transformers are generally supplied with a presruro rcliof duvico mounted on s handhole am the transformer cover. This is bet to open iw a pressure holow that which would mptrue tha transformer tank. Xt la fittod with a flange which permits the attachment cf piping to carry the released gas and vapor to tha ale outside tho building or vault. These devices ere designed to reseel ae soon ee tha Is relieved which prevents the intamel propoum . ontxanco of moisture into the txanefprmer tank. 1.PUT TYPE TRANSFORMERS Another solution to tho problem of e sewer transformer which een satoly bo . Installed inside buildings and factories eloee to the load it serves was developed by the Wostlnghouse Electric Corporation and put into prodoetion in 1936. This wee tho HONS 045644  dry typo 4 or transformer designed for a tempera ro riso by realetance of eoc. Xa tho vary first transformer produced and In all subsequent dosign* every effort ties mads to ollminete organic notarial* from tSe dosign. For uso in factorioa end buildings the dry type transformer has largely replaced eskarol transformer, which era now used mainly for sons network transformers and small power eonter transformers* For uso in factories the use of dry type transformers la almost universal In the United States. They have boon manufactured in tlsaa up to $000 KVh. throe phaeo at 1SOOO volts in aolf ooolod dosigns, with fans tho rnting of a solf cooled disign can at low coat ba nearly doubled. f modified disign ean be built and hsntstically coaled in a case filled with dry nitxogon or a high molecular weight fluro-carbon gas which can be used outdoors or in atmospheres where explosive or excessive dust in present. This transformer reprosants tho ultimate in transformer safety. Safety tssts made on such a transformer aro discussed in. tho transactions of tho American Institute of Electrieel Engineers, technical paper No.53-245, April 1953. tiko any other machine dosign which must bo- s compromise between cost, tho desirable dosign features which eon be included, and-dhs undesirable features which most be excluded, the soC rlso dry type transformer has advantages end disadvantages tfcan compared to other typos of transformers. , ' A0VNR3MBS OP DRT TTP* POWER.T*AN*FORMERS ' 1* Safety '* Safety *ls tho primary advantage of the dry typo power transformer, such tranoformors am ba and aro located on platforms in tho factory, directly over tho machine for which * they supply-th' voltage. ''hey aro erjmglete.'y explosion proof rnd wrt,'*??y flra preef. While the molsturo proofing varnish on tho eolla and core esn bo made to bom st very high temperatures, it la present in very smell quantity, and bum* feebly end ia nearly alf extinguishing when the power is switched off. Sines it contains no liquid tho transformer can safetly bo mounted in s light wight rhoot metal eaeo louvered for ventilation. 3. Low Installation and Malntonaned cost Sines the insulation*!* a dry typo transformer la mulnly air tho slxc and weight of tbs coro and ceils is somewhat greater than that of an equivalent oil or oakarel transformer. However tho combination of the heavy liquid and uso of s light weight sheet metal ease results in an over all weight reduction. Tho tranoformor ean ta located on a platform over factory machines. 4pus runs of either bio duet of insulated cables in oonduit can be usod. No provision, for handling liquid insulation are required. Zf the transformer Is supplied with fans arranged to blow sir through the coll ventilating duet tho weight reduction over equivalent liquid installations is quite large. In tho dry typo transformer the eooling alb flrwn directly over the heat gcnor.iiing surfaces, Ja much moro efficicm( lhc*r> (.he ctw>Mng fens directed on external tank or radiator surfaces. - The dry typo transfrrxwr is nearly maintonanee free. Usually all that is required is to periodically chuck to sea that the coil surfaces do not become cevurcd tith dust. If dust doss accumulate it must h? wiped off or blown off with dry oomprusaad sir. 3, Uso ss Rower Centers ' Iw (ho ItiJ i od simp,? dry type . : :>*uv are tvwnK.nl )y provided vi.h a (riprf ruoi. MONS 0 -5 6 ^ 5 sir witch . the high voltage side. This switch i- counted in an extortion of tho transformer caia. xt ean bo provided with coble terminals tot connection to either lead eovorod or rubber insulated power cables. Also it --* ------ -1------------------ -* * f. the ease design Is made to match that of standard In such e power canter the high voltage switch compartment, the a'ir cooled dry type DISADVANTAGES OP DAY TYPE TRANSFORMERS 2. Cost A self cooled dry typo transformer has a coat ceeparablo to that of an aakarel transformer of equivalent rating. The higher cost Is duo to the largor Insulation clearances which are requirod by air insulation and by the high coot of Insulating materials capable of with-standing a temperature of 150c. these factors are partially Cosponsatod by the elimination of liquid insulation and by the lower cost of the transformer case. ^ ( Dry type transformers provided with fans ean on a eost per XVA basis be built With easte comparable with oil filled transformers. ' A few words about the insulation materials used for dry type transformer at present end in tho past might.clarify tho reasons for higher costs. For turn Inflation highly purified asbestos paper* glees,fiber oovared wire applied with e varnish, and e paper made of fibers of poly-amideimide have been used. Asbestos is sn erratic insulator and is hygroscopic, rifeer glass providen.no Insulation but maintains an air separation which is tho insulation. It has been successful -for voltage up to * 19 XV. (pedal coating machines era required to apply glass fiber to the conductor. POlyeaidelmlde paper such os Women made by Dupout Co. le an excellent Insulation and ean he applied with standard paper taping machines. Xt is possible to make 29 XV. transformer with this material. nadlal spacers betoan disc type onils may bo made of poreolain blocks 9*i e.a. thick for windings <9 to 19 KV. Plates of glass-plastic such ae glastic can alao be used. ' pecore for vortical 0811 ducts eon be made from pyrex glass tubes or glastic type glaea-plastlc strips. lbs end blocks for the coils are best mads of cylindrical poreolain insulators with sharp edges corrugations. %0p to 19 KV. the high to low and low to ground barriers eon bo mado of s high- the lew to ground harrier but by providing a dear air duet. In the high to low epoee a elaar air duct may bo used If glaea-silleone or silastic shoots oco wrapped around the outside of the L.V.coil and coiled .inside the ir.V.coil. Per protection against moisture both Utw coze und coils of dry typo transformers must be eoated with a high temperature varnish. A Ncstinrjlwmuu vantlsh b>*od on pelyemidoimidae end called Doryl is excellent . There aro also available silicone varnishes. ' ^ 2. impulse Strongth ' Dry type transformers do not have Impulse strengths as high as do liquid MGNS 045646 filled tranafoivwwo. Tho 'ncriesn Standard* permit a lower baiie impulse level and a lowor power frequency tost at follow* ' Insulation ClM XV 1.2 . 5.0 8.(6 ' 15 D-\siC Imnulso levels Liquid Dry lypo 50 10 75 25 S 35 110 SO Con raw Liquid 10 19 26 34 TCSt Dry Type 4 12 19 31 Tho Custlficnti.cn la barad on econosd.es and ono tha fact that tha transformers aro u*ad lnaid* buildings and factories on protected circuits. Usually tho connection to tho tranoKiaaion circuit is by noons of cables and a lightning arrestor it esquired CO ba located at tho polo wlvcro tho connection la node. Kllliona of connectod KVA. and yoara of earvico hova justified the practice. * foaled dry type transformers which aro suitable for outdoor uee have the aana insulation lovela at liquid filled transformer. ' ). Unitod voltage Asugo 1 Ventilated dry typo transformers wore originally mad# for voltages IS XV. and below . Uttar the uao of nates paper turn insulation and interleaved windings have peualtted designs for 25 xv. in the Uhitod States higher voltages are eeldeai used Inside bu.Llfir.gs and i v :orica. 4* When inoialled in locauioaa where there la extssivo duet such' et udsent sllbi axpiosita duet as in grain alavators, explosive gasos as in mines, or corrosive goose, tho ventilated dry typo transformer cannot )>c unod unless such mstorials are excluded and a suyply of clean cooling sir ia pipo^ to tha trsnsformor. MONS 045647