Oil Filled Cables Terminals

5/28/2018 Oil Filled Cables Terminals

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Oil-FilledTerminalsforHighVoltageGablesBY EUGENE D. EBY1

Associate,. I. . E.Synopsis, Underground cables for transmission of powerat33,000ft. and abovehave only recentlycomeinto usein America,

or received much attention here. In connectionwithsuch cablessuitable terminals are necessary and presentanimportant problemin highvoltagedesign.A marked tendencyisnoted toward the oil-fillingofcablejoints;terminal conditions makethisprocedure both logical and desirable.Dielectricstrength must first be specified, and should exceedthatof thecable; flashover should occur without puncture; lightningvoltages should be guarded againstinthe design. Proper d-c. testsar estill undetermined for various combinationsofsolid and liquiddielectrics andarigid practise can not yet be establishedwith assurance. Atpresent, highvoltagecable lines are intended for a-c.operation and safety factors should be determined for that kind ofservice. High voltaged-c. operation may come into practise later,andresearchind-c. testing shouldbepushed.Standard ratings ofterminals areproposed, correspondingtothe accepted standard ratings for other high voltage apparatus.

Consistency with other terminal insulation, such as apparatus

bushings and line insulators, isdesirable. Cable insulation mayeventually experience similar standardization. The methodofrating single-conductor and three-conductor cables should beharmonized, and both based on operating-linevoltage.Four typical designs of highvoltage cableterminals are describedrepresenting a carefully worked out andeffective solution oftheproblem. These are(a)87,000-volt three-conductor; b)60,000-volt single-conductor; (c)73,000-volt, single-conductor; and d)110 000-voltsingle-conductor. Flashover tests and time tests,corresponding to breakdown and endurance tests on equivalent cables arereportedto illustrate the ability of the terminals to withstand factoryand field tests on the cables, and to show the ample factors of safetyunder operating conditions. Resultsof anexperimental installation of the110,000-volt terminals demonstrate thesafety of thedesign,predictedfrom calculations and confirmed bylaboratorytests.Fo r temporary testing purposes these oil-filled terminals are mostconvenient andeconomical,andcontributetothe uniformity andreliability ofthe results in cable testing, which are the factors ofgreatestimportance.

OIL-FILLED TERMINALS FOR HIGH VOLTAGE CABLESW ITH the introduction of lead-encased cables intothe field of high voltage power transmission,there has arisen the necessity of providing, at theends of these cable lines, suitable terminals or end-bells,capable of maintaining safe connection between thecable and the apparatus or the overhead line with whichit is to operate. Three-conductor 33,000-volt cableshave been used inthiscountry for only a few years, andthe number of such installations isstillsmall. Operation of single conductor cablelinesat higher voltages isstillmore recent. Interest in high voltage power transmission over cablelineshas been rapidly increasing because of the large blocks of power which it is necessaryto deliver through congested urban sections, and whichcannot be handled on overhead lines. At present thereis active interest in cable for132,000-voltoperation anda reasonable prospect of attainingthisrating in the not-far-distant future. This rapid increase in cable voltagehas led to an intensive effort in the development of thenecessary joints and terminals for these higher voltagecables. It is the purpose ofthispaper to present someof the results ofthiswork as related to the problem ofterminals.

TENDENCY TOWARD OIL FILLINGThere is evident in the development of high-tension cable joints a definite tendency toward oilfilling, i. e., complete filling of the enclosing shellwith a fluid oil under sufficient pressure fromauxiliary reservoirs to eliminate all voids or pockets1. Engineer High Voltage Bushing Eng. Department, GeneralElectric Company, Pittsfield, Mass.Presented at the Regional Meeting of District No. 1,Swampscott, Mass., May 7-9, 1925.

within the joint. The use of hard compounds isaccompanied with certain well-known and seriousdisadvantages, among them being the shrinkage uponcooling which leaves unfilled cavities, separation fromthe surfaces of the insulating materials thus invitinglocal breakdown, and incomplete sealing of the jointagainst entrance of moisture through holes in the shelland the lead wipes. These disadvantages are effectively corrected by the use of afluidfillerunder pressure.

The tendency toward the oil-filling of cable jointshas already received expression by the substitution ofsofter compounds as fillers in place of the harder compounds commonly used with the lower voltage joints.Of the soft compounds petrolatum is the most common,while in one or two prominent cases a mixture of transiloil and petrolatum has been employed. Auxiliarypressure reservoirs of several types have been advocatedand used to maintain complete filling of the joints.The use of these softer fillers has been fully justified bythe results obtained, and these point the way to theoil-filled joint as a logical and promising solution ofsplicing in high tension cable lines.

TERMINAL CONDITIONSFrom similar considerations the oil-filled terminalis the logical solution of the problem of insulating theends of high tension cables. In two prominent particulars the terminal offers a simpler problem than thejoint,in that space limitations are largely removed, andthe bared conductor does not have to be enclosedwithin grounded metal. On the other hand, theterminal presents difficulties not met with in jointdesign, since it combines the joint problem of insulatingthe cable end from the sheath by solid and liquid dielectrics, with the further problem of insulating it at the


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594 EBY: OIL FILLE D TERMINALS FOR HIGH VOLTAGE CABLES Journal. I. . E.same time from the sheath through the air. There isalso the greater temperature variation, and completeexposure to the elements. In addition tothesephysicaldifferences, the location of a terminal at the junctionpoint between underground cable and overhead linemay at times impose far greater potential strains uponits insulation than most of the cable system is evercalled upon to withstand. The cable terminal ingeneral, therefore, is subject not only to those potentialstresses originating or developing within the cablesystem, but also to those stresses occurring in theoverhead system as well, including lightning. Thislatter source of over-voltagestressis usually far moredestructive to solid insulations than to liquids. Theuse of oil in a cable terminal offers the best known meansof dealing with the lightning problem, at the same timecontributing very effectively to the quality of thecable near the terminal by serving as an oil reservoir.

DIELECTRIC TESTSThe dielectric strength is the first feature of thedesign to be determined. Obviously the terminal mustbeable to withstand alltestsapplied to the cable afterinstallation. This it might do butstillbe weaker thanthe cable. Properly, it should be strongerthan thecable so that it will not fail at any voltage, either momentary or sustained, which the cable can withstand.By failure is meant internal breakdown. The internalstrength should be greater than external flashover, thevalue of which should be consistent with theflashoverofbushings on connected apparatus and insulators onconnected lines. The desirability of flashover without

puncture in the case of bushings and insulators is wellestablished, and the same should be true of cableterminals. The terminal, therefore, should be able towithstand both the short breakdown tests and thelonger time tests, it shouldflashoverexternally withoutinternal failure at normal frequency, and it should be asnearly lightning proof as possible.Inthe foregoingparagraph,alternating voltages havebeen inmind. Ofcourse, where d-c.testingis employedthe terminals must be able to withstand the appliedd-c. voltages. Whetherthismeans a greater-strengththan for a-c.testingwill depend largely upon the kind ofinsulation and the ratio of d-c. to a-c. voltage. The

proposed ratio of 2.4 is probably not far from correctfor cable paper; for oil a smaller ratio exists, nearer1.5; and for a combination of oil and paper an intermediate ratio would probably apply, expected to varywith the proportions of these materials. At present,data on the strength of various materials under highdirect voltages are too meager to draw definite conclusions or establish a rigid practise. It should be remembered,however, that the terminals and cable areto operate with alternating current, and it is of firstimportance that they withstand a-c. potentials successfully. The difficulties of applying a-c.teststo longlinesof high capacitance will encourage the use of d-c.

testing equipment; and some cable lines installed atfirstfor a-c. operation may even experience later on aconversion to d-c. operation. Research in d-c.testingmust,therefore, be pushed vigorously, in order that onlyproperd-c.testsshall be employed. It is not unlikelythat both kinds of tests will become a part of thedesigner's check upon his product.STANDARD RATINGSIn order that both manufacturer and consumer maybenefit by standardization of parts and designs, standard ratings should be adopted and designs developedaccordingly. It seems logical that the standard voltageratings for high-tension apparatus should apply tocable terminals as well. This would harmonize terminaldesign with that of bushings, insulators, switches,metering transformers and lightning arresters. Standard ratings, as shown in the following tabulation; arealready in general accepted use forthese devices.

15,000 88,00025,000 110,00037,000 132,00050,000 154,00073,000 220,000These ratings, when applied to cable terminals,should represent the practise for both Y and deltacircuits, grounded and ungrounded, except as modifiedfor other apparatus using terminal insulators. Thecable should not be equipped with terminals of lowerflashover voltage than that of the bushings of transformers, circuit breakers, and lightning arresters connected thereto.Intermediate ratings of cable may be found necessary for economic reasons. This will not prevent theuse of standard terminals, however, which is highlydesirable so that their line-to-ground flashover strengthshall not be inferior to other apparatus on the system.It is already generally recognized that the insulation ofapparatus located at different points on a high tensionsystem should have a uniformly high value, even thoughsomeof the apparatus is located on a part of the systemnormallyoperating at lower voltage than another part.Cableinsulation may eventually experience thissamestandardization.This proposed standardization of voltage ratingsof terminals for high-tension cables emphasizes the desirability of a uniform practise in applying voltageratings to the cables themselves. Three-conductorcables are rated in terms of line-to-line voltage and theirtestsdetermined bythisrating. In the case of single-conductor cables, however, it has been the practise todetermine theirtestvoltage on the basis of their workingvoltage between conductor and sheath. This has oftenresulted in the working voltagefromconductor to sheathbeing used as an expression of the operating voltage,whereas in the case of all other apparatus the operatingvoltage is understood to be the voltage from line to line.It would appear rather inconsistent to rate singleconductor terminals in terms of the working voltage of


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June1925 E B Y : O I L F I L L E D T E R M I N A L S F O R H IG H V O LT A G E C A BLE S 595the cables, asthiswould give the terminal an entirely-different rating from the other apparatus to whichit will be connected. For example, a single conductorcable for a 50,000-volt system, although its workingvoltage is 28,900 volts, should have a terminal whoserating is 50,000volts. Furthermore, if the cable andits terminalsareto have their voltage ratings determinedon the same basis, both would properly be assigned arating of50,000volts.

TYPICAL DESIGNSIn light of the foregoing considerations, a carefulstudy of the high-voltage cable terminal problem wasmade,which resulted in certain definite design features,including oil filling. To illustrate the principal features of these oil-filled terminals, four sizes will bedescribed; (a)37,000-volt,three-conductor, corresponding to the highest rated satisfactory three-conductor

cable yet produced, viz., 33,000volts; b)50,000-voltsingle-conductor, applicable to the lowest rated singlecables above the present three-conductor range; (c).73,000-volt, single-conductor, corresponding to thehighest rated single cable in practical operation inthiscountry;and (d) 110,000-volt,single-conductor, as nowbeing used in the highest voltage experimental cableinstallation thus far undertaken. Terminals of this

FIG. 137,000VOLT, THREE-CONDUCTOR, OIL-FILLED TERMINALFOR LEAD-COVERED CABLE

last rating were used also in the elaborateseriesoftestsconducted by the Electrical Testing Laboratories ofNew York at the Pittsfield Worksof the General ElectricCompanyduring the summer of1924,when severalmanufacturerscontributed samples of their best effortstoward132,000-voltcable.37,000-voltthree-conductor terminal. This design isillustrated in Fig. 1. The three insulators are in thesameplane, an arrangement which makes the tank verymuch larger than a triangular arrangement, and thespreadof the insulators twice as great; but it lendsitselfreadily to wall or pole mounting. Sufficient space in

the tank has been allowed for transposition of conductors for phasing, without intermediate link connectors. No side opening in the tank is necessary, as theinsulators and cover areremovable. The insulators areof wet-process porcelain formed in one piece, withsmoothly ground ends, against which bakelized corkgaskets are compressed by means of the metal clamping rings cemented around the ends of the porcelain.

FIG. 2DRAWING SHOWING CONSTRUCTION OF 37 , 000 -VOLT, THREE-CONDUCTOR OIL-FILLED TERMINAL FOR LEAD-COVERED CABLE

The glass cylinder mounted above each insulator servesas a sight glass registering the level of the oil with whichthe terminal is filled. An auxiliary reservoir should beprovided in connection with a tank ofthissize, to carefor the expansion of the large volume ofoil,in thiscase about 22 gallons. The lower end of the iron tankis flanged and bolted to the brass wiping sleeve. Thejoints here and from tank to cover, as well as at theends of the porcelain insulators and glass gages, aremade oil-tight with treated cork gaskets. Filling anddraining is accomplished through a pipe connectiontothewiping sleeve.

The internal construction is shown in Fig. 2. Thecable sheath terminates just within the wiping sleeve.A thin copper band slipped under the end of the leadprotects the paper from sharp edges and damage insoldering. The belt insulation is removed in steps,and the three conductors separated and spaced by aporcelain block. A reenforcement of insulating tapeis built up around the outside of the three conductorsto give predetermined shape and dimensions, so chosenas to secure a safe distribution of both radial and lateralstresses. Upon the lower portion ofthisreenforcementis wound an overlay of metal tape, soldered at the lowerend to the cable sheath. The upper end ofthismetaloverlay approaches close to the inner surface of thewiping sleeve, which then recedes from the reenforcement and the conductor insulation to meet the lowerflangeof the tank. The shape ofthesemetal surfacesvitally influences the potential stresses radially andlaterally, and largely controls the circumferentialstressaround the conductor insulation, which seems tobe a large factor in so-calledcrotchfailures.


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596Above the reenforcement, the conductors pass ingentle curves to porcelain tubes mounted in a treatedwoodsupport, and thence in straightlinesto the terminal studs soldered to their bared upper ends. Fairlyclose fitting paper tubes preserve the concentric alignment of the conductors within the grounded metalshields in the cover. These shields present a smooth,uniformand definite surface to the electric field fromthe conductors, and also serve to improve the potentialdistribution on the external surface of the porcelainshells.The terminal stud at the end of each conductor islocked in place by a double-threaded nut in the recesseddepression of the top washer. A terminal cap engageswith the stud andsealsthe top against moisture by anenclosed cork gasket. External connection is madethrough attachment to the threaded stud extending

FIG. 3FLASHOVER TEST AT 17 0, 00 0 VOLTS FROM CAP -TO-CAP ON 37 ,0 00 -VOLTS, THREE-CONDUCTOR OIL-FILLED TERMINAL FORLEAD-COVERED CABLE

fromthe top of the terminal cap. A drain cock in thetop washer above each gage glass permits the escapeofair duringfillingof the terminal with oil.Samplesofthisterminal have been tested with three-phase potential to flashover at170,000volts, from capto cap, as shown in Fig. 3. Theflashovervoltage fromcap to case is about 150,000 volts. Time testshavebeen made at76,000volts for seven hours, followed by100,000volts for four hours. Subsequent examinationfailed to disclose any signs of deterioration.Whilesuch terminals for three-conductor cables areperfectly practicable from the standpoint of design, theyhave some disadvantages in installation, cost, andmaintenance, that encourage the use of single-conductor terminals on short single cables spliced to thethree-conductor cable, a short distance from the end ofthe line. This practise is likely to displace the former

Journal . I. . E.forsound technical as well as physical reasons. Withalength of single cable at the end of the line, extrainsulation may be provided against the higherstressesin a connected overhead line, or, due to proximity toreflection points in connected apparatus. Single termi-

FIG. 450,000-VOLT SINGLE-CONDUCTOR OIL-FILLED CABLETERMINALS WITH TEST PIECE OF2 0 / 3 2 IN. PAP ER INSULATEDLEAD-COVERED CAB LE. RIGHT HAND TERMINAL DISMANTLEDTO SHOW CONSTRUCTION

nals which can be mounted directly underneath overheadlinesof greater spacing than the three-conductorterminalwould have, are much smaller and lighter inweight and hence easier to handle, and require muchless oil for filling with consequently less expansion

FIG. 5 D RAW ING SHOWING CONSTRUCTION OF 50 ,0 00 VOLTSINGLE-CONDUCTOR OIL-FILLED TERMINAL FORLEAD-COVEREDCABLE

capacity in the oil reservoir. The glass gage at the topofthe single type of terminal is sufficient for the expansion of the oil in the terminalitself Single-conductorterminals for higher voltages are described in the following paragraphs. Designs for the lower voltages where

EBY: OILFILLED TERM INALS FORHIGH VOLTAGE CABLES


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June1925 EBY: OILFILLED TERMINALS FO R HIGH VOLTAGE CABLES 597three-conductor cables are used, as at22,000volts and33,000 volts, have been worked out along the samelines.

50,000-VOLT SINGLE TERMINALThis design is illustrated in Fig. 4. In this casethe wiping sleeve and support are in one piece, thelatter taking the form of a square flange for bolting tohorizontal brackets. The one-piece porcelain shell iscemented into the clamping ring at its lower end, andinto the recessed cap at its upper end, with corkgaskets in the joints. The cap shown in the illustrations was designed for petrolatum-filled terminals;

for oil-filled terminals a glass gage would be used togive a constantly visible indication of the oil level.The connection details at the top are similar to thosedescribed for the37-kv.terminal.The internal construction is shown in Fig. 5. Theread sheath terminates, as before, just within the wiping

sleeve, and the copper band is inserted under the edge ofthe lead. A reenforcement of insulating tape is applied directly upon the cable insulation, and the lowerend ofthisreenforcement is overlaid with a metal tapeup to its greatest diameter. The reenforcement pro-

drical ducts, thus increasing the insulating value of theoil, and directing its circulation.Samples of thisterminal assembled with500,000cir.mils20/32in. paper cable1have received repeated flash-overtests at an average of 190,000volts, as shown in

Fig. 6, and have subsequently withstood, in consecutiveorder, the combination of all the several cable tests

FIG. 6FLASHOVER TEST AT 190 ,00 0 VOLTS ON A 5 0 , 0 0 0 -VOLT SINGLE-CONDUCTOR OIL-FILLED TERMINAL FOR LEAD-COVERED CABLE

jects above the supportflangesand within the groundedmetalshield, with dimensions so chosen as to keep theradial and lateralstresseswithin safe values. A papercylinder surrounding the conductor insulation, andspaced from it by means of narrow strips of press-boardkeeps the conductor straight and concentric withinthe ground shield, and divides the oil space into cylin-

FIG. 7 Tw o 73 ,0 00 -VOLT SINGLE-CONDUCTOR OIL-FILLEDTERMINALS FOR LEAD-C OVERED CAB LE, ASSEMBLED W ITHTEST PIECE OF66,0 00 VOLT, 30 /3 2 IN.PAP ER INSULATED CABLE

for 40,000-volt cable taken from the proposed Edisonspecifications, as follows: 114,000 volt for 5 min. (breakdown test)71,000 volt for 15 min. (full reel test)57,000volt for 8 hours (high voltage time test)No disturbance of any kind developed during thesetests, and no deterioration could be observed upon laterexamination.73,000-VOLT SINGLE TERMINAL

In Fig. 7 there is shown the terminal developed foruse with 66,000-volt cable. In external appearanceand construction it closely resembles the -50,000-voltterminal just described. Internally, the only prominent difference is in the insulating of the metal groundshield by embedding its upper end in varnished cambricsupported directly upon the paper cylinder. Perforations through the flange of the ground shield permitdownward flow of the circulating oil in the duct outside of the cylinder. These details and the generalconstruction are illustrated in Fig. 8.A typical flashovertest onthis terminal at 290,000volts is shown in Fig. 9. Timetestson sample terminals have been made at200,000volts for several hours.These terminals have also been used with great satisfaction in making time tests on samples of 30/32 in.1. Cable of this size and insulation is being installed atColumbus,O.,foroperationat 4 0 0 0 0volt,three-phase.


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598 EBY: OILFILLED TERM INALS FORHIGH VOLTAGE CABLES Journal.I. . E.papercables at200,000volts and in one instance, atestof 225,000volts for 36 hours was made on such a cablewith no terminal trouble.

V/EWINDIRECT/ON fiRROWFIG. 8DRAWING SHOWING CONSTRUCTION OF A 7 3 , 0 0 0 -

VOLT SINGLE-CONDUCTOR OIL-FILLED TERMINAL FOR LEAD-COVERED CABLE

110,000-VOLT SINGLE TERMINALThe terminals shown in Figs. 10, 11 and 12 were

developed first for testing of cables having 30/32

in assembly, the wiping sleeve is separate from thehorn-shaped support casting, with a cork gasket between bolted flanges to form an oil-tight joint. A reenforcement of insulating tape encircles the cableinsulation from the termination of the lead sheath to a

FI G. 1 0 , - VOLT SIN- FIG. 11110 ,000-VOLT SINGLE-CONDUCTOR OIL-FILLED GLE-CONDUC TOR OIL-FILLEDTERMINAL FOR LEAD-COV ERE D TERMINAL FOR LEAD-COVE REDCABLE CAB LE, SHOWING TERMINAL

REMOVED FROM CAB LE, E X POSING RE-ENFORCEMENT OFCABLE INSULATION

point opposite the top of the ground shield. Radialand lateralstressesare controlled as before bythisreenforcement, together with the configuration of theenclosing metal. A bare ground shield is possible by

FIG. 9FLASHOVER TEST AT 29 0, 00 0 VOLTS ON A 73 , 000VOLT SINGLE-CONDUCTOR OIL-FILLED TERMINAL FOR LEAD-COVERED CABLE

in. paper insulation, to determine how near anapproach had been made to a safe and reliable132,000-volt cable. For economy and convenience

FIG. 1 2 T w o , -VOLT SINGLE-CONDUCTOR OIL-FILLEDTERMINALS FOR LEAD-COVERE D C ABLE, ASSEMBLED WITHTW O PIECES OF 3 0 / 3 2 IN . PAPER-INSULATED CABLE SPLICEDWITH AN OIL-FILLED JOINTreason of its diameter, and two paper cylinders breakupthe oil space into vertical ducts. In addition to theground-shield, a terminal-shield is provided inside theupperend of the porcelain shell toimprovethepotentialdistribution along the outside surface of the porcelain.


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June1925The porcelain shell and its two clamping rings, andthe glass oil-gage and the two adjacent castings areborrowed without change from the standard interchangeable oil-filled bushing for transformers, oilcircuit breakers, and lightning arresters, and illustrate

again the value, and convenience of standardized

FIG. 13DRAWING SHOWING FIG. 14 FLASHOVER TE STCONSTRUCTION OF A 110 ,00 0- AT400,00 0-VOLT ONA 110 ,000 -VOLT SINGLE-CONDUCTOR OIL- VOLT SINGLE-CONDU CTOROIL-FILLED TERMINAL FORLEAD - FILLED TERMINAL FORLEAD-COVERED CABLE COVERED CABLE

material. The details are shown in Fig. 13. Theconstruction is such that, with the exception of the topconnector and the wiping sleeve, this terminal may beassembled complete before installing over the preparedend of the cable. This is of great advantage in a terminal of this size and weight, which must be handledwith a hoisting tackle of some kind. During installation the long stud, into which the end of the cable issoldered, is passed through the tube in the top of theterminal arid secured at the proper elevation by a locknut above the cover casting. The wiping sleeve is thenraised into position, bolted to the support casting, andwiped to the lead sheath.In thetestson the proposed132-kv.cable, for whichthis terminal was first developed, it successfully withstood the breakdowntestsup to its flashover voltage of350,000volts. These breakdowntestswere made, asusual, by starting at some predetermined voltage, andincreasing in 10 per cent steps at short intervals.Momentary flashover voltages of 400,000 volts, asshown on Fig. 14, have since been measured on thesesameterminals, when raising the voltage steadily up totheflashoverpoint. Timetestsof six hours at275,000,and eight hours at240,000volts were obtained on somesamples of cable, the terminals functioning with complete satisfaction. With sufficiently good cable, it is

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probable that time tests as high as 300,000 volts foreight hours could be made without trouble developingin these terminals.It is of very real practical interest, also, that an experimental or trial installation of similar cable equippedwith these terminals is receiving a service test on the110,000-volt system of the Adirondack Power andLight Company, atAlbany, . Y. Fig. 15 shows theinstallation. This piece of cable is lying exposed on theground with the ends extending into the terminals,which are mounted on a frame work several feet high.An auxiliary reservoir of oil maintains the oil in theterminals at the proper level, and supplies whateverabsorption into the cable there may be. No current iscarried by this cable, which fact, coupled with itsexposure above ground, imposes a much more severetemperature variation than would be the case with aloaded cable buried in the ground. The test has beenrunning now for four months (to Feb. 1,1925),and theterminals have given no trouble whatever.

TERMINALS FOR TESTING PURPOSESIt may not be inappropriate to say a word aboutthe use of such terminals, as are here described, fortemporaryor testing purposes. Whileat low voltagesit is usually sufficient to immerse the ends of the cable in

FIG. 15 EXPERIM ENTAL 110, 00 0 VOLTS CABLE UNDERGOINGFIELD TEST AT NORTH ALBANY STATION OF ADIRONDACKPOWER AND LIGHT CORPORATION, EQUIPPED WITH 110,000VOLTS SINGLE-CONDU CTOR OIL-FILLED TERMINALSoil orcompound,either in a tank or by means of cones ofmetal or paper surrounding the ends of the cable, yet athigher voltages, such as more than200,000volts, suchtemporary methods often become both inconvenientand unsatisfactory in results. The oil-filled type ofporcelain terminal, as described above, lends itself mostadmirablyto testing purposes,2as well as to permanent

2 . See also paper on Testing High-Tension ImpregnatedPaper-Insulated, Lead-Covered Cable by Everett S. Lee, pre*sented at the MidwinterConvention . I. . E., New York,Feb. 9 -12 , 1925 .

EBY: OIL FILLED TERMINALS FOR HIGH VOLTAGE CABLES


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600 EBY: OIL FILLED TE RM INALS FOR HIGH VOLTAGE CABLES ourn l. I. . E.service installations. The preparation of the cableends for assembly with the terminal is a simple andrapid process, the terminal parts are easily andaccurately adjusted to the cable, the oil filling can bedone without delay, and thetestingcan proceed almostas soon as the filling is completed. Withequal facility,the oil can be drained, the terminal removed, and a newpiece of cable prepared for test with the same set ofterminals. Not only does this method speed up thework, but it makes necessary only a small amount ofequipment, avoids the use of large tanks of oil, andsaves the loss of much temporary material wasted insome methods of testing. Of even far greater importance are the uniformity and reliability of results obtained, which factors are well nigh indispensible to thevalue of thetest data on the cables themselves.

CARRIER CURRENT TRANSMISSIONFOR UNDERGROUNDCOMMUNICATIONA report has recently been issued by J. J. Jakoskyand D. H. Zellers, both of the Bureau ofMines,on theeffect of Metallic Conductors on Underground Communication. This report contains (1) a discussion ofthe chief factors affecting carrier-current transmissionwith particular reference in application to undergroundcommunication for mines; (2) a description of typicalunderground circuits used for power and lighting; and(3) description of results obtained in carrier-currentexperiments in coal mines near Pittsburgh, and conclusions as to power requirements and limitations formine use.

FACTORS GOVERNING CARRIER-CURRENTTRANSMISSION

Indesigning apparatus for carrier-current telephonyfor underground purposes, the general behavior andcharacteristics of mine circuits as high-frequency channelsmust be considered. A typical mine power systemusually consists of combined aerial and cable circuits;on the surface the aeriallinesare, as a rule, well insulated and with effective spacing between wires; underground,the cables may be run in conduit, twisted pair,or duplex, depending upon type of service, or spacedsix or more inches, depending on the prevailing conditions. The leakage of undergroundlinesis usually highas compared to surface lines, on account of moisture andthe fact that wires are run within a few inches of, oroften touching, the wet mine walls, or are incased inmetal conduits.In practically all direct-current mine installations,large copper conductors are used, in order to decreasethe voltage drop in the distributing system. Suchlarge low-resistance conductors, with an absence oftransformers, etc., provide almost ideal conditions forcarrier-current operation. The distances to be worked,usually not greater than5000to10,000feet, allow underfavorable conditions comparatively, low powers to beused for transmitting and very simple equipment forreceiving.

Operating carrier-current over alternating-currentmine systems offers considerably greater difficulties.The alternating-current systems, as a rule, use highervoltages and smaller conductors from the power houseand surface lines into the mine; and transformers orsynchronous converters are used for obtaining propervoltages at distribution points. Whena carrier-currentset operates over conductors containing transformersand equipment of similar electric characteristics, suitable by-pass and shunt condensers should be used, tocarrythe high-frequency current past the transformer,etc.GENERAL CONCLUSIONS1. A carrier-current apparatus to be of practicalmining use must be so designed that it will operateefficiently over conductors of widely varying electricalcharacteristics.2. Short-wave carrier-current apparatus is toocritical in adjustment and operation for practical mineuse. Every change in the metallic circuit, such asbreaks in the line, grounds, etc., necessitatesreadjustment of the apparatus if efficient operation is to beobtained over distances of a few thousand feet ormore.3. The long-wave apparatus, while not as criticalin operation as the short-wave, still requires moreadjusting incasesof changes in the circuit than is nowbelieved practical for underground mine use, especiallyincasesof disaster or when emergency communicationmust be established.

4. Studies and investigations of different mine accidents after explosions and fires show that overheadwiring, such as trolley wires, and power, lighting, andtelephone circuits, are usually broken or destroyed toan extent to render unreliable their use in casesofdiaster. In times of emergency and during mine-rescueoperations, whatever means of communication is employed must be not only reliable but portable, simple,andquick to operate. In such cases, the use of an apparatus that requires tuning or adjusting for satisfactoryoperation over broken conductors whose electricalconstants have been changed by such disasters mightbe of doubtful benefit to the rescue crew, although itmight be of some value to the entombed men who, bytheir enforced waiting and idleness, can tune or adjusta setifthey have one. Morethan the simplest typeofadjustment, however, is out of the question becauseofthe lack of training of the average miner, absence oflight, and adverse physiological and psychologicalconditions.

5. Practically the only metallic conductors in minesthat are not destroyed at one or more points by a disaster are buried conductors (power and lighting circuitsin underground circuit), water and compressed air piping, and track returns. Such conduits are poor carriersforhigh-frequency currents. The low-frequency T. P.S. and voice apparatus carries much more efficientlyover such conductors.

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Oil Filled Cables Terminals