formers Mr. Lee, for example, feels thatcycling once a day should be sufficient andthat high-potential and induce tests afterheat aging and after humidity exposure areunnecessary. Both of these are a functionof the eventual application of the trans-former and of what a designer chooses tocall the failure point. We see no objectionto cycling once a day if this is more repre-sentative of the application nor do we insiston a failure point at a dielectric stress levelhigher than that required for normal oper-ation as long as the data are properly docu-mented.

Similarly, referring to oil-filled electronictransformers, Mr. Fenoglio suggests omit-ting humidity exposure altogether and ad-justing the cycling schedule to accommodatelarger thermal capacities. These are verylogical innovations which fit into the phi-losophy of a test code designed for eachtransformer type and for particular servicerequirements within each type. In addi-tion to these differences another has beennoted in correspondence with electronicequipment designers. They have observedthe lack of provision in the proposed testcode in the paper for a cold soak at -65 Cprior to the beginning of the weekly heataging. This observation is well foundedsince some of their equipment must passcertain specifications of which the cold soakphase is a part. If the transformer is des-tined for this equipment, then definitely theexposure at -65 C should be included toproperly evaluate the behavior of the insu-lation system.Mr. Fenoglio's discussion in particular

should serve as a basis for the formulationof a test code for small oil-filled transformers

used in electronic equipment.Mr. Garbarino has expressed the opinion

that if the basic characteristics of an insu-lation system could be independent of sizeor type of equipment, then a less restrictedtest code would allow for more commonground for exchange of information amongdesigners. This statement would be true ifhe had used the word material instead ofsystem. Unfortunately, as Dr. Narbut'sdiscussion implies, there are many combi-nations of materials that form systems andjust as many applications for which thesesystems must be designed. This is thereason that the AIEE Power Division Com-mittees are finding it necessary to writefunctional test codes to classify insulationsystems properly. This, of course, is inaddition to the classification of materialsestablished by the AIEE Basic SciencesCommittee.Mr. Garbarino has also expressed an ob-

jection to the evaluation of the over-all con-struction rather than the insulation system.It is not only true that the over-all construc-tion is evaluated but it is also necessary.Variations in manufacturing technique mustbe taken into account, for example, but, asthe paper points out, it is very importantto find out where in the system failure occursto interpret the data properly. Mr. Gar-barino further states that the dielectricstresses in a transformer are well belowaverage for the material and the mechanicalstresses they are subjected to are unknown.The operating dielectric stresses are a frac-tion of what the insulation can stand ini-tially. However, the designer must makeprovision for transient stresses and for thedeterioration of dielectric strength as the

insulation is aged. It is true that the mag-nitude of the mechanical stresses are un-known although in many applications thesestresses are known to be present. There-fore, their presence must be simulated bywhat is considered to be a representativevalue in a test code.Mr. Lee suggests that the relative humid-

ity and temperature should be more closelydefined. The authors agree with this sug-gestion and with the 93% relative-humidityvalue. The 65-C temperature value, how-ever, seems high. A 35-C42-C valuewould seem adequate. Mr. Lee feels that a10,000-hour life for class-A insulation at100 C is inadequate. This is an instancementioned previously of what is selected asthe failure point of the insulation. For thedata in Fig. 1 the test dielectric stress wasdetermined by the relation given in step 4of the proposed test code. When the insu-lation failed to pass this value, it was con-sidered to have failed-not when it would nolonger operate at normal voltage and loadas Mr. Lee probably interpreted the curve.In addition, to simulate the conditions in abroadcast transmitter or other specializedapplication the test cycle could be madeless severe, thereby increasing the test life ofthe transformers.We believe that the test code as proposed

in the paper with modifications enumeratedin the closure is basically sound. Beingsufficiently flexible, it should closely simu-late and realistically evaluate in the labora-tory the service conditions that most insu-lation systems in electronic transformers areexposed to. It should therefore form thebasis for the writing of a test code for smalldry-type transformers.

Crepe Papers and Crepe-Paper Cables

G. CAMILLIFELLOW AIEE

L. MULLIGANNONMEMBER AIEE

Synopsis: Varnished cambric tape hasbeen used as electric insulation very exten-sively in the past because of its high dielec-tric strength, stretchability to fit irregularsurfaces, and relatively high resistance tohumidity. However, this material is rela-tively expensive, and seriously contaminatesthe Askarels.Two new high-density crepe papers have

now been developed to replace the varnishedcambric. For the same thickness andwidth, the tear strength of one of these newcrepe papers is equal to that of the varnishedcambric, while its lengthwise tensile strengthis almost three times as high. The dielec-tric strength of this crepe paper at both 60cycles and impulse tests compares veryfavorably with the varnished cambric.The second of these new crepe papers is

characterized with 2-way stretch and is suit-able for producing cables with great flexi-bility. The electrical, physical, and chemi-cal characteristics of this new paper are alsocomparable to those of varnished cambric.Cables insulated with this new crepe papercan be advantageously used for both oil andAskarel transformers.

E. L. CRANDALLASSOCIATE MEMBER AIEE

IN THE manufacture of transformers aconsiderable amount of varnished

cloth has been used for the taping of ir-regular surfaces such as joints, terminals,etc., and an even greater amount in themaking of flexible cables used in liquid-filled transformers. As varnished cam-bric is rather expensive, and deleteriousto the Askarels, two new kinds of crepepaper have been developed equal to thevarnished cambric in electrical and phys-ical characteristics, harmless to theAskarels, and far more economical.Although crepe-paper insulation has

been used in the electrical industry formany years,1-' until recently it did notpossess the tensile strength of varnishedcloth or the electrical characteristic ofthe plain kraft paper. Nevertheless, thematerial played an important role in thedesign and insulation of high-voltageequipment. It is worth noting, for in-

stance, that the large reduction in thesize and weight of high-voltage currentand potential transformers brought aboutin recent vears was due to a very largeextent to the use of crepe paper. It wasrealized, however, that if crepe paper wasto replace varnished cambric in trans-formers, and have more diversified use,some of its characteristics had to be im-proved. This has now been largely ac-complished.

Crepe Paper as a Substitute forOther Taping Materials

An analysis of the old industrial crepepaper, which included physical and elec-

Paper 54-78, recommended by the AIEE Trans-formers and Insulated Conductors Committees andapproved by the AIEE Committee on TechnicalOperations for presentation at the AIEE WinterGeneral Meeting, New York, N. Y., January 18-22,1954. Manuscript submitted September 14, 1953;made available for printing November 27, 1933.G. CAMILLI, L. MULLIGAN, and E. L. CRANDALL arewith the General Electric Company, Pittsfield,Mass.

The authors wish to acknowledge the contributionsof Dr. C. W. Stillwell of the Dennison Manufactur-ing Company in the development of the crepe pa-pers, and the assistance of C. H. Matthews, M.Guarnier, F. J. Turner, and J. E. McDonough, allof the General Electric Company, in carrying outthe developmental testing.

Camilli, Mulligan, Crandall-Crepe Papers and Crepe-Paper Cables610 JUNE 195-4

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Fig. 1. Photomicrograph (100 times) of a sec- Fig. 2. Photomicrograph (200 times) of a sec-

tion of commercial-grade crepe paper showing tion of an improved crepe paper described in

ruptures of the fibers made during the creping the paperprocess

trical tests, microscopic examination,and a study of the creping manufacturingmethod, uncovered the presence of in-ternal ruptures produced during thecreping process. Fig. 1 shows a crosssection of one of the old crepe papers;the clearly visible internal ruptures areresponsible for the paper's relativelypoor physical characteristics. Thesepapers were formed on the continuouswire screen of a Fourdrinier-type papermachine and it was apparent that thepulp-refining methods used to make thebase paper were unsuitable for obtainingtensile strength and fiber cohesion neces-sary to withstand the creping process.In addition, the density of the base paperswas not great enough to give good elec-trical characteristics in the finished prod-uct. The improved papers are muchdenser and in some cases consist of a mix-ture of 70-per-cent kraft (wood pulp)and 30-per-cent manila fibers. Manilafibers, from 0.125 to 0.5 inch in length,are almost invariably obtained from oldcordage which has been disintegrated byboiling in caustic soda. The wood pulpis first chipped and then chemicallytreated to dissolve certain constituentswhich are undesirable in the paper. Thefibers of the wood pulp range in lengthfrom 0.1 to 0.3 inch. The addition ofvery strong, highly flexible manila fibersto the pulp mixture gives the base paperthe necessary strength to withstand thecreping process. In addition, the newbase papers are made on a cylinder papermachine which characteristically producesa paper which has the highest tensilestrength in the lengthwise direction andthe highest tearing strength and elonga-tion in the crosswise direction. Increaseddensity has been obtained by selectedpulp-refining methods.That the new paper withstands the

creping process without excessive internalcorruption may be appreciated by com-paring Fig. 2 with Fig. 1. The physicaland electrical properties of this new crepepaper are compared with those of bias-cut varnished cambric in Table I.

Dielectric tests in uniform field weremade using the electrodes shown in Fig. 3.Those in nonuniform field were madewith the electrodes shown in Fig. 4.The knife edge of one of the electrodesshould be noted.

In the application of crepe paper as asubstitute for varnished cambric thequestion arises whether or not the dielec-tric strength is a function of the amountof stretch removed from the paper. Fig.5 shows that for this type of material thedielectric strength is highest when thethickness of the tape (by tension) is re-duced to approximately 60 per cent of theunstretched tape. The value quoted, of

course, may be different for differenttypes of material and should be deter-mined for the particular crepe paper underinvestigation. The effect of tension onthe thickness of a typical crepe paper isshown in Fig. 6.

Crepe-Paper Cables

In the past, cables for use in power anddistribution transformers have been in-sulated either with varnished cambric ormanila paper. Because varnished cam-bric contaminates the Askarels the use ofvarnished cambric cables has been lim-ited to oil-filled transformers. The useof manila (uncreped)-insulated cableshas not been extended to high-voltagepower transformers because they lack theflexibility and over-all electrical propertiesof the varnished-cambric cables. Askarelpower transformers are used in relativelylow-voltage systems where the requiredcable insulation is relatively thin and thestiffness of low-voltage manila cables hasbeen tolerated without serious disadvan-tages.

It should be noted that cables used forthe internal connections of transformersshould be very flexible. In many casesthese cables are bent to a radius equal totwice their insulated diameter. In high-voltage power transformers, where the useof cables with heavy wall thicknesses is a

necessity, flexibility is an important prob-lem. Until recently, flexibility was ob-tained by the use of varnished-cambriccables at a sacrifice of other characteris-tics. It is well-known, for instance, thatoil-impregnated kraft paper has superiorelectrical characteristics over manilapaper and varnished cambric, and that

Table 1. Comparative Data Between Biased Varnished Cambric and Crepe Paper for Taping

BiasedCrepe Varnished

Characteristics Paper Cambric

Original thickness of base material, inches ................................ 0.005 ...... 0.012Available elongation, per cent* ......................................... 100 12Thickness of crepe paper before elongation is removed, inches ........ 0.025.Apparent density, grams per cubic centimeter (elongation removed) ......... 1.00 ....... 17Pounds required to remove:80 per cent of available elongation .................................... 27 3890 per cent of available elongation .................................... 35

No. of 1/2 lap wraps per 100-mil thickness ............................... 7 6Coverage of square yards per poundt .................................... 3.85 ....... 3.28Approximate relative cost per pound, per cent ............................ 40 100Average tensile strength, pounds per inch width per mil ..... .............. 11.6 .... 4Tear strength cross machine direction grams force per milt ................. 25 21.560-cycle breakdown, volts per milUniform field ....................................................... 508 ....... 400Nonuniform field ................................................... 425 400

Impulse breakdown, volts per mil§Uniform field ....................................................... 1,100 1,360Nonuniform field (positive polarity) ................................... 1,200 ....... 970

(negative polarity) .................................. 1,180 ....... 930

* Crepe paper can be made with any elongation from 10 to 300 per cent.

t Assuming 80 per cent of available stretch removed during taping.$ In determining tue physical properties of crepe paper, the thickness refers to the base paper.§ All electrical properties apply to samples vacuum oil treated and tested under oil and having an insu-lation wall thickness of 200 mils.

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200OUTER ELEGTROI(SPUN COPPER)

Di- ~~~REPE PAPER TAF

(MATERIAL BEING T

INNER ELECTRODE

SECTION A-A

oil-immersed paper-insulated cables maybe operated at slightly higher tempera-tures than varnished cambric. Thislatter means that for the same operatingtemperatures paper-insulated cablesshould have better aging characteristicsthan cables insulated with varnishedcambric. Kraft-paper insulated cable,of course, can be used in both oil andAskarel transformers. This problem offlexibility has been solved recently by theuse of a special 2-way crepe paper for thedielectric of the cable.A study reveals that great flexibility

would be obtained if the conductor were

insulated in such a manner that, in thefinished cable, the corrugations of thecrepe paper would be perpendicular to

CREPE PAPER FOR TAPING

UNIFORM FIELD1/2 LAP - 14 WRAPS

cx 100

=) 50

20 40 60 80 100

THICKNESS IN % OF THE UNSTRETCHED PAPER

Fig. 5. Effect of stretch on the dielectricstrength of crepe paper. Test with the crepe

paper between two rounded electrodes

Fig. 4. Tape-testing elec-trodes, nonuniform field.Note knife edge of the inner

electrode

the axis of the cable. It can be seen thatin this case, when the cable is bent, thecrepe paper will yield with an accordion-like effect.

These ideal conditions could be metby using a creped tape in which the cor-

rugations make an angle with the axis ofthe tape approximately equal to the anglewhich the axis of the tape makes withthe perpendicular to the axis of thecable.

In applying an insulating paper tapeabout a conductor, the tape is woundspirally around the conductor and makesan angle with respect to the axis of thecable designated as the angle of laySimultaneously the cable is advancedaxially at a constant speed. While thepitch of the tape remains constant theangle of lay changes because of the in-creased circumference of the insulatedcable. Thus, the ideal conditions cannotbe met for all layers of the insulation.A solution to the problem of obtaining

maximum cable flexibility is obtained bythe use of 2-way-stretch crepe paper.

Regardless of what the angle of lay maybe for any particular layer of the insula-tion, the 2-way-stretch crepe paper can

supply the necessary components ofelongation when the cable is bent on a

CD 30-j

z 20

z 0

0

TENSION IN POUNDS PER I" WIDE TAPE

Fig. 6. Effect of tension on the thickness ofcrepe paper used for taping

short radius. A 2-way-stretch crepepaper of the type shown in Fig. 7, havingexcellent electrical quality, is now availa-ble and has been used in the manufactureof cables for use in power transformers.

Cable Manufacture

For this application the base paper usedfor the tape has a thickness of approxi-mately 0.004 inch and, after creping athickness of approximately 0.018 with astretch of approximately 50 per cent inthe lengthwise direction of the tape andapproximately 20 per cent in the cross-wise direction. The lengthwise elonga-tion is so set (with creping compound)that only one-half the available elonga-tion is removed at 5-pound tension perinch width.

Slitting the master rolls of crepe-paperinsulation into the required tape widthin pad form requires good tension controlon the supply roll and pickup pads.This is required to prevent excessivestretching of the crepe paper.

Application of the crepe-paper tape tothe cable also requires exceptionally goodtension control and a precision type oftape-insulating machine, the same asused for high-voltage paper-insulatedpower cables. After insulating, the usualsingle cotton braid is applied over-all.To prevent contamination of the under-

Fig. 7. A cable insulated with 2-way-stretchcrepe paper can be easily bent to small radii ofcurvature, because during the bending bothcomponents of elongations (A and B) are

supplied by the crepe paper

Camilli, Mulligan, Crandall-Crepe Papers and Crepe- Paper Cables

Fig. 3. Tape-testing electrodes, uniform field

612 JUNE 1954

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Table 11. Comparison Between VarnishedCambric and Crepe-Paper Insulated Cables-450/24 American Wire Gauge Conductorwith 0.375-Inch Insulation Wall Thickness*

Varnished CrepeCkaracteristics Cambric Paper

60-cycle breakdown, kvLooped cables ................ 203. 196Crossed cables............... 203 .....216

Impulse breakdown (l'/2x4O-microsecond wave), kvLooped cables ................ 545 ..... 610Crossed cables................565 .....690

60 cycles per cent powerfactor at

25 degrees centigrade..........3.2 ..... 0.8100 degrees centigrade 9. 1..... 6.5

* All tests made with the samples vacuum oil-treated and tested under oil.

lying untreated crepe-paper tapes, and toprevent any delay during final drying andtreating operation, this braid is nottreated with varnish.

Electrical Tests

Comparative test results made onvarnished cambric and 2-way-stretchpaper-insulated cables are shown inTable II. The data apply to cables witha 450/24 American Wire Gauge conductor

with 0.375-inch insulation wall thickness.Two methods' of positioning the cablesduring the tests were used: one withstraight cables "crossed" at right anglesat their centers and the other with cables"looped" to form bights, the loops beingin the same plane and the contact beingat the looped portions. The radii of theloops were made approximately fourtimes the insulated diameter of the cable.Tests were made with samples under oilafter being vacuum-dried and vacuum-filled with deareated oil. Repeated bend-ing of these cables does not materiallyeffect the results obtained in Table II.

Exposing the cables for a short timeto an atmosphere of high humidity aftervacuum treatment seems to effect the 60-cycle strength of both varnished-cambricand crepe-paper insulated cables to thesame degree. The impulse strength ofboth types of cables does not seem to beadversely effected by high humidity asmuch as the 60-cycle strength.

Bending Characteristics of the NewCables

Repeated tests seem to indicate thatthese cables can be bent to a slightly less

degree than the varnished cambric cables.The minimum radius of curvature, whichcauses no breakage of the tapes at present,seems to be four times the outside diam-eter of the cable. Improvements in themanufacture of the 2-way crepe paperwill certainly improve this limitation.

Conclusions

Two kinds of crepe paper are nowavailable which are physically and elec-trically equal or superior to varnishedcambric. The 1-way-creped papers havebeen successfully used for general tape in-sulation, and the 2-way-creped papershave been used advantageously in themanufacture of flexible cables which areapplicable in both oil and Askarel-filledtransformers.

References1. NEw DEVELOPMENTS IN CURRENT-TRANS-FORMER D SIGN, G. Camilli. AIEE Transactions,vol. 59, 1940, pp. 835-42.

2. NEW DEVELOPMENTS IN POTENTIAL-TRANS.FORMER DESIGN, G. Camili. AIEE Transactions(Electrical Engineering), vol. 62, July 1943, pp.483-87.3. VACUUM-PLASTIC-FILLED INSULATED VOLTAGETRANSFORMERS, E. C. Wentz, F. B. Colby. Elec-trical Engineering, vol. 71, July 1952, p. 631.

DiscussionL. Meyerhoff and W. H. Cortelyou (GeneralCable Corporation, Bayonne, N. J.): Theauthors state that manila-insulated cableslack the flexibility and over-all electricalproperties of varnished cambric cables andalso state that the dielectric strength of 2-way-stretch crepe paper on both 60 cyclesand impulse tests compares favorably withvarnished cambric.

Tests performed in our laboratory haveshown that properly made manila insulatedcable has flexibility comparable to that ofsimilar cable insulated with 2-way-stretchcrepe paper, retains its shape much betterafter bending, and has consistently greater-in some cases 50 per cent greater-dielectricstrength. For our tests we used a 4/0American Wire Gauge rope-stranded con-ductor, insulated with 250 mils of the de-sired insulation. The insulation of two ofthe samples consisted of 2-way-stretch crepepaper, the tapes being applied with light

Table Ill. Torque in Pounds-Feet For VariousBends

Bend Radius Two-Way Two-Wayin Terms of Crepe, Crepe,

Cable Light Medium 5-MilDiameter Tensios; Tension Manila

1.5t...... 20 ........20. 19.5 ....... 28.52. 22.5.. 20.5. 243. 17.5. 19. 21.54 .. 15.5.5 15

tension in one sample and medium tensionin the other. A third sample was insulatedwith 5-mil manila tapes 1/2-inch wide.To compare flexibility, specimens were

bent over mandrels of various radii, from1.5 to 4 times the cable diameter. Thespecimens, clamped at one end to the man-drel, were fed through a loose-fitting guidetube tangent to the mandrel which wasturned at a constant angular rate of 1.1turns per minute. The torque requiredwas measured and is given in Table III.

After the bending force was released thecables in all cases sprang back somewhat,the crepe-paper samples springing backmuch farther than the manila sample. Theprocedure was to hold the samples in thebent position for 5 minutes and then to per-mit them to spring back for 5 minutes.Results are shown in Table IV.Although the manila paper cable requires

slightly greater bending forces for thesmaller diameter bends, the much smallerspring-back is a benefit which might easilyoutweigh the slight disadvantage of in-

Table IV. Spring-Back, Per-Cent Increasein Loop Diameter

Bend Radius Two-Way Two-Wayin Terms of Crepe, Crepe,

Cable Light Medium 5-MiuDiameter Tension Tension Manila

1.5. 29. 30. 92 . 36. 41. 203 . 45. 51. 204 ........... 58 ........ 60 . 14

creased stiffness. For a bending radius offour times the cable diameter, the bendingforce required is the same for all three cables,but the spring-back for the manila papercable is less than one-quarter that for thecrepe-paper samples.

Samples were also subjected to 1.5x40-microsecond impulse tests and to 60-cyclebreakdown tests. They were bent aboutmandrels of the desired radii and equippedwith flat-strip ground electrodes, which werebent to the required curvature and tiedagainst the outside surfaces of the bentsamples. The samples were then driedunder heat and vacuum, impregnated intransformer oil, and immersed in a containerof transformer oil for the tests. The typeof sample used is believed to approach moreclosely the actual conditions in a trans-former than do the types of samples de-scribed by the authors. Results for thetwo kinds of breakdown tests are given inTable V.

Table V. Impulse and 60-Cycle BreakdownTests

Bend Radius Two-Way Two-Wayin Terms of Crepe, Crepe,

Cable Light Medium 5-MilDiameter Tension Tension Manila

Impulse Breakdown, I.5x40-Microsecond Wave, Kv2. 230.. 245. 3553. 260.. 290. 345

Breakdown at 60 Cycles, Kv2. 80. 95. 1053. 95. 85. 115

Camilli, Mulligan, Crandall-Crepe Papers and Crepe-Paper Cables

. _~V~

613JurNE 1954

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It was observed that whereas the manilatapes showed no breaks or severe creasingin any case as a result of bending, the 2-way-stretch paper exhibited severe circumferen-tial indentation folds in the inner part ofthe loop in the case of the smaller radiusbends. This creasing resulted in reduced di-electric strength, as evidenced by the factthat the breakdown path in the crepe-papersamples started generally from the side ofthe conductor on the inside of the bend, andfollowed a helical path around to the groundstrip on the outside of the loop. In the caseof the manila samples the failure path wasgenerally radial on the outside of the loop.Although it is realized that comparisons

of different types of samples and perhapsdifferent test conditions are not fully justi-fied, it may nevertheless be of value to com-pare breakdown values reported by the au-thors with those obtained by us. From thedata in Table IV an impulse value of 815 to920 volts per mil may be calculated for theauthors' crepe-paper samples. This com-pares with 920 to 1,160 volts per mil onour crepe-paper samples and with 1,380 to1,420 volts per mil on our manila sample.The 60-cycle value calculated from TableII for their crepe-paper samples is 260 to290 volts per mil, as compared with valuesof 320 to 380 volts per mil on our crepe-paper samples and 420 to 460 volts per milon our manila sample.Apparently the authors were concerned

merely with producing a transformer leadwhich would have characteristics approach-ing those of a varnished-cloth-insulated leadand be capable of being used in Askarels aswell as in transformer oils. Our tests, onthe other hand, had for their purpose theproduction of a paper-insulated lead whichwould have the best obtainable electricstrength compatible with the mechanical re-quirements of transformer leads. Withmanila paper the electric strength, while notquite as high as can be obtained with woodpulp under ideal conditions, is materiallyhigher than with crepe paper.

It is well to remember that crepe papermust have inherently lower dielectricstrength than good flat paper because crepepaper, due to its folds, must produce lowerdensity insulation than flat paper, or in otherwords, must have a lower ratio of papersubstance to oil.

B. R. Hubbard (John A. Roebling's SonsCorporation, Trenton, N. J.): The authorsare to be congratulated on the presentationof a paper of timely interest to engineersinterested in oil-filled transformers. It isappreciated that the subject is too broad tocover all the aspects of it in a paper like this.There are, however, a few points that areworthy of further consideration.For instance, the tabulation of compara-

tive characteristics shown in Table I ap-pears to refer primarily to measurementsmade on dry crepe paper. In practice thepaper is submerged in transformer oil.I would be interested to know what effectthe immersion of the paper in transformeroil would have on tensile strength, tear re-sistance, and the force required to remove agiven percentage of the available elongation.

Since the primary function of the crepingis to improve flexibility, the ability of thepaper to retain its creping after it has beenstretched, for example, by bending the cable

around a small radius, would seem to be ofconsiderable importance. I can visualizeloose insulation on the conductor if thecrepe paper loses its elasticity when sub-jected to stretching or bending. Perhapsthe authors will say something about theamount of permanent set that takes placewhen the dry paper is stretched and alsowhen it is stretched after saturation withtransformer oil.

In a report' that dealt with the effects oftemperature on paper insulation in trans-former oil it was noted that some types ofpaper tended to absorb the high power-factor materials resulting from the oil de-terioration; also that, while this appearedto reduce the power factor of the oil, at thesame time it increased the power factor ofthe paper.The report showed also that while the

acidity of the oil itself increased only slightlyafter a prescribed aging, there was a markedincrease in oil acidity when certain combi-nations of paper and oil were subjected tothe same aging.The same report indicated that some kinds

of paper had a tendency to become brittleafter exposure to transformer oil at ele-vated temperatures. Both the kraft andrag papers appeared to resist heat muchbetter than manila paper.

Perhaps the authors will be able to say alittle about the aging characteristics and theeffects of elevated temperatures, such asmight occur during short overloads, on thecharacteristics of the crepe paper theydescribe.

REFERENCE1. DETERIORATION OF TRANSFORMER OIL ANDPAPER INSULATION BY TEMPERATURES, F. J. Vogel,C. C. Petersen, L. W. Matsch. AIEEE Trans-actions, vol. 70, pt. I, 1951, pp. 18-21.

J. H. Nicholas (G and W Electric SpecialtyCompany, Chicago, Ill.): The authors, intheir synopsis and conclusions, have maderather definite statements that a crepe-paper tape is now available which is superiorboth physically and electrically to present-day varnish cambric tape. I wonder whe-ther their unqualified statements were in-tended to cover only specific applications intransformers and not in the entire field ofinsulations.

I refer specifically to the footnotes inTables I and II which state that the sampleswere vacuum oil treated. By vacuum oiltreatment I assume that the test mandrelswere wrapped with the tapes, vacuum oilimpregnated, and then tested in oil. If thisis the case, then a word of caution is war-ranted to avoid the possible substitution ofcrepe paper for varnished-cambric tape in anapplication not favorable to its use.There are many applications of wrapped

insulation which are not subject to vacuumoil treatment and where the use of the oil-impregnated crepe paper tape might bedisastrous because of oil migration from thetape. Such would be the case in high-volt-age power cable joints and terminationsused on solid-type cables with asphaltic orvinsol resin compound filled casings.

I have made many electrical tests withvarious types of oil-impregnated crepe-paper tapes (impregnated with and withoutVistanex compound) using test mandrelssimilar to Fig. 3. Very erratic results were

obtained when the test mandrels of crepepaper were wrapped and merely immersedand tested in oil. The low-loss varnished-cambric tape mandrels were quite consis-tent and in general gave higher dielectricbreakdown values. On the other hand,vacuum treatment of the crepe-paper tapemandrel and impregnation with degasifiedoil raised the radial dielectric strength con-sistently above the varnished-cambric tapeeven when the varnished-cambric tape man-drels were similarly treated. This wouldindicate that the dielectric strength of crepepaper is highly dependent on oil impreg-nation and absence of voids.

I have had quite a few test experienceswhich lead me to believe that crepe-papertape, even when vacuum-treated and testedunder oil at 50 to 100 pounds per squareinch, does not exhibit the longitudinal oraxial dielectric strength of half-lapped, low-loss varnished-cambric tape when subjectedto high-voltage impulse stresses. I believethat this weakness can be attributed to thelongitudinal channels formed as the tape isapplied on cylindrical devices such as cablejoints or terminations.

I appreciate that the data presented areintended to be relative but it would be inter-esting to know what type of electrical testwas used to determine the 60-cycle dielectricstrength (short-time or long-time) andwhether the dielectric strengths are maxi-mum or average stresses.

B. P. Kang (Anaconda Wire and Cable Com-pany, Hastings-on-Hudson, N. Y.): Theauthors have ably pointed out the versa-tility and advantages of the use of crepepapers for various applications as an elec-tric insulator. Their excellent ability tocombine with practically all insulating com-pounds, their ease of application, and theirlow cost are well recognized. Their even-tually successful use in many fields of high-voltage insulation will be a major contri-bution of the authors.From the several discussions offered at the

meeting, it was clearly indicated that theelectrical properties such as dielectricstrength, power factor, and aging stabilitywere not up to their parent material, im-pregnated manila rope paper. However, inthe discussions no explanation was voicedfor the cause or causes of this degradatioll.

Fig. 8 (left). Electric stress on a single-conductor cable insulated with impregnated

plain paper

Fig. 9 (right). Electric stress on a single-conductor cable insulated with impregnated

crepe paperC-conductorP-impregnated pdperS-shieldE-electric stress (over-all)ER normal component of electric stressET-tangential component of electric stress

Camilli, Mulligan, Crandall-Crepe Papers and Crepe-Paper Cables614 JUNE 1954

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p

ER E

ETFig. 10. Enlarged vector diagram of electricstress on a single-conductor cable insulated

with impregnated crepe paperSee subcaption of Figs. 8 and 9 for meaning of

symbols

From some of my experiments and with alittle reasoning, two causes appear quitereasonable for this degradation. The aver-age value of dielectric constant of manilarope paper impregnated with a standardcable oil is about 3.25, while the crepe paperimpregnated with the same oil rarely has adielectric constant above 2.50. The dielec-tric constant of this oil alone is about 2.25.The first cause of degradation thus seemsdue to the increase in the oil/paper ratiowhich is shown by the decrease in dielectricconstant of the impregnated crepe paper incomparison with plain manila rope paperimpregnated under the same conditions.Since oil itself has a lower dielectric strengththan that of impregnated paper, the pres-ence of the excessive quantity of oil natu-rally tends to reduce the dielectric strength.The second cause is the introduction of tan-gential stress even in a single-conductor cableor a shielded three-conductor cable. Theconditions may be illustrated by Figs. 8-10.From Fig. 8 it can be seen that the electric

stress on a layer of impregnated paper insu-lation in a shielded cable is practically nor-mal to the paper surface and no appreciabletangential component of the stress is pres-ent. Fig. 9 represents a cable insulatedwith crepe paper, the enlarged stress dia-gram of which is shown in Fig. 10. Sincethe diagrams represent single-conductorshielded cables, the general direction of thestress can be represented by straight linesfrom the conductor to the shield. With re-spect to the entire cable or the over-all insu-lation, the stress is essentially radial. How-ever, if one takes into consideration eachsingle layer of the crepe-paper tapes, thecondition as illustrated in Fig. 10 exists.The stress with respect to each single

layer of crepe-paper tape may be resolvedinto two components, one normal to thesurface of the paper and the other parallelto it. This parallel component is the tan-gential stress on the paper insulation. Themagnitude of this component varies frompoint to point, depending on the bending orfolding of the paper. It is well known thatthe effects of tangential stress on impreg-nated paper cable insulation are much moresevere than those of radial stress. This mayaccount for the low dielectric strength, highpower factor, and relatively poor electricstability as reported in the discussions.Once the causes are known, further workmay be initiated to eliminate these short-comings.

G. Camilli, L. Mulligan, and E. L. Crandall:The data presented by Mr. Meyerhoff andMr. Cortelyou are very interesting; how-ever, their results are based on one set ofsamples representing one type and size ofcable with only a 1/4-inch wall insulation.No data were given as to the test conditionssuch as the humidity and temperature ofthe paper when the bending tests were per-formed. We believe that a marked differ-ence in the test results between our crepe-paper cables and their manila cables wouldhave been obtained if a full range of con-ductor size and insulation wall thicknesswere used and the tests conducted under awide range of humidity conditions. We areat a loss in understanding why the smallerspring-back obtained in their manila cablesis of any benefit. It seems to us that thisindicates a permanent set in the insulationwhich will cause a loose insulation if thecable is subjected to repeated bendings.There is a marked difference in the methodof electrical testing used by the authors andthat used by Meyerhoff and Cortelyou. Webelieve that our methods simulate moreclosely the manner in which cables are usedin power transformers. Meyerhoff andCortelyou are, of course, well aware of thefact that in our nonuniform field testing,with much thicker insulation than in theirtest, naturally lower volts per mil are ob-tained. When we tested cables with 1/4-inch total thickness, dielectric strengths ofthe order of 400 to 440 volts per mil at 60cycles and of the order of 1,060 to 1,210volts per mil at impulse were obtained. Itmight be of interest also to mention that weget the same breakdown values when testsare performed with the cables in oil at 75degrees centigrade on a 24-hour step-by-step method, as we do at 25 degrees centi-grade on a 1-minute step-by-step method.Probably the denser insulation of the manilacables will not exhibit this long-time stresscharacteristic.

It is well recognized by the industry that,from an electrical standpoint, manila paperis inferior to kraft paper nor is it as uniformin over-all quality. Manila paper also haspoorer thermal mechanical aging char-acteristics than kraft paper. Crepe-papercables have now been produced for severalyears with completely satisfactory results.With reference to Mr. Hubbard's com-

ments, we wish to mention that the datashown in Table I refer to tests made undernormalized conditions, that is, 50-per-centrelative humidity and 70 degrees Fahren-heit. Transformer oil as,such, without heattreatment of the paper, has no effect on thetensile strength and tear resistance of thecrepe paper if the paper which has beenwetted with oil has the same amount ofmoisture as it had in the dry condition.The original available stretch and tensile

strength of a given crepe paper is -usuallylowered by heat treatment. Our dielectricstrength tests were made after several bend-ings of the cables, and the results shown inTable II were obtained after these bendings.It should of course be recognized that cableswhich are used in power transformers areusually bent before impregnation with oil.The conclusions reached in the paper by

Vogel, Petersen, and Matsch mentioned inMr. Hubbard's discussion are that kraft andrag papers exhibit better thermal and elec-trical stability than manila paper. As faras we can determine, the particular agingcharacteristics of the crepe papers underdiscussion would be represented by the oil-impregnated kraft paper cited in the article.We would like to point out, however, thatthe rate at which the tensile strength of thekraft paper decreases under similar test con-ditions is not quite as rapid as indicated byVogel, Petersen, and Match. Manila paperdoes not age as well as kraft paper.With reference to Mr. Nicholas's discus-

sion, our comparison between varnished-cambric and crepe papers deals only withapplications in fluid-filled equipment inwhich the insulating paper is vacuum-treated and impregnated with a fluid afterit has been wound in place. Oil-impreg-nated crepe-paper tape for joint or terminaluse is an entirely different application thanthe present paper covers. Suffice to saythat preimpregnated crepe-paper tape forcable accessories was developed severalyears ago, has proved to be most satisfac-tory, and is still used for fluid-filled cableaccessories at all ratings. The tape has en-joyed an excellent service record. Usersfind it easy to use in the field and appreciateespecially being able to build up heavy coneswithout the danger of slippage which isalways a problem with varnished cambric.

Crepe paper intended to be used for jointsis vacuum-treated and kept under oil (incans) until it is used. After completion ofthe joints, these are evacuated and thenfilled with deaerated oil. We believe thissame procedure was followed when varn-ished-cambric tape was used in making thejoint.The dielectric data on the crepe papers

used for taping, which are reported in TableI, are the results of short-time tests and arethe average values from many samples.With reference to Mr. Kang's discussion,

we question the reference to manila paperas the parent material because the crepepaper for cables is made from kraft paper.We do not agree that kraft paper cables areinferior to manila cables from the dielectricstrength, power factor, and the electricalstability standpoints. We agree with Mr.Kang's comments on the effect of the ratio ofoil to paper and their dielectric constants onthe dielectric strength of a cable if we areconcerned only with short-time tests. Nodoubt Mr. Kang is familiar with the resultsof an investigation carried on by Prof.Whitehead' at The Johns Hopkins Univer-sity in which it was shown that low densitypapers have much higher long-time dielectricstrength than denser papers.Mr. Kang's comments on tangential and

radial stresses appear to be based on a cablefor transmission of electric power ratherthan for the particular application discussedin our paper.

REFERENCE

1. THE DIELECTRIC STRENGTH AND LIFE OF IM-PREGNATED PAPER INSULATION-I. THE INFLUENCEOP DENSITY OF THEt PAPER, J. B. Whitehead.AIEE Transactions, vol. 59, 1940, pp. 715-20.

Camilli, Mulligan, Crandall-Crepe Papers and Crepe-Paper CablesJUN-E 1 954 615

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