32 IEEE TRANSACTIONS ON ELECTRICAL INSULATION, VOL. EI-6, NO. 1, MARCH 1971

REFERENCES wound pre-insulated coils," AIEE Test Procedure 511, Oct.1956.

[1] W. L. Marshall et al., "Adhesive silicone rubber sheet [5] W. T. Starr and H. S. Endicott, "Progressive stress-A newmaterial and tapes, and method of preparing same," U. S. accelerated approach to voltage endurance," AIEE Trans.Patent 2 789 155, Apr. 16, 1957. (Power App. Syst.), vol. 80, Aug. 1961, pp. 515-522.

[2] E. A. Boulter, H. N. Galpern, and W. D. Bartlett, "A [6] "A-C capacitance, dielectric constant, and loss characteris-simulated service test for evaluation of high voltage turbine tics of electrical insulating materials," Amer. Soc. Test. Ma-generator insulation systems," AIEE Conf., Paper CP61-163. ter., Test Method D-150.

[31 E. A. Boulter, "Functional evaluation of high voltage tur- [7] "Dielectric breakdown voltage and dielectric strength of elec-bine generator insulation using the generette core model," trical insulation materials at commercial power frequencies,"AIEE Conf., Paper CP 59-1083. Amer. Soc. Test. Mater., Test Method D-149.

[4] "Proposed test procedure for evaluation of systems of in- [8] "Test for electrical resistance of insulating materials," Amer.sulating materials for electric machinery employing form- Soc. Test. Mater., Test Method D-257.

Detection and Identification of ConductingPaths in Capacitor Tissue

ERIC KELK, ROBERT M. HINDE, AND IAN 0. WILSON

Abstract-A mercury electrode tester is described, with an existent in a good-quality tissue, these weak spots mayoperating range of 10-500 V, for recording the incidence of conduct- range from pinholes to the presence of conducting particlesing paths in capacitor tissue. bridging the sheet. More usually, they comprise a range

Conducting inclusions detected with this instrument have been b ing th e usually, te coprise airanestudied by scanning electron microscopy and electron probe micro- oanalysis techniques. They have been shown to include both car- quality requires methods for determining the distribu-bonaceous and metallic particles. tion of such weak spots and identifying their nature.

It is concluded that the mercury electrode tester can detect The service life of capacitors is also, of course, depen-gross conducting particles short circuiting the sheet at the lowest dent on the power factor characteristics of the dielectrictest voltage; thin spots and porous areas that break down at higher w d i . Tvoltages; and smaller particles embedded in the sheet that give which determine its thermal stablity. These characteris-rise to low resistance areas by partial breakdown at higher voltages. tics are controlled by the dielectric properties of the cellu-The techniques described provide means for rapid, quantitative losic insulation and of the impregnant, and by their free-

assessment of capacitor tissues and also for identification of the dom from ionic contamination. This aspect of capacitornature of the conducting particles found. tissue quality has been discussed previously [1] and will

not be dealt with in this paper, except to point out thatINTRODUCTION some of the types of conducting inclusions that have

N A TYPICAL capacitor, two to five layers of thin been identified can be sources of ionic contamination.

paper, each measuring from 8-20 tm in thickness, STUDY OF THE NONUNIFORMITY OF TISSUEare interleaved with thin aluminum foil electrodes.

Vacuum drying and impregnation of these wound ele- Nonuniformity of Paper Structurements are then essential to achieve optimum performance The structure of paper, being a network of bondedof the dielectric. With this type of configuration, where fibers, is fundamentally heterogeneous and numerous at-the total area of paper may be as large as 10 m2, the tempts have been made to put its study on a quantitativeelectrical breakdown strength of the winding is largely basis. It has been shown [2] in these laboratories thatdetermined by the distribution of weak spots, rather the electrical breakdown of paper is related to small-than by the intrinsic strength of the dielectric. To take scale imperfections in its structure. In extending thisthe extreme cases, which will obviously be rare or non- work (which was originally on cable papers) to capaci-

tor tissues in the thickness range 10-25 um, many tech-

Manuscript received April 29, 1970; revised July 30, 1970. niques have been used to study the fine-scale nonuniform-The authors are with the Central Research and Engineering ity of the tissue structure over areas of the order of

Division, British Insulated Callender's Cables Ltd., London W12, 510~ ndaee.TeeicueEngland.'2510u ndaee.Teencdd

KELK et al.: CONDUCTING PATHS IN CAPACITOR TISSUE 33

30 and impulse strength of cable paper [2]. It has sincea) THICKNESS, MEASURED WITH 3mm DIAMETER BALL ANVILS, ON SINGLE LAYER OF TISSE' % E (200 READINGS) been applied to capacitor tissues for both impulse and

20 Sh 1 r direct voltage breakdown measurements. Typical resultsXTm 1k, 01 ovo*CC for the distribution of breakdown values are shown as a1 I>< s histogram in Fig. 1 (c).

10 s 3 As has already been pointed out, because of the con-

+z 12 l nfiguration of the usual capacitor winding, the effective4l 6 8_&_______ 12 MIC,ON strength of the dielectric in service is controlled by the

2 4 6 a 10 12 14 M ICRON

b) IMPERMEABILITY, MEASURED ON CIRCULAR_ low-voltage tail of the breakdown voltage distributionAREA OF 2mm DIAMETER (LoREADING) (which can be seen clearly in the histograms) rather

20 than the mean value. It is, therefore, clearly desirable toa have an electrical test method that enables the incidence

ci10- of weak spots to be measured at any given voltage, prefer-o

-F]ably over a large area of tissue. The various types of

6 , 1 apparatus that have been used for this purpose fall into100 IpOO 10,000 lOOpOO pOOpOO GURLEY SECONDS three groups: the conducting particle tester, usipg one or

C) D.C. BREAKDOWN VOLTAGE, SINGLE LAYER OF UNIMPREGNATED TISSUE, two rollers as electrodes; higher voltage versions, some-7mm DIAMETER ELECTRODES ( S times referred to as formation testers; and apparatus

50 using mercury electrodes.40-

Detection of Conducting Paths Conducting ParticleTesters

20- T | | 1 These are designed to record the presence of large10o particles, which effectively short-circuit the sheet of tis-

200 400 600 S 1000 V sue, using a plate and roller or twin-roller electrodeFig. 1. Nonuniformity of capacitor tissue (10 Azm, 1.0 g/cm3). configuration, and a low test voltage, typically 10 V.

They will not be described in detail here, being generallyfamiliar, and a test of this type is usually incorporated in

1) variability of thickness, using an electromicrometer cpctrtsu pcfctoswith sphericalanvils; ~~~capacitor tissue specifications.wlth spherlcal anvlls;

2) variability of substance (gram per square meter) HV Testers with Roller Electrodesby an indirect method with an optical density scan-ner, and also by more direct techniques such as The simple conducting particle counter has also beenalpha- and beta-ray absorption; modified, so that higher voltages (up to 5 kV) can be

3) variabilityinairimpermeability. employed, using a plate and roller electrode system. In

varicablireuty afthirknessandimpermeability,varia instruments of this type, known as formation testers,Typical results of thickness and impermeability varia- electrical breakdown occurs at weak spots. It can easily

tion are shown in Fig. 1 (a) and (b) as histograms, indicat- be shown, by the application of the alcohol stain test,ing the degree of nonuniformity observed. A further sim- that each count recorded at the higher voltages (e.g., 500ple technique for flaw detection is the alcohol stain test, V) is a breakdown creating a hole in the tissue (provid-in which the presence of small areas permeable to alcohol ing that a counter of adequate reset time is employed,are revealed by placing the tissue over a sheet of carbon to avoid counter saturation, or double counting of break-duplicating paper. Porous areas penetrated by alcohol downs that can occur at slow speeds of roller traverse).applied to the top of the sheet are shown as permanent Even with a satisfactory counter design, however, thisstains on the underside, which may then be counted. type of tester has two inherent drawbacks: the plate andThese techniques for studying the physical structure of roller rapidly become pitted when operating at high vol-

paper can only be regarded as indirect methods for as- tages; and the actual area of contact, and hence the areasessing tissue quality, and are, in any case, limited to de- of tissue being tested, are dependent on the precision oftecting the presence of thin or porous areas; they do not machining of the electrodes. Both these difficulties aregive any indication of the presence of conducting particles. overcome with the mercury electrode tester.For this reason, electricalI test methods, which are capableof detecting any form of weak spot, are to be preferred. Mercury Electrode Tester Design and Operation

The tissue is pulled through mercury electrodes, theElectrical Breakdown Measuremzents speed of traverse being controlled at 1 in/mmn by anThe technique for measuring the single-sheet break- electric motor-driven reel-up. The electrode area is de-

down strength of small areas of paper was developed in fined by a V-shaped slot, 25 x 0.5 mm at the base, cutorder to study the relationship between physical structure in a paxolin block, which serves as the upper electrode

34 IEEE TRANSACTIONS ON ELECTRICAL INSULATION, MARCH 1971

MEQCURtY COUNTS/m2

D.C. + 1 \\\b\b\\\\CT5UE5FOWER T t J t COUNTER 1Xw T ]

T1~~~~~~~~~~RR220ka1l

TW

Fig. 2. Mercury electrode tester: simplified diagram. 10,0 s

0when filled with mercury. The tissue passes between this celectrode and the lower mercury bath, also contained in 000la slot in a paxolin block.The two blocks are clamped together sufficiently

tightly to prevent drag-out of the mercury, while allow-ing the tissue to pass freely. The circuit is shown in 100°simplified form in Fig. 2.A dekatron counting unit was built for this apparatus,

designed to achieve a high counting rate, a sufficient sen- 1lsitivity to operate at low voltages (and a reasonable 10 20 30 40 50 10 Vspeed of traverse of the tissue), and a circuit registering Fig. 3. Mercury electrode test: results for 15-,gm tissues, densitya single count for each pulse from the electrodes. The 0.75 g/cm3.circuit diagram is given in the Appendix.The counter sensitivity is adjustable, and is normally zp0O,000

set to record pulses of ten percent, or more, of the inputvoltage. For measurements at low voltages the counter COUNTS/rn7may be set to accept 50 percent of the input voltage.

If the counter is set to record pulses caused by a ten 100°000 635percent rise in voltage across R (Fig. 2), it will, in effect, 629

detect areas in the tissues where the resistance falls to 3/v / 8<9R or less. Conducting paths are thus defined as areas 0759pwith a resistance of less than 2 Mu. 13 9/cm5An important feature is that the counter can record

pulses caused by successive falls of resistance to 9R, 4R,etc., even if the original conducting path that caused 11/the resistance to fall to 9R is still between the electrodes.

Typical Results

Fig. 3 shows curves of counts/voltage for a number oflow-density 15-MAm tissues over the range 10-100 V, whichis generally found to be the most useful for assessing the,quality of tissues. 50 100 200 300 400 500 VThe counter is, however capable of operating at up to

500 V, which is often useful when studying thicker tissues, Fig. 4. Mercury electrode test: results for 10-pum tissues, densityor those of high density (Fig. 4). At these higher voltages, 1.0 g/cm3 (627, 629, 635, 655, 657).a maximum count may be reached, falling as the voltageis increased. This phenomenon is illustrated in Fig. 4, for in resistance. [This enables it to record counts higher thana range of medium density 10-Mm tissues, where it will the limit of one conductance per unit electrode areabe seen that the tissues giving the highest counts at vol- (80 000/M2).] Instead, saturation is reached when, by par-tages below 100 V, are the first to reach their maximum tial or complete breakdown, the local resistance fallsas the voltage is raised, so that at 500 V the order of to a value equal to R/9 or less (Fig. 2). Further conduc-ranking is reversed. This maximum is not caused by a sim- tances or breakdowns in this area cannot then be counted.ple saturation of the counter, which, as described above, As the voltage is increased, this condition occurs moreis able to record the presence of more than one conduc- frequently, and the recorded counts decrease. The poorertance within the electrode area ( 25 x 0.5 mm), provided tissues then give lower counts, since counter saturationthat the second and further particles cause sufficient falls has not been reached for the better tissues (Fig. 4) .

KELK et al.: CONDUCTING PATHS IN CAPACITOR TISSUE 35

TABLEI TABLE II

Mean Life under Mercury Electrode Tissue Impulse Strength (V)Tissue Test (h) Count at 50 V/m2

C 290O 1300 80 0 300C 1300 200 S 190S 450 480 B 220B 220 870 TW 170TW 70 3100 T 80T 70 13000 _

do not bridge the sheet initially, but may be revealed bypartial breakdown of the overlying fibers.

It is of interest to compare (Table I) the order of rank- The simple concept of electrical breakdown of the pa-ing of the low-density tissues shown in Fig. 3 with their per at thin spots is inadequate to explain the counts ob-performance in three-layer capacitor windings subjected served, as a comparison with impulse breakdown tests,to accelerated life tests. Obviously, the performance of also carried out with mercury electrodes, shows.the tissues in accelerated aging tests of this type depends Table II gives the results of impulse tests with 25-also on their power factor/temperature/stress character- mm-diameter mercury electrodes on the tissues whoseistics, but it is interesting, nevertheless, that the ranking conducting path counts are shown in Fig. 3.order deduced from the mercury electrode test is very These impulse strengths are much higher than wouldsimilar to that from the life tests. be predicted from the mercury tester counts, if each count

It should be pointed out that the results shown in Figs. corresponded to a breakdown. With an impulse test area3 and 4 have been taken from our files as examples of of 0.0005 m2, the upper limit for breakdown voltagecomparative tests on a range of tissues of similar type should be that at which the count reaches 2000/M2, orbut varying origin, to show the utility of this test in one conductance in each test area on average, assumingassessing tissues. They are not to be taken as being uniform distribution. In fact, this voltage (Fig. 3) isrepresentative of the latest grades, since, through the approximately -X the observed impulse strength. Con-efforts of pulp and paper makers, guided by such test re- versely, it is found that up to the actual impulse break-sults, a steady improvement has been achieved in recent down level, an average of 25 conductances/breakdownyears. test area are recorded on the mercury tester, which, as

NATURE OF THE CONDUCTING PATHs DETECTED described above, detects a fall in resistance across thesheet, and not necessarily an actual breakdown.

BY THE MERCURY ELECTRODE TESTER These results suggest that the conductances recordedAt 10 V the conducting path count with the mercury on the mercury tester may be of several types, and a di-

electrode tester is usually of the same order (though rect study of their nature was undertaken, using theslightly higher) as the count recorded with roller techniques of scanning electron microscopy and X-rayelectrodes. These conducting paths obviously correspond microanalysis, to locate particles in the paper structureto the presence of particles bridging the sheet, and this and to identify them.has been confirmed by microscopic examination of thearea, which almost always reveals the presence of a visi- IDENTIFICATION OF INCLUSIONS BY SCANNING ELECTRONble inclusion. The composition of these inclusions is dis-

cussed in thenextsection. MICROSCOPY AND ELECTRON PROBE MICROANALYZERcussed in the next section.The mercury electrodes give a more intimate contact The Stereoscan scanning electron microscope [31 com-

with the sheet of tissue, which can vary locally in thick- bines high resolution with a very large depth of focus.ness to a marked degree (Fig. 1), so that a higher count The micrographs of solid surfaces produced by this in-would be expected with the mercury electrode tester. The strument have an ultimate resolution of 250 A andvery marked voltage dependence of the mercury elec- possess a marked three-dimensional appearance. The in-trode conductance count indicates, however, that new strument is thus particularly useful for the examinationtypes of conducting paths are detected as the voltage of the variable surface topography of paper and otherincreases. fibrous materials.The counts recorded may indicate the following. In the electron probe microanalyzer [4] the elemental1) A breakdown of the sheet at nonuniformities in the composition of particles and inclusions as small as 2-3

paper structure, i.e., thin spots where the breakdown ,um in diameter may be obtained by spectroscopicallystrength of the paper is exceeded at the test voltage, or analyzing the X-ray emission resulting from the electronporous areas where the mercury can penetrate the sheet. beam bombardment, and the elemental distribution can

2) The presence of smaller conducting inclusions that be displayed on the screen of a cathode ray tube.

36 IEEE TRANSACTIONS ON ELECTRICAL INSULATION, MARCH 1971

The experimental procedure therefore adopted was toexamine a specimen in the Stereoscan electron micro-scope to determine the disposition of any particle presentwith respect to the paper fiber matrix, and then to trans-fer the specimen to the X-ray microanalyzer to deter-mine the elemental composition of the particle.

Specimen Selection and PreparationA selection of capacitor tissue samples has been ex- Fig. 5. Scanning electron micrograph X78.

amined. In each case a preliminary electrical test of thetissue was carried out in the laboratory using the mer-cury bath apparatus described above. Such areas werethen suitably marked and circular sections, 1 cm in di-ameter, which contained the inclusions, were cut from thetissues and mounted for electron microscopy and X-raymicroanalysis examination. Since the tissues were non-conducting it was necessary to evaporate a layer of alu-minum, approximately 500 A thick, onto the surfaces tobe examined to avoid charging under the electron irradia-tion and to minimize localized heating by the electron Fig. 6. Scanning electron micrograph X78.beam.

Using electron probe techniques particular care is re-quired in the selection of the beam current since the lo-calized electron bombardment can result in embrittle-ment and charging of the tissue fibers with consequentrisk of loss of the particles. Owing to the fragmentarynature of the inclusions and uneven surface topography,accurate quantitative X-ray microanalysis was precludedand therefore semiquantitative results only were ob-tained.A further batch of capacitor tissues from another Fig. 7. Scanning electron micrograph x39.

source was examined by electron probe X-ray micro-analysis only and the results of this investigation arealso reported.

Experimental ResultsEighteen specimens of faulty capacitor tissue, 15 ,am

in thickness, were selected for examination in the Ster-eoscan. Foreign particles, in the size range 100-400 Km,were found to be present in all the samples and in themajority of cases were embedded in the tissue fibers;in several instances the particles were overlaid by fibers.Micrographs of two samples are shown in Figs. 5 and 6and it can be seen that both the inclusions have a frag- Another particle examined was not deeply embeddedmented structure; Fig. 6 also shows several fibers over- but appeared to be lying on the surface of the tissue;laying the embedded particle. These samples were then this is shown in Fig. 7. The particle, which did not havetransferred to the electron probe microanalyzer for the the broken appearance observed in the previous samples,determination of elemental composition but no major was found to contain mainly iron together with a traceelements were detected in either case, only traces of of aluminum. The measured iron content was approxi-calcium, silicon, sulphur, and aluminum being found. An mately 60 percent.attempt was made to detect the presence of carbon, but Fig. 8 is a Stereoscan micrograph of a tissue specimenthis was not possible owing tQ the delicate nature of the that was found to contain a broken inclusion associatedtissues and the consequent charring effect of the high with a pore in the fiber structure. On examination in theelectron beam current required to produce sufficient car- microanalyzer only small amounts of calcium and alumi-bon X-ray intensity. In such cases it was concluded that num were detected.the particles were in all probability essentially calbon- The micrograph of a further specimen, in which theaceous. inclusion was embedded in the fibers and had a flaky

KELK et al.: CONDUCTING PATHS IN CAPACITOR TISSUE 37

(a) (b) (a) (b)

(c) (c) (d)

Fig. 10. (a) Scanning electron micrograph X80. (b) Micro-Fig. 9. (a) Scanning electron micrograph X38. (b) Electron analyzer electron image X74. (c) Iron distribution X74. (d)

image X74. (c) Iron distribution X74. Sulphur distribution X74.

TABLE III

Main ConstituentsTissues Number of Carbon- Size Range

Examined Particles Fe Fe + S Ca Brass Cu Zn aceousa (pm)

First series 12 7 1 4 100-400Second series 94 22 16 19 7 6 24 1-20

1 Assumed to be carbonaceous because no major elements were detected.

appearance, is shown in Fig. 9 (a). Microanalysis re- A second series of capacitor tissues from another sourcevealed that the main constituent was iron together with was also examined. These were found to contain numer-a small amount of aluminum: the measured iron content ous particles that were located and whose elemental com-was approximately 62 percent. The electron image ob- position were determined by X-ray microanalysis. In thesetained in the microanalyzer and corresponding iron dis- tissues the particles were much smaller, being in the sizetribution are shown in Fig. 9(b) and (c), respectively. range 1-20 ,am, and were found to contain iron, calcium,Of the 18 tissue specimens in this series examined in brass, copper or zinc. In some tissues clusters of small

the Stereoscan microscope, 12 were selected for electron particles all of the same composition were found. A con-probe microanalysis; eight of these were found to con- trol sample taken from part of a tissue some distancetain mainly iron and in the remaining four, only trace from a region containing conducting particles was alsoamounts of various elements were found. examined. Particles were again found in this sample butOne of the iron-rich particles differed from the others these were markedly smaller, 1-6gm, and were predom-

in that it also contained an appreciable amount of sul- inantly calcium containing or concluded to be carbon-phur. The particle was embedded in the tissue and the aceous.overlying fibers can be seen in the Stereoscan micro- The results obtained in these investigations are sum-graph shown in Fig. 10(a). The electron image of the marized in Table III.same area obtained in the X-ray microanalyzer is shownin Fig. 10(b) and the iron and sulphur distributions are DISCUSSIONshown in Fig. 10(c) and (d), respectively. A semiquan- The Stereoscan studies have shown the detailed struc-titative analysis showed that there was approximately ture of the tissues and particles and revealed the degree58 percent iron and 42 percent sulphur present, cor- of penetration of the particles in the surrounding fiberresponding to the composition of iron sulphide Fe3S4, matrix. In the samples examined only one particle waswhich contains 56.6 percent iron and 43.4 percent sulphur. not firmly embedded in the tissue fibers whereas the re-

38 IEEE TRANSACTIONS ON ELECTRICAL INSULATION, MARCH 1971

mainder were firmly impressed, and in several instancesfibers were observed to overlay the inclusions. The ma-

> +°jority of the inclusions have thus passed through at ,least part of the calendering process and this is supportedby their broken or flaky appearance. The occurrence ofclusters of small particles of the same composition alsosuggests the initial presence in the tissue of relatively OZIlarge particles that have been subsequently crushed into %9;0OZ9smaller fragments during the manufacturing process.Of the particles found in each of the tissues examined, 6, 05

although accurate quantitative analysis was not possible, omz >

it was considered that the iron-rich inclusions were es- I oo isentially iron oxide. When no detectable element was v 14 O

found it was concluded that the particle contained a highconcentration of carbon; this has also been suggested in ozthe case of similar conducting particles examined in alaser excitation emission spectroscopy study carried outby the Electricite de France. m 0 IdoogThe finding of trace amounts of aluminum in certain

of the samples is accounted for by the conducting coatingapplied in the laboratory to minimize charging and heat- mozzing effects. The sulphur detected could be derived fromthe sulphate (kraft) processing of the paper pulp, and theiron and other metallic elements from the pulp- andpaper-making machinery, especially the beaters, andTwater pipe lines. Deposits in the latter could also ac-count for the presence of calcium. The asbestos incorpo- I mrated in the paper rolls, often paired with steel rolls dur- |ing calendering, could be an additional source of ironcontamination and also account for the silicon found.In the second series of tissues the exceedingly small par- luozoticle size also suggests the possibility that a certainamount of this contamination might be airborne. k-Ll

Scanning electron microscopy reveals details of sur-face structure, and electron probe microanalysis is cap- Idoogsable of detecting isolated particles within the paper ma- _t |trix; lhence not all the particles shown to be present maynecessarily create a short circuit of the sheet at low vol- CZ) batages. This is supported by the fact that in the controlsample of tissue examined, where low-resistance pathswere not detected at 10 V, particles were still found. Itappears likely that the counts at higher voltages occurwhere these smaller embedded particles are associatedwith pores in the fiber structure, or where several parti-cles are in juxtaposition so as to create a conducting paththroughout the thickness of the tissue when a sufficientlyhigh voltage is applied.

CONCLUSIONS 10/15 WMOZ

The mercury electrode tester detects the following.1) Conducting particles that effectively short-circuit

the tissue; these are detected at 10 V and are essentiallythe same as those detected by the roller electrode testers;they mnay be carbonaceous or inorganic (metallic or metalLsalts) in composition. l0 u

2) iFlaws (thin spots or porous areas) in the paper Istructure; comparison with electrical breakdown test re-

IEEE TRANSACTIONS ON ELECTRICAL INSULATION, VOL. EI-6, NO. 1, MARCH 1971 39

sults, however, shows these to constitute only a pro- drives the first counting tube. The output from the firstportion of the mercury tester counts at higher voltages. tube is used to drive a pulse coupling circuit, giving the

3) Low-resistance areas, caused by small inclusions second stage. There are four pulse-coupling units in all,embedded within the sheet of tissue; these are of similar giving a five-stage counter.constitution to the larger particles detected at 10 V, but The counter is zeroed by applying a 120-V negativeare only recorded by the counter after partial break- pulse to each Ko pin of the counting tubes by closing adown of the sheet at higher test voltages; correlation with relay having five contacts so that each Ko pin is isolatedthe results of accelerated aging tests suggests that they during normal running.may be associated with the reduction of capacitor life by The counter has a maximum rate of about 4 kHz,ionic contamination. which is ample for the usual count level encountered. ItThe techniques described have been applied to both is sufficiently sensitive to detect pulses down to 0.5 V, and

the rapid, quantitative assessment of capacitor tissue the circuit is designed so that each pulse from the elec-quality (with the mercury electrode tester), and the iden- trodes can register only one count.tification of embedded particles (with the Stereoscanelectron microscope) and determination of their composi- ACKNOWLEDGMENTtion, and hence their probable origin (with the electron The authors wish to thank Dr. A. L. Williams, Direc-probe microanalyzer.) tor of Research and Engineering, BICC Limited, for per-

mission to publish this paper. They are also indebted toAPPENDIX D. R. Groombridge for the scanning microscopy, and

DEKATRON COUNTER other colleagues who assisted in the experimental work.

FOR MERCURY ELECTRODE TESTER REFERENCESThe circuit of the dekatron counting unit is shown in [1] E. Kelk and I. 0. Wilson, "Constitution and properties of

Fig. 11. The input signal V1 from the electrodes is fed paper for high-voltage dielectrics," Proc. Inst. Elec. Eng.onto the first grid of a monostable multivibrator circuit (London), vol. 112. Mar. 1965, pp. 602-612.

[2] H. C. Hall and E. Kelk, "Physical properties and impulsevia blocking capacitor (0.01 pF). This grid is also bi- strength of paper," Proc. Inst. Elec. Eng. (London), vol.ased via a potentiometer to a voltage V2, such that V1 + 103A, Dec. 1956, pp. 564-570.

is sufficient to trip the circuit. The vibrator circuit [3] W. C. Nixon, "Scanning electron microscopy," Contemp.V2 1S sufficlent to trlp the clrcult. Tne vlDrator c Phys., vol. 10, no. 1,1969, pp. 71-96.is arranged so that the resultant rise time dn/dt = 108 [4] S. A. Bergen, "X-ray determination of element distribu-

V/s tosuitthe integrated pulse drive circuit, which tion in ultra-microsamples," Res. Dev., vol. 1, Sept. 1961, pp.V/s to SU1t the lntegrated pulse drlve clrcult, 58-61.

Voltage Surge Performance of Vacuum-InsulatedCryo-Cable

PETER GRANEAU, SENIOR MEMBER, IEEE, AND JOHN JEANMONOD, MEMBER, IEEE

Abstract-This paper describes the results obtained from two relation between the energy of the source and the damage producedshort experimental lengths of single-phase vacuum-insulated cables, by sparks. Hence, the chances seem good that a cryo-cable con-one of which can be cooled by liquid nitrogen. Using a variety of nected to a bulk power transmission system will not suffer catas-high-voltage sources, it has been shown that there is little cor- trophically with each internal spark.

Paper 70 CP 170-PWR, recommended and approved bh the INTRODUCTIONIEEE Insulated Conductors Committee of the IEEE PowerGroup for presentation at the IEEE Winter Power Meeting, New - T IS the purpose of this paper to give utility engineersYork, N. Y., January 25-30, 1970. Manuscript received January I is lms ftevr nsa eairo ih14. 1970; revised August 18, 1970. afis lmsoftevrunulbhvo fhg-

P. Graneau was with Simplex Wire and Cahle Company, voltage vacuum insulation and allow them to be-Camhridge, Mass. He is now with Underground Power Corpora- com usdt-h data,on a,cbe a eation, Weston, Mass. 02193.coeuetotedethtonda,absmybes

J. Jeanmonod was with Simplex Wire and Cahie Company, resistant to discharges as are overhead lines. The non-Camhridge, Mass. He is now with Boston Insulated Wire and dsrcientr fvcu icagsi ninsileCahle Company, Dorchester, Mass. 02125. dsrclentr fvcu lcags1 nlnslle