IEEE Transactions on Electrical Insulation, Vol. EI-12, No. 1, February, 1977

MEASUREMENT OF AC INSULATION LOSSES

AT CRYOGENIC TEMPERATURES

William E. Anderson & Richard S. DavisHigh Voltage Measurements Section

Electricity DivisionNational Bureau of Standards

Washington, D.C. 20234

ABSTRACT

The design of superconducting high voltage trans-mission lines requires engineering data which, untilrecently, have been largely unavailable. Theselection of a suitable dielectric for a tape-insulatedac cryogenic cable, for example, requires the knowledgeof insulation dissipation fagtors at high voltage,which are typically 20 x 10 or smaller. We presenthere dissipation factor measurements made on severaldielectric tapes under consideration by a super-

conducting power transmission line project as wellas on epoxies which appear mechanically suitableas low-tempetature bushing material. The measurementtechnique and instrumentation are described.

INTRODUCTION

The design of superconducting transmission cablesrequires a systems approach which includes consider-ation of the conductor, insulator, cryogenic envelopeand refrigerator. Each component of the system mustfunction satisfactorily without disrupting the othersystem elements. The cable insulation must meetdemanding requirements of breakdown strength, corona

inception level,and mechanical integrity. But it mustaccomplish all this without burdening the refrigeratorwith a heat load due to ac dielectric losses.

For a flexible ac cable designed with plastic tapeinsulation, the power (P) continuously dissipatedin the dielectric follows the relation

p a f V E:Itan 6,

where f is the power frequency and V the operatingcable voltage. Reduction of P to acceptable valuescan only be accomplished by minimizing the producteC' tan 6 where £' is the real part of the complexpermittivity, £* = Et + £" and tan 6= s/c', is thedissipation factor. Ideally, one would wish theinsulating material to have as low an C' as possiblebut in practice most plastic materials have values of£ Cno lower than 2.

Finally one is left with tan as the parameter whichultimately determines the suitability of plasticinsulating materials in regard to refrigeration load,all other constraints having been assumed to be met.Present thinking by cable designers [1,2] places an

upper limit on the tan 6 of taped insulation some-

where between 4 x 10-6 and 20 x L0-6. In rigid cable

designs it is possible to use liquid helium as theinsulator. Solid dielectric spacers, however, must

be used to keep the inner and outer conductorsconcentric. These spacers are allowed to have loss

tangents more than an order of magnitude higherthan taped insulation because the spacers are but

a small fraction of the total volume of insulation.Nevertheless, reliable values of tan 6 for spacerscast in epoxy are useful to the design engineer.

EXPERIMENTAL

There are several techniques now available toobtain necessary dissipation factor data. Acalorimetric technique which has high precisionnear the absolute zero of temperature has been usedat Stanford University for several years [3]. Withthis calorimetric technique one can apply largestresses to thin ("400 pm) samples and varyfrequency over many decades. It is, therefore, a

useful tool for looking at the intrinsic behavior ofdielectric samples.

We have chosen a method in which the electricalproperties of a material are measured directly. Our

technique places no limits on sample size althoughwe are restricted to using power frequency.

In order to measure the dissipation factor of a

sample, it is placed between the plates of a capacitorwhich is inserted into liquid helium and measuredusing the NBS current comparator bridge [4]. Asimplified schematic of this instrument is shownin Fig. 1. The capacitor containing the sample is

;C,

Cx = (N, / N2) Cston 8 I/ RxwCx = (N3/N,) (I/RscoCfI

Fig. 1 Simplified schematic of NBS current

comparator bridge.

51

Manuscript received June 1, 1976. This paper wasreported at the 1976 IEEE International Symposiumon Electrical Insulation, Montreal, Canada.

+This work supported by ERDA.

represented by a parallel combination of an idealcapacitor, C, and a resistor, Rx. The currentin C is electrically balanced against the currentin axcompressed-gas standard capacitor, Cs. Placinga resistor in parallel with Cs would not be apractical way of balancing out the current in Rbecause of the high-voltage characteristics ofavailable resistors. Instead an operationalamplifier using capacitive feedback produces asmall voltage proportional to, but 7r radians out ofphase with the high voltage. This voltage acrossa standard resistor, Rs, yields the necessarycurrent for balancing out the current in R . Theestimated uncertainty of the bridge at 60 Hz is+1 ppm for the capacitance value and +1 x 10 forthe dissipation factor. The resolution of the bridgeis a factor of 10 better than this.

Since the compressed-gas capacitor is used as astandard, any uncertainty in its value of tan 6propagates directly as uncertainty in the measurementof the unknown. For the majority of high voltagemeasurements, uncertainties in the capacitor standardare negligible. The necessary cryogenic measurementsdemand an unprecedented measurement uncertainty(±1 x 10-6), however, so that knowledge of thestandard's tan 6 to less than +1 x 10-6 becomesnecessary. One novel solution to this problem wasdevised by Nelson who used a capacitor filled withliquid helium as his dissipation factor standard [5].There is good reason to believe that liquid heliumhas no intrinsic loss mechanisms and that at 4 Kany loss due to surface films on the electrodeswill be "frozen" out.

Our approach to establishing the value of thedissipation factor of our standard of our standardcapacitor was to compare it against an in-housestandard. NBS has recently developed a dissipationfactor standard [6] which is based on a theoremfirst discovered by Thompson and Lampard [7]. TheNBS standard has a dissipation factor of zero withan uncertainty of less than 1 x 10-7 at 1600 Hz.The dissipation factor of our compressed gascapacitor was found to be (1.8 + 0.5) x 10-6 at60 Hz. The uncertainty is primarily due to a lossof precision at power frequency.

Our sample probe is shown in Fig. 2. A guardedparallel-plate capacitor serves as sample-holder.The capacitor plates are made of stainless steelpolished to a mirror finish. The active low-voltage electrode is 10 cm2 in area. It is setin epoxy which provides insulation from the guardring. A spring-loaded polytetrafluoroethyleneclamp (not shown in Fig. 2) compresses theelectrode-dielectric sandwich.

High voltage is supplied to the capacitor by avacuum-insulated feedthrough. Commerciallyavailable ceramic bushings terminate the high-voltage feedthrough at each end. In normaloperation, the probe is immersed in liquid heliumat 4.2 K. The liquid level is maintained above thelower bushing during all high-voltage measurements.

To avoid measurement errors, it is important thatthe signal from the low-voltage electrode be shieldedalong its entire path from the capacitor to thecurrent comparator bridge. A miniature coaxialcable shielded with stainless steel accomplishesthis in the probe shown in Fig. 2. The shieldof the cable is connected to the capacitor guardring and is grounded at the bridge, care beingtaken to avoid accidental grounds elsewhere alongthe cable path.

52

The total resistance of the leads is less than 20At 60 Hz, this resistance would cause an addeddissipation factor of 1 x 10 in a 1300 pF capacitor.Since the capacitance of all samples reported in thispaper has been smaller than 700 pF, the lead resistanceis seen to have a negligible effect on the measurementof tan 6.

RESULTS

As a consistency check we measured the dissipationfactor of liquid helium at atmospheric pressure. Wefound the liquid helium filled capacitor to besomewhat noisy at elevated electric stress. At1.5 kV/mm the noise was absent and a measurementyields a 60-Hz tan 6 of (+0.3 + 0.7) x 10-6.

The results of measurements on single layers ofinsulators are shown in Tables I and II. Allmeasurements reported in this paper were made atatmospheric pressure and 4.2 K. The voltage dependenceof tan 6 is in most cases quite small. Measurementswere terminated at the first signs of corona onset asevidenced by noise in the bridge detector. Therepeatability of polymeric samples from the samemanufacturer is generally good although polymers withthe same generic name may differ markedly.[8] Severalof the samples are seen to have tan 6's less than20 x 10-6 at design stresses of the order of 10 to 20kV/mm. However these materials all have rather poormechanical properties. The mechanically-sound 6materials have tan 6's in the range of 50 to 100 x 10An attempt by the manufacturer to improve the tan 6 ofone of these materials (green polysulfone) by reducingthe sodium concentration was somewhat successful.

GUARDEDCABLE TO- I, TBRIDGE I

I

I I

I E t HI

I I

LOW VOL

TODIFFUSION

PUMP

IGH VOLTAGE ELECTRODE

SAMPLE DIELECTRIC

UARD RING.TAGE ELECTRODE

Fig. 2 Sample holder for tan6 measurements.

Table I. Summary of Results at 60 Hz and 4.2 K

Material [8]

Polyamide (Rilsan Nylon 11,non-oriented)

Maximum VoltageThickneSs Applied

40 pm 1000 V rms

Tan 6 atMaximum Voltage

27 x 10-6

Polycarbonate (Makrofol KG,uniaxially oriented)

Polyethylene (Valeron 4000,biaxially oriented, crosslamina ted)

Polyethylene (Cryovac D-330-SW,biaxially oriented)

Polypropylene (Hercules N-400,non-oriented)

Polypropylene (Hercules EK-500,biaxially oriented)

Polypropylene (Hercules EK-503,biaxially oriented)

Polysulfone (Rowland-clear)

Polysulfone (Rowland-blue)

Polysulf one (Rowland-green)

Polysulfone (Rowland-green,fewer Na impurities)

Polyether-sulf one (Kimberly-ClarkExperimental, biaxially oriented)

Polye ther-sulfone (Kimberly-ClarkExperimental, non-oriented)

The significant decrease in dissipation factor withincreasing stress for the two polyether-sulfonesamples is unique in our experience. The resultsof the epoxy measurements are tabulated in Table II.Several of the epoxy samples exhibited a significantdirect dependence of tan 6 on applied stress. Furtherwork must be done to determine the cause of thisdependence. For the present, we only report values oftan 6 extrapolated to zero stress. Sample preparation(including catalyst concentration) may account forthe varying results on identical materials ofdiffering thicknesses. The apparent increase oftan 6 with sample thickness may be a real effectresulting from the increased probability of defects.

In order to make these measurements, it was necessary

to coat the electrode-sample interfaces with paraffinoil. Presumably this procedure suppresses corona

at the interfaces. Measurements by King and6Thomas[3]and us have placed an upper limit of 1 x 10 on thetan of paraffin oil at 60 Hz. In similar tan 6studies carried out in the United Kingdom, metalelectrodes were evaporated onto the polymer samples[5]accomplishing the same purpose as paraffin oil. Ithas been our experience that, when our samples havenot been coated with paraffin oil prior to insertioninto the sample capacitor, the measured tan 6 exhibitsa strong voltage coefficient.

To examine further the effects of paraffin oil on

various interfaces, we studied several stackingarrangements of 30 pm thick polypropylene films. The

dissipation factor of this material has a very smallvoltage coefficient over a wide range of voltagestress. We measured the dissipation factor of three

sheet stacks of this material. In one case bothsides of each sheet were coated with paraffin oil.In another instance only the metal-insulator inter-

faces were coated. We also measured a two-sheetstack with oil only between the metal-insulatorinterfaces.

Table II. Summary of Epoxy Results

Material [8]

G-10, 1.6-mm thick

Stycast, 2850 FT (Blue)Catalyst 11, 2.8-mm thick

Stycast, 2850 FT (Blue)Catalyst 11, 480-pm thick

Stycast, 2850 FT (Blue)Catalyst 24, LV, 360-pm thick

Stycast, 3850 FT (Blue)Catalyst 24 LV, 560-pm thick

Stycast, 2850 FT (Blue)Catalyst 24 LV, 630-pm thick

at 60 Hz and 4.2 K

Tan

560 x 106

340

280

185

210

210

The results are plotted in Fig. 3. The slopesare consistent with measurements made on polyethyleneusing the calorimetric method [3] but it is clear thatthe nature of the interfaces plays an important role.Present designs call for the insulation of a

flexible superconducting cable to be impregnatedwith helium at temperatures of 7 to 9 K and

53

Tan 6 at200 V rms

24 x 106

2000 69

2000 9

800 18

2000

75

100

40

125

30

40

100

140

140

110

7

1200 26

1200

1000

15 .5

100

61

6.5

7

3

24

17.5

96

113

105

79

62

70

3000

1200

126

1000

110

10

82

400

30

28

800 42

pressures from 1 to 1.5 MPa (10 to 15 atmospheres).It is therefore our intention to continue ourmeasurements in an apparatus which will operateat these conditions.

60XI-6

cc-0

CD

LL

z0

0-co

(I)C')

50X 106 _

40x10-6

30X10-6

20 10-6

0

0

* SINGLE SHEET OF POLYPROPYLENE,COATED WITH PARAFFIN OIL

,O 2 SHEETS OF POLYPROPYLENE,METAL- INSULATOR INTERFACES COATED

O 3 SHEETS OF POLYPROPYLENEMETAL-INSULATOR INTERFACE COATED

+ 3 SHEETS OF POLYPROPYLENE,+0 ALL INTERFACES COATED

0 +ot 0

+e+O

0 0

0 10 20 30 40

kV/mm, rms

Fig. 3 Dissipation factor data on layered stacks of30- thick polypropylene films.

REFERENCE S

1. E. B. Forsyth, G. A. Mulligan, J. W. Beck andJ. A. Williams, Trans. IEEE, PAS 94, 161 (1975).

2. D. C. Swift, Proc. IEEE 118, 1237 (1971).

3. C. N. King and R. A. Thomas, Appl. Phys. Lett. 26,406 (1975).

4. 0. Petersons and W. E. Anderson, Trans. IEEE,IM-24, 336 (1975).

5. R. L. Nelson, Proc. IEE 121, 764 (1974).

6. J. Q. Shields, Trans. IEEE, IM-21, 365 (1972).

7. A. M. Thompson and D. G. Lampard, Nature 177,888 (1956).

8. Certain commercial materials are identified inorder to adequately specify the experimentalresults. In no case does such identificationimply recommendation by the National Bureau ofStandards, nor does it imply the material is thebest available.

CONCLUSIONS

It has been demonstrated that a high-voltage currentcomparator bridge of advanced design can be usedto acquire significant engineering data for thedesign of superconducting cable insulation. Thedissipation factor of a number of materials hasbeen measured at stresses up to 40 kV/mm. Measure-ments will soon be extended to experimentalconditions which closer approximate cable geometryand environment.

ACKNOWLEDGMENTS

The authors would like to acknowledge John Q. Shieldsfor his measurement of the tan 6 of our standardcapacitor. J. C. Palla's and E. D. Sims' assistancein the preparation of the manuscript is alsoappreciated.

54

n -+