Vacuum insulation for cryogenic cables. A brief review

Vacuum insulation A brief review P Graneau,

for cryogenic

Simplex Wire and Cable Company, Cambridge, Massachusetts,

cables. USA

Vacuum-tight pipe enclosures are an essential feature of any cryogenic cable and the added advantage of the thermal insulation afforded by an evacuated space makes the use of a vacuum dielectric for high voltage insulation an attractive economic proposition. The areas of fundamental and applied research which might contribute significantly to fhe technology of cryogenic cables are outlined. Dielectric coatings on electrodes, the gas effect, impluse breakdown, and solid insulators in vacuum are discussed. 1. Introduction

The benefits derivable from certain thin dielectric layers deposited on the electrode surfaces, as revealed by Jedynak6 and Rohrbach3, demand much deeper study. Field emission currents across the cable insulation represent a dielectric loss, and the resulting heat has to be removed, at considerable cost, with refrigerators. What is worse, this loss occurs also with dc voltages when conventional dielectrics are virtually loss free. Anodizing of an aluminium cathode is sufficient to suppress the cold emission current by several orders of magnitude. It remains uncertain whether this is due to modifications of the microscopic field distribution or the scarcity of emission electrons in the dielectric coatings. Only a few barrier materials, such as epoxy films and anodic layers, have been found suitable for vacuum insulation and are compatible with the low substrate temperature of cryogenic conductors. It is remarkable that sparking through the cathode coating does not destory its effectiveness until irreversible damage occurs on the underyling metal surface. Experience with electrostatic separators suggests that a certain amount of sparking through the cathode covering is always beneficial. The findings of Powell et al’ are significant in that they focus attention on the trapping of adsorbed gas between coating and electrode metal.

High-voltage vacuum insulation has been proposed for use in low temperature transmission lines of electrical energy1’2. As the thermal insulation of such lines will, in any case, call for vacuum-tight pipeenclosures, there exist distinct cost advantages favouring the vacuum dielectric. Its low permittivity and loss factor are of particular interest in the development of ac cables. Besides, vacuum is a sufficiently good thermal insulator for isolating two coolant streams. In this latter respect it has no rival amongst the low temperature dielectrics so far investigated. Vacuum insulation already plays an important role in the field of electrical energy transmission through the application of mercury arc rectifiers and vacuum circuit breakers. These two examples differ from a high-voltage cable in that they require a dielectric which at times can sustain the flow of large electronic and ionic currents. In an effectively sealed cable system employing contamination-free pumps, the surfaces of conductors and solid insulators need not undergo any form of structural change with time and this should enhance stability, so often found lacking in other applications of high-voltage vacuum insulation. There is reason to believe’* that cold electrode and insulator surfaces confer more advantages than disadvantages. The purpose of this paper is to sketch out areas of fundamental and applied research which might contribute significantly to the technology of cryogenic cables. 2. Dielectric coatings on electrodes Much of the work carried out on electrostatic particle separators3’4*10’11is relevant to low temperature transmission lines. The size of separator electrodes, some electrode materials and coverings, gaps up to 10 cm, air-to-vacuum bushings and the range of voltages applied, are all factors in separator research not too far removed from the requirements met in cryogenic cables. An essential difference from separator practice may be that sparking should never be allowed in a cable dielectric, either during high-voltage conditioning or in service. Any semi-permanent change of the conductor surface endangers long-term stability. On the other hand, it seems feasible to operate the cable insulation at a stress level below the onset of appreciable field emission currents, microdischarges and Xradiation. This condition is enforced by the need of the cable to withstand lightning and switching surges which are far greater than the transmission voltage. Vacuum/volume

W/number 7.

Pergamon Press LfdlPrinfed

3. Gas effect In recent years it has been demonstrated that the control of residual gas composition and pressure is capable of lowering pre-breakdown currents and lifting sparking potentials. A relatively high pressure, in the range lo-” to lo-* torr, of certain inert gases appears to improve the insulation. The limited experience gained in exploiting this effect, unfortunately, contains some indication that it may not be permanent. At present the phenomena responsible for the gas effect are not we11understood, and it is therefore difficult to speculate about its permanency in the sealed and cold space of a cryogenic cable dielectric. In analyzing the gas effect, Chatterton concluded that with large gaps it is more likely to be the result of stripping negative charges off hydrgoen ions in collision with neutral gas molecules, rather than the removal of electron emitting asperities at the cathode by positive ion bombardment. Chatterton and Cooke’ and Rohrbachs stressed the dependence of the gas effect on gap length which is difficult to reconcile with the sputtering theory. Cooke made the interesting suggestion that the gas effect could

in Great Britain

395

P Graneau: Vacuum

insulation

for cryogenic

cables

be enhanced by covering the cathode with a low emission coating. His hypothesis is that increasing the gas pressure would continue to raise the breakdown voltage, were it not for the onset of glow discharges, which is dependent on the ymechanism of secondary electron production. Systems employing coated electrodes in a controlled dilute gas atmosphere have, as yet, receieved very little attention. 4. Impulse Breakdown DenholmO discovered ten years ago that, in contrast to common cable dielectrics, the 50 c/s ac breakdown strength of a small vacuum gap was greater than the dc strength. Even if Denholm’s results prove to be characteristic only of small gaps, prospects must be good of insulating cryogenic cables at ac stress levels comparable with the dc insulation levels achieved in electrostatic particle separators. There is an urgent need for research on the ac power frequency performance of 1 to 10 cm gaps. According to Denholm’s findings, the development of vacuum sparks takes long enough to produce significant differences in the breakdown 1eveIs of gaps subjected to transient voltages. He went on to show that the breakdown strength to pulses of 62 psec duration is greater than that obtained with 50 c/s sinusoidal waves. This behaviour is being used by Smith and MasoP in the development of an impulse particle separator. They measured the time lag to breakdown on a two centimetre gap. With a pulse having a 4 psec front, time lags of the order of 20-30 psec were observed. No attempts have been made to delay the build-up of the discharge by electrodecoatings, control over residual gas or cooling of electrodes. These factors may hold the key to improvements of the impulse strength of vacuum insulation. 5. Insulators Load bearing dielectric supports for conductor spacing and terminal bushings are essential parts of a vacuum insulated cryogenic cable. Conductor spacers have to be mechanically strong and operate in a cold environment and unusually high electric fields. There is little doubt that the spacers will be the weakest points of the insulation system, and on this basis they should attract the greatest research effort. Two centimetre long Pyrex glass stand-off insulators were studied by Shannon et aP. This group concluded that dc insulation levels of 80 kV/cm should be achievable. Their experiments were mainly concerned with insulator profiles and spark conditioning. Watson and ShannotPextended the investigation to pulsed flashover of conical glass insulators and found a steep dependence on cone angle and polarity. With nanosecond pulses they achieved average breakdown stresses of 200 kV/cm and concluded that sparks were initiated by thermionic emission of electrons from the glass which leaves the insulator surface

393

charged positively. This prompts many speculations about the ac performance of cold glass dielectric supports. Metal shielding of the cathode-dielectric-vacuum triple junction is an important aspect of insulator design. It has been discussed by Finke15 and forms a feature of most vacuum bushings”. There is much scope for experimental research on shielding, if only to resolve whether the beneficial effects are due to field modifications or the interception of ion trajectories. In ac applications, shielding might possibly be combined with capacitive voltage grading and this suggests yet another topic of research.

References ‘A H Powell. R J Slauehter and D R Edwards. “An Aoonrarus for Investigating 5b c/s Voltage Pre-Breakdown Electron Emissionin Vacuum Insulation at Low Temperature,” International Journnl of Electronics, 21, 393 (1966). 2 (a) W T Norris and D A Swift, “Developments Augur Design of Superconducting Cables” Electrical World, July 24, 1967, p. 50. (b) “Practical Superconducting Cable Designs”, Electrical Review, Feb 3, 1967, p 155. s F Rohrbach, “Some Studies of High Voltage Vacuum Breakdown Across Large Gaps: Investigation of the Properties of Oxide-Coated Aluminium Electrodes,” CERN Report 64-50, NPA Division, Nov 1964. 4 C Germain, L Jeannerot, F Rohrbach, D Simon and R Tinguely, “Technological Developments of the CERN Electrostatic Separator Programme,” Proceedings of the Second International Symposium on Insukztion of High Voltages in Vacuum, MIT, Sept, 1966 p 279. 5 L Jedynak, ” Vacuum Insulation of High Voltages Utilizing Dielectric Coated Electrodes,” Journsl of Applied Physics, 35, 1721 June 1964. e P A Chatterton, “The Effect of Gas Species and Concentration on Prebreakdown and Breakdown Phenomena,” Proceedings of the Second International Symposium on Insulation of High Voltages in Vacuum, MIT, Sept 1966,~ 195. ’ C M Cooke, “Residual Pressure and its Effect on Vacuum Insulation,” Proceedings of the Second International Symposium on Insulation of High Voltages in Vacuum, MIT, Sept 1966,~ 181. e F Rohrbach, “Pre-Breakdown Currents and Microdischarges Across Large Gaps in Clean Vacuum,” Proceedings of the Second International Symposium on Insulation of High Voltages in Vacuum, MIT, Sept 1966, p 83. 9 A S Denhohn, “The Electrical Breakdown of Small Gaps in Vacuum,” Canadian Journal of Physics, 36, 476, (1958). lo W A Smith and T R Mason, “Preliminary Measurements of Time Lags to Breakdown of Large Gaps,” Proceedings of the Second Internationai Symposium on Insulation of High Voltages in Vacuum, MIT, Sept 1966~ 97. I1 K D Scrivastava and R G Fowler,” Demountable Air-to- Vacuum Input Bushing,” Rutherford High Energy Laboratory, Report No RHELIM-10 July 1965. I2 D J Degeter, “Cryogenic Cooling Reduces High Voltage Arcing Between Electrodes Operating in a Vacuum, ” AEC-NASA TECH BRIEF 66-10499, Nov 1966. Is J P Shannon, S F Philp and J G Trump, “Insulation of High Voltage Across Sohd Insulators in Vacuum,” Journal of Vacuum, Science and Technology, 2, 234, (1965). I4 A Watson and J P Shannon, “Pulsed FIashover in Vacuum,” Proceedings of the Second International Symposium on Insulation of High Voltages in Vacuum, MIT, Sept 1966, p 245. I5 R C Finke, “A Study of Parameters Affecting the Maximum Voltage Capabilities of Shielded Negative Dielectric Junction Vacuum Insulators,” Proceedings of the Second International Symposium on Insulation of High Voltages in Vacuum, MIT, Sept 1966, p 217.

Dielectric strength of polymers temperatures under vacuum”

at cryogenic

Rigidit di6lectrique des polym&es cryogBniques sous vide

aux tempgratures

J Bobo

Route de Nozay 91, Marcoussis,

B Fallou France

and M Perrier, Centre de Recherches de la Compagnie G&&al and J Galand,

Laboratoire Central des industries

d’Electricit4,

Electriques, 33 Avenue du GtWral

France

Leclerc, 92 Fontenay aux Roses,

Dielectric strength of solid insulation (especially polymers) when measured in a cryogenic medium (gas or liquid) is found to be highly dependent upon pre-breakdown discharges. These discharges may act in two different ways. On the one hand, especially when a fluid like helium. (whose dielectric strength is poor) is concerned, discharges may extend very far from the electrodes and thus involve high electric stresses upon large areas of the samples under test. In extreme cases, these discharges can even produce flash-over. On the other hand, discharges may lead to important deterioration and consequently localised reductions in dielectric strength. In order to reduce the effect of these discharges on the apparent dielectric strength observed in cryogenic media, the behaviour of solid insulation under high vacuum was studied. Special care was taken to minimize the effects of pre-breakdown discharges. A large increase of dielectric strength was observed in relation to certain polymers, especially polyethylene-terephthalate. The results are in good agreement with values published by different authors as intrinsic strength values. It is concluded that high vacuum may afford the solution to many insulation problems occurring at cryogenic temperafures. 1. Introduction

1. Introduction

In liquid helium, the values of dielectric strength of solids which were measured’ are low compared with those obtained at normal temperature in various liquids, as for example, in transformer oil. It is to be expected, according to measurements of intrinsic dielectric strength such as those published by Ball*, that the dielectric strength of non-polar materials will be approximately constant below a certain temperature, but that those of polar materials will increase with reduced temperature. Considering the low dielectric strength of liquid helium, it might be thought that prebreakdown discharges, produced in this fluid by a relatively low electric field, would be responsible for the low values measured in samples immersed in that medium.

Dans l’helium liquide, les valeurs de rigidite dielectrique des solides que nous avons mesun% sont faibles par rapport a celles obtenues a temperature ambiante dans divers milieux liquides comme par exemple l’huile de transformateur. Or, on peut s’attendre, d’apres les mesures de rigidite di&ctrique intrindque, telles que celles publ&s par Ball*, a ce que la rigidite dielectrique des materiaux apolaires soit approximativement constante en dessous dune certaine temperature, mais que celle des materiaux polaires croisse lorsque la temperature diminue. Compte tenu de la faible rigidite dielectrique de l’helium liquide, on peut penser que des d&charges predisruptives, produites dans ce fluide par un champ electrique relativement faible, soient responsables des faibles valeurs mesurees sur des echantillons solides immerges dans ce milieu.

2. Effect of partial discharges at normal temperature As a first step, we studied the effects of these discharges in two fluids, transformer oil (resistivity of 5 x 10XL cm and dielectric strength of the order of 30 kV/mm) and a creosote base liquid (with a low resistivity of 2 x 1OQ. cm and a dielectric strength of the order of 25 kV/mm at normal temperature), with the same experimental arrangement as that employed later under vacuum. The electrodes consist of two carefully polished stainless steel spheres, 30 mm in diameter, which are placed on each side of the sample under a minimal pressure to put them in contact with it. An alternating 50 Hz frequency voltage is applied between those electrodes and progressively increased at the rate of 500 V/s. The samples are films of polymer 50 microns thick: polyethylene-terephthalate (Mylar), polyamide 11 (Rilsan), polytetrafluoroethylene (Teflon FTFE), tetrafluoro*This study was conducted

Vacuum/volume

within the framework

W/number 7.

of a research programme

Pergamon Press LtdfPrinted

2. Infiuence des d6charges partielles ti temperature ambiante

Dans une premiere &ape, nous avons CtudiC l’itiuence de ces d&charges dans deux fluides, huile de transformateur (resistivite 5 x 10” a. cm et rigidite dielectrique de l’ordre de 30 kV/mm) et liquide, a base de creosote, de faible resistivite 2.1OQ cm, et de rigidite dielectrique de l’ordre de 25 kV/mm, a temperature ambiante, avec le m&me dispositif experimental que celui utilise ensuite sous vide. Les electrodes sont deux spheres d’acier inoxydable, de 30 mm de diametre, soigneusement polies; elles sont plac6es de part et d’autre de l’echantillon, en appliquant la pression minimale pour maintenir celui-ci. On applique une tension alternative, de frequence 50 Hz, en augmentant regulibrement sponsored

by the DBkgation

in Great Britain

G&kale

&la Recherche

Scientifique

et Technique.

397

J Bobo, M Perrier, B Fallou and J Galand: ethylene hexafluoropropylene copoIymer mide (Kapton) and layered kapton-teflon The

results

of measurements

are given

Dielectric (Teflon

strength

FEP),

(Kapton

FM).

in Table

I.

polyi-

Table 1.

Mylar

Average dielectric strength MV/cm peak amplitude ambient medium: ambient medium: transformer oil low resistivity fluid ____~_____-.--... El Ez ~. 2.7 5.8 ~. -__

Rilsan

2.6

PTFE

3.0

FEP ~__ Kapton

Materials __~

Kapton

FM

-~

3.7

E&

-.-.

1.4 _.~ 1.4

2.0

2.5

1.2

3.1

4.4

1.4 .-

___~ 7.7

1.8

Each value in this table is the average of ten measurements. For a mylar film of 50~~ for example, the following values are obtained (Table 2). Table 2. Material

Ambient medium Transformer oil

Mylar 50#

_ Creosote

--____ _ Breakdown voltage (kV r.m.s) 10 10.1 10.1 11.4 10.6 8.7 9.3 8.1 8.1 7.9 Average value 9.5 .~__ 20.3 20.6

20.3 20 20.3 20.3 21.6 20.5 Average value 20.5

20.6 20.3

It is found, by making a measurement of partial discharges in a conventional way3, in the fluid with low resistivity, that there is no partial discharge of amplitude greater than IO PC, up to breakdown; in transformer oil, for voltages above 75 per cent of the breakdown voltage, discharges, the amplitude of which can attain 60 PC, may be observed. These observations have been made for Mylar but should be valid for other materials. The experiments show the very important role played by partial discharges in lowering the dielectric strength of these films immersed in liquids of high resistivity. These discharges can act in two different ways: on the one hand they produce very rapid significant degradation of solid materials, especially in the case of polymers which have a low erosion resistance and, on the other hand, they can, by creating superficial high charges at the surface of the solid insulating materials, locally increase the field strength they are submitted to and therefore allow breakdown to occur at relatively low voltages between the electrodes.

3. Influence of discharges at cryogenic temperatures 3.1 Experimental apparatus. Figure I shows the cell used for measurements of dielectric strengths under vacuum at cryogenic temperatures. The cylindrical vessel, evacuated down to lo-’ torr, is immersed in a bath of liquid helium which, by conduction, ensures cooling of the film and electrodes. A remote control system permits unwinding of the insulating strip between the high voltage electrode and the facing one. Figure 2 shows the electrodes. An arrangement enables the assembly to be moved up and down in the cryostat. Temperature measurement of the film in contact with both electrodes is made by two thermocouples. This device allows one hundred measurements to be made on a length of 5 to 6 metres of film. 398

at cryogenic

temperatures

under vacuum

la tension d’essai B la cadence de 500 V/s. Les &hantillons sent des films de polymlires de 50 microns d’kpaisseur: polytCrCphthaIate d’Cthyl&e (Mylar), polyamide I1 (Rilsan), polyt&afluoroCthyl&ne (TCflon PTFE), copoIym&e GtrafluoroCthyBne-hexafluoropropyBne (T&on FEP), polyimide (Kapton) et stratifiC kapton-tbflon (Kapton FM). Les rCsultats des mesures sont don&s dans Ie tableau I : Chaque valeur de ce tableau reprCsente la moyenne de dix mesures. Pour un film de mylar de 50 p, par exemple, on obtient les valeurs suivantes (tableau 2).

2.2

4.2

3.2

of polymers

On a constat& en utilisant un mesureur de d&charges partielles cIassique3, que dans Ie fluide g faible rCsistivitC, il n’apparaissait jusqu’a Ia disruption aucune d&charge partielle de charge apparente supkrieure g 10 PC; alors que, dans I’huile de transformateur, on observe, pour des tensions supkrieures g 75 pour cent de la tension disruptive, des dCcharges dont la charge apparente peut atteindre 600 PC. Ces observations ont et6 faites dans Ie cas du Mylar mais doivent &tre valables pour les autres matkriaux. Ces expkriences mettent en Cvidence le r8Ie t&s important des dCcharges partielles dans l’abaissement des rigidit& diCIectriques de ces films, immergCs dans des Iiquides de grande r6sistivitC. Ces dCcharges peuvent agir de deux facons diffkrentes: d’une part elles provoquent t&s rapidement une dkgradation importante des matCriaux solides soumis B leur action, particulikrement dans le cas des polymkres qui prksentent une faible rCsistance B I’Crosion, d’autre part elles peuvent, en creant & la surface des isolants solides des charges superficielles importantes, augmenter Iocalement la valeur du champ Blectrique auquel ils sent soumis et favoriser ainsi la rupture pour des tensions relativement faibles entre les Electrodes.

3. Influence des dkharges

aux temperatures cryogbniques

3.1 Appareillage. La figure I reprisente la ceIluIe permettant la mesure des rigidit& diBIectriques sous vide aux tempiratures cryogkniques. L’enceinte cylindrique, dans Iaquelle on rCaIise un vide de lo-’ torr, est plongee dans un bain d’helium Iiquide qui assure, par conduction, Ie refroidissement du film et des Clectrodes. Un systeme de commande B distance permet de faire d&filer Ie ruban isolant entre I’Clectrode haute-tension et la contre-Clectrode. La figure 2 represente ces Electrodes. Un dispositif assure Ie d&placement de l’ensemble dans Ie cryostat. Une mesure de la tempkrature du film au contact des deux electrodes est effectuCe g I’aide de deux thermo-couples. Cette cellule permet de faire une centaine de mesures sur une Iongueur de 5 B 6 m&es de film. 3.2 Exptkimentation. Cette etude a Ctt? faite sur Ies mat6riaux solides prCc&demment @it&s, dans I’azote liquide, dans I’hCIium Iiquide et sous vide B 4,2”K. 11 est difficile de comparer directement les caractCristiques diClectriques de ces 3 milieux; mais un ordre de grandeur de leurs propriCtCs respectives, dans des conditions voisines de celles utiIi&es dans Ies experiCnces considCr&s, est don&e sur la planche I en fonction de 1’Ccartement entre Ies Clectrodes. La courbe relative & I’azote liquide g 77°K est celle publiCe par Mathe+; elle a cte obtenue en utilisant 2 sphkres en acier de