Influence of Electrodes on D-C Breakdown in Gases at High Pressure

J. G. T R U M P R. W. C L O U D M E M B E R A I Ε Ε A S S O C I A T E A I Ε Ε

TH E I N V E S T I G A T I O N of the influence of electrodes on breakdown in gases was prompted by the steadily increasing importance of gases at high pressure as the

principal insulating medium for high-voltage apparatus. Particularly for Van de Graaff electrostatic generators and other direct forms of particle accelerators compressed-gas insulation has become essential to the attainment of high-energy particles in a compact and economic design. 1 It is likely that high-pressure gas insulation will gradually in­vade the realm of high-power engineering where high-voltage cables and transformers are the most obvious imme­diate applications.

In this study the d-c breakdown strength in the gap be­tween metallic electrodes shaped to produce a uniform elec­tric field was determined for a variety of gases over a pres­sure range from atmospheric to 400 pounds per square inch gauge. In the course of this work a marked difference be­tween the breakdown strength of aluminum and of stainless-steel electrodes at the higher pressures was observed, and this important fact became a particular point of inquiry. Measurements were made in air, in carbon dioxide, in ni­trogen, and in equal mixtures of nitrogen and carbon diox­ide, using electrostatically generated constant voltages up to 900 kv.

The voltage source was a Van de Graaff test generator with a 12-inch-diameter ter­minal contained in an 18-inch-diameter steel pressure tank. At the higher pressures in air this apparatus is capable of producing steady voltages of over 1,000 kv. The volt­age of the terminal was continuously measured by a gen­erating voltmeter 2 with an estimated absolute accuracy of =±=1 per cent.

In all cases the electrodes between which breakdown took place were shaped with the Rogowski 3 contour to secure an essentially uniform and maximum electric field in the cen­tral area of the gap. Their maximum diameter was six inches. Both the aluminum electrodes of 2S grade and the 304 stainless-steel electrodes were carefully machined, smoothed with successively finer emery, and finally buffed

J . G. T r u m p and R. W. Cloud are with Massachusetts Institute of Technology, Cam­bridge, Mass . ; J . G. M a n n is with Duke Power Company, Greenville, S. C , and E. P . H a n s o n is with the High Voltage Engineering Corporation, Cambridge, Mass.

T h e work described in this article was assisted by the joint program of the Office of Naval Research and the Atomic Energy Commission.

J . G. M A N N E. P. H A N S O N A S S O C I A T E A I E E

to a mirror finish. The interelectrode gap could be ad­justed by moving the upper electrode through an insulated screw-thread arrangement in the upper tank cover. The lower electrode was fixed to the high-voltage terminal of the generator. Only relatively high prebreakdown currents to the upper terminal were regularly measured, using a 0- to 20-microampere d-c ammeter.

The test gases were introduced into the main pressure vessel and served also as the insulating medium of the electrostatic generator. Since the selected electrode gaps were one-fourth inch, one-half inch, and three-fourths inch, the limitation in voltage in this system was always clearly defined by the gap itself. The generator tank was dried by evacuation overnight with dry ice in the vacuum line. Air and commercial nitrogen and carbon dioxide passed through dry silica gel insured a thoroughly dry gas system.

Sparkover voltage measure­ments continue to suffer from the arbitrary determination on the part of each experi­menter of a workable spark-over criterion and from the relatively wide variations in the reported results due both to the complex nature of the discharge and to the impor­tance of experimental factors. Even the selection of a specific average rate of sparking was found to be dependent on the direction in and rapidity with which the final value was ap­

proached. Influential factors include the size and shape of the electrodes, the smoothness and polish of the surfaces, and especially the amount and kind of electrical condition­ing used.

In order to secure stable testing conditions, it was found desirable to condition the electrodes by permitting limited sparkover at the highest gas pressure and with the 3/4-inch gap. For the stainless-steel electrodes this had the effect of gradually increasing the voltage which could be insu­lated, presumably because of the clean-up action of the dis­charges on the contamination of the electrode surface. The final voltages were usually about 50 per cent higher than the initial sparkover values and were attained after several hundred sparks. The aluminum electrodes showed relatively less improvement and reached their limiting volt­age more rapidly under conditioning by sparkover. The performance of the aluminum electrodes often could be impaired by permitting continued sparkover at the higher

961 NOVEMBER 1 9 5 0 :> Trump, Cloud, Mann, Hanson—Influence of Electrodes on D-C Breakdown in Gases

D-c breakdown measurements u p to 900 kv between metallic electrodes producing a uniform field and immersed in compressed gases show markedly higher values for stainless steel after conditioning by sparkover than for aluminum electrode systems. Sparkover voltage measure­ments were made in air, in carbon dioxide, in equal mixtures of carbon dioxide and nitrogen, and in nitrogen. The relative insulating strength of these gases diminished in the order listed, and a similar electrode dependence on higher pressure was shown in all of them.

voltages and pressures. This work confirmed some of the observations of Howell 4 and others that in the case of steel electrodes the ultimate sparkover voltage could be improved by conditioning with limited energy in the discharge and that the required amount of conditioning became less the more highly finished and highly polished the electrode sur­face was.

With conditioning completed, the voltage-breakdown measurements were made by raising the gap voltage in steps of about five per cent of the expected breakdown value. Each succeeding voltage level was held for three minutes until one or two sparks were obtained on the average in the 3-minute interval. This arbitrary criterion established the limiting voltage value for that gap. It was found that at pressures below about 100 pounds per square inch the spark­over voltage was more clearly defined and could be ap­proached more rapidly. At least three independent sets of

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6 0 0

4 0 0

Figure 1. Insulating strength in com­pressed air with stainless-steel and with aluminum elec­trodes producing uniform fields over 1/4-, 1/2-, and 3 /4 -

inch gaps

Pressure of Air in Lbs. per Sq. In. Gage

data were taken for each pressure and each gap in order to demonstrate the variations which might be expected in this phenomenon.

COMPARISON OF STAINLESS STEEL AND ALUMINUM ELECTRODES

FIGURE 1 compares the d-c breakdown strength of uniform electric fields in compressed air between stainless-steel

electrodes with that for aluminum electrodes. While sub­stantially the same insulating strength is realized at pressures below 100 pounds per square inch gauge, the stainless-steel electrode system exhibits progressively superior perform­ance with higher pressures. At 400 pounds per square inch about 50 per cent higher voltages could be insulated with stainless steel than with aluminum. Figures 2, 3, and 4 show the d-c breakdown strength for stainless-steel and aluminum electrodes in carbon dioxide, an equal mixture of nitrogen and carbon dioxide, and nitrogen. These curves incorporate the breakdown values for the several gaps which were studied, including 1/4-, 1/2-, and 3/4-inch lengths.

No dependence of any breakdown strength upon gap length or total voltage was observed, the breakdown gradi­ent for the several gaps being a constant at a given gas pres­sure within the limits of experimental error. The shaded area of each curve indicates the observed divergence in the breakdown values during the several experimental runs. Because of the complex and statistical character of the breakdown phenomenon, as well as its dependence upon local electrode geometry, surface contamination, and the degree of electrical conditioning, such divergence is recog­nized as an almost inevitable part of voltage breakdown in gases.

In comparing these curves, it should be remembered that, although the aluminum electrode systems received less conditioning by controlled sparkover than those of stainless

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CL 400

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0 100 200 300 400

Pressure of CO2 in Lbs per Sq. In. Gage

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- 1 0 0 0

8 0 0

6 0 0

4 0 0

2 0 0

100 2 0 0 3 0 0 4 0 0

Pressure of N2 and CO2 Mixture in Lbs. per Sq. In. Gage

Figure 2 (left). In­sulating strength in compressed CO2 with stainless-steel and with aluminum elec­

trodes

Figure 3 (right). In­sulating strength in equal compressed mixtures of N2 and CO2 with stainless-steel and with alumi­

num electrodes

962 Trump, Cloud, Mann, Hanson—Influence of Electrodes on D-C Breakdown in Gases ELECTRICAL ENGINEERING

steel, this process was in both cases carried to the extent required for maximum performance. Only in the case of nitrogen gas with aluminum electrodes did the prebreak-down current consistently show values in excess of two microamperes. It can be observed that the same depend­ence of breakdown strength upon electrode materials exists for these several gases, that the superiority of stainless steel is most marked in the case of air, and that at pressures above ten atmospheres and for systems that can be electrically con­ditioned it appears advantageous to use stainless steel in preference to aluminum because of the substantial improve­ment in voltage insulating strength.

I N F L U E N C E O F C A T H O D E A N D A N O D E E L E C T R O D E

M A T E R I A L

FIGURE 5 shows the variation of d-c sparkover voltage with pressure for a uniform field in air as a function of the

cathode and anode material. The upper and lower curves represent the average of the d-c sparkover characteristics shown in Figure 1 for all-stainless and all-aluminum elec­trode systems. The intermediate curves show the marked reduction of insulating strength resulting from a mixed gap of one aluminum and one stainless-steel electrode. This reduction was greatest when the cathode was aluminum and resulted in a sparkover characteristic close to, though generally slightly better than, that of an all-aluminum gap. With a stainless-steel cathode and an aluminum anode the sparkover pattern at lower pressures approached the per­formance of all stainless steel, but deteriorated at higher pressures.

Since it would be expected that the cathode would be of determining importance, several confirming check tests on this observation were made using freshly polished elec­trodes. Evidence 5 recently has been presented which states that the anode material is without influence on the break­down potential of a mixed gap at high pressure in which only small predischarge currents are permitted to flow. In any gap-testing procedure involving sparking, the cathode emis­sion characteristics may be modified by sputtered or evapo­rated anode metal. It is also very possible that the emission characteristics of the anode material could produce observ­able effects on both predischarge currents and the sparkover criterion.

In any event, although the cathode emission character­istics are believed to be the primary factor, it seems clear that in any practical measurement involving sparkover, the insulating characteristic at high gas pressures of the inter-electrode gap can also be significantly affected by the use of an inferior electrode material on the anode.

The relative insulating strength of uniform gaps between stainless-steel electrodes and between aluminum electrodes may be examined as a function of gas pressure as in Figure 6 or as a function of the gradient on the more limited elec­trode as in Figure 7. The insulating strength of a pressure gap is dependent on the α, β, and γ mechanisms developing primarily in the interelectrode space. The effect of higher pressures, by reducing the probability of these gaseous ioni­zation processes, is to permit the application of progressively higher gradients to the electrode surfaces, which may then become the principal limitation. From Figure 6 it can be

seen that the ratio of maximum gradients for the two metals increases steadily with pressure, that this ratio is different for the several gases and roughly proportionate to their relative insulating strength. Figure 7 shows more clearly that the limiting influence in the insulating strength of the gap is determined primarily by the electrode gradient. The increasingly upward slope of these curves suggests that a surface gradient is gradually being reached at which the most limited (aluminum) electrode is contributing expo­nentially to the sparkover of the gap. This contribution must come primarily from the cathode surface and consist of photoelectric, secondary, and high field emission. High field emission must be regarded as a probable mechanism even at gradients of 5 0 0 kv per inch since this average value is increased by localized projections and since the surface is

Figure 4. Insulat­ing strength in com­pressed N2 with stainless-steel and with aluminum elec­trodes producing uniform fields over 1/4-, 1/2-, and 3/4-

inch gaps

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Pressure of Νε in Lbs. per Sq. In. Gage

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•c 1000

% 800 > ο 5

• 600 2

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«////)

St. St. - and St. St. +

St. St. -and A l * A l - and St. St. + Al-and Al*

«////)

St. St. - and St. St. +

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St. St. - and St. St. +

St. St. -and A l * A l - and St. St. + Al-and Al*

St. St. - and St. St. +

St. St. -and A l * A l - and St. St. + Al-and Al*

St. St. - and St. St. +

St. St. -and A l * A l - and St. St. + Al-and Al*

Figure 5.

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Insulating strength in compressed air between elec­trodes of similar and dissimilar materials

NOVEMBER 1 9 5 0 Trump, Cloud, Mann, Hanson—Influence of Electrodes on D-C Breakdown in Gases 9 6 3

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4 0 0

Figure 6. Ratio of sparkover gradients on stainless-steel and on aluminum electrodes as a function of gas pressure

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φ 1.0,

-/ A i r

- N 2 / y C 0 2

1 ι 1 2 0 0 4 0 0 6 0 0 8 0 0 1000

Sparkover Gradient on Aluminum Electrodes

Figure 7 . Ratio of sparkover gradients on stainless-steel and on aluminum electrodes as a function of gradient on the aluminum

electrode

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800r

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C 0 2

COa+NaJ

S t St. electrodes

Air \ Al. CO* J electrodes N 2 — StSt. elec. COa+Nfj j

f electrodes N 2 J

0 100 200 300 400

Pressure of Gas in Lbs. per Sq. In. Gage

Figure 8. Comparative insulating strength of several gases at high pressures for uniform fields between stainless-steel electrodes

and between aluminum electrodes

necessarily contaminated by relatively low-work-function material.

These fields may also serve to enhance secondary and photoelectric emission to many times their ordinary value. 6 While the nature and number of gas molecules in the interelectrode space continue to affect the sparkover cri­terion, it appears that the nature of the electrode surface, particularly the emission coefficients of the surface of the cathode, become the controlling factors at the higher voltage gradients.

PERFORMANCE IN OTHER GASES

ACOMPARISON of the d-c sparkover strength in several gases for stainless-steel-electrode and for aluminum-elec­

trode systems over a pressure range up to 400 pounds per square inch is made in Figure 8. In addition to the marked superiority at higher pressures of the stainless-steel-elec­trode systems, these curves show that the highest insulating strength is obtained in air and progressively lower strengths are realized in carbon dioxide, in an equal mixture of car­bon dioxide and nitrogen, and in nitrogen—in the order in which they are named.

The observation of the superiority of air over the other permanent gases has often *been confirmed in studies with pressurized Van de Graaff accelerators. The advantage of nitrogen and carbon dioxide lies mainly in their chemical inertness, and it is believed that for most general appli­cations where a fire hazard exists a mixture of carbon di­oxide and nitrogen is desirable. Other studies have indi­cated little difference in the performance of Van de Graaff generators operating at a given total pressure for mixtures in which the carbon-dioxide content was varied from 20 to 80 per cent.

It is very probable that the influence of carbon di­oxide in improving the insulating strength of a given elec­trode system is due primarily to the electronegative char­acter of the oxygen atoms, although minor benefits are prob­ably obtained because of the larger collision cross section of this molecule. This study has not concerned itself with certain halogen compounds, such as Freon 12 (CC1 2F 2) and sulfur hexafluoride (SF 6 ), which are known to be distinctly superior to air at a given pressure, but which may have limi­tations where continuous ionization and sparkover would ultimately result in their chemical decomposition and con­sequent failure. 7

REFERENCES 1. Electrostatic Sources of Electric Power, J . G. T r u m p . Electrical Engineering, volume 66, J u n e 1947, pages 525-34.

2. Generating Voltmeter for Pressure-Insulated High-Voltage Sources, J . G. T r u m p , F. J . Safford, R. J . V a n de Graff. Review of Scientific Instruments (New York, Ν. Y.), volume 11, February 1940, pages 54-6 .

3. Die Ausbildung von Pruefelektroden noch Rogowski, Car l Stoerk. Elektrotechnische Zeitschrift (Berlin, Germany) , volume 52, May 1939, pages 43 -5 .

4. Breakdown Studies in Compressed Gases, A. H . Howell . AlEE Transactions, volume 58, May 1939, pages 193-204.

5. Electrical Breakdown in C O 2 From Low Pressures to the Liquid State, D . R. Young . Technical Report 22, Laboratory for Insulation Research, Massachusetts Institute of Technology, Cambridge, Mass., August 1949.

6. Ejection of Electrons by Ions at High Fields, R. R. Newton . Physical Review (New York, Ν . Y.) , volume 73, May 1948, page 1122.

7. Effect of High-Voltage Electrical Discharges on Sulfur Hexafluoride. W. C, S c h u m b , J . G. T r u m p , G. L. Pr ies t . Industrial and Engineering Chemistry (Easton, Pa . ) , volume 41 , Ju ly 1949, pages 1348-51.

964 Trump, Cloud, Mann, Hanson—Influence of Electrodes on D-C Breakdown in Gases ELECTRICAL ENGINEERING