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THE DTSUIATIKG STRENGTH OF GASES AT HIGH EREGSUBES John G. Trump Department of E l e c t r i c a l Engineering MaesachuBetts I n s t i t u t e of Technology Cambridge, Mass. The increasing use of gaseous d i e l e c t r i c s at elevated pressures i s ore of the s i g n i f i c a n t trends i n high-voltage engineering today. Such compressed gases can he superior t o s o l i d and l i q u i d d i e l e c t r i c s and t o high vacuum ir i s u l a t i o n , p a r t i c u l a r l y f o r long gaps and high voltages. I n the passage of e l e c t r i c charge between electrodes immersed i n a gas, the TownsendoC process i s the most proDific source of ioniza- . t i o n . Unless removed by attachment t o a gas molecule, each free electron i n an interelectrode space subject t o a s u f f i c i e n t l y high e l e c t r i c f i e l d i s accompanied by an exponentially growing number of electrons produced by c o l l i s i o n s as i t moves the remaining distance t o the anode. Concur- r e n t l y , certain processes renew the free electrons on the cathode side of the gap. These regenerative sources of i o n i z a t i o n include photoelectric emission from the cathode and photoionization of the gas as the excited and ionized gas atoma radiate t h e i r energy, and secondary electron emission from the cathode under bombardment by the positive ions cleared from the gap by the e l e c t r i c f i e l d . At products of pd (pressure x gap length) higher than about 500 mm Hg X cm, there i s an increasing tendency f o r theoC ion i z a t i o n process t o proceed as independent avalanches which a t t a i n progressively higher ion densities. An increase i n gas pressure reduces the mean free path between electron c o l l i s i o n s and hence permits an increased voltage across the gap. I n a uniform f i e l d the sparkover voltage increases l i n e a r l y with pressure up t o about ten atmospheres and then more slowly. This i n i t i a l l i n e a r i t y between voltage and pressure suggests that the factors contributing t o - ward e l e c t r i c a l i n s t a b i l i t y are not sensitive, at moderate pressures, on the t o t a l number of io n i z a t i o n events occurring between the electrodes. At higher pressures the cumulative e f f e c t of the increased ionization i n the gap and the growing importance of new ioni z a t i o n mechanisms, p a r t i c - u l a r l y at the cathode, account f o r the gradual diminution i n the rate of voltage increase with pressure. These additional mechanisms include high f i e l d emission from the cathode, field-enhanced photoelectric and secondary emission from the cathode, as well as increased photoionization of the gas. The diminished a b i l i t y of the positive ions t o diffuse out of the active f i e l d i s also i n f l u e n t i a l at higher pressures. Since the investigations of ITatterer there has been a growing awareness that many compounds i n the gaseous state, p a r t i c u l a r l y those which contain chlorine, f l u o r i n e , bromine, oxygen, and other electroneg- ative atoms, may have s i g n i f i c a n t l y higher insulating strength than a i r at
- h - the same pressure. The d i e l e c t r i c superiority of these molecules arises from t h e i r a b i l i t y to'remove free electrons hy attachment. These electro- negative molecules may also exert t h e i r influence by extracting a port i o n of the energy of incident electrons; these compounda are usually of higher molecular weight and complexity and thus o f f e r increased opportunity f o r i n e l a s t i c c o l l i s i o n s . Figure 1 shows the insulating strength i n a uniform f i e l d of a va r i e t y of gases from Ee t o SF5 as a function of gas pressure. These constant p o t e n t i a l sparkover measurements by Kusko extend t o higher t o t a l voltages and higher products of pressure x gap than have been reported heretofore. Lowest insulating performance i s obtained with noble gases such as helium. Nitrogen, the most stable and i n e r t of gas molecules, exhibits a d i e l e c t r i c strength s l i g j h t l y lower than that of a i r . The better performance of CO2 over Wg may be accounted f o r by incident electrons and t o attach them, thus forming limocuous negative ions, \lhen the pressure of CO2 was Increased t o 60O p s i at which saturation occurs at 20^0, no discontinuity between the e l e c t r i c breakdown strength of gaseous CO2 and l i q u i d CO2 was observed. At elevated pressures the breakdown voltage may be observed t o increase as much as 100^ a f t e r a number of interelectrode sparks of l i m i t e d energy, Howell and others have ascribed t h i s improvement under controlled sparkover t o the conditioning of the electrode surfaces by the reduction of high f i e l d emission areas such aS'might be caused by surface i r r e g u l a r i t i e s . - I f the surface roughness i s reduced by highly b u f f i n g the electrodes, conditioning i s accomplished with much less sparking. Differences i n reported breakdown values i n gases have been found t o vary widely among observers, depending on such factors as: the surface condition of the electrodes, the degree of conditioning, the electrode area, the electrode geomertry, and the c r i t e r i o n f o r breakdown.
- 5 - 1600 1400 X o 1200 UJ 5 Q 1000 800 UJ S 600 < I 400 < UJ m 200 0 He STAINLESS STEEL ALUMINUM 1 From A Kusko . M I T 1951 1 0 200. 1000. 1200 400 600 . 800 GAS PRESSURE, PSI GAGE DC BREAKDOWN GRADIENTS IN VARIOUS GASES AND MIXTURES FOR 0.5" UNIFORM GAP BETWEEN STAINLESS STEEL AND ALUMINUM ELECTRODES
