IEEE Transactions on Electrical Insulation Vol. EI-18 No.6, December 198363

MEASUREMENTS OF BREAKDOWN POTENTIAL AND

IONIZATION AND ATTACHMENT IN SF6-C02 MIXTURES

Z. Y. Lee

Department of Electrical EngineeringHuazhong University of Science and Technology

Wuhan, China

ABSTRACT

The swarm coefficients (ionization and attachmentcoefficient) and sparking potentials have beenmeasured in SF6-CO2 mixtures over the range of60<E/P'110 V/mPa and gas pressures varying between0.7 and 75 Pa, gap spacings between 2 and 40 mm.The limiting values of E/P, (E/P)Zim, were obtainedby three methods: measuring E/P as a function of Pd;computing (a-n)/P as a function of E/P; and Debitetto-Fisher criterion. The rate of increase of theattachment coefficient with increasing percentageof SF6 in the mixtures was much larger than the rateof change of the first ionization coefficient.Addition of SF6 increased the sparking potentialunder uniform and non-uniform fields.

INTRODUCTION

Sulfhur hexafluoride (SF6) is an insulating mediumfor compact substation components and gas insulatedcables because of its high dielectric strength andgood heat transfer properties. Recently there is anincreased interest in the possible applications ofmixtures of SF6 and other common gases [1,2,3], be-cause mixtures are suitable particularly for applica-tions where low ambient temperatures could result inliquifying SF6 if operated at higher pressures; theuse of a mixture of SF6 and a cheap inert gas couldeliminate some of the problems associated with pureSF6 and reduce the insulation cost.

The design of improved gaseous dielectrics is beingpursued by controlling both the number and the ener-gies of free electrons. The number is reduced byattaching the electrons to molecules found to attachelectrons efficiently over a wide energy range. Thefree electron energies are controlled by other mole-cules, with two benefits: electrons are not only keptfrom attaching sufficient energy for ionization, butthey are forced into a lower energy range whereattachment is more efficient than at higher energies.Thus, improved gaseous dielectrics are designed asmulticomponent systems on -he basis of detailed,quantitative, fundamental knowledge. When a simplegas such as CO2 is used for electrical insulation,its electrical breakdown strength may be increased byaddition of SF6. Several investigations have beenmade concerning the breakdown characteristics of

SF6-CO2 mixtures. Measurements of static breakdownvoltages under sphere-plane electrode geometry forSF6-CO2 mixtures have been reported by Christophorouet al. [4] for a single value of Pd=100 Pa-m. Tothe author's knowledge, investigations into morebasic aspects of the ionization current growth and asystematic and extensive study of breakdown charac-teristics in the SF6-CO2 mixtures appear scarce.

The purpose of the present investigation therefore,is to accurately determine the fundamental swarmparameters Vs, a/P and n/P over the range 60'E/P`110V/m-Pa with gas pressures varying between 0.1 and 10kPa for vatious concentrations of SF6 and CO2 in theirmixtures and to study their effect on the insulationstrengths of the various mixtures. These are neededin order to understand the overall dielectric charac-teristics of SF6-CO2 mixtures leading to an optimaldesign of SF6-CO2 mixtures insulated systems.

EXPERIMENTAL APPARATUS AND PROCEDURE

The experimental system (Fig. 1) consisted of astainless steel vacuum chamber (volume about 80 Z)which contains two electrically isolated 15 cm diam-eter aluminum electrodes with Rogowski profiles. Oneof these electrodes was mounted on a movable platformwhich can be adjusted to produce any desired elec-trode spacing in the range 1 to 40 mm to an accuracyof 0.05%. For electrode spacings in this range theelectric field was uniform. The upper electrode(cathode) had a quartz window ground flush with the

O018-97.67/83/ 1200-0637S$01 .00 C. 1983 IEEE

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IEEE Transactions on Electrical Insulation Vol. EI-18 No.6, December 1983

surface, set into its central region. A thin layerof gold about 10 nm thick was deposited onto thecathode so as to overlap the quartz aluminum junction.When the layer was illuminated from the rear with UVlight, photo-emission of electrons occurred which pro-vided the initial cathode current. This method of ob-taining cathode emission had advantages over the pre-vious methods whereby the UV light illuminated thecathode through holes in the anode. Clearly, suchholes disturbed the electric field, particularly atsmall gaps. After evacuating the chamber to about0.4 mPa, the required mixtures were obtained first byadmitting the smallest constituent until the requiredpartial pressure was read on a digital multimeter whichwas connected to a pressure transducer (MKS Baratron),and then the second gas was added into the chamber togive the required total pressure. The chamber wasthen sealed and left to stand for one hour.

Fig. 1: Layout of the experimentaZ system

1 FZuke 410B Power Supply 5 EZectrodes2 InsuZators PA MK current meter3 UV Zamp DG Dia1Z gauge4 Chamber

In order to determine whether the gas mixing periodof one hour was sufficient, the following check wascarried out. A 90% SF6 + 10% C02 mixture was intro-duced into the chamber and left to stand for one hour.The measurements were then made at E/P=l10 V/m*Pa forspacings from 2 to 40 mm with steps of 2 mm. At eachelectrode separation (under the same E/P value) 3 to4 minutes were needed for pre-breakdown current stabil-ization, i.e. it took more than one hour to finish themeasurements. After one additional hour of gas mixingthe measurements were repeated and the results werefound to be in close (2%) agreement with those ob-tained previously, suggesting that adequate mixingwas obtained within one hour.

To obtain weaker mixtures, it was necessary to openthe cahmber after the mixing period and pump out apredetermined amount of gas. The gas pressures weremeasured to an accuracy better than 0.5%. SF6 withstated purity of 99.99% and C02 with purity of 99.98%were used throughout this study. The gap spacing wasmeasured to an accuracy of 0.01 mm by a dial gauge.

Applied voltages from a Fluke 410B 10 kV dc stabilizedpower source were measured using a digital voltmeterto within 0.5% while the ionization currents weremeasured to about 0.5% using a MK2 current meter.

The uv light was obtained from a Pen-Ray quartz lampof about 5 W output. The initial currents were foundto be stable to within 3% over the whole range ofparameters P and d studied. The ct/P and n/P valueswere evaluated using a linearized least-squares tech-nique by curve-fitting the modified Townsend equation[5]. Before and after using SF6-C02 mixtures in thechamber, to test the equipment, current growth curveswere obtained in N2 and the values of the ionizationcoefficient calculated from these curves were foundto be very close (1%) agreement with the data ofFolkard et al. [6]. The measurement of the staticbreakdown voltages Vs was done after conditioning theelectrodes by about 20 preliminary sparks. To avoidthe effect of the preceding spark on the followingone, the measurement was made after an interval ofmore than 4 minutes.

0-N .

~-cx

a.3

13

1151

105

98

90

58l

LIZI° 20 40 60 80 100

Sf:b (o)

Fig. 2: VaZues of ionization coefficient (a/P) as afunction of the concentration of SF6 in C02 forconstant vaZues of E/P

RESULTS AND DISCUSSION

Measurements of spatial growth of prebreakdown cur-rents were made at constant values of E/P over therange 60<E/P'll0 V/m Pa, over the range of values ofd but at a given pressure, in SF6-C02 mixtures, theconcentrations of the SF6 varying from 0 to 100%.The a/P and n/P values obtained, neglecting electrondetachment, are shown in Figs. 2 and 3. In pure SF6agreement with the a/P and ri/P data of Bhalla andCraggs is very close over the given range 60'E/P'110V/m Pa [5]. Also the o/P values in pure CO2 are ingood agreement with those of Bhalla and Craggs [7].From these data, it can be seen that the rate of in-crease of attachment coefficient- /P with increase inpercentage of SF6 in the mixtures is larger than therate of change of the ionization coefficient, especi-ally at low values of E/P. The ionization coefficientsappear to vary very little from one concentration ofmixtures to another, especially at high values of E/P.Over the range of gas pressures studied the ct/P andn/P coefficients both in SF6 and CO2 were found to be

Lee: Breakdown. Ionization, and Attachment in SF6-C02 Mixtures6

independent of gas pressure. It is sometimes impor-tant to know the variation of the effective ioniza-tion coefficient, (a-n)/P, of the gas mixtures as afunction of E/P around (E/P) ,im where (E/P)Zim is theE/P at which a/P=n/P, and below which breakdown doesnot occur. Fig. 4 shows the relationship between(a-qJ/P and E/P. The values of (E/P) Zim for variousSF6 concentrations in the mixtures are given inTable 1.

Table 1

Values of (E/FP Zim (V/m*Pa) for variousconcentrations of SF6 in C02

%° 5i6 o1 30 50 70 90 100

o(btsVpevdfE/Pro61 70 7B.9 84,6 88

pLots (Fi9.u)

obtai ned fromIoIp(Lot9iosobtcin4d frOmVs 4 4.3 62mAsare"ntLs(TE 9:

4s. 5 61I. 68.8 '774 84.8

7o.3 79.7 86a.5

88

88.7

Under the conditions where ca/P=q/P the currentgrowth equation in a uniform field is given byDebitetto and Fisher [8] as

E/p (V/m r') I = Io (1+ad).

Fig. 3: VaZues of eZectron attachment coefficient(n/P) as a function of E/P1 SF62 90% SF63 70% SF6

Cy.

9

-0

'A

4 50% SF65 30% SF66 10% SF6

That is, a plot of I vs d is a straight line at

(E/P)lZim It is shown by Geballe and Reeves [9] thatin attaching gases this condition occurs at the limit-ing E/P, i.e. (E/P)Zim. Using the above criterionthe (E/P) Zim for various concentrations of SF6-CO2mixtures is determined and the results are given inTable 1.

pd(p. -m)

Fig. 5: VaZues of E/P as a function of Pd

4: VaZues of (a--n)/P as a function of E/P

SF690% SF670% SF6

4 50% SF65 30% SF66 10% SF6

(1)

Fig.

123

123

SF690% SF670% SF6

4 50% SF65 30% SF66 10% SF6

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IEEE Transactions on Electrical Insulation Vol. EI-18 No.6, December 1983

voltages depend on the polarity of the applied voltage.Generally corona threshold is lower when the highlystressed electrode is the cathode, while breakdownvoltage is lower when the highly stressed electrodeis the anode. However, the breakdown voltages areabout 5% higher in positive sphere-plane gaps than innegative sphere-plane gaps for the Pd range studied.Obviously, before drawing any firm conclusions, itwill be necessary to extend these measurements tohigher Pd values.

0 67 733Pd (jp * TR)

Fig. 6: Breakdown voZtages versusconcentrations of SF6 in C021 CO22 10% SF63 30% SF6

>)

io0

Pd for various

4 50% SF65 SF6

(+) 30 mm diam. sphere-plane

(-) 30 mm diam. sphere-plane

o L. G. Christophorou

Sparking potentials are also measured in SF6-CO2mixtures under uniform fields, the concentration ofSF6 varying from 10 to 100%. The values of E/P as a

function of Pd for various concentrations of SF6 are

shown in Fig. 5 which shows that as Pd increases thevalues of E/P fall rapidly at first, then graduallyand eventually approach limiting values. For compari-son purposes the limiting values of E/P obtained bythis method are also included in Table 1. The resultsgiven in Table 1 show that good agreement within 3% isobtained, confirming the accuracy of evaluation ofa/P and n/P. The maximum dielectric strength for a

gaseous insulator is obtained when electrode configur-ations produce uniform distribution in the gap. How-ever, in most of the practical applications, non-

uniform field gaps are unavoidable. As mentionedabove, measurements of breakdown voltages under sphere-plane electrode geometry for SF6-CO2 mixtures have beenreported for a single value of Pd=100 m-Pa. In presentstudy, more measurements have been done in a 30 mm

diameter sphere-plane gap and a 1 mm diameter rod-rodgap. The results are shown in Figs. 6 and 7. Thevalues at Pd=-100 m-Pa are in very good agreement withthose of Christophorou. The results show that theaddition of SF6 by 10% by volume can increase thebreakdown voltage of C02 by about 35% for rod-rod gap.With addition of SF6 above 20% the rate of increaseof Vs will be reduced. In non-uniform field, breakdown

Fig. 7: VaZues of Vs versus Pd for various concentra-tions of SF6 in C02

1 C022 10% SF63 30% SF6

4 50% SF65 SF6

30 mm diam. sphere-sphere---------- 1 mm diam. rod-rod

CONCLUS IONS

Ionization and attachment in SF6-CO2 mixtures inswarm conditions have been studied. The values ofa/P and n/P in the range 60'E/P'll0 V/m.Pa were com-

puted. The rate of increase of n/P with increase inpercentage of SF6 in the mixtures is larger than therate of change of a/P. With agreement within 3% thevalues of (E/P)Zim have been obtained by threemethods for various concentrations of SF6 in 002. Theresults confirm the accuracy of evaluation of n/P andq/P. Measurements of sparking potentials under uni-form and non-uniform fields have been done over therange of 6 to 200 m-Pa. Indications are that a slightsaturation effect exists.

5

10

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5

7 1315pd ( p6 m)

()

640)

_,I,

Lee: Breakdown. IoniZation, and Attachment in SF6-C02 Mixtures

REFERENCES

[1] L. G. Christophorou, D. R. James, R. Y. Pai,M. 0. Pace, R. A. Mathis and D. W. Bouldin, "HighVoltage Research (Breakdown Strengths of Gaseousand Liquid Insulators)". Oak Ridge NationalLaboratory, Report ORNL/TM-5604, 1976.

[2] N. H. Malik and A. H. Qureshi, "A Review of Elec-trical Breakdown in Mixtures of SF6 and otherGases". IEEE Trans. on Electr. Insul., Vol. EI-14,No. 1, 1979, pp. 1-13.

[3] 0. Farish, "Breakdown in SF6 and its Mixtures inUniform and Nonuniform Fields". Proc. of theFirst Inter. Symposium on Gaseous Dielectrics,Knoxville, Tennessee, 1978.

[4] L. G. Christophorou, D. R. James, R. Y. Pai,M. 0. Pace, R. A. Mathis and D. W. Bouldin,"High Voltage Research (Breakdown Strengths ofGaseous and Liquid Insulators)". Oak RidgeNational Laboratory, Report ORNL/TM-5917, 1977.

[6] M. A. Folkard and S. C. Haydon, "ExperimentalInvestigations of Ionization Growth in Nitrogen".J. Phys. B: Atom. Molec. Phys., Vol. 6, 1973,pp. 214-226.

[7] M. S. Bhalla and J. D. Craggs, "Measurements ofIonization and Attachment Coefficients in CarbonDioxide in Uniform Fields". Proc. Phys. Soc.LXXVI, 3, 76, 1960, pp. 369-377.

[8] D. J. Debitetto and L. H. Fisher, "Second Town-send Coefficient in Oxygen at High Pressures".Phys. Rev. 111, 1958, pp. 390-394.

[9] R. Geballe and M. L. Reeves, "A Condition onUniform Field Breakdown in Electron-AttachingGases". Phys. Rev. 92, 1953, pp. 867-868.

Manuscript was received 23 December 1982, in revisedform 7 July 1983.

[5] M. S. Bhalla and J. D. Craggs, "Measurement ofIonization and Attachment Coefficients in SulphurHexafluoride in Uniform Fields". Proc. Phys. Soc.,Vol. 80, 1962, pp. 151-160.

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