NBSIR 80-1 966/\

Minutes of Workshop on

Gaseous Dielectrics for Use in

Future Electric- Power Systems

Richard J. Van 8runt

S

Electrosystems Division

Center for Electronics

and Electrical Engineering

National Bureau of Standards

September 10-1 1, 1979

Issued January 1980

Final Report

Sponsored by

The Electric Power Research institute.

The Department of Energy and The National Bureau of Standards

NBSIR 80-1966

MINUTES OF WORKSHOP ON

GASEOUS DIELECTRICS FOR USE IN

FUTURE ELECTRIC-POWER SYSTEMS

Richard J. Van Brunt

Electrosystems Division

Center for Electronics

and Electrical Engineering

National Bureau of Standards

September 10-11, 1979

Issued January 1980

Final Report

Sponsored by

The Electric Power Research Institute,

The Department of Energy and The National Bureau of Standards

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

Introduction

This workshop was held at the National Bureau of Standards

laboratories in Gaithersburg, Maryland, on September 10-11, 1979.

The agenda and a list of active participants is attached. The

workshop was organized for the purpose of addressing questions

concerning current and planned research on development of new

gaseous dielectrics viewed as possible substitutes for SFg. Focus

was on identifying technological barriers to use of new gases and

gas mixtures and questions concerning properties of gases in need

of further investigation. It brought together many individuals from

industry, government, and academia with diverse interests and

opinions concerning the needs for future research in the gaseous

dielectrics area. It was assumed that the workshop would provide

a stimulating prelude to the forthcoming International Symposium on

Gaseous Dielectrics where unresolved questions concerning the needs

and desirable directions of future research can again be debated.

Editors Comments

This document has been prepared primarily to serve the purpose of

a record for the benefit of those who participated in the Gaseous

Dielectrics Workshop. The minutes presented here were extracted from

tape recordings made during all sessions of the workshop. This is not

a verbatim transcription. The recordings of formal talks, discussions,

and comments have been edited and in some cases reordered and paraphrased
- 2 -

for the sake of coherence and clarity. Where possible, individual

speakers and their affiliations have been identified. No attempt

was made to transcribe conversations in those few cases where voice

recording was undecipherable or where intended meaning was unclear.

It is hoped, however, that all major points are covered herein. The

editor apologizes in advance for any misinterpretations or misplaced

emphasis. The actual tapes used to prepare these minutes are on file

at the National Bureau of Standards, Washington, D.C. 20234. Copies

of these minutes have been distributed to all active participants in

the workshop as indicated on the attached list.

Minutes

September 10, 1979 - T. F. Garrity, Chairman

I. 1:00 p.m. - Chairman's Opening Comments, T. F. Garrity,

Department of Energy

Concern was expressed that those engaged in research on

"improved" gaseous dielectrics should be working toward common

objectives. There should be mutual understanding among researchers

in this field about what is meant by a "good" or "improved" gas

dielectric. One purpose of the workshop was to place in perspective

the research currently carried out under EPRI and DOE sponsorship

on development of new gaseous dielectrics. It was noted that there

are several criteria to be considered in deciding what is meant by

an improved dielectric and these include: (1) cost of gas,

(2) performance, and (3) system requirements. It was pointed out,

as an example, that the cost of gas could be as much as three times
-3-

presently used SF

g

, but total system cost could be reduced by 20%

with improved dielectric performance, so that mere cost of gas

has to be tempered by overall system demands.

Questions to be considered are the following: Can performance

of currently used SF^ be improved so as to make better use of its

inherent capabilities? What are we designing the gases for? Are

we, for example, designing these gases for meeting lightning impulse

requirements, switching impulse requirements, resistance to particles

and contamination, temperature and heat transfer requirements, or

60-Hz withstand voltage? Which of these criteria is the most important

in a given application? It is clear that one must take a look at the

total system in deciding these issues.

It was argued that it is not the purpose of this meeting to

come away with a precise definition of what is meant by a gaseous

dielectric superior to SF^. It is rather intended that this meeting

should instill ideas and raise questions which will enable those

actively involved in research to work toward the same basic

objectives. This workshop might be considered the first step in

answering the important question concerning what is meant by a

superior gaseous dielectric. Hopefully, it will lead to a better,

more intelligent discussion of this subject at the next International

Symposium on Gaseous Dielectrics. The need was expressed for a two-way

exchange between industry on the one hand and research establishments

on the other in development of new gaseous dielectrics.
-4-

II. 1:15 p.m. - D. E. Massey, GPU Service Corporation,

Users Look at Insulation Systems

Those at the workshop were reminded that the utility's

objective is to provide power at the lowest cost possible. This,

however, is becoming increasingly difficult with increasing fuel

costs and incidental costs at generating stations as required, for

example, to meet environmental standards. In considering costs of

a 500 kV transmission line, it was noted that normal operating

capacity of a line is only 50% of maximum design capacity. On

a cost basis, overhead transmission lines still win out over

underground lines, except where there are corridor limitations or

restrictions

.

For a compressed gas system, the cost of gas is not necessarily

a significant consideration. It was noted, for example, that in a

transmission system, if one had a gas that could improve the

dielectric strength of SFg by 100%, then one could afford to use

that gas at five times the cost of SFg because of the resulting

reduction in the sheath cost corresponding to its reduction in size.

It was mentioned that 44% of the overall cost is associated with

the sheath.

In discussing transmission costs, it was noted that the

reactive losses in an SFg system are high. Maintenance costs for

SFg lines are also considerably higher than for comparable overhead

lines. If failure occurs, underground lines are out of operation for

longer times than overhead lines, weeks compared to hours. The

maintenance downtime for a line is a significant cost consideration.
- 5 -

It was noted for circuit breakers that the gas used has a high

handling cost compared to oil because of the stringent purity

(cleanliness) requirements. The need was thus expressed for

developing simplified gas handling methods which can be dealt

with either by improving properties of the gaseous medium so that

it is less susceptible to contamination or by improving the method, e.g.,

the gas handling equipment and procedures. It was emphasized that

gas requirements in breakers are different from those in transmission

lines and other substation equipment. There is no reason for a

si ngl

e

gas in a breaker. It has been found that ^

is very good as

an interrupting medium even though it is not particularly good as

an insulant afterwards.

Concerning substations, it was indicated that the objective here

is size reduction to be achieved by increasing the dielectric strength

of the insulating medium. Again cost of the insulating gas need not

be a major consideration. It is always the overall owning cost that

is significant.

Questions :

Qr. N. Grao Trinh of Hydro-Quebec added to the presentation by

pointing out the gap that exists between the theoretical and practical

limits on compressed gas insulated (CGI) system size reduction.

At the Hydro-Quebec research laboratories, they are looking at CGI

bus system performance improvement. Associated with this, inhere is

the problem of reducing the size of a conductor for a given current

capacity as well as the problem of increasing operating temperature

particularly for the insulating spacers. In dealing with the problem
- 6 -

of corona onset, one must take into consideration the onset field

gradient for different size conductors.

Tom Garrity of DOE mentioned that it had been pointed out to

him by people at AEP that the cost of maintenance on their four

compressed gas insulated substations had recently gone down to about

a third of that for comparable open air substations.

Loucas Christophorou of ORNL asked how cost would depend on

gas pressure. The reply was that this is not a significant

consideration, i.e., the cost figures presented in the talk do not

change significantly with operating gas pressure provided pressures

are not excessively high.

The question was raised about how the utilities feel about the

issue of toxicity, and it was made clear that they do not want to

use anything that is toxic. For one thing, the handling costs of

toxic fluids is too high.

III. 1:30 p.m. - R. E. Wootton , Westinghouse R&D Center,

An Experimental Study of Dielectric Gases for

Practical Applications

The discussion in this talk centered around results obtained as

part of an EPRI-sponsored project that was recently carried out with

the cooperation of E. I. du Pont de Nemours and Company. A final

report on this work is now in preparation. In this project, they

looked at a variety of gases from both the insulation and interruption

point of view.

It was noted that many aspects of the gas must be considered in

determining whether or not it has any clear superiority or advantages
-7-

ovsr commonly used ~.-g. .ness aspects include (1) electric strength,

(2) carbonization, (3) vapor pressure, (4) toxicity, (5) environmental

acceptability, (o) immunity to adverse effects of moisture, contamination,

and particles, (7) stability and overall compatibility with other likely

components of the system. They did not conduct practical laboratory

work on a ; 1 of these aspects, although this may be needed to determ

n e ultimate suitability of gases as useful dielectrics. They did

laboratory measurements on: (1) breakdown (dielectric strength),

(2) carbonization, and (3) stability and compatibility with materials

of construction. They carried out a comprehensive literature survey

and did calculations on the oroblem of vapor pressure determination,

and also did a literature survey on toxicity. Some cost analysis

was done although difficulties and uncertainties remain on this

question. Excluded from the work scope were the influence of

humidity, moisture, contamination and particles.

Examples were given of the types of data obtained. They, for

example, looked at molecular weight dependence of relevant electrical

and physical properties, e.g., breakdown, varor pressure.

Generally as molecular weight goes up, breakdown strange- o~ tne gas

goes up and vapor pressure goes down. They also correlated dielectric

strength with heat of atomization and boiling ooinc.

They tried to formulate rules based on vapor pressure, electric

strength, etc., for selecting and screening gases for further study.

They considered thirty pure gases as well as mixtures with $ z

-

Measurements of electrical breakdown strength were re*'* 0

’',~ec

quasi -un i form fields using spnere-sohere electrodes a

n

d 50- '-z voltage.
- 8 -

Of the thirty pure gases examined, some had higher dielectric strengths

than SFg.

Consideration was given to dependence of breakdown strength on

temperature. The maximum dielectric strength that can be achieved

with any gas at a particular temperature is limited by its vapor

pressure. In general, maximum dielectric strength increases rapidly

with increasing temperature due to increased vaporization. To

determine if a gas is indeed superior in performance to SFg, one must

decide on the temperature range of interest. In any application, the

best choice of gas depends on operating temperature range. It was

pointed out as an example that CF^SFg has a higher dielectric strength

than pure SFg at higher temperatures for a gas pressure limited to

400 kPa.

Some of the gas mixtures examined included: CBrF^+SFg, CClFg+SFg,

CF3S F5+S Fg , CFgS02F+SFg, and CF 9 =CC 1 F+SFg . They measured dependence of

breakdown in a quasi -uniform field on gas mixture ratios and usually

found a linear relationship. Some of these mixtures show promise and

are perhaps worthy of additional consideration. The gas CF3SO2 F was

noted as an example to have a dielectric strength considerably higher

than SFg and exhibit a linear relationship in mixtures with SFg; also,

it does not carbonize and is believed to be nontoxic.

In some cases, nonlinear mixture characteristics were observed.

In a few mixtures, such as the CF-CF=CF2+SFg mixture, a synergistic

effect was observed, i.e., the breakdown strength of the mixture was

higher than either of the constituents. This effect was observed in

sphere-sphere and sphere-plane electrode systems and it was discovered
-9-

that the degree of synergism appears to be enhanced with increasing

field uniformity. It was noted that the simple linear, weighting

formula for predicting net ionization and attachment coefficients

in the gas, although explaining some nonlinear characteristics,

cannot be applied to explain this synergistic effect.

The importance of vapor pressure was again emphasized. They

considered two minimum operating temperatures, namely 0 °C and

-30 °C. The latter was found to be too restrictive on the gases

that could be used. However, all gases and mixtures considered

could be operated wi thout condensation down to 0 °C. For mixtures,

this condition puts a limit on the mixture ratios that can be used.

Most of the stronger dielectric gases do not exert as high a vapor

pressure as SFg. In considering, for example, the tertiary mixture

0.44SFg+0.06CF

3

SF

5 +0.50N2, the 6 % CF

3

SF

5 indicated is the maximum

amount of that gas that can be added without condensation at

-30 °C.

Carbonization was Dointed out as an important problem from a

practical point of view. Some gases such as CF

3

~C=C-CF

3

that initially

have dielectric strengths greater than SFg show marked decline in

breakdown strength after a breakdown has occurred. In the case of

CF

3

-CeC-CF

3

, large carbon deposits are left on the electrodes after

several breakdowns, and these may eventually build up to the extent

that they bridge the electrode gap. A computer model

of the carbonization process was described which uses information

on the heats of formation and latent heats of the material to predict

decomposition modes at high temperatures 1000 °K). Among the molecule
- 10-

formed is elemental carbon, and the model predicts the amount of

carbon formation as a function of temperature. The degree of

carbonization as predicted by the model decreases with addition of

SFg to the gas. The explanation for this is that the carbon is

absorbed in the formation of gaseous compounds such as CF^. An

extreme example of this is a mixture of CF^+SFg which was found to give

no carbon formation at any temperature. They verified the effect of

SFg on suppressing carbonization in arcing experiments in which

decomposition products were collected. They report that the degree

of carbonization generally decreases with increasing SFg concentration,

often up to a sharp cut-off point beyond which there is no carbonization.

The problem of high temperature aging was also examined. Here

it was noted that one must not only look at the stability of the gas

but also at what the gas or its decomposition products do to the solid

insulation system. Aging was studied by monitoring gas pressure

as a function of temperature and by analyzing the gas by gas

chromatography and other methods. They also looked at the effect of

various gases on deterioration of resins.

The question of cost was difficult to answer because costs

depend on volume of production which depends on volume of use,

environmental restrictions, etc. In general, however, gas costs

go down with decreasing dielectric strength. To get a dielectric

strength higher than SFg one must pay a higher price.

Concerning needs for future research it was suggested that

long-time-to-breakdwon studies should be undertaken. The reasons why

one cannot attain, in practical systems, more than 50% of the intrinsic
- 11 -

strength of SFg need to be examined. Is this limitation due to dust

and particles or surface roughness? It was also recommended that

emphasis be put on the study of synergistic effects in gas mixtures.

In conclusion, it was noted that according to this study no single, pure gas has been found that is "superior" to SF C in all

6

respects. On the other hand, some mixtures may be superior to SF*

6

in limited respects, e.g., they have lower cost, wider operatina

temperature, immunity to particles, etc. Such mixtures may show

promise for future applications. The promising gases considered in

this project need to be examined further to determine their resistance

to adverse effects of particles and dust.

Questions ;

Dr. Mastroianni of Allied Chemical commented that they had

observed breakdown with SF^-fl uorocarbon mixtures in the laboratory

using sphere-plane geometry that had as much as 18% improvement over

SFg, but showed no such improvement in an actual test section of bus

line.

A question was raised about the field configuration dependence

of the breakdown results. It was again emphasized that synergism

was observed in only three gas mixtures and the effect becomes enhanced

as the field becomes more uniform.

IV. 2:00 p.m. - M. J. Mastroianni, Allied Chemical Corporation,

Breakdown and Stability Properties of Some New

Gaseous Dielectrics

The program described was twofold in which they locked at:

(1) dielectric strengths of potentially new dielectrics and (2) arc
- 12 -

stability. Concern was expressed about the acceptability of gas

mixtures by the utilities, in particular the worry about complexities

introduced by mixing gases. Particular attention was paid to the

problem of carbonization and methods for quantifying it.

It was pointed out that dielectric strength is almost a simple

function of molecular weight. For certain classes of compounds,

dielectric strength increases nearly linearly with molecular weight.

It was also noted that the dielectric strength depends on characteristics

of the molecular bonding, often increasing with bond multiplicity,

e.g., triple bonded molecules tend to have higher dielectric strengths

than the analog double bonded molecules.

In designing a new gaseous dielectric one must realize that vapor

pressure goes down with molecular weight, and toxicity generally goes up

with multiplicity of molecular bonds, trends which are counter to those

just mentioned that give higher dielectric strength.

In this study, they looked at gases that were fluorine containing,

examples of which included: CF^SFr, CF^OSF^, (CF^S,

(CF-^N, CF

3

CF

2

CN, CF

2 CF

2

CF

2

CN, CF^SOpF. As an example, CF^SF^ is a

known good dielectric. It has a boiling point of -21 °C and a high

dielectric strength. Unfortunately , it is relatively unstable, and

rapidly decomposes in a low current (n, 1 ma, 30 sec.) arc into CF^

and SF^ with an associated increase in pressure. Only gases with

boiling points below 0 °C were investigated.

The gases selected were screened for dielectric strength in

breakdown tests using a sphere-plane ( 0.1 inch) gap at pressures of

1, 2, and 3 atm. For gross stability of the gas they looked at both
-13-

carbonization at the electrodes and pressure rise in the system after

breakdown occurred. Pressure rises of 70% were observed in some cases. They noted that improvement in dielectric strength over SF

g depended on gas pressure. The breakdown results shown were consistent

with thus far unreported results obtained in tests using sections

of CGI line. It was mentioned that one compound was found which

looked superior to SF

g

under the tests performed although it was not

identified.

Arc stability for different gas mixtures at 2 atm total pressure

was determined by arcing the gas for 30, 60, and 90 seconds at low

current and then performing analysis for decomoosition on recovered

samples. The results were reported in terms of percent of gas mixture

decomposed. The greatest degree of decomposition was observed for pure

fluorocarbons.

No arc induced decomposition of SF

g could be observed when the gas

was analyzed with a gas chromatograph. An example was discussed for

an SFg+CF

2 Cl

2 mixture where the percent decomposition initially went up

with mole percent content of CF9 C1 9 , but a large increase in the

decomposition occurred in the region of a 50-50 mixture. Similar results

were obtained for other gases. As another example, the decomposition

(carbon deposition) in test cells containing C

2 F

g +SFg

mixtures was revealed

in photographs, and again decomposition depended on mixture ratio. he

mixture 90% C

2

F 6

+10% SF

g

showed relatively little carbonization comDared

to 100% C

9

F

g

. They also looked at mixtures of fluorocarbons with other

gases like air, N

2

and N

? 0, and found no dramatic improvement over using

SF C . The conclusion of this study was that fluorocarbons are not very

b
-14-

arc stable although arc stability can be improved by adding SFg.

In summary, the conclusions of this study were: (1) no dramatic

improvements relative to SF. were found with the new gases studied,

with perhaps one possible exception; (2) SF^ gas blends are possibly

useful but the incentive for trying these is at present rather small ;

(3) the commercially available fluorocarbons are unstable and at this

time not considered suitable as a way of achieving gas cost reduction;

(4) if fluorocarbons are to be used, then they must be mixed with an

arc stabilizer, e.g., SF^ ; (5) particle effects on mixtures are

unknown and more work on this problem is required. It was also noted

that no significant improvement in dielectric strength over SF^+N^

or SF^+air mixtures could be achieved using SF^ mixed with available

fluorocarbon mixtures.

Questions :

Dr. A. Chutji an of JPL asked if any work was done on arc stabilizers.

Other than adding , N

? Q, CC

?

, and SF^, little had been done.

Dr. Mastroianni expressed the opinion that even a 90* improvement by

adding an arc stabilizer was not necessarily sati sfactory . Those at

Allied Chemical feel that no level of carbonization is tolerable, and

unless this condition can be met, the gas should not be considered for

use in practical systems. D. E. Massey of GPU Service Corporation

pointed out that, from the utility's viewpoint, carbonization in a

transmission system, particularly in a transmission line, is not

necessarily bothersome since if it results from a flashover, the line

will have to be cleaned up or replaced anyway. In a breaker, however,

there is concern about effects of carbonization. Again, he emphasized
- 15 -

that we should not be looking at a single gas as a replacement ror

SFg, but rather several depending on application, operating

conditions, etc.

Bruce 3ernstein of EPRI pointed out that there was an apparent

inconsistency between the results obtained for carbonization in

SFg-fl uorocarbon mixtures reported in this talk and these discussed

in the previous talk by Roy Wootton. Particular concern was expressed

about the enhanced carbonization in the 50-50 mixture range.

R. Wootton of Westinghouse mentioned that black deposits that appear

on electrodes after arcing are not always simply carbon. Depending

on electrodes used they found very dark deposits that upon analysis

revealed little carbon. For example, in the presence o* aluminum it is

possible to form aluminum fluoride which has a black color.

V. 2:30 p.m. - M. R. Kegelman, E. I. du Pont de Nemours and Conoany,

Calculation of Vapor Pressure rcr Dielectric 2as ,,J xtures

This project was carried out as part of the EPRI supported coocerativ

effort with the Westinghouse Research and Development Center previously

mentioned by Roy Wootton.

It was again enphasized that vapor pressure or volatility is among

the more important secondary physical properties of a dielectric gas t"at

must be carefully considered after the primary concern with breakdown

strength and arc stability. It was pointed out that vapor pressure

effects are sometimes misunderstood by those working in t

n e gaseous

dielectrics area. In -.any respects, this talk served as a kind o~

tutorial on how to do calculations of vapor pressure for any given

mixture of gases or fluids.
-16-

One must first consider the different possible binary

mixtures. These are primarily of two kinds: (1) the case where

the liquid phases of the two gases are soluble in each other,

and (2) the case where they are not soluble in each other. The

case where the liquid phases are insoluble is the simplest, for

in this case each component exhibits a vapor pressure independent

of the other. An example of this is the mixture of water and toluene.

The vapor pressures are additive in this particular case, and this

explains why the mixture boils at 85 °C whereas toluene and water

by themselves boil respectively at 110 °C and 100 °C. The composition

of the vapor at a given temperature is independent of the relative

amounts of liquid present in a mixture of immiscible liquids.

It was pointed out that another simp!

e

case of a two component

system is the case where one has a so called "permanent" gas such

as N

2

and a condensable gas such as SFg. Nitrogen can be considered

a permanent gas because its critical temperature is -147 °C, i.e.,

this is the temperature above which N

2

cannot be liquefied at any

pressure. Moreover, liquid SFg can dissolve only small amounts of

N

2

; therefore, SFg in the liquid phase will exert its full vapor

pressure in the presence of Ng.

The greatest misconceptions often creep in for the case where

the liquids in a binary mixture are soluble. In a mixture of miscible

liquids A and 8, the molecules of A or B do not experience the same

intermol ecu! ar forces that would be present in the pure state, i.e.,

in an environment of their own kind. Therefore, the molecular

restraining forces in a liquid will be different than in pure liquids.
-17-

The vapor pressure of the constituents in this case are no longer

additive. Unlike the case of immiscible liquids, the vapor pressure

and gas composition does not depend on the proportions of the liquids

in the mixture. Liquids that are totally miscible generally obey

Raoul t's Law which states that "the vapor pressure exerted by a

liquid in solution is proportional to the mole fraction of that

liquid in the solution." Using this law, one can generate plots

such as were shown of vapor pressure versus concentration of some

gas X, for example, in SFg at a given temperature say -30 °C. If X

is less volatile than SFg, one wants to know how much of X can be

tolerated for a given temperature and pressure requirement without

condensation. An example of application of Raoul t's Law was considered

for a mixture containing a gas X with a 0 °C vapor pressure of 3.27 atm

in SFg which has a 0 °C vapor pressure of 12 atm. Assuming an initial

room temperature loading pressure requirement of 0.44 MPa, one finds

that it is possible to have a mixture containing up to 75% of gas X

without condensation at 0 °C. It was reported that calculations like

this were performed for various SFg-fl uorocarbon gas mixtures. Good

agreement was found when calculated results were compared with

measurements

.

They also considered some ternary gas mixtures. The same procedures

used for binary mixtures can be extended and applied to mixtures or

higher component multiplicity.

It was finally noted that Raoult's Law does not apply in all cases,

in particular it will not work for nonideal liquids. In such cases,

one must resort to laboratory measurements.
-18-

VI. 3:30 p.m. - L. G. Christophorou, Oak Ridge National Laboratory,

The Oak Ridge Program on Gaseous Dielectrics

The ORNL program was described as comprehensive in that it includes:

(1) basic studies of fundamental processes, (2) applied testing,

(3) practical measurements, (4) impulse studies, (5) environmental

effect studies, and (6) efforts to interface with industry. Highlights

from each of these areas were discussed.

Considering first the basic studies it was noted that the main

effort here is to understand factors that control free electron energies

and number densities in dielectric gases under conditions of electrical

stress. There was also concern in this effort to obtain the necessary

data on fundamental processes, particularly of gas decomposition, to

assess the long term stability and environmental effects of gaseous

dielectrics. One should ask the question: what is it that makes a good

dielectric? For optimum high dielectric strength, for example, what

counts is the ability of the gas to remove the ever present free electrons. A "good" dielectric in this sense will consist of molecules

that have both a high electron attachment probability and high electron

energy loss cross-secti ons, so that the mean electron energy can be degraded

to reduce the probability for ionization and at the same time enhance

attachment so that electrons are readily removed via negative ion formation.

The negative ions have a much lower mobility and a much lower probability

for inducing ionization by collision than free electrons. In designing

a gaseous dielectric to obtain highest dielectric strength, one must,

therefore, attempt to maximize the integral

CO

/

CT a (e)f(e,E/P)de

o
-19-

and minimize the integral

00

/ a. (c)f (e ,E/P)de

1

1

where a

a (e), cr.. (e), and f(e,E/P) are the electron attachment cross

section, ionization cross section and electron energy distribution

function respectively, and e, E/P, and I are the electron kinetic

energy, field-to-pressure ratio, and gas ionization potential respectively.

From this, one can understand why detailed microscopic properties,

e.g,, attachment rates, collision cross sections, etc., are

useful in preparing a good dielectric.

Ideally, the gases used as dielectrics should be good electron

thermal izers in the sub-excitation energy range. Preferred compounds

should also have high electron attachment cross sections over a wide

electron energy range, say between 0 and 5 eV. It is especially desirable

to have as high an attachment cross section as possible at higher electron

energies. It was pointed out that there is probably no si ngl

e

gas

that has all the desired microscopic properties, and, therefore, the

best dielectrics are likely to be mixtures.

They have discovered in their work that perfluorination of large

hydrocarbons is beneficial in enhancing a (e). Perfl uorocarbons typically

have higher (factor of two or more) dielectric strengths than SFg.

They also have high ionization potentials and relatively high electron

energy loss cross sections. At present, these compounds are viewed as

possible additives to multicomponent dielectric systems. In some cases,

one may need to use only relatively small amounts of these to enhance
- 20-

dielectric properties. They may not always be suitable as the

predominant gas component in power transmission applications

because of their relatively low vapor pressures. It was suggested

that CHFg might be a desirable replacement for Ng in SFg-Ng mixtures.

Concern was expressed that compounds used as gaseous dielectric

components to enhance cr (e) should if possible capture the electrons

a nondissociatively. It was pointed out that decomposition due to

dissociative electron attachment (e+AB A"+B) can occur for some

compounds even for electrons with near zero energy. It was reported,

for example, in the recent mass spectrometric work of I. Sauers, et al

.

(J. Chem. Phys. 71_, 3016 (1979)) that for 1 ,3-C^Fg the yield of

dissociative attachment products far exceeds the yield of the parent

molecular negative ions in the relevant electron energy range from

0-9 eV expected for the electron energy distribution, f(e,E/P),

under typical practical operating conditions. On the other hand,

the compounds c-C^Fg, 2-C^Fg, and c-C^Fg were not found to easily

decompose by dissociative attachment at these electron energies.

They have looked into the question of electron detachment, i.e.,

how effectively the electron is bound to a negative ion. Various

detachment mechanisms were examined such as electron collisional

detachment, photodetachment and field induced detachment. They concluded

that the most important electron detachment mechanism is collisional

detachment, either associative or nonassociative. By theoretical

calculation. Professor Harold C. Schweinler who has worked with them

finds that detachment is not likely to be important under oractical

conditions. Fields on the order of 10-20 MV/cm are, for example,

required to detach, with significant probability, negative ions with

electron affinities of 1 to 1.5 eV.
- 21 -

Experiments in this program have shown the trend that increasing

electron scattering cross sections increases breakdown strength in a

gas. In this process, the formation of temporary negative ion

resonances (quasi-trapping of electrons by a molecule) is important

since this mechanism enhances the collision cross section. Nitrogen

is a relatively good electron thermal izer because it has resonanceenhanced inelastic electron scattering at low electron energies

(around 2.0 eV). Also, molecules with multiple bonds, high electric

dipole moments, and high static polarizabilities tend to have high

electron scattering cross sections.

It was argued that ionization cross sections are relatively

unimportant compared to attachment and inelastic electron scattering

cross sections. One generally cannot improve dielectric strength of

the gas much by reducing the net ionization cross section. If the

system is effective at thermalizing and otherwise removing electrons

with sufficient energy to cause ionization, then ionization is not of

signi ficance

.

They have studied mixtures of perfl uorocarbons with nitrogen

and found that most of these mixtures have higher dielectric strengths

than SFg particularly in nonuniform fields. Randy James at ORNL

observed synergistic behavior in uniform field breakdown strengths for

the mixtures 1 “ C

3 F

5 +SF

6 and 1 -C

3 F

6

+c-C

4

F 8

, i.e., the breakdown strength

for some mixture ratios exceeds that for either component alone.

A study was carried out with various binary mixtures with one

electron attaching and one dipolar gas component as a thermalizer.

In these cases, synergisms were observed. However, for non-dipc! ar
- 22 -

plus attaching gas mixtures no synergisms were found. It was thus

argued that observed synergistic effects were due to enhanced electron

thermal izati on in dipolar gases resulting from higher scattering

cross sections.

In discussing the practical studies it was mentioned that they

have looked at (1) breakdown in different electrode geometries,

(2) surface roughness effects, (3) contaminant effects in the gas,

(4) impulse breakdown, (5) corona onset, (6) nonuniform field effects,

and (7) effects of temperature. They are also planning to investigate

accelerated life tests.

Some of the preliminary conclusions from these practical studies

are:

(1) All perfluorocarbon gases and mixtures which are superior to SFg

in uniform field measurements continue to be as superior under practical

conditions. It was stressed that the perfl uorocarbon-N^ mixture under

cylindrical electrode testing suffers less than comparable SF^-N^ mixtures.

The synergistic effects in SF^-N^ become less as field becomes more

nonuniform, whereas this is not the case for similar perfl uorocarbon-Nmixtures. As the field becomes more nonuniform, the percentage of

perfluorocarbon needed to achieve a given dielectric strength becomes

increasingly less than the corresponding amount of SFg needed to achieve

the same conditions.

(2) Perfl uorocarbon-Ng mixtures perform satisfactorily as surface

roughness increases as compared, for example, to SFg-Ng mixtures.

(3) The breakdown strengths for SFg and SFg-N^ mixtures were found to

be independent of temperature.
-23-

(4) The C^Fg-SFg mixture was found to have much higher negative

polarity impulse breakdown voltage in a point-plane configuration than

pure SFg.

They have looked at gas decomposition due to initial electron

collisions (previously mentioned dissociative attachment) and decomposition

under stress and breakdown. The breakdown products have been identified

using a gas chromatograph-mass spectrometer with selective mass-to-charge

ratio positive ion monitoring. One observes many decomposition products

under conditions of continuous sparking and many of these are perfluorocarbons, contrary to what many have been led to believe. They found that

CF^ production in sparking actually went down as the perfluorocarbon

content in perfluorocarbon-SFg mixtures went up, i ndicati ng that the source

of CF^ is really derived from reactions involving products from SFg.

It was argued that there may be mixtures that optimally inhibit the

production of certain gaseous decomposition products. The monitoring

of CF^ was in fact proposed as a quantitative method for testing

optimization of mixtures to minimize undesirable decomposition. It was

noted that the addition of N

2 to perfl uorocarbons decreases formation

of CF^.

It was suggested that the following mixtures deserve further study:

c-C

4

F

8

+SF

6

, c-C

4

F 8+SF

6

+N

2

, c-C

4

F 8

+SF

6

+CHF 3> and c-C

4

F

8

+SF

6

+l ,1 .I-CjHjFj.

These mixtures, for example, do not appear to have serious carbonization

probl ems

.

Questions :

R. J. Van Brunt of NBS questioned the importance placed on using

dissociative electron attachment as an indicator of gas stability since it is
-24-

known, for example, that this occurs in SF

g even at very low electron

energies, yet the decomposition products seem to recombine after a time

to reconstitute the gas. Perhaps the subsequent chemical reactions

that occur after dissociative attachment are more important.

It was pointed out by Dr. Chri stophorou that in the case of SFg

the F" production is predominantly at higher electron energies (greater

than 2 eV) and thus may not be important in a practical system where

the number of "high energy" electrons above 2 eV would be very small.

Ara Chutjian of JPL noted, however, that dissociative attachment

in SFg leading to formation of SF-” can occur even down to threshold.

In his reply. Dr. Chri stophorou recalled that the cross section

for SF^" production is roughly two orders of magnitude smaller than

that for SFg" formation for electron energies below 0.1 eV.

There was a question by Dr. J. Dutton, University College Swansea,

about the downgrading of the importance of ionization. He pointed out

that although this may be true under static breakdown conditions where

the electron energy distribution is thermal i zed , it may not be true for

impulse conditions where the electron energy distribution can be quickly

brought up to a high mean energy value.

It was admitted that under highly inhomogeneous field conditions

ionization may indeed be an important consideration.

VII. 4:00 p.m. - J. Castonquay, Hydro-Ouebec Institute of Pvesearch,

Analysis of Trace Decomposition Products in Gaseous

Die! ectrics

In the research described in this talk, they have taken the

practical point of view of developing analytical techniques to observe
-25-

gaseous decomposition in real power systems. It is known that under

electric discharge conditions SFg can decompose readily in the presence

of impurities and this is a problem that needs to be addressed in the

development of new gaseous dielectrics that might be considered as replacements for SFg.

In their work, they have performed analysis of gases in practical

high voltage systems designed to operate with a flashover voltage of

1 MV with 3 atm SFg which had undergone breakdown tests. In this analysis,

they were able to identify certain decomposition products such as S0 9

up to the 1 25 ppm level

.

Laboratory decomposition studies were performed in a teflon cell

equipped with a steel needle electrode and aluminum cathode. Recent

investigations of decomposition in such a cell caused by discharges

and partial discharges included the following artificially "contaminated"

mixtures: pure SFg, SFg+10* air, SFg+9*£ CC^ , and SFg+10* CO. It was

discovered that the decomposition for all of these mixtures was roughly

at the same rate. From this, one might conclude that the presence of

most atmospheric constituents in SFg does not appreciably accelerate

the rate of decomposition over that in "pure" SFg.

To determine the extent of decomposition, they looked for the following

contaminants: S0F9 , SO

2 F

2

, SOF^, F

9 C0, and CH^. It was noted that oxygen

as well as COwere consumed in reactions leading to many of these products.

Also they found that white crystals were deoosited at the bottom of the

cell which hydrolyzed in air, and thus were assumed to be aluminum

fluoride. Moreover, with the aid of x-ray fluorescence they found that

after the tests aluminum was deposited throughout the cell.
-26-

They also looked at the following mixtures: SFg+11% CH^ and

SF

g +9.3* C2 H

2

> and found that the decomposition was faster than in pure

SFg. In these cases, SOF^ was one of the most prevalent

decomposition products. These results indicate that hydrocarbons

apparently induce a more rapid decomposition of SFg than even ^

0 ,

which has always been considered one of the most harmful forms of gas

contamination. It was noted that C

2 H4

in SF^ gave the highest observed

rate of decomposition in a high-voltage, low-power arc.

It was emphasized that the fluorine atom is the main primary

decomposition product in SF-, and this atom reacts readily with whatever

it encounters in a system. It would thus appear that hydrocarbons

are readily attacked by free fluorine.

In their work, analysis of the gases was done by syringe injection

into a chromatograph. With 200 microliters of contaminated SFg, they

can now measure concentrations of contaminants down to the ppm level.

There are, of course, some contaminants that are difficult to observe.

In -particular, it was pointed out that HF and SF^ will attack oxides

and are difficult to analyze. For SFg, it was argued that S0F

?

, S0F„

,

and S 0

? F

p are the main decomposition products that should be monitored

at the ppm level in order to determine quality of the gas. The most

powerful recommended analytical technique is the combined gas

chromatograph-mass spectrometer.

In these studies, they used what might appear to be unusually high

gas impurity concentrations. This was done, however, to simulate what

might happen, for example, during flashover near an epoxy spacer where

contaminant levels due to release of gases from solids might be considerably

higher than in the gas under ordinary conditions.
-27-

In their analytical technique, they used a chromatograph with a

column constructed of Teflon tubing and Porapak-Q support material.

A quadrapole mass spectrometer was used as a detector. To enhance

chromatographic sensitivity, very short columns were used since short

columns require less clean up. This was done, of course, at the sacrifice

of separation in column elution times for the various decomposition

products of interest. It was pointed out that if one is to observe

SF^, one must repeatedly inject the same mixture into the column for up

to one hour in order to precondition it before attempting an analysis

for this gas component. The SF

4

will react with Si0

2 or H

? 0 in the

column and convert to SOF^. In chromatographic analysis of fluorine

compounds, one of the problems is the presence of gaseous silicon

compounds particularly Si F^ which can result from such reactions as

2SF

4

+Si0

2

-> SiF

4

+2S0F

2

and

4HF+Si0

2

-> SiF

4

+2H

2

0 .

For this analytical work, a silicon rubber membrane separator was

used at the gas chromatograph-mass spectrometer interface. Helium was

the carrier gas used in the column.

Various loss mechanisms in the gas chromatograph-mass spectrometer

system which limit sensitivity were discussed. These include: (1) losses

within the column due to gas transformati ons or conversions, which increase

with column length, (2) losses in the membrane separator, particularly
-28-

if the gas of interest has low solubility in the membrane such as is

the case for SF^ in a silicon rubber membrane, (3) losses in the

mass spectrometer as results, for example, from the finite ionization

efficiency (typically only about 1%) and the finite ion detection

efficiency. Despite these losses, analytical sensitivity down to a

few ppm should be possible with a gas chromatograph-mass spectrometer

system. Because the ionization chamber was operated at 180 °C, some

SFg was slightly decomposed on the chamber walls giving rise to artificial

ion peaks in the mass spectra which are more characteristic of SOF^.

Results from the gas chromatograph-mass spectrometer were compared

with other "standard" methods of analyzing SF^ such as ASTM 2284. It

was found that the recommended procedures do not always give the

sensitivity for SOF

2 that is claimed and should perhaps be reexamined.

Questions :

Dr. Ed Walsh of Westinghouse asked about the toxicity of SOF

2 and

SO

2 F2

. It was noted in reply that one can breathe these at the 1 ppm

level for ten hours per day, five days per week without ill effects.

Dr. Christophorou of ORNL commented that in mixtures of SFg with

fl uorocarbons and/or with Ng some of the byproducts of SFg decomposition

are picked up by the fluorocarbons.

Dr. Devins of GE asked if S

2

F-J

q

was observed. It was stated that

they did not try to find it, and, moreover, i ts presence at small

concentrations might be difficult to observe. If you can detect SOF

2 you will certainly not detect SgF-jg.

Dr. R. J. Van Brunt of NBS asked if they had tried to use a jet

separator at the gas chromatograph-mass spectrometer interface. They have
-29-

not tried a jet separator yet but have plans to do so. The comment was

added that the performance of a jet separator is very sensitive to

carrier gas flow rate in the chromatograph.

VIII. 4:30 p.m. - R. J. Van 3runt, National Bureau of Standards,

Study of Corona Characteristics and Corona Chemistry

in Gaseous Dielectrics

In this talk, a description was given of a DOE-sponsored

research program at N3S that is concerned in part with gaseous

dielectrics. The main focus of this work is on providing fundamental information and data useful in the development of more

meaningful tests and models of gaseous dielectric performance.

Particular emphasis at the present time is on the problem of long term

effects, such as chemical changes that result from subjecting a gas to

continuous high stress conditions. The four primary objectives of the

program include: (1) partial discharge (corona) characterization,

(2) chemical diagnostics development, (3) swarm and breakdown data

evaluation, and (4) fundamental mechanism identification.

Considering the first objective, it was noted that partial discharges

(corona) occur in practical systems around particles, sharp protrusions,

at interfaces, etc., and may be difficult to avoid or detect. It

represents a power loss and can in some cases be a major cause of

insulation deterioration. Very little fundamental work has been done

on corona in SFg and other electronegative gases or gas mixtures that have

possible usefulness as gaseous dielectrics. Thus far in this program,

they have looked at the problem of measuring and defining corona inception

for ac and dc voltages which has included performance of measurements to

\
- 30 -

electrically and optically characterize the phenomenon. Corona in SFg

appears primarily in the form of pulses for both positive and negative

polarity. Attempts are now underway in their laboratory to characterize

the corona pulse frequency and pulse height distributions as a function

of operating conditions as defined, for example, by gas pressure, applied

voltage and electrode configuration. They have also looked at the pulsed

optical emission from corona in SF^ and SF^+N^ mixtures in an attempt

to determine optical spectra, intensity distributions, and correlation

with simultaneously observed electrical pulses.

The second objective indicated above involves development of chemical

diagnostic techniques which can help provide information about corona

induced chemical changes in the gas that might be correlated

with observed changes in corona characteristics. The primary technique

under development now is a gas chromatograoh-mass spectrometer system

for in situ analysis, which is similar to that described in the previous

talk. It was noted, however, that this technique has its limitations

in that it only indicates the initial and final conditions of the gas,

i.e., the relative amount of decomposition that has occurred in the gas

after it has been subjected to corona or discharges over a given Deriod

of time. It tells very little about the intermediate chemistry that

occurs during corona. Other techniques are under development to learn

more about fundamental processes that occur in discharges and these

include: laser induced opto-gal vani c spectroscopy, and optical emission

and absorotion spectroscopy

.

At the present time, the third item in the list of objectives is

concerned entirely with evaluation of swarm data for electronegative gases.
-31-

This work is being conducted at the Atomic Collisions Data Center,

Joint Institute for Laboratory Astrophysics, University of Colorado

and National 3ureau of Standards, 3oulder, Colorado 80309.

Professor Jack Dutton from the University College of Swansea is providing

assistance in this effort. Consideration is being given to extension

of this project to include fundamental data on electrical breakdown

and sparking potentials.

There are also plans to do experiments on fundamental mechanism

identification. The purpose here is to identify collision

processes and determine their relative importance in high pressure gas

discharges. Processes of interest include, for example, collision-induced

metastable quenching, field-enhanced collisional detachment, dissociation

rates, etc.

The remainder of the talk dealt with recent results obtained on

characterization of corona phenomena in SFg and SF-+N? mixtures. The

measurements were performed using a stainless steel point-plane electrode

configuration. Results were reported on measurement of dependence of

corona pulse rate on applied voltage and current. Pulse detection and

counting techniques were used to determine onset of corona under ac and

dc conditions. For ac conditions, corona observations 'were restricted

to selected phase intervals so that separate measurements of positive

and negative onset within each half cycle could be made. The negative

corona onset in SF^ under both ac and dc conditions always occurs at the

lower voltage. It was observed that the negative corona onset voltage

was the same for both ac and dc under all conditions. The positive

corona onset voltage, however, was in general different for ac and dc, and
-32-

this difference increased with increasing gas pressure, This would

appear to be due to the fact that as pressure increases the ionization

on one half cycle begins to have more influence on development of corona

on the next half cycle. This was suggested also by the frequency

characteristics of negative corona which, at higher pressures, display

abrupt discontinuities at voltages corresponding to the positive corona

onset. The results of these measurements indicate that measurement of

dc positive corona onset is not necessarily a good indicator of what

would happen under ac conditions.

It was noted that the pulse shape characteristics are quite different

for positive and negative corona in SFg and SF^+N^ mixtures. Plots of the

pulse height distributions for positive and negative corona for various

operating voltages and gas pressures were shown. It was pointed out that

over the pressure range of 50 to 500 kPa the mean amplitude of the

negative corona pulses is typically an order of magnitude lower than

the mean amplitude of the positive corona pulses. The repetition rate

of negative corona pulses, however, is considerably higher, which explains

why the average corona current can be roughly the same for both polarities.

Positive corona starts as low level avalanches which eventually develop

into a pulse burst activity preceded by a "large" streamer pulse. The

average length and intensity of the bursts tends to increase with decreasing

gas pressure and increasing voltage. The burst activity manifests itself

as structure in the pulse height distribution, and this structure seems to

depend on gas composition, being quite different, for example, in SFg-N^

mixtures compared to pure SF^.

Negative corona, particularly at lower pressures (< 300 kPa),

starts as single relatively large pulses 10 pC) and then quickly
-33-

changes to a quasi-glow condition consisting of many closely spaced

pulses as the voltage is increased. Trichel pulses such as occur in

No contaminated with electronegative gas were not observed in SF^ in the

pressure range 50 to 500 kPa. This type of activity was also not

observed in the SFC+N0 mixtures studied.

VJ L. There appears to be some indication that pulse height distributions

are sensitive to small changes in gas composition resulting from

contamination or electrical discharges.

Questions :

Dr. A. H. Qureshi of the University of Windsor pointed out that

the results described in this presentation appear to be quite consistent

with recent results obtained at the University of Windsor. He noted that

their observations of optical pulses correlated well with electrical

observations of corona pulses.

Dr. Van Brunt noted that although they also obtained good one-to-one

correlation in recent measurements, there were systematic differences

in pulse shape between the optical and electrical signals, possibly due

to variations of the emission spectra with time.

Dr. Ken Davies of Westinghouse asked if observed pulse frequency

correlates with ion transit times. The answer was that this has not been

looked at. This question is difficult to answer for the conditions under

which the measurements are made where space charge distortion of the field

might be significant.
-34-

September 11, 1979 - B, S. Bernstein, Chairman

IX. 9:00 a.m. - C. M. Cooke, Massachusetts Institute of Technology,

Basic Factors Influencing Reliability

This talk was concerned with variations and causes for variations

of breakdown voltage in gas insulated apparatus. One question of

relevance with regard to reliability is the expected breakdown voltage

for a single or small number of applied pulses without regard to pulse

shape or pulse length. This can then be extended to ask what happens

after many years of operation? In this case, one wants to know, what

are the rare breakdown voltages after many pulses have occurred?

The question of concern is what is the probability that a given pulse

will cause breakdown?

This further leads to the question of how to test for acceptable

performance and how one system should be compared with another in the

cases where the systems differ, e.g., relate the performance of one

particular kind of gas with that of another. In beginning the discussion,

the following assumptions were introduced: (1) the system is operated

in a partial discharge free mode which implies no decomposition in the

gas medium, (2) the electric field is relatively uniform and thus space

charge distortions are neglected. Allowance will, however, be made

for surface roughness which can cause localized nonuniform fields. The

problem then is to find a way of predicting the performance of a gas

insulated system. It was pointed out that at high gas densities electrode

effects on breakdown begin to become important.

The breakdown under the conditions specified here is not a glow

discharge but rather a sudden hot spark. In their work, they had
-35-

assembl ed data on breakdown versus tine. A kind of universal

breakdown curve was shown that indicated how the withstand voltage

increases dramatically as the time duration of applied voltage is reduced.

The well known fact was again emphasized that for practical sized systems

operated at SF

g gas pressures considerably above 1 atm the breakdown

voltage after long times is considerably below the ideal theoretical

limit of (89 kV/cm-bar). However, at 1 atm the long time results

come close to the theoretical limit.

The data from which the universal breakdown curve was derived were

for natural irradiation, i.e., there were no external sources of electrons

such as result, for example, from deliberate irradiation with UV, x-rays,

or gamma rays. The question was then raised concerning the origin of the

universal curve shown, i.e., how does one explain the observed time delays

for breakdown?

The problem was broken up into three parts, i.e., three origins of

delay were separately considered. These are: (1) the time required in

waiting for an initiation event such as a free electron in the apprcoriate

place, a particulate or a plasma, e.g., a surface oxide layer breakdown

which causes a pulse of charged particles; (2) consideration of false

starts which would correspond to the situation where an initiating event

occurs but then fizzles out and does not develop into breakdown; and

(3) the time delay associated with the formation of a conducting channel.

Each of these were next examined in some detail.

Item (3), the formation time, was considered first. This is the

time necessary for the voltage to collapse after the initiating electron

appears. If a slight overvoltage is applied this is usually a very short
-36-

time, less than a nanosecond. One can argue then that the formation

time is not significant in many situations.

Next the question of false starts was looked at. This was done

by considering the electron avalanche as being a link that takes the

gas from its insulating state to its conducting state. Either the

Townsend or Streamer criterion can be considered in which a large

avalanche growth is what is required to initiate breakdown. The

question was posed: given n starting electrons some place, what is

the probability that there will be a sufficiently large number of

3 electrons produced, say 10 , required by the criterion considered?

The function P(n,x) was defined as the probability of having n electrons

after the avalanche develops to a distance x. A sum was then performed

over all n greater than some critical value n

c determined by the chosen

criteria

.

The result is not sensitive to the choice of n . This

avalanching probability function has been evaluated for SF^ gas assuming

a sharp protrusion on a surface. Plots were shown of relative applied

voltage to the gap versus the probability of growth to breakdown P for

8

n

c = 10 and a given number of initiating electrons. The probability

function is computed by asking the question that as an electron (or group

of electrons) is moving through the gas under a defined set of applied

field and pressure conditions, so that there are well defined average

ionization, attachment coefficients, etc., what is the probability that
-37-

in the next increment of time the avalanche will either have grown or decayed? This is answered by solving a probability differential

equation that describes the probability of increase in size per unit

of travel

.

The novel feature of this analysis is that a nonuniform field

has been introduced. (The uniform field breakdown case has previously

been solved). The probability curve generated is useful in test

evaluation. One makes a test for a given period of time at some voltage

and then backs off on voltage and waits. One then asks the question,

how long must one wait on the average for breakdown to occur under the

new conditions? Based on purely the statistics of avalanche growth

considering false starts only, i.e., avalanches that do not achieve

8

n c = 10 , one can make a prediction. The problem of false starts is

thus handled by merely considering avalanche growth statistics.

Finally, other ways that one can get a time delay in breakdown

were considered, i.e., those included in category (1) of waiting for

an initiating event. One assumes a voltage pulse applied to a gap.

Two things can then happen. First of all in a gap to which no voltage is

applied there will be a certain number of initiating electrons produced

by cosmic radiation and background radiation. One can predict the

distribution of these. As soon as a voltage is applied, these charge

carriers will be swept out of the gap. Thus, as voltage is raised higher

and higher there will be a time at which there are no more carriers in

the gap. It was noted, however, that the clearing time depends on mobility

of the electrons and rate of voltage increase. It is evident then that

even though the applied voltage exceeds the breakdown voltage, breakdown
- 38 -

will not occur due to the absence of initiating electrons. The concern

thus shifts to the mechanism by which more charge carriers are introduced

once the high voltage level is reached. Given a carrier, it can be assumed

that breakdown will occur. The question to be asked then is, once the

gap is cleared and voltage exceeds breakdown how long does it take before

a new initiating electron appears? This can apparently take considerable

time. Without initiating electrons one can apply five or perhaps even

ten times over-voltage without occurrence of actual breakdown. Of course,

at sufficiently high fields initiating electrons can come from field

emission, although it was mentioned that contributions from field emission

may be difficult to determine. Their work seemed to show less effect from

field emission at a given field than might be expected, particularly

from experiments performed in vacuum.

At this point, the discussion turned to the effect of protrusions.

Around some of the higher protrusions there is a volume in which avalanche

growth can take place despite the fact that this may not be possible in

the ambient applied field E

q

, or around smaller protrusions. It was

pointed out that the active volume around n protrusions should be

roughly proportional to

This is because the equipotential surfaces are nearly spherical around

any protrusion. The total active volume in a gap can grow very rapidly

with applied electric field. For breakdown to occur, the initiating

electron must appear in the active volume. According to this model, the
- 39 -

probability that an effective initiating electron will actually appear

obviously increases with E q

, surface roughness, and duration of overvoltage,

i.e., length of voltage pulse.

It should be possible using the vol tage-time-to-breakdown curve to

predict breakdown after n units of time from data on one unit of time.

It was pointed out in closing that the whole picture is not merely

waiting time for electrons. There are other sources of breakdown

initiation such as from particles and along solid dielectric insulators,

the internal charging of which can sometimes introduce sufficient field

distortion to enhance breakdown.

Questi ons :

Dr. J. Devins of GE brought up the question of attachment as a

mechanism for removing initiating electrons and effectively diminishing

the life time of a free electron. It was stated in reply that inherent

in the calculation is the tacit assumption that the number of carriers at

the initial time is in a steady state which would necessarily include

attachment rates.

Dr. L. Chri stophorou of ORNL asked about the mobilities used in the

calculation. In answer, it was noted that the mobilities used were those

for electrons in typical fields.

The question was raised concerning effect of system size, e.g.,

area effects. Dr. Cooke pointed out that the model results do indicate

size effects.
-40-

X. 9:30 a.m. - J. Dutton, University College of Swansea,

Swarm and Breakdown Data for Gaseous Dielectrics

The purpose of this talk was to show in a general way how data

obtained from the study of electron swarms and breakdown are related

to answering some of the central questions which arise naturally in

considering improved gaseous insulation. For any practical system, the

three main questions that come up concerning electrical breakdown are:

(1) what is the V

$

(lowest voltage at which breakdown will occur),

(2) what is the space and time dependence of current growth I (x ,t ) when

V > V

s ,(3)and how do V

s and I(x,t) change with time and as a result of

current flow.

To begin to answer these questions, the first thing that one

must do is define the system. One needs to know (1) the electrode

geometry, (2) the free electron availability, (3) the type of voltage

applied, i.e., ac, dc or impulse, (4) the physical properties of the

system namely the type of gas and electrode materials as well as the state

of those electrodes. One can then either directly measure the quantities

of interest, namely and I(x,t) or use a physically based model. These

two procedures are, of course, compl ementary . One wants to know

under what conditions one can with some confidence use physically based models

and under what conditions one still needs to rely on direct measurements.

It is first necessary to consider the input requirements for a modeling

effort. These are: (1) initial electric field distribution E(x,t) since

all processes depend on electric field, (2) processes going on at the

electrodes, although in many cases rather inexact knowledge of

that is adequate for modeling, (3) gas number density N, and (4) either the
-41-

cross sections for gas collision processes as a function of energy,

or alternatively the swarm coefficients, i.e., the drift velocities,

diffusion coefficients, excitation, ionization, attachment and detachment

coefficients, etc., as a function of E/N, and N.

If cross sections are to be used, then one must first set up an

energy balance from which one can get an electron energy distribution.

The cross sections are integrated over that energy distribution and

inserted into the charge conservation equations to obtain the sparking

potential V

$

and I(x,t). The use of swarm coefficients can be somewhat

easier since in the determination of swarm coefficients the integration

of the cross sections over the electron energy distributions has already

been performed. The charge conservation equations must then be solved

with appropriate boundary conditions.

The questions to be asked are: what can we predict from the model

and how much confidence can we have in the results? It was noted that

we can calculate sparking potentials at low fields and this has been

done for quite some time. We can also simulate I(x,t) and this has, for

example, been done for current growth in nitrogen discharges in the low

pressure region.

In considering the question of confidence in modeling it was mentioned

that our present capability is limited. We can make reasonable predictions

for uniform fields for V

s and I(x,t) up to at least 600 kV provided

E < 10

7 V/m. For nonuniform fields we can determine dc corona onset

voltages and model early stages of current growth I(x,t) provided the field

is of sphere-plane configuration and is not too convergent. The limitation

here is determined by the combination of computing power and computer methods.
-42-

For the near future, within the next few years , it would appear possible

to extend modeling further into the more convergent nonuniform field

regions and predict later stages of I(x,t) and eventually, therefore,

the actual breakdown voltage V . For the distant future, there is

hope of extending the modeling to the region where fields at the

cathode exceed 10^ V/m such as occurs in the high pressure situation

discussed in the previous talk. At present, it appears unlikely that

modeling could be attempted in cases where there is an ill defined

initial field.

It was noted that the above discussion applies to single gases.

The case for gas mixtures was considered next. The question here is:

what are the prospects for modeling breakdown in mixtures? It was

emphasized that a mixture is in general not simply the sum of its

constituent parts. Examples of special problems that appear for mixtures

were discussed.

The first mixture considered was that of N 90+0«. In this

C_ Lm mixture 0^ is weaker, having a dielectric strength only about

three fifths that of N„0. However, adding a small quantity of

0 9 actually increases the breakdown strength of N 9 0. This is another

example of the synergistic effect noted in previous talks. Synergisms

have also been observed in mixtures of ^

and freons . The ionization

coefficients in these mixtures were found to exhibit a very peculiar

dependence on relative gas composition.

In mixtures there can be marked changes of the electron energy

distributions as compared to those expected from pure gases. Also,

collision processes can be introduced which are not possible in single

gases. For example, in the ^O+C^ mixture it is believed that the reaction
-43-

NO +0

2

-> N0+0

2

“ is introduced which effectively competes with the

electron detaching reaction N0”+N

2

0 N 2

0+N0+e. This mechanism may

explain the enhancement of N

2

0 dielectric strength by addition of the

weaker dielectric 0

2

. For mixtures one cannot reliably use swarm coefficients

to do the modeling. Except as a first approximation, one must resort in

this case to use of basic collision cross sections.

Although modeling for gas mixtures is indeed more complex than

modeling for pure gases, there is good evidence that modeling for

mixtures can nevertheless yield reliable results. Examples were

cited of recent successes in modeling of gas mixtures used in laser

discharges and in prediction of breakdown in SF^+He mixtures.

It was concluded that ready access to critically evaluated collections

of swarm coefficients and collision cross section data were essential to

a successful modeling effort. Mention was made of the existing program

to compile and evaluate swarm data at the JILA-NBS Atomic Collisions

Data Center. In closing, the question was put forth to the meeting

concerning the need for an organized program for collection and critical

evaluation of breakdown and sparking potentials. Would a handbook giving

this kind of information be useful?

Questi ons :

Dr. D. Miller of Gould-Brown Boveri brought up the question of

the importance of photoelectric processes at the cathode and thus the

importance of cathode materials. Dr. Dutton replied that at low fields

the effect of the cathode is indeed included in the boundary conditions.

In the high field region the surface becomes dominant. The question in

this region is: what is the feature of the cathode that is dominant,
-44-

e.g., is it surface roughness, surface deposits, geometry, etc.? This

was discussed in the previous talk by Dr. Cooke.

Dr. Cooke of MIT noted that swarm parameters are an average

quantity and wondered how long it takes an electron at rest suddenly

appearing in the gap to achieve the "average" conditions. In reply,

it was said that this depends on the number of collisions with gas

molecules, but should happen very quickly.

Dr. L. Chris tophorou of ORNL mentioned that relaxation times

for electrons are typically on the order of sec./Torr. He also

again emphasized that in mixtures it is not sufficient to merely

weight relevant parameters according to the partial pressures of the

components

.

Panel Discussion

3. Bernstein, Chairman

Bruce Bernstein (EPRI) :

In his opening remarks, Mr. Bernstein expressed concern about

the needs for future research. He pointed out that there are academic,

industrial and utility needs and perspectives that should be examined.

He then discussed highlights of the previous talks. The panel was

reminded of Dave Massey's point that if substation size can be reduced,

cost is not an extremely important factor. In their talks, Roy Wootton

and Martin Mastroianni offered the opinion that it was an unlikely

possibility that a single gas could be found that would be superior to

SFg in all respects. Various promising mixtures, however, were mentioned

by Loucas Christophorou and Roy Wootton. Marti n -Mastroianni expressed

concern about degradation in mixtures containing about 50% fluorocarbons.
-45-

Carbonization was discussed and the question was raised about how

important this problem really is in an actual power transmission

line since clean up would be necessary following flashover independent

of the degree of carbon deposition. Mat Kegelman described a method

for calculating the maximum amount of additive dielectric gas that

could, for example, be put in SF^ and still meet the necessary vapor

pressure requirements. Concerning basic, fundamental studies, the

questions there are how do we fit in, utilize and justify measurements

for example of ionization coefficients, attachment processes, etc., from

the point of view of aiding development of the utility’s needs for

future power systems? How do the measurements of R. Van Brunt at NBS

and others on fundamentals of partial discharges fit into the picture?

The question of toxicity was raised by Ed Walsh but not addressed in

depth in any of the formal talks.

Dave Massey (GPU Service Corp.) :

He noted that $900,000 might be paid for a 500 kV breaker that

is tripped only once in five years. The utility people would like to

know if there is any way that this can be done at lower cost. A better

gas might be part of the answer. Again it was emphasized that the tripping

rate for breakers is roughly only one per 100 miles per year. Thus, one

is paying a high price for protective equipment that remains idle for

long periods of time. Concerning carbonization, this may not be a big

problem in a transmission line, since clean up is always required after

a failure. One should be concerned though that the clean up operation

can be performed thoroughly and safely. The concern with environmental

acceptability of gaseous decomposition byproducts may be too great at

this point. The power industry is after all used to handling toxic and
-46-

hazardous materials, and problems in transmission systems are not

likely to be comparable to those, for example, in nuclear power

plant operation.

Bruce Bernstein (E?RI) :

He asked if substation size reduction should take priority.

Dave Massey (SP'J Spryjce Corp.l :

He replied that it should, and added the comment that the present

SF, insulated substation is a plumber's nightmare.

Jacques Castoncuay (IREQ) :

He mentioned that Hydro-Quebec has up to now installed one metal

clad SF^ substation. He also pointed out that one should be careful

to distinguish between insulation used in static systems compared to

that used in dynamic systems. In a static system, even if decomposition

occurs , the products which result are quite likely to be good dielectrics

and the system may continue to ooerate for many years. The byproducts

o^ SF,, for example, are generally good dielectrics since they tend to

be highly electronegative. Therefore, from the simple dielectric strength

point of view, the effect of gas decomposition may not be extremely

important. He noted also that a 50% SF.-h. mixture is nearly as good

a dielectric as pure SF. . Specialized mixtures could be used for

breakers. If these same mixtures could also be put into a metal clad

cable that would be desirable, but the mixture criteria for these two

applications may be quite different. Again we must ask, what are these

criteria? Is it immunity to particles, impurities and surface defects

for example? He pointed out that SF, is not particularly ideal in beina

o able to ~eet these criteria. Also SF, has a peculiar, unpredictable
- 47 -

behavior in terms of its pressure effect.

Michael Pohl (Air Products) :

He noted that they used the standard ASTM test to determine

contamination levels in the SFg gas which they manufacture, and he

was disturbed about findings reported by J. Castonquay at t h is ~eeting

concerning sensitivity of this test. However, they do not view it as an

absolute test, i.e., they cannot detect exactly how much hydrolyzable

fluoride is present. All they say from the test is that the concentration

is below a particular level. He also pointed out that Air Products

provides a service of performing analysis for users of gas that has

been in a breaker. This requires extraction of a gas into a cylinder

which is then shipped to the testing laboratory.

Jacques Castonquay (IREQ) :

Those at the Hydro-Quebec Laboratory have taken the view that it

might be preferable to develop techniques to analyze gases on s’te in

the field) since it is quite likely that fluoride content will be

perturbed and change during the time of sampling and shipping.

Michael Pohl (Air Products) :

At Air Products, they have been investigating these changes in

fluoride content due to sampling by using samel es of gas wit” contro ec

levels of fluoride contamination. They have indeed observed ’ arge

changes in fluoride content after the samp e has been ~e c t

n e

cylinder for some time. The decomposition oroduct concentrations

observed immediately after arcing may be quite di r ~erent •'c - those

observed a day or so after arcing.
-48-

Loucas Chri stophorou (ORNL) :

He also noted that Dr. I. Sauers in their laboratory has observed

that fractional content of certain decomposition products changes with

time after arcing. He brought up again the question of costs discussed

by Dave Massey.

Dave Massey (GPU Service Corp.) :

He reminded everyone that it was the total cost that was significant

and increase in gas costs could certainly be tolerated if these were

accompanied by compensating factors such as size reduction or increase

in efficiency. He noted that the cost of losses in a transformer

are greater than the cost of a transformer.

Bob Noberini (Consolidated Edison Company) :

The utilities have a big interest in 345 kV transformers that

are cooled by SFg and/or other gases having desirable insulating

properties. He mentioned that the Consolidated Edison Company is

acting as a host utility for DOE on testing of such transformers in

the single phase 200-400 MVA range. Right now there are problems in

dealing with oil cooled transformers of this power level. Considerable

time, for example, is required in the installation of such equipment.

With gas insulated systems, it is hoped that the equipment can be

manufactured, sealed and tested at the factory and be ready for immediate

installation in the field. Other desirable aspects of gas insulated

systems in transformers would include the fact that they are

non-flammable, non-explosive, relatively non-polluting, have a lower

noise level than oil systems and a lighter shipping weight. He pointed

out that they now have an SFg insulated substation on the west side
- 49 -

of Manhattan with five transformer banks and a capacity for another

five. All the transformers , however, are oil cooled, and it is their

hope to eventually replace these with gas cooled equipment. They

could, he noted, reduce the size of this substation considerably

by going to gas cooled transformers because the transformer is the

single largest piece of equipment. One of the hopes from future use

of SFg or other gas insulated equipment is that it will help solve the

current and impending corridor crisis. They simply cannot pack any

more power into existing right-of-way by present methods.

Consolidated Edison has a "now" problem in terms of high load density,

very tight corridor, and zero right-of-way expansion capability.

Future increases in power delivery must go underground.

Ed Walsh (Hestinghouse) :

He again broached the topic of toxicity. He pointed out that

aluminum fluoride formed in arcs is a highly toxic material. He

wondered if anyone had ever been poisoned or otherwise adversely

affected by the arcing byproducts of an SF^ system. The answer from

the utility's viewpoint seems to be no_. He also expressed concern

about toxic contamination in substations due to rupture failures.

Mike Pohl (Air Products) :

He mentioned that the recommended procedure for cleaning up fluoride

contaminated SF

g is to dump the gas into a gas cart, preferably with

a blower. Technicians performing this task should leave no cart of the

body exposed, e.g., they should have sleeves and pant legs taoed. wear

gloves, gas masks, etc.
- 50 -

Loucas Chri stophorou (ORNL) :

He pointed out that one desirable and promising feature of some

new gas mixtures is that they may be more resistant to production of

toxic gases in arcing. There is no doubt that this is an important

feature deserving careful consideration. He also argued that one

should look into scavenging techniques for removal of toxic byproducts.

Vasu Tahiliani (EPRI

)

:

He mentioned that EPRI already has a project addressing this

problem of developing scavenging techniques for toxic substances.

Martin Mastroianni (Allied Chemical) :

Circuit breakers, he noted, have built-in gas cleaning systems.

In a two pressure breaker, for example, there is an active system where the

gas is pumped through a cleaning system in the breaker. In a single

pressure breaker, there is a cleaning system on the side of the port to

clean gas up from normal arcing operation. One of the goals of the

EPRI project mentioned will be to look at how effective various cleaning

agents are. Also, one must determine if there is sufficient cleaning

capacity. In a transmission bus, the system is compartmentalized and

gas can be cleaned, or scrubbed, by pumping it through a cart with a cleaning

system. He pointed out that if a burn-through in a power line occurred,

the release of toxic gas would only be for a short period, and it is

unlikely, unless a worker is very close at the time of failure, that local

toxicity would prevail and be maintained at hazardous levels for very long.

Data on long term toxicity does not apply in this case. He also mentioned

that there have thus far been no recorded human injuries or deaths due

to exposure to SFg gas used in power systems. The major concern with
-51-

hazards from burn-throughs would be in enclosed gas insulated substations

that have inadequate venting to prevent trapping of decomposed gas.

Jack Dutton (University College Swansea) :

Turning to the academic perspective, he again emphasized that

once all of the fundamental atomic and molecular collision processes

are identified and understood and incorporated into a complete model,

one does not have to measure every sparking potential to know how a

given system (dielectric gas mixture) will behave. Better theoretical

models are needed, and better models require better fundamental input

data. As more reliable fundamental data become available, one can also

hope to extend the range of the modeling effort to include other gases

and other operating conditions. Considerable good basic data exist

already for simple, common gases like and N^, but the data is

sparse for many of the various exotic fluorocarbon gases

discussed in this workshop as dielectric additives or as oossible

replacements for SF C

. Extension of modeling will also require improved

computation techniques. In considering nonuniform fields, modeling

must also be linked with experiments on the growth of space charge.

He recommended a basic experimental program on the study of space charge

development. Particularly at high fields one also needs to Know more about

fundamentals of what happens at the interface between the gas and electrodes.

Experiments are needed to look at microscopic details of what happens

on a surface in a short time scale (say on the order of microseconds or

less) in fields greater than 10

7 V/m at high gas pressures. It is his

opinion that the technology does not yet exist that wil 1 allow satis;ac^ory

performance of these kinds of experiments, at i eas ^ under conditions
- 52 -

that approximate practical situations. The collection and critical

analysis of fundamental data are needed to support future modeling

efforts. These data include collision cross sections and electron

swarm coefficients. One critical output from modeling is the sparking

potential. Again he posed the question concerning the need for

critical evaluation of electrical breakdown data. Should one, for

example, produce a handbook of critical data for gaseous dielectrics?

Input from many different laboratories would, of course, be required

to generate such a handbook.

N . Giao Tri nh ( I REO) :

He expressed the opinion that results of fundamental studies or

theoretical models can never be a substitute for testing. The

extrapolation of models which apply to ideal situations cannot be

made easily to practical situations. Testing must always be relied

on as the final method of determining acceptance of insulating systems.

He questioned whether the results obtained from fundamental research

can really be made applicable to engineering problems, and expressed

the need to establish more of a link between basic and applied studies.

Jack Dutton (University College Swansea) :

He agreed that one would never get to a situation where modeling

based on fundamental physics could replace engineering testing. That

is not the ultimate purpose of modeling. It should rather provide

guidance in design of future high voltage systems and help determine

which are the more promising systems that should be subjected to

extensive testing.
-53-

Ara Chutjian (JPL) :

He agreed that fundamental data and modeling can help in finding

the best choices for gases to be tested. He noted that one of the

present gaps in fundamental data was in the low electron energy region

Many cross sections and rate coefficients are not determined in the

important energy region below 15 meV where, for example, attachment

cross sections are rapidly increasing. He then proceeded to describe

in some detail a new experimental method developed at JPL to measure

line shapes for attachment of threshold electrons to electronegative

gas molecules such as SF^ and CFC1 ^ * With the technique described,

it is now possible to measure attachment cross sections at electron

energies in the range of 0-50 meV which is not covered in measurements

by swarm techniques such as those used in the ORNL program. The

technique involves the production of a narrow band of low energy

2 electrons via direct photoionization of Xe gas to the P-| /7 state or

Xe

+

in a field free region. The electronegative gas of interest is

mixed in with Xe, and negative ions such as SF^ or Cl produced by

attachment of the photoelectrons are detected with a mass spectrometer

The electron energies in the experiment are varied by changing the wav

length of the photon source. The photon source for these experiments

is a discharge lamp with a monochromator, ihe measured production, 0

,

of a negative ion, say X' at an incident photon energy E q is obtained

from the expression

P(EJ = S *(<
-54-

where S is the incident photon bandwidth, Oj and cr^ the Xe photoionization

and electronegative gas electron attachment cross sections, respectively,

and where * denotes convolution. The Xe photoionization cross section

is assumed to have the usual step function behavior and the function S

is taken to be a Gaussian with a width determined by the slit width

(or resolution) of the monochromator. They have measured a width of

30 meV for the low energy electron attachment cross section in SF^.

They can put their measured attachment profiles on an absolute cross

section scale by normalization to the results of "good" swarm measurements

at electron energies above 100 meV. He noted that they can also

look at dissociative attachment channels.

R. J. Van Brunt (N3S) :

He commented that one of the purposes of the NBS swarm data

evaluation program is to identify possible serious gaps in the fundamental

data. An example of this was just pointed out in the discussion by

Dr. Chutjian, namely that there is very little reliable cross section

data at low electron energies. He also questioned where the emphasis

in the modeling effort should be placed. Clearly some of the newer,

exotic gas mixtures such as those involving perfl uorocarbons are not yet

ripe for modeling since very little fundamental cross section data are

now available. Perhaps modeling efforts should concentrate more for

now on the familiar gases and gas mixtures such as SFg, SF.-CC^,

SFg^, SFg-N

2

, and CC^-^ for which considerable fundamental data

exist, and where much yet remains to be done in understanding fundamental

gas discharge phenomena. Eventually modeling can be extended to other
-55-

gases and mixtures as reliable swarm and cross section data for these

become available. He urged a vigorous experimental program to

obtain fundamental data on some of the newer, more promising electronegative gas additives to gaseous dielectrics. One should, for example,

try to obtain a fundamental understanding of why synergisms occur in some

cases but not in others, and why some mixtures work better than others.

Loucas Christophorou (QRNL) :

He commented that they actually have measured cross sections down

to zero energy, although indeed in their swarm measurements the cross

sections derived from attachment rates are quite uncertain for energies

below ( 3/2 ) kT ('v 0.038 eV). He questioned the importance of knowing

exact values for these very low energy cross sections in understanding

the electrical behavior of a gaseous dielectric, since the relative number

of electrons at very low energies (at the low energy tail of the

distribution) is very small. He commented further about the disturbing

fact that there is not a single laboratory in the world today doing

D/y (diffusion/mobility) measurements. He also pointed out that although

there are many laboratories performing measurements of relative cross

sections, there are only a few doing the more important absolute cross

section measurements. Of course, absolute cross section measurements

are more difficult to make. However, absolute cross sections are needed,

particularly in the subexcitation energy range.

Jack Dutton (University College Swansea) :

He agreed that there is a lack of effort on D/y measurements.

However, he noted that the situation is more serious than that since we

cannot at this time measure attachment and detachment coefficients to

better than 50% except in very special cases.
- 56 -

Bob Hebner (NBS) :

He brought up the question about where the effort in

modeling should be placed. He pointed out that modeling programs are

underway to predict reliability of gas insulated systems , and the important

considerations here are not necessarily processes that occur in the gas

itself, but rather such things as particle dynamics, exchange of electronic

charges at metal -particle interfaces, etc.

R. J. Van Brunt (NBS) :

In reply to this comment, he argued that there are two different

points of view, one being that of evaluating the dielectric performance

of a gas by itself for comparison purposes, and the other being that

of evaluating total system performance. Most of the discussion at

this workshop has focused on consideration of the gas by itself, such

as its breakdown strength or its interaction with other materials.

There is the prevailing understanding nevertheless that this is far

from the complete picture, and in a practical system there may be

many other more important things going on that one should consider.

Jack Dutton (University College Swansea) :

He emphasized again that one needs guidance about the behavior

of the gas. The effects of particles and surfaces can in fact be

viewed as part of the gas problem.

Roy Nakata (GE) :

He expressed the opinion that when considering the question of

a gas system versus a practical system one should distinguish between

the point of view of near and long range future. Although there is a

definite need for information on fundamental behavior of gases for the
-57-

future , there are anomalous behaviors in complete practical systems

that need examination now. For example, there are questions in considering

electrodes or conductors of how sharp is "sharp", and how divergent

is a "divergent" field. Whereas in a gas transmission line the field

may be easy to determine, in a transformer this is not necessarily the case. Should we not design systems in the near future to take better

advantage of gases at hand? Use could perhaps be made of new materials

and corona stabilization.

Jack Dutton (University College Swansea) :

He noted that corona stabilization depends entirely on the

development of space charge, and consequently if one can acquire a basic

understanding of space charge development, then one can get a handle on

the problem of corona stabilization.

Martin Mastroianni (Allied Chemical) :

He felt that the concern of this workshop is with future electrical

systems. For electric power systems, there are the needs of three

general types of equipment, namely (1) transmission lines, (2) circuit

breakers, and (3) transformers . For a circuit breaker, the important

characteristic of the gas dielectric used is its arc interrupting

capabilities, and for this application, the insulating properties are

of secondary importance. For a transformer, the heat transfer properties

of the gas are important. In a GIT line the breakdown voltage or insulating

quality takes precedence. The point again is that the type of gaseous

dielectric used should depend on application and equipment requi rements

.

There is still much that remains unknown about the electrical properties

of SFg, and as a result we presently design equipment that can use only
- 58 -

50% or less of the intrinsic dielectric strength capability of this

gas. He is of the opinion that emphasis for future research should

address the problem of particles. If a gas is found that has an equal

or higher dielectric strength than SF^ or other gas presently used,

it must be demonstrated that this remains true for conditions of

particle contamination or particle initiated breakdown. He expressed

the need for more fundamental work on the breakdown problem, in

particular the investigation of mechanisms that prevent optimum use

of the intrinsic strength of the gas. Attempts should be made to

look at a gas mixture from the point of view of adding a new kind of

"magic fairy dust" that will enable one to use the full capability

of the gas

.

Roy Wootton (Westi nghouse) :

He brought up again the question of the benefits of corona

stabilization mentioned by Roy Nakata. Corona stabilization occurs,

for example, on the tip of a particle or surface protrusion. However,

if an initiating electron does not appear early on, corona stabilization

will not happen under impulse, whereas a later initiating electron will

lead to breakdown. The delay in an initiating electron is thus of no

benefit in a nonuniform field regime where one is relying on corona

stabilization to prevent breakdown. He expressed the need to look at

a practical system as a "whole" system. In considering aging one must

not only look at the chemistry that goes on in the gas but also

interaction between the gas and the resin. Also, resins at high temperature

undergo deterioration and give off their own gases. In the short term,

it appears that it would be desirable to learn more about what goes on
- 59 -

in SFg so that its full potential as a dielectric can be utilized.

A case can be made that flashover on solid insulators in compressed

gas systems deserves priority in future research. In the long term,

one has to look more carefully at the basic data for gases.

Jack Dutton (University College Swansea) :

The question of preconditioning of high voltage compressed gas

insulated apparatus was raised by several including Alexander Stewart

of Harry Diamond Labs. In response to this, Dr. Dutton pointed out that

although the conditioning effect is well known, it is not necessarily

well understood. For example, in some systems a polished electrode

surface can become pitted as a result of conditioning, yet the overall

dielectric strength of the system improves. Why does this happen?

Chat Cooke (MIT) :

He pointed out that as systems are pushed to higher and higher

stresses one is less and less able to predict and understand insulation

performance. The predictions become less capable of meeting reality.

When looking for a "better" gas dielectric one is trying to meaningfully

compare the electrical performances of various gases. Chat proposed

that serious consideration be given to a standardization of electrodes

for testing with the objective that meaningful comparisons can be made

of different gases. The electrodes will have to be special because

it is no longer possible to consider uniform field conditions, since surfaces

are in general nonuniform. Many tests have shown that if one produces

spark breakdown between electrodes in different gases, the resulting

physical surfaces of these electrodes can be quite different. The nature

of electrode surfaces changes completely as a result of breakdown and

this change is dependent on the gas used. He would like the electrodes
-60-

used to be useful from a calculation! point of view. As one suggestion,

he proposed that one use an electrode system in which there is a known,

well defined, small asperity on the cathode surface. This asperity should

be defined so that the initial field distribution is well characterized.

Such a protrusion could have a spherical end radius and a height-to-radius

ratio less than ten. The electric field for a hemispherical protrusion or

sphere on a pedestal can be solved exactly. In comparing gases, one

should look at the initiation or inception of breakdown. He suggested

that the partial discharge study carried out at NBS by Van Brunt is

perhaps a useful approach. He emphasized that the "standard" electrode

system chosen should be a compromise of some kind in which it is possible

to make calculations and yet will allow one to make the leap to prediction

of what might happen in practical systems. The cell must be small enough

so that it can be used readily, but have the characteristics that enable

one to infer what might happen in a practical system. Of course, there

is really no hope of using such a laboratory test to predict exactly what

will happen in a big practical system. However, it can serve as a basis

for a better, more meaningful comparison of the proposed insulating

materials, namely the gases, and provide information that is related as

best possible to a practical condition. In any case, it should not be

designed to give misleading information.

Bruce Bernstein (EPRI) :

He suggested that this question of standardized electrodes be fed to

the forthcoming International Symposium on Gaseous Dielectrics. He also

indicated that he would alert (and has in fact already done so)

Dr. Alan Cookson, chairman of the "Insulation Systems for Gas Insulated

Cables" Subcommittee (5-12) of the Insulated Conductors Committee about
- 61 -

this problem with hopes that it be considered at the next subcommittee

meeting

.

R. J. Van Brunt (NBS) :

He expressed sympathy for the need to develop standardized

electrodes for testing and indicated the interest of NBS in this effort.

He reminded the group that it is the function of MBS to assist industry

in development of more meaningful test, monitoring and measurement

procedures. Although they are not in the business of developing new

materials, namely in this case gaseous dielectrics, they take the view

that a better understanding of basic physical phenomena can help not only

in the development of new materials but also in development of more

meaningful comparative tests of these materials. They have recently

concentrated their laboratory effort on the study of partial discharge

(corona) phenomena with the hope that observation of prebreakdown partial

discharges in nonuniform fields might offer a better way of evaluating

relative dielectric performance of different gases. He raised the question

about the urgency of the need for new gaseous dielectrics in all

applications. He could see, for example, an urgent need in the specialized

case of high power switching, as used for large lasers and beam machines,

where gas decomposition is significant, but the urgency of the need in

electric power transmission applications was not entirely clear. Perhaps

more could yetbedone with existing gases.

Ara Chut.jian (JPL) :

He raised the question about possible effects of fl uorocarbons used

in dielectrics on the ozone layer in the earth s upper atmosphere.
-62-

Loucas Chri stophorou (ORNL) :

In response to Van Brunt's question, he expressed the opinion

that there is a need that must be satisfied now for new gaseous

dielectrics in power systems application. Industry should, for

example, be able to adjust itself to new dielectrics and consider

design of equipment to accommodate different kinds of gases. One

should not strive now for the optimum systems. Optimization of systems

will naturally occur as need arises and as more scientific information

is acquired about behavior of materials.

Roy Wootton (Westi nghouse) ;

Going back to Dr. Cooke's proposal for standardized electrodes,

he indicated that in their EPRI program to examine new gases they also

encountered the difficulty mentioned, namely that comparative breakdown

tests on 1 atm and 4 atm gases in uniform fields did not necessarily

give a good indication of what happens for tests performed using geometries

that simulate practical conditions.

Dr. V. L. Kenyon, III (Naval Surface Weapons Center) :

He brought up the problems associated with the special application of

a gaseous dielectric as a high power switching medium. The big problems

here are how to avoid total decomposition of the gas and limit the

deleterious effects of the decomposition products on the rest of

the switch. He pointed out that this use of gaseous dielectrics, although

not particularly significant now, can be expected to increase in the future.

Bruce Bernstein (EPRI) :

He commented that EPRI has made a conscious effort to avoid funding

research on SFq-^ mixtures, since considerable work on this has been

conducted by others.
- 63 -

Vasu Tahiliani (EPRI

)

:

He argued that one of the challenges for the future is to

devise ways of exciting the insulation system to screen out particular

defects like particles, protrusions, broken insulators, absence of

gas, presence of air, etc., without having to apply high voltage.

EPRI is currently interested in funding research directed to this problem.

Dave Miller (Gould-Srown Boveri

)

:

He wondered if modelers and those involved with design of measurements

are going in the direction that will help those designing equipment

decide why they may, for example, be achieving only a fraction of the

total insulating capability of a gas, such as is the well known case for

SFg discussed several times in this workshop. He expressed concern

that those doing basic gas studies are going in the direction of

controlled experiments which do not readily relate to practical situations.

Perhaps one needs to steer modeling more in the direction that makes

it applicable to practical equipment. Should there not be a number of

different models, e.g., models that consider separately particle effects,

breakdown along insulator surfaces, etc.?

Jack Dutton (University College Swansea) :

In reply to Dr. Miller, he emphasized again that in the case of

SFg something other than the gas dominates the breakdown condition at

high fields and high pressures. In this situation, one is injecting a

lot of electrons from a small site somewhere in the system. The question

then is, can one model a situation in which one injects a space charge over

a very small volume? At present, this is a difficult situation to model ,

but some progress is being made.
- 64-

Loucas Chri stophorou (ORNL) :

He pointed out that a large number of experiments and tests under

controlled conditions are needed to supplement modeling. They are,

for example, investigating under controlled conditions effects of surface

roughness, particles, etc. These kinds of investigations as well as

modeling are required before it is advisable to proceed with any large

scale industrial testing. There might be some questions, however,

concerning the appropriate types of tests to which new gases should be

subjected

.

R. J. Van Brunt (NBS) :

He commented that the fundamental work primarily provides guidance

and does not solve immediate problems associated with practical systems.

He cited the example of Castonquay's work on effects of contamination

on rate of gas degradation. A fundamental understanding of the chemistry

involved here could help in the design of gaseous insulation which is

resistant to contamination. The fundamental research often works

around the fringes of the practical problems, and benefits of this

work may not be realized until long after the work is completed. It is

his opinion that basic research should be viewed in general as being

geared more toward long range, future technology and adding to the store

of knowledge upon which those in the future can draw in designing better

systems. It may, nevertheless, provide insight into the solution of

existing problems, but should not necessarily be steered in that direction.

Modeling and basic laboratory tests and measurements will never be a

substitute for engineering testing; on the other hand engineering testing

alone cannot provide the insight acquired from basic research.
-65-

Loucas Chri stophorou (ORNL) :

In closing this workshop, he made an announcement concerning the

forthcoming Second International Symposium on Gaseous Dielectrics that

will be held at the Hyatt Regency Hotel, Knoxville, Tennessee,

March 9-13, 1980. It will be hosted by the Oak Ridge National

Laboratory and sponsored by the Department of Energy in cooperation

with the IEEE Power Engineering Society. Those who have not been

notified of this meeting and want to attend should contact:

Dr. L. G. Christophorou

Chairman, Second International Symposium

on Gaseous Dielectrics

Oak Ridge National Laboratory

Building 4500S, H-162

P.0. Box X Oak Ridge, Tennessee 37830 .

It is hoped that questions, ideas, and opinions generated at this

workshop will be brought to the symposium for further discussion. Again,

the symposium will bring together those from many countries involved

in a broad spectrum of research from basic physics to applied engineering.
_
AGENDA

WORKSHOP ON GASEOUS DIELECTRICS FOR USE IN FUTURE ELECTRIC-POWER SYSTEMS

National Bureau of Standards

Gaithersburg, Maryland

September 10 and 11, 1979

Monday, September 10 -- Lecture Room D, Administration Building (101)

11:00 am — Registration

1 :00 pm

1:15 pm

1 :30 pm

Chairman’s Opening Address

T. F. Ganntty, Vzpantmznt oh Enzngy

Users Look at Insulation Systems

V. E. Maiaey, GPU Sznvtzz Conponation

An Experimental Study of Dielectric Gases for Practical Applications

R. E. Wootlan, Wz&ttingkou&z PSV CzntZA

2:00 pm -- Breakdown and Stability Properties of Some New Gaseous Dielectrics

M. J. Mcu>tnotannt, Atttzd Chznu.zat Conponation

2:30 pm — Calculation of Vapor Pressure for Dielectric Gas Mixtures

M. R. Kzaelman, E.I. duPont dz Nzmouu & Company

3:00 pm — Coffee

3:30 pm — The Oak Ridge Program on Gaseous Dielectrics

L. G. Ck/cLitopnonou, OaJz Zidgz National Labonatony

4:00 pm — Analysis of Trace Decomposition Products in Gaseous Dielectrics

J. Caitonquay, Hydn.o-Qu.zbzc. In&tctutz ah PzizancJn

4:30 pm — Study of Corona Characteristics and Corona Chemistry in Gaseous Dielectrics

R. J. Van B^nunt, National. Sunzan. oh Standandj,

Tuesday, September IT — Room A340, Metrology Building (220)

3:30 am

9:00 am

9:30 am

10:00 am

10:30 am

1:30 pm

2:30 pm

Registration

Basic Factors Influencing Reliability

C. M. Cookz, Mai4 achuzzttz, hiztttutz Tzzhnotogy

Swarm and Breakdown Data *or Gaseous Dielectrics

J. Vutton, Unlvzsutty Cotizgz oh 3uxum,za

Coffee

Panel Discussion — Needs for Future Research

Chapman - S. Bznnstztn, EPRI Panzt Mzmbz/ii - J. Vutton, J. CoAtcncuau , V. E. Moa&zu,

M. J. Ma&tnotanni, R. J. Van Bnunt,

and J. Bzzktzn

Lunch - provided for participants in the NBS Senior Lunch Club

NBS Openhouse — Gaseous and Liquid Dielectrics Research

Laboratories

LIST OF ATTENDEES

Workshop on Gaseous Dielectrics for Use In Future Electric-Power Systems

National Bureau of Standards

September 10 and 11, 1979

1 . Earl C. Beaty

JILA Atomic Collisions Information Cntr.

Joint Institute for Laboratory Astrophysics

National Bureau of Standards and

University of Colorado

Boulder, CO 80309

2. Bruce S. Bernstein

Electric Power Research Institute

1800 Massachusetts Avenue, N.W.

Suite 700

Washington, D.C. 20036

3. Jacques Castonquay

Science des Materiaux

Institute de Recherche de 1' Hydro-Quebec

1800 Montee' Ste-Julie

Varennes, Quebec

Canada

4. Loucas G. Christophorou

Atomic, Molecular and High Voltage Physics

Laboratory

Oak Ridge National Laboratory

P.0. Box X

Oak Ridge, TN 37830

5. Ara Chutjian

Jet Propulsion Laboratory

California Institute of Technology

4800 Oak Drive

Pasadena, CA 91183

6. Chatham M. Cooke

High Voltage Research Laboratory

Building N-10

Massachusetts Institute of Technology

Cambridge, MA 02139

7. D. Ken Davies

Research and Development Cntr.

Westinghouse Electric Corp.

Beulah Road

Pittsburgh, PA 15235
8 . John Devins

Room 21, Radiation Bldg.

General Electric Company

River Road

Schenectady, NY 12301

9.

Jack Dutton

Department of Physics

University College Swansea

Swansea, Wales

Great Britain

10. Thomas F. Garrity

Electric Energy Systems Division

Department of Energy

20 Massachusetts Avenue, N.W.

Washington, D.C. 20545

11. Robert E. Hebner

National Bureau of Standards

Electrosystems Division

Washington, D.C. 20234

12. David R. James

Atomic, Molecular and High Voltage

Physics Laboratory

Oak Ridge National Laboratory

P.0. Box X Oak Ridge, TN 37830

13. Philip E. Jones

Allied Chemical

Specialty Chemicals Division

20 Peabody Street

Buffalo, NY 14210

14. Matthew R. Kegelman

E.I. DuPont de Nemours and Co.

Petrochemicals Department

Bldg. 136, Room 139

Experimental Station

Wilmington, DE 19898

15. Edward F. Kelley

National Bureau of Standards

Electrosystems Division

Washington, D.C. 20234
16 . William Kennedy

Transformer Department

General Electric Company

100 Woodlawn Ave.

Pittsfield, MA 01201

17. Van Kenyon III

Naval Surface Weapons Cntr.

White Oak Laboratory

Silver Spring, MD 20910

18. David A. Leep

National Bureau of Standards

Electrosystems Division

Washington, D.C. 20234

19. Dave Massey

GPU Service Corp.

Route 183, Reed Road

Box 1018

Reading, PA 19603

20. Martin J. Mastroianni

Allied Chemical

Specialty Chemicals Division

20 Peabody Street

Buffalo, NY 14210

21. Ronald H. McKnight

National Bureau of Standards

Electrosystems Division

Washington, D.C. 20234

22. J. Robert McLoughlin

General Electric Company

Research and Development

Room 12, Radiation Bldg, K1

P.0. Box 8

Schenectady, NY 12301

23. David B. Miller

Gould-Brown Boveri

Research and Development Division

Norristown Rd. and Route 309

Spring House, PA 19477

24. Martin Misakian

National Bureau of Standards

Electrosystems Division

Washington, D.C. 20234
25 . John H. Moore

Department of Chemistry

University of Maryland

College Park, MD 20742

26. Roy Nakata

Bldg. 9, Room 110, HVL

General Electric Company

100 Woodlawn Avenue

Pittsfield, MA 01201

27. Robert N. Noberini

Consolidated Edison Co.

4 Irving Place

New York, NY 10003

28. Oskars Petersons

National Bureau of Standards

Electrosystems Division

Washington, D.C. 20234

29. Michael C. Pohl

Air Products and Chemicals, Inc.

Specialty Gas Department

Box 351

Route 1 Tamaqua, PA 18252

30. A. H. Qureshi

Department of Electrical Engineering

University of Windsor

Windsor, Ontario N9B 3P4

Canada

31. John D. Ramboz

National Bureau of Standards

Electrosystems Division

Washington, D.C. 20234

32. I. Sauers

Atomic, Molecular and High Voltage

Physics Laboratory

Oak Ridge National Laboratory

P.0. Box X Oak Ridge, TN 37830

33. Alexander L. Stewart

Harry Diamond Labs

2800 Powder Mill

Aurora Facility

Adel phi, MD 20783
34 . Vasu Tahiliani

Electric Power Research Institute

3412 Hill view Avenue

P.0. Box 10412

Palo Alto, CA 94303

35. N. Grao Trinh

Institute de Recherche de V Hydro-Quebec

1800 Montee' Ste- Julie

Varennes, Quebec

Canada

36. Richard J. Van Brunt

National Bureau of Standards

Electrosystems Division

Washington, D.C. 20234

37. A. P. Vitols

General Electric Co.

100 Wood! awn Avenue

Pittsfield, MA 01201

38. Edward J. Walsh

Westinghouse Electric Corp.

Sharon, PA 16146

39. Roy E. Wootton

High Voltage and Gas Physics

Westinghouse Research and Development

Center

Beulah Road

Pittsburgh, PA 15235

NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 80-1966

2.Gov’t. Accession No. 3. Recipient’s Accession No. iMBi m

4. TITLE AND SUBTITLE

Minutes of Workshop on Gaseous Dielectrics for Use in

Future Electric-Power Systems -- September 10-11, 1979

5. Publication Date

8. Performing Organization Code

7. AUTHOR(S)

Richard J. Van Brunt

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Proj ect/Task/Work Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State. ZIP)

Electric Power Research Institute, 3412 Hillview Ave.,

P.0. Box 10412, Palo Alto, California 94303

Department of Energy, 20 Massachusetts Ave., Washington , DC 2054

National Bureau of Standards, Washington, D.C. 20234

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

[ | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200~word or leas factual summary of moat significant information . If document includes a significant bibliography or

literature survey, mention it here.) This workshop was held at the National Bureau of Standards laboratories in

Gaithersburg, Maryland, on September 10-11, 1979. The agenda and a list of active

participants is attached. The workshop was organized for the purpose of addressing

questions concerning current and planned research on development of new gaseous

dielectrics viewed as possible substitutes for SFg. Focus was on identifying

technological barriers to use of new gases and gas mixtures and questions concerning

properties of gases in need of further investigation. It brought together many

individuals from industry, government, and academia with diverse interests and

opinions concerning the needs for future research in thegaseous dielectrics area.

It was assumed that the workshop would provide a stimulating prelude to the forthcoming International Symposium on Gaseous Dielectrics where unresolved questions

concerning the needs and desirable directions of future research can again be debated

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons) .... ...... , , . Basic and applied research; chemical stability; corona; dielectric strength;

electric power systems; gaseous dielectrics; insulation; SFg; sulfur hexafluoride;

swarm data

18. AVAILABILITY I f Unlimited

| y | For Official Distribution. Do Not Release to NTIS

f-

] Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No, SN003-003-

j—

;

Order From National Technical Information Service (NTIS), Springfield,

VA. 22161

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(THIS REPORT)

UNCLASSIFIED

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