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,.I.N,S.T- 0F STAND & TECH R.I.C.

A 11 ID 5 )

i 1980 Annual Report:

, Assistance for Future

> Systems Research

Technical

Insulation

R. J. Van Brunt, M. Misakian, and D. A. Leep

Electrosystems Division

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

K. J. Moy, Graduate Student

University of California

E. C. Beaty

Quantum Physics Division

U.S. Department of Commerce

National Bureau of Standards

Boulder, CO 80303

1

*

NBSIR 81-2242

1980 ANNUAL REPORT: TECHNICAL

ASSISTANCE FOR FUTURE INSULATION

SYSTEMS RESEARCH

R. J. Van Brunt, M. Misakian, and D. A. Leep

NATIONAL. BVRRAU

or standards

libra rt

NOV 2 3 1981

nCrt Cue

QCtbO

no.

lift

Electrosystems Division

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

K. J. Moy, Graduate Student

University of California

E. C. Beaty

Quantum Physics Division

U.S. Department of Commerce

National Bureau of Standards

Boulder, CO 80303

January 1981

Issued April 1 981

Sponsored by

Office of Electric Energy Systems

Department of Energy

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
• -J.u . j» , -^1 " - 

» •AMI • s voivM
TABLE OF CONTENTS

Abstract .............................. i

Summary. ......... ............... iii

I. Introduction 1

II. Technical Progress ..... 5

II. A. Characterization of Corona Pulses in SFg and

SFg - N 2 Mixtures .......... 5

II.A.l Motivation 5

II. A. 2 Apparatus ........ 6

II. A. 3 Data Analysis ................. 8

II. A. 4 Instrumental Factors 11

II. A. 5 Results and Conclusions ............ 26

II. B. Comparison of AC and DC Corona Inceptions in SFg. ... 29

II.B.l Motivation 29

II. B. 2 Apparatus and Measurement Technique ...... 29

II. B. 3 Results . ....... 31

II. B. 4 Discussion 38

II. C. Identification of Corona Induced Decomposition

Products in SFg 47

II.C.l Motivation 47

II. C. 2 Apparatus and Measurement Technique ...... 48

II. C. 3 Results 57

II. C. 4 Discussion 65

II. D. Effects of Trace Decomposition Products and H

2 O on SFg Positive DC Corona Characteristics. ........ 67

II. D . 1 Motivation 67

II. D. 2 Measurements and Data 68

II. D. 3 Discussion 81

II. E. Laser Induced Changes in N 2 Corona Characteristics . . . 83

II.E.l Motivation 83

II. E. 2 Experimental Arrangement 84

II. E. 3 Results and Discussion 84

II. F. Other 87

III. Conclusions and Summary 88

References 92

i

Abstract

An experimental technique has been developed to quantify the electrical

characteristics of pulse- type partial discharges (corona) in compressed

electronegative gases. By this method the pulse repetition rates, shapes, and

height distributions are determined together with the average current. The

technique has been applied to the investigation of corona behavior in

compressed SF5 and SFg/^ mixtures, and to measurements of corona

inceptions in SF5 for both dc and 60-Hz ac voltages. A gas chromatographmass spectrometer has been employed to monitor production of trace contaminants

by corona-induced degradation of SF5. The most abundant gaseous by-products

that were detected from dc corona were H2O, SOF2, and S02F2* The

results of chemical analysis were correlated with measurements of the

electrical characteristics of the corona. It was discovered that the pulse

height distributions and repetition rates were sensitive to the presence of

trace contaminants at the ppm level. Experiments were also performed on the

effect of trace levels of water vapor, introduced by heating a wire, on

positive dc corona pulse burst behavior in SF5. It was noted that the burst

activity was significantly inhibited by trace amounts of H2O. These results

indicate that measurement of pulse characteri sties of positive corona can be

used as a sensitive monitor of gaseous contamination levels. The effects of uv

radiation and optical radiation from a tunable dye laser on electrical behavior

of corona in SF5 and M2 were investigated. Ultra-violet light can have,

under certain conditions, a large effect on initiation of electron avalanches

in SF5 near onset. Optical radiation in the wavelength range 580 to 620 nm
was found to produce a measurable effect on corona in M2 when it impinged on

the electrode surfaces. A bibliography of electron swarm data needed to

predict theoretically breakdown and to model discharge phenomena in gases was

prepared.

Di sclaimer:

This is a report of an ongoing research project. Therefore, some of the

results presented here, although believed to be correct, must still be

considered preliminary. The conclusions based on the data given in this report

must also be viewed as tentative and subject to possible revisions as

additional information is acquired.
Summary

Progress was made on the development of new measurement methods to study

fundamental physical and chemical properties of partial discharges (corona) in

SFg and other gaseous dielectrics. A fast multichannel analyzer system was

adapted to the measurement of point-plane corona pulse characteristics. The

electrical properties of corona pulses, which include pulse height

distributions, pulse repetition rates, pulse shapes, and average corona current,

were measured for SFg and SF 5 /N 2 mixtures as a function of gas pressure,

composition, applied voltage, and polarity. The measurement system was used

for investigations of corona inception voltages for dc and 60-Hz ac conditions

in compressed SFg. The results of inception voltage measurements indicate

that observations made under dc conditions cannot always indicate what will

happen for 60-Hz ac. At pressures .above 200 kPa (^2 atm) the effects of field

enhancement due to space charge in the gap become significant in determining

positive inception under ac conditions. The effects of uv radiation on

measurement of SFg corona inception under ac and dc conditions were also

examined. For some conditions of gas pressure and gap spacing, uv radiation

was found to have a very large effect on the probability of electron avalanche

development near onset. This is particularly true for positive dc corona.

A gas chromatograph-mass spectrometer system was used to analyze the

content of SFg which had been subjected to continuous low level positive

corona for extended periods of time (up to 333 hours). The most abundant end

decomposition products that were identified are HgO, S0 F 2 , and SO 2 F 2

.

Quantitative analysis of the latter two species has been possible, whereas the
! V

H2O content could only be determined on a relative scale. The H2O appears

to arise from heating of the electrodes by action of the discharge and thus its

presence at trace levels is difficult to avoid. After 24 hours of corona at

the 2.0 yA level, SOF2 and SO2F2 were found to be present at roughly the

10 and 30 ppm levels respectively. These arise from reactions of the primary

decomposition products, namely lower valence sulfur fluorides, with H2O.

These reactions also lead to production of HF.

The results of chemical analysis were correlated with measurements of the

electrical characteristics of the corona. It was found that discernible

changes in the positive corona pulse repetition rates and pulse height

distributions occurred after only a few hours of continuous corona. The burst

character!' sties of positive dc corona in SF5 seem to be quite sensitive to

trace levels of polar contaminants such as H2O. This was verified in

experiments where trace amounts of H2O were introduced into the gas by

heating a wire, i.e., via thermally-induced desorption from a surface. The

presence of H2O and, by inference, other polar molecules appears to inhibit

pulse burst development. This behavior is believed to be related to the effect

these molecules have on the dynamics of positive ion space charge in the gao.

The results of this study imply that positive corona pulse height distributions

are sensitive indicators of gaseous contamination. The information obtained

here should help in the design of future tests to determine relative

performance of various gaseous dielectrics with respect to dielectric strength

and long-term stability under electrical stress.

Attempts were made to observe the optogalvanic effect in N2 corona

discharges using coaxial electrodes and a tunable dye laser which could be
operated between 580 and 620 nm. Although no optogalvanic signals were

observed when the laser beam passed through the gas, the corona current was

affected by light impinging on the electrode surfaces. The physical basis for

this effect is not yet understood, but it might prove to be a useful probe of

surface conditions.

A bibliography of electron swarm data needed to predict breakdown and to

model discharge phenomena in gases was prepared. A compilation of swarm data

with critical evaluation is near completion.
J
I. Introduction

The objective for this project is to develop diagnostic techniques for

monitoring, identifying, and predicting degradation in future compressed gas

electrical insulating systems under normal operating conditions. The focus is

on providing fundamental information and data needed to improve test design and

performance evaluation criteria. The major emphasis has been on a laboratory

study of partial discharge (corona) phenomena in gaseous dielectrics. The

research effort reported here has been concerned with 1) characterization of

corona phenomena under dc and 60-Hz ac conditions, 2) monitoring of

corona-induced chemical changes in gaseous insulation, and 3) obtaining

fundamental data needed to model and predict discharge inception and corona

behavior. A compilation and critical evaluation of existing fundamental

electron swarm data for electronegative gases has been underway.

Because of the importance and prevalence of corona in practical high

voltage systems, there is need to better understand this phenomenon and the

effect it can have on system performance. There is particular concern about

the degradation that can result if the insulation system is subjected to

continuous corona for an extended period of time. Not only can the insulating

qualities of the gas deteriorate as the result of corona, but also highly toxic

substances might be produced which could be a hazard to human health and safety

if released into the environment.

It is desirable to know the effect that gaseous contaminants can have on

dielectric strength and corona discharge characteristics such as inception

voltage, current growth, discharge pulse rates, and pulse height distributions.

An understanding of the fundamental mechanisms that determine corona discharge
- 2

-

characteristics and eventual breakdown would be useful in the design of

improved gaseous dielectrics, and in devising physically meaningful and

relevant tests of dielectric performance. In the measurement and observation

of corona phenomena in gaseous insulation, there is the need for improved

measurement methods that can be used in the laboratory to meaningfully quantify

corona characteristics in a reproducible way so that results obtained in one

laboratory can be directly compared to results from another. Much of the early

research on corona phenomena has produced an abundance of qualitative

information which, although interesting, is often of limited usefulness. The

question of defining partial discharge inception in compressed electronegative

gases for different voltage waveforms needs to be addressed. In considering

the problem of corona inception it is necessary to understand the statistics of

electron avalanche initiation and growth. A model to predict electron

avalanche statistics may also prove to be a useful way to predict time- tobreakdown in practical systems that are locally overstressed.

In developing models to predict breakdown, corona inception, and electron

avalanche statistics, it is necessary to understand the basic ionization

processes that occur in a gas. In particular, it is necessary to have

reliable, fundamental electron swarm data such as electron drift velocities,

electron kinetic energy distributions and electron ionization, attachment, and

detachment rates. Progress in the theoretical modeling of discharge phenomena

can be greatly hindered by the lack of good swarm data. Often reliable swarm

data in electronegative gases cannot be obtained by traditional measurement

methods, and must instead be generated from data on electron collision cross

sections. There is a clear need for the generation, compilation and
-3-

evaluation of electron swarm data in support of future modeling efforts. There

is also a need for a more thorough compilation and evaluation of electrical

breakdown data in gases. This data is not only needed by those involved with

design of future high voltage insulating systems, but also by those who want to

test theoretical models to predict breakdown.

During the past year research was conducted at MBS to meet the objectives

discussed above. The problems of measuring and defining corona inception in

SF§ were addressed. Measurements were made of corona inception voltages for

compressed SFq under both dc and 60-Hz ac conditions. A model was proposed

to explain observed differences between the ac and dc results. The effects

of uv radiation on corona discharge inception and electron avalanche initiation

were also examined for both ac and dc conditions.

A technique to measure pulse height distributions of partial discharges in

gases as a means of statistically quantifying the discharge activity was

evaluated. A system has been devised to determine the electrical

characteristics of pulse- type corona which includes measurement of pulse

shapes, pulse repetition rates, pulse amplitude distributions, as well as

average corona current. Electrical characteristics of positive and negative dc

corona in SF5 and SF5-N2 mixtures have been measured and the results

reported.^

A gas chromatograph-mass spectrometer (GC/MS) system has been developed

and calibrated for use in sensitive monitoring of trace contaminants and

decomposition products in gaseous dielectrics. The instrument can presently be

used for quantitative analysis to the ppm level of some of the more important

decomposition products in SFq. It has been used to identify and measure
build-up of decomposition products in SFg produced by continuous positive dc

corona. A hot wire technique was developed and tested for artificially

introducing trace amounts of water vapor into compressed SFg which could be

monitored using the GC/MS. The effects of trace levels of corona-induced

decomposition products as well as artificially introduced H 2O on the

electrical characteristics of dc corona in SFg have been investigated and

reported. 6 An investigation of the effects of an intense tunable laser beam

on development of dc corona discharges in N 2 was also carried out.

A program to compile and evaluate electron swarm data relevant to

high-voltage insulation research has been formulated and discussed^ and a

comprehensive bibliography of electron swarm data has been prepared. 6

Serious consideration is now being given to setting up a data center to

collect, evaluate and disseminate electrical breakdown data for gases.

In other activities related to this research, a report6 was prepared

containing the minutes of the Workshop on Gaseous Dielectrics for Use in Future

Electric-Power Systems which was held at the National Bureau of Standards on

September 10-11, 1979. This meeting was sponsored jointly by the Department of

Energy, National Bureau of Standards, and the Electric Power Research Institute

and held for the purpose of discussing technological barriers to the

development and use of new gaseous insulation. A report? was also prepared

on the safe handling of SFg for use in high voltage electrical apparatus.

This report was primarily for the benefit of those at NBS who must work with

this material in the laboratory.
-5-

II . Technical Progress

II. A. Characterization of Corona Pulses in SFg and SFg - N 2 Mixtures

II. A. 1 Motivation

Until very recently there has been, in general, a lack of information on

the fundamental properties of corona in pressurized SFg and other

electronegative gases, excluding, of course, air. This fact has previously

been noted in the li terature.^»9 There is also a need to be more

quantitative in the description of corona phenomena. It is known from previous

studies that dc corona in SFg, at pressures above 50 kPa, occurs

predominantly in the form of pulse bursts from inception to break down. 9-1

1

A complete quantitative electrical description of this phenomenon, which is

statistically useful, would require information on distribution of pulse shapes

and pulse heights, pulse burst length, repetition rates and the relationship of

these to the observed total average corona current at a given applied voltage,

gas pressure, and electrode configuration.

As a first step in quantifying the corona activity we have, in this work,

placed emphasis on the problem of measuring the pulse height distribution (PHD)

characteri stic using a fast multichannel analyzer (MCA). With this system the

electrical characteri sties of dc corona in SFg and SFg/N2 mixtures have

been measured under a variety of conditions of gas mixture, total pressure, and

applied voltage using a point-plane electrode system. Preliminary results of

these measurements have already been reported.^

It should be noted that the pulse height analysis technique for observing

partial discharges is not new. It has been applied in previous work to monitor

degradation in insulating material s. 12-15 j^ e technique, however, has met
-6-

with only limited success thus far, due mainly to difficulties associated with

interpretation of the results. The physical basis for changes in PHD's with

time under stress has not been well understood. To our knowledge there have

been no attempts to apply this technique to fundamental characterization of

partial discharges in electronegative gases.

II. A. 2 Apparatus

For these measurements, corona was generated by applying high voltage to a

cell containing point-plane electrodes. Using the standard convention, the

polarity of the corona refers to the polarity of the voltage applied to the

stressed electrode, namely the point. A schematic giving essential features of

the system used for these investigations is shown in Fig. 1. More details of

the multichannel analyzer system have been given in an earlier report 16

and will not be discussed here. It should be noted however that some

modification of the instrument as described in Ref. 16 was necessary for the

study of gas discharges.

In pointing out the essential features, we note that the system can be

used to measure either 60-Hz ac or positive and negative dc partial discharges.

For ac measurements a gating arrangement is employed to control the phase

interval of observation. Using this it is possible to measure separately ac

positive and negative partial discharge inceptions. The pulse rate of the

partial discharges is measured with a gated scaler for both ac and dc voltages.

The PHD's are measured with a multichannel analyzer (MCA) the input of which is

a peak-and-hol d circuit. The peak-and-hol d circuit is designed to detect the

peak of a pulse in a time interval of 2 ys once the input voltage exceeds a

level determined by a di scrimi nator. The dead time (minimum sampling interval)
PARTIAL

DISCHARGE

CELL - 7 -

t/i 3O

0>

1/1

ft3 CT1 C

00OJcn

s_03u

COr— "O

03

5_03CLU

-a

-ac

03U

03MXI oLO4-Oco

+-> 03NV

<uu

03

s_03

03u

s_ -MU

CD

O)

-a i/i a> u

00 -r- 3 St:

CL) fL

co CL' >i — t/O —
-8-

of the MCA can be varied from 5 ys to any arbitrarily large interval. Not

shown in Fig. 1 are the electrostatic voltmeter used to measure either the dc

or rms ac voltages applied to the cell and an electrometer which can be

switched in place of the impedance Z to measure an average corona current.

A schematic of the preamplifier circuit which was used as a corona

detector for all of the PHD measurements reported here is shown in Fig. 2.

With this arrangement, it is possible to detect individual electron avalanches

as small as 0.02 pC (i.e., ^1 x 10^ electrons). For these measurements

Z = 5.0 kft.

The brass corona cell could be evacuated to a pressure of 1.3 x

i0“5 p a ('Xi i x 10"7 jorr) prior to introducing a gas sample. Polished

stainless steel electrodes were used. The gap spacing and tip diameter of the

point electrode could be varied.

II. A. 3 Data Analysis

An example of the results of a "complete" electrical characterization of

positive dc corona in SFg at 300 kPa K3 atm) pressure is illustrated in

Fig. 3. Shown in this figure are the measured pulse height spectra on a charge

level scale with corresponding current, pulse rate, and pulse shapes as a

function of voltage. To place the pulse heights on a charge level scale, it

was necessary to calibrate the system by applying pulses of known amplitude V"

to a precision variable capacitor (see Fig. 2) which gives an injected charge

q = C V" where C is the value of capacitance, usually within the range of

1.0-10.0 pF. The level correspond!

- ng to an injected charge q appears at a

particular channel n of the MCA where n is roughly proportional to q.
-9-

ICL Z

CD U

1

— 0) -1 -J

< 3

o QFIGURE

2.

Preamplifier

circuit

used

in

pulse

height

distribution

measurements

as

corona

detector.
RELATIVE

INTENSITY

-10-

SFC6 A

i

P = 300kPa

I=4.6XIO"

6

A

CORONA PULSES

5 us/DIV.

35-4kV

1

f = l0 4

/s

A'* 1= 1 .6X IO“

6A 3I.OkV

ss

f = 6 X 10 /s ^ 1= 3 X I0”7A

• /

i

27.8kV

fiysinii

f = I.5XIQ

3

/S

1=2 X I0’

8A 25-6kV

• • i *4 m

J

• •

I '1 »l VL. f= 130/s

10 30 50 70 90 NO

DISCHARGE LEVEL (pC)

FIGURE 3. Electrical characteristics of positive corona

in SFg at an absolute pressure of 300 kPa.

Shown are the corona pulse shapes, pulse

repetition rates, average corona currents

and pulse height distributions at the

indicated applied dc voltage.
-nA least-squares straight-line fit to a set of observed points ( q-j , n-j )

,

i > 3 is used to establish the charge level scale from the MCA output. The

uncertainty in charge scale calibration depends on charge level and increases

as the level decreases. At the present time errors at all charge levels may be

as great as 20 percent due in part to uncertainties in matching the time

constants of the calibration circuit to the actual corona detection circuit.

Further investigations are required to evaluate and reduce this uncertainty.

The PHD data shown in Fig. 3 have been arbitrarily normalized to the

maxima in relative intensity. For these data the cut off point at the low

charge end is determined by the discriminator level setting used to eliminate

noise at the MCA input. The minimum charge level that can be recorded by the

MCA is determined by a combination of the gain setting of the variable gain

linear pulse amplifier and the discriminator level setting into the MCA.

(Note: the discriminator for the MCA is not specifically indicated in Fig. 1,

but is an integral part of the peak-and-hol d circuit.)

II. A. 4 Instrumental Factors

There are several instrumental factors that can have a significant effect

on the measurement of PHD's such as the time constant of the detection circuit

and the dead time of the MCA. These must be carefully considered lest the

results be misinterpreted.

We first consider the effect of MCA dead time (sampling interval). This

could be significant if the spacing between pulses such as in a burst is

smaller than the sampling interval. If in a burst, the pulses show a regular

dependence of amplitude on time during the sampling interval, then the form of
- 12 -

the measured PHD, i.e., the way in which the MCA characterizes the burst, will

depend on how much of the burst is actually sampled.

The effect of MCA dead time on the measured PHD is illustrated in Fig. 4

for the case of positive dc corona in SF 5 at a pressure of 200 kPa. The

corona in this case occurs predominantly in the form of bursts during which a

large primary event is followed by many smaller secondary events; although at

this pressure and voltage there are some single, isolated, lower level pulses

not associated with bursts. This is illustrated by the oscilloscope trace

inset (also see traces in Fig. 3). In Fig. 4, we also show a comparison of PHD

data taken under identical conditions with the exception that the MCA dead time

is 7 us in one case and 50 ps in the other. It can be seen that in going from

the longer to the shorter dead time there is a major enhancement of the

relative contribution of the low charge pulse features. This can be explained

by the fact that for the 7 ps time the smaller secondary pulses which occur

during burst activity are being sampled, whereas at 50 ps there is little or no

contribution from these events since, for the operating conditions chosen,

nearly all burst lengths are less than 50 ps.

The 50 ps data, therefore, simply correspond to the PHD for primary

initial events of which in this case there appear to be two types as previously

noted from the oscilloscope trace of pulses shown. These are namely larger

streamer pulses, generally the precursors of bursts, and lower amplitude single

avalanche type pulses. 3 These measurements confirm earlier speculation^

that much of the observed structure in PHD spectra for positive corona in SFg

is associated with characteristics of pulse bursts, and demonstrates the

advantages of using a fast MCA sampling rate for PHD corona characterization.
RELATIVE

INTENSITY

-13 -

T T

sf6

P-200 kPa

25.1 kV

10 jjis/DIV.

MCA DEAD TIME = ~

s

. 7 |iS

. * MCA DEAD TIME

50 |js

w

4.0 8.0 12.0 16.0 20.0 24.0

DISCHARGE LEVEL (pC)

26.0

FIGURE 4. Measured partial discharge pulse height distributions in SFg for

different multichannel analyzer dead times. Also shown in the inset

is an oscilloscope trace displaying pulses from three separate

partial discharge events, one of which corresponds to a burst.
-14-

We next examine the effect of the finite time constant of the measurement

circuit. Although this problem has been briefly mentioned (see Ref. 2), it

deserves careful consideration. We now consider this question here in more

detail

.

To a good approximation, the discharge-pulse detection circuit can be

modeled as an equivalent circuit consisting of a low pass filter connected to a

current source of infinitely high internal impedance corresponding to the

discharge cell as shown in Fig. 5. In this arrangement, it is the output

voltage V(t) which is measured. It should be recognized, however, that the

real measurement circuit may contain several time constants, and therefore

consist of a higher order low pass filter than considered here. For the sake

of simplicity, we treat only a single filter, since it is expected that a

single RC combination dominates. Moreover, the conclusions reached from the

analysis below are largely independent of the detailed form of the instrument

transfer function, i.e., the effects of higher order filtering.

The instantaneous discharge current i ( t ) at any time t is related to the

instantaneous currents through the capacitor C and resistor R by

i (t) = i c ( t ) + i R (t) . (1

)

Equation (1) can be rewritten in terms of the instantaneous charge q(t) on

the capacitor C to give

i(t) q(t) dq(t)

RC dt

' ( 2 )
-15-

FIGURE 5. Equivalent circuit for partial discharge detection

system.
-16-

Having solved Eq. (2), the desired output voltage is simply given by

V ( t) = q(t)/C. (3)

Corona pulses, associated with motion of electrons in the electrode gap,

are typically of very short duration, i.e., < 10 ns. Fast oscilloscope traces

of typical dc corona pulses in our cell with pressurized SFg show that they

have a duration of less than 4 ns. In Figs. 6 and 7, we show the output

display of a fast transient digitizer used to record the shapes of pulses

observed when the detector shown in Fig. 2 was replaced by a fast preamplifier

for which Z =50 ft. The shapes recorded here are typical of those seen for

positive and negative dc corona in SFg and indicate a width of about 1 ns or

less for these events. Some variation in the shapes of the corona pulses was

noted in these high resolution measurements, but this topic will not be

discussed further here. More measurements of pulse shape characteristics are

underway.

The important point to note here is that the RC time constant of the

detection circuit used for PHD measurements (in this case roughly 2.0 us) is

much greater than the corona pulse duration At, i.e., At << RC. It should be

noted that the choice of RC time constant for this instrument is made on the

basis of limitations imposed by the sampling time of 2 us of the peak-and-hol

d

circuit (see Ref. 16). There is no point in having a detector RC which is much

smaller than this.

We now show that if this condition holds (At << RC), then the amplitude of

the pulse sampled by the pulse height analyzer is proportional to the area
- 17 -

FIGURE 6. Oscilloscope trace of transient digitizer output

for a single pulse from positive dc corona in SFg.

The time scale is 0.5 ns/di v.

0

FIGURE 7. Oscilloscope trace of transient digitizer output

for a single pulse from negative dc corona in SFg.

The time scale is 1.0 ns/div.
-18-

1

under the corona current and hence directly proportional to the total charge Q

in the pulse. To do this we first consider a simplified case where it is

assumed that the corona pulse is rectangular and of width At, so that

i(t) = i 0 for t <_ At and i(t) = 0 for t > At; so the net charge in the corona

pulse is i 0At. For t _< At the solution of Eq. (2) is

q(t) = RCi 0 [1 - exp(-t/RC)], (4)

and for t > At,

q(t) = RCi o exp ( -t/RC)[exp(At/RC)-l], (5)

so that the output has a maximum (peak value) at t = At given by

V max

= Rio ^ ( - At/RC)].

If At << RC, then by expanding the exponential we obtain, to a good

approximation.

(7)

The fractional error of the charge measurement in this case is given mainly by

the second term in the expansion and is thus proportional to (At/RC) which is

negligible provided At << RC.
-19-

In the more general case of arbitrary i(t) one can calculate the output

response using the standard Green's Function approach to solve Eq. (2). The

Green's function G(t,t^) in this case must satisfy the differential equation

dG&’~~ ' RT

= S(t-t') (8)

subject to the boundary conditions: G

s 0 at t < t" and G -* 0 as t-t"-* 00

.

Here 6 ( t - t^) is the usual Dirac delta function. One can verify that the

appropriate function is given by

,0, t < t'

G(t,t') = O)

exp[-(t-t*)/RC], t > t' .

With this function the solution to Eq. (2) is:

G(t,t") i ( t")dt" . ( 10 )

At the peak of the output pulse (V max ) the condition dq(t)/dt = 0 must be

satisfied, and this yields the time t^ at which the peak occurs, from which
- 20 -

we have

vmax

= q(tm)/C. ( 11 )

The fractional error e in determining the charge Q is given by the

difference

e = (Q - q(t) )/Q ( 12 )

or using Eqs. (9) and (10)

£ i (t")dt i(t")exp[-(t

m t")/RC]dt (13)

Mow assuming a relatively short corona pulse so that i(t) =0 for t > t^ + 6t

where (t^ + 6t) « RC, we have

£ i(t")dt" +

t -r

(14)

This reveals that the fractional error is proportional to a term on the order

of tm/RC plus an integral over the tail of the corona pulse extending 5t

beyond t^. This latter term is also small provided 5t << t^, or if

6t << RC. In the simple example considered above, for a symmetric triangular
- 21 -

corona pulse of full width At, one can easily show using Eq. (10) and the

condition dq/dt = 0 that

tn, = At/( 1 + At/RC), ( 15 )

and, therefore.

<St * At - tn,

= A tC ( At/RC ) / ( 1 + At/RC)],

( 16 )

from which it is seen that indeed 6t - 0 provided At/RC << 1.

There are, of course, cases where one cannot assume <5t << t^ such as are

known to occur for some Trichel pulses that have an extended tail associated

with ion conduction following the initial electron charge transport across the

gap. a phenomenon, similar to Trichel pulses, which exhibits this

behavior has been observed in our laboratory for negative corona in SFg at

pressures less than 60 kPa, and at voltages close to onset. In Fig. 8, we show

an oscilloscope trace of these negative corona pulses and the corresponding

PHD. They are like Trichel pulses observed in other electronegati ve gases in

that they occur with nearly constant amplitude and exhibit a long tail which

can extend more than 100 us beyond the initial pulse. Unlike true Trichel

pulses, however, the spacing between them is random. The PHD profile is that

expected from sharp pulses of nearly constant amplitude followed by a long

tail. The high background level on the low charge side of the main peak in the

PHD is not due to small pulses, but rather results from the sampling several
RELATIVE

INTENSITY

sf6

P=50kPa

V = 5.78 kV

0.0

a

a

a

a

a a

° a a a ' a

0 " “a ° °o® Jf

_ a “ a tT

I

» °° a %

aa

ai. 1

5.0 10.0 15.0 20.0 25.0

DISCHARGE LEVEL (pC)

30.0

FIGURE 8. Pulse shapes and corresponding pulse height distribution for negative

"Trichel like" corona pulses in SF

g at an absolute pressure of 50 kPa.

The corona was generated at a voltage of 5.78 kV using a point plane

electrode geometry with a point diameter of 0.08 mm and gap spacing of

1 .24 cm.
-23-

times the long tail which necessarily decays with a time constant

characteristic of the measurement circuit. The positive upswing in the

oscilloscope trace corresponds to the point where the current associated with

the long tail abruptly extinguishes. It is clear, in this case, that the

condition fit << tmis not satisfied and the charge level at the PHD peak near

22.4 pC does not include the charge in the tail associated with ion transport

across the gap.

In the case of pulse bursts such as have been observed for positive dc

corona in SFg, there are complications in the interpretation of PHD

measurements (in addition to those noted previously) that depend on the

multichannel analyzer dead- time. The added complications arise if the spacing

between burst pulses is less than the RC time constant of the measurement

circuit. This is certainly the case for our observation of positive corona

pulses in SFg at pressures above about 100 kPa. In Fig. 9 are shown

oscilloscope traces of positive corona pulse bursts observed with a detection

circuit having a time constant of 0.1 ys which is to be compared with our usual

PHD measurement circuit with a time constant of ^2.0 ys as noted above. There

seem to be two relatively distinct regions of the burst following an initial

streamer event. Immediately after the initial event a group of pulses appears

with a spacing of ^0.1 ys, then a later group appears with a larger spacing of

0.5 ys. The spacing in either case is less than, or at most comparable to, the

RC time constant of the circuit.

In examining the problem that occurs here, we consider two extreme cases,

namely when tc >> RC and when tc << RC. In the first case, one is

obviously measuring the charge of single pulses in the burst, i.e., the PHD

-25-

corresponds to isolated corona events. For the other extreme, this is not the

case, and it can be shown that what one is measuring here is the sum of the

charges included in the burst pulses sampled. To demonstrate this, we consider

the simple example of two rectangular pulses of the same amplitude i 0 and

width At such that the spacing ^ between them satisfies the condition At +

t

c << RC. The solution to Eq. (2) gives a maximum of

q (

t

m ) = RC i 0 Cl-exp(-At/RC)] [1 + expC ( »tc -At)/RC]] (17)

which for + At << RC reduces to

q(t,n) - i o ( 2At ) , (18)

i.e., the sum of the charges in the two pulses. In reality, the situation is

intermediate between these extreme cases, and the measured amplitude for a

burst in which ^ < RC will be in general greater than that associated with

charge in a single pulse, but less than the net charge in all pulses included

in the sampling interval.

We have considered in the above discussion some of the fundamental

limitations of the PHD measurement technique which should be understood if one

is to properly interpret results. In particular, because of the necessity of

using a detection circuit with a finite time constant and because of the finite

sampling rate of the multichannel analyzer, one is not always measuring the

true pulse height or charge distribution spectrum of partial discharges. The

result of the measurement is rather a convolution of the instrument impulse
-26-

response function with the input signal from the discharge as is the case with

any measurement system. Nevertheless, the PHD measurement is a useful method

for statistically quantifying the characteristics of corona in a reproducible

way and, as will be shown later, is a sensitive indicator of gas contamination,

electrode surface conditions, and changes in these induced by the corona. This

technique should be further studied with the goal of optimizing the information

content of the data so that it can be readily correlated with data from other

diagnostic measurements.

II. A. 5 Results and Conclusions

The results of many measurements of the electrical characteri sties of dc

corona pulses in SFg and SFg/N2 mixtures have already been summarized in

Ref. 2. In particular, a report is given there of the voltage and pressure

dependence of corona pulse height- distribution and pulse repetition rates

measured using the apparatus described here. - In addition to the data given in

Ref. 2, measurements have recently been made to check the dependence of SFg

corona characteristics on gap spacing and on irradiation of the gap with uv

light from an Hg-discharge lamp. The previously reported PHD data^ were

obtained using a point-plane electrode gap of 1.24 cm. Measurements were also

made for a 2.28 cm gap. In this case, the partial discharge PHD

1 s for positive

and negative corona exhibited features quite similar to those seen with the

smaller gap, i.e., the essential PHD characteri sties remain unchanged as the

gap is increased.

The effect of irradiating the gap with uv light was also investigated for

positive SFg corona. Particular attention was given to cases, to be noted

later, where relatively large effects of radiation on pulse repetition rates
-27-

were observed. It has thus far been discovered that, even in cases where

radiation causes an order of magnitude or more increase in repetition rate, the

PHD characteristics remain virtually unchanged.

The results presented in Ref. 2 will not be reproduced here, but the

general conclusions are worth repeating. The conclusions drawn from this work

pertain to point-plane positive and negative dc corona in SFg at pressures in

the range of 50 to 500 kPa. For positive corona the conclusions are listed

be! ow.

1) Positive corona starts as low level avalanches which rapidly develop

into streamer pulses or pulse bursts that increase in mean amplitude

and repetition rate as the applied voltage is increased. Typical

pulse amplitudes are in the range of 10 - 100 pC.

2) The burst length of positive corona pulses decreases and the mean

amplitude increases with increasing gas pressure.

3) Positive corona PHD's broaden and pulse burst lengths increase with

increasing applied voltage.

These conclusions are consistent with other observations of positive

dc corona in SFg. 10 * 11 It has been proposed 10 that the burst

characteristics of the corona, such as burst length and mean spacing of pulses

in the burst, are controlled by the development of space charge in the interelectrode gap. The initial event of largest amplitude corresponds to an
-28-

electron avalanche or streamer which propagates into a gap free of space

charge. The negative ion space charge which this event creates moves towards

the point, locally enhancing the electric field and producing, via detachment,

avalanche-initiating electrons which give rise to secondary events in the

burst. These secondary avalanche events cannot propagate as far into the gap

due to the shielding effect of the residual positive ion space charge, and thus

will necessarily be of lower amplitude than the primary event. Once the level

of positive space charge builds up to some critical point the burst will

extinguish. One sees then that the duration of the burst will be controlled by

the rate at which the positive ion space charge builds up which, in turn, is

governed by the rate at which positive ions move toward the plane. At higher

pressures (or lower applied voltages) the positive ions will move more slowly,

thus positive ion space charge will build up more rapidly and consequently the

pulse bursts will diminish in length. In this way, one can, at least

qualitatively, understand the observed pressure and voltage dependence of

positive dc corona burst characteristics in SFg.

The conclusions for negative dc corona in SFg are listed below.

1) The predominant mode of negative corona in SFg is a quasi-glow

consisting primarily of many small (<3 pC) closely spaced pulses,

although there is a tendency as pressure decreases below 400 kPa for

negative corona to begin as somewhat larger pulses ^10 pC, see

Fig. 8.

2) The pulse repetition rate for negative corona in the quasi-glow mode

is usually an order of magnitude greater than for positive corona

at comparable average current.
-29-

There is evidence from measurements made using SF 5 /N 2 mixtures that

the corona characteristics, particularly the PHD's, are quite sensitive to

small changes in gas composition. Further investigation of this is underway,

and more will be said about this later.

II.B. Comparison of AC and DC Corona Inceptions in SFg

II.B.l Motivation

There has been some concern about the extent to which data on dc corona

inceptions can be used to predict what will happen under 60-Hz ac conditions.

The argument is often heard that 60 Hz is slow enough compared to the duration

of a typical corona event that it is essentially like dc. There exists reason

to believe however that even at gas pressures slightly above 100 kPa K 1 atm)

the development of ion space charge in a gap can be quite different under ac

and dc conditions so that differences in corona phenomena should be evident.

The extent to which this is true has not previously been clear. Again this

work has been motivated by the general lack of useful fundamental data on SFg

corona. There also exists the problem of defining inception voltage in such a

way that a meaningful comparison can be made between ac and dc conditions. We

have performed measurements, discussed below, which help examine this question.

Preliminary results of this study have already been reported (see Ref. 1).

II.B. 2 Apparatus and Measurement' Technique

The apparatus used to generate and detect corona in SFg is the same as

that previously described in Sec. II. A. (see Fig. 1). For both ac and dc

voltages the onset of corona was determined by observing and recording

individual corona pulses. Because SFg corona in the pressure range 50 to

500 kPa appears initially in the form of pulses, it was deemed that pulse
-30-

detection would be the most satisfactory way of making comparative inception

measurements (preferable, for example, to a measurement of current). For the

ac measurement, a gating circuit arrangement (see Fig. 1) was employed so that

corona pulses could be observed only during predetermined phase intervals of

the 60-Hz sine wave. In this way it was possible to separate positive and

negative inceptions for ac.

The voltages applied to the electrodes were measured for both ac and dc

conditions with the same electrostatic voltmeter. In the case of ac, this

meter gives the rms value. Thus it was necessary to calculate peak values

assuming a pure sine wave. The ac voltage values reported here for onset are

the calculated peak values.

Measurements were made both with and without irradiation of the gap with

light from an Hg-discharge lamp through a quartz window. Inception was defined,

in all cases, as the intercept of a straight line fit to the data on a semi-log

scale with the voltage axis determined by the 0.1 count/s level. For these

measurements, the level of sensitivity for corona pulse detection was in the

range of 0.05 to 0.02 pC.

Applying the same definition of inception to both ac and dc conditions

leads to some difficulties in making a meaningful comparison. One could

reason, assuming identical instantaneous field conditions, that near onset, the

measured discharge repetition rate for 60 Hz should be reduced relative to that

for dc by roughly the fraction At/T where At is the time interval in the sine

wave during which the voltage exceeds the threshold value and T = 0.0167 s

corresponding to the period for 60 Hz. One would thus expect, given the

definition for inception chosen here, that the onset for 60 Hz should be
-31-

somewhat higher than for dc. This is indeed observed to be the case as will be

seen later when comparison is made between ac and dc negative corona inceptions

where, because these both occur for gaps presumed initially free of space

charge, instantaneous onset field conditions are the same. However, for

sharply rising onsets such as occur for negative corona in SFg, the error

associated with neglecting the factor At/T in comparing ac to dc is relatively

smal 1

.

II. B. 3 Results

Examples of data on corona pulse repetition rate versus voltage are shown

in Figs. 10 - 14. The data shown are for positive and negative dc corona in

SFq at different gas pressures both with and without uv irradiation of the

gap. The open symbols correspond to data obtained with radiation, and the

different symbols correspond to measurements made at different times. The data

in Figs. 10 - 12 were obtained with a poi nt-to-plane gap spacing of 1.24 cm

whereas the data in Figs. 13 - 14 correspond to a gap of 2.28 cm. The point

electrode tip diameter was 0.09 mm for both cases. The measurements were

performed using polished stainless-steel electrodes. At each pressure, a fresh

SFg gas sample was used after the cell had been evacuated. It is apparent,

from the data shown here, that positive corona in SFg initially develops at a

much slower rate than negative corona, and the curve of repetition rate versus

voltage exhibits a sudden increase in slope several kilovolts above threshold.

This increase in slope appears to be associated with the development of the

pulse burst activity as evident from corresponding pulse height distribution

measurements (see previous section).
REPETITION

RATE

FIGURE 10. Partial discharge repetition rate versus applied voltage for

positive and negative, point-plane dc corona in SFg at an absolute pressure of 100 kPa. The open symbols correspond to

measurements made with an irradiated gap.
REPETITION

RATE

FIGURE 11. Partial discharge repetition rate versus applied voltage for positive

and negative, point-plane dc corona in SFg at an absolute pressure of

300 kPa. The open symbols correspond to measurements made with an irradiated gap.
REPETITION

RATE

-34-

10

'

10

'

10

FIGURE 12. Partial discharge repetition rate versus applied voltage for

positive and negative, point-plane dc corona in SFg at an absolute

pressure of 400 kPa. The open symbols correspond to measurements

made with an irradiated gap.
REPETITION

RATE

(I/s)

FIGURE 13. Partial discharge repetition rate versus applied voltage for positive

and negative, point-plane dc corona in SFg at an absolute pressure of

200 kPa. The open symbols correspond to measurements made with an irradiated gap. The point-plane gap spacing is 2.28 cm and the point

diameter is 0.09 mm.
REPETITION

RATE

(1/s)

36-

FIGURE

10 15 20 25 30 35

APPLIED VOLTAGE (kV)

40

14. Partial discharge repetition rate versus applied voltage for

positive and negative point-plane dc corona in SFg at an absolute

pressure of 400 kPa. The open symbols correspond to measurements

made with an irradiated gap. The point-plane gap spacing is

2.28 cm and the point diameter is 0.09 mm.
-37-

For SFg gas pressures in the range of 100 to 500 kPa, the effect of uv

radiation on negative dc corona onset is slightly more pronounced for the

1.24 cm gap than for the 2.28 cm gap. As seen from the figures, however, even

in this case the presence of radiation reduces the onset voltage by less than

10 percent of that for no radiation. There is a tendency, though, for results

to become somewhat more reproducible under irradiation.

For the 1.24 cm gap, the presence of uv radiation seems to have little or

no effect on the development of positive dc corona. In the case of the 2.28 cm

gap, however, the effect of radiation inception is seen from Figs. 13-14 to be

quite pronounced. The difference between positive inceptions measured with and

without radiation is found to increase with increasing gas pressure. The

effect is clearly greatest near inception where corona is predominantly in the

form of low level avalanches and progressively diminishes as voltage increases

to the point where self-sustaining streamer bursts develop corresponding to an

abrupt increase in repetition rate.

For the 60-Hz ac measurements, the presence of radiation has little or no

observable effect on inception at either gap spacing. One could argue that

this is expected because, under ac conditions near threshold, the fraction of

time that the voltage exceeds threshold is small compared to 100 percent for

the dc case and, therefore, the radiation may not cause observable enhancement

in partial discharge count rate for light intensities used in these

measurements. For the ac case, there is simply not as much time available for

photons to initiate avalanches. Also as noted earlier, near onset the measured

discharge repetition rate for 60 Hz should be reduced relative to that for dc

by roughly the fraction At/T. Since this fraction might be quite small it
-38-

would be difficult to observe changes in pulse repetition rates close to

threshold where the already low (0.1 to 1 count/s) pulse rates for dc must be

reduced by At/T.

The results of determining inception values from data such as shown in

Figs. 10 - 14 are summarized in Figs. 15 and 16 which show positive and

negative ac and dc inception voltages as a function of SFg gas pressure for

the two point-plane gap spacings.

II. B. 4 Discussion

An understanding of the physical basis for the rather large effect (see

Figs. 14 and 16) of irradiation on the development of electron avalanches at

positive dc corona inception in SFg would be desirable. At this point one

can only speculate. One plausible explanation is that the uv enhances

i nitiati ng-electron production in an active volume near the point via

photodetachment of negative ions. Table I shows the predominant Hg emission

lines from the lamp used with their correspond!' ng energies and relative

intensities. The table also shows electron affinities of various negative ions

that might be present in the gas. Although the available photon energy is not

sufficient to photoionize SFg, it is sufficient to photodetach negative ions

likely to be present. As gas pressure is increased, one might expect formation

of negative ion clusters of the type (SFg)(SFg) n where n >_ 1 (see

Refs. 18 and 19). Unfortunately the electron affinities of these species are

not known, but detachment from these are likely to be at least as probable as

from simpler molecular negative ions; also their mobilities are expected to be

lower enabling them to remain longer in the active volume where free electrons,
PARTIAL

DISCHARGE

ONSET

(kV)

-39-

GAS PRESSURE (kPa)

FIGURE 15. Comparison of ac and dc positive and negative corona inceptions

for SF5

. The measurements were performed at room temperature

using stainless steel point-plane electrodes with a gap spacing

of 1.24 cm and a point diameter of 0.09 mm. The gap was irradiated with uv light from a mercury discharge lamp.
PARTIAL

DISCHARGE

ONSET

(kV)

-40-

FIGURE 16. Comparison of ac and dc positive and negative corona inceptions for

SFf;. The measurements were performed at room temperature using

stainless steel point-plane electrodes with a gao spacing of 2.28 cm

and a point diameter of 0.09 mm. The open points correspond to data

taken with the gap irradiated by a mercury discharge lamp.
-41-

Table I: Predominant atomic emission lines from Hg di scharge lamp used fo

gap irradiation indicating relative photon intensities and energies, and

electron affinities of various negative ions possibly present in SF§

.

Photon Photon Relative Electron

Wavelength (nm) Energy ( eV

)

Intensity Ion Affinity

295 5.46 0.419 SF6 0.6 eV

305 4.13 0.562

325 3.8 0.351 SF‘ 2.7eV

355 3.49 0.654

365 3.40 1.00 _ F" 3.4eV

395 3.14 0.357

405 3.07 0.442 30

2

l.leV

425 2.92 0.525

435 2.85 0.622 o' 1.5eV

535 2.32 0.422

545 2.28 0.659

565 2.20 0.339

575 2.16 0.641
-42-

rel eased by detachment, can initiate avalanches thus explaining pressure

effects.

The photon intensity and wavelength dependence of avalanche formation

probability in an irradiated gap should be checked. Preliminary measurements

indicate that the avalanche formation rate increases with light intensity.

However, it has not yet been determined that there is, for example, a

saturation effect which might be expected once the intensity is high enough to

photodetach all of the ions in the active volume where free electrons can lead

to avalanche formation. Wavelength dependence measurements might give an

indication of the negative ions involved if photodetachment is indeed the

initiation mechanism. By checking dependence of avalanche production on light

beam position, one might also learn about the extent of the active region where

avalanches originate.

Referring to Figs. 15 and 16, it can be seen that under all conditions the

ac and dc negative inceptions appear at lower absolute values than the positive

inceptions. Also there is reasonable agreement between the ac and dc negative

inceptions, the small difference probably being accounted for by the assumed

definition for onset as discussed above. The polarity effect seen here has

previously been noted H>20 f or <jC point-plane corona in SFg. At the

present time, however, there exists no theoretical prediction of the magnitude

of this effect. One could speculate that this effect is physically reasonable

because the corona-initiating electrons for a negative point are necessarily

generated in the high field region near the point as opposed to the lower field

region away from the point where positive corona originates. Thus negative

corona might be expected to appear at lower voltages. On the other hand, the
-43-

polarity difference may also be due in part to different roles that the point

electrode surfaces play in initiation of positive and negative corona. The

reasons for this effect thus remain open to question.

It is also seen in Figs. 15 and 16 that, in the case of positive corona,

the dc inceptions are always higher than the corresponding ac inceptions, and

there is a tendency, at least for results obtained without irradiation, for

this difference to increase with increasing gas pressure. This difference, we

believe, can be attributed to the effect of ionization from corona in the

negative half cycle influencing development of corona on the subsequent

positive half cycle.

To determine if this explanation is plausible, we have estimated the time

required for various negative ions formed in corona to clear the gap before

polarity reversal has occurred. Negative ions that do not reach the positive

plane electrode before polarity reversal can be attracted back to the point

electrode, locally enhancing the field and thus lowering the inception voltage.

The velocity v of a negative ion moving across the gap on the x-axis, defined

by a line from the point electrode perpendicular to the plane, is

v(x,t)

dx

dt

= k E(x) cos cot. (19)

where k is the ion mobility (drift velocity per unit field) assumed independent

of field and E(x) is the magnitude of the x-component of electric field. This

equation can be integrated to yield a limiting mobility value k^, below which
-44-

no ion at a distance i from the plane would reach the electrode in a quarter

cycle, i.e.,

where T is the period of the 60-Hz field.

To calculate the electric field between the point- plane electrodes, space

charge in the gap was neglected and the electrode system was conveniently

approximated as a hyperboloid of revolution over a plane. The static field is

where n and £ are meridian coordinates in a prolate spheroidal coordinate

system, f is half the focal distance, and $

0

is the potential of the point. A

tracing of an enlarged photograph of the point electrode permitted its

characterization as a surface of constant n (n 0

). For the special case of an

ion trajectory along the electrode axis ( £

= 0) we obtain, using Eqs. (19) and

(21) and x = f cosh S cos n,

dx/E(x)

,

( 20 )

described then by the expression^!

( 21 )

( 22 )

which, when integrated, replaces Eq. (20) to give
-45-

The mobilities of negative ions produced in SFg discharges have been

measured. 18,19 j^ e mobility values given by Patterson^ for the

ions SFg, SF§, SFg (SFg) and SFg ( SFg )2 have been adjusted for

pressure P assuming a P“1 dependence, and are indicated in Fig. 17.

Schmidt et al.,^ concluded that their data indicate a P“1 dependence

below ^100 kPa, but a p

_ l*25 dependence above this pressure suggesting

different physical processes occur as pressure increases. Mobility values of

Schmidt et al . , are also shown in Fig. 17 and compared with k^ values

calculated for two ion trajectories corresponding to £=0 and £=0.5.

Regardless of the choice of mobility data, it can be seen from Fig. 17

that negative ions produced in SFg may not clear the gap before polarity

reversal and can thus lead to a space-charge-enhanced field on the positive

half cycle. As pressure is increased, more of the negative ions are expected

to survive the period of polarity reversal, thereby increasing the possible

buildup of space charge. This can explain the increase with pressure of

differences between ac and dc positive corona inception voltages. Calculations

are presently being made to check the model proposed here. These involve

examination of electric field enhancement near the positive point electrode

which would result from a negative-ion space charge cloud of reasonable charge

density. The objective is to determine if the field can be sufficiently

enhanced by space charge in the gap to account for the corona inception

differences observed.
in 3

in 0) r— in

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3 E •p— o -m cn •T— 3 c s_ 3

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3 > o 4-4 O

> c c (3 E

•f— f0 <u •r- Qcn i- -c O -X C

4-4 4-> CL o

in o

3 "O >> <3

•t— 3 -Q 4-> f— 3 • s- <3 13 o -o *3 3 ->-CO

4-1 103 > 4-1 r— u U. 4-4 3 O <3

0) U~) <3 3 .a cn - •03 U a. f3 34-

<3 3 T- o C 3

s_ •r- -a in CC

4-> c 3 3

o •<- -3 S- O

3 3 H- 3 r— o U 0) in m ns

•r— 3 in in 3

•o o • 3 44 O O -3 r— i- i— 3

3 £- 4-4 rc CL f3 Q. in > « 3 s- O 4-1 3 in <3 3 4-1 4-4-0 -3 3 4= > C-«“

4-> O 4-4 3 T3 3 E

S- 3 in.3

CT s- 4-13 0

3 <d a; >) (3 i.l/7 o > .

4-> i— CL 1— •r— r-— O 3 >,

<3 4-> <3 s_ CL S..Q

<3 S (3 (3 oio in r—1 i- in i3 .x aj o 4-1 4-4 Q_ 2: m CL X 3-4

3 3 *o* >5 *r— o OO 4-1 • 4-> 4-1 CLO

3 T- CD r—

r— S_ r— 3 i— (3

• r- 3 •!- 4-4 3

-O in j3 • r— • 13 > o in o 4-> 4- -O O

E 3 E 3 -Q

i. 3 C£ -O <3 c Cl 3 J3 3

o > 4-4 in in r— 103 3 0 3

Ll_ _3 3 O i— s- 4— oo > in o 3

O 4-4 •r— J- in 4- in > 3 3 in

in ° 2 S— 4-1 SZ 3

0) E 3 4-1 h- S- 3 3 in (3 CL

r— O m Q_ 3 •1- 0) o • 4-1 > 4-1 -f- 4-1 4- 3 (3 o s_ O U

CD c o s_ 3*0

3 3 +J 3 in 3 3

•i— 4- O -3 4-4 -O S_

4-1 aj i- r— 3 3

•i— <3 O 3 3 in £ <3 cn Q. 13

in i- 3 3 3 3

i3 4-> •I-— L O E

r*»

UJ

cc

ZDO
-47-

II. C. Identification of Corona Induced Decomposition Products in SFg

II.C.l Motivation

Although there have been numerous studies of decomposition in SFg

produced by high power discharges, arcs, circuit breaker operation,

etc . 22-28

^ very Tittle has been done to look at decomposition in

low level corona discharges which do not significantly heat the surrounding

gas. 29 Decomposition in corona is more difficult to study because it

occurs more slowly, and even after hours of operating the discharge, the

products may only appear at trace levels. Thus sensitive analytical techniques

must be employed. On the other hand, the rate of decomposition can be

controlled more readily than in an arc by controlling the discharge power

level. A controlled corona discharge may prove to be a superior way of

evaluating relative stability of gaseous dielectrics under conditions of

excessive electrical stress. Also one should not infer from arc data what can

happen for corona i since the nature of these discharge processes are quite

different.

Again the importance of understanding how gaseous dielectrics degrade in

partial discharges is reemphasized. Corona occurs, in practical systems,

around conducting particles, surface protrusions, and at interfaces, and is

known to cause insulation deterioration. As metal -cl ad gas insulated systems

are designed to operate at higher and higher stresses, it may be more difficult

to avoid corona. The long term stability of an insulating system can be

seriously affected by the occurrence of corona if even on an intermittent

basis. There are practical situations where corona cannot be avoided, and, in

fact, for some applications, such as in the insulation of high energy particle
-48-

accelerators, corona is deliberately generated in the insulating gas to insure

uniform field distributions.

In our studies reported here, we have chosen to use a gas

chromatograph-mass spectrometer (GC/MS) system to perform chemical analysis of

the gas which has been subjected to continuous corona generated by applying dc

voltage to a point-plane electrode system. This method has its limitations in

as much as highly reactive species like HF, or species that easily hydrolyze

such as SF4, require very specialized, complicated gas handling and sampling

techniques in order to be observed with any reasonable degree of sensitivity.

In general, however, the GC/MS is one of the most sensitive analytical

instruments available and can be used to detect most species of interest. In

this work, we have identified the decomposition products generated in

pressurized SF5 for point-plane corona operated under conditions of both

constant voltage and constant current for many hours.

II.

C

.

2

Apparatus and Measurement' Technique

Chemical analysis of SFg gas degraded by corona was performed with a

modified commerci ally-available GC/MS system equipped with a data processor.

The gas chromatography process can be briefly described as follows. A steady

stream of inert "carrier gas»" usually helium, is made to flow through a

tubular "column" designed so that its surface is encountered frequently by each

gas molecule in the stream. The column may be a fine capillary or a wider tube

packed with one of a great variety of porous materials known as column

supports. The mixture of gases or vapors to be analyzed is introduced near the

entrance of the column and is swept into it by the carrier gas stream.
-49-

Molecules of different species progress along the column at different rates,

because the molecules repeatedly get retained by adsorption on the surface (or

in solution, if the column has a liquid coat) for different lengths of time,

owing to their different degrees of affinity for the stationary solid or liquid

of the column. Different molecules of the same species progress at nearly the

same rate, despite some inevitable variation due to nonuniformities of the

column and to diffusion. Passage through the column, therefore, separates a

gas mixture, causing each component of the original mixture to emerge, or to

“elute," from the column at a different time. A detector at the column exit,

which in the present case is a mass spectrometer, measures the arrival rate of

material other than carrier gas, and the time record from this detector is the

chromatogram, showing "abundance" versus "retention time." A column will be

unable to separate some components of a mixture if they are too similar in

their chemical behavior toward the materials used in that column. Which kind

of column to use, under what conditions of temperature and flow rate, is a

matter of judicious choice, and the lore of chromatography is vast.

For most of the results reported here, the column in the GC consisted of

Porapak (80/100 mesh) in 3.2 mm diameter Teflon^ tubing which

was 90 cm in length. For most of the measurements, the column was operated at

24°C with He carrier gas flow rate of about 32 mL/min. Some measurements were

performed using a longer glass column with the same support material. In this

case, retention times were longer but the same species were observed with

roughly the same degrees of sensitivity as for the column mentioned above.

For the column conditions noted above, the absolute pressure at the

injection port was 250 kPa. The gas samples to be analyzed (both in
-50-

calibration and corona experiments) were extracted from a vessel using a gastight sampling syringe. In the usual sampling procedure, for SFg at 300 kPa

for example, the volume of the sampling syringe was expanded to 1.33 times its

original size to give an estimated resulting gas pressure in the syringe of

225 kPa. This was slightly less than the pressure at the injection port; thus,

when the syringe was connected to the injection port and unlocked (valve

opened), gas did not flow into the chromatograph until the plunger of the

syringe was pushed. At the time that the syringe was unlocked, a small amount

of carrier gas flowed into the syringe, but this did not affect the

measurements. Under the conditions mentioned above, the retention time for

SFg was 0.5 minutes from the time of injection.

The gaseous species eluting from the column were detected with the mass

spectrometer. A membrane separator was used between the GC and MS. The mass

spectrometer, in order to separate electrically molecules of different masses,

must first ionize the molecules, by bombarding them with 70-eV electrons.

These not only ionize the molecules but dissociate many of them as well, in a

characteri Stic pattern. The ion fragmentation pattern, i.e., mass spectrum,

serves as a unique signature for identification of the species. Abundant

standard reference mass spectral data are available to allow identification of

nearly all gaseous species of interest. The data processor for the system, in

fact, contains a library of standard mass spectra which can be used for

immediate, on-line comparison with measured spectra. The mass spectra of all

decomposition species reported here have been verified.

Once the mass spectra of the decomposition products had been identified,

it was advantageous to operate the system in the single- ion monitoring mode.
- 51 -

In this procedure, the mass spectrometer is set to look only at selected ion

species of a given mass-to-charge ratio (m/e), namely those ions most abundant

in the mass spectra of the products to be detected. By this mode of operation,

one can achieve higher detection sensitivi ty--down to the ppm by pressure

level

.

For some of the important decomposition products, namely the oxyfluorides

SOF2 and SQ2F2, as well as SO2, an attempt was made to calibrate the

GC/MS so that quantitative analysis of gas concentration could be performed.

Because of difficulties in preparing reliable standard gas samples of known

concentration, we have thus far been unable to calibrate the system for the

important species H2O, HF and SF4. In the case of the latter two species,

no serious attempt has yet been made to detect these species. They could be

detected only with extreme difficulty and much reduction in sensitivity with

the GC column conditions described here.

In performing the calibration, checks were made on the proportional i ty of

the GC/MS response to the concentrations of SOF2 and SO2F2 in SFg.

Using 0.4 mL samples of 500 ppm, 250 ppm, 125 ppm, 63 ppm, 31 ppm,

16 ppm, and 8 ppm of SOF2 in SFg, we found the response ratio

[S0F2(m/e=67 )/SFg(m/e=70 )] to be equal to the known sample concentration

ratio times 102 ±1 (one standard deviation), where it is suspected, however,

that the observed agreement is somewhat fortuitous. The single- ion

chromatograms for m/e=86, correspond!

- ng to these measurements, are shown in

Fig. 18. With 1.0 mL samples and larger uncertainties, [SOF2/SF5] could be

measured down to the 1 ppm level.

For S02F2, using 0.4 mL samples of concentrations of 500 ppm.

250 ppm, 125 ppm, and 63 ppm, we found the response ratio
-52-

FIGURE 18. Single ion chromatograms for ion of mass-to-charge ratio of 86

from gas samples containing the indicated concentrations of S0F~

in SF*. Shown are the SOF^ peaks and an ion feature from SFg

as a function of retention time.
-53-

[$02^ 2

m/ e= 102 ) /SF 5

( m/e=70 ) ] to be equal to the concentration ratio times

59 ±3 (one standard deviation). Again, with 1.0 mL samples, S02F 2 /SFg

could be measured down to 1 ppm, but the uncertainties were large. The chief

difficulty with some of our measurements of SO 2 F 2 is that the signal

associated with ions from this species appears just after the SFg response

which, because it is so much more intense, perturbs the baseline in an

unpredictable way. This problem is illustrated by the single ion chromatograms

shown in Fig. 19 for the ion S 02F+ from SO 2 F

2 with m/e=83. In this

case, the S 02F + peak is preceded by a dip in signal level which we believe

is due to a momentary reduction in the ion multiplier gain resulting frcm the

intense bombardment of ions from SFg. Recently, our measurements of

SO2F 2 have been more accurate due to the discovery of GC column conditions

which allowed greater separation between the SO 2 F 2 and SFg features.

In preparing the standard samples for calibration, a large syringe was

used to extract the trace constituent of interest from a lecture bottle

containing pure gas which was then injected into a 4.0 liter volume into which

the desired quantity of pure SFg was later introduced. For SOF 2 and

SO 2 F 2 , we injected 4.0 mL of both gases at 150 kPa into the 4.0 liter cell,

and then filled the cell with SFg to a pressure of 300 kPa. In this way, we

obtained a well -mixed 500 ppm mixture of SOF 2 and SO 2 F

2 in SFg.

After analyzing this mixture, we made it twice as dilute by doubling the

SFg pressure allowing the gas to mix for a few minutes, then pumping half of

it away (until the pressure was 300 kPa as before). This procedure of analysis

and dilution was repeated until the SOF 2 and SO 2 F 2 concentrations

constituted only 1.0 ppm of the mixture. Although we have no proof that the
-54-

FIGURE 19. Single ion chromatograms for ion mass-to-charge ratio of 83 from

gas samples containing the indicated concentrations of SCLF+ peak

from S0«F« and a dip in background associated with passage of SFr througlvtne column.
-55-

SF 5 supply was initially free of SOF2 and SO2F2 at the ppm level, the

results of our analysis indicated that this was most likely the case. The ion

chromatograms shown in Figs. 18 and 19 were obtained in analysis of the

standard samples prepared by the method described above.

In addition to the checks mentioned above on proportionality of response

to sample concentration, measurements were also made to check on

proportional i ty of GC/MS response to sample volume. For example, the areas

under the m/e = 70 peaks of SF5 for 0.4 mL and 0.2 mL injections at the same

concentration were compared. If the response is linear, the signal from the

0.4 mL sample should be twice that of the 0.2 mL sample. An average of more

than five such comparisons, however, showed that the response ratio was (18 ±6)

percent higher than the expected value of 2.0. Comparing the areas under the

m/e = 86 peaks for SOF2 in 1.0 mL and 0.4 mL injections gave a ratio of these

which was ( 9 ± 11) percent less than the expected value of 2.5. A similar

comparison of peak heights for m/e = 102 from SO2F2 showed a ratio (33 ±11)

percent lower than 2.5; and finally, comparison of area under the m/e = 18 peak

of H2O gave ratios (20 ± 13) percent smaller than the actual volume ratios.

These tests demonstrate that the response of the system is, in general,

nonlinear with respect to volume of gas injected. It is speculated that this

problem is most likely due to either nonlinearities in the efficiency of the

membrane separator to passage of polar molecules, or consumption of gas in the

process of column conditioning. The results of this test provide an indication

of the relative error that would be introduced in comparing samples of
-56-

different sizes. To avoid this error, comparisons were made using samples

having the same injection volume.

Measurements were also performed to check on the constancy of the GC/MS

response to repeated injections of identical samples. In one test, 8 samples

of 0.2 mL of SFg injected 1.5 minutes apart yielded responses uniform to

2 percent (one standard deviation), and 8 samples of 0.4 mL of SFq at various

times throughout one day yielded the same response to 4 percent. There exists

evidence, however, that the reproducibil ity is not always as good as is

suggested by these results. At the present time, we have not undertaken a

thorough investigation of all possible sources of error that might contribute

to uncertainties in quantitative analysis using the GC/MS. It is our

conservative estimate that the trace concentration levels reported here are

accurate to within ±30 percent. A significant source of error might be

associated with nonuniformity of trace contaminant concentrations within the

corona cell. This could be reduced using improved gas sampling procedures.

Further investigations of this are now underway.

In acquiring the data, the mass spectrometer was usually programmed to

monitor preselected ions with the following m/e (mass-to-charge ratio): 16, 18

(indicative of H

2O); 64, 48, ( SO 2

) 5 32, 70 ( 02 , SF 2 , SF 4 ); 67, 86

(SOF 2 ); 83, 102 (SO 2 F 2 ); 105 (SOF 4

). In some cases, other ions were

included when it became evident that other species such as 0CS, CO 2 , CO and

CF 4 might be present. In the determination of absolute concentration levels,

the areas under the ion chromatogram peaks were compared with those for the

standard gas samples. This was done, for example, using the relationship
-57-

[moles of S0F2/moles of SFg] unknown

[moles of S0F2/moles SFg] calib.

[A ( 86 )/A( 70 ) ] unknown

[A ( 86 )/A( 70 )] calib.

where A (86) is the area under the m/e=86 chromatogram peak for SOF2 and A( 70

)

is the corresponding area for the m/e=70 ion from SFg.

Usually the H2O ions (m/e=16, 18), SFg ions (m/e=32, 70), and SOF2

ion (m/e=86) were integrable by the data processor, whereas SOF2 (m/e=67) and

SO2F2 (m/e=83, 102) had to be measured by hand from plots. In the latter

cases, areas could be accurately estimated from knowledge of peak heights and

widths provided there was sufficient signal. For each of the species listed

‘there are responses in two (or more) ion channels, of which one was more

reliable by virtue of greater size or a steadier baseline and these were: for

H2O, m/e=18; SFg, m/e=70; SOF2, m/e=86; SO2F2, m/e=102.

II. C. 3 Results

Figures 20 and 21 show single ion chromatograms for samples of SFg

degraded by dc positive point- plane corona at a constant current of 2.0 yA.

Indicated are the accumulated times during which the discharge was on. The

SFg gas pressure for the measurements was 300 kPa (^3 atm). Stainless-steel

electrodes were used with a gap spacing of 1.27 cm and an initial point

diameter of 0.16 mm. At the end of 32.5 hours of corona the point electrode

was examined under a microscope. The tip, in addition to being discolored, had
-58-

SE, P= 300 kPa 6

m/e = 83 m/e = 86

SINGLE-ION CHROMATOGRAMS

FIGURE 20. Single-ion chromatograms for ions of mass-to-charge ratios of 83 and 86

obtained from gas samples extracted after the indicated accumulated times

during which SFg at an absolute pressure of 300 kPa was subjected to

continuous positive dc corona at a constant current level of 2.0 yA.

Shown are features associated with SFg, SOF 2

, and SO2

F

2

-
-59-

Sf|, P= 300 kPa

m/e=48 m/e = 16

SINGLE ION CHROMATOGRAMS

FIGURE 21. Single ion chromatograms for ions of mass-to-charge ratios of 16 and

48 obtained from gas samples extracted after the indicated accumulat

times during which SFg at an absolute pressure of 300 kPa was subjected to continuous positive dc corona at a constant current level

of 2.0 uA. Shown are features corresponding to SFg, SO

2

F

2

, SO

F^

and
-60-

increased in diameter by about 12 percent, thus indicating that some melting

had occurred.

A direct visual comparison of the relative peak heights in the

chromatograms shown in Figs. 20 and 21 with those in Figs. 18 and 19 would

suggest that after 26.3 hours of corona, SO2F2 is present at roughly the

40 ppm level and SOF2 at the 20 ppm level. The results of more careful

analysis are consistent with these estimates giving respectively 39 ppm and

16 ppm. In the case of H2O, only relative concentrations could be determined

for reasons previously noted.

The primary decomposition products that could be observed with the GC/MS,

after running corona continuously at 2.0 yA for 32 hours, were SOF2,

SO2F2 and H2O. In this experiment, the H2O concentration appeared to

build up within the first 10-15 hours of operation and then level off, whereas

the SOF2 and SO2F2 concentrations continued to increase with time. This

is indicated by the data shown in Figs. 22 and 23. In the measurements

performed to date, the presence of H2O was always the first indicator of

contamination in SFg subjected to corona. It is speculated that H2O is

driven into the cell by the effect of the discharge in, for example, heating

the electrode surfaces by ion or electron bombardment.

In Fig. 22, two sets of data are shown which represent an attempt to

identify the source of H2O. Starting with new electrodes that had been

recently cleaned and polished, a corona discharge was run in 300 kPa SFg, at

a level of 2.0 uA, for 7.4 hours, and during this time the build-up of H2O

was monitored. Then the cell was evacuated to high vacuum and a fresh, dry

SFg sample was introduced, again at a pressure of 300 kPa. Analysis of the
-61-

TIME (hrs)

FIGURE 22. Time dependence of relative H^O concentration in SFg at 300 kPa which had

subjected to continuous positive dc corona discharges at a level of 2.0 yA.

For the data corresponding to the x's "clean" electrodes were used, whereas

for the data corresponding to the o's the electrodes had been preconditioned

by prior discharges in SFg.
-62-

new gas revealed no H 2O content. The corona discharge was then resumed at

the same level and again the relative H 2

O concentration was monitored as a

function of time. In Fig. 22, the x's correspond to the initial discharge

which was run with "clean” electrodes and the o's are from the second gas

sample. Clearly the H 2O content builds up less rapidly in the second sample,

which would suggest that the discharge in the first sample drove off a

significant amount of H 2O and, in that sense, conditioned the electrodes for

the subsequent test with the next fresh SFg sample. This result is

consistent with the suggestion that the electrodes are the primary source of

H2O.

For times up to about 100 hours, the concentrations of SOF 2 and

SO2 F 2 seem to increase linearly with time. This is indicated by the

results shown in Fig. 23. For times longer than about 100 hours at 2.0 yA,

additional decomposition products begin to become detectable. In Fig. 24, we

show single ion chromatograms indicating the presence of SO2 and OCS after

333 hours of continuous corona at a level of 2.0 yA. The SO 2 was first

detected at 160 hours.

After 333 hours the point electrode showed considerable erosion which was

visible without the aid of a microscope. Examination under the microscope

revealed the presence of several nodules of yellow material, possibly pure

sulfur, deposited near the tip of the electrode. Also after this time the

quartz window on the side port of the cell showed definite signs of being

etched, thus indicating that a significant amount of HF had been produced,

which as noted before, could not be detected with the GC/MS.

-o

03

u

I

-C2

03 Q_

-O' O

oCO

03LO

Ll_ 00 c = 3. O

i- o 4— CM

C\J

Li_ 4- CMO

oO0 r—

cu -a > c a>

03 i

CM03

LL. O +-> OO 03 « (/) OO S_ O 3 O O

C\TO O CO

OO CO

4- 4- O O

a -a

•r- o

-M -r- L0 S_

•r- O)

s- Q.

a>

03 u

03 is- o

03 4- U 03c

ui o

c s- o o

•f- u

S- o

o-o

4-

a>

L0 >

E-r034->

5-

-r- 05 on o o

4-1 Q.

03= c

o-r-

$- i=-0

U O) •O d 03 O J_ •<— 0

1

o

aj-o

cc

c a; •r-<U OO.Q

oCM

UJ

cc

33CO
-65-

II. C. 4 Discussion

The production of H

2O in the corona discharge is somewhat surprising.

The results shown in Fig. 22 strongly suggest that it is driven into the cell

by the action of the discharge in heating the electrode. These results also

show that the H 2O concentration tends to level off after several hours,

presumably because an equilibrium condition is reached in which water is driven

from the point at the same rate as it is consumed by other mechanisms, e.g.,

reabsorption. Also, the rate of water production is significantly affected by

the preconditioning of the electrode with operation of a discharge for several

hours before introducing the gas sample to be studied.

These measurements imply that it may be quite difficult to avoid

introduction of H 2O into a test cell used to measure breakdown or study long

time "aging" effects in corona. A further implication of these findings is that

if partial discharges occur in practical systems insulated with gas, water

vapor will be produced. The amount will, of course, depend on the duration of

the discharge and area over which it occurs.

Water, if present, will significantly affect the chemistry that occurs

during and subsequent to decomposition of the gas in a discharge.

In SFg, free oxygen results from dissociation of H 2 O in the discharge,

and one expects the following reactions^ to occur:

SF

6

0 $0F

2 + 4F

SF

6

0

-*• S0Fd + 2F

SF

6

-> SF

4 + 2F

sof

4

0 + ^

inside of the discharge
- 66-

Outside the discharge region, SF4 and SOF4 will eventually hydrolyze in the

presence of H2O and be converted to SOF2, SO2F2 and HF via the

reactions

I

SF 4 + H 20 -* S0F 2 + 2HF

SOF4 + H 20 + S0 2F2 + 2HF.

In this way, we can explain the occurrence of SO2F2 and SOF2 as

predominant, observable, end decomposition products.

The fact that there was no evidence of HF, SF4 or SOF4 from analysis

reported here is not sufficient basis for concluding that these compounds were

not present. As noted before, these are very reactive gases and will not

readily pass through the GC column used. In an attempt to evaluate the

sensitivity of our GC/MS to detection of SF4 in both M2 and SFq, no ions

were observed in the mass spectrometer that could be directly attributed to

SF4. Instead we observed features corresponding to SOF2 which we assume

came largely from hydrolysis of SF4 with trace amounts of H2O in the mixing

vessel or gas sampling syringe.

The mechanisms that account for production of SO2 and OCS, which became

detectable much later than the oxyfl uorides, are not understood at this time.

The appearance of these seemed to be associated with an apparent build up of

CO, CO2, and CF4 concentrations, but further investigation of this is
-67-

required. It is clear though that SO 2 and OCS cannot result from primary

decomposition of SFg, and must involve secondary reactions, perhaps on the

electrode surface. The source of carbon is not immediately obvious, although

it might come directly from evaporation of the stainless- steel electrode. As

noted, considerable deterioration of the electrode was evident.

The concentrations of the oxyfluorides seem to increase linearly with time

as suggested by the data in Fig. 23. It would be desirable to observe in

future experiments how the rate of production of these species depends on

discharge power level. One might speculate that the slopes of the production

curves should increase in proportion to the power level.

II. 0. Effects of Trace Decomposition Products and H?0 on SF^ Positive DC

corona unaracteri sties “ ’

II.D.l Motivation

At the present time, we know of no other systematic studies of the effects

of trace contaminants, such as H 2 O, on the behavior of low level electric

discharges in compressed SFg. There is reason to suspect, as previously

noted^ in our study of corona in SF 5 /N 2 mixtures, that the pulse

characteristics of corona are quite sensitive to small changes in gas

composition. In fact, a measurement of corona pulse characteristics might

prove to be a useful diagnostic of insulation deterioration. This idea has

been pursued already in earlier work. 12,14 in particular, measurement of

changes in pulse height distributions (PHD 1 s) might be one of the most

sensitive indicators of contamination or deterioration

.

It is, moreover, desirable to learn about the effect that trace

contamination can have on corona inception and breakdown measurements used for
-68-

evaluation of dielectric performance of gases. This information could prove

useful in the design of tests. If, for example, in performing a series of

breakdown measurements, the first event alters the gas composition, then

subsequent measurements with the same sample might be affected. Furthermore,

it would be desirable to know, on a relative scale, how susceptible various

gaseous dielectrics are to the presence of contamination as compared, for

example, to SFg. Some gases can be expected to be more resistant than

others.

In the work described here, we have monitored the PHD's of partial

discharges as a function of time for continuous dc corona in SFg under

conditions of both constant current and constant voltage. At the same time,

the content of the gas sample was monitored using a gas chromatograph /mass

spectrometer (GC/MS). Experiments were also performed in which PHD

characteristics of degraded gas were compared with those of a fresh gas sample

under the same electrode conditions. In another experiment, pure SFg gas

samples were artificially contaminated with trace amounts of H 2O

using a hot wire technique, and the effects on the PHD and pulse repetition

rates of the corona were noted. Preliminary results have already been

reported.^

II. D. 2 Measurements and Data

The PHD and pulse repetition rate measurements for SFg corona were

performed using the multichannel analyzer system previously discussed in

Sec. IIA (see Figs. 1 and 2). The chemical analysis of the gas was performed

with the GC/MS and corresponding operating and sampling conditions described in

the previous section. The relative concentration of H 2

O and the absolute
-69-

concentrations of the oxyf1 liori des SOF 2 and SO 2 F 2 were determined by the

technique of single-ion monitoring (see previous section, also Ref. 31).

Figure 25 shows changes that occur in the positive corona PHD spectra as a

function of time when SFg at a pressure of 200 kPa in a volume of 3780 cnr*

is subjected to continuous corona discharges for a period up to 260 minutes at

constant voltage. Stainless-steel electrodes were used with a poi nt-to-plane

gap spacing of 2.28 cm and an initial point diameter of 0.09 mm. Also

indicated in the figure are the corresponding average corona current and

partial discharge repetition rates. The decreases in current and repetition

rate are suspected to be associated mainly with changes in point electrode

conditions. Some melting of the tip was, for example, evident after several

hours of operation giving rise to an effective increase in tip diameter.

The changes in PHD seen in Fig. 25, however, seem to be more indicative of

permanent changes in gas composition caused by the discharge. The general

tendency, observed in several tests of this type, is for the PHD's for positive

dc corona to exhibit greater contribution from small single pulses, and

relatively less contribution from bursts, as time progresses. This is also

evident in measurements performed under conditions of constant corona current.

It appears to suggest that there is a decreasing tendency for the positive

corona to develop as pulse bursts as the gas becomes more "degraded."

The most convincing evidence that the corona characteristics are sensitive

to gas contamination is provided by comparative PHD measurements performed

immediately before and after changing from a degraded to a pure gas sample. In

this way effects due to changing electrode conditions are minimized. In

Fig. 26a is shown the measured PHD for a sample of SFg which has been
RELATIVE

INTENSITY

-/U

FIGURE 25. Time variation in partial discharge pulse height distribution for

positive corona in SFg at a pressure of 200 kPa. Also indicated

are correspond!' ng average corona current and observed pulse

repetition rates. The measurements were performed at a gap spacing

of 2.28 cm with a point of diameter 0.09 mm, and a gas volume of

3780 cm^.
71

co

<T> ro

ii <

CO

'oXOn

FIGURE

26.

Partial

discharge

pulse

height

distributions

for

positive

dc

corona

in

SFfi

at

an

absolute

gas

pressure

of

200

kPa

and

applied

voltage

of

46.6

kV

for

(a)

gas

degraded

by

corona

at

constant

current

of

1.5

pA

for

the

period

of

32

hours

immediately

before

changing

the

sample,

and

for

(b)

pure

SF

fi

immediately

after

introducing a

fresh

gas

sample.

Indicated

are

the

corresponding

corona

currents

and

pulse

repetition

rates.

The

charge

level

scale

is

the

same

for

both

displays.
-72-

degraded by corona at a constant current of 1.5 yA for a period of 32 hours.

Immediately following this measurement, the cell was evacuated to high vacuum

and a fresh sample of pure SFg was introduced at the same pressure (200 kPa)

and the PHD was again measured at the same voltage of 46.6 kV. The result of

the latter measurement is shown in Fig. 26b. Note that not only is there a

dramatic difference in the PHD spectra, but also both the corona current and

pulse repetition rate are down two orders of magnitude relative to the degraded

sample. The tendency for pulse bursts to dominate is restored upon adding a

fresh gas sample as indicated by the PHD (also compare Fig. 26 with Figs. 3, 4

and 25). In Fig. 27, we show oscilloscope traces illustrating how the initial

burst characteristic of the discharges becomes more diffuse and separated with

time and eventually gives way to a phenomenon dominated by single, isolated

pulses. After 14.0 hours of corona, burst activity ceases.

During the same tests described above, gas samples were periodically

extracted from the cell and analyzed with the GC/MS system. Such analysis was

performed immediately after extraction with the syringe to avoid contamination

from atmospheric constituents, primarily H 2O. The results of one such

experiment which ran for 32.5 hours are shown in Fig. 28. It should be noted

that the pulses shown in Fig. 27 correspond to this particular experiment. For

this case, a positive dc corona was operated at a constant current of 2.0 yA in

SFq at an initial absolute pressure of 300 kPa (^3 atm). Stainless steel

point-plane electrodes were used with a gap spacing of 1.27 cm and an initial

point diameter of 0.16 mm. The gas was contained in a 3780 cm^ volume. At

the end of 32.5 hours the point electrode was examined and the tip, in addition

to being discolored as usual, was found to have increased in diameter by
-73-

SF. CORONA 6

PULSES

t = O-Ohrs

t =9.0 hrs

GURE

27.

Changes

in

observed

positive

corona

pulse

shapes

in

SFg

corresponding

to

the

experiment

which

yielded

the

data

in

Figure

28.
-74-

FIGURE 28. Time dependence of measured concentrations of H2 O, SOF2 , and SO2

F2

in SFg at 300 kPa which had been subjected to continuous positive

dc corona discharges at a level of 2.0 yA. Shown are data obtained

from analyses performed immediately after the corona was turned off

and from analysis after corona had been off for three or more hours.
-75-

12 percent, thus indicating that some melting had occurred. The effect of

melting in this case, however, was not nearly as severe as it was for the

sharper point used to obtain data presented in Fig. 25. For the experiment

considered here, it was not necessary to turn up the voltage with time to

maintain constant current. In fact, the required voltage actually decreased

slightly from 32.8 kV initially to 27.7 kV at the time the experiment was

terminated thereby demonstrating that effects of electrode melting can be

significantly reduced by using larger electrode tip diameters and higher

operating voltages.

Two sets of data are shown in Fig. 28; one correspond!

- ng to analysis

performed immediately after the corona was turned off, and the other

corresponding to analysis performed after the corona had been off for several

hours so that the gas had ample time to equilibrate. On the basis of this

rather limited data, it would not be possible to conclude that there are

significant, systematic differences between the results obtained from these two

sampling procedures, although in a few cases there are clearly large

di fferences.

Again like the data shown in Figs. 22 and 23, the oxyfluoride

concentrations seem to be increasing nearly linearly with time, and the H 2 O

concentration shows evidence of reaching saturation. It is significant to

point out from a comparison of Figs. 27 and 28 that, by the time that there is

a pronounced change in pulse characteristics of the positive corona, the

concentration of SOF 2 is only about 4 ppm and that for SO 2

F

2 is only

about 15 ppm. Even keeping in mind the possible 30 percent uncertainty in
-76-

these values, it is remarkable that such a large change in discharge behavior

can be associated with such small increases in contaminant concentrations.

The experiments on the effect of trace gaseous decomposition products on

corona were followed by experiments in which H

2O was deliberately introduced

into samples of pure SFg* Evidence from the degradation experiments

described above suggests that the presence of small amounts of H 2 O could have

a large effect on corona. In these experiments, water was injected into a cell

containing pressurized SFq by heating a wire. Two wires were used in

separate experiments. One was a stainless steel wire having a diameter of

0.06 mm and length of 30 cm and the other was a nichrome wire having a diameter

of 0.051 cm and a length of 20 cm. In both experiments, while the wires were

heated for several hours by passing a constant current through them, the build

up of H 2O in the cell was monitored with the GC/MS. The results are shown in

Fig. 29. For the stainless steel wire, the SFg pressure was 300 kPa and, for

the nichrome wire, it was 200 kPa. It is seen that the rate of H 2

O injection

from both types of wires is similar. Both exhibit a saturation effect similar

to that which occurs when H 2O is produced by running the corona discharge.

This observation lends further support to the hypothesis (see the previous

section) that in the operation of a discharge, H

2 O is driven into the cell by

local heating of the electrode.

To examine the effect of H 2O on positive dc corona, the electrical

characteristics were determined at the following times: 1 ) immediately after a

fresh SF 5 sample was introduced before injection of water, 2) after water was

injected by heating the wires for roughly 15 hours (see Fig. 29), and 3) after

the SF 5 sample contaminated by H 2O had been replaced with a fresh, pure
RELATIVE

WATER

VAPOR

CONCENTRATION

-77-

FIGURE 29. Time dependence of relative concentrations of H ? 0 vapor in SFg measures

with a GC/MS for two experiments involving electrical heating of wires

described in the text.
-78-

sample at the same pressure. The electrode conditions were the same in all

cases since the discharge was run at a relatively low level for a short time

sufficient to characterize it by performing PHD, pulse repetition rate, and

current measurements (typically less than 5 min.). During these short periods

the corona is unlikely to have noticeable effect on the electrodes (see data in

Fig. 25).

The best indication of the effect of H2O on the corona character!' sties

is obtained by comparing the results for the contaminated samples with the

results for the subsequent fresh samples. This is because the total gas

pressure in the cell will be reduced slightly (perhaps as much as 10 percent)

due to the sampling with the GC/MS to monitor H2O concentration during

heating of the wire. It is known that even relatively small changes in gas

pressure can give rise to detectable changes in corona. 2

In Figs. 30 and 31, we show comparisons of electrical characteristics of

SF5 corona observed for fresh and contaminated gas samples. In Fig. 30, we

also show correspond!' ng single-ion chromatograms which indicate the relative

concentrations of H2O. The large peaks in these scans correspond to a

feature associated with SFg which should be of comparable intensity for both

gas samples assuming identical GC/MS conditions. For the data in Figs. 30

and 31, the tip diameters were respecti vely 0.12 mm and 0.096 mm. In both

cases, the poi nt-to-plane gap was 1.28 cm. For the data in Fig. 30, H2O was

introduced using the stainless steel wire, whereas for Fig. 31 the nichrome

wire was used.

In Fig. 31 we show the observed PHD's, repetition rates f, average corona

current I, and observed pulse shapes. A comparison between pure and
RELATIVE

INTENSITY

10.0 20.0

DISCHARGE LEVEL (pC)

FIGURE 30. Pulse height distribution and frequency of positive dc corona in

SFg at 300 kPa with and without trace H^O contamination as indicated.
=

182

kPa

O O o o
-81-

contaminated gas samples is made at the same voltages, namely 18.7 kV and

21.1 kV, and at the same current, namely 2.0 x 10"? A.

II. D. 3 Pi scussion

As seen from the results given in Figs. 30 and 31, the presence of trace

amounts of H 2O in compressed SFg can have an enormous effect on the

electrical characteristics of positive dc corona. In Fig. 30, we see that,

compared to the pure gas, the repetition rate of the corona for contaminated

gas is four orders of magnitude greater at the same applied voltage of 20 kV.

These results, together with those given in Fig. 31, indicate that the effect

of H 2

O is most dramatic close to the onset voltage, as might be expected.

Water vapor clearly degrades the dielectric strength of the gas. The effect of

H2

O on the PHD characteristics is seen to be similar to that which results

from degradation of SFg in a continuous discharge, i.e., the presence of

H 2

O diminishes the tendency for the corona to develop as bursts. This is

evident by comparing the before- and- after PHD's in Fig. 26 with those* for

21.1 kV in Fig. 31.

On the basis of the information acquired here, one could presumably argue

that it is H 2O that accounts for most of the change in PHD's when the gas is

degraded by a discharge (see Fig. 25).

On the other hand, it could also be that the effect of the other

decomposition products is similar to that of water. With the data that is

presently available, it is not possible to separate the effect of H

2 O from

that of all other known trace gases in degraded SFg. This is partly because,

at the present time, it is not possible to perform a reliable quantitative

analysis of H

2O content with the GC/MS. However, by comparison with the
-82-

res tilts for other products, one might guess that the maximum absolute

concentrations of H 2O in Figs. 28 and 29 are on the order of 100 ppm.

Further work on this is required.

A meaningful quantitative comparison of the effects of H 2O and dischargeproduced gaseous contaminants on corona behavior is not possible for other

reasons. In the present experimental procedure there are, for example, no

assurances that the electrode conditions are the same, since in the degradation

experiment the corona characteri sties are measured with the same electrodes

used to operate the continuous discharge, and the point electrode is known to

undergo some deterioration when the discharge is run for extended periods.

One can conclude, on the basis of the measurements reported here, that the

presence of trace contaminants such as H 2O, SOF 2

, SO 2

F

2 and Hfr can have

a large effect on the burst characteristics of positive dc corona in

pressurized SF 5

. The effect which H

2 O and the other contaminants have in

inhibiting burst activity can perhaps best be understood in terms of the

influence that these molecules have on the development and dynamics of ionic

space charge in the point-plane gap. As already noted in Sec. 1 1 A (also see

Refs. 10 and 11) the burst characteristics are controlled by the manner in

which the positive ion space charge develops; thus anything that perturbs this

space charge will likewise change the bursts. Unlike SFg, the molecules

H

2 O, SOF 2 , SO 2 F 2 and HF are all polar, and polar molecules,

particularly H 2 O, have a tendency to form clusters around ions. Due to

clustering, the positive ion mobilities in the gap will be decreased and this

leads to an enhancement of the rate of positive space charge build-up near the

point. A rapid increase in the positive space charge will tend to quench
-83-

secondary electron avalanche development and thereby inhibit bursts. Other

processes such as charge transfer collisions could also affect ion mobilities

and thus the way in which space charge develops.

The enhancement in the level of corona, i.e., pulse rate and average

current, due to trace decomposition products and/or H 2 O is most likely the

result of an increase in the effective ionization coefficient of the gas.

Insufficient information exists at this time however to determine if this

explanation is realistic.

I I.E. Laser Induced Changes in N 2 Corona Character!* sties

II.E.l Motivation

To our knowledge, the effect of intense optical laser beams on development

and characteri sties of corona discharges has not previously been investigated.

The purpose of this work has been to evaluate the possibility of using the

laser stimulated opto-gal vanic effect as a microscopic diagnostic of

fundamental processes in corona discharges. The idea is that by optically

pumping selected atomic or molecular species with a tunable light beam incident

on the discharge region one induces a change in the effective ionization

coefficient of the gas, which manifests itself as a measurable change in

discharge current. This kind of effect has been observed in several types of

low pressure discharges, primarily hollow cathode di scharges .32-34

It is also of interest to investigate the effect which laser light might

have on modification of electrode conditions, e.g., via photon enhanced field

emission. The discharge was chosen as the starting point for this

exploratory investigation because it is one of the few molecular gases for

which opto-gal vanic signals have been reported.^3
-84-

II. E. 2 Experimental Arrangement

Working with a coaxial electrode system, an attempt was made to observe

the opto-gal vanic effect in N 2 with positive dc corona. The cylinder was

fabricated from aluminum and the center corona wire was made of

5.7 x 10

-

3 Cm diameter stainless steel wire. To obtain optimum

sensitivity in this measurement, the coaxial electrode system was designed (see

Fig. 32) to permit a dye-laser beam to be directed coaxially with the corona

wire and into the visible glow of the corona. The measurements were carried

out for the pressures ranging from 1.3 kPa (10 Torr) to 13.3 kPa (100 Torr).

Laser powers typically near 100 mW were used, but smaller and greater powers

were occasionally employed. The maximum discharge current was near 1.1 mA.

The beam from the dye laser was mechanically chopped with a rotating

slotted disc and the in-phase component of the resulting change in discharge

current was measured using a phase sensitive detector which employed a lock-in

amplifier. The current was sensed as a voltage across a resistor in series

with the discharge cell. Opto-gal vanic signals observed with a neon-filled

hollow-cathode lamp were used for wavelength calibration of the laser.

II. E. 3 Results and Discussion

The opto-gal vanic effect, in the form of discrete lines correspond!

- ng to

electronic transitions in excited N 2 or N 2 +

, was not observed for

positive dc corona generated in this experiment. A change in corona current

due to the presence of the incident laser light was detected, however. This

signal, which was continuous, exhibited some wavelength dependence, and could,

on occasion, be made to change polarity by changing the voltage applied to the

cylinder. When the signal was positive, it decreased in magnitude as the
]

-85-

FIGURE

32.

Change

in

corona

current

as

a

function

of

dye

laser

beam

position.

The

data

has

not

been

corrected

for

a

slow

decrease

in

average

discharge

current.
- 86 -

wavelength was increased from 580 nm to 620 nm. When negative, the signal

increased as the wavelength was increased. The effect was most pronounced when

the laser beam was incident on the cylinder and not the corona wire. Figure 32

shows the spatial dependence of the effect.

The signal when observed as a function of time at a given wavelength is

unstable and may be coupled to changes in the basic physical processes

occurring in the partial discharge related, for example, to changes in surface

conditions or gas composition. Reproducibility of measurements was, as a

result, very difficult to achieve. The greatest signals appeared to occur near

the breakdown voltage at a given pressure.

Because the maximum signal, as a function of position, occurs at the

cylinder surface, it appears likely that we are observing a surface phenomenon.

The cylinder surface does provide some electrons to the discharge process when

positive ions and uv photons impinge on it. The laser light may be influencing

this process, e.g., via photon-enhanced electric field emission, but such an

interpretation is, at this time, speculative. In view of these results, and

considering the importance which surface conditions are known to have in high

field breakdown of compressed gases, it would be desirable to explore further

the use of a tunable laser beam as a diagnostic probe of electrode surface

conditions.

The results of the measurements performed suggest that the ordinary

opto-gal vanic effect, which is due in part to laser-induced optical pumping of

metastables in a discharge, is not a particularly useful diagnostic in the

study of corona discharges in molecular gases. The reasons for this are not

entirely clear, but might be related to the relatively high noise level in most
-87-

corona discharges as well as the fact that molecular species are generally more

effective in colli si onally quenching metastables.

II. F. Other

During this reporting period there were several other activities and

accomplishments that deserve mentioning. Minutes were prepared and distributed

for the Workshop on Gaseous Dielectrics for Use in Future Electric-Power

Systems which was held at the National Bureau of Standards on

September 10-11, 1979.6 These minutes were extracted from tape recordings

made during the meeting. The workshop was held for the purpose of addressing

questions concerning current and planned research on development of new gaseous

dielectrics viewed as possible substitutes for SFq. Focus was on identifying

technological barriers to acceptable use of new gases and gas mixtures and

questions concerning properties of gases in need of further investigation.

A report concerned with the safe handling of SF5 used in laboratory type

electrical apparatus was prepared. ^ This report is intended to benefit those

within NBS who are not familiar with the properties of SFg, but who must use

this gas for electrical insulation or arc interruption. The information it

contains was derived from the National Institute of Occupational Safety and

Health's Registry of Toxic Effects of Chemical Substances and from other

sources believed to be reliable, but is not the result of any critical data

evaluation or testing activity performed at NBS. A report entitled "A

Bibliography of Electron Swarm Data"^ was completed and copies are now

available for distribution from the JILA Atomic Collisions Data Center in

Boulder, Colorado. This report allows a user to find electron swarm data (and

some related information) in the regular literature. The types of data
r -88 -

included are for example: ionization, attachment, excitation, and

recombination rates (or coefficients) for electrons in various atomic and

molecular gases. Also included are data on electron drift velocities, energy

distribution functions, diffusion and mobility. The bibliography is quite

comprehensive and includes experimental and theoretical data on nearly all

gases of interest as components in gaseous dielectrics, as well as for other

applications where discharges are important, e.g., lasers, light sources, high

voltage switches, etc. Copies of each of the papers listed have been saved on

microfilm. A compilation of electron swarm data has also been completed and a

report containing this data together with critical commentary is in

preparation.

III. Conclusions and Summary

A measurement technique for quantitatively characterizi ng partial

discharges for laboratory studies of gaseous dielectrics has been developed and

evaluated. This technique provides a relatively complete description of the

electrical properties of corona such as the pulse repetition rate, pulse height

distribution, pulse shapes and average current. By this approach, one can

obtain reproducible numerical data on corona which presumably can be compared

with similar measurements in other 1 aboratori es. The instrument, which employs

a relatively fast multichannel analyzer, has inherent limitations due to the

finite sampling rate and time constants of the circuits involved. It is fast

enough at present, however, to give a useful statistical signature of pulse

shapes and pulse bursts. Possible problems in i nterpretation of pulse height

distributions occur mainly when the corona pulse duration exceeds the RC time
- 89-

constant of the detection circuit or when the mean interval between pulses,

particularly in a burst, is small compared to the analyzer sampling time.

The pulse characteristics of dc corona in SF5 and SF5/N2 mixtures

were measured for a point-plane electrode geometry. It was discovered that

these characteri sties are quite sensitive to gas pressure, gas composition and

applied voltage. There is also a very significant dependence of corona pulse

characteristics on electrode polarity. The pulse height distributions were

insensitive to irradiation of the gap with uv light, and the trends in voltage,

pressure, and polarity dependence were relatively insensitive to changes in

electrode gap spacing. This was not necessarily the case, however, for pulse

repetition rates. At the larger gap spacings and higher pressures, for

example, the positive dc corona pulse rate was seen to be very sensitive to uv

radiation near onset. Negative corona pulse characteristics are more sensitive

to point electrode conditions, whereas positive corona is more sensitive to gas

composition.

It was discovered, in fact, that positive dc corona pulse height

distributions are sensitive indicators of trace contamination in SF5. When

trace amounts of H2O are introduced into pressurized SFq, the burst

activity in positive corona is found to be inhibited. This is also true after

SFq has been degraded by continuous corona for several hours. In both cases,

the level of corona increases with contaminant concentration indicating a

reduction in dielectric strength. It is speculated that the changes in corona

burst characteristics are related to the effect of polar molecules on

development of positive ion space charge in the gap. The reduction in
-90-

dielectric strength is probably due to contaminant-induced changes in the net

ionization rate of the gas.

The corona-induced decomposition of SF 5 was monitored periodically with

a gas chromatograph-mass spectrometer. When SFg was degraded by corona at a

constant level of 2.0 yA for about a day (24 hours), the primary detectable

decomposition products were H 2O and the oxyfluorides SOF 2 and SO 2 F 2.

The existence of these also imply the presence of HF and SF 4 which could not

be detected with the procedures used. After much longer times, there was also

evidence of other gaseous species such as SO 2 , XS, CO, and C 02 * The

concentrations of SOF2 and SO 2F 2 after 24 hours were, respectively, at

the 10 and 30 ppm levels. The H 2 O is apparently driven from the electrodes

due to heating by the discharge. It would appear from these studies that,

unless precautions are taken to precondition electrodes such as by extensive

baking under vacuum, it is impossible to avoid introducing H 2O when operating

a corona discharge. The H

2 O content appears to level off after the first 10

to 20 hours of operation. The pulse burst characteristics of corona show a

dramatic change after 4 hours of corona, thus suggesting that these are

significantly affected by decomposition products present at only the ppm

level

.

The questions of defining and measuring corona inception voltages in SF 5

under dc and 60-Hz ac conditions were also addressed. The technique employed

here was one of measuring pulse counts versus applied voltage (peak voltage in

the case of 60 Hz). It was found that dc measurements of inception do not

always give a good indication of what will happen under 60-Hz ac conditions,
-91-

particularly at gas pressures above 200 kPa. Although the ac and dc negative

inceptions are close in value, the positive inceptions may differ by nearly as

much as a factor of 2.0 at 500 kPa. This difference can be explained in terms

of the build up of negative ion space charge in the gap under ac voltage. The

dc inceptions are also affected more by the presence of uv radiation in the

gap.

In summary, the results of the laboratory measurements reported here help

provide a better fundamental understanding of corona phenomena in compressed

gaseous dielectrics. The measurement techniques developed will be useful in

further studies of corona and should be helpful in designing tests to evaluate

relative performance of various new dielectric gases. With the methods

considered one can gain a better understanding of processes leading to aging or

deterioration of the insulation. The information acquired may also aid in

design of techniques to monitor and predict insulation aging in practical

systems.

As an aid to those involved with theoretical prediction of breakdown and

modeling of discharges in gaseous dielectrics we have prepared an extensive

bibliography of electron swarm data. We have also compiled and evaluated the

existing swarm data which is likely to be most relevant to dielectrics

research.
-92-

References

1. R. J. Van Brunt and M. Misakian, "Comparison of DC and 60-Hz AC Positive

and Negative Partial Discharge Inceptions in SF 5

," Proc. Conf. on

Electrical Insulation and Dielectric Phenomena, National Academy of

Sciences Press, Boston (1980).

2. R. J. Van Brunt, J. S. Hilten, D. P. Silver, "Partial -Di scharge Pulse

Height Distributions and Frequencies for Positive and Negative DC Corona

in SFg and SF 5 -N 2 Mixtures," Gaseous Dielectrics II , Proceedings of

the Second International Symposium on Gaseous Dielectrics, Pergamon

Press, pp. 303-311 (1980).

3. D. A. Leep and R. J. Van Brunt, "Pulse Characteristics of Positive DC

Corona in SFg: Effects of Trace Decomposition Products," Proc. 33rd

Gaseous Electronics Conference, to be published in Bull. Am. Phys. Soc.

-(1980).

4. E. Beaty, J. Dutton and L. Pitchford, "Electron Swarm Data Relevant to

High-Voltage Insulation," Gaseous Dielectrics II , Proceedings of the

Second International Symposium on Gaseous Dielectrics, Pergamon Press,

pp. 25-31 (1980).

5. E. C. Beaty, J. Dutton, L. C. Pitchford, "A Bibliography of Electron

Swarm Data," JILA Information Center Report No. 20, Univ. of Colorado

(1979).

6 . R. J. Van Brunt, "Minutes of Workshop on Gaseous Dielectrics for Use in

Future Electric-Power Systems," U.S. National Bureau of Standards

Interagency Report, NBSIR 80-1966 (1980).
-93-

7. D. Ac Leep and R. J. Van Brunt, "Safety Considerations for Handling SFg

Used in Electrical Equipment," U.S. National Bureau of Standards

Interagency Report, NBSIR 80-2063 (1980).

8. A. H. Cookson and R. E. Wootton, "AC Corona and Breakdown Characteri sties

for Rod Gaps in Compressed Hydrogen, SFg and Hydrogen-SFg Mixtures,"

IEEE Trans. Power App. Syst., Vol. PAS-97, pp. 415-422, (1978).

9. 0. Farish, 0. E. Ibrahim, and A. Kurimoto, "Prebreakdown Corona in SFg

and SFg/N 2 Mixtures," 3rd Int. Symp. on High Voltage Eng., Milan,

(1979).

10. R. Hazel and E. Kuffel , "Static Field Anode Corona Characteristics in

Sulfur Hexafluoride," IEEE Trans. Power App. Syst., Vol. PAS-95,

pp. 178-185 (1976).

11. 0. Ibrahim, "Corona Stabilization and Breakdown in. SFg and SFg/N 2

Mixtures," Ph. D. Thesis, University of Strathclyde, Glasgow, Scotland,

(1980).

12. R. Bartnikas, "Use of Multi-channel Analyzer for Corona Pulse-Height

Distribution Measurements on Cables and Other Electrical Apparatus," IEEE

Trans. Instru. and Meas., Vol. IM-22, pp. 403-407 (1973).

13. R. Bartnikas, "Note on Multi-Channel Corona Pulse-Height Analysis," IEEE

Trans. Electr. Insul., Vol. EI-8, pp. 2-5 (1973).

14. K. N. Mathes, "Monitoring Voltage Aging in Liquid Impregnated Solids with

Pulse Analysis," Proc. EPRI Seminar on Solid Dielectric Materials Applied

to the Electric Power Industry, Monterey, CA (1979).
-94-

15. R. Bartnikas, "Corona Pulse Counting and Pulse Height Analysis

Techniques," in Engineering Dielectrics I, Corona Measurement and

Interpretation , American Society for Testing and Materials, Philadelphia,

(1979).

16. W. E. Anderson and R. S. Davis, "Measurements on Insulating Materials at

Cryogenic Temperatures, " Final Report, U. S. National Bureau of Standards

Interagency Report, NBSIR 79-1950 (1980).

17. L. B. Loeb, Electrical Coronas - Their Basic Physical Mechanisms ,

University of California Press, (1965).

18. W. F. Schmidt, H. Jungblut, D. Hansen and H. Tagashira," Ionic and

Electronic Processes in Sulfur Hexafluoride," Gaseous Dielectrics II ,

Proceedings of the Second International Symposium on Gaseous Dielectrics,

Pergamon Press, pp. 1-11 (1980); W. F. Schmidt and H. Jungblut, "Ion

Mobility and Recombination in Compressed Sulfur Hexafluoride," J. Phys.

D: Appl. Phys. Vol. 12, pp. L67-L70 (1979).

19. P. L. Patterson, "Mobilities of Negative Ions in SFg," J. Chem.

Phys. 53_, p. 696 (1970)

20. N. H. Malik and A. H. Qureshi, "The Influence of Voltage Polarity and

Field Non-Uniformity on the Breakdown Behavior of Rod-Plane Gaps Filled

with SFg," Trans. Electr. Insul., Vol. El -14 , pp. 327-333.

21. E. Weber, Electromagnetic Theory , Dover, New York, p. 509 (1965).

22. C. C. Bondene, J. Cluet, G. Keib, and G. Wind," Identification and Study

of Some Properties of Compounds Resulting from the Decomposition of SFg

Under the Effect of Electrical Arcing in Ci rcui t-Breakers , " Revue

Generale De L'Electricite (RGE)-Numero Special, op. 45-78 (1974).
-95-

23. K. Hirooka, H. Kuwahara, M. Noshiro and Y. Jitsugiri, "Decomposition

Products of SFg Gas by High-Current Arc and Their Reaction Mechanism,"

Elec. Eng. in Japan, Vol . 95, No. 6 (1975).

24. S. Tominaga, H. Kuwahara, and K. Hirooka, "Influence of Arc-Decomposed

Gas on Cast Epoxy Insulators for Gas Insulated Switchgear," IEEE Trans.

Power Apparatus and Systems, Vol. PAS-98, No. 6, pp. 2107-2114 (1979).

25. J. Castonquay, A. Theoret and R. Gilbert, "Arc Degradation of SFg in

the Presence of Polymeric Insulating Materials," Conference Record of

1978, IEEE International Symposium on Electrical Insulation,

Philadelphia, pp. 162-166 (1978).

26. T. Suzuki, S. Nakayama and T. Yoshimitsu, "Degradation of Insulating

Materials, Due to SFg Gas Dissociation Products," IEEE Trans, on Elec.

Insul., Vol. EI-15, No. 1, pp. 53-58 (1980).

27. A. K. Vijh, "Nature of Metal -Electrodes /SFg Reactions in SFg

Decomposition Due to Di rect-Current Interruption Linder Simulated CircuitBreaker Conditions," IEEE Trans, on Elec. Insul., Vol. El -11, No. 4,

pp. 157-160 (1976).

28. W. Becher and J. Massonne, "Contribution to the Decomposition of Sulfur

Hexafluoride in Electric Arcs and Sparks," Elektrotech. Z., Vol. A91,

pp. 605-610 (1970).

29. To our knowledge the only published work on degradation of SFg in

corona is by B. Burta'kova', J. Krump and V. Voshlik, Electrotechnicky

Obzor, Prague 67, p. 230 (1978).
-96-

30. Certain commercial equipment, instruments or materials are identified in

this report in order to adequately specify the experimental procedure.

In no case does such identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply that the material

or equipment identified is necessarily the best available for the

purpose.

31. I. Sauers, T. J. Havens and L. G. Chri stophorou, " Carbon Inhibition in

Sparked Perfl uorocarbon - SFg Mixtures," J. Phys. D: Appl . Phys.,

Vol . 13, pp. 1283-90 (1980).

32. K. C. Smyth and P. K. Schenck, "Opto-Gal vanic Spectroscopy of Neon

Discharges: Mechanism Studies," Chem. Phys. Letters, Vol. 55, pp. 466-472

(1978).

33. D. Feldman, "Opto-gal vanic Spectroscopy of Some Molecules in Discharges:

NH 2 , NO2 , and N 2

," Opt. Comm., Vol. 24, pp. 67-72 (1979).

34. W. C. Bridges, "Characteristics of an Opto-gal vanic Effect in Cesium and

Other Gas Discharge Plasma," J. Opt. Soc. Am., Vol. 68, pp. 352-357

(1978).
NBS-114A (REV. 2*6C

)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions

)

4. TITLE AND SUBTITLE

PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

NBSIR 81-2242

1980 Annual Report: Technical Assistance for Future Insulation

Systems Research

5. AUTHOR(S)

R. J. Van Brunt, M. Misakian, D, A. Leep, K. J. Moy, and E. C. Beaty

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

Task No. AQ53-EES

8.

Type of Report & Period Covered

Annual Report 1980

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Department of Energy

Office of Electric Energy Systems

Washington, D.C. 20461

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant in formation. If document includes a significant

bi bl iography or literature survey, mention it here)

An experimental technique has been developed to quantify the electrical characteristics

of pulse- type partial discharges (corona) in compressed electronegative gases. By this

method the pulse repetition rates, shapes, and height distributions are determined

together with the average current. The technique has been applied to the investigation

of corona behavior in compressed SFg and SFg/N2 mixtures, and to measurements of corona

inceptions in SFg for both dc and 60*Hz ac voltages. A gas chromatograph-mass

spectrometer has been employed to monitor production of trace contaminants by coronainduced degradation of SFg. The most abundant gaseous by-products that were detected

from dc corona were H2 O, SOF2 , and SO0 F2

. The results of chemical analysis were

correlated with measurements of the electrical characteristics of the corona and it was discovered that the pulse height distributions and repetition rates were sensitive to

the presence of trace contaminants at the ppm level. Experiments were also performed on the effect of trace levels of water vapor, introduced by heating a wire, on positive dc

corona pulse burst behavior in SFg, It was noted that the burst activity was significantly inhibited by trace amounts of H2 O. These results indicate that measurement of

pulse characteristics of positive corona can be used as a sensitive monitor of gaseous

contamination levels. The effects of uv radiation and optical radiation from a tunable

dye laser on electrical behavior of corona in SFg and N? were investigated. Ultra-violet

light can have, under certain conditions, a large effect on initiation of electron avalanches in SFg near onset. Optical radiation in the wavelength range 580 to 620 nm was

found" tovu produce a measurable effect on corona in N? when it impinged on the electrode h. icl.-ihfior_£iLLCAl T y breakdown surfaces. A bibliography of el ectron swarm data needed. n. KEY

‘‘f’' r

ds fcy s 'mie° lors> corona;’decomposition; electrical insulation; gas chromatograph; inception voltages;

mass spectrometer; partial discharge; sulfur hexafluoride; swarm data; water vapor

13. AVAILABILITY

fxl Unlimited

I For Official Distribution. Do Not Release to NTIS

1

j Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

15. Price

USCOMM-OC 5043-P 30

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