Reference

NBS

PUBLICATIONS

NBS1R 85-3112

AlllDb 3 M 7 3

Development of Power System

Measurements -- Quarterly Report

April 1 , 1 984 to June 30, 1 984

R 5. Hebner, Editor

\

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Gaithersburg, MD 20839

July 1984

Issued March 1 985

QC100

.1156

35-3112

1984

Prepared for: Department of Energy

ision of Electric Energy Systems

>0 independence Avenue, SW

ihington, DC 20585

NBSIR 85-3112

DEVELOPMENT OF POWER SYSTEM

MEASUREMENTS - QUARTERLY REPORT

APRIL 1, 1984 TO JUNE 30, 1984

R. E. Hebner, Editor

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Gaithersburg, MD 20899

July 1984

Issued March 1 985

Prepared for: Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Foreword

This report summarizes the progress on three technical investigations

during the third quarter of FY 84. Although reasonable efforts have been made

to ensure the reliability of the data presented s it must be emphasized that

this is an interim report so that further experimentation and analysis may be

performed before the conclusions from any of these investigations are formally

published. It is therefore possible that some of the observations presented in

this report will be modified, expanded, or clarified by our subsequent research.
TABLE OF CONTENTS

Page

Foreword . iii

LIST OF FIGURES v

LIST OF TABLES v

Abstract 1

1. INTRODUCTION 1

2. ELECTRIC FIELD MEASUREMENTS

Subtasks Nos. 01 and 02 . . 1

2.1 Flow Induced by A Draft Down A Hole 1

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. 03. ... 8

3.1 Results for Oxyfluoride Yields from SF 6 Corona. . 9

3.2 Comparisons with Previous Observations. .... 16

3.3 Possible Mechanisms for Oxyfluoride Formation ... 16

3.4 Influence of 0

2 and H

20 on Oxyfluoride Production ... 20

3.5 Dependence of Oxyfluoride Production on Discharge Current and Time. . 21

3.6 Polarity Effect 22

3.7 Magnitude of Production Rates 22

4. NANOSECOND BREAKDOWN IN POWER SYSTEM DIELECTRICS

Subtask No. 04 24

5. REFERENCES. ...................... ... 30

iv
LIST OF FIGURES

Page

Figure 1. The boundary conditions . 3

F igure 2. Estimating u^ near 5

Figure 3. Air speed versus height (non-dimensional | u

J ) . The

measured values were obtained using three flow rates

through the opening, 0.37 x 10“ 2 nr/s ( ) s 0.69 x 10" 2 m

3 /s (o). and 1.7 xlO” 2 m

3 /s ( ). The

data have been normalized to unity at d = 0.5 cm ....... . 7

Figure 4. Measured charge rates of production for S0F 4 , S0F 2 , and S0 2

F

2 versus discharge current for negative

corona in 200 kPa SF 6

. 14

Figure 5. Measured energy rates of production for SOF^, S0F

2

, and S0 2 F

2 versus discharge power for negative

corona in 200 kPa SF

g

. .............. . . 15

Figure 6. Schematic representation of transmission line pulse

generator to study fast risetime breakdown

processes in dielectrics .................... 26

Figure 7. Examples of input-output pairs used in deconvolution

studies of capacitive voltage sensors. Data were acquired using a fast waveform recorder, (a) output

from mercury wetted relay pulser. (b) output from sensor under test. . . .............. 27

LIST OF TABLES

Page

Table 1. A comparison of theory to experiment for normalized

air speed into a small opening (5.1 cm x 5.1 cm),

u (0.5 cm) = 1 5

Table 2. Limiting values for S0F 2 production rates 10

Table 3. Limiting values for S0 2 F

2 production rates 11

Table 4. Limiting values for S0F 4 production rates 12

Table 5. Estimated production rates for minor species from

negative corona in SF

6 12

v

DEVELOPMENT OF POWER SYSTEM MEASUREMENTS ~ QUARTERLY REPORT

April 1, 1984 to June 30, 1984

R. E. Hebner, Editor

This report documents the progress of three technical

investigations sponsored by the Department of Energy and performed by or under a grant from the Electrosystems Division, the National Bureau

of Standards. The work described covers the period April 1, 1984 to

June 30, 1984. This report emphasizes the errors associated with

measurements of dc electric fields, the properties of corona in

compressed SF 6 gas, and the measurement of voltage pulses on nanosecond time scales.

Key words: deconvolution; electric fields; gaseous insulation;

partial discharges; pulsed voltage measurement; SF 6 ; solid insulation.

1. INTRODUCTION

Under an interagency agreement between the U.S. Department of Energy and

the National Bureau of Standards, the Electrosystems Division, NBS, has been

providing technical support for DOE's research on electric energy systems. This

support has concentrated on the measurement of electric and magnetic fields, the

measurement of interfacial phenomena, and of partial discharge phenomena, and

the measurement of voltage and current on nanosecond time scales. The technical

progress made during the quarter April 1, 1984 to June 30, 1984 is summarized in

this report.

2. ELECTRIC AND MAGNETIC FIELD MEASUREMENTS

Subtask Nos. 01 and 02

The objectives of this investigation are to develop methods to evaluate and

calibrate instruments which are used, or are being developed, to measure the

electric field, the space charge density, and current density in the vicinity of

high-voltage dc transmission lines and in apparatus designed to simu^te the

transmission line environment; to provide electrical measurement support for

DOE -funded efforts to determine the effects of ac fields on biological systems,

and to provide similar support for biological studies which will be funded by

the State of New York.

During the current reporting period, an improved model was developed for

fluid flow through an opening in a plane. The present calculation is threedimensional in nature and provides better agreement with air speed measurement

results than an earlier two-dimensional analysis. The calculations are relevant

to predicting perturbations of ion motion near the aperture of aspi rator-type

ion counter operating in the ground plane. A knowledge of how rapidly such a perturbation becomes negligible provides guidance as to the accuracy with whicn

the NBS-devel oped parallel plate facility can provide a constant ion density and

thus can serve as a standard ion source.

2.1 Flow Induced By a Draft Down a Hole

An aspirator type ion counter, operated in the ground Diane, extracts r r from the high field side of the ground plane. This fluid 'air) Mow can oer^jr1
the ion distribution. This perturbation, while it may be less important for

measurement near dc transmission lines, may be significant in biological

exposure facilities and in laboratory calibration systems.

A distribution of point sinks is used to model the three-dimensional,

steady air flow which is induced in a half-space by a known outflow across the

boundary. In a rotational ly symmetric case, velocities are found analytical ly.

In the general case, numerical integration may be applied to approximate the

fl ow.

Air is drawn into an opening in a broad flat surface at a speed much below

that of sound. To determine the flow above the surface, the flow is modeled as steady, irrotational , and incompressible in the upper half-space consisting of

points x = (x 1# x 2 , x 3 ) such that x >0. We wish to find the velocity, u, satisfying the boundary conditions for x in the boundary (x 3 = 0)

:

u

n (x) = u o for x in ft (la)

u„(x) = 0 for x not in ft n

' ' (lb)

Here, u n is the component of u normal to the boundary and ft is the opening,

an open, bounded set of points in the boundary. We also require the velocity at

infinity to be zero:

I' m |u(x)

X

Thus, in the plane x

3 = 0, we have the situation shown in figure 1.

The central idea is to represent the flow field as a linear superposition

of point sinks over the region ft [1]. It emerges from our analysis that the

boundary conditions are satisfied and that for one simple, but important region,

ft, the velocity can be found analytically.

The continuity equation for incompressible flow is

V • u = 0. (2)

Irrotational ity is given by v x u = 0. The solution for a point sink at x

1 with

volume flux m is [2]

x-x

I

x-x

I 3 ( 3 )

The superposition of point sinks of constant strength which are uniformly

distributed over the region ft, gives rise to the velocity field:

u. u(x) = / /

Q

O

4tt

x-x

1 dx 1 i dx

3

( 4 )

where m and u 0 are related by m = 2u 0

. area (ft) and m is the volume flux

into the sink ft from both the upper and lower half spaces. In the case that the

flux comes from the upper half only, one must replace the constant

2
Figure i. The boundary conditions.

3
(5)

it o

4tt by

u o

2 it

*

In the interior of the flow, for x in the proper half space x

3 > 0, the

integrand in (4) is absolutely integrable and one may differentiate through the

integral to establish that u satisfies the differential equation (2).

Some care must be taken in establishing the boundary conditions (1). When

x lies outside ft but in the plane x

3 = 0, the vector x-x', which appears in the

integrand^ has a zero component in the normal direction. The integral converges

and so u(x) is defined and has a normal component of zero, i.e.,

u n (x) = 0 . (6)

For x in ft, the integrand in (4) has a singularity at x

1 = x and is not

summabl_e. Consider, however, the velocity at a small distance, d, above the

point x = (X]_, x 2

, 0). At xd = (x

l

, x

2

, d), the velocity can be estimated and

lim u n (x d ) = u Q

. (7)

d-*o

To show this, consider a disk centered at x and lying entirely in ft. Let

the disk radius be R as shown in figure 2, and let r = Ix'-xl. Integrating

separately over the disk and its complement in ft, and converting to polar

coordinates for the disk, one sees that at xd , d « R, the normal component of

the velocity is well estimated by

U

n

2tt /

R/

d

( r

2 +d 2)3/2

rdrde

( 8 )

= u (1 } . 0 (R2+d2)l/2

(9)

As d+0, we get u n-*u 0 , which was to be shown.

Note that when ft is a disk, the velocity along the central axis, x

L given by eq (9). Thus for a circular hole

u(0,0,x 3 ) = (0,0,u

o (l- a (1+ a

2 )- l/2 )), a = £

3

.

x

2 = 0, is

( 10 )

The validity of this model has been verified with measurements made along

the central axis (see below). For a more complicated ft, the evaluation of u(x)

may require a quadrature. Geometric complexity is of no difficulty as long as ft can be decomposed into triangles and disks. In this case computer software is

available for the necessary calculations.

For application of this method to flows in a half space, the velocities

away from the boundary are accurately determined. Neglecting viscosity gives

good results because there are no upstream boundaries and consequently no sources of vorticity in the far field. The procedure runs into difficulty at

4
Figure 2. Estimating u n near a.

b
the edge of the region a where the velocity given by this method is infinite.

This non-physical solution is the result of neglecting the boundary layer. In the apparatus under consideration, the Reynolds number ranges from 5000 to

30000. For such- values, a more accurate determination of flow field in the

upstream region is possible, because, for slightly viscous flow, a jet issues on the downstream side of the hole. One must characterize the jet and use it to

determine the velocity, un, at the hole. Then eq (10) may be used to find the

upstream flow more accurately.

The central velocities given by eq (10) agree with experiment.

Measurements of velocities are made along the central axis lying over a rectangular opening. The data is used to fit the model eq (10) for a circular

opening. Table 1 shows the results of experiment and theory. The measurements

are made at different flow rates through the same opening and all figures are normalized so u(0.5 cm) = 1. Two techniques are used to find an effective R and

the corresponding u 0

. In case 1, two measurements (d, u(d)) yield two

simultaneous equations in u0 and R. These are solved iteratively to find the

model parameters. In case 2, one selects R so that ttR 2 is equal to the known area of the opening and u0

is chosen so that u(0.5 cm) = 1. Figure 3 displays

the measurements and the theoretical curve of case 2.

Table 1. A comparison of theory to experiment for normalized air speed into a small opening (5.1 cm x 5.1 cm), u(0.5 cm) = 1.

Measurement Theory

R

d

"W ' 2 ( •

)

3.19

Case 1 2.98

m

2.91 —Case2 2.88 cm

0.0** Not Measured 1.18 1.20 1.26 1.21

0.5 1.0 1.0 1.0 1.00* 1.00* 1.04 1.00*

1.5 .68 .72 .66 .68* .66* .68* .65

3.5 .29 .27 .25 .31 .29 .29* .27

5.5 .13 .14 .12 .16 .14 .15 .14

7.5 .08 .08 .10 .09 .09 .09 .08

* Agreement is an artifact of data fitting

**No measurements made, theory gives u 0

.

Uncertainties in the air speed measurements range from 5% for the highest

values, to 10% for the lowest values. The position of the probe is known to

within -1 mm. Assuming the model calculation to be correct, a linear error analysis shows that the uncertainty in u0

is <10%.

We have presented an analytic form of the flow induced by a draft through a hole in a flat boundary. The model is fully three-dimensional and provides a closed form for the velocity in a problem of interest; i.e., for an aspirationtype ion counter operated in the ground plane. Some work is needed to implement

this method on the computer. It is interesting to note that in the evaluation

of eq (4) by quadrature, the boundary condition, eq (la), can be generalized to

allow u n (x) to vary over the opening. The inclusion of the boundary layer in

the model would extend its usefulness to the edge of the half-space, but the

influence of this boundary layer at the ground plane is probably not important

for the description of the perturbations introduced by the ion counter.

6
Figure 3. Air speed versus height (non-dimensional |u ). The measured values

were obtained using three flow rates through the opening,

0.37 x 10- 2 m

3 /s (a), 0.69 x 10“ 2 m

3 /s (•), and 1.7 x 10”* ms a).

The data have been normalized to unity at d = 0.5 cm.

7
During the next quarter, a measurement system for characterization of

ambient ac magnetic fields will be developed. Electrical measurements

associated with the operation of an ion counter operating in the ground plane

will be performed under a test line and in the NBS parallel plate apparatus.

For further information contact Dr. M. Misakian at (301) 921-3121.

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. 03

The objective of this project is to develop diagnostic techniques to

monitor, identify, and predict degradation in future compressed gas electrical

insulating systems under normal operating conditions. The focus is on the

fundamental information and data needed to improve test design and performance

evaluation criteria. The scope of the project encompasses the following

investigations: 1) measurement and calculation of electric discharge inception

in compressed electronegative gases; 2) measurement, calculation, and

compilation of fundamental data on electron transport and electrical breakdown

in gases; 3) measurement of absolute electric discharge-induced decomposition

rates in gaseous dielectrics; and 4) examination of the influence of

contaminants like water vapor on the performance of compressed gas-insulated

systems. Emphasis is given in these investigations to the development and

evaluation of new measurement techniques.

The planned activities for FY84 include:

1) Preparation of conference and archival papers presenting results of our measurements on the production rates for the oxyfluorides S0F 2 , S0 2 F

2 and

S0F 4

, and S0 2 and H

2 0 generated by corona discharges in SF 6 or SF 6 /H 2 0

mixtures;

2) Extension of the measurements on by-product production rates in gas

discharges to the mixtures, SF 6 + 0

2

, SF 6 + N

2

, SF 6 + C0 2

, and

SF 6 + c-C 4 F

8 + CHF 3 ;

3) Compilation of a computerized bibliography of electrical breakdown data

in insulating gases and publication of this bibliography in an NBS report;

4) Investigation of the effects of trace quantities of water vapor on the

enhancement of electron avalanche growth and corona discharge development

in SF

6

.

The bibliography mentioned in activity 3 has been completed and is available as an NBS Technical Note [4]. The bibliography contains 1) a list of archival

papers and books that present data on electrical breakdown in gases and which

have been published since 1950; 2) a computer generated index indicating the

references that give particular types of data for each gas; 3) an author index;

and 4) a list of relevant technical conferences.

The citations given in the bibliography contain experimental or theoretical

data on breakdown which include: 1) sparking potentials; 2) breakdown voltages;

3) critical fields, or field-to-gas density ratios; 4) corona inception

voltages; 5) voltage-time characteristics; 6) relative and absolute dielectric

strengths; and 7) breakdown probabilities. The types of data which were

8
considered include those which apply to: 1) uniform and nonuniform electric

fields; 2) ac, dc, and impulse voltages; and 3) possible special effects

associated with particles, surfaces, interfaces, and corona. The bibliography

is intended to serve as a guide in locating data on breakdown which are most

relevant to particular applications. All gases likely to be encountered in

electrical insulation applications are included. Any omissions are due either

to a failure to locate relevant references or to a complete lack of data.

Copies of the bibliography are available from the National Bureau of Standards,

Electrosystems Division, Gaithersburg, MD 20899, and from the U. S. Government

Printing Office, Washington, DC 20402.

The emphasis of our effort during the past quarter was on activity 1. An

analysis of all previously acquired data on S0F 4 production in SF

6 corona discharges was carried out to determine the absolute production rates for this

species. New procedures for minimizing errors associated with GC/MS column and

ion source conditioning effects in quantitative analysis of gases were developed

to improve the accuracy and reliability of measurements on discharge-generated

gas species production rates.

The conference paper entitled "Production Rates for Discharge Generated

S0F 2 , S0 2 F

2 , and S0 2 in SF 6 and SF 6 /H 2 0 Mixtures" by R. J. Van Brunt,

T. C. Lazo, and W. E. Anderson has been published in the Proceedings of the

Fourth International Symposium on Gaseous Dielectrics, Knoxville, Tennessee,

April - May, 1984 [5]. This paper presents the results of our measurements of

the absolute production rates for S0F 2

, S0 2 F

2

, S0 2, and H

2 0 generated by

point-plane corona discharges under a wide range of conditions in SF 6 containing

trace levels of water vapor. An archival paper describing all of the results on production of stable, oxygenated, gaseous species from corona discharges in SF & as well as giving detailed information about the analytical techniques employed

has been prepared and is undergoing review. A discussion of the results

presented in this paper is highlighted in this report. This includes discussion

of a proposed mechanism for SF 6 decomposition in corona discharges that is

consistent with previous observations of electrical , thermal , and laser-induced

decomposition of SF 6 and SF 6 /0 2 mixtures.

3.1 Results for Oxyfluoride Yields from SF

6 Corona

The most abundant, long-lived, stable, gaseous species generated by corona discharges in SF

6 gas containing trace levels of 0

2 and H2 0 are the oxyfluorides

S0F 2

, S0F 2 F

2

, and S0F 4

. Results for the limiting constant production rates of

S0F 2

, S0 2

F

2

, and S0F 4 from corona are given in tables 2-4. Both the energy

rates-of-production , ru , and the charge rates -of -producti on , rq, are listed

as functions of discharge conditions (polarity, current, and power) and gas

pressure. Gas pressures range from 114 kPa (-1.1 atm) to 300 kPa (-3 atm).

Discharge currents from 1.5 yA to 64 yA with corresponding power dissipations

from 0.054 W to 4.3 W are included. The limiting constant production rates were derived from linear least squares fits to the measured concentrations C versus either net energy dissipated, u, or net charge transported in the discharge, Q,

for u > um -j n and Q > Qmin* where uml

- n and Om-j n are the lowest values for

which the assumptions of linearity hold [5].

Production rates were estimated in some cases for observable minor species,

namely S0 2 , 0CS, and C0 2 as indicated in table 5. The rates for these are see'i

to be an order of magnitude or more smaller than those for the predominant

oxyfl uorides.

9
Table 2. Limiting values for S0F 2 production rates

Discharge Discharge Gas Production

Polarity Power Current Pressure Rates

(W) (uA) (kPa) (n moles/J) (u moles/C

)

Pos. 0o804 2U70 116 1.28 49.5

0.054 1.5 200 5.20 181

0.335 8.0 200 4.69 195

0.777 16.0 200 3.80 180

0.198 4.0 300 4.06 187

0.430 8.0 300 3.09 178

0.945 16.0 300 2.64 172

Neg. 0.898 25.0 114 1.21 45.0

0.230 8.0 200 1.52 35.2

0.586 16.0 200 0.93 34.7

0.821 20.0 200 0.68 34.1

2.225 40.0 200 0.54 32.4

4.290 64.0 200 0.44 33.7

0.764 16.0 300 0.73 31.2

1.140 21.0 300 0.41 23.9

1.820 30.0 300 0.33 22.3

10
Table 3. Limiting values for S0 2F 2 production rates

Polarity

Pos.

Neg.

Discharge

Power

(W)

Discharge

Current

(uA)

Gas

Pressure

(kPa)

Production

Rates

(n moles/J) (y moles/C)

0.804 20.0 116 1.83 69.7

0.054 1.5 200 4.51 151

0.154 4.0 200 4.07 158

0.335 8.0 200 3.27 147

0.777 16.0 200 2.10 109

0.198 4.0 300 2.32 119

0.430 8.0 300 2.03 113

0.945 16.0 300 1.29 111

0.898 25.0 114 0.52 18.0

0.230 8.0 200 0.62 16.0

0.586 16.0 200 0.47 16.9

0.821 20.0 200 0.36 14.8

2.215 40.0 200 0.25 13.9

4.290 64.0 200 0.16 12.6

0.764 16.0 300 0.38 17.9

1.140 21.0 300 0.29 16.9

11
Table 4. Limiting values for S0F 4 production rates

Polarity

Pos.

Neg.

Discharge

Power

(W)

Discharge

Current

(uA)

Gas

Pressure

(kPa

)

Production

Rates

(n moles/J) (u rnoli

0.804 20.0 116 7.02 290

0.054 1.5 200 7.04 260

0.335 8.0 200 8.60 373

0.777 16.0 200 8.45 418

0.198 4.0 300 5.79 292

0.430 8.0 300 5.92 343

0.945 16.0 300 7.08 431

0.898 25.0 114 0.85 30.1

0.230 8.0 200 0.92 23.8

0.586 16.0 200 0.97 34.6

0.821 20.0 200 1.05 41.5

2.215 40.0 200 0.90 49.7

4.290 64.0 200 0.94 63.6

0.764 16.0 300 0.44 20.6

1.140 21.0 300 0.39 21.0

12
The rate values given in tables 2 and 3 for S0F 2 and S0 2 F

2 have been

assigned a maximum uncertainty of ±35 percent. The rates for S0F 2 typically

have less uncertainty than those for S0 2 Fj. Results from positive corona have

higher uncertainty than those from negative corona because of the greater

fluctuations in the discharge current in the former case. For S0F 4 the

uncertainties are higher, but estimated to be always less than ±57 percent. The

rates given in table 5 for the minor products are only estimates since a reasonable error assessment was impossible.

Table 5. Estimated production rates for minor species from negative

corona in SF 6

.

Species

Discharge

Current

(uA)

Pressure

(kPa)

Production

(n moles/J)

Rates

(u moles/C)

SO 2 25 114 0.008 0.3

O

I/O 40 200 0.002 0.1

ocs 40 200 1.3 x 10" 5 7.2 x 10- 3

co 2 40 200 0.035 2.0

The results for r

u and rq are shown in figures 4 and 5 respectively for

negative corona at the pressure of 200 kPa. Important observations from the

data in tables 2-4 and figures 4 and 5 include:

The production rates for S0F 2

, S0 2 F

2

, and S0F4 are of comparable

magnitude to within roughly a factor of 3. For most cases,

particularly at the higher pressures and power levels, the order of

production rates is rq(S0F 4 )>rq(S0F

2 )>rq(S02 F2

).

Oxyfluoride production does not change dramatically with pressure. One

exception is the large relative drop in the S0F 2 and S0 2 F

2 rates

compared to S0F 4 in going from 200 to 116 kPa for positive polarity.

Oxyfluoride production increases by a factor of five or more in going

from negative to positive polarities for discharges of comparable oower

1 evel s.

The charge rates-of-production for S0F 2 and S0 2 F

2 vary less with

discharge level than the corresponding energy rates; whereas, the

opposite appears to be true for S0F 4

.

The trends noted for S0F 2 and S0 2 F

2 from the fourth observation imply that

the time rates-of-producti on for these are more nearly directly proportiona 1 to the current than the power, i.e., dc/dt « i. Because of the relative 1 / large

uncertainties, it can only be stated that for S0F 4 , the rates behave like :c it

13
DISCHARGE CURRENT CpA)

Figure 4. Measured charge rates of production for S0F 4 , S0F 2 , and S0 2 F

2 versus discharge current for negative corona in 200 kPa SF 6

.

14
PRODUCTION

RATE

(n

Moles/

J)

Figure 5. Measured energy rates of production

discharge power for negative corona

for S0F 4 , S0F 2 , and S0 2 F

2 versus

in 200 kPa SF 6

.

15
« i a

, with Ka<2. The results in figure 5 would in fact suggest that dc/dt «

p for this species, where p is the power dissipation in the discharge.

3.2 Comparisons with Previous Observations

The oxyfluoride production rates found here are comparable in magnitude to

many of those previously reported in other types of SF 6 electric discharges.

However, any comparisons with other results is of questionable significance,

since all previous measurements were performed under dissimilar conditions. The

configuration used by Chu, et al . [6] was perhaps most nearly like that of the

present experiments. These measurements were performed with corona generated by

60-Hz ac voltage applied to aluminum electrodes in SF 6 at a pressure of about

155 kPa. The corona evidently occurred at the peaks of each half-cycle and was mainly characterized by pulses with an average magnitude of 10 3 pC and a repetition rate of 2 kHz. From the information provided, the total S0F 2 + S0 2 F 2 production rate from this measurement is estimated to lie between 620 and

1400 y moles/C. The conditions most like this in the present work would

correspond to the positive discharge at 1.5 yA which yielded a net rate for

S0F 2 + S0 2F 2 + S0F 4 production of 592 y moles/C.

Although the present result is close to that of Chu, et al., it is

disturbing that they do not observe S0F 4

. It is conceivable that this can be

explained by a conversion of S0F 4 to S0 2 F 2 via the reaction S0F4 + H 2 0 •+ S0 2 F 2 + 2HF in the gas sampling process. Other cases [7,8] where there was failure to see S0F 4 from corona or "weak-current" discharges in SF 6 , even though S0 2 F 2 was reported to be a predominant product, might also be accounted for by hydrolysis

of S0F 4

. There are documented cases [9-12] where S0F4 was observed in

low-current discharges and even appeared as the dominant product.

Boudene, et al . [13], Sauers, et al . [14], and Castonguay [15] all report

total oxyfluoride energy rates-of-production in SF

6 arcs and sparks with

stainless steel electrodes that are comparable in magnitude to some of the rates

found here. However, these types of discharges appear to generate predominantly

S0F 2

. Also, in these cases metal vapors and thermal dissociation may play an important role because of the high levels of power dissipation. The present

results are consistent with previous observations and arguments [7,8,16,17] that

S0 2 F

2 and S0F 4 should become more prevalent as by-products when the discharge

power level is reduced.

Recent relative measurements by Ophel , et al . [18], of the total

hydrolyzable fluoride production from stainless steel, point-plane corona in

flowing SF 6 at 440 kPa show net time rates-of-production that are directly

proportional to discharge current rather than power. This behavior would be

consistent only with the results found here for S0F 2 and S0 2 F

2

. Were S0F 4 to

become a major component among the hydrolyzable species detected, then the

present results indicate that proportionality to current need no longer be

expected.

3.3 Possible Mechanisms for Oxyfluoride Formation

It is useful as a guide in the i nterpretation of the results to consider

the plausible sequences of reactions which can lead to production of S0F 2

, S0 2 F

2 , and S0F 4 in a corona discharge. It should be cautioned, however, that

chemical processes in electric discharges are generally expected to be complex.

16
involving many intermediate steps. An exhaustive listing of the possible

processes is beyond the scope of this work. It is intended here to draw

attention to some of the important reactions which appear to be consistent with

the present observations and which should be included in any serious attempt to

describe the decomposition of SF 6 theoretically. A corona is a relatively weak

plasma in which the electron temperature greatly exceeds the gas temperature and

nonequilibrium conditions prevail [19]. It is therefore expected that the

initial stage of SF 6 decomposition involves dissociation of molecules by

electron collisions. At electric field-to-gas density ratios, E/N, close to the

critical value where ionization growth is possible, the mean electron energies

in an SF 6 discharge are theoretically estimated [20=22] to be between 5 and

10 eV. Thus some of the processes that are likely to contribute include:

+ sf 6 SF 5 + F + e. Eth = 4.0 eV (id

+ SF 6

-*> SF 4 + 2F + e. Et h = 6.1 eV (12)

+ SF

6

->* SF 3 + 3F + e. Eth “9.5 eV (13)

+ SF

6 - SF 2 + 4F + e. Eth - 12 eV (14)

+ SF 6 SF 5 + + F + 2e, Et h

= 15.0 eV (15)

+ ’sf 6 SiV + F, Eth ~ eV. (16)

Indicated are the threshold energies E^h based on determined ionization,

attachment, and bond dissociation energies [20=26]. Other processes that can occur include dissociative attachment, leading to F”, dissociative ionization

leading to smaller or multiply charged fragments; and multistep dissociation,

e.g., dissociation of the fragments such as e + SF4 + SF

3 + F + e. The cross sections or relative probabilities for these processes are expected to be lower

than those listed above [21,27]. Of those processes given, the greatest

contribution will probably be from dissociation leading to the larger fragments

SF X , x = 3, 4, 5 which is supported by calculations by Masek, et al . [28], of

dissociative fragment production rates from electron swarms in SF 6 and SF 6 /0 2 mixtures using numerical solutions of the Boltzmann equation. Cross sections

for processes ( 1 1

) - ( 14 ) are not known, but are included in the total electronic

excitation cross section which is estimated [20-22] to be comparable to or larger than the cross sections for attachment and ionization. The latter

processes are not energetically favored for E/N values near the ionization limit

since attachment probabilities are peaked at very low energies where the

electron energy distribution function rapidly decreases, and the thresholds for

ionization occur in the high energy tail of the distribution. The thresholds

for the favored dissociation processes occur near the peak of the energy

distribution.

It is known [9] that if oxygen containing species are not present in the ;as or on surfaces to react with the SF 6 dissociation products, then they will

rapidly recombine through relatively fast processes such as

sf

5 + sf5 sf

6 + sf 4 ( 17 )

SF

5 + + SF 4 +

+ F ( 18 )

17
F + SF 4 t SF 5 (19)

F + SF 5 * SF 6 (20)

F

2 + SF 4 - SF 5 + F (21)

F

2 + SF 5 > SF 6 + F (22)

The list, of course, represents only a portion of the possible reactions. The

reaction sequence ( 19 ) -

( 22) has been proposed by Gonzalez and Schumacher [29] to

explain the observed rapid thermal conversion of SF 4 + F

2 into SF 6

. It has been

argued [30] that reaction (17) is favored because of its low activation energy.

The enthalpy change aH ° 298 for this reaction is -49.4 kJ/mole. From available

thermochemical data [31], one obtains aH ° 298 = -434.1 kJ/mole for either

sequence (19)-(22) or (17) and (19) -

( 21 ) . There is evidence [32] from mass

spectral data that SF 5 + can be formed in metastable states that decay according

to (18). The ions SF 4 + and SF

5 + are expected to be highly reactive.

Other reactions among the SF

6 products can yield long-lived sulfur

fluorides like S

2 F

2 and S2 F10 . Both of these have been reported as minor

products of SF 6 decomposition in some arc or spark discharge studies [16]. It

is known [33] that S

2 F

2 (and its isomers) are more likely formed by the reaction

S + SF 2 - S

2 F2

, (23)

rather than

SF + SF > S 2 F 2 . (24)

Formation of S 2 F

2 is thus not expected in a cool discharge which yields

predominantly the larger SF X fragments. It is reported [29] that although

S 2 F

10 can be formed by the reaction

SF 5 + SF 5 + S 2 F 10 , (25)

its rate is much lower than those of the competitive reactions (17) or

( 19 ) -

( 22 ) . The product S 2 F 10 is important because of its high

toxicity [13,17]. Due to its thermal instability, it could not be observed by

the present chromatographic method. The species SF 4 is reported to be the most

stable and most abundant primary product from both thermal dissociation of SF 6 at moderate temperatures [34-36] and from relatively cool electric

discharges [37,38]. The SF 5 radical has not been observed even at relatively

low temperatures, and this suggests that the reactions which convert it to SF 4 and SF 6 are quite fast.

The presence of oxygen, or oxygen containing species in the discharge region

will interfere with the recombination process and give rise to the formation of

oxyfl uorides , free fluorine, and HF. Possible, energetically favorable,

bimolecular reactions that could directly lead to oxyfluoride formation in the

discharge are [39]:

SF

5 + OH H. S0F 4 + HF, aH° 298 = -464 kJ/mol

e

(26)

SF

5 + 0 + S0F 4 + F, aH° 298 = -327 kj/mol

e

(27)

18
SF 4 + 0 > SOF 4 , AH° 2g g = -549 kj/mole (28)

SF 4 + 0 - S0F 2 + 2F, aH ° 298 = -124 kj/mole (29)

SF

3 "+ 0 + S0F 2 + 2F, AH ° 298 = -213 kJ/mole (30)

SF 2 + 0 - S0F 2 , aH ° 298 = -496 kj/mole (31)

SF 4 + OH > S0F 2 + HF + F, aH ° 298 = -130 kJ/mole (32)

SF

3 + OH > S0F 2 + HF, aH ° 298

= -351 kJ/mole (33)

SF 3 + 0

2 * S0 2 F

2 + F, AH ° 298

= -260 kj/mole (34)

SF 2 + 0

2 - S0

2 F

2

, aH ° 298

= -461 kj/mole (35)

Reactions (29), (30), and (34) have previously been invoked [40,41] to account

for rapid S0F z and S0 2 F

2 formation from pyrolysis of SF b in the presence of 0

2

. Reactions (27) -(30) have been mentioned by d'Agostino and Flamm [9] as mechanisms for oxyfluoride production from SF b /O z mixtures in low-pressure, rf

discharges. Leipunskii, et al . [30] have mentioned reaction (32) and suggested

that it proceeds through the intermediate S0F 3

.

From low-pressure SF b /O z mixtures with relatively high 0 2 content there is

evidence [29,42,43] that ion and neutral species such as S0F., +

, SO z F

:j +

, SO z F

b +

, S 3 0F y +

, SF b O z , SF b 0, SF b O z SF b , and SF b 0^SF b are also formed. Some of these may

act as intermediates in the formation of the observed oxyfl uorides . Because of

the relatively high pressures and low oxygen content in the present experiments,

it can be argued [29] that formation of large sulfur oxyfl uorides such as SF b O b SF b is unlikely.

If the concentrations of the by-products become sufficiently high, then

secondary reactions can play a role in the discharge. Included here are reactions such as

S0F z + 0 + S0 2 F 2 , (36)

whereby the stable oxyfluorides undergo conversion from attack by free radicals

or ions generated in the discharge. Because the observed oxyfluorides always

remained at trace levels, reactions of this type are expected to be unimportant

here provided that they are confined to the small volume of the discharge.

Secondary reactions involving SF

b * in the ion drift region could be more important. A possibly significant reaction is the F“ exchange

SF b - + S0F 4 + S0F b - + SF b (37)

which affects S0F 4 production.

Within the bulk of the gas or on the walls, slower reactions may occur which would modify the gas composition. Processes in this catagory are:

SF 4 + H

z 0 hk S0F z + 2HF , AH° Zbb = -84 kj/mole

19
SOF 4 + H 2 0 -> S0 2 F

2 + 2HF , AH° 2g 8 = -2 kj/mole

S0F 2 + H

2 0 - S0 2 + 2HF, aH° 298 = +54 kJ/mole

(39)

(40)

All of the reactions (38)-(40) are possible at room temperature. However,

consistent with the present results, only reaction (38) is rapid enough to

significantly influence the observed oxyfluoride production. Its rate has been

estimated by Sauers, et al . [37] to lie between 1.0 and 2.6 x 10-19 cm

3 /s at a temperature of 350 K. Assuming that this is a gas phase reaction, and using

typical water vapor concentrations found in the present experiments, it is

estimated that SF 4 has a half-life in the cell of between 0.15 and 0.40 h.

Since this is small compared to typical gas sampling intervals, it can be

assumed that $F 4 is completely converted to S0F 2

. Reaction (38) has frequently

been invoked to account for S0F 2 production in previous investigations of SF 6 decomposition; however, it has never been verified that it is actually a gas-phase reaction. If it does occur in the gas phase, then most likely, as suggested by Herron [39], it proceeds through the intermediate SF

3 0H in two

steps.

Reaction (39) has previously been proposed [13-17] to explain S0 2 F

2 production in electric discharges. The present results indicate that its rate

in the gas phase is too slow to account for most of the S0 2 F 2 formed. It could

nevertheless, as noted above, explain the absence of gaseous S0F 4 in some earlier experiments. The extremely low rate of S0 2 production [5] suggests

furthermore that reaction (40) is also not important for the conditions

considered here.

It is evident from consideration of reactions (34) and (35), that

production of S0 2 F

2 directly from primary SF 6 dissociation products requires

0 2. The important role of 0 2 in enhancing S0 2 F

2 production has been verified in

earlier work [9,13,30,44], and is consistent with the present observations. For

decomposition at lower pressures [9], addition of 0 2 appears to increase S0 2F

2 production at the expense of S0F 4

. Boudene, et al . [13] noted that both S0 2

F

2 and S0F 4 production rates in hi gh-pressure arcs are more sensitive to gas-phase

0

2 content than S0F 2 production.

Failure to observe significant quantities of S0 2 F

2 and S0F 4 in

hi gh-temperature arcs could be accounted for in part by the higher degree of gas

dissociation in these cases compared to low-power discharges. Certainly it

could be expected that availability of undissociated oxygen and the larger SF X fragments would be reduced at higher temperatures so as to inhibit reactions

( 26 ) -

( 35 ) . This hypothesis is supported by equilibrium calculations at high

temperatures [34,40]. Since S0F

2 could be produced primarily by reaction (38),

its formation need not depend on 0

2 content. The fact that S0 2 F

2 is generally

observed here and by others [9,10,30,40] to have the lowest rate of formation in

relatively 1 ow-temperature or low-power SF 6 decomposition processes can be

understood by its dependence on the availability of the lower valence SF 3 and

SF 2 fragments. In cool discharges these are likely to be produced with lower

probabilities than the larger fragments SF 4 and SF 5 which would preferentially

convert to S0F 4 and S0F 2

.

The sources from which oxygen is derived for production of S0F 2

, S0 2 F

2

, and

S0F 4 have not been positively identified in the present experiments, although

3.4 Influence of 0

2 and H 2 0 on Oxyfluoride Production

20
measurements are now underway that could significantly narrow the range of

possibilities. The following three sources deserve consideration: 1) gaseous

0 2 contamination including that initially present plus that introduced during

discharge operation; 2) oxygen contained on surfaces including that which is

chemically bound in insulating materials such as Si

0

2 ; 3) H2 0 present either in

the gas or on surfaces. Interpretation of the present results suggests that the

primary oxygen sources are not necessarily the same for all species. Source (2)

will be considered unimportant in the discussion that follows because of the

highly confined nature of the discharge which was located far from insulating

surfaces.

At least at low levels (less than 1 percent), the measurements show that

oxyfluoride production is not extremely sensitive to variations in the gas phase

0 2 and H

20 concentrations. In fact, when 0 2 content was increased above 10%,

total oxyfluoride yield decreased. This implies that the production rates are controlled mainly by the dissociation rates of SF 6 , i.e., by the availability of

SF X fragments. The decrease in oxyfluoride yield with increasing 0

2 content

may be due to the influence of 0 2 on the SF 6 dissociation rate, e.g., by

modification of the electron energy distribution as suggested by Masek [28].

The observed [5] consumption of gaseous H

2 0 can be accounted for by its role in

S0F 2 formation through reaction (37) and by providing 0 and OH radial s for

reactions (26 ) -

( 33 ) in the discharge.

It is proposed on the basis of the foregoing discussion that the primary

oxygen sources for S0F 2 and S0 2 F 2 could be derived respectively from H 2 0 and 0 2 , whereas S0F 4 could derive oxygen from both of these. This assignment is

strengthened by the interpretation of the dependences of the rates on discharge

current and time as discussed below.

3.5 Dependence of Oxyfluoride Production on Discharge Current and Time

The condition dc/dt * i, observed here for S0F 2 and S0 2 F

2

, is expected if:

1) the formation process involves only one electron impact generated

dissociation fragment; and 2) changes in i do not significantly modify the

electron energy distribution in the discharge. The first condition is satisfied

by the predominant mechanisms suggested above to account for SOF^ and S0

2 F2 production. The second condition is open to question, but is evidently a reasonable assumption [19] for glow or corona-type discharges in which the

electric field is controlled by ion space charge. Increases in applied voltage

required to increase the current will presumably expand the volume of space

charge without significantly altering the mean E/N. Independent of i and

correspond!' ng space charge development, dissociation is expected to occur predominantly in regions of comparable E/N.

Certainly if dc/dt « i for S0F 2 and S0 2 F

2 , then it is expected, on the

basis of the proposed mechanisms, that dc/dt * i 2 for S0F 4 , since its formation

involves two fragments from electron impact dissociation. The production of

S0F 4 increases with i as seen in figure 4, but probably somewhat more slow’y

than i 2

. The deviation from i 2 dependence, if significant, could be accounted

for by secondary reactions such as reactions (37) and (39) which could remove

S0F 4

.

As production of S0F 4 becomes predominant, it might be expected to affect

the product rates of the other oxyfluorides since it competes in the consuPr*.

21
of SF 4 and SF 5

. Rates for both S0F 2 and S0 2

F

2 show a slow fall off with

increasing i which may indicate competition with S0F4 production. The extent to

which S0F 4 production is competitive depends, however, on the extent to which it

preferrential ly consumes free radicals used in S0F 2 or S0 2 F

2 formation as opposed to those that would otherwise recombine.

If the initial portions of the production curves are ignored [5] and it is

assumed that the curves do indeed approach linearity, then reactions involving

initial gas dissociation products would appear to dominate in oxyfluoride

production. Deviations from linearity can occur if secondary reactions

contribute by which one oxyfluoride is transformed into another, e.g., reactions

(36) and (39). Because of the low oxyfluoride content, reactions such as (36)

which occur in the discharge are considered unlikely, and measurements made at

times when the discharge was off indicate that reactions like (39) and (40) that

could occur in the bulk of the gas are too slow to observe.

3.6 Polarity Effect

The large effect of reversing polarity on the production rates of the

oxyfluorides can be understood from consideration of corresponding pronounced

changes in the discharge characteristics [45]. In evaluating this effect the

following observations are noted: 1) the effect is greater for S0 2F 2 and S0F4 than for S0F 2 ; 2) the power dissipation at a given current was always higher for

positive polarity; 3) the discharge was more pulsating under positive polarity;

and 4) the point electrode surface suffered considerably more damage under

positive polarity. In positive discharges the point electrode surface is

evidently subjected to bombardment by more highly energetic ions or electrons

from electron avalanches originating in the gas than is the case in negative

discharges where a more uniform ion space charge is effective in shielding the

electrode surface and moderating the energies of impacting ions. Higher mean electron energies and higher electrode surface temperatures would be consistent

with a higher dissociation rate in the positive case. In addition to

dissociation by electron impact, pyrolysis of SFg at the positive point surface

could conceivably contribute to decomposition. The enhancement in oxyfluoride

production in going from positive to negative polarity is consistent with the

postulated increase in the gas dissociation rate.

Greater heating of the electrode in the positive case could also be

accompanied by a higher rate of oxygen release into the discharge volume. This

would influence oxyfluoride production and might account for a more pronounced

effect for the species S0 2 F 2 and S0F 4 that, according to the above model, are most affected by the presence of oxygen.

3.7 Magnitude of Production Rates

Information is not available at the present time on the rates for all of

the various chemical processes invoked here to account for oxyfluoride

production. Thus, accurate predictions of the production rates are not yet

feasible. Nevertheless, sufficient information does exist to at least allow

upper limits to be placed on the magnitude of the total SF 6 decomposition rates

for discharges in which dissoication is controlled mainly by electron collision

processes. Based on the evidence provided here that the rate determining factor

is the initial SF 6 dissociation rate, i.e., the rate for processes such as

22
e + SF (41) 6

•+ SF X + (6-x) F + e,

then an upper limit on the total oxyfluoride yield can be estimated using the

following two assumptions: 1) all SFX fragments convert to oxyfluorides ; and

2) electronic excitation leads predominantly to dissociation so that the total

electronic excitation rate k ex coincides with the total dissociation rate.

The rate k ex could also be adjusted to include fragmentation of $F 6 leading to

various ion products.

In addition, a one dimensional approximation is made that electrons

released into the active ionization volume at a constant rate dn e /dt, e.g., by

field emission from the point cathode, traverse the gap distance d along the

point-to-plane axis. These assumptions enable one to compute the total charge

rate-of -production rq? t defined by

ri* t * ili

(42)

where the rq j

are the rates for the individual oxyfluoride components.

Allowing for’the variations of electron production in the gap by ionization and

attachment, one obtains the expression

r q>t * i-l(N/N

0 )(dn

e/dt) /

d

ex ^ /*(aU')-nU'))di'] dt, (43)

where a and n are respectively the usual ionization and attachment

coefficients [16-18], v is the electron drift velocity, and N

0 is the Avogadro

constant required to express the rate in units of mol es-per-coulomb. In general, the quantities k ex , v, a > and r\ will depend on E/N and hence on position i' in the gap.

Precise variations: of E/N within the gap are not easily predicted when the

discharge is present due to the expected strong influence of ion space charge on the field. However, the active region near the point in which signficant

ionization and excitation occur must satisfy the condition

a( l ' ) > n( l

1 ) • ( 44

)

When this condition no longer holds, electrons are quickly thermal ized and

removed from the gas by attachment to SF 6 molecules. Lacking knowledge of the

exact dependence of E/N on z', the active discharge volume will be modeled here

as a region of extent i in which eq (44) holds, and E/N assumes a constant value

close to the critical value (E/N

)

c where a = n» This choice takes into

consideration the expected moderating effect of the ion space charge which tends

to reduce the field near the point.

Expressing N in terms of pressure p and temperature T using N = p/kT (k is

the Boltzmann constant), the approximation for rq> t becomes

evkT N

°

(45)

23
where e is the electron charge, and k ex and "v assume the values for the

electronic excitation rate coefficient and drift velocity respectively at

E/N - (E/N) c

= 370 x 10“ 21 Vm 2 corresponding to the known [46] critical

value for SF 6

.

- In deriving eq (45) it is assumed that the measured discharge

current i corresponds to electron motion only. It should be noted that 1 is, in

general, pressure dependent so that rq> t may not be proportional to pressure

as implied by eq (45).

There appears to be some disagreement in the literature [20,21,47] about

the correct total electronic excitation cross sections needed to determine the

rate coefficient k ex for SF 6. It will be assumed here that

2 x 10 -15 m

3 /s _> ^ex 2. 1 x 10“ 15 m

3 /s,

where the lower value corresponds to that calculated by Masek, et al. [28] at

E/N = 100 x 10 -21 Vm 2 and the upper value allows for the expected increase in

k ex for E/N = (E/N

)

c

. The values of k ex considered here are comparable in

magnitude to those reported in the literature for other molecules [48]. The

observed [46] value for v in SF 6 at (E/N

)g

is about 2 x 10 5 m/s. The extent

of the active region is expected [49] to Tie between about 1 and 2 percent of

the total gap spacing, i.e., 0.24 rren > i > 0.12 mm. Using eq (45), the

predicted upper limit on the rate for a pressure of 200 kPa and temperature of

300 K lies in the range, 1800 ymoles/C > rq

s t

> 450 ymoles/C. This can be

compared with the values obtained from tables 2-4 for a pressure of 200 kPa.

For positive discharges at 8.0 yA and 16.0 yA respectively, it is found, for

example, that rq^ = 634 ymoles/C and rq jt = 717 ymoles/C, and for a negative discharge at 40 yA, rq^ = 96 ymoles/C.

The results for positive discharges generally lie within the range of

predicted upper limit values, whereas results for negative discharges lie well

below the range. It should be noted that the model used is perhaps more appropriate for negative discharges where electrons originate from the point

electrode surface and the space charge is expected to be uniformly distributed

about the point. For positive discharges, the space charge is probably much

less uniform, and as previously suggested, there may be contributions to

dissociation from processes other than electron impact.

Undoubtedly, the mean value of E/N in the active volume is greater than

( E/N

)

c for the positive case. However, independent of polarity, the total

rates rq^ do not, and should not, lie significantly above the range of upper

limits estimated here.

During the next quarter emphasis will be given to activity 2). Further

measurements will be made on oxyfluoride production from SF 6 /0 2 mixtures in

negative corona. These investigations will also be extended to SF 6 /N 2 mixtures

for which nitrogen oxide as well as oxyfluoride production rates will be

measured as a function of N

2 content.

For further information contact Dr. R. J. Van Brunt at (301) 921-3121.

4. NANOSECOND BREAKDOWN IN POWER SYSTEM DIELECTRICS

The objectives of this effort are to develop measurement capabilities and

to provide data in support of the DOE program concerned with the effects of fast

rising electrical signals on electric power systems.

24
The response of the dielectric components of electrical equipment to

waveforms such as those generated by lightning or switching surges is reasonably

well understood. However, there exists only a limited set of information

regarding breakdown phenomena when dielectrics are subjected to fast rising

pulses such as those generated by nuclear electromagnetic pulses or by

disconnect operations in gas insulated equipment. Conventional equipment used

in breakdown studies is, at best, marginally adequate for use in nanosecond

breakdown studies. Test facilities must provide suitable pulse shapes, test

cell geometries, and electrical diagnostics with time resolution in the

nanosecond range.

The test facility shown schematically in figure 6 illustrates some of the

features which must be an integral part of a system which is to be used to study

sub-microsecond breakdown in dielectric materials. The pulse forming network

and triggered switch provide the desired voltage waveform, the test cell

contains the dielecric specimen, and, in the transmission line shown in

figure 6, the termination prevents a reflected wave from being imposed across

the test cell. While not all systems will have this appearance, the salient

features must be the same. Of critical importance are the voltage and current

sensors indicated, since accurate measurements are required to determine both

the voltage and temporal characteristics of breakdown in the dielectrics. If,

for example, the effects of trace impurities on formative time lags in SF 6 are

of interest, then both the voltage at which breakdown occurs and the time to

breakdown after application of the voltage pulse must be determined. Since

these time lags may be as short as 1-2 nanoseconds, severe requirements are placed on the voltage sensors. In addition to experimental studies in the

laboratory, the use of analytical techniques to characterize these sensors

appears necessary.

The characterization of sensors and many other electrical measurement

devices such as waveform recorders can, under rather general conditions, be

reduced to deconvolving an integral equation. We present a method of

deconvolution and discuss its applicability to the problem of sensor characterization.

In our laboratory an input signal, x(t), to and an output signal, y(t),

from a device are recorded (see figure 7). When such a system is linear,

causal, and time-shift invariant, linear systems theory allows one to infer that

y(t) = JqX( s) g (t-s) ds (46)

where g is the impulse response of the system [50].

Equation (46) can be the basis for determining g or its integral, the step

response S(t). Alternatively, one may determine the input signal, x, if j and

either g or S are known. Mathematically and computational ly these two problems

may be solved by identical means.

The principal difficulty in the solution of eq (46) is that one has an integral of the first kind which is singular or nearly so. In our case, the

data x and y are finite collections of discrete, digitized measurements }no j-

subject to error. When discretized, eq (41) gives rise to a linear equation

25
To data acquisition

Figure 6. Schematic representation of transmission line pulse generator to

study fast risetime breakdown processes in dielectrics.

26
nS

Figure 7. Examples of input-output pairs used in deconvolution studies

capacitive voltage sensors. Data were acquired using a ”'ast recorder, (a) output from mercury wetted relay pulser. ' s from sensor under test.

of

waveform

output

27
x g = y (47)

which may be non-singular but is ill-conditioned. Thus eq (47) has a unique

solution, g, but small variations in y produce large variations in g. Such

variations are a character!' sti c of noisy systems. Because eq (47) is

ill-conditioned we consider that the system to be solved has the form

X g = yT + n , (48)

where n is noise superimposed on the true output, y . If we knew yT we might find g T (the correct impulse response). Of course, one can't know

y T

. In effect this means there is a large class of solutions to eq (48).

From this class we seek functions, g, which satisfy auxiliary, a priori

conditions. These are typically that g or its derivatives are not too large.

Intuitively, these conditions require that g not be wildly oscillatory. One may

use as a measure of size some combination of the root mean square (RMS)

magnitudes of g and its derivatives, for example.

(49)

R is called a regularizing operator and

seek solutions to the regularized versio

minimizes

I 2 is the RMS norm [51]. We then

eq (42) by finding y r which

y

- Xg Rg

(50)

y r is a positive number called a regul ari zation parameter. Thus we abandon

exact data consistency for a combination of faithfulness to the data and

reasonableness of the solution.

An equivalent linear problem is given by the Euler equation for eq (50)

(XT

X + -fR 2

) g = XT

y. (51)

We solve eq (51) using a variety of R's (identity and first difference operators

as well as the R given above).

The results of these calculations for model data derived by perturbing

analytic functions are very good. For experimentally derived data, initial

results suggest that the numerical method can construct g r close to the limits

of detectability imposed by the measurement of x and y.

The utility of this technique is limited by the absence of error bounds or other measures of the sensitivity of g r to noise in x and y. The digitizer we use has a frequency response which is flat to about 400 megahertz, and which has

a system rise time of 0.8 ns.

We have computed the discrete Fourier transform (DFT) for the x and y.

Above 2 Ghz the noi se-to-si gnal ratio is high, judging by the variation observed

in repeat shots. Meaningful data above 1 Ghz would not be expected, and it is

unknown what the transient response of the waveform actually is. Other staff

28
members of the Electrosystems Division are actively pursuing methods which can

be used to characterize waveform recorders. The solution of the regularized

problem, eq (50), effectively filters high frequency components of the solution

vector g r

. The amount of filtering is dependent on the parameter n the

selection of which is not directly related to the noise.

One area of investigation in the next year is the inclusion of error estimates in the computation of g. One method of finding confidence intervals

is based on the work of O'Leary and Rust [52]. Their methods are not entirely

appropriate to the general problem of finding g. A second method is to relate n to the noise, n e

A second area of study is the fast computation of g r

. The storage

requirements of the current algorithm are proportional to N

2 and the computation

time to N

3 when x and y each has N measurements. We will apply the conjugate

gradient method of Hestenes [53]. The storage requirements of our implementation are proportional to N, and by exploiting the structure of eq (46) we reduce the computation time to no more than N

2 log N Both are substantial

savi ngs.

In addition to the future analytical work, experimental work will continue

on the development of a general system and the adaptation of existing equipment

to look at fast breakdown in dielectric liquids.

For further information, contact Dr. R. H. Mcknight at (301) 921-3121.

29
5. REFERENCES

[1] Sir Horace Lamb, Hydrodynamics (Sixth Edition), Dover (1945).

[2] G. K. Batchelor, An Introduction to Fluid Dynamics, Cambridge University

Press (1967) (Chapter II).

[3] NBS Report NBSIR 83-2761, Development of Power Systems Measurements--

Quarterly Report, January 1 - March 31, 1983.

[4] R. J. Van Brunt and W. E. Anderson, "Bibliography of Data on Electrical

Breakdown in Gases," NBS Tech Note 1185 (1984).

[5] R. J. Van Brunt, T. C. Lazo, and W. E. Anderson, "Production Rates for

Discharge Generated S0F 2 , S0 2F

2 , and S0 2 in SF 6 and SF 6 /H 2 0 Mixtures,"

Gaseous Dielectrics IV (L. G. Chri stophorou, ed.) Pergamon Press, New York

(1984) pp. 276-285.

[6] F. Y. Chu, H. A. Stuckless, and J. M. Braun, "Generation and Effects of

Low Level Contamination in SF 6 -Insulated Equipment," Gaseous

Dielectrics IV (L. G. Chri stophorou , ed.) Pergamon Press, New York (1984)

pp. 462-472.

[7] H. Grasselt, W. Ecknig, and H. J. Polster, "Application of Gas

Chromatography to the Development of SF^ -Insulated Switch Gear and

Apparatus," Elektrie, Vol. 32, pp. 398-371 (1978).

[8] F. B. Waddington and J. Heighes, "Sulphur Hexafluoride Insulation for

Current Transformers , " AEI Eng., Vol. 7, pp. 196-199 (1967).

[9] R. d'Agostino and D. L. Flamm, "Plasma Etching of Si and Si

0

2 in SF 6 -0

2 Mixtures," J. Appl. Phys., Vol. 52, pp. 162-167 (1981).

[10] G. Bruno, P. Capezzuto, and F. Cramarossa, "Inorganic Volatile Fluorides

Obtained from Electrical Decomposition of Sulfur Hexafluoride in a Quartz

Tube," J. Fluor. Chem., Vol. 14, pp. 115-129 (1979).

[11] H. J. Emeleus and B. Tittle, "Synthesis of Pentaf luorosul phur Chloride and

Sulphur Oxide Tetraf 1 uori de in the Microwave Discharge," J. Chem. Soc.,

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[12] H. Gutbier, "Mass Spectrometric Investigation of the Decomposition of

Sulfur Hexafluoride in a Glow Discharge," Phys. Verh., Vol. 17, pp. 163

(1966).

[13] C. Boudene, J. Cluet, G. Keib, and G. Wind, "Identification and Study of

Some Properties of Compounds Resulting from the Decomposition of SF 6 under

the Effect of Electrical Arcing in Circuit Breakers," Rev. Gen. Electr.,

No. Special, pp. 45-78 (1974).

[14] I. Sauers, L. G. Chri stophorou , and S. M. Spyrou, "Negative Ion Formation

in SF

6 Spark By-Products," Gaseous Dielectrics IV (L. G. Chri stophorou,

ed.) Pergamon Press, New York (1984) pp. 292-305.

30
[15] J. Castonguay, "Kinetics of the Arc “Decomposition of SF 6 -Gas Mixtures,"

Conf. Record, IEEE Int'l Symp. on Elect. Insul., Boston (1980).

[16] W. Becher. and J. Massonne, "Contribution to the Decomposition of Sulfur

Hexafluoride in Electric Arcs and Sparks, "Elektrotech. Z. Ausg., Vol . A91,

pp. 605-610 (1970).

[17] A. Baker, R. Dethlefsen, J. Dodds, N. Oswalt, and P. Vouros, "Study of Arc

By-Products in Gas-Insulated Equipment," Elec. Power Res. Inst. Rept.,

EPRI-EL-1648 (1980).

[18] T. R. Ophel, D. C. Weisser, A. Cooper, U K. Fifield, and G. D. Putt,

"Aspects of Breakdown Product Contamination of Sulphur Hexafluoride in

Electrostatic Accelerators, " Nuc. Instr. and Methods, Vol. 217,

pp. 383-396, (1983).

[19] M. Goldman and A. Goldman, in Gaseous Electronics, Vol. 1, (N. M. Hirsh

and H. J. Oskam, ed.) Academic Press, New York (1980) pp. 219-290.

A. Goldman and J. Amouroux, in Electrical Breakdown and Discharges in

Gases, (E. E. Kunhardt and L. H. Luessen, ed.) Plenum Press, New York

(1981) pp. 293-346.

[20] H. Itoh, M. Shimozuma, and H. Tagashira, "Boltzmann Equation Analysis of

the Electron Swarm Development in SF§ and Nitrogen Mixtures," J. Phys. D: Appl . Phys., Vol. 13, pp. 1201-1209 (1980); T. Yoshizawa, Y. Sakai,

H. Tagashira, and S. Sakamoto, "Boltzmann Equation Analysis of the

Electron Swarm Development in SF 6 ," J. Phys. D: Appl. Phys., Vol. 12,

pp. 1839-1852 (1979).

[21] L. E. Kline, D. K. Davies, C. L. Chen, and P. J. Chantry, "Dielectric

Properties for SF 6 and SF 6 Mixutres Predicted from Basic Data," J. Appl.

Phys., Vol. 50, pp. 6789-6796 (1979).

[22] M. S. Dincer and G. R. Govinda Raju, "Monte Carlo Simulation of the Motion

of Electrons in SF 6 in Uniform Electric Fields, "J. Appl. Phys., Vol. 54,

pp. 6311-6316 (1983).

[23] A. P. Modica, "Shock Tube Study of Sulfur Hexafluoride and Sulfur Chloride

Pentafluoride Equilibrium Decomposition," J. Phys. Chem., Vol. 77,

pp. 2713-2719 (1973).

[24] P. J. Hay, "Generalized Valence Bond Studies of the Electronic Structure

of SF 2 , SF 4 , and SF 6 ," J. Am. Chem. Soc., Vol. 99, pp. 1003-1012 (1977 ).

[25] D. L. Hildenbrand, "Mass Spectrometri c Studies of Some Gaseous Sulfur

Fluorides," J. Phys. Chem, Vol. 77, pp. 897-902 (1973).

[26] L. M. Babcock and G. E. Streit, "Ion-Molecule Reactions of SF

G

: Determination of I. P.(SF 5 ), A. P. (SF

5 + /SF 5 ), and D(SF s -F)," J. Chem.

Phys., Vol. 74, pp. 5700-5706 (1981).

[27] T. Stanski and B. Adamczyk, "Measurements of Dissociative Ionization Cross

Section of SF 6 by Using Double Collector Cycloidal Mass Spectrometer,

Int. J. Mass. Spec. Ion Phys., Vol. 46, pp. 31-34 (1983).

31
[28] K. Masek, L. Laska, V. Perina, and J. Krasa, "Some Peculiarities of the

SFg and the SF 6 + 0 2 Discharge Plasma," Acta. Phys. Slov., Vol. 33,

pp. 145-150 (1983).

[29] A. C. Gonzalez and H. J. Schumacher, "The Kinetics and the Mechanism of

the Thermal Reaction between Sul furtetraf luori de and Fluorine," Z.

Naturforsch. , B: Anorg. Chem., Org. Chem., Vol. 36B, pp. 1381-1385

(1981).

[30] I. 0. Leipunskii, A. K. Lyubimova, A. A. Nadeikin, A. I. Nikitin, and

V. L. Tal'roze, "Investigation of the Process of Formation of

Oxygen-Containing Products as a Result of Multiphoton Dissociation of SF 6 by Radiation from a High-Power Pulsed C0 2 Laser," Sov. J. Quantum

Electronics, Vol. 12, pp. 413-417 (1982).

[31] D. D. Wagman, W. H. Evans, V. B. Parker, R. H. Schumm, I. Halow, S. M.

Baily, K. L. Churney, and R. L. Nuttal, "The NBS Tables of Chemical

Thermodynamic Properties," J. Phys. Chem. Ref. Data, Vol. 11, suppl. 2 (1982).

[32] V. H. Dibeler and F. L. Mohler, "Dissociation of SF 6

, CF 4 , and SiF 4 by

Electron Impact," J. Res. Nat. Bur. Stand., Vol. 40, pp. 25-29 (1948).

[33] F. Seel, "The Isomers of disulfurdifluoride," Chemia, Vol. 22, pp. 79-83

(1968); F. Seel, E. Heinrich, W. Gombler, and R. Budenz,

"Sulfurdifluoride," Chimia, Vol. 23, pp. 73-74 (1969).

[34] R. L. Wilkins, "Thermodynamics of SF 6 and Its Decomposition and Oxidation

Products," J. Chem. Phys., Vol. 51, pp. 853-854 (1969)’.

[35] K. P. Brand and J. Kopainsky, "Particle Densities in a Decaying SF 6 Plasma," Appl. Phys., Vol. 16, pp. 425-432 (1978).

[36] J. F. Bott and T. A. Jacobs, "Shock -Tube Studies of Sulfur Hexafluoride,"

J. Chem. Phys., Vol. 50, pp. 3850-3855 (1969).

[37] I. Sauers, H. W. Ellis, L. C. Frees, and L. G. Chri stophorou, "Studies of

Spark Decomposition Products of SF 6 and SF 6 /Perf 1 uorocarbon Mixtures,"

Gaseous Dielectrics III (L. G. Chri stophorou , ed.) Pergamon Press,

Ne/z York (1982) pp. 387-401; "Detection of SF 4 in Sparked SF 6 ," IEEE

Trans. Elec. Insul., Vol. El -17, pp. 284-286 (1982).

[38] D. C. Frost, S. T. Lee, C. A. McDowell, and N. P. C. Westwood, "A Versatile, Fast Pumping Ultraviolet Photoelectron Spectrometer for the

Study of Transient and Unstable Species," J. Elec. Spec, and Rel

.

Phenomena, Vol. 12, pp. 95-109 (1977).

[39] The enthalpies of reaction listed apply only at 298 K and in some cases

are only approximate due to possible large uncertainties in the heats of

formation of species like SF 2 , SF 3 , S0F 2 , and S0F 4 (J. H. Herron, National

Bureau of Standards, private communication). The values given were derived from several sources including Ref's 27, 28, and JANAF

Thermochemical Tables, J. Phys. Chem. Ref. Data, Vol. 7 (1978) pp. 917;

Vol. 3, pp. 387 (1974).

32
[40] B. Siegel and P. Breisacher, "Oxidation of Sulfur Hexafluoride," J. inorg. nucl . Chem; Vol. 31, pp. 675-683 (1969).

[41] K. L. Wray and E. V. Feldman, "The Pyrolysis and Subsequent Oxidation of

SF 6

," Proc. 14th Int'l. Symp. on Combust., pp. 229-238 (1972).

[42] L. C. Frees, I. Sauers, H. W. Ellis, and L. G. Christophorou , "Positive

Ions in Spark Breakdown of SF 6 ," J e Phys. D: Appl. Phys., Vol. 14,

pp. 1629-1642 (1981).

[43] A. C. Gonzalez and H. J. Schumacher, "The Kinetics of Thermal Reaction

between Sulfurtetrafluoride and Flourine in the Presence of Oxygen,"

Zeit. Phys. Chem. Vol. 127 (1981) pp. 167-177; J. Czarnowski and

H. J. Schumacher, "The Kinetics and the Mechanism of the Thermal

Decomposition of Bis-Pentafluorosulfurtrioxide (SF

5 OOOSF5

)

, " Int. J. Chem. Kinetics, Vol. 11 (1979) pp. 613-619.

[44] A. M. Velichko, I. 0. Leipunskii, A. K. Lyubimova, A. A. Nadeikin,

A. I. Nikitin, and V. L. Tal'roze, "Role of Heterogeneous Processes in the

Formation of Oxygenated Products During the Multi photon Dissociation of

SF 6 by C0 2 Laser Emission," High Energy Chem., Vol. 17 (1983)

pp. 124-127.

[45] R. J. Van Brunt and D. Leep, "Characterization of Point-Plane Corona

Pulses in SF 6 ," J. Appl. Phys., Vol. 52 (1981) pp. 6588-6600.

[46] J. W. Gallagher, E. C. Beaty, J. Dutton, and L. C. Pitchford, "An

Annotated Compilation and Appraisal of Electron Swarm Data in

Electronegative Gases," J. Phys. Chem. Ref. Data, Vol. 12 (1983)

pp. 109-152.

[47] J. P. Novak and M. F. Frechette, "Transport Coefficients of SF 6 and SF 6 /N 2 Mixtures from Revised Data," J. Appl. Phys., Vol. 55 (1984) pp. 107-119.

[48] J. Dutton, "A Survey of Electron Swarm Data," J. Phys. Chem. Ref. Data,

Vol. 4 (1975) pp. 577-856.

[49] R. J. Van Brunt and M. Misakian, "Mechanisms for Inception of DC and 60-Hz

Ac Corona in SF

s ," IEEE Trans, on Elec. Insul., Vol. El -17 (1982) pp.

106-120.

[50] A. V. Oppenheim and A. S. Will sky , Signals and Systems, Prentice-Hall,

1983.

[51] Tikhonov & Arsenin, Solutions of Ill-Posed Problems, Winston, (1977)

[52] O'Leary & Rust, Confidence Internal Estimates for 111 -Posed Problems

with Inequality Constraints, to appear in SIAM J. for Scientific and

Statistical Computing.

[53] Hestenes & Stiefel, "Methods of Conjugate Gradients for Solving Linear

Systems", J. Res. Nat. Bur. Standards, Section B, 49, 1952, pp. 409-436.

33
U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 85-3112

2. Performing Organ. Report No 3. Publication Date February 1985

4. TITLE AND SUBTITLE

Development of Power System Measurements —

Quarterly Report April 1, 1984 to June 30, 1984

| ii

5. AUTHOR(S)

R. E. Hebner, Jr.

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) ! NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, O.C. 20234

7. Contract/Grant No.

S. Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, DC 20585

10.

SUPPLEMENTARY NOTES

1

i Document describes a computer program; SF-185. FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

This report documents the progress of three technical

investigations sponsored by the Department of Energy and performed by or under a grant from the Electrosystems Division, the National Bureau

of Standards. The work described covers the period April 1, 1984 to

June 30, 1984. This report emphasizes the errors associated with

measurements of dc electric fields, the properties of corona in

compressed SF 5 gas, and the measurement of voltage pulses on nanosecond time scales.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) deconvolution; electric fields; gaseous insulation; partial discharges;

pulsed voltage measurement; SF^; solid insulation

13.

AVAILABILITY

tkl Unlimited For Official Distribution. Do Not Release to NTIS

1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, G.C.

' 20402.

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PRINTED PAGES

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