Research for Electric Energy

Systems -- An Annual Report

A 13, ID 2 b^bEA^

NBSIR 87-3643

R. E. Hebner, Editor

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Gaithersburg, MD 20899

December 1986

Issued September 1987

Prepared for Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

r-QC ->n, DC 20585

100

• U56

#87-3643

1987

C.2

NBSIR 87-3643

Research Information Center

Naiionai Bureau of Standards

Gaithersburg, Maryland 20899

RESEARCH FOR ELECTRIC ENERGY

SYSTEMS - AN ANNUAL REPORT

R. E. Hebner, Editor

/X).

mn

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Gaithersburg, MD 20899

December 1986

Issued September 1987

Prepared for Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Clarence J. Brown, Acting Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Foreword iv

Abstract . v

1. Introduction 1

2 . Electric Field Measurements . 1

2.1 Introduction 1

2.2 Ion Mobility 2 2.3 Ion Mobility Measurements 2 2.4 Corona Detection by AM Radio 8

10

10

11

27

31

37

4. Interfacial Phenomena in Liquids 42

4.1 Introduction. 42

4.2 Streamer Initiation: Electro-optic Field Measurements 43

4.3 Experimental Observations... 42

4.4 Order of Magnitude Analysis 45

4.5 Conclusion 46

5. Nanosecond Breakdown in Power System Dielectrics 47

5.1 The Use of E-Dot Sensors in Measuring Fast Rise Pulses 47

5.1.1 Introduction 47

5.1.2 Sensor Characterization 47

5.1.3 Discussion 51

5.2 Streamer Propagation in Transformer Oil under the 51

Influence of Submicrosecond Rise-Time Pulses

5.2.1 Introduction 54

5.2.2 Apparatus 54

5.2.3 Experimental Results 57

5. 2. 3.1 Slow Pulse Results 57

5. 2. 3.Fast Pulse Results 57

5.2.4 Discussion 60

6. References 67

3. Gaseous Dielectrics Research

3.1 Introduction

3.2 Evaluation of the Consistency among Electron.

Collision, Transport, and Dielectric Strength

Data for Binary Gas Mixtures

3.3 Gas-Phase Hydrolysis Rates for SOF^ and SOF^.

3.4 Identification of Corona Discharge- Jgduced^ .

SFg Oxidation Mechanisms Using SF,/ 0«/H„ 0

and SF,/ O^/H« 0 Gas Mixtures ’ 1 3.5 Transfer of F in SF^ 4- SOF^ Collisions

and Its Influence on SOF^ Yield from Corona

Discharges in SF^- Containing Gases

iii
Foreword

This report summarizes the progress of five technical investigations

during FY 86. Although reasonable efforts have been made to ensure the

reliability of the data presented, it must be emphasized that this is an interim progress 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.
RESEARCH FOR ELECTRIC ENERGY SYSTEMS AN ANNUAL REPORT

Robert E. Hebner, Editor

Abstract

This report documents the technical progress in the five investigations which

make up the project "Support of Research Projects for Electrical Energy

Systems", Department of Energy Task Order Number 137, funded by the U.S.

Department of Energy and performed by the Electrosystems Division of the U.S.

National Bureau of Standards. To support the measurement of ions in the

vicinity of dc transmission lines and in biological exposure facilities

designed to simulate the line environment, techniques to measure ion mobility

were evaluated. In addition, techniques were developed which biological

researchers could use to determine the sensitivity of an AM radio to detect

partial discharges in a biological exposure facility. Within the project to

develop measurement techniques and obtain basic data for gaseous dielectrics,

a theoretical method was developed to evaluate the consistency among electron

collision, transport, and dielectric strength data for binary gas mixtures.

In addition, the gas phase hydrolysis rates for SOF

2 and SOF^ at room

-23 3

temperature were measured to be (1.2 ± 0.3) x 10 cm /s and (1.0 ±0.3) x

-21 3

10 cm /s respectively; the corona discharge oxidation mechanisms were

16 16 18 18 identified using prepared samples containing 0^ and 0 or 0^ and 0;

and the role of SF^ negative ions on the SOF^ yield was also determined.

Progress in the measurement of the electrical behavior at a liquid- solid

interface included the optical measurement of the dynamic electric field

distribution as a streamer initiates. Progress in developing measurements for

nanosecond dielectrics was primarily in two areas . One is in the

characterization of the sources of error in the measurement of voltage pulses

using E-dot probes. The other is the identification of the reduction of

breakdown voltage in oil as the characteristic time of the voltage pulse is

reduced.

V
.
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 electrical energy systems.

This document summarizes the technical accomplishments of this program during

fiscal year 1986. Section 2 of this report describes the work in ac electric

and magnetic fields and dc field and ion measurements. Section 3 summarizes

the development of the measurement techniques and the accumulation of

fundamental data needed for gaseous dielectrics research. Section 4 describes

the development of techniques to measure interfacial phenomena in liquids.

The final technical project, which deals with the measurement of nanosecond

breakdown in power system dielectrics, is summarized in section 5.

2. ELECTRIC FIELD MEASUREMENTS

Tasks 01 and 02

Martin Misakian and P. Michael Fulcomer

Electrosystems Division

National Bureau of Standards

2 . 1 Introduction

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

calibrate instruments which are used, or are being developed, to characterize

the electrical parameters in the vicinity of high voltage transmission lines

and in laboratory apparatus designed to simulate 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 are funded by the State of

New York.

The electrical parameters of interest include the electric field

strength, magnetic flux density, ion current and charge densities, ion

mobility, and ion species. Earlier NBS studies have examined measurement

techniques and sources of error associated with the determination of the

electric field strength [1-14], magnetic flux density [1,3,6,10,15], ion

current density [5,8,12,16], and ion charge density [17-20]. During 1986, an examination of techniques for measuring ion mobility spectra was begun with

the view of identifying methods that could be readily adapted for use in

bioeffects exposure systems. A brief investigation was also conducted of the

practice of using portable am radios as a means for detecting corona in ac electric field exposure systems. NBS has also been active in the development

of standards which provide guidance for the measurement of power frequency

electric and magnetic fields. In June 1986, the IEEE Standards Committee

approved the final draft of a standard for measurement of ac power line fields

[21] and following a 6 -month balloting period, the members of International

Electrotechnical Commission Technical Committee 42 approved in July 1986 a standard for measuring power frequency electric fields. The final drafts of

both documents were prepared at NBS and include the results of NBS studies.

1
2.2 Ion Mobility

The mobility of an ion, K, as it moves through a gas under the influence

of a weak electric field is defined as the ratio of ion drift velocity to

electric field strength. Corrections for pressure and temperature are normally made to obtain a "reduced mobility." The measurement of average ion

mobility, K, is useful for determining charge density, p, from the

*4 -4 —4 relationship J - pKE where J is the ion current density and E is the electric

°4 «4 field strength. Measurements of J and E have been routinely performed under

dc transmission lines and in bioeffects exposure systems. Thus, the

additional measurement of mobility permits for the determination of charge

density. The charge density can be determined with an aspirator- type ion

counter but the calibration of such devices is difficult [20] and the

measurement of J, E, and P provides a means to calculate the accuracy of the

ion counter.

Perhaps a more important reason for measuring ion mobilities is that

the mobility provides a qualitative measure of ion size [22]. Ideally, the

ion species are of interest when considering possible bioeffects due to

exposure to ions, but sampling of ions at atmospheric pressure with a mass

spectrometer is difficult and requires use of complex apparatus. Ion mobility

spectra can be measured at atmospheric pressure with less complicated

instrumentation and monitoring mobility spectra during a bioeffects study can

serve as a form of "quality control" to insure that the types of ions that

test animals are exposed to do not change dramatically. Such data also may be

useful in establishing the equivalence of bioeffects investigations conducted

in different laboratories.

2.3 Ion Mobility Measurements

Ion mobility can be determined directly by measuring the time required

for an ion to travel a given distance in a known dc electric field. A device

that has been used extensively for this measurement is the drift tube. Two

prototype parallel plate drift tubes have been examined to determine their

suitability for measuring ion mobilities at atmospheric pressure. Schematic

views of the drift tubes are shown in figures 1 and 3 . Each device may be

regarded as an ac time of flight drift tube because the electrical shutters

which allow the passage of ions at certain times are controlled with ac voltages. Figure 1 shows a drift tube similar in principle to that employed

by Bradbury and Nielsen to measure electron mobilities in gases [23]. The top

shutter and bottom shutters consist of grid wires in parallel plates

electrodes separated by a distance D and having a dc potential difference

V. The alternate wires in each shutter are connected electrically and

sinusoidal voltages of opposite polarity are applied to adjacent wires to

produce a localized electric field between the wires. The spacing between

adjacent wires in the NBS drift tube was nominally about 0.5 mm and the wire

diameter was 0.076 mm. The voltages applied to each shutter are in phase.

When the ac voltages are at near zero, the shutters allow the passage of ions.

Ions which pass through the top shutter in figure 1 and travel to the bottom

shutter in half a cycle of the ac voltage will pass through and be measured

with an electrometer A. Measurement of ion current as a function of frequency

results in a current maximum of a characteristic frequency, F, and at integral

2
BRADBURY-NIELSON TYPE ELECTRICAL SHUTTER

FOR ION MOBILITY MEASUREMENTS

Drift Velocity = 2DF/N

Mobility = 2D2 F/NV

Figure 1. Schematic view of parallel plate drift tube similar in principle

to that used by Bradbury and Nielsen to measure electron

mobilities in gases. The shutters open when the ac voltages

applied to the grid wires are at or near zero volts.

3
POSITIVE

ION

CURRENT

(arb.

units)

0 20 40 60 80 100 120

FREQUENCY (Hz)

Figure 2. Sample data of positive ion mobility as a function of frequency

using the parallel plate drift tube shown in figure 1.

4
TYNDALL TYPE ELECTRICAL SHUTTER

FOR ION MOBILITY MEASUREMENTS

Drift Velocity - 2DF/N

Mobility - 2D2 F/NV

Figure 3. Schematic view of parallel plate drift tube similar in principle

to that used by Tyndall and Powell to measure ion mobilities in

gases. The top and bottom shutters open alternately near the

peaks of the voltage.

5
multiples of that frequency. The drift velocity, v , of the ion in the

uniform field region is given by v

d - 2DF/N where D is the spacing of the

2 parallel plates and N is an integer. The mobility is given by K = 2D F/NV

where V is the voltage applied to the parallel plates

Figure 2 shows positive ion current measured as a function of frequency.

The positive ions were produced by corona in air in a parallel plate apparatus

that has been described previously [7]The top shutter of the drift tube was positioned in the ground plane of this apparatus. The spacing between the

shutters was near 0.05 m and the electric field strength (V/D) in the drift

tube was 35 kV/m. The nns voltage applied to the grid wires in each shutter

was about 12 volts. The presence of more than one current maximum with its

associated mobility is not unusual because more than one ion species typically

results from corona activity in air [24]

A potential problem with the measurement results is the possible overlap

-4 2 of spectra. Ions with mobility 1.27 x 10 m /Vs near 45 Hz can also pass

through the shutter system and be measured at 90 Hz, possibly distorting the

higher frequency portion of the spectrum. The Bradbury -Nielsen type drift

tube may excessively broaden the spectra [25] so the seriousness of the

overlap is not clear. A second problem encountered was difficulty in

fabrication of the shutters and subsequent shorting of adjacent wires when the

ac voltages were applied. After some use, the shorting problem became

sufficiently bad to make the grids unusable.

Figure 3 shows a schematic view of a drift tube similar in principle to

that employed by Tyndall and Powell to measure ion mobilities in pure gases at

below atmospheric pressure [26]. During the NBS measurements, the shutters

were normally closed with a bias voltage of 9 volts and opened periodically

with ac voltages applied to one grid of each shutter. Each grid was made with

no. 14 screen that had been made more transparent by removing every other wire

in one direction. The spacing between the grids in each shutter was nominally

0.8 mm. The expressions for ion drift velocity and mobility are the same as for the Bradbury -Nielsen type drift tube. However, the shutters open at and

near the peaks of the ac voltages as shown schematically in figure 3. The

resolution of the shutter system used by Tyndall and Powell is greater than

that used by Bradbury and Nielsen, but end effects reportedly prevent an

absolute determination of mobility value [25]

Figure 4 shows sample preliminary data obtained with the prototype drift

tube shown in figure 3. The electric field strength in the drift tube was near 31 kV/m and the rms voltage applied to the shutters was 12.5 volts. Only

i 2 one current peak with a mobility of 1.11 x 10 m /Vs is evident in figure 4

and it may correlate with the lower mobility current maximum observed in

figure 2.

Future plans include a further evaluation of ac time of flight drift

tubes, construction and evaluation of a pulsed- type time of flight drift tube,

comparison of the ac and pulsed time of flight measurement techniques, and

design of a high pressure ion source for the NBS mass spectrometer.

6
POSITIVE

ION

CURRENT

(arb.

units)

Figure 4. Sample data of positive ion mobility as a function of frequency

using the parallel plate drift tube shown in figure 3.

7
2.4 Corona Detection by AM Radio

A limited study on the use of portable AM radios as detectors of corona

onset in ac electric field exposure chambers was completed during the past

year. This study was prompted by the use of such devices to detect corona in

biological exposure systems. Three radios were used in the study and were

found to be highly sensitive for detecting either ac or dc corona in a parallel plate apparatus, provided the radio was placed in close proximity to

either the high voltage supply wire or ground return wire of the apparatus . A

radio simply placed in the same room with the apparatus did not give a reliable indication of corona. Both ac and dc high voltage were utilized in the testing. The dc tests

simplified determination of the magnitude of corona avalanche at inception and

enabled investigation of polarity effects on sensitivity. Corona was

generated by means of a sharp point or points of specified radius attached

electrically to the bottom plate of the parallel plate apparatus. Two

different points were used - one of radius 0.007 mm and the other 0.07 mm. In some tests, both points were used at a spacing of 0.95 mm. Other test

variables were (1) the distance between the parallel plates, (2) the polarity

of the dc corona and (3) the polarity of the corona generating point.

Variation in plate spacing produced gap spacings (distance between the corona

generating point and the opposite flat plate) of between 2.5 cm and 10.5 cm. Figure 5 is an example of the information collected for one set of

variables. Here a point of radius 0.07 mm at ground potential produces

negative corona when positive high voltage is applied to the top plate. The

plate spacing 15 cm and the corona generating point is 4.5 cm high, producing

a gap of 10.5 cm. Plotted against voltage are the average current due to

corona and the approximate amount of charge per corona pulse. The inception

of corona induced radio noise is indicated on the diagram [27] . The first

measureable current (not shown on the diagram) was approximately 0.0005 /xA at

12.15 kV.

The voltage was measured with an electrostatic voltmeter and the average

current with an electrometer. The approximate amount of charge per avalanche

was determined by graphically measuring the area beneath the current waveforms

of the avalanches. The current waveforms were observed with an oscilloscope

that was connected across a 1000 0 resistor which was in series with the

parallel plate ground lead.

Corona avalanches with 10 to 20 picocoulombs of charge were readily

detected by means of the radio. The onset of corona as a function of voltage

was also readily determined as shown in figure 5. The radius of the

initiating point, the presence of two points and the polarity of the points,

had no significant effect on the results. Positive dc corona was easier to

detect, than negative corona.

For the measurement conditions described above, the inception voltage for

negative corona was less than that for positive corona. As a result, 60-Hz

corona could contain corona of both polarities or only negative corona. The

difference in radio noise produced by different polarity corona is distinct

because of the difference in magnitude of the positive and negative avalanches

and because the frequency (number of pulses per unit time) of the positive

8
AVERAGE

CURRENT

(11A)

1.0 1000

100

10

Figure 5. Typical data obtained during tests of AM radio noise as an

indicator of corona onset in a parallel plate structure. Average

corona current and approximate charge per corona avalanche pulse

are plotted vs. voltage for negative corona.

9

CHARGE/PULSE

(pC)
avalanches was normally less than for negative avalanches. The radio noise

from positive corona pulses tends to overwhelm that from the negative corona

if both are present.

Not mentioned above, but important from the stand point of the detection

dc corona, is the observation that as the voltage continues to be increased

beyond corona onset a region of glow discharge, free of pulses, will occur. Because of the absence of pulses, radio noise in the glow discharge region is

not readily detected. Therefore, to determine if there is dc corona in a system, it is advisable to increase the voltage from zero in order to pass

through the region of corona onset. AC corona can always be detected at

elevated voltages, however, because the voltage will pass through the

avalanche region at some point in the cycle. In fact, ac corona is somewhat

easier to detect because the radio noise resulting from 60 Hz corona has a pulsating character compared to the more steady noise of dc corona. The results of NBS measurements cannot be extended to all portable AM

radios because there does not appear to be any industry standard which sets

limits on minimum sensitivity of portable AM radios with ferrite-core loop

antennas. However when "calibrated" against instruments providing

quantitative information on corona discharges, and when placed in an appropriate location, AM radios appear to be useful detectors of corona.

3. GASEOUS DIELECTRICS RESEARCH

Task 03

Richard J. Van Brunt, Motaganahalli C. Siddagangappa, and Arthur V. Phelps

Electrosystems Division

National Bureau of Standards

3.1. Introduction

The objectives of this project are to develop measurement capabilities

and to provide fundamental data as part of the Department of Energy's basic

research concerned with the development and evaluation of advanced compressedgas -insulation technology.

To reduce space requirements and improve the efficiency of high-voltage

transmission systems, the electric power industry has turned toward more

extensive use of compressed-gas insulation. To design meaningful tests of

system performance and establish specifications for the quality of materials,

more information is needed about the fundamental physical and chemical

processes which lead to insulation deterioration and electrical breakdown.

This research project is directed toward the evaluation of advanced

measurement techniques as diagnostic tools of insulation performance;

applications of gas chromatography -mass spectrometry to characterize corona

by-products; and the acquisition of fundamental data, e.g. corona inception

voltages, production rates of corona by-products, and the effects of

contaminants on discharge initiation.

This report highlights four significant technical activities. The first

is concerned with a theoretical method that has been developed to determine

the consistency among electron transport, collision cross section, and

10
The

dielectric strength data for binary gas mixtures. The second technical

activity concerns measurements of the hydrolysis rates for SOF^ and SOF^.

third concerns an investigation of SF^ oxidation mechanisms in corona

16 1 6 discharges using SF^/0^/E^0 mixtures in which normally occurring 0^ or 0

18 18

were replaced with 0^ and 0 respectively. The fourth is concerned with

a determination of the rate for F transfer in SF " + SOF. collisions and the

6 4

influence of this process on SOF^ production in negative, point-plane corona

discharges

3.2. Evaluation of Consistency among Electron Collision, Transport, and

Dielectric Strength Data for Binary Gas Mixtures

This work was motivated by the need to find a convenient, objective

method for evaluating data on dielectric strength or electrical breakdown in

electronegative gas mixtures used as insulating or dielectric media in highvoltage systems. The method to be proposed here is relatively easy to use and

offers not only the means for data evaluation, but also provides insight into

the physical bases for observed behavior as a function of gas mixture

composition such as pressure dependent or pressure independent synergistic

effects

Details of this method together with a critical review of previous

methods used to parameterize breakdown data in terms of electron transport or fundamental collision cross section data are included in a recent archival

publication [28] . The discussion given here is restricted to a brief

description of the “pressure- independent" model and its application to the

analysis of data for the gas mixtures SF^/N^, SF^/CCJi^F^, and which

have been the subject of previous extensive experimental and theoretical

investigations and for which there exist controversies in either reported

values for dielectric strengths or in relevant cross section data. From

analysis applied to these mixtures, it has been possible to determine the sets

of dielectric strength, transport, and collision cross section data that

exhibit the highest and lowest degrees of consistency.

The proposed method of analysis is based upon three assumptions. The

first is that the kinetic energy distribution function, f^Cs), is maxwellian

of the form

f (*) - 2*(irkT )' 3/2exp (-t/KT ) . (1) m m m

which satisfies the normalization requirement

ff (c)c 1/2

dc - 1 , (2)

* o m

11
and where k is the Boltzman constant and is the electron temperature that

depends on the electric field- to-gas density ratio E/N, and is related to the

mean electron energy c by

T - 2e/3k . (3) m

'

This assumption is reasonable provided the computation of the electron

transport parameters needed to determine dielectric strength are insensitive

to the detailed shape of f (c), since it is well known [28] that in general

this function is not maxwellian. The relevant transport parameters are the

ionization coefficient, a /N, the electron attachment coefficient ri /N, and m m

the drift velocity that are given respectively by

f. (2/m)

1/2

[FIml + (1 - F)I

m2

]/Wm w

t - (2/») 1/2 [TMra i + (1 - F)Hn2

]/Wm

(5)

W - - f(2/m) 1/2

f

S m 3 N mm

( 6 )

where e and m are the electronic charge and mass respectively, and

I.-/00 c f (e) a r

. (£)de , mj J nr Ij

(7)

H . - SZC fm

( £ ) a

A -5^ £ ^ d£ * mj * o m Aj

( 8 )

M2 de

(9)

2 .

In Eqs . ( 7 )

- ( 9 ) , a_ .

a A . , and aw . are the ionization, attachment, and n

Ij Aj Mj

momentum transfer cross sections for the j th gas constituent respectively, and

is the corresponding ionization potential. For a binary mixture

consisting of gas components A and B corresponding respectively to j = 1 and

2 , F is given by

F - [ A] /( [ A] + [B]) . ( 10 )

12
In the case of electronegative gases considered here where rj ^ 0, it is m

convenient to define "dielectric strength" as the critical E/N = (E/N) at

which

a /N

m VN (id

The second assumption is that the electron temperature for the gas

mixture can be represented as a weighted linear combination of the electron

temperatures for the individual gas components , i . e

Tm

= g(F) T^E/N) + (1 - g(F) ) T

2 (E/N)

where g(F) satisfies the conditions

g - 1 at F - 1 ,

g - 0 at F - 0 ,

g > 0 for 1 > F > 0

( 12 )

(13)

The third and final assumption provides a prescription for finding the E/N

dependencies of T^ and using known electron transport parameters for the

individual gas components. The requirement from this assumption can be

expressed mathematically as

X (CjOO/N)^ VxVT

j

)/N) th (14)

where x - E/N, (x|.(T.)/N) , is the product of x and a transport parameter for

the jth gas component calculated using Eqs . (1) together with known cross

sections, (£.(x)/N) is the experimentally known transport parameter, and 0.

J ex J is a "correction factor" which can be used as an adjustable parameter to

obtain the best fit to critical E/N data, (E/N) , that have been determined

c , m

experimentally. It is required that 0 > 0, and it is evident that if the

maxwellian distribution applies and the cross section and transport data are consistent, then

0^ — 1. Large deviations of0^

from unity obtained in

fitting (E/N) data would indicate a failure of the maxwellian

c , m

approximation, significant inconsistencies among the breakdown, transport, and

cross section data, or a combination of both problems. In a sense, the 0. can

be viewed as consistency parameters.

The first step in making fits to dielectric strength, (E/N)

c data is

to determine the predicted F-dependence of T^ using Eqs. (4) and (5) in

Eq. (11), i.e. solving the equation

° " -C (F(<7 I1 (£) • ffAl (£)) + a F)(<7 12 (0 - »A2 (*))]«f“(«, Vd« (15)

13
for T

ffl

. The next step is to find the E/N dependencies of and using

Eq. (14). For the examples considered here, the ionization coefficient data

were selected for use in Eq . (14), i.e. = a

j

Using linear representations

of aj/N given by

*j/N - A

j0 + A (E/N) , (16)

Eq. (14) assumes a quadratic form

2

A. . X

Jl

+ A

jQ

X 3 (2/m) 1/2

0 . [I. .(T.)/S. .(T.)

j

3 33 J JJ J 0 (17)

which can be readily solved numerically to find the x and 0

^ dependencies of

T.. In Eq. (17) I.. and S.. are respectively integrals like Eqs . (7) and (9)

3 33 33

where f^c) for the mixture is replaced with f (c) f° r the jth pure gas

component

Having found T (F)

T^(E/N), and T

2 (E/N), Eq . (12) can be solved

numerically to give the predicted (E/N)

c In making the fits to

experimental critical field data discussed below, it was initially assumed

that g(F) - F, and the parameters 0

^ and 0^ were adjusted within the

restricted range 5.0 > 0

^ >0,25 to give the best least squares fit to

(E/N)^ versus - F data. Slight improvements to the fits could sometimes be

found using one of the two parameterized forms for g(F) given by

g(F,j9

3

) - ^

3

F1/2 + (1 - 0

3

)F , (18)

or

g(F,^

4

) - /34

F

2 + (1 - £

4 )F , (19)

where 0 ^ and 0^ are adjustable constants that can assume any positive or negative value. It was found, however, that if 0

^ and 0 ^ are close to unity

when g(F) =» F, then further improvements in the fits using Eqs. (18) or (19) are either not possible for 0 ^ , 0^ * 0 , or are relatively minor and correspond

to small parameter values, i.e. \ 0 ^\, I I < 0.2. This suggests that

Eq. (12), in which g(F) - F, generally gives a reasonable representation of

T (F) when consistent data sets are used.

A standard FORTRAN computer program was written to carry out the steps

indicated above. If the parameters 0

^ or 0^ were found to lie outside of the

range indicated above, then the corrections to the calculated ionization

coefficients were considered to be excessive and it was assumed that either

14
oc/N(m

2

x

10

21

)

Figure 6. Ionization coefficient data for from the various indicated

sources, and linear representations of these data used in fitting

dielectric strength data for SF^/^ and Sas mixtures.

15
a/N(m

2

x

10~

21

)

Figure 7. Ionization coefficient data for SF^ from the various indicated

sources, and linear representations of these data used in fitting

dielectric strength data for SF^/N^ and SF

^ gas mixtures.

16
oc/N(m

2

x10

21

)

5.0 T 7

E/N (Vm 2 x10 -19

)

Figure 8. Ionization coefficient data for from various indicated

sources, and linear representations of these data used in fitting

dielectric strength data for 2^2 SF^/CC^^ Sas mixtures

17
the model did not apply or there was a serious inconsistency in the data set

used.

Data on ionization coefficients used in the present analysis for the

gases N^, SF^ , and CCJl^F^ are shown respectively in Figs. 6-8 together with

different linear representations of the data that were considered as indicated

by the lines. The numbers 1, 2, and 3 in the figures indicate different

parameters used in fitting the data as indicated in Tables I to III. These

data were derived from various sources given in Refs. [28-32]. It is seen

that over the narrow E/N range of concern for dielectric strength

determinations, the quality of the data is generally not sufficient to justify

using representations that are higher than first order in E/N.

Available numerical cross section data for these gases were used to

compute the integrals in Eqs . (7) -(9). The ionization cross section data for

N£ and SFg were obtained from [33] and the ionization cross section data for

CCi^F^ from [34]. The electron attachment cross section data used for SF^ was that reported by Kline, et al. [35] and represents the sum of all attachment

processes including dissociative attachment leading to SF^ and F formation as well as low-energy, collision stabilized attachment to form SF, . Two

D different sets of electron attachment cross section data were

considered as reported in [34] and [36] and shown in Fig. 9. It is obvious

that the two data sets do not agree. The solid line in Fig. 9 is a convenient

analytical representation of the cross section data from [32] of the form

V£) A-,B.i=-l l

-a. £ i

( 20 )

where B^, b^, and a^ are adjustable parameters, the values for which can be

found in [28]. The momentum transfer cross section data for SF^ and were

obtained from Phelps, et al . [37-39]. In the case of CCJi^F.^, there exist at

the present time no experimental data on the momentum transfer cross section.

Following a suggestion by Govinda Raju and Hackam [40]

the momentum transfer

cross section for this molecule was assumed to equal a constant of 3.0 x 10

2 cm , which approximates the average value determined by Novak and

Frechette [41] using fits to diffusion coefficient and mobility data from

Boltzmann type calculations coupled with estimates of momentum transfer

associated with inelastic collisions.

In Figs. 10-12 are shown examples of fits based on the model discussed

above to experimental critical (E/N)^ ^data obtained from the literature for

the binary gas mixtures SFg/^ , and CCJl^F^/SF^. The fitting

parameters , ionization coefficient representations, attachment cross

sections, etc. that corresponds to the fits shown in these figures are given

in Tables I through III. The data selected for fitting were obtained from the

18
ATTACHMENT

CROSS

SECTION

(x

10“

18

cm 2)

ELECTRON ENERGY (eV)

Energy dependencies of electron attachment cross section data for

from different indicated source. The solid line is a fit

to the data of McCorkle. et al . using Eq. (20).

19

Figure 9
(E/N)

c

m

x10

F = CSF6 ]/(CSF6

] +cn2 ])

Figure 10. Best fits to the indicated dielectric strength data for SF

6

/N 2’

where the dash-dot curve is a fit to the data of Siddagangappa and

co-workers [43] and the solid and dashed curves are fits to the

data of Verhaart [42] using respectively numerical and analytical

representations of the relevant cross sections (see Table I)

.

20
(E/N)„

m x

10

Figure 11. Best fits to the indicated strength data for

the highest degrees of consistency (see Table II).

21
CM

05

O

X

E»Q

UJ

F = CSF6 ]/(CSF6

] + [CCI 2 F2 ])

Figure 12. Dielectric strength data for SFc /CC2 0 F0 mixture with best fits of

o L Z highest consistency to the data of Wootton, et al . [48] (see Table

III).

22
a> x:

4-5 XO

X.

>,oco

4J

co —

cocooXo

CO

<15 CD • X CM

CO 2

(D \

TO NO

Cij 4-5 oo

COa) c

x: o

co

-4 05 x: X

05 0 •to JO

4J x:

4-5 x: CO

4-> c

CD 3 XX

CO co

4-5 *H o X xX

4-5 X

CO o

<D CD X3 rH

<D

CD rH

jC T3

4-5 rH

•o 05 rH 4-5 <D C

•H CD >> SX 4J X

05 0

jc Q.

4-> X0

CO

X. "O

CD 0

4-5 4-5 <D 0 c O

co X X T3

05 C

a- X

IH0

rHn

03 E- 0Co 0Xo

4-5 o

05 X,co

rH

4-5 o0XXoo

4-5 >.

I—I 05c<

OJ

CM ca

no

Cfcl 0Q

coco

4-5 o

a; oo

co

cooXO

CM 2

CM ca 2 ca

in

CM ooo o

ON

coo c

o

COo

LO

ON OO oo • • • o o o

TO X c

0-4 0 0o NO oc 0 o CO X X

4-5 Cz-i 0 

' NX 0 c 0 o > X X3

CJ

CM CM CM

on

m oo

05Q.

CL c 0 0 CO

4-> TO so 03 X c 0 0 0 0 0 CO • X 4-5 0 3 0 rH 0 X x: x: TO 0 a X X o 00

CX4 0 0 X 4-5 > < CO 0

23
Ionization

coefficient

curves,

CCi^F^

attachment

data,

and

correction

factors

that

correspond

to

the

fits

shown

in

Fig.

11

for

CC9,^F

CM

CM ca

CM

CijCM o?oo

LOo o

CM O

ONon

\o <0 • . o o

COc

4-S O C -m 0 -U CMS O

U, SZ <D CMC CO 0? 03 CJ 4-> co 0 CO < O

t. 0

e—

-3-

<m

0) CC

00

-=T

<M0

cc

CO-3’

<m0

OS

Cm0Oo

CO

TZC

03 <0

CM

C14CM o?Oo CM CM CM co

coCO

4-> 03

Cdco

H-t

bn

COo>

s33 CM O 25

on

m

m

HHM

CD fH

£>

03 E-

•a

03 0 U jj

03 -U a w

Cij 0r—

r—

•M>

TDC0 •M

sz S- u fH 0 0 0 0 •M 0 r-H e rH s. pH 0 4-> 0 :3 05 CO 0 S Of 2:

24
Table

III.

Parameters

corresponding

to

the

fits

to

the

SF^/CCJl F

data

shown

in

Fig.

10 u3

CO GO £, C CD

4-J

m

CQ

C\J

Ct4CM o?oo

CM ca

vo

Eij cnc

UJ o C •'-( CD 4Jo

CD

CMi

Cti J CMO CO o? CO o 4J m O 4-3 CO < o

£_ o

oo • X3

0-4 C

<*-! CO

CDO t*— CJC CO o tc

•f-J *w

4J Ct, CON

1-i CO C CD O > M S_oo

CM

CxiCM ©?

CJo

vO

txa oo

C0 ^ U CM CM

CO rX3 • • O (D bO

43 C H

co e

4J o

•f-i s_ Du <i-t Oo oo

CMO

OO O • o O t-

=r on on

in ir>

m . . . <*h <4-. Q-. CD 0) CD cc DC OS

CM CM CM

CM t— »—

CM on

25
following references: SF

6

/N

2 [32,42-44]; [44-47];

CCi

2

F

2

/SF

6

[44,47-50].

For the mixture SFg/N

2 the data of Aschwanden [32] and Verhaart [42] are in close agreement; therefore only the fits to the Verhaart data are shown in

Fig. 10 together with the best fit to the data of Siddagangappa, et al . [43].

The solid and dot- dashed lines correspond respectively to fits for the data of

Verhaart and Siddagangappa et al. made using numerical cross section data.

The dashed curve is a fit to the Verhaart data using analytical expressions

for the cross sections as given for example, by Eq. (20) and specified

in [28]

.

All of the curves in Fig. 10 correspond to the assumption that

g(F) - F in Eq. (12). Slight, almost imperceptible improvements in the fits

were found for the three data sets selected when g(F) was given by Eq. (19)

with parameter values in the range |/3^| < 0.5. It is seen from Table I, as judged by the closeness of the parameters and /? 2 to unity, that the results

of Verhaart [42] and Siddagangappa, et al. [43] yield respectively the highest

and lowest degrees of consistency with the assumed swarm and cross section

data. It is worth noting that the results of Boltzmann type calculations

[35,39,51,52] also tend to be in better agreement with the data of Verhaart.

As expected, the fits made using numerical cross sections yield higher degrees

of consistency than those using approximate analytical forms. It was also

found that for all three data sets, the use of ionization coefficient curves 3

and 2 respectively for N„ and SF, (see Figs. 6 and 7) always gave the best

fits for the highest degrees of consistency. In the case of SF^ , curve 2 best

represents an average of the available data. Within the relevant breakdown

range 3.7 x 10 ^ Vm^ > E/N > 1.6 x 10

^

Vm^ , curve 3 for N

2 gives an

accurate representation of the data at low E/N, but falls somewhat below the

average at high E/N.

In the case of the CCi

2 F2 /N2 mixture (see Fig. 11 and Table II) separate

fits were made here using the attachment cross section of data of McCorkle,

et al. [36] and Pejcev, et al. [34]. Since the data of Mailer [53] and Mailer

and Naidu [46] are in close agreement, only the best fit to the Mailer data is

shown as indicated by the dashed line in Fig, 11. Also, because the recently

reported data of Berril, et al. [47] are in close agreement with the data of

Somerville, et al . [44], only the latter data set was used for fitting. The

best fits to the data of Malik and Qureshi [45] and Somerville, et al . [44]

correspond respectively to the dot- dashed and solid lines in Fig. 11.

The curves drawn in Fig. 11 represent again the best fits obtained in

each case for the highest degree of consistency with g(F) =» F. The

ionization coefficient representation indicated by curve 2 in Fig. 8 yielded

the highest degree of consistency in all cases and is seen to best represent

an average of all the data. As in the case of SF^/N

2

, curve 3 for N

2 gave the

best consistency. The fit to the data of Somerville, et al . [44] using the

attachment cross section of McCorkle et al. [36] yielded the highest degree of

consistency. The data of Somerville, et al . also agree best with results of

Boltzmann type calculations [41]. The data of Mailer [46], which is seen to

26
disagree significantly with that of Somerville, et al . , yielded the lowest

degree of consistency. For both the data of Mailer and Malik and Qureshi

[45]

the best fits for the highest levels of consistency required use of the

attachment cross sections given by Pejcev, et al. [34].

As shown in Fig. 12, all the available experimental (E/N) data for

c , m

SF6/CCi2F2 tend to agree and all exhibit slight degrees of positive synergism

in which the breakdown strength of the mixture is greater than that of either

constituent. The results, not shown here, of Boltzmann type calculations by

Okabe and Kouno [50] and Frechette and Novak [54] also exhibit this positive

synergism. The fit to the data of Wootton, et al. [48] yielded the highest

degree of consistency with swarm and collision cross section data. The curves labeled 1 through 3 in Fig. 12 correspond to fits made only using the data of

Wootton, et al. with the corresponding parameters indicated in Table III. It was found that the positive synergistic behavior could be obtained with a high

degree of consistency and close to unity) by using the attachment cross sections of McCorkle, et al . [36], but not by using the cross sections of

Pejcev, et al. [34]. This is also consistent with the Boltzmann

calculation [54]

The fits corresponding to curves 1 and 2 in Fig. 12 were obtained

assuming g(F) - F. The dashed curve was obtained using Eq. (19) with =

0.2. This is an example of a case where a noticeable improvement in the fit

is found using Eq. (19) under conditions of highest consistency. It was generally found that significant improvement to fits using either Eq. (18) or

(19) most often occurs when the degree of consistency is low.

From the examples considered here it would appear that the proposed

method of analysis shows promise as a convenient approach for determining the

consistency among various types of fundamental data relevant to uniform- field

electrical breakdown in electronegative gas mixtures

3.3. Gas-Phase Hydrolysis Rates for SOF^ and SOF^

This work was carried out in collaboration with I. Sauers of the Oak

Ridge National Laboratory and the results have recently been published in the

Journal of Chemical Physics [55] . The significant data obtained from this

investigation are summarized here.

It is known that SOF^ ( thionylfluoride) and SOF^ (thionyl tetrafluoride)

hydrolyze according to the reactions

( 21 )

and

SOF. + H_0 -

2 S0 o Fo + 2HF

4 2 2 2

( 22 )

27
Because both SOF^ and SOF^ are known [56,57] to be major long-lived

stable gaseous by-products from SF^ decomposition induced by electrical

discharges in the presence of oxygen and water vapor, a knowledge of the rates

k^ and for the above reactions is needed for the design of chemical

diagnostics applied to SF^- insulated, high-voltage systems [58,59].

Ruegsegger, et al. [60] recently reported a gas -phase rate constant for

-20 3 reaction (21) of (4.0 ± 1.5) x 10 cm /s at a temperature of 340 K. This

rate is about two to three orders of magnitude greater than the rate estimated

by Sauers, et al. [61] at a comparable temperature, and is also greater than

that expected from measurements of oxyfluoride and SO^ yields from electrical

discharges in SF^ reported by Van Brunt [56] and Latour-Slowikowska,

et al. [62]. There has been no previously reported information on the SOF^

gas-phase hydrolysis rate k^

.

The rate constants k^ and k^ were measured directly in the present work

using a gas chromatograph-mass spectrometer (GC/MS) to periodically monitor

SOF^, SOF

2

, SO

2 F2

, SO

2

, and 1^0 concentrations from a vessel containing SC^

(or SOF^) in either ^

or SF^ buffer gas at a total pressure of 200 kPa

(~2 atm) and at a room temperature of 298 K. Details of the gas analysis and

mixture preparation procedures can be found in Refs. [55,56,63]. Examples of

the data from which the rate constants were derived are shown in Figs. 13 and

14 which display measured concentrations -versus -time for the observed

reactants and products. The second order rate constants were derived from

data such as shown in these figures assuming that the time dependences of the

observed species are governed by the following set of rate equations:

4DLL

dt - k

i

[Y][H

2 0] (23)

dlYl

dt

k

i

[Y][H

2

0] (24)

d[H 0] - - k.[Y][H

2 0] + k'A[H2 0] , (25)

where [Y] and [X] are the reactant and product concentrations, i.e. or SOF^ and SC>2 or SO

2 F2 for i - 1,2 respectively. The term k'Af^O] is included

to account for absorption or evaporation of water vapor from surfaces. This

rate is assumed to be proportional to the deviation Af^O] = [i^O^ - [^0] , of the water vapor concentration from equilibrium with the walls, where [ H^O

]

g is the normal equilibrium level. The rate factor k^[Y] in the first term on

the right-hand side of Eq. (5) was sufficiently small compared to k' in the

second term that decreases in water vapor concentrations due to the hydrolysis

reaction were not discernible. The rate constants were computed using

28
CONCENTRATION

(n

moles/cm

3)

Figure 13. Measured SO^, SOF^ and 1^0 concentrations versus time in SF^

buffer gas.

29
CONCENTRATION

(10~

8

moles/cm

3)

Figure 14. Measured SO^F^, SOF^, and ^0 concentrations versus time in SF^

buffer gas

30
Eq. (23) and the averages of the slopes of the [X] -versus- time production

curves

Based on the analysis discussed above, the second- order rate constants

for gas -phase hydrolysis of SOF

2

and SOF^ at 298 K in both N

2 and SFg buffer

-23 3

gas are assigned the values (1.2 ± 0.3) x 10 cm /s and

-21 3

(1.0 ± 0.3) x 10 cm /s respectively. The results were found [55] to be

consistent with reactions that occur predominantly in the gas phase. The rate

for S0F

2

hydrolysis is in agreement with previous estimates of Sauers,

et al. [61]but is three orders of magnitude lower than the value obtained by

Ruegsegger et al. [60]

The value of from Ruegsegger, et al. was obtained indirectly from mass spectrometric observation of S0

2 production during gas -phase hydrolysis of SF^

at relatively low gas pressures. The mass spectral data of Ruegsegger, et al

was reanalyzed by Van Brunt and Sauers [55] using an isothermal chemical

kinetics code developed by Brown [64]

and it was shown from this analysis

that the S0

2

in this case was formed directly from SF^ by a surface reaction

mechanism since there is evidence [61,65] that hydrolysis of SF^ in highdensity or liquid-phase H

2

0 need not involve S0F

2 as an intermediate.

The values for the rate constants k^ and k

2

reported here for reactions

(21) and (22) are undoubtedly the most reliable available. These rates are needed for interpretation of results obtained from gas analysis applied to

SF^- insulated high-voltage systems because water vapor is usually the most

prevalent contaminant in the gas. The possibilities of observing either S0F

2 or SOF^ will certainly depend on H

2

0 contamination in the system as well as that introduced during the gas sampling process

3.4. Identification of Corona Discharge -Induced SF^ Oxidation Mechanisms

1 O -]£ 1 g TO

using SFg/ 0

2 /H2 0 and SF^/ 0

2/H

2 ° Gas M^xtures "

An understanding of the primary SF^- oxidation mechanisms is required to

predict chemical activity associated with dissociation of SF^ in electrical

discharges such as can occur in gas -insulated, high-voltage apparatus. The

motivation of the present work is to obtain information about the dominant

processes that lead to formation of the oxyfluorides S0F

2

, SOF^, and SC> 2

F

2 from SF,/0o mixtures containing water vapor in a highly localized corona

o 2. 16 16

discharge using gas mixtures in which normally occuring C> 2 and H

2 0 are

18 18 replaced or enriched with 0

^ and H2 0 respectively. The approach used here

is similar to that taken by Velichko, et al. [66] in the investigation of SF^

31
oxidation induced by multiphoton dissociation. The predominant sources of

oxygen for formation of the oxyfluorides have been identified here by mass

16 18 spectrometric determination of the relative 0 and 0 content in these

18 18 species formed in different gas mixtures containing either 0^ or 0. A

preliminary report of these observations was given at that 1986 Gaseous

Electronics Conference [67], and a complete discussion of the work is covered

in an archival paper recently submitted for publication [67]

The experimental procedure used to obtain the results reported here is

identical to that previously described [56]. A continuous, direct current,

negative, point-plane corona discharge was operated for up to 30 hours at

40 /^A in the gas mixture of interest at a total pressure of 200 kPa in a 3.7 liter cell. During operation of the discharge, small gas samples were

periodically extracted with a gas tight syringe and injected into the column

of a GC/MS.

The GC/MS was operated in the single- ion monitoring mode whereby ions of

particular mass- to -charge ratios (m/e) characteristic of the 70 eV mass

spectra for the various gaseous species of interest were sequentially detected

during the time that the injected gas sample eluted from the GC column. To

determine the influence of molecular oxygen on oxyfluoride formation,

comparable measurements were performed using the following two sets of similar

16 18 16 18

gas mixtures: SFg/20% 0

2

, SFg/20% 0

2

, and SFg/2% 0

2

, SF

g /2% 02

* In all

of these mixtures , varying trace amounts of water vapor were present in the

form of H^^O.

The influence of water vapor on oxyfluoride formation was investigated

18 using mixtures initially prepared with 97.5% 0 enriched H^O by the method

previously described [27,35], It was found, as expected for a static gas

system, that because water vapor exchanges with the walls of the vessel

18 16 enclosing the discharge, the H

2 0 concentration relative to 0

diminishes with time.

Examples of data obtained for S0

2 F

2 are shown in Figs. 15-17. Results

for the other oxyfluorides can be found in [67]. The data shown in Figs. 15

and 16 are single -ion chromatograms for ions with m/e - 87 and m/e = 83

18 16 respectively showing features associated with S ^2^2 anc* ^ ^2^2 ^orme<^ various indicated gas mixtures after comparable amounts of charge Q had been

transported, where Q is related to discharge current i, and time t by

Q - it . (26)

18 These chromatograms indicate that S ^2^2 P reva^- ent ®ixtures containing

^^0

2 and S^0

2

F

2 is prevalent in mixtures containing ^C>

2

thus suggesting that

32
RELATIVE

INTENSITY

m/e = 87

SF6 /20% 02

T

sf6

/ 16 o2 /h2

18 o

°%ll

l SFS ' ,6 °2/H2' 6 °

0

Figure 15.

2.0 4.0 6.0

SF

n

/o

e /H°o

°hromat

S VVH

°Srams at ">/e - 87 from the different indicated

2

0 ®as “^tures and net charge transported, Q. of

significance here are the relative changes in the feature

associated with S °

2 F

2 33
RELATIVE

INTENSITY

RETENTION TIME (min)

Figure 16. Single- ion chromatograms at m/e - 83 showing features associated

with S^C>

2 F2 from the different indicated SF^/C^/^O gas mixtures

and net charge transported Q.

34 -
0 1.0 2.0 3.0 4.0 5.0 6.0

NET CHARGE TRANSPORTED (C)

.16 Figure 17. Measured quantity of S ^2^2 §enerate<^ by the corona discharge

versus net charge transported, Q, from the various indicated gas

16, mixtures

All mixtures contained at least trace amounts of H^^'O

The solid lines represent linear fits to data previously reported

56] for the mixtures SF./trace

35

16

0^ and SFg/1%02
SO^F^ formation preferentially utilizes molecular oxygen. This is indicated

more quantitatively in Fig. 17 which shows absolute yields of S^^O^F^ versus Q

from various SF^/O^ mixtures. The present data for mixtures containing 2 and

18 16

20% O

2

(or 0^) are shown for comparison with previously reported data from

16 16 SFg/trace and SFg/1% O

2

mixtures obtained under comparable

conditions [56]The present results for the ^0

^ mixtures are consistent

with the earlier data. The most significant observation from the results

16 18 shown in Figs. 16 and 17 is the dramatic drop off in S °2^2 w^en °2

is substituted for '^1 ^s observation is consistent with the previous

suggestion [56] that SO

2

F

2

is formed predominantly by the relatively slow [68]

reaction

SF

2 + 02 - S02 F2

, (27)

which occurs at ambient temperature in the main gas volume surrounding the

active discharge region.

Similar data obtained for SOF

2 show that its production is insensitive to

18 the presence of 0 It was found, however, that S OF

2 appears in significant

18 18 quantities when 0 is present in the gas. * The rate of S OF

2

production

18

was, in fact, found to be proportional to the Hj 0 concentration, thus

suggesting that SOF

2

is predominantly formed by the relatively slow [61] gas

phase reaction

SF4 + H

2

0 - S0F

2 + 2HF , (28)

which again occurs predominantly in the main gas volume of the discharge

vessel as proposed by Van Brunt [56].

The rates for SO

2

F

2

and SOF

2

production should thus be controlled by the

rates at which SF

2

and SF^ diffuse out of the active discharge volume. The

fact that SO

2

F

2

and SOF

2 production rates are observed [56,69] to be

relatively insensitive to either or 1^0 content is consistent with the

assumption that SF

2

and SF^ react preferentially with 0^ and 1^0 respectively

in the main gas volume.

The results for SOF^ production are found also to be consistent with the

proposed [56] mechanisms for its formation involving the fast reactions

36
( 29 ) SF C + OH ->• SOF, + HF

5 4

SF

5 + 0 h. S0F4 + F , (30)

SF

4 + 0 - S0F

4 , (31)

which occur in the small active discharge volume near the point cathode. Thus

S0F4 can utilize both 0

2

and H^O in obtaining oxygen for its formation. At

relatively low 0^ concentrations (below about 5% by volume) the data indicate

that reaction (29) predominates, i.e. the S0F

4 yield is not significantly

dependent on 0

2

content. This may be the consequence of an enhancement in OH

concentration in the discharge by the fast reaction

F + H

2 0 - HF + OH . (32)

When the 0

^ content is increased to about 20%, it is found that reactions (30)

and (31) become relatively more important, i.e. S0F

4 appears to derive

increasingly more of its oxygen from 0^ as expected.

3.5. Transfer of F in SF, + SOF. Collisions and Its Influence on SOF. Yield

6 4 4

from Corona Discharges in SF^- Containing Gases.

This work was carried out in a collaboration involving R. J. Van Brunt of

the NBS Electrosystems Division, L. W. Sieck of the NBS Chemical Kinetics

Division, I. Sauers of the Oak Ridge National Laboratory, and

M. C. Siddagangappa of the Indian Institute of Science. A preliminary report

of the results was given at the 1986 Gaseous Electronics Conference [70] and

the work has now been prepared for archival publication [70]

This report

briefly summarizes the significant results and conclusions from this work. A

detailed account of the experimental procedures and theoretical model are given in [70]

It is evident from the discussion given in the previous section that S0F

4

is a major and often predominant by-product of SF^ oxidation in glow and

corona type electrical discharges [56,71-74]. In any attempt to understand

the magnitude of the S0F

4 yield from electrical discharges, it is necessary to

have information about the rates for reactions that lead to its formation and

destruction. It is known [75] that the energetically favorable reaction

SF," + SOF, S sof " + SF C , (32)

6 4 5 5 occurs quite rapidly at near the collision rate. Because of the possible

importance of SF£ as a predominant charge carrier in the discharge gap, it

o

37
has been speculated [70,76] that reaction (32) can account for observed

nonlinearities in SOF^ yields from corona discharges.

In the present work the temperature (T) and electric field- to -gas

pressure (E/P) dependences of the rate constant k for reaction (32) were

measured using different mass spectrometric techniques [70,75,77,78]. The

results for the temperature and E/P dependences of k are shown respectively in

Figs. 18 and 19. Figure 19 also shows for comparison the thermal rates

obtained from Fig. 18 at the indicated effective temperatures T^^, plotted at

approximately equivalent E/P values estimated using the relationship [79]

T

eff - T[1 + (M(m

o

P

o

) 2/31c T)(E/P) 2 (33)

where T is the actual gas temperature (350 K) in the E/P measurement

experiment, M is the SF^ ion mass, is the reduced SF^ mobility,

100 kPa, and k. is the Boltzmann constant

P

To within the uncertainties

of the data the rate constants k determined at low E/P are in agreement with

the thermal results as required.

The temperature dependence for the rate constant can be represented by

the empirical expression

k(cm^/s) - 0.124 exp (-3.3 In T(K)) , (34)

-9 3 for 433 K > T > 270 K, and k assumes a constant value of 1.2 x 10 cm /s for

T < 270 K. The abrupt leveling off of k at low temperatures is a behavior

observed for other ion-molecule reactions that at present is not thoroughly

understood [80-82]. Because neither the dipole moment nor the polarizability

of SOF^ are known, it is not possible to predict the limiting classical

collision rate for reaction (32). It can be expected from comparison with

other similar fast reactions that the maximum value for k will not be

-9 3 significantly greater than 1.0 x 10 cm /s

The E/P dependence of k for a gas temperature of 350 K, can be

represented by the approximate formula

^ -10

k(cm /s) - 7.0 x 10 exp (-0.022 E/P) , (35)

where E/P is expressed in units of V/cm®torr (1 torr - 133.3 Pa) and lies in

the range of 60 to 130 V/cm®torr. For E/P < 60 V cm® torr, k assumes an

- 10 3 approximate constant value of 2.5 x 10 cm /s. The results for the E/P

dependence given here are consistent with the earlier data of Sauers [75].

Although the results at low E/P tend to agree with the thermal results (see

Fig. 19), such agreement need not be expected at high E/P. Increasing the

electric field primarily increases the mean translational energy of the

reactants, whereas increasing the gas temperature increases both the mean

level of internal excitation and translational energy of the reactants. To

38
ro

CD

®. O CVJ CVJ CVJ CVJ ru ® <d cj • ^ •

( 0,01 X V|)U|

in

39

Figure

18.

Natural

logarithm

of

the

measured

rate

constant

k

(cm

/s)

x

10

versus-the

natural

logarithm

of

temperature

T

in

degress

Kelvin

for

reaction

(32).
CO

co

o

0

T"

1o

X

E/P(V/cm-torr)

Figure 19 Measured E/P dependence of the rate k (closed circles) . Shown for

comparison are values of k obtained from the temperature

dependence results at the indicated effective temperature s (

)

that correspond to particular E/P values (open circles).

40
the extent that k depends on internal energies, there will be differences

between the temperature and E/P dependences at higher E/P, and at very high

fields, the concept of "effective temperature" associated with the reaction is no longer meaningful.

A theoretical model was proposed [70] which utilizes the information

obtained here on the rate constant k for reaction (32) to interpret the

observed behavior of SOF^ production rates for negative, point-plane corona

discharges in gas mixtures containing SF^. It was shown that the charge rateof-production for SOF^ can be expressed as

d[S0F

4 ] _ kfSOFJ exp{(-k[S0F

4 ]/v - k')ZQ

}- 1

~dQ rQ

+

eVr

t

(k[S0F

4 ]/v + k')

’ (36)

where r^ is the charge rate -of -production in the discharge volume due to

reactions like (29)-(31), e is the elementary electron charge, Z

q is the

point- to-plane gap spacing, is the volume of the gas containing vessel, and

k' a decay constant associated with a process that competes with reaction (32)

in the destruction or deactivation of SF^ . The drift velocity v of SF^ is

assumed to be given by

V - Vo (Eav/P) ’ (37)

where E is the average value of the electric field in the ion drift region.

All previously reported data from our laboratory on S0F

4 production from

negative corona discharges in mixtures like SF^/O^, SF^/Fl^

, SF^/Ne (see

Refs. 56, 69, and 76) were successfully fit using Eq. (36). It was found,

however, that in order to account for the observed deviations from linearity

in the S0F

4 production curves, it was necessary to invoke a competing process

for SFg destruction, i.e. k' ^ 0. Although it was not possible to identify

this process unambiguously, there are at least three that appear plausible.

These include formation of the anion complexes SF^ (H^O)^, SF^ (HF)^, n = 1 , 2 , 3 , . . . which could be considerably less reactive than SF^ , and F

exchange with trace amounts of SiF

4

that might be present due to reactions of

HF with insulating materials in the discharge cell. It could be concluded

from the model calculations that SF, ceases to be the predominant negative

o ion charge carrier in the discharge gap within a distance of less than 10% of

the gap spacing. Further measurements are now underway using the NBS pulsed

electron-beam, high-pressure mass spectrometer system to identify the terminal

negative ions in the ion-drift region of a corona discharge gap.

41
4 . INTERFACIAL PHENOMENA IN LIQUIDS

Task 04

Edward F. Kelley

Electrosystems Division

National Bureau of Standards

4.1. Introduction

The objective of this research is to develop techniques to measure the

electrical behavior of liquid- solid composite insulation for use in highvoltage systems. Model systems of practical interest are being investigated,

such as the interface between transformer oil and pressboard. The effects of

contamination and temperature up to 150 °C can be documented in these model

systems in two ways: 1) breakdown measurements using high-speed photography,

and 2) electro-optical measurements of the electric field in the vicinity of

the interface. Such field measurements provide an understanding of the role of

space charge and surface charge in high-voltage apparatus. Because of

possible standards applications afforded by these electro-optical techniques,

NBS has a leading role to play in developing and perfecting the measurement

techniques

Electrical failure in high-voltage apparatus often occurs in the vicinity

of or upon an interface which serves as an insulator between two conducting

regions of differing potential. However, in well-characterized systems, the

interface will not necessarily cause the failure [83,84]. Additionally,

preliminary measurements of the electric field in the vicinity of a pressboard

interface either parallel or perpendicular to the field have been made

previously and reported in the literature [85,86]. Those measurements

demonstrated that the interface parallel to the field showed no dramatic

surface charging which would give rise to large field enhancements, whereas

the interface perpendicular to the field demonstrated surface charging as would be expected. Thus, the cause of interfacial failure in practical

apparatus does not seem to originate from macroscopic field irregularities

arising from the presence of the interface parallel with the field in clean

systems

The lack of macroscopic field irregularities suggests that attention to

microscopic details in the vicinity of the electrodes is warranted. Much of

the thrust of the experimental effort in the last year has been to explore

some of the details of the microscopic behavior of the liquid at the electrode

surface. Although this represents a slight deviation from electro-optical

studies of transformer oil, it is an important departure which prepares the

laboratory for the observation of breakdown phenomena with increased spatial

and temporal resolution. The techniques employed to examine the liquid system

will have immediate application to a composite, interfacial system. The

following data represents the most outstanding accomplishment in this area for

the last year. A fuller presentation was made at a conference in October 1986

[87]

42
4.2. Streamer Initiation: Electro-Optic Field Measurements

When considering initiation of prebreakdown phenomena in liquids it is

often assumed that charge injection heats and vaporizes the liquid giving rise

to an ionized gas or plasma phase. The plasma then acts like a conductor and

a process continues whereby the streamer grows across the gap. In this

report, estimates are provided of the electric field, amount of injected

charge, and joule heating in the vicinity of a needle tip which is the cathode

in a needle-sphere electrode geometry. Kerr-effect electro-optical

observations of the impulse field in the vicinity of the tip show a distortion

from the geometrical field due to charge injection from the tip prior to any

streamer development. The injected charge is not distributed evenly over the

surface of the tip, but appears to reside mainly in a relatively narrow

channel. Estimates reveal that the injected charge densities are on the order

3 of 100 mC/cm . The streamer initiates where this charge- inj ection channel

touches the needle tip.

4.3 Experimental Observations

Nitrobenzene is used in this measuremnt because of its large Kerr

coefficient. Although nitrobenzene is chemically different from transformer

oil, similarities in prebreakdown processes in a wide variety of liquids

suggest that the specific chemical nature of the liquid used is not important

in determining properties of prebreakdown in liquids . The nitrobenzene had a resistivity of about 200 fl»cm before the experiment and 50 Q«cm after.

A xenon flash tube provides illumination for the high magnification

photography using an image converter camera, one frame of which observes an area of 0.76 mm x 0.65 mm at 22X at the tip. The wavelength dependence of the

Kerr-effect necessitates color filtration. Higher fields produce more fringes

which cannot be resolved if the light bandwidth is too large, so a dielectric

filter was used [500.7 nm, full-width-half -maximum 3.7 nm]

The standard configuration for Kerr-effect observations is to sandwich

the liquid cell between quarter-wave plates and polarizers so that circularly

polarized light enters the cell [88]

Aligned polarizers (maximum

transmission for zero field) or crossed polarizers (minimum transmission for zero field) are used in this experiment. Quarter-wave plates are not used

because better image quality was obtained without them. With an axially

symmetric, nonuniform geometry it can be shown that after the first few

fringes the Kerr-effect fringe pattern is nearly the same with or without the

quarter-wave plates. This is because the fringe fields elliptically polarize

the incident linearly polarized light before that light reaches the highest

field regions. Without quarter-wave plates, isoclines (regions where the

strongest field is aligned with the polarization axes) are observable only in

the first fringe.

The camera can be triggered so that conditions at any time during the

applied voltage pulse can b£ photographed (see Fig. 20). Fig. 21 shows the

Kerr effect over the width of the pulse as compared to shadowgraph results.

Distortions in the fringe pattern can be clearly observed at the tip as the

voltage nears the peak. This distortion persists to the end of the pulse.

Comparing the results with crossed and aligned polarizers (Fig. 21b and 21c)

43
Figure 20. Region observed by the camera, shape of voltage pulse, and framing

sequence relative to the pulse

(e) (d)

Figure 21. Kerr effect at needle tip (22X): With aligned polarizers the

first dark region ahead (left) of the tip is fringe n - 1; pulse

rise (a) is compared to the peak and fall (b) which is compared to

crossed polarizers (c) where the seventh fringe (white) is just

coming out of the needle. Shadowgraph results (d) were made under

same conditions as (b) and (c) , 19.3 kV (± 3%). The framing rate

is 2 x 10 s frames/s.

44
shows that the highest fringe to exist near the point is fringe n = 7 at the

voltage peak. The distortion of the field due to charge injection amounts to

a decrease in the fringe count by at least one fringe (fringe shift An =* -1)

and possibly two (An = -2) , as can be seen by the bending of the fringes into

the tip. Although the irregular fringe patterns give the impression of

streamer development, no streamers can be observed using shadowgraph

techniques (Fig. 21d)

4.4 Order of Magnitude Analysis

Referring to Fig. 21, if there were no charge injection the fringe

(n — 7) would not bend back into the tip and the seventh fringe would be fully

developed at the tip. The information contained in these fringes is sufficient

to make estimates of the quantity of charge injected and the resulting heating

of the liquid.

For the Kerr effect, the birefringent phase difference for any light ray

path (parallel to the x-axis) is given by an integral of the square of the

component of the electric field which is perpendicular to the path

* Fafe*

B ( y

2

< 38 >

2 where B is the Kerr coefficient (3.24 pm/V for nitrobenzene). This phase

difference is related to the fringe number by $ - n, , whereby the normalized

2 2 transmitted light for each path is given by T - cos ($/2) = cos (n7r/2) for

2 aligned polarizers, and T - sin (n?r/2) for crossed polarizers.

Order -of -magnitude approximations can be made to estimate this phase

integral: 1. assume that the perpendicular component of the field within a tip radius (r in Fig. 29) of the tip has the constant value E over the length

L - 3r along the light ray (x-direction) ; 2. assume chat this region provides

the main contribution to the phase shift. The integral simplifies to

2 $ - 2ttBLE . Then, the approximate field in terms of the fringe number is

E - (n/2BL) 1/2

. (39)

Differentiation allows us to determine the change in the field corresponding

to a fringe shift An, dE/dn = E/2n. For example, with r - 25 /an, n = 7, and

An = -1, a field strength of E = 120 MV/m and a decrease in the field of

AE — -8.6 MV/m obtains. This change AE from the Laplacian field due to charge

occurs in the direction of the charge channel (perpendicular to the x- direction) or z-direction, the axis of symmetry. Poisson's equation is then

AE/Az - q/e. With a relative permitivity for nitrobenzene of 35, the

permitivity is 310 pF/m, and assuming that the distance involved is one tip

3 radius, Az =* r, then the charge density is q - -110 C/m . Referring to Fig. 21, it would appear that the charge extends about four

tip radii into the liquid, z - 4r, with a width of about the tip diameter or

2 less, (area A =» nr ). Assuming the injection process occurs in t = 1 ps , then

45
the current density is J - qz/t - 11 kA/m , and the total charge Q contained

in the volume (V - Az) is Q - 21 pC. To determine the energy expended by

joule heating we need the resistivity, taken to be R - 1 ft»m, the density of

3 nitrobenzene D - 1200 kg/m , and the heat of vaporization of typical

hydrocarbons h - 420 kJ/kg„ The energy density in the channel is then

3

K * RJ2t - 110 MJ/m , and the energy density needed to vaporize the liquid is

3

H — Dh - 500 MJ/m . The ratio of the energy supplied to the heat needed is

K/H - 0.22. It should be noted that no streamers initiated at the voltages

which would produce only seven fringes at the tip. Higher voltages were

needed. Unfortunately, it was not possible to resolve the fringes at the

higher voltages with higher magnification because of the poor image quality.

However, if we were to repeat the above calculation for seven fringes (n = 7)

and assume a fringe shift of two (An =* -2) then we would find the same field

3 of E - 120 MV/m at the tip, a charge density of q - 210 C/m , and an energy

3 density of K — 450 MJ/m which is approximately 90% of the required energy

density to vaporize nitrobenzene. Similarly, if n - 9 (13% field increase)

3 and An - -2, then the energy density is K - 350 MJ/m which is 70% of the

energy density needed.

4.5 Conclusion

Electro-optic Kerr-effect observations of the field at the tip of a needle indicate that a large amount of charge is being injected within a relatively narrow channel at the tip of the needle. An order-of -magnitude

calculation reveals that the injected charge density results in joule heating

of the liquid which is seen to be of the same order as the heat needed for

liquid vaporization.

46
5. NANOSECOND BREAKDOWN IN POWER SYSTEM DIELECTRICS

Task 06

Ronald H. McKnight

Electrosystems Division

National Bureau of Standards

5.1 The Use of E-Dot Sensors in Measuring Fast Rise Pulses

5.1.1 Introduction

The measurement of fast rise high voltage electrical pulses with rise

times less than 100 ns and total pulse duration less than 1000 ns challenges

the methods conventionally used by power system engineers. Such pulses may be

produced in various ways including disconnect operations in gas insulated

equipment, or as a result of nuclear electromagnetic pulse (EMP)

Conventional power system dividers have inadequate bandwidth, and so field

coupled devices such as capacitive dividers, E-dot (or V-dot) sensors,

Rogowski coils and fluxmeters, which find wide application in pulse power and

simulator systems, must be considered. Other specialized resistive dividers,

using liquids as elements have also been shown to have applicability in

measuring fast rise pulses. Although of limited accuracy, these devices are easily constructed and modified to meet experimental conditions. They exhibit

a lack of long term stability, however, and must be recalibrated frequently to

verify acceptable performance.

The field coupled devices have certain attractive features including a non- intrusive installation, simplicity of construction and potentially wide

bandwidth. They share the difficulties of calibration and response

characterization common to all fast high voltage sensors. In addition, the

most widely used system consisting of a derivative (E-dot) sensor coupled to an integrator has a very low sensitivity and has practical application only

for large voltages. However, the system may be useful for some of the

evaluations being made of the effects of fast rise pulses on power system

equipment where large signals are being used. In particular, these devices

may be useful in large transformers where contacting probes are not feasible.

As a first step in a study of field coupled sensors, an analysis of the

response of such E-dot sensors to EMP- like waveforms has been made.

Experimental work is presently in progress which is concerned with determining

errors associated with E-dot sensors as well as methods for calibration. This

report provides a general description of these sensors and presents results of

numerical calculations for a selected fast rise waveform.

5.1.2 Sensor Characterization

The equivalent circuit of an E-dot sensor and its attendant passive

integrator is shown in Fig. 22. The physical sensor itself is an electrode

which is installed in a grounded part of an enclosure so that it is field

coupled to the high voltage element. The signal from the electrode is carried

to the integrator by a terminated coaxial line. This high-pass circuit is

described in circuit analysis texts as a differentiating circuit, and the

47
-DOT

(E-DOT)

SENSOR/INTEGRATOR

N

vVW

vWV

o

O

6 o

48

Figure

22,

Schematic

of

a

capacitively

coupled

E-dot

sensor

with

integrator

Circuit

component

values

used

in

the

present

study

are

summarized

in

Table

IV.
output from the circuit is given by = description of the circuit is valid for waveforms whose highest frequency

components have periods which are long compared to the time constant

r^-R^C^ of the differentiator. A similar analysis and appropriate limiting

conditions can be used to show that the low pass filter made up of C^, R^

acts as an integrator, again over a limited range of frequencies. For the

integrator the requirement is that the integrator time constant =» R^C^. be

long compared with the period of the lowest frequencies of interest in the

pulse to be measured. A frequency domain analysis of this system may be

summarized as Vl/VQ “ jw^/Cl + jwr

D

) , (40)

(R^C^) dV^/dt [89]. This approximate

and

V7! " 1/(1 + ja,rI

} • (41)

Here is the input voltage to the differentiating circuit, is the

differentiator output and V^is the integrator output. If R^»R^, and

the response can be written as W“lWi (42)

It must be emphasized that these are approximate expressions and provide only

qualitative guides as to what the pulse response of the circuit to different

applied waveforms. In addition, the analysis does not include the effects of

stray circuit elements such as in Fig 22. A circuit analysis program

written for use with a personal computer has been used to study the

characteristics of an E-dot/integrator circuit. Table IV summarizes the

circuit elements chosen for this study. They are representative of those

found in real laboratory systems.

Table IV: Circuit Parameters Used in Sensor Calculations

C

c - 1 P F

h

~ 5° 0

r j - 1, 5, 50 jzs

C

s - 1, 5, 50 pF

It is useful to display the frequency response of the circuit shown in Fig. 22

for the circuit parameters of Table IV. Fig. 23 shows the frequency response

49
50

Figure

23.-

Calculated

frequency

response

for

the

circuit

of

Figure

1

for

the

circuit

values

shown.
calculated for the parameters indicated. The frequency independence over an extensive bandwidth predicted by eq. 42 is clearly shown as is the effect of

changing the integrator time constant. The primary effect of the shunt

capacitance, , is not shown here but results in a loss of high frequency

response. Note that the magnitude of the transfer function is very small; 100

dB attenuation corresponds to a ratio of 10 ^ . For proper operation, the

integrator must be loaded with a high impedance (for example, the 1 megohm

input impedance of an oscilloscope) . This presents a practical limit to the

upper limit on the bandwidth of the measuring system which will be discussed

later.

In order to determine the affects of the finite bandwidth of the measuring

system on a given waveform, a double exponential waveform similar to that used

in EMP computations was chosen. This waveform was used as an input waveform

and the output from an E- dot/integrator circuit with selected components was calculated using the circuit analysis program mentioned previously. The

calculated output was multiplied by the theoretical ratio of the circuit and

the percentage error between the input and output waveforms was determined.

Examples of these calculations are given in Figs. 24-25. In Fig. 24, the

errors are shown for three different integrator time constants for the time

span up to 1000 ns. The short time calculations (up to 200 ns) are summarized

in Fig. 25. It is clear that sizable errors are associated with the 1 ns integrator are present, even on the shortest time scales. Even the 50 ns integrator errors are substantial over the 1 ,us time range.

5.1.3 Discussion

The use of E-dot sensors to measure fast rise pulses in power system

equipment presents an attractive option for selected measurements . These are situations where contacting sensors are impractical. While the field coupled

sensor could be used in an open geometry and calibrated against a well

characterized sensor using longer pulses, such a calibration is valid only for

a fixed geometry and probably is not practical for configurations where

different test objects are to be used. The requirement that the integrator

be loaded with a high impedance limits the available recording devices to a bandwidth of approximately 200 MHz, which is adequate for the waveforms of

interest. The low sensitivity of the system is not important where large

signals are being used. Because of the extensive use of digital data

acquisition systems, it is straightforward to numerically integrate the output

of the differentiating circuit, which would result in an increase in available

bandwidth since there would be no restrictions on the input impedance of the

recording system and substantially higher sensitivity.

The use of the E-dot sensor to measure fast rise pulses is presently

being investigated using the NBS test line and several sensor configurations

Means of calibrating the sensors are under consideration as are sources of

error. The present numerical analysis will be correlated with the

experimental results.

5.2 Streamer Propagation in Transformer Oil Under the Influence of

Submicrosecond Rise-Time Pulses.

51
CM

2

HI

i

H

H0HU3 XN30U3d

52

Figure

24.

Percentage

errors

for

the

double

exponential

waveform

described

in

the

text.

Time

constants

chosen

for

the

integrator

are

indicated.
25.0

o

o

M

o

o Lii

U0UU3 ±N30U3d

53

Figure

25.

Percentage

errors

for

the

double

exponential

waveform

for

the

first

200

ns.

Integrator

time

constants

are

shown

on

the

graph.
5.2.1 Introduction

A variety of phenomena can produce pulses which have risetimes of order

100 ns or less. The nuclear electromagnetic pulse (EMP) is by no means the

only source of such steep -wave front phenomena; rather, the largest problem

comes from conventional sources. Because of nearby switching operations,

nearby lightning, or due to gas- insulated- systems connected to apparatus,

conventional power apparatus can be exposed to such fast-rise pulses [91].

Thus, there is good reason to investigate the prebreakdown phenomena

associated with fast- rise pulses to see if the phenomena differs from

prebreakdown behavior associated with pulses in the microsecond regime. It is

particularly important to determine if insulation configurations exist for

which the fast -rise pulse breakdown occurs at a lower voltage than for

microsecond pulse breakdown.

Investigations were made of the prebreakdown behavior of transformer oil

when subjected to fast-rise pulses as compared to microsecond pulses. A

simple needle-sphere liquid gap geometry was used, and the prebreakdown

phenomena monitored using high-speed photography. The results show that the

fast- rise prebreakdown structures can be very different than for microsecond

breakdown. On the basis of comparisons of 60-Hz ac and dc breakdown voltages

with microsecond pulse breakdown voltages, one might be tempted to assume that

the breakdown voltage would continue to rise as the pulse rise-time decreases.

Such is not the case in general, for the prebreakdown structures may be

similar for ac, dc and microsecond pulses, but the similarity need not persist

for fast-rise pulses.

5.2.2 Apparatus

Figure 26 shows the electrode geometry and the image -converter - camera'

view of the gap. The gap was 5 mm, and the camera was set to capture ten

exposure of the gap. Three framing rates were used with the camera: 2.0 x

7 6 6 10 , 4.8 x 10 , and 2.0 x 10 frames/s (50 ns, 210 ns, 500 ns between the

start of the frames, respectively). The needle's tip radius of curvature was approximately 200 fm. After a number of shots, the gap was reset to 5 mm so that the gap never changed more than 5%. A xenon flash tube was used to

provide the illumination to produce shadowgraphs of the streamer phenomena.

Three types of pulses were applied to the electrodes as shown in Fig. 27.

A slow pulse with a rise time of several microseconds, and two types of fast

pulses. The slow pulse was generated by the discharge of a capacitive pulse

forming network into the primary of a pulse transformer. The fast pulse was generated by a specially designed Marx system which had a characteristic rise

time of 80 ns. The rise time could be further shortened by a sharpening gap.

The sharpening gap was placed just outside the cell which contained the

electrodes. It is anticipated that the rise time of the sharpened pulse was

less than 10 ns, but the rise time of the protected detection system employed

prevented a faster measurement. There was never any difference observed in

streamer propagation characteristics for the two fast pulses within the

accuracy of these data (±10%) , therefore no distinction is made between the

fast pulses in the data presented. This does not mean, however, that upon a more careful examination a difference would not be resolved.

54
I > <D P

CO O

S-i C

<DE w

cO cO Oi P u c

<D CD P E P a

(D o > pH C <D o > o CD 1 T3

CDbO P

cO <D E E

i-i CO

<D T3 5-i C P

CO

w

>•> <D U X p P

(D <D E W O O P C

(D o CD bO p 3

•H cr

<D c CD T3 o w O E U bO P CO C o 5-4 »H

(D (D E I-1 E cO

<D cO P O tp

<DP (D "3

<D JZ <D rC H P

Cu <D w & 1 ss

CD a 3

rP cO C

TD bO

CD <D

<D cp 2 O P

CN

<DU3

bO

•p

55
{

56

Figure

27.

Three

types

of

pulses

applied

to

the

electrodes.

The

slow

pulse

is

compared

to

a

fast

pulse

having

an

80

ns

rise

time

which

can

shortened

by

a

gap

external

to

the

cell

to

a

rise

time

of

less

than

10

ns
The cell and sharpening gap are shown in Fig. 28. The sharpening gap and

water resistor chamber was removed for the application of slow pulses.

Transformer oil is circulated through the sharpening- gap chamber and the cell

and is filtered continuously during the experiments. The response of the

water resistor was tested using step-response techniques to assure that the

recorded waveforms were not cluttered with artifacts of the measuring system.

Overall voltage measurement inaccuracy is estimated to be ±10% with a shot - to

shot precision of ±5%. For this preliminary work it was not considered

important to maintain better accuracy.

5.2.3 Experimental Results

5. 2. 3.1 Slow Pulse Results

Results have been obtained for both positive and negative pulses and

although there are observed differences, for reasons of brevity detailed

discussions of only positive data are displayed here. Figure 29 shows the

streamer structure for the slow pulse for positive needle. In photographs

a-e, the charging voltage on the pulse forming network is increasing;

photographs e-g were taken at the same charging voltage. Increasing the

charging voltage linearly increases the output voltage of the pulse

transformer, but for overvoltage conditions the peak is never reached and the

increase in charging voltage has only the effect of increasing the rate of

voltage rise. In general it is observed that as the voltage rises an anode

streamer is initiated and grows across the gap with a quasi- spherical shape at

a supersonic speed of roughly 4 x 10^ cm/s . The sonic speed is defined as the

speed of the post-breakdown shockwave which is (1.80 ± 0.05) x 10^ cm/s. It

should be noted that in Fig. 29a the streamer branching either increases or broadens as the streamer nears the electrode. This feature occurs sooner in

the slow streamer growth as the rate of voltage rise increases. However,

rather than growing steadily across the gap, at a critical field at one of the

tips of the streamer the streamer growth changes to a faster mode of

propagation. This critical field is reached sooner for the higher rate-ofrise voltages, and the streamer spends less time in the slower propagation

mode. The differences in the initiation of the faster propagation mode seen

in Figs. 29e-29g, are presumably due to the random nature of the streamer's

development since the charging voltage and the applied pulse shape was the same for each. Whenever the faster propagation mode initiates early in the

streamer development, the breakdown voltage will be lower than if it initiated

later in the development of the slow streamer. The statistical nature of the

initiation of the faster event will produce large deviations in the value of

the breakdown voltage as can be seen in Figs. 29e-29g show.

5. 2. 3.

Fast Pulse Results

Figure 30 shows the streamer structure for the fast pulse for a positive

needle. For low voltages the slow event propagates much as it does for the

slow pulse. However, as the voltage level of the fast pulse is raised, we reach a critical voltage over which the slow phenomena hardly appear at all.

For the positive point this occurred at approximately 86 kV. We can expect

some shot-to-shot variability in this critical voltage since during each

breakdown the tip of the needle is changed by the breakdown arc. Even so,

57
G o

HC/> <

pH 4J pH Cm

m

•pH CO G G X T3 •pH pH 0 M i X C CM T5 00 C/1 }H G G •pH

•pH G G X 3 u pH X H «pH Sh cr 0 E 0 •pH X cO T3 M pH

(—1 X • G co • pH s G H 1/1 H p E g O •pH O CO G G o U 0 pH G pH •U 00 G G U .3 G C • M CM •pH >- 5 CO c 0 G CO X G 0 G ,3 C G

T5 X G «C «M pH pH X Li M - CD Cm pH

CO •pH G G G E G g 3

r“ ,3 G • G < G

!-! H T3 .3 <D G ,C G

"O G Sh r“

CD •pH • C •’H G G M X CO G •M .C M c H G 3 G 0 o

CO cO ^ 3 •pH u

4-1 o a 4J C •U G -p-) G G a o X G 00 pH Li

CO G •pH O pH G

OO E CO co oom •pH G O G G 3 pH G C

OO Sh *m G j- c v£> C C < O 0

»r^ o U G G c r-* g a G

a) G3 •U

M • G G a G G G G O G C

•O Vh 3 X G 3

CO 3 G CO Dh T3 rH X G U •p-) *M G G C ^ G G !h G

CO G DC G 00

T5 CO G G U 3 a G C G G 4J

CO Vh pH pH CO CO 00 pH

•Q pH pH £ *M O

pH G G *pH oo >

pH CO G E G 00 c 1 0) co J-> C

r- G CO G C *pH c

•pH X G 0 CD G •i-i C u G G a x 5 G JZ !h o U X C Sh G G » -o G 00 •m G X T5 p* X 3 G .3 G G G

CO E O MEG pH G CO U C G H G pH

)H X X O X 3 p •pH S3 G 4J G G a. 73 CM

58
(b) 111 kV B, 1.05 M*, 10 kV chg (210 n*)

Figure 29. Streamer characteristics for microsecond pulses with positive

needle. The experimental conditions appear beneath each picture:

The first voltage is the breakdown voltage (kV B) , the first time

is the estimated duration of streamer propagation, the second

voltage is the charging voltage used in the pulse -generating

circuit (kV eg) , and the time in parentheses is the time between

the start of each frame. (For all photographs in this figure the

framing rate is 48 x 106 frames/s or 210 ns between frames.)

(a) 77 kV B, 1.4 fs , 5.6 kV chg (210 ns) • (b) 111 kV B, 1.05 fs, 10 kV chg (210 ns)

(c) 130 kV B,.,630 ns, 15 kV chg (210 ns)

(d) 141 kV B,' 520 ns, 20 kV chg (210 ns)

(e) 151 kV B, 500 ns, 25 kV chg (210 ns)

(f) 131 kV B, 250 ns, 25 kV chg (210 ns)

(g) 146 kV B, 600 ns, 25 kV chg (210 ns)

59
less than a 5% change in voltage about these critical voltages would switch

the streamer phenomena between their slow and fast modes of propagation.

At least four types of streamer propagation modes are visible in Fig. 30

with the positive needle: Two slow modes and two fast modes. The slow modes

start with a narrow filamentary structure which gives rise to a slightly

faster and either more branched or heavier branched (or both) structure which

moves slightly faster; see Figs. 30a and 30b. The second slow mode gives rise

to a fast mode which is much less branched but the branches are heavy. This

fast mode can then give rise to an even faster mode of propagation which shows

narrower branches; see especially Fig. 30d. We could venture explanations for

these different modes: They could be electrohydrodynamic in nature where the

slow phenomena becomes unstable and produces the faster phenomena, or we could

be observing a descrete ionization processes where deeper molecular electronic

levels are being effected providing more electrons per collisional event as the field increases at the tip of the streamer branches. However, no certainty attaches to either hypothesis, and an accurate description awaits

more refined experiments and more complete computer modeling efforts.

The critical voltage at which the slower phenomena no longer have a chance to form for the positive needle is around 90 kV. In Fig. 30b a slow

event almost bridges the gap, whereas in Fig. 30c only a small portion of the

slow event exists before a faster event takes off to bridge the gap.

In Fig. 31 we show the streamer's formative- time vs. the applied voltage

for both the slow and fast pulses. The formative time (borrowing from an older terminology: formative time lag), or streamer duration or growth time,

is the gap length divided by the average streamer speed. Fig. 32 shows the

average streamer speed as a function of applied voltage comparing the slow and

the fast pulse results . For the lower voltages where the slow events dominate

the streamer's propagation time, the apparent continuous increase in streamer

speed with voltage is an artifact of the switching from slow to fast

propagation modes and is not a continuous increase in the speed of the

streamer. The high-voltage propagation behavior shows a similar increase in

the average streamer velocity with applied voltage. However with the present

resolution of the camera it is not possible to determine if the streamer is

truly increasing its speed or if it is switching between two fast modes of

propagation giving the overall appearance of acceleration with voltage. The

medium-voltage behavior is interesting, since there is where the difference

between the streamer propagation under fast or slow pulses becomes apparent.

Here the fast-pulse streamer switches from the slow to the fast propagation

mode with a small change of voltage producing a discrete step in both the time

and average speed curves. On the other hand, the slow-pulse streamer's curves

exhibit continuity because both fast and slow propagation modes are explored

by the streamer to different extents.

5.2.4 Discussion

We have seen that there are a variety of streamer propagation modes and

structures in the prebreakdown phenomena in liquids depending upon the voltage

pulse applied. Generally speaking, slow events begin the prebreakdown

phenomena for both polarities and for slow pulses (and we would expect the same for ac and dc)

.

However, for fast pulses the streamer need not begin

with the slow mode, but can be initiated in the faster modes directly. The

60
(e) 156 kV 6, 63u ns, £5 kV eng (210 ns)

Figure 30. Streamer characteristics for fast pulses with positive needle.

Experimental conditions are noted beneath each picture: The

breakdown voltage (approximately the peak voltage) , the estimated

duration of the streamer, and the time between the start of each

frame. Note the increase in branching of the slowest phenomen in

(a) and (b) . Also note the evidence of an ultra-fast event

emi'nating from the second fast phenomena in (d) . These

photographs show at least four different types of streamer

propagation modes

(a) 55 kV B, 1.6 fs (210 ns)

(b) 86 kV B, 1.05 fs (210 ns)

(c) 86 kV B, 320 ns (210 ns)

(d) 165 k\)r B, 180 ns (50 ns)

(e) 249 k\ir B, 46 ns (50 ins)

61
TIME TO CROSS GAP VS APPLIED VOLTAGE

(a)

CL

<

C3

CO

(/)

O

QCO

O

Positive Needle

i

1 Pulse Shape

§D + SLOW

q* FAST

+ +

+

+

°B + * .

+

+ +

+ +

a £b J ^ _l 1 1 §

p c&ctpq

0 100 200 300

APPLIED VOLTAGE (kV)

(b)

CL

<

a

c/)

O)

O

cc

o

o

5

4

3

2

1

0

0 100 200 300

APPLIED VOLTAGE (kV)

Negative Needle

+

+

Pulse Shape

+ + SLOW

+ FAST

+

&V** *

+

9 V

^CtP+

1 OJ

Figure 31. Streamer growth time vs applied voltage for slow and fast pulses

with (a) positive and (b) negative needle polarity. Abrupt

transitions form slow to fast phenomena are visible when using the

fast pulse. As the slow pulse overvoltages the gap, the fast

phenomena dominate and the breakdown occurs more rapidly which

tends to even decrease the breakdown voltage.

62
AVERAGE

VELOCITY

(cm/s)

S

AVERAGE

VELOCITY

(cm/s)

AVERAGE STREAMER VELOCITY VS VOLTAGE

108

107

106

10s0 100 200 300

APPLIED VOLTAGE (kV)

108

107

106

105

0 100 200 300

APPLIED VOLTAGE (kV)

Negative Needle

1 1 1 1 Pulse Shape

+ SLOW

FAST

—i

off*

^

— ££] +

+

+-H-

+

+

i i i

a

i

Positive Needle

Pulse Shape

+ SLOW

FAST

cfip

SP

o+

+V++

+

Figure 32. Average streamer speed as a function of applied voltage for slow

and fast pulses with (a) positive and (b) negative needle

polarity. Average speed on the order of 107 cm/s (approaching

0.1% the speed of light) are noteworthy.

63
entire picture is diagrammed in Fig. 33. The slowest streamer growth occurs

for a slow pulse just above the initiation threshold. As the voltage of the

slow pulse increases, the rate of rise increases, and the breakdown will occur

at or before the peak. As a slower event moves toward the opposing electrode

we can imagine the field at the tips of the streamers will increase due to the

conductivity of the streamer, but at the same time, due to the proximity of

the nearby branches of the streamer, the field is graded somewhat [92]. When

a critical field is reached at any tip, a faster phenomena can be initiated.

Under the conditions of a faster-rise pulse, that critical field is reached

sooner in the development of the slow phenomena. If the pulse rate -of- rise is

fast enough, then most of the streamer development is dominated by the fast

events. Since for these extreme overvoltage conditions little time is spent

in the slow mode, the breakdown voltage can actually be lower that for a pulse

which has a slower rate-of -rise

.

With the application of the fast pulse, it is possible to reach the

critical field at the tip of the needle before or, at most, just after the

streamer initiates. In such a case the breakdown voltage can be lower than

for the extreme overvoltage slow pulse. In these experiments it was not

possible to breakdown the gap with a fast pulse at a lower voltage than the

minimum breakdown voltage for the slow pulse. Based on these measurements,

the inception voltage of the slow event would be roughly half that of the fast

event. The statistical nature of the initiation of the slow event is

suggested by the trend observed that as the rate-of-rise of the slow pulse

increased the initiation voltage of the slow event increased, however no attempt was made to resolve the initiation process in detail and under high

magnification.

It must be stressed that the system under investigation here is simple:

a divergent electrode geometry in oil. Although the breakdown voltage for the

fast pulse is not as large as expected compared to the slow pulse because of

the complications due to streamer propagation modes, we need not think that

fast pulses pose no danger for more complicated systems involving composites

such as pressboard, paper, and oil. Indeed, the growth of an initiated event

in the liquid medium, either via a partial discharge or microsecond pulse, may

be stopped by any solid insulation material in the direction of its growth.

The slow phenomena generated because of the more gentle pulse may not have the

energy to penetrate or puncture the solid material, and the solid dielectric

will accumulate surface charges to grade the field and prevent further growth

of the streamer. However, if a fast pulse initiates a fast event, there may

be sufficient energy to puncture the solid insulation material. Such a scenario might render the composite system to be more vulnerable to fast

pulses than microsecond pulses. However, we have not produced data here to

confirm this possibility, and only offer it as a hypothesis which will be

subjected to experiment in the near future.

5.2.5 Conclusion

The simple needle-sphere electrode system in oil investigated here has

produced different streamer propagation characteristics for fast, nanosecond

pulses than for slow, microsecond pulses. Because of the changes in the

propagation modes of the streamer as it grows, it is possible to produce

breakdown by a fast pulse at a lower voltage than would be expected from

looking at the breakdown voltage produced by extreme overvotage conditions

64
TIME

VOLTAGE

Figure 33. Effects of a fast pulse vs a slow pulse

As the slow pulse overvoltages the gap,

increasing in overvoltage,

less of the streamer

development is found in the slower propagation modes. Eventually,

the slower pulse can reach such an overvoltage condition that the

streamer spends little time in the

the breakdown voltage can be lower

slower mode. Under such

for the fast rise pulse.

a case Breakdown voltages with fast rise pulses can be even lower

65
from a slow pulse. Although it was never possible to cause the liquid gap to

fail at a lower voltage with the fast pulse than with the minimum breakdown

voltage from the slow pulse, concern is expressed over the possible

implications the different streamer propagation modes can have on composite

insulation systems

.

66
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[87] E. F. Kelley and R. E. Hebner, "Electro-Optic Field Measurement at a Needle Tip and Streamer Initiation in Nitrobenzene," Annual Report,

Conference on Electrical Insulation and Dielectric Phenomena,

pp. 171-177, October 1986.

[88] E. C. Cassidy, R. E. Hebner, M. Zahn, and R, J. Sojka, "Kerr-Effect

Studies of an Insulating Liquid Under Varied High-Voltage Conditions,"

IEEE Trans. Elec. Insul., Vol. EI-9, No. 2, pp 43-56, June 1974.

[89] J. Millman and H. Taub , "Pulse, Digital, and Switching Waveforms",

McGraw Hill, 1965, pp 38-50.

[90] M. Wilkinson and E. Chu, "Calibration of Capacitive Voltage Probes in

Water-Dielectric, High Power Pulse Generators", in measurement of

Electrical Quatntities in Pulse Power Systems, R. H. McKnight and

R. E. Hebner, eds .

,

NBS Spec. Pub. 628, 1982.

[91] Conference Record of the 1986 IEEE International Symposium on Electrical

Insulation, see session D -- Steep Wavefront Phenomena, pp . 145-169. In

particular, see "Impact of Steep Front, Short Duration Impulses on Power

System Apparatus and Insulation: A Critical Review," L. M. Burrage,

P. F. Hettwer, B. W. McConnell, p. 159 (June 1986).

72
[ 92 ] E. F. Kelley and R. E. Hebner, "The Electric Field Distribution

Associated with Prebreakdown Phenomena in Nitrobenezene , " J. Appl.

Phys., Vol. 52, No. 1, pp 191-195, January 1981.

73
U.S. OKPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSXR 87-3643

2. Performing Organ. Report No. 722

3. Publication Date September 1987

4. TITLE AND SUBTITLE

Research for Electric Energy Systems - - An Annual Report

5. AUTHOR(S)

R. Hebner

6. PERFORMING ORGANIZATION (If joint or other than N BS, see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7e Con tract/ Grant No. Task Order #137

I. Type of Report & Period Covered

Annual - 1986

9.

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

Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, D.C. 20585

10.

SUPPLEMENTARY NOTES

PH Document describes a computer protram; 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

bibliography or literature survey, mention it here)

This report documents the technical progress in the five investigations which

make up the project "Support of Research Projects for Electrical Energy Systems",

funded by the U.S. Department of Energy. To support the measurement of ions in the

dc transmission line environment, techniques to measure ion mobility were evaluated.

In addition, techniques were developed to determine the sensitivity of an AM radio

to detect partial discharges in a biological exposure facility. Within the project

to develop measurement techniques and obtain basic data for gaseous dielectrics, a theoretical method was developed to evaluate the consistency among electron collision,

transport, and dielectric strength data for binary gas mixtures; the gas phase

hydrolysis rates for SOF2 anc* ^4 were measured; the corona discharge oxidation

mechanisms were identified and the role of negative ions on the SOF4 yield was determined. Progress in interfacial measurements included the optical measurement of

the electric field distribution as a streamer initiates. Progress in developing

measurements for nanosecond dielectrics was in the characterization of the errors in

the measurement of voltage pulses using E-dot probes and the identification of the

reducation of breakdown voltage in oil for faster pulses.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s)

13.

AVAILABILITY

"

Unli mi ted f~l For Official Distribution. Oo Not Release to NTIS

CD 9

rder From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20-402.

Order From National Technical Information Service (NTIS). Springfield, VA. 22161

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PRINTED PAGES

15. Price

USCOMM-OC S043-P80