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> Research for Electric Energy

Systems - An Annual Report

NBS

PUBLICATIONS

R. E. Hebner, Editor

U.S. DEPARTMENT OF COMMERCE

National 3ureau of Standards

Center for Electronics and Electrical Engineering

Gaithersburg, MD 20899

December 1985

Issued March 1986

Prepared for: Department of Energy

QC >f Electric Energy Systems

iqq ependence Avenue, SW

on, DC 20585

. U56

86-3316

1986

C. 2

HBS BESEARCH INFORMATION

NBSIR 86-3316

RESEARCH FOR ELECTRIC ENERGY

SYSTEMS - AN ANNUAL REPORT

R. E. Hebner, Editor

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Gaithersburg, MD 20899

December 1985

Issued March 1 986

Prepared for: Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

Foreword

This report summarizes the progress of five technical investigations

during FY 85. 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.

iii
LIST OF FIGURES

Page

Figure 1. AMFM-I frequency response as function of signal

amplitude. Frequencies at which clipping occurs

(see text) for the various scales are indicated 4

Figure 2. Oscilloscope display of current waveform

superimposed on waveform of output signal from

AMFM-1. Amplitudes of two waveforms have been

made equal for purposes of comparison

Figure 3 . Spectrum analyzer display indicating frequency

components of the waveforms shown in figure 2.

The traces for the two waveforms are superimposed

Figure 4. Schematic view of NBS parallel plate apparatus.

The arrangement shown is appropriate for generating

an electric field with positive space charge. The

top and bottom plates of the parallel plate system

are located in the planes z = 0 and z = d. The

corona wires are oriented parallel to one another

and are perpendicular to the plane of the figure.

Figure 5. Normalized ion density profiles for two current

- 6 2 - 6 2 densities, 1 x 10 A/m and 0.1 x 10 A/m . The calculated results assume that V^_ - 2650 V,

K = 1.3 x 10 ^m^/Vs , and d - 0.17 m

6

6

8

10

Figure 6. Schematic view showing perturbations of electric

field and charge density due to an aperture in

the ground plane and air motion do not extend

across the parallel plate spacing

Figure 7. Measurements of space potential perturbation

due to 5 cm x 5 cm aperture in ground plane as

a function of height above ground plane. . . .

Figure 8. Ion trajectories for air speed of 1 m/s in plane

of aperture. The solid curves show influence

of 15 kV/m electric field. Dashed curves are for no electric field. The time-of-flight for

all trajectories is 50 ms. The volume occupied

by the ions that enter aperture when E * 0,V',

exceeds volume of air,V, which enters in the same time. Inset shows effect of mobility value

on trajectory near aperture

Figure 9. Ion trajectories for air speed in plane of

aperture of 6 m/s. The solid curves show

influence of 15 kV/m electric field. Dashed

. 10

13

18

vi
5.6 Liquid Breakdown Phenomena 49

5.7 Partial Discharge Measurements and Their Photography 52

5.8 Summary 53

6. NANOSECOND BREAKDOWN IN POWER SYSTEM DIELECTRICS

Subtask No. 05 53

6.1 Introduction 53

6.2 Test Facilities 54

6.3 Mathematical Methods 55

6.4 Electro-Optic Methods 60

6.5 Streamer Growth in Dielectric Fluids under Fast Risetime Pulses . 62

6.6 Summary 66

7. REFERENCES. 71

v
TABLE OF CONTENTS

Page

Foreword iii

LIST OF FIGURES vi

LIST OF TABLES x

Abstract 1

1. INTRODUCTION 2

2. AC ELECTRIC AND MAGNETIC FIELDS

Subtask No. 01 2

2.1 Introduction

2.2 Ambient Magnetic Field Meter 2 2 . 3 Summary 5

3. DC FIELDS AND ION MEASUREMENTS

Subtask No. 02 5

3.1 Introduction

3 . 2 NBS Parallel Plate Apparatus and Approach

3.3 Space Potential Perturbation Measurements 12

3.4 Discussion 14

3.5 Ion Trajectories and Losses 15

3.6 Summary 19

4.

GASEOUS DIELECTRICS RESEARCH

Subtask No. 03 20

4.1 Introduction 20

4.2 Influence of Trace Amounts of Water Vapor

on Electron Avalanche Growth in SF^ 20

4.3 Common Parameterizations of Swarm and Breakdown

Data for Binary Gas Mixtures 23

4.4 Corona- Induced Decomposition Rates of SF,

D in SF^/N-, SF,/Ne, and SF,/0o Mixtures 30

O Z O O Z 4.5 Models for Decomposition of Gases in Corona Discharges 3 3 4.6 Summary -*0

5.

INTERFACIAL PHENOMENA IN LIQUIDS

Sub task No. 04 40

5.1 Introduction *0

5.2 High Precision Field Measurements: Room Temperature AC & DC 2 5.3 Space Charge in Transformer Oil at Room Temperature - - AC & DC

5.4 Space Charge in Transformer Oil at 125 °C - - AC & DC . . . 5.5 Electrode Material Effects on the Generation of Space Charge. *3

iv
18

curves are for no electric field. The time-offlight for all trajectories is 50 ms. The

volume occupied by the ions that enter aperture

when E ^ 0,V', exceeds volume of air,V, which

enters in the same time

Figure 10a. Measured pulse height distributions for electron

avalanches formed near a positive point electrode

in SF, at a pressure of 200 kPa for the indicated

b

different water vapor concentrations and applied

voltages

Figure 10b. Measured electron avalanche repetition rates

versus applied voltage at the indicated water

vapor concentrations for avalanches larger

than 0 . 08 pC .

Figure 11. Calculated differential electron detachment

probabilities versus location between the point

electrode surface d and the point x

c at which

the ionization and attachment coefficients for

the gas are equal for two different applied

voltages. The different lines designate

different assumed energy dependences of the

cross sections for collisional detachment of

negative ions with different indicated detachment

thresholds e

Figure 12. Fits (solid lines) to normalized critical fieldto-gas density ratios for the mixtures SF,/N0 b Z and CCi

2 F2 /N2

Figure 13 . Examples of data on measured oxyfluoride

concentrations versus net charge transported

in the corona discharge for different indicated

SFg/Ne and SF^/t^ mixtures from which production

rates were determined

Figure 14. Examples of data on measured oxyfluoride

concentrations versus net charge transported

in the corona discharge for different indicated

SF,/Ne and SF-/0- mixtures from which production

rates were determined

Figure 15. The complete set of electron collision cross sections for SF, used to calculate electronb

induced dissociation rates of SF, in the

b

mixtures SF,/N0 , SF-/0-, SF,/Ho 0, and SF,/Ne

b Z b Z b Z b

from numerical solutions of the electron

22

22

24

27

31

32

vii
transport equation. The attachment, ionization,

momentum transfer, vibrational excitation, and

electronic excitation cross sections are

i eff respectively denoted by Q^, Qq

, and

Figure 16. Measured charge rates -of -production for the

oxyfluorides and S0o normalized to the SF,

Z o concentration versus percent concentration of

, 0 and Ne in the mixtures SF^/^ , SF^/O^,

and SFg/Ne. Also shown are total oxyfluoride

plus SC>2 rates (r ) in comparison with the

predicted upper limits for the total rates for

SFg decomposition

Figure 17. Electro-optical, Kerr-effect, dc field measurements

in transformer oil at room temperature. Solid

lines show the negative polarity, dashed lines,

positive polarity. The numbers in parentheses

are the average applied field. In b) the

normalized field is shown to emphasize the

enhancement. In c) the normalized fields are shown for the 63 kV profiles to illustrate the

temporal behavior of the enhancement after

changing polarity. The dotted line is the field

found just after the polarity is changed from

positive to negative. After a few minutes the

enhancement for the negative applied voltage

gradually becomes that shown by the solid line . .

Figure 18. Normalized electric fields for 60 Hz ac at room temperature. NP and PP refer to negative peak

and positive peak respectively. The sudden drop

in the positive peak profile near the HV electrode

is an artifact of the detector and not some unusual

decrease in the field near the plate. The average

peak field is 232 kV/cm

Figure 19. Electric fields for dc at 125°C. At lower fields

there is less space charge near the cathode

Figure 20. Electric fields for 60 Hz ac at 125°C. NP and

PP refer to the negative and positive peaks of

the 60 Hz waveform respectively. The average

peak field is 224 kV/cm. The space charge,

which is proportional to the derivative of

the field, is shown for the negative peak profile. .

Figure 21. Multi-metal electrode configuration to reveal

dramatic difference between electrode metals

in production of space charge while employing

34

39

44

46

48

viii
the same liquid. The gap is 0.635 cm, plate

width is 1.27 cm and the length of the plates

is 15 cm (into the page) 50

Figure 22. Intensity profiles for different electrode

materials under the application of 50 kV dc,

both polarities, room temperature. No attempt

has been made to compensate these data for

nonuniform incident light intensity. Since

large diameter windows having a residual

birefringence were used the intensity profiles

on the average have a slight slope. The

important feature is that the same space -charge

behavior is noted independent of electrode

material 51

Figure 23 . Waveforms used in studies measured directly

from pulse generator. The test waveform does

not replicate beyond 40 ns as indicated. ....... . ... 56

Figure 24. Output signals from a capacitive sensor for

test line input signals shown in figure 1. ..... . . ... 56

Figure 25. Comparison of deconvolved and input signals

for the capacitive sensor. Signals have been

offset for comparison. ................ . ... 57

Figure 26. Comparison of the output signal from a resistor

divider with the input signal. ............ . ... 57

Figure 27. Deconvolved and directly measured input

signals as obtained using the resistor divider.

Signals have been offset for comparison. ....... . ... 59

Figure 28. Schematic of system to measure electrical

quantities using electro-optic sensors ........ . ... 61

Figure 29. Trapezoidal waveform used in Pockels effect

response calculation. For the calculations

shown in figure 30, only the maximum electric

field is changed . ... 63

Figure 30. Normalized intensity of light for Pockels

effect measurement described in text and for

waveshape shown in figure 29. Raggedness in

waveshape is due to coarseness of time grid

used for calculations, a. E =* 0 . 5 Mv/m. m

b. E -1 Mv/m. c. E =2 Mv/m

m m

. ... 64

Figure 31. Configuration used to determine the response

of the water resistor used to measure the

voltage waveform applied to the gap. Response

is shown schematically on the figure . ... 65

ix
Figure 32

Figure 33

Figure 34.

Cross sectional view of system used to study

liquid dielectric breakdown processes under

nanosecond pulse conditions. Construction

details, such as types of materials, have been

omitted for clarity. The flow of oil through

the sharpening gap and the test region is

indicated, as is the water capacitor used for

pulse shaping 67

Representative images obtained from high speed

photography of the breakdown process. The

voltage waveform changes from zero to the

maximum value between frames 2 and 4. The

initiation of the supersonic streamer growth

is observed in frame 4, and the breakdown

process is complete in frame 6. There is no slow bush- like growth like that observed in

studies using slower waveforms

Comparisons of framing camera pictures of

streamer growth in N- Hexane showing two

different streamer speeds. a. Breakdown

voltage of 120 kV, with time between exposures

of 500 ns. Order of exposure is that shown in

(fig. 33). b. Breakdown voltage of 240 kV,

with 50 ns time between exposures. 7.05, 6.28

(±0.4)

68

69

Table 1.

Table 2.

LIST OF TABLES

Space Potential Perturbations . . .

Possible negative ion formation and destruction

processes in binary, molecular gas mixture . . .

12

29

x
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's

Office of Energy Systems Research and performed in the

Electrosystems Division of the U.S. National Bureau of

Standards. To support the measurement of ac electric and

magnetic fields, a magnetic field meter was designed,

constructed, and calibrated. The meter has bandwidth and

sensitivity sufficient to measure ambient magnetic fields. The

measurement support for dc fields and ions focussed on techniques to calibrate ion counters. A standard ion source was characterized and the effects of air flow were modeled. Within

the project to develop measurement techniques and obtain basic

data for gaseous dielectrics, the influence of trace amounts of

water on avalanche initiation in SF, has been measured and the

6 results have been explained in terms of electron detachment of

negative ions. In addition, a method for fitting the variation

of the breakdown strength of binary gas mixtures with changes in

concentration was developed and evaluated using published data.

The decomposition rates induced by corona in mixtures of SF^

with nitrogen, with oxygen, and with neon were measured. The

results are explained using a theoretical model for chemical

decomposition of gases in high-pressure corona. Progress in the

measurement of the electrical behavior at the interfaces in

liquid- solid composite insulation included improvement of the

instrumentation used to measure the electrical field and the

space charge in transformer oil so that the inaccuracy is less

than 1% and the imprecision is less than 0.2%. Using this

system, it has been shown for the oils and field strengths used

in this study that the space charge density is relatively

unaffected by a change of electrode material from steel to

copper to aluminum to brass. Using high speed photography, it was shown that hydrostatic pressure has a significant effect on

the prebreakdown streamers. Finally optical and electrical

measurements were combined to obtain insight into the nature of

partial discharges in insulating fluids. Techniques to evaluate

sensors used to measure nanosecond voltage and current pulses

have been explored in the project dealing with nanosecond

breakdown in power system dielectrics. Deconvolution methods to

evaluate sensor behavior have been developed and used. Initial

measurements suggest that the propagation of prebreakdown

streamers in power system dielectrics may be different under

nanosecond pulses than under microsecond pulses in oil.

1
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 1985. The Interagency Agreement specifies five tasks which are to

be performed and a description of the technical progress in each task

constitutes a section of this report. Section 2 describes the work in ac electric and magnetic fields and section 3 describes the investigation of dc

field and ion measurements. Section 4 summarizes the development of the

measurement techniques and the accumulation of fundamental data needed for

gaseous dielectrics research. Section 5 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 6.

2. AC ELECTRIC AND MAGNETIC FIELDS

Task 01

Martin Misakian and P. Michael Fulcomer

Electrosystems Division

National Bureau of Standards

2.1 Introduction

The objective of this investigation is to provide electrical measurement

support for efforts to determine the effects of power frequency electric and

magnetic fields on biological systems. To accomplish this objective, NBS

establishes and maintains facilities to evaluate the performance of probes

used by researchers in biological investigations as well as fieldmeters used

in the vicinity of transmission lines.

Recent speculation on the possible bioeffects of weak power frequency

magnetic fields has focused attention on the characterization of such fields.

NBS has developed an ac magnetic fieldme ter- spec trum analyzer combination for

measuring ambient magnetic fields with frequencies from about 10 Hz to 3 kHz

A description of the fieldmeter's characteristics is given below.

2.2 Ambient Magnetic Field Meter

A portable, battery powered, magnetic field meter has recently been

developed to provide improved accuracy in the measurement and analyses of low

level ambient magnetic fields produced, both indoors and outdoors, by 60 -Hz

power lines and equipment. It is designated "AMFM-1" for Ambient Magnetic

Fieldrneter-Version #1. The fieldmeter consists of two parts: (1) a magnerie

field probe consisting of 2000 turns of electrically shielded wire wound -'n t donut-shaped non-inductive core, approximately 10-cm in diameter, which : s connected by standard 50-Q coaxial cable to (2) a chassis box approximate lv

7-cm x 20-cm x 25-cm deep, which contains a detector consisting of amplifiers

an integrator, buffers, and battery-power supply. Also included on the

chassis box is a small analog display for ease in determining the maximum

field at a given location and for checking battery voltage. The rms value >: the output signal is read with a portable multimeter (provision has been made

2
for later incorporating a digital display on the chassis box). A portable,

low-frequency (5 Hz to 50 kHz), spectrum analyzer is also included as part of

the measurement system. Connection of the spectrum analyzer to the AMFM-1

permits the determination of the harmonic components in the magnetic field.

The percentages of the harmonics can be measured with an uncertainty of less

than ±5 %

.

-7 -4

Five ranges from 0.2 x 10 tesla (0.2 milligauss) to 2 x 10 tesla

(2 gauss) full scale are provided by the AMFM-1. Measurements down to

20 x 10

^

tesla (20 microgauss) are possible on the most sensitive range with

an uncertainty of less than ±2%. The frequency response is, at present,

limited mainly by the probe (pickup coil) and varies less than -ldB from

approximately 10 Hz to 3 kHz, thus allowing complex fields consisting of a 60 Hz fundamental and various percentages of harmonics and subharmonics to be

characterized. Between 30 Hz and 600 Hz, the frequency response is flat to

within ±1%. The frequency response of the detector (not including the probe)

varies less than ±ldB from 10 Hz to more than 20 kHz.

Current drain from each of two 9 -volt alkaline batteries is approximately

6 mA thus allowing at least 24 hours of continuous operation of the AMFM-1

(and somewhat longer total hours of operation when used intermittently) on one set of batteries.

The signal from the probe is proportional to the derivative of the

magnetic field. Consequently, the percentage of each harmonic present in the

magnetic field is weighted by its harmonic number. Thus, in addition to

amplifying the signal from the probe, the detector integrates the signal to

eliminate the undue weighting. However, the integrator must follow some stages of amplification to avoid signal- to-noise problems in the integrator.

One potential difficulty resulting from this approach is that signals due to

higher order harmonics, with their undue weighting and amplification, can

overload the last amplifier stage i.e., "clipping'' occurs. A reduced overall

output would solve this problem but at the expense of good low noise

performance on the most sensitive scale. Other solutions would be to reduce

the dynamic range of the instrument or to place the integrator in the middle

of the amplifier stages and use a more complicated range switching

arrangement. None of these alternatives were used in the present instrument

because each has an offsetting disadvantage and because even at full scale

output (where the clipping would first occur), the AMFM-1 responds to signals

- l\ up to 600 Hz without clipping on all but the 2 x 10 tesla range. This means

that in most practical situations (where ambient fields are less than

-4

-0.2 x 10 tesla) the 10th harmonic of 60 Hz could be equal to 100% of the

fundamental and still be measured correctly by the AMFM-1. For output signals

which are a lower percentage relative to the fundamental, the harmonic

frequency that can be measured (i.e., no clipping occurs) increases as shown

in figure 1. As an example, the harmonics of a signal whose fundamental

amplitude is at the low end (15% or less) of full scale can be measured to

beyond 3 kHz, even if the harmonics are equal in amplitude to the fundamental.

The same approximate 3 kHz limit applies to harmonics which are less than

approximately 15% of a full scale fundamental. This performance will enable

accurate measurement for nearly all practical situations and further

complication of the design was not warranted.

3
*

o

*

CO

*

CO

*

o

o

CO

o

o

CO

o

o

o

co

o

CO

N

X

>

o

z

HID

o

UJX

(©ibos iin* *0 %) aarundwv ivnois i-wjwv

4

Figure

1.

AMFM-1

frequency

response

as

function

of

signal

amplitude.

Frequencies

at

which

clipping

occurs

(see

text)

for

the

various

scales

are

Indicated.
- q. On the least sensitive range (2 x 10 tesla) , clipping of harmonics

which are 100% of the fundamental will occur at full scale output for

frequencies above 120 Hz. The clipping point increases to 240 Hz for 50%

harmonics and to 1.2 kHz for 10% harmonics. Figure 1 gives a summary of the

above results.

A check to determine if clipping is occurring can be made by switching

the detector to the next higher (less sensitive) range. The signal reaching

the integrator will now be a smaller percentage of full scale and less, or no clipping will occur. The readings for the two ranges will remain essentially

unchanged if no clipping is occurring.

Calibration of the AMFM-1 on all but the two most sensitive ranges was performed using a magnetic field produced with a Helmholtz coil. Because of

the perturbations of ambient magnetic fields, a voltage injection technique is

used to check the calibration of the two most sensitive ranges.

Figure 2 shows an oscilloscope display of the current waveform to the

Helmholtz coils (which coincides with that of the magnetic field) superimposed

on the waveform of the output signal from the AMFM-1. The current to the

Helmholtz coil is produced with a programmable waveform generator-power

amplifier combination and contains a 3rd harmonic with a magnitude of about

40% of the fundamental, a 5th harmonic which is 19% of the fundamental, a 7th

harmonic which is 8.5% of the fundamental, and a 9th harmonic which is 4% of

the fundamental. The phases of the 3rd and 5th harmonics have been shifted

+90° relative to the 60 Hz fundamental and the 7th and 9th harmonics are in

phase with the fundamental. The amplitudes of the two waveforms in figure 2 have been made the same for purposes of comparisons. Figure 3 is the spectrum

analyzer display for the two waveforms shown in figure 2. No differences

between the two superimposed oscilloscope traces in figures 2 and 3 are discernable, indicating that for the frequencies shown the AMFM-1 adequately

characterizes the spectral composition. A fuller account of the design,

calibration procedure, and measurement uncertainties for the AMFM-1 is being

prepared.

2 . 3 Summary

To aid in the characterization of ambient ac magnetic fields, a portable

magnetic f ieldmeter- spectrum analyzer system has been developed to measure

fields as low as 20 x 10

^

tesla for frequencies from about 10 Hz to 3 kHz.

3. DC FIELDS AND ION MEASUREMENTS

Task 02

Martin Misakian and Ronald H. McKnight

Electrosystems Division

National Bureau of Standards

3.1 Introduction

The objective of this project is to develop methods to evaluate and

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

electric field, the space charge density, the current density, and related

5
Figure 2. Oscilloscope display of current waveform superimposed on waveform

of output signal from AMFM-1. Amplitudes of two waveforms have

been made equal for purposes of comparison.

Figure 3. Spectrum analyzer display Indicating frequency components of the

waveforms shown in figure 2. The traces for the two waveforms are superimposed.

6
electrical parameters in the vicinity of high-voltage dc transmission lines

and in apparatus designed to simulate the transmission line environment.

To carry out this objective, it is necessary to evaluate and/or to

develop instrumentation required to characterize the electrical environment in

exposure facilities and near transmission lines. The specific investigations

required include the determination of the effect of ion current on electric

field measurement, the characterization of the effects of electric fields on

the collection efficiency of aspiration- type ion measurement instruments , and

the identification of the size and type of ion which makes up space charge.

The primary focus during this reporting period was the evaluation of the

performance of ion measurement instruments.

A method for experimentally calibrating an aspirator- type ion counter

presently does not exist because of the lack of a standard ion source. In

earlier reports [1-3]

,

the NBS parallel plate system [4] was theoretically

examined to determine if it could be used to produce a known ion density for

calibration purposes. Originally designed for establishing a known dc

electric field with space charge (for calibration of dc electric field

strength meters) , the ion density profile between the parallel plates is

readily calculated. We present here the results of recent measurements which

establish, under certain conditions, the suitability of the parallel plate

apparatus as a means for producing a known unipolar ion density. In addition,

the effects of an electric field and air motion on ion transport into an aspirator- type ion counter operating in the ground plane are examined by

calculation of ion trajectories into the ion counter.

The design and operation of the NBS parallel plate apparatus has

previously been described in detail [4] and is discussed here only briefly. A

schematic view of the apparatus is shown figure 4. Ions generated by

stainless steel corona wires are directed both upward (to the cap) and

downward to the first screen. Ions not collected on the first screen continue

downward to the second screen which is the top "plate" of a parallel plate

system. The ions which pass through the top plate travel a distance d to the

bottom plate and form the current density J. Electrically isolated current

sensing patches mounted on the bottom plate are used to measure J. The side

dimensions of the parallel plates are 1.7 m x 1.7 m and the spacing is near

0.17 m for most of the present studies.

The solution of Poisson's equation in one dimension yields expressions

for the electric field strength, E(z)

,

and electric space potential, $(z)

,

between the parallel plates [4]

;

3.2 NBS Parallel Plate Apparatus and Approach

( 1 )

$(z) = V -(Kc /3J)[(E 2 + 2Jz/Kc ) 3/2

-

E

3

]

,

t o o o o

( 2 )

7
CAP

Figure 4. Schematic view of NBS parallel plate apparatus. The arrangement

shown is appropriate for generating an electric field with

positive space charge. The top and bottom plates of the parallel

plate system are located in the planes z - 0 and z - d. The

corona wires are oriented parallel to one another and are perpendicular to the plane of the figure.

8
where is the potential of the top plate (z=0) , E

q

is the electric field at

the top plate, K is the average ion mobility, and e is the permittivity of

free space.

Following procedures described in [4]

,

the values of K and E

q can be

determined experimentally, thus permitting the calculation of E from eq (1)

for a given current density. The constitutive relation for conduction current

density

,

J = pKE (3)

can then be used to determine the charge density, p, as a function of z. It

should be noted that J is approximately constant between the parallel plates

because there are no sinks or sources of ions in this region. The ion density

decreases and the electric field increases as a function of distance, z, from

the top plate.

Figure 5 shows the results of model calculations of normalized positive

r r) / 0 ion density for J = 1.0 x 10 A/m , K = 1.3 x 10 m /Vs, V^_ = 2650 V, and

d = 0.17 m. A decrease of 49% in ion density occurs between the top and

bottom plates. Decreasing J by a factor of ten while keeping K, V and d

fixed results in a decrease of only 6.0%. The relatively small change in p for the second case suggests that the parallel plate apparatus might be used

for generating a "known" or calculable ion density (i.e., in this example,

known within about ±3%) for an aspirator- type ion counter operating in the

ground plane. However, in order to use eqs (1) and (3) to calculate the

unperturbed charge density profile, it must be shown that (1) the air motion

due to the operation of the ion counter perturbs the ion density only in a volume near the aperture of the ion counter and does not extend significantly

to the upper regions of the parallel plate system, and (2) it must also be

demonstrated that the perturbation of the ion density due to the aperture of

the ion counter is restricted mainly to the volume near the aperture.

Figure 6 shows schematically the conditions described above.

An estimate of the electric field perturbation due to an aperture in the

ground plane can be made by numerically calculating the electric field for the

geometry shown in figure 6, in the absence of space charge. Such a calculation has been performed at NBS [1]

,

but the uncertainties in the field

values and the associated space potentials are not well characterized.

Analytic expressions for the electric field and potential for a circular

aperture in the ground plane of a large parallel plate system have been

derived by Fry [5]. The electric potential, $' , and field, E', along the

symmetry axis (z-axis) are given by

- E^ [z/2 + R/7r + (z/tt) tan "'‘(z/R)], (4)

and

E' = E [ (1/2) + zR/tt (R

2 +z

2

) + (1/tO tan"

1 (z/R) ] , (5)

9
igure 5. Normalized ion density profiles for two current densities, 1 x — P 10 A/m and 0.1 x 10 A/m . The calculated results assume

-4 2

that V = 2650 V, K - 1.3 x 10 m /Vs, and d = 0.17 m.

Z

Figure 6. Schematic view showing perturbations of electric field and charge

density due to an aperture in the ground plane and air motion uo not extend acros-s the parallel plate spacing.

10
where is the unperturbed uniform electric field (prior to introduction of

aperture) and R is the radius of the aperture. Unlike eqs (1) and (2), z is

taken to be zero in the ground plane in eqs (4) and (5)

.

Expressing eqs (4)

and (5) as normalized by the unperturbed potential (E^z) and unperturbed

electric field (E^) , respectively, we have

($'/E z) = [1/2 + R/ttz + (1/tt) tan"

1 (z/R) ] , (6)

and

(E'/E ) = [1/2 + zR/tt(R

2 +z2

) + ( 1/tt) tan"

1

( z/R) ]

. (7)

As z increases, the departure of $>' and E' from the corresponding uniform

field values decreases and the magnitude of the electric field perturbation

approaches twice the value of the space potential perturbation [6], i.e.,

limit I (E ' -E )/E I - 2|($'-E z)/E I . (8)

Z-KB ' u u

1 1 u u'

At a height of two diameters above the aperture (along z-axis)

,

the

perturbation of is + 0.16% and the perturbation of E' is -0.31%. At a given height, the perturbations decrease as a function of distance from the

z-axis [ 5 ]

.

The perturbation of the electric field and charge density due to the air

motion has not been calculated, but knowledge of the air speed as a function

of distance from the aperture provides some insight into the problem. The air

motion above the aperture has been modelled theoretically and calculated air

speed values along the z-axis compare favorably with experimentally determined

values [3]. The air speed along the z-axis is given by [3]

u = u

q

[1 - z/ (R

2 +z2

) 1/2

] , (9)

where u

q

is the air speed in the plane of the aperture and R is the radius of

the aperture. The "effective radius" of the square hole used for the

2 experimental measurements was found by setting ttR equal to the area of the

hole. At a point which is four radii above the aperture, the calculated air

speed is 3% of u . To put this result into perspective, it should be noted

that the air speed at the entrance of a commercial ion counter can be as high

as 1 m/s. This compares with a drift velocity of about 1.4 m/s for a positive

ion in a 10 kV/m electric field. Thus, close to the aperture, the air speed

can be comparable to the ion drift velocity and significant perturbation of

the ion density (in addition to perturbation due to aperture) may occur. However, it appears that the influence of the air motion caused by the fan in

the ion counter at a point which is four radii above the aperture, -3 cm/s in

the above example, will be small.

11
The above discussion suggests that a parallel plate apparatus with a spacing greater than four times the ion counter aperture and with an ion

density profile that changes by less than about 6% may be suitable as a standard ion source. It is recalled, however, that the discussion regarding

perturbation of the electric field due to an aperture in the ground plane did

not consider the presence of space charge. Because the introduction of an aperture affects the surface charge density on the parallel plates as well as the volume charge density between the plates, field perturbations due to the

aperture are expected to be greater than for the space -charge free case considered above. Measurements of the electric space potential have been made

in the NBS parallel-plate system as a function of distance above an aperture

in the bottom plate and compared with space potential values without an aperture. The effect of air flow through the aperture on the space potential

has also been examined. An estimate of the electric field perturbation is

then made using eq (8)

.

3.3 Space Potential Perturbation Measurements

Measurements of the electric space potential between the parallel plates,

in the presence of space charge, were made with a space potential probe very

similar in design to one reported- by Penney and Matick [7] and Matick [8].

- 6 2 With V

t

=2650 V, d=0.17 m and J=0.25 x 10 A/m , measurements of the space

potential were made as a function of height above the bottom plate, with and

without a 5 cm x 5 cm aperture. The space potential probe was located over

the aperture and could be made to move vertically with a screw- driven vertical

displacement apparatus located outside the parallel plate system. The "noaperture" condition was established by sliding a sheet of stretched aluminum

foil over the aperture. Figure 7 shows examples of graphs of probe potential,

V , versus guard potential, V , near the intersections with the line V -V at

P

6 P g P g

various heights above the bottom plate, with and without the aperture. The

perturbation of the space potential due to the aperture are expressed in volts

and percentages at each height. For purposes of comparison with perturbations

predicted by eq (6), which is valid only for space-charge free conditions, the

2 2 "radius" of the square aperture is made to satisfy the relation ttR -25 cm , i.e., R=2.82 cm. Table 1 shows the results of perturbation measurements made over a period of several days. Uncertainties in the perturbation measurements

are shown in parentheses in table 1 and are due mainly to uncertainties in

reading the electrostatic voltmeters used to measure the probe potential and

to small drifts in the electrostatic voltmeter reading. As expected, the

measured perturbations of the space potential exceed the values predicted bv i theory that ignores space charge

.

Table 1. Space Potential Perturbations

Height Above

Bottom Plate

% Perturbation

of $>' fea(6)

]

% Perturbation

Measured ( o**Q)

4R 0.16 0.37, 0.31, 0.48, 0.37 (±0.10)

3R 0.37 0.70, 0.68 (±0.15)

2R 1.16 1.86, 1.60, 1.83, 1.80 (±0.20)

1.1R 5.96 7.05, 6.28 (±0.40)

12
11.3 cm

1820

(4xR) V

<TJ II <(O 930

1800

w> Aperture A No

>

920

> 1790 \ 1 j J | aperture

Ql > 910

1780

/6.5 V(0.4%) T

900

1770 i Li i 890

1600 1700 1800 1900

V„(V)

Vp=Vg

Aperture

800 900 1000 1100

Vg(V)

Vg(V) Vg(V)

Figure 7. Measur eraents of space potential perturbation due to 5 cm x 5 cm aperture in ground plane as a function of height above ground

plane.

13
The additional perturbation due to air motion was examined by drawing air

through the aperture. The air flow was measured with a turbine -type flowmeter

and the air speed in the plane of the aperture was calculated by assuming a uniform velocity across the opening. No measureable (i.e. >0.1%) change in

space potential could be observed at a height of 11.3 cm (4R) above the

aperture for average air speeds between 1.6 m/s to 6.6 m/s in the plane of the

aperture. The apparently negligible influence of the air motion at 11.3 cm is an interesting result because the calculated air speed, when u

q is 6.6 m/s, is

about 10% of the calculated drift velocity at the same height. This result

will be examined further.

3.4 Discussion

The measurements of the preceeding section (for V

t=2650 V,

K=1.3 x 10 ^ m^/Vs , d=0.17 m, J=0.25 x 10 ^ A/m^) suggest that the space

potential perturbation due to the aperture at a distance of four radii above

the aperture is about 0.4% and the effects of air motion due to the fan in the

ion counter are negligible when the air speed is as large as 6.6 m/s in the

plane of the aperture. Measurements at significantly greater distances above

the aperture were prevented by possible contact between the space potential

probe and a portion of the top plate which extends downward along the

perimeter (fig. 4)

.

If the electric field perturbation at four radii above the aperture is

estimated using eq (8), a perturbation of less than about -1% is expected.

The perturbations of E and p are expected to be much smaller than 1% at the

top plate which is 0.17 m (6R) above the aperture. For points off the z-axis,

the perturbation decreases further. Thus, the requirement that significant

perturbations of the field not extend to the top plate of the parallel -plate

system is realized for the values of J, V , K and d indicated above.

Calculation of the unperturbed charge density profile indicates that p increases by 16.5% from the bottom plate to the top plate; the increase to a height of four radii is only 10% [9] . The small perturbation of p due to the

aperture and the absence of a measurable perturbation due to air motion at a height of four radii above the bottom plate suggests that the sampling region

of the ion counter is below this height. Thus, the range of unperturbed ion

densities sampled by the ion counter varies less than about +10% from the

value at the bottom plate. Taking the median unperturbed value of p to be the

"known" Ion density, the uncertainty in p becomes ±5% in the absence of other

uncertainties

.

To determine the total uncertainty in p, the steps in its calculation

must be considered. It is recalled that p is given by, from eq (3),

p(z) = J/KE(z),

where

E(z) + 2Jz/KE ] o 1/2 11 )

14
Using techniques described in Reference 4, K can be measured with an uncertainty of less than ±6%, J is measured with an uncertainty of less than

±6%, and E(z) can be calculated using the values of J and K with an uncertainty of less than ±1%. The parameters J and K are independent but E(z)

is correlated with J and K. Adding the uncertainty in E(z) to that of K, the

rms uncertainty in the median value of p is less than ±10%. Finally, adding

±5% uncertainty to take account of the range of (unperturbed) charge densities

being sampled by the ion counter, the total uncertainty in p becomes less than

The transport of ions into an aspirator- type ion counter under the

influence of an electric field and air motion is examined in a semiquantitative manner in this section. In the model that is considered, a circular aperture of radius R is used to approximate a square aperture of an ion counter of side dimension 2R. It is assumed that the ions travel along

trajectories determined by the addition of the ion drift velocity (product of

ion mobility and electric field strength) and air velocity, and do not

contribute to the magnitude or direction of the electric field which is

essentially space-charge free. The trajectories are taken to be in the

vertical y-z plane.

The electric field is found by taking the negative gradient of the

electric potential derived by Fry for a uniform space-charge-free electric

field perturbed by a circular aperture of radius R in the ground plane [5],

±15%.

3.5 Ion Trajectories and Losses

E' =• -V*', ( 12 )

where

(13)

A and p are oblate spheroidal coordinates and are related to Cartesian

coordinates by the equations

y

= R coshA cosp,

z =» R sinhA sinp,

(14)

(15)

and A is E R/2

.

u

The horizontal and vertical drift velocities, v and v^ , are given by

v = KE' , and

y y

(16)

v = KE' , z z (17)

where

E'y(AS sin/i cosp sinhA/B)-(AT sin^u cosp coshA/B) , (18)
2 2 Z’ - - (A sin n coshA/B) + (AT cos /i sinhA/B) , (19)

S = [coshA + (2 /tt) coshA tan ^ (sinhA) + (2/7r) tanhA] , (20)

T = [sinhA + (2/n) + (2/tt) sinhA tan ‘'‘(sinhA)], (21)

and

B = R (cosh

2 A-cos

2

/z) . (22)

An expression for air velocity for points above the aperture is [3]

u(r) = -(u /2tr) // |r-r'

|

3 (r-r' ) dx'dy', (23)

&

where u

q

is the air speed in the plane of the aperture, r and r' are position

vectors above and in the plane of the aperture respectively, x'and y' are Cartesian coordinates in the plane of the aperture and is the area of the

aperture. Equation (23) is used to obtain the air velocity components above

the aperture. Along the z-axis, eq (23) reduces to eq (9) and as noted

earlier the air velocity values are well predicted by eq (9) when an "effective radius" is used for R. The off-axis values of velocity predicted

by eq (23) have been compared with preliminary measurements and appear to be

too high. However, the off-axis inaccuracies should not affect the semiquantitative conclusions of this section.

For a square aperture of side dimension 2R, eq (23) can be integrated and

yields the following expressions for the air velocity components:

u^ = - (u

q/27t) log [ (R+F) ( -R+G)/( -R+F) (R+G) ] , and (24)

u

z = - (u

q/ 27t) [ tan"

1 (R(y+R)/zG) - tan"

1 (R(y-R)/zF) ] , (25)

where

F = [R

2 +(y-R)

2 + z2

] 1/2

, and

G = [R

2 +(y+R)

2 + z2

] 1/2

. (27)

In the y-z plane where the ion trajectories are calculated, the edges of the

circular and square apertures coincide. It should be noted that the

velocities along the z-axis predicted by eq (25) for a square aperture agree

well with the results obtained using the simpler expression for a circular

aperture, eq (9), if the radius of the circular aperture is taken Co be the

"effective radius."

16
For z>0, the equations of motion for positive ions under the influence of

the electric field and air motion through the aperture are

dy/dt = v + u , and

y y

(28)

dz/dt = v

z + uz

> (29)

where the ion drift and air velocities are given by eqs (16) -(17) and eqs

(24)-(25). Below the plane of the aperture, the air speed is approximated by

a constant velocity, u

q

, parallel to the z-axis.

Equations (28) and (29) have been solved on a computer using a variable

step-variable integration method with software developed at NBS [10].

Examples of trajectories for E^ =* 15 kV/m, u = 1 m/s, R = 0.025 m, and

-4 2 K = 1.3 x 10 m /Vs are shown with solid lines in figure 8. The time-of-

-3

flight to the plane of the aperture for each trajectory is 50 x 10 s. Also

shown in figute 8, with dashed lines, are ion trajectories assuming no electric field; the time-of -flight is again 50 x 10 s. The surfaces that

bound the trajectories with and without an electric field define volumes of

charge that reach the aperture during the assumed time interval. Ignoring

possible loss mechanisms in and below the plane of the aperture, the

significance of the different volumes can be seen by examining the expression

for measured ion density,

%

- I/M ' (30)

where I is the ion current and M is the volumetric flow rate. Expressing I as charge per unit time, Q/t, and M as air volume per unit time, V/t,

%

- (31)

The charge Q can be expressed as the product of average charge density, p , 3. and volume, V', occupied by the charge. Then,

Pm

,aV/V (32)

When there is no electric field, the volume of charge that enters the aperture

coincides with the volume of air (V' = V) and the measured charge density

equals the average charge density in space. With an electric field, V' is

greater than V and the measured charge density will exceed the average value

in space and thus be in error.

As the air speed is increased, the difference between V' and V decreases

and approaches p . Figure 9 shows ion trajectories with and without an electric field for u

q equal to 6 m/s. While the air speed may be

unrealistically high for typical ion counter operation, the smaller difference

between V' and V is apparent.

17
Figure 8. Ion trajectories for air

speed of 1 m/s in plane of

aperture. The solid curves show influence of 15 kV/m

electric field. Dashed

curves are for no electric

field. The time-of-flight

for all trajectories is 50

ms. The volume occupied by

the ions that enter aperture

when E » 0,V', exceeds volume

of air,V, which enters in the

same time. Inset shows

effect of mobility value on

trajectory near aperture.

z

Figure 9. Ion trajectories for air

speed in plane of aperture of

6 m/s. The solid curves show

influence of 15 kV/m electric

field. Dashed curves are for

no electric field. The tlmeof-f light for all

trajectories is 50 ms. The

volume occupied by the ions

that enter aperture when E • 0,V', exceeds volune of

air,V, which enters in the same time.

18
Losses in and below the plane of the aperture can occur because of ion

trajectories which terminate near the edge of the aperture. For example, the

inset in figure 8 suggests that some trajectories less than =1 mm from the

-4 2 edge of the aperture (K = 1.3 x 10 m /Vs) will not "clear" the aperture.

This would represent approximately 8% of the ions that reach the aperture,

assuming a uniform charge density across the aperture.

It should also be noted that most of the trajectory calculations in

figure 8 have been performed for a single ion mobility value,

-4 2 1.3 x 10 m /Vs. A distribution of mobilities normally exists and ions with

-4 2 mobilities greater than 1.3 x 10 m /Vs could also fail to clear the aperture

as shown in the inset of figure 8. Interestingly, if the product of mobility

and electric field (drift velocity) is kept constant, the trajectory remains

unchanged for a given air speed.

If a duct is introduced below the aperture, loss of ions will occur via

collisions with the wall of the duct. Assuming for simplicity that the

trajectories in figure 8 remain unaffected by the duct, calculations show that

ions entering the aperture 5 mm from the edge (y = 2 cm) will collide with the

duct about 1.3 cm below the plane of the aperture. This loss, as well as the

loss of other ions passing through the aperture further from the z-axis, would

amount to about 36% of the total number of ions reaching the aperture, again

assuming that the ion density were uniform over the area of the aperture.

The excess ions entering an ion counter due to an external electric field

which attracts ions to the ion counter and the ion losses which subsequently

occur to the walls of the ion counter was noted by Swann early in this century

[11]. He concluded that the ion current to the walls was just equal to the

excess ion current due to the electric field, independent of air speed, and

that the electric field did not influence the ion density measurement [12].

However, because Swann appears to assume that all the electric field lines

terminate on the walls of the ion counter, his conclusion that all of the

excess ion current goes to the walls must be qualified. Some field lines

could terminate on the electrodes of the ion counter and contribute to a higher ion density measurement. For example, calculations show that field

lines within a radius of 5 mm of the center of the aperture will travel at

least 4.5 cm below the plane of the aperture before reaching an imaginary wall

at a radius of 2.5 cm. Loss of ions due to Coulomb repulsion and other

mechanisms will be treated later in a fuller account of the problems

associated with operation of aspirator- type ion counters in the presence of an external electric field.

3 . 6 Summary

Progress has been made in characterizing an apparatus for producing a known unipolar ion density for calibration of aspirator- type ion counters

operating in the ground plane in the presence of a large external electric

field. The external field causes more ions to enter the opening of an ion

counter than would occur in the absence of the field. Whether the extra ions

cause a measurement error depends, in part, on whether any electric field

lines terminate on the collecting electrodes of the ion counter. The

percentage of extra ions arriving at the opening of the ion counter increases

19
as the air velocity decreases, for a given electric field strength. It is

noted that the above results are true for ion counters operating in the ground

plane and extension to cases where the ion counter is operated above the

ground plane is not straightforward.

4. GASEOUS DIELECTRICS RESEARCH

Task 03

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

Electrosystems Division

National Bureau of Standards

4.1 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

byproducts; and the acquisition of fundamental data, e.g. corona inception

voltages, production rates of corona byproducts, and the effects of

contaminants on discharge initiation.

This report highlights four significant technical activities. The next

section describes the determination of the influence of trace amounts of water

vapor on electron avalanche growth. This is followed by a theoretical

discussion of a technique for the common parameterization of swarm and

breakdown data for binary gas mixtures. Experimental and theoretical results

on corona- induced chemical decomposition are then discussed.

4.2 Influence of Trace Amounts of Water Vapor on Electron

Avalanche Growth in SF^

The work presented in this section has been described in detail in a paper [13] that has been accepted for publication in the Journal of Applied

Physics, and therefore only a summary of the method, results, and conclusions

is presented here. This work was also discussed in a paper presented at the

1985 38th Annual Gaseous Electronics Conference, in Monterey, California

Measurements were made of the statistical size distributions and

repetition rates of electron avalanches formed near a positive point e 1 ec t m4'

in a uv- irradiated point-plane gap containing pressurized SF^ with varying,

known, trace amounts of H^O from about 10 to 300 parts-per-million bv volume

20
The method for measuring electron avalanche pulses is the same as described

previously [14]. The gas chromatograph-mass spectrometer method of measuring

the water vapor in SF^ was developed as part of this project in the previous

year and has been described in a recent publication [15]. The results

obtained from the measurements verify the trends reported in an earlier

preliminary work [16] , namely that the introduction of trace amounts of H^O

into SF, has the effect of enhancing both the mean size and repetition rate of

o electron avalanches formed near a positive point electrode by an order of

magnitude or more.

Examples of results obtained for the pulse height distributions and

repetition rates for electron avalanches as a function of [H^O] are displayed

respectively in figure's 10a and 10b. Plotted in (fig. 10a) are the

arbitrarily normalized relative numbers of avalanches that were recorded in a given time interval versus the avalanche size expressed in picocoulombs (1 pC

is roughly equivalent to 6 x 10 electrons) for different indicated [H^O]

values and voltages applied across the gap. The vertical arrows in this

figure mark the largest avalanches which were recorded for SF^ at the

indicated voltages for the least amount of water vapor content

([1^0] = 14 ppm^) . The smallest avalanches observed, i.e., the low side

cutoff points for each data set, were determined by the discriminator setting

of the pulse detection system. Shown in (fig. 10b) are measured average

avalanche count rates versus applied voltage for all avalanches in excess of

0.08 pC.

It is of interest to note in figure 10a that the avalanche size

distributions exhibit a bimodal structure once a significant number of

avalanches exceed the level of about 1.5 pC (~10^ electrons).

Theoretical calculations, the details of which will not be discussed

here, were performed in an effort to understand both the basic, microscopic

mechanisms by which avalanche size is increased by low concentrations of water

vapor and the reasons for the development of bimodal distributions. The

calculations of mean avalanche size, using computer codes previously developed

as part of this project [17,18], demonstrated that it would be necessary for

the ionization coefficient to increase by as much as three orders of magnitude

to account for an order of magnitude increase in the mean avalanche size.

With assistance from A. V. Phelps of the NBS Quantum Physics Division,

calculations were made of the ionization and electron attachment coefficients

for SF^/H^O mixtures using numerical solutions of the electron transport

equation. In agreement with estimates based on the Wieland Approximation

[19], the calculation showed that the introduction of 1% H o 0 causes the

ionization coefficient for SF, to drop by about 1% when the ratio of electric

o field- to-gas density is at the critical value where ionization growth is

possible. It was therefore concluded that the avalanche enhancement effect of

is not due to a change in the ionization rate of the gas.

21
RELATIVE

NUMBER

(Arbitrary

unlta)

V 1 5.0 kV V * 14.2 kV V *13.5 kV

Figure 10a. Measured pulse height distributions for electron avalanches

formed near a positive point electrode in SF^ at a pressure of

200 kPa for the indicated different water vapor concentrations

and applied voltages

.

APPLIED VOLTAGE (kV)

Figure 10b. Measured electron avalanche repetition rates versus applied

voltage at the indicated water vapor concentrations for

avalanches larger than 0.08 pC.

22
A more reasonable mechanism has been proposed [13] to account for the

observed enhancement effect, which considers the appearance of minor negative

ion charge carriers associated with water vapor such as the clusters (^0)^,

OH n = 1,2,... that have a much lower energy threshold for collisional

detachment than the expected predominant negative ions associated with SF^

,

e.g. SFg (^2^n‘

'^ie more weakly bound negative ions, which are formed when

enough ^0 is present, can experience electron detachment at much lower

electric fields than the SF, -related ions and thereby can give rise to

o formation of larger avalanches at a higher rate.

This hypothesis is supported by theoretical calculations [13] which were

performed of collisional detachment probabilities for negative ions having

different detachment thresholds as a function of position in the critical

initiation volume [17,18] near the point electrode surface. Examples of

results from such calculations are displayed in figures 11a and lib. Shown

are computed probabilities for electron detachment per unit distance p^ as a function of position in the critical volume for ions with different assumed

detachment cross sections as indicated with different threshold energies e .

Thresholds of 7 to 10 eV for collisional detachment of F and SF, are

o consistent with other recent work [20-22]

.

It is seen that for lower

thresholds of, for example, c = 0.5 eV, the value of p^ increases by orders

of magnitude over that for ions with e = 7 to 10 eV. This is more than

sufficient to account for the observed avalanche enhancement effect.

An attempt has been made, using a computer model of electron avalanche

growth statistics based on an extension of Legler's theory [23], to determine

the theoretical shape of the avalanche size distributions and the conditions

under which the distributions might become bimodal. It was found [13] that

the modified Legler theory, which applies for nonuniform fields and which

incorporates the statistics of initiating electron release through p^

mentioned above, always predicts a monotonically decreasing probability with

increasing number of electrons in the avalanche. This theory cannot,

therefore, account for the development of bimodal distributions, and it is

proposed that this behavior can only be predicted by a three dimensional model

which properly includes the influence of ion and electron space charge.

4.3 Common Parameterizations of Swarm and Breakdown Data

for Binary Gas Mixtures

The work described in this section was discussed both at the 1985 Joint

Symposium on Swarm Studies and Inelastic Electron-Molecule Collisions,

Tahoe City, California, and at the 1985 38th Annual Gaseous Electronics

Conference, Monterey, California, and is now being prepared for archival

publication. Again, only the main ideas are discussed here without detailed

mathematical derivations.

23
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03

C-3

(30 —4Lu

24
A theoretical model is proposed for use in fitting and analyzing data on electrical breakdown in binary, electronegative gas mixtures which makes use

of available cross section and electron swarm data for the individual

components of the mixture. The model affords more physical insight and allows

more flexibility in fitting data than the simple Wieland Approximation [19]

.

It was shown that the model is useful in resolving controversies or disagreements in results and in determining whether or not data are physically

reasonable. It also permits examination of the conditions under which

pressure dependent (or independent) synergistic effects are possible.

Three basic assumptions are required in the model when pressure dependent

effects are neglected. (Pressure dependence will be considered later.) These

assumptions are: 1 ) the electron kinetic energy distributions in the gas are Maxwellian under all conditions, i.e.

F (e) = (2A1/2

)A 3/2

£ 1/2 exp(-A e) , (33)

7 7 7

where c is the electron kinetic energy and 7 = 1 , 2 , m designate the two

individual gas constituents A and B and the mixture respectively; 2) the

electron temperature (T^ = 1/kA^, k = Boltzmann's constant) for the mixture is

given by a weighted sum of the temperatures for the individual components,

i.e.

T

ffi = q(F)T

1 + (1 - q(F))T

2

, (34)

where for 1 -+ A and 2 -* B,

F = [ A] /( [ A] + [B]), 1>F>0 , (35)

and

q ( 1) = 1, q(0) = 0, q(F) > 0; (36)

3) the variables F and E/N (field- to-gas density ratio) are separable so that

the ionization (a) and attachment (

rj

)

coefficients for the mixture can be

represented again as weighted sums respectively given by

am = h(F)a

1

(E/N) + (1 - h(F) )c* 2 (E/N) , (37)

and

- g(F)ry

1 (E/N) + (1 - g(F) )

r, 2 (E/N) , (38)

where the functions h(F) and g(F) satisfy the same conditions as q(F) . It

should be noted that in assumption 2 above, one could equivalently take an identical linear combination of the mean electron energies.

A computer program was written to find least squares fits to measured

critical electric field-to-gas density ratios (E/N)^

^ using the proposed

model. The values of (E/N) correspond to the condition,

c , m

25
a (39) m m

at which ionization growth, and therefore electrical breakdown in the gas

mixture, is possible. The adjustable parameters for fitting are the electron

temperatures and for the two gas components (or equivalently A^ and A^)

.

These temperatures are adjusted within reasonable limits and a and r? are m m computed using the expressions

g(F)

A, S ,L.

1 ml 11

A

2

S m2H

22

(40)

and

h(F) =

A

2

I 22

Sm2 + VW + Sm2

+

A

l

S mlI ll

' A

2

Sm2I 22

(41)

where

2 r® -1

~ X 3

€ smp " Vt £ W£ > e d£ ' (42)

-A £ y h. £<V£)e d£ ' (43)

-A £ H =* J eo .(e) e d£

,

a/3 J o a/3 (44)

Am = A (q(F) ) , and m m

(45)

a ,3 = 1 , 2 , m (46)

These expression are derived from the basic assumptions, and definitions of

the ionization and attachment coefficients using the electron collision cross sections a . , a . , and a., for momentum transfer, electron attachment, and

mp ap lp

ionization respectively (e^ = ionization potential). The (E/N)^

^ values are computed from the requirement of eq (39) and compared with experimental data.

Examples of cases to which this fitting procedure have been applied are shown in (figs. 12a and 12b) respectively for the mixtures SF^/N 0 and

CCJi^F^/N^ for which there exist disagreements among the various measured >n<i calculated (E/N)^ ^ values. Indicated in those figures are the energies

corresponding to the peaks in the Maxwellian distributions for and the

electron temperature ratios that gave the best fits. In figure 12a, "he upper

solid curve is a fit to the normalized experimental data of S iddagangapp.i . et

al . [24] which shows a high degree of synergism, and the lower solid curve is

26
CO6 ;=;MzN

CO u° b _*• oo O UcMooi-JN

CM U. d

27

Fits

(solid

lines)

to

normalized

critical

field-to-gas

density

ratios

for

the

mixtures

SF,/N„

and

CCd F

/N.
a fit to the normalized data of Aschwanden [25] which shows less synergistic

behavior and is more in agreement with the results of other measurements

[26-28], and theoretical Boltzmann [29-32] and Monte Carlo [33] type

calculations. The significant point to be made here is that to obtain a high

degree of synergism as implied by the data of Siddagangappa , it is necessary

that and T

2 be quite different, i.e. by approximately an order of

magnitude. The values for T^ and from the fit to the Aschwanden data are nearly the same, consistent with temperatures estimated from the Boltzmann

calculations. The condition T^ = T^ is sufficient for the Wieland

Approximation to apply [19], and indeed this approximation [34] is seen to

satisfactorily represent the lower sets of data. The same behavior is found

for the fits to the data for shown in (fig. 3b) [35]. A fit to the

data exhibiting the highest degree of synergism again requires a large

difference in the electron temperatures for the individual components and

CCJ! 0 F • in this case e. for is also unrealistically high. In general, it

2 2 max 2 jo

can be stated that in order for there to be a high degree of pressure

independent synergism, such as a positive synergism where (E/N)

c ^ > (E/N)c ^ and (E/N) _, there must be a shift in the electron energy distribution with F

^ so as to enhance the attachment rate in the gas. In the model proposed here,

this requires that

To account for pressure dependent synergistic behavior, one must consider

three-body electron stabilization processes as pointed out in a recent work of

Hunter and Christophorou [36], In particular, for a binary, molecular gas

mixture designated here by AX/BY, one must consider the various possible

negative ion formation and destruction processes such as are included in

table 2.

The reactions that are underlined in this table are electron detachment

processes. The model discussed above for fitting (E/N)

c ^ data can be

extended to include pressure dependent effects. In this case, however, a separation of the variables F and E/N is no longer possible, and one obtains

the following complicated forms involving the various reaction rate constants

given in table 2:

>7 m

= G

1

( N,F)r?1 + G

2 (N,F)r?2 , (47>

where

g(F) (k_+k (l-F)N+k +k_FN) (k +FNk_+( 1 - F)Nk ,

)

q = ^ 2 2 2 2 ±_ (<4g

1 (k

5 +FNk

3

) [(k

4

+k

7 )(l-F)N+(k

3 +k

6 )NF+k2 +k

5

]

and

(l-g(F))(k'+k' (l-F)N+k'+k'FN)(k'+(l-F)Nk>FNk^)

G

2 = (k'+(l-F)Nk^) [ (k^+k' ) (l-F)N+(kdk^)NF+k'+k:

28
Table 2. Possible negative ion formation and destruction processes in binary,

molecular gas mixture

k

e + AX -*•

k

e + BY ->

A + x

(AX')* + AX -> AX~ + AX

(AX')* + BY - AX' + BY

(AX')* + AX + e

(AX')* + AX -» AX + AX + e

(AX')" + BY AX + BY + e

B + Y"

(BY')* + BY - BY' + BY

(BY')* + AX - BY' + AX

(BY')* -» BY + e

(BY")* + BY - BY + BY + e

(BY')* + AX - BY + AX + e

29
It is of significance to note that if electron detachment processes are unimportant, so that one can set all the rate constants for the detachment

processes equal to zero, then one simply obtains = g(F) and G 0 = 1 - g(F) , i.e. eq (47) reverts back to the form of eq (38). It can be concluded,

therefore , that pressure dependent synergistic behavior occurs only if

detachment is important. When pressure dependent synergisms do occur, they

are a consequence of the influence of one gas component on enhancing, or reducing, the electron detachment rate of the other component.

4.4 Corona- Induced Decomposition Rates of SF^

in SFr /N0 , SF,/Ne, and SF,/0 Mixtures

b Z b b Z

Some of the results of this work have been published in a recent archival

paper [15], presented at the 1985 Gas Discharge Conference in Oxford, England

[37], and at the 1985 38th Annual Gaseous Electronics Conference in

Monterey, California, and have been submitted for presentation at the forth

coming 1986 IEEE International Symposium on Electrical Insulation. Only

selected significant features of this work are reviewed here.

The charge rates of production for the gaseous oxygenated species SOF 0 , SO

2 F2

, SOF^, and SO

2 were measured in mixtures of SF^ with ^ , with O

2

, and

with Ne as a function of SF^ content for a continuous, 40-/rA, dc , negative,

corona discharge at a total gas pressure of 200 kPa (-2 atm) . Examples of

data from which production rates were determined are shown in figures 13 and

14. The experimental method has been described previously [15].

The maximum SF^ decomposition rates in a corona discharge for the

mixtures of interest were computed and compared with total observed rates

given by the sums of the measured rates for all oxygenated products. The

maximum charge-rates-of-production r^ which are related to the time rates of

production, dc/dt, by

r^ = i *"( dc/dt ) , (50)

where i is the discharge current, were computed using the expression [15]

r

q

k Pi ex evkT

( 51 )

where k , P, i, e, v, k, T, N A are the SF, dissociation rate constant,

ex A 6 partial pressure of SF^, the mean path traveled by an electron in the gas

(approximately the radius of the active discharge volume), electronic charge,

electron drift velocity, Boltzmann's constant, gas temperature, and Avagudro ; constant respectively. The SF^ dissociation rate is given by

30
a)

ii --—i

1 1

—

r

NfG CORONA

c)

b)

d)

Figure 13. Examples of data on measured oxyfluoride concentrations versus net

charge transported in the corona discharge for different indicated

SFV/Ne and SF../N- mixtures from which production rates were

6 o 2

determined.

31
70

co jgoE

z

2

H

<

IUJo

M

It,<v o

w

60

50

40

30

20

10 -

-—i——r

•

1 1 V" i *

T

8Fa : 02 y u

« 100 : 0 A 0 * © 08 : 1 t’ 'AV m

© 05:5

* 00 : 10 o' > -

0 75:25

« 50 : 50

»t “ P-200 kPa % \ 1-40 uA

•

*V*

A * (=*/

»X**e

A?*

X X X X 0 1.0 2.0 3.0 4.0 5.0

NET CHARGE TRANSPORTED (C)

a)

c)

b)

d)

Figure 14. Examples of data on measured oxyfluoride concentrations vers-js net

charge transported in the corona discharge for different Indicate:

SFg/Ne and SF^/O^ mixtures from which production rat*js wer- determined. 32
k ( 52 ) ex

= £ C « 1/2

««)<!,_ («)<u

1 1

at E/N = (E/N)

c (a = r?) .

In eq (52), f(s) is the electron energy distribution which was computed for

each mixture using numerical solutions of the Boltzmann transport equation by

the method previously described [38]. The quantities Qx;j_(0 are the cross sections for electronic excitation of SF^ by electron impact which were

assumed to coincide with dissociation processes. The complete set of SF^

electron collision cross sections used in the calculations of k are shown in ex figure 15. Consistent with recent arguments [39], it is assumed that the

chemically active region of the corona discharge has a uniform E/N close to

the critical value. The size i of the active volume was estimated

photographically by measuring the width of the small uniformly luminous region

of the discharge near the point electrode surface. Figure 7 shows examples of

the measured SF^ -normalized (r /[SF^]) production rates for the individual

products observed in the different mixtures as well as comparisons of the

measured total SF, -normalized decomposition rates with the theoretical maximum

o values. Indicated also are maximum decomposition rates determined by

including possible maximum contributions to SF, dissociation from metastable

o excitation transfer in and Ne . This contribution is unlikely to be

important for SF^/N^ because there is experimental evidence that the

predominant metastable state of N^ is not quenched by SF^ [40]

.

Several important conclusions have been derived from this work. These

include

:

1) In "pure" SF^ , the discharge dissociation products predominantly

recombine back into SF,

.

6

2) The decomposition rates for SF^ increase and approach the maximum

values as the SF, content decreases. This behavior indicates that the

6 primary effect of diluting the SF, gas with N Ne, or 0_ is a

O Z Z reduction in the rate of recombination of SF, dissociation fragments.

6 &

3) The appearance of S0_ in SF,/N0 and SF,/Ne at low SF, concentrations

L o L b b indicates that inhibition of SF^ recombination by dilution enhances

multistep electron- impact dissociation processes since S0 0 can only be

formed from the smaller fragments S or SF which are only efficiently

produced by multiple electron collisions.

33
CROSS

SECTION

(m

2)

ELECTRON - SF6 CROSS SECTIONS

,-16

10-2 10-1

1 10 10 2

ELECTRON ENERGY (eV)

10 3

F i gur e 15. The complete set of electron collision cross sections fur 3F, used

to calculate electron-induced dissociation rates of SF, in the

mixtures SF^/N^, SF^/O^, SF^/H^O, and SF^/Ne from numerical

solutions of the electron transport equation. The attachment, ionization, momentum transfer, vibrational excitation, and

electronic excitation cross sections are respectively denoted by

v «o- <£" QVIB’

and Q

x
4) The lack of SC>2 and. the appearance of S deposition on the anode in

SF-/0- mixtures suggest formation of S by a rapid charge transfer

o Z process, possibly involving 0

^

, which prevents SO^ formation by

removing S atoms from the discharge region.

5) In all SFg/T^ mixtures, nitrogen oxide yield was small compared to

oxyfluoride yield. This indicates that the gas decomposition is

predominantly associated with SF^ even in very dilute mixtures.

6) There was no evidence of nitrogen fluoride formation in SF^/^

mixtures in 40 -/jA discharges. Evidence of trace amounts of NF^ was found in SFg/98%N2 mixtures subjected to discharges with currents

greater than 100 /J.A. for long periods (more than 20 h) .

7) For SF-/0 mixtures, CO- formation was observed and the behavior of

b Z Z its production with time and with O

2 content suggests that it is

formed from reactions of O

2 with carbon on the point electrode

surface

.

4.5. Models for Decomposition of Gases in Corona Discharges

This work is preliminary and has not yet been discussed at a technical

conference. The proposed chemical kinetics models have general applicability

to decomposition of high pressure gases in cool glow or corona type discharges

and provide insight into certain observed features of decomposition such as dependences on discharge current and gas composition. A few examples of how

the model can be applied are considered here. A relatively simple example

which has recently been considered is the decomposition of CF^ in a corona

discharge. I. Sauers [41] reported that the primary product of corona

chemistry in CF^ is ^

2 ^^’

anc* c^e t:^me ' rate ' 0 f -production is proportional to

the discharge current, i.e. the charge-rate-of -production is constant. A

simplified reaction scheme which applies in this case is:

k

l e + CF. -* CF_ + F + e 4 3

k

2 CF_ + F - CF.

3 4

k

CF

3 + CF

3 - C

2 F6

F +

35
The following assumptions are now made: 1) C_F, is always at trace levels,

Z o i.e. [CF^]»[C

2 Fg] ; 2) steady state conditions exist, i.e. d[CF^]/dt = d[F]/dt

= 0; 3) recombination predominates, i.e. k

2»k3 ,k^. The relevant rate

equations are:

d[CF ] , -jr~ - k

l

n

. [CF4J- k

2

[ CF

3

1 [Fl- k3[CF

3

] (53)

(54)

d l C

2 V

dt = k3 [CF3

; (55)

where n^ is the electron density, assumed to be proportional to the current.

Because recombination predominates, one can take [F] - [ CF

3

]

,

which when used

in eqs (53-55) with the above assumptions gives

d tSFfii dtk

3 k

l

(56)

thus indicating that the production rate is indeed proportional to

current

.

The model has been applied to corona- induced decomposition of SF^

containing small quantities of 1^0 and 0 ^. Although in this case the reaction

scheme is much more complicated, one can, using assumptions like chose used

for CF^, obtain reasonable qualitative descriptions of the dependences of

oxyfluoride (S0F o , SO

2 F2

, and SOF^) production on the concentrations [H o 0] and

[

O

2

]

.

and the discharge current which are consistent with experimental

observations [15]

.

For production of SOF

2 and SOF^, which is in general competitive , one can consider a restricted set of reactions:

e + SF, -* 1 SF C + F + e

O J

k

2 SF C + F + (SF.) - 2SF,

D 0 0

k

3 SF C + SF C SF. + SF,

5 5 4 6

36
e + C^-^O + O + e

k

5 0 + 0 + (SFg) - 0

2 + (SF

g

)

k

6 + H

2 0 -> SOF2 + 2HF

k

7

SF C - SOF, + F

5 4

k

8

F + F + (SF

g

) - F

2 + (SFg)

k

9

SF, + 0 + (SF,) -> SOF. + (SF, ) , 4 b 4 b

where the possible three body nature of some reactions has been indicated.

The localized nature of the corona discharge must be considered in this case

where hydrolysis of SF (of rate k, ) is assumed to take place throughout the

4 b

entire gas cell, whereas the other reactions involving SF^ take place in the

highly restricted volume of the discharge. Therefore, it is necessary to

introduce the rate k

s [SF^] at which SF, escapes the discharge volume. The

value of the effective rate constant, k , can be determined from solutions of

s the diffusion equation for SF^ which apply under steady-state conditions.

{

0 +

It will be assumed that steady state conditions apply and that

recombination of 0, SF C and F back into 0_ and SF, predominate. The

D Z b

predominance of recombination in relatively pure SF^ is indicated by the data

shown in figure 16 which, from comparison with the computed maximum

decomposition rates, shows that less than 3% of the dissociated SF, converts

b to oxygenated by-products. The results of solving the appropriate set of rate

equations are:

d[S0F

2

] k

S k3 Kne dt " Jh k

s + k

9 ^

I°2)l SF

6

(57)

1/2 _ 1/2

and

37
d[SOF

z dt

/K k

4

[0

2

] k

5

[ sf

6

]

vl/2

k_,n

7 e + K^tSFJn 3 /2

6 e k

s + k

9^ t° 2

lt SF6M1/2n

e 1/2 (58)

with

K =

k

2 + k8

(59)

The current dependences of the charge-rates-of-production for these species,

as implied by eqs (58) and (59), are:

r

q

(SOF

2

) = a

1

/(a

2 +a

3 i

1/2

) (60)

and

r

q

( S0F

4

) = b

x + "2'

1/2

(b

3 + b4

i

1/2

)

(61)

where a., b., i = 1,2, 3, 4 are constants. This model then indicates that S0F o l i 2 and S0F

4 production rates will respectively decrease and increase with

increasing current, consistent with observations [15] . Neither rate depends

on [ H^O ] , which is again consistent with observations, and both have a weak

dependence on [ 0^ ] . with S0F

4 increasing and SOF 0 decreasing with increasing

[0

2

] . Although this latter behavior appears to be qualitatively consistent

with previous results for SF,/0o [15,37], and the data shown in (fig. 16). it

o L obviously does not apply at high oxygen concentrations (above 30%) and the

failure of S0F

4 production to change significantly with 0

2 suggests that the

oxygen for its formation can come from another source. This question deserves

further consideration.

The production rate of S0

2 F

2

, which is postulated to require molecular

oxygen for its formation via reactions involving the lower valence fragments

SF

2 and SF^, can also be predicted with this model. In this case, one obtains

r,(S°2F2> - C

1 [02 ]/(C2 [0

2

] C

3

i

1/2

),

where C. , i = 1,2,3 are constants. Again one finds a weak dependence on both

[C> 2

] and i consistent with observations. It should be noted that, in the

present model, formation of S0

2 F2 does not compete with either SOF, or SOF^

formation

.

38
a)

_ c ) Figure 16. Measured charge rates-of-production for the oxyfluorides and SO^

normalized to the SF^ concentration versus percent concentration

of N_, 0_, and Ne in the mixtures SF,,/N-, SF c /0_, and SF £ /Ne.

<Z d b d b <L o Also shown are total oxyfluoride plus SO rates (r ) in comparison

with the predicted upper limits for the total rates for SF^

decomposition.

39
4.6 Summary

This report highlights the results from four significant technical

activities associated with the NBS project on gaseous dielectrics. The

influence of trace amounts of water vapor on electron avalanche initiation

probability in SF^ near a positive point electrode has been experimentally

verified and theoretically explained in terms of likely electron detachment

mechanisms for negative ions in the electrode gap. A new method of fitting

data on electrical breakdown for binary electronegative gas mixtures has been

proposed and applied to the cases of SF-/N- and CCi 0 F0 /N0 mixtures for which

b z z z z discrepancies exist in previously measured and calculated data. The method

allows one to evaluate data and provides physical insight into various effects

such as synergistic behavior. The corona discharge - induced chemical

decomposition rates of SF, have been measured for the gas mixtures SF-/N-

,

o o Z SF,/0 o , and SF,/Ne, and the results have been compared with calculations of

D Z O the maximum possible decomposition rates which were made by using computed

electron impact dissociation rates for SF^ in a model for the discharge. From

this comparison, and from data on the production rates for oxygenated byproducts like SOF^ , SO

2 F

2

, SOF^, and SC> 2, it was possible to gain a better

understanding of the fundamental processes that determine the chemical

stability of SF^ in an electrical discharge. A theoretical model for chemical

decomposition of gases in a high-pressure corona, or glow discharge has been

proposed and applied to the interpretation of experimental results obtained

for CF^ and SF^. Using this model, it was possible to explain the observed

dependences of oxyfluoride production from SF^ on discharge current and water

vapor and oxygen concentrations.

5. INTERFACIAL PHENOMENA IN LIQUIDS

Task 04

Edward F. Kelley

Electrosystems Division

National Bureau of Standards

5.1 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 investigated . such

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

contamination and temperature up to 150°C is being documented in these model

systems in two ways: 1) breakdown measurements using high-speed photograph-.’,

and 2) electro - optical measurements of the electric field in the vi 'ini tv

the interface. Such field measurements provide understanding of the role o*

space charge and surface charge in high-voltage apparatus.

Electrical failure in high voltage apparatus often occurs in the ' of or on an interface which serves £s an insulator between two conducting

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

40
interface will not necessarily cause the failure [42,43]. 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 [44,45]. Those measurements

demonstrated that an interface parallel to the field showed no significant

surface charging, i.e. charging which would give rise to large field

enhancements. An interface perpendicular to the field, however, 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.

However, the field measurements reported above were only accurate to

within ±5%, and extensive studies were only performed on clean systems.

Subtle effects due to slight field enhancements would not be observed due to

the imprecision of the old apparatus. Additionally, little attempt has been

made to determine the role played by contaminants or additives. Preliminary

studies have suggested that additives in the oil influence the space -charge

generation in the oil, particularly at high temperatures. Also, particulate

contamination, and most importantly contamination by conducting particles,

influences the breakdown charateristics of systems. Because the overall

picture of apparatus failure is complex, it has become necessary to improve

our measurement capabilities and widen our scope of investigation.

This report emphasizes four areas in which progress has been made in the

understanding of interfacial and liquid breakdown processes:

1) The electro -optical field-measurement apparatus has been improved so that field measurement inaccuracies are within 1% and the relative

imprecision of any single field measurement can be as low as 0.2%.

2) Measurements have been performed on the influence of electrode

material on the space-charge generation in transformer oil.

3) Studies in breakdown of liquids have been extended by an investigation

of streamer propagation characteristics under high pressures. The

study was accomplished using a high-speed image -converter camera.

4) A preliminary study was made of the nature of partial discharge

activity in transformer oil. Photographs of the partial discharge

phenomena using a high-speed image -converter camera were taken.

The first item, improved accuracy of field measurements, provides us with a better diagnostic tool to determine differences in the field due to changes in

the liquid, either by chemical or particulate contamination, and then to

distinguish the liquid's behavior from that arising from the interface. It is

important to understand the characteristics of the liquid itself so that no phenomena are incorrectly attributed to the presence of an interface. The

electrode material study attempts to determine the role of the electrode

material in space-charge production in the liquid. Such knowledge is

particularly important for studying interfaces which will cover the electrode

surface like a paper wrapping around the coils of a transformer. The liquid

breakdown study contributes to our fundamental understanding of the breakdown

process in liquids and will guide any modeling efforts. In particular, the

41
intiation process needs attention, since once a streamer initiates, it

requires less voltage to continue to grow. Inhibiting initiation of streamers

is, therefore, vital to inhibiting failure. The partial discharge study is

not an attempt to characterize partial discharge behavior in liquids, but to

provide high-speed photographic evidence for the phenomena which give rise to

the partial discharges. If particulate contaminants or voids are responsible,

then this fact will provide guidance in the study of interfacial failure in

practical apparatus.

5.2 High Precision Field Measurements: Room Temperature AC & DC

Measurements of the electric field have been made with an imprecision of

about 0.2% which render field enhancements of 1% quite measurable. Absolute

measurements of the field with an uncertainty of 1% is possible. In previous

interfacial studies, long plate lengths were required to get useable Kerreffect measurements. Long interfaces were, therefore, also required. When

using paper or pressboard as the interfacial material, numerous small "hairs"

of the cellulose material cover the interfacial surfaces. When examined under

crossed polarizers to observe the Kerr effect, these "hairs" scatter and

depolarize the light seriously reducing the image quality. With the present

improvements in the electro-optical measurement system, it is possible to

reduce the size of the electrode system and interfaces employed from about

30 cm to about 3 cm. The reduction not only increases the quality of the

image, but also increases the speed with which new interfaces can be

installed.

In previous studies of the electro-optical Kerr-effect in liquids, such

as transformer oil, which have a small Kerr coefficient, residual and

temperature -dependent birefringence in the windows used have seriously

impaired accuracy and sensitivity of field measurements except at the highest

withstand voltages. Special glass windows were used in the new apparatus

which exhibit a birefringence of less than 1 nm per cm of glass thickness over

the temperature range from room temperature to 150 °C. (A birefringence of

1.4 nm would be roughly equivalent to uncrossing a pair of crossed polarizers

by 1° -- such an amount of light would be 0.03% of the incident light, or of

U

the same order as the light transmitted by the crossed polarizers with a 10

extinction ratio.) At least at the center of the windows where measurements

were made, the windows performed within specification.

The windows used in earlier experiments had a residual birefringence at

least ten to twenty times worse than exhibited by the new glass. This

prevented precise intensity measurements and produced field-measurement

imprecision of 5% or more. Because of the new windows, careful measurements

of small amounts of space charge at room temperature could be made for r :.< first time.

In the following experiment, parallel-plate electrodes were used having .» length of about 15 cm, a width of about 2.2 cm, and a plate separation or about 0.5 cm. The transformer oil was circulated through the cell during i.

.

measurements. Compared to the turbulence during the application of the

voltage, however, the relatively slow speed of the liquid due to the

circulation pump could be entirely ignored. The liquid pump served to n<l

gap area of postbreakdown bubbles and particulate byproducts. The laser .

42
source which was used in earlier investigations was replaced by an incandescent light bulb. The bulb provided a more uniform iliuminacion which

was not readily affected by the turbulence of the liquid in the cell.

5.3 Space Charge in Transformer Oil at Room Temperature -- AC & DC

With dc, the maximum enhancement of the field due to space charge was about 5% above the average field. The breakdown field was (193 ± 14) kV/cm,

and the 5% enchancement was measured at an average field of 187 kV/cm -- within the expected breakdown range. The space charge was found to fill the

3 gap rather uniformly having a density of -7 nC/cm . In earlier measurements

using heated oil, the same polarity of charge was found to reside nearer the

anode than the cathode and was therefore identified as heteropolar charge.

However, the space charge observed here appeared to fill the region between

the plates uniformly as indicated by the constant slope of the electric-field

profiles shown in figure 17.

Figure 17c shows the relatively long time constant associated with the

development of the space-charge under dc voltage. Positive polarity was maintained on the right plate until equilibrium was reached in the

distribution of space charge -- the space charge was uniformly negative. The

polarity was then quickly reversed (within about 3 minutes) and the

enhancement of the field remained at the same location initially, but within a minute or so, the enhancement slowly returned to the anode which was now the

left electrode -- thus the space charge returned to being uniformly negative.

However, immediately after the polarity was changed to negative, the

enhancement was nearer the negative electrode which indicates that the charge

between the electrodes was initially positive -- only the direction of the

field changed, the derivative of its magnitude did not. This type of behavior

suggests that the dc enhancement is due to features of the liquid- electrode

interface, and not to migration times of the charges. The turbulence

associated with these voltages would quickly mix any low-mobility charges

throughout the gap

.

If one assumes that the migration of the space-charge is slow (ignoring

the turbulence for the sake of argument) and that the uniform negative space

charge which was produced while the high voltage electrode was positive

persisted after the voltage was turned off, then when the negative voltage was applied with the negative space charge present the slope of the enhancement

would have immediately changed -- the enhancement would have immediately

appeared at the anode. This is not observed. Upon polarity reversal the

ehancement remains at the same physical electrode (now negative) which means

that the polarity of the uniform space charge is positive. A picture which

may explain these data is that during dc voltage application charge injection

is impaired at one electrode and not the other. Upon polarity reversal the

conditions which inhibit injection at the one electrode are not immediately

changed but require time to be reduced and build up at the other electrode.

This hypothesis is, of course, very tentative. More work is needed to

document the response of transformer oil before serious modeling attempts can

be made

.

The behavior of transformer oil subjected to ac , 60 Hz, is quite

different than for dc as far as space-charge characteristics are concerned.

43
NORMALIZED

FIELD

a) ELECTRO-OPTIC DC FIELD PROFILES AT 24°C

POSITION, z

b) NORMALIZED FIELD c) EFFECTS OF POLARITY CHANGE

rigure 17. Electro-optical, Kerr-effect, dc field measurements in transformer

oil at room temperature. Solid lines show the negative poiaritv,

dashed lines, positive polarity. The numbers in parentheses are the average applied field. In b) the normalized field is shown to

emphasize the enhancement. In c) the normalized fields are shown

for the 63 kV profiles to illustrate the temporal behavior of the

enhancement after changing polarity. The dotted line is tne run

found just after the polarity is changed from positive to

negative. After a few minutes the enhancement for the negative

applied voltage gradually becomes that shown by the solid line.

44
Figure 18 shows the field enhancement at room temperature for ac . The

breakdown peak voltage for ac, around 130 kV (256 kV/cm) , was too large to be

measured with the present apparatus. The data were taken at 118 -kV peak

(232 kV/cm) which was probably within a standard deviation of the breakdown

3 voltage. The space-charge density is about +2 nC/cm , and is relatively

uniform through the gap. Thus, the space charge at room temperature for 60 Hz

ac is opposite in sign and smaller in magnitude than that observed using dc

.

5.4 Space Charge in Transformer Oil at 125 °C -- AC & DC

At temperatures around 125°C, more space charge is found for both ac and

dc than at room temperature. With dc , the breakdown fields are reduced from

193 kV/cm at room temperature to (146 ± 14) kV/cm at the high temperature --a

24% decrease. Field enhancements of about 20% next to the anode are shown in

figure 19. For such data to be taken it was necessary to shorten the data

accumulation time because breakdown would occur within a few seconds. These

fields indicate that a relatively uniform negative space charge is present of

3 about -20 nC/cm . At lower fields, the space charge was larger near the anode

than the cathode

.

Under the application of 60-Hz ac the breakdown fields were lower:

(228 ± 14) kV/cm which is 11% lower than the approximate value at room

temperature. The field profiles are shown in figure 20. The U-shaped fields

are produced by bipolar homocharge having a larger positive component at the

anode, that is, a negative space charge is observed near the cathode (about - 3 16 hC/cm ) , but a positive space charge having a larger magnitude is observed

3

near the anode (about 24 nC/cm ) . Despite these rather large charge densities

the field is enhanced less than 3% mid- gap and is decreased as much as 5% next

to the anode

.

Although the field is decreased at both electrodes for the hightemperature ac data, a decrease in the strength of the oil is observed. The

11% decrease in strength is a tentative result and further research is needed

to confirm it. It does suggest, however, that the breakdown process in

liquids may not be dependent solely upon the average field strength at the

electrodes. Other processes no doubt are involved. The advantage of electrooptical measurements is that, by knowing the electric field, one can separate

field effects from other factors which could influence the breakdown process

like the particulate content of the liquid, electrohydrodynamic motion,

chemical nature, etc.

5.5 Electrode Material Effects on the Generation of Space Charge

Through Kerr-effect studies on other liquids, it has been established

that space charge can depend upon the type of metals used for electrodes.

This was noted by E. C. Cassidy, et al. [46] and more recently M. Zahn has

extended the results to water [47]. This project attempted similar

measurements in transformer oil.

To determine space charge as a function of electrode metal, an electrode

system was constructed which mounted four different metals in one cell, see figure 21. Because of the electrodes' proximity, small differences in charge

45
NORMALIZED

ELECTRIC

FIELD

-d/2 d/2

POSITION, z

Figure 18. Normalized electric fields for 60 Hz ac at room * empe r'at and PP refer to negative peak and positive peak respectively,

sudden drop in the positive peak profile near the HV electrode is an artifact of the detector and not some 'in usual decrease in the

field near the plate. The average peak field is 232 <7 cm.

46
NORMALIZED

FIELD

DC FIELDS AT 125°C

POSITION, 2

POSITION, z

Figure 19. Electric fields for dc at 125°C. At lower fields there is less

space charge near the cathode.

47
SPACE

CHARGE

(nC/cm

3)

NORMALIZED

ELECTRIC

FIELD 60 Hz AC FIELDS AT 125°C

-d/2 d/2

Figure 20. Electric fields for 60 Hz ac at 125°C. HP and PP reefer to tne

negative and positive peaks of the 60 Hz waveform respect i vel y

.

The average peak field is 224 kV/cm. The space charge, w.ni ;n ;

o

proportional to the derivative of the field, is shown for the

negative peak profile.

40
injection may not be observable. If the charge injection process were

significantly different for different electrode materials, however, such a configuration should render the differences obvious. Since transformer oil

tends to change its characteristics regarding space charge production as it

ages or is cycled through temperatures, the same oil sample was used for all

the metals at the same time.

The results using dc voltage were surprising in view of similar studies

with other liquids. No differences were observed in the space-charge

production, independent of the metals used. These results were obtained near

the breakdown voltage of the system. Figure 22 shows the light intensity data

obtained at room temperature. This is not the electric field, bur the

intensity data; the electric field exhibits much less enhancement. The data

are also obtained using windows which have some residual birefringence which

could not be eliminated. This has the effect of giving each profile a slight

slope. The important point is that each metal exhibits the same enhancement

upon polarity reversal within the error of measurement which could be rather

large for this preliminary experiment -- as high as 10% to 20%. No attempt

has been made to scale the data to account for nonuniform incident light

illumination which causes the height of each profile to be slightly different.

To reduce synergistic effects between neighboring electrodes, two of the

inner electrodes were removed so that the differing metals were more widely

separated. There was no change in the electro-optical behavior noted above.

Thus, in order to detect subtle differences, if they exist, the apparatus

must be refined or changed. Further, a comparison with 60-Hz ac must also be

made. It must be emphasized that this is a result of an exploratory

experiment and further work will be done.

5 . 6 Liquid Breakdown Phenomena

A liquid handling system has been installed which permits the

characterization of prebreakdown streamers in liquids as a function of

pressure from atmospheric pressure (0.1 MPa) up to 30 MPa. Initial

experiments were conducted up to 10 MPa on n-hexane. The breakdown voltage

for both polarities was found to increase as the pressure increased. The

increase was linear within the imprecision of the breakdown voltage

measurement, typically 10%.

As the pressure increased, the thickness of the streamer branches became

thinner for both the anode and cathode streamers. At higher pressures, near

1 MPa, the subsonic cathode bush, which appears routinely at atmospheric

pressure would not be found. At 0.6 MPa, the subsonic bush would occasionally

occur. In addition, near 1 MPa and near the threshold of the breakdown the

streamer would initially be quasi-sonic, but as it extended across the gap it

would slow down, stop growing and disappear just as if the voltage had been

removed, suggesting a minimum voltage of propagation [48], The increase in

breakdown and the suppression of streamer development with increasing pressure

are consistent with the assumption that the breakdown of a liquid at room temperature involves a change-of -phase . The increase in the hydrostatic

pressure would impede the growth of the lower density region which could lead

to the observed behavior.

49
<\J

<30

50

Multi-metal

electrode

configuration

to

reveal

dramatic

difference

between

electrode

metals

In

production

of

space

charge

while

employing

the

same

liquid.

The

gap

is

0.635

cm,

plate

width

is

1.27

cm

and

the

length

of

the

plates

Is

15

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(into

the

page).
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HV 0 0 2 60 0 J0

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A1ISN31NI

51
5.7 Partial Discharge Measurements and Their Photography

Partial discharge measurements are of interest since they tend to be an indicator of problems which can lead to apparatus failure. A detection system

was assembled which could detect 1-pC discharges. The system was calibrated

by discharging a 2-pF capacitor charged to 9 V using a mercury-wetted relay

with the cell's electrodes connected. The cell was filled with transformer

oil and the electrode spacing was 2 mm or 4 mm, depending upon the experiment.

Both electrodes had spherical surfaces with a radius of curvature of 1.3 cm. The transformer oil was circulated through a 2-/xm paper filter prior to the

application of voltage. It is anticipated that most of the particles are removed; however, as soon as voltage is applied, particles are drawn from the

sides of the cell. Even if the entire system is carefully cleaned with

various solvents and an ultrasonic cleaner, one breakdown produces many small

particles presumably from the electrodes and cell walls which were not removed

despite the careful cleaning. Thus, after several breakdowns, the liquid is

circulated through the filter. It is believed that this treatment produces

the minimum particle content.

Since breakdown voltage levels were employed, it was necessary to provide

protection for the oscilloscope's amplifier so that the breakdown currents

would not damage the oscilloscope while 'observations were being made on a millivolt scale. In addition to the electrical measurements, two optical

methods were used to view the partial discharges. Either a photomultiplier

detected light emitted from the gap, or high-speed photographs were taken

using an image -converter camera after a partial discharge occurred.

When the transformer oil was clean and even after a few breakdowns had

occurred, it was necessary to wait on the order of one minute or longer for a partial discharge having a magnitude of 1 pC or larger, to be detected by the

oscilloscope. A coincident light pulse was almost always observed with the

photomultiplier. The partial discharge could be observed with the unaided

eye, provided several minutes were allowed for the eye to adjust to the

darkened room. In general, it is anticipated that the pulse from the

photomultiplier will be somewhat proportional to the partial discharge

magnitude, however, because of the spherical electrode geometry used, the

photomultiplier does not see the entire surface of the electrode equally, and

a strict correlation of pulse magnitudes was neither expected nor observed.

To test the effect of conducting particles on the partial discharge

behavior, a few cubic millimeters of iron filings were added to the cell.

Upon the application of voltage, there was an increase in partial discharge

activity. A dim glow could be seen on both electrode surfaces in the vie in: tv

of the gap, and occasionally, what appeared to be a larger particle would

bounce between the electrodes emitting flashes of light each time it hit an electrode. A peculiar rare event was also observed, we might term it a partial breakdown. One could see a breakdown bridge the gap but it was no - complete in the sense that it did not lead to voltage collapse. The optical

and acoustical emissions from these partial breakdowns were less than those

from a full breakdown. High-speed photography of partial breakdowns did not

reveal any difference from the full breakdowns; however, in all the high- ’ photographs, particles could be seen along the pos t -breakdown , high- current

chanel. Presumably the presence of the particles influenced the postbreakdown phenomena.

52
To isolate the location of the partial discharge to a small area, the

ground electrode was changed from a sphere -like surface to a needle with a tip

radius of curvature of approximately 30 /im. The gap was 5 mm. Upon the

addition of conducting particles, partial discharge activity was readily

observed by the detection system. Attempts were then made to photograph the

partial discharge activity. In almost all cases, some streamer initiation

activity existed in the vicinity of the tip of the needle whenever a partial

discharge occurred. Unfortunately it was not possible to identify the source

of the initiation since all the photographs were taken after the discharge

occurred. More work is clearly needed. Most of this investigation, however,

will be done as a part of a new DOE- funded project at the University of

Tennessee

.

5 . 8 Summary

The objective of this task is to understand failure in high voltage

apparatus. Usually failures involve interfaces, so research to understand

interfacial phenomena associated with liquid- solid composite insulation is

necessary. Diagnostic techniques have been developed to monitor the

electrical behavior of oil, and a measurement system has been devised to

provide 1% field measurements in liquids with imprecisions of 0.2%. With such

accuracy, subtle changes in the liquid can be monitored with respect to

additives and particulate contamination. Partial discharge activity can also

be monitored and its influence on practical insulation systems can be

evaluated in terms of modification of the space-charge characteristics of the

oil. Coupling these measurements with high-speed photography provides a more

complete picture of the time evolution of failure in a practical system with

interfaces. Simultaneous measurments , electro-optical field measurements,

partial discharge measurements, and high-speed photography, are quickly coming

within the realm of possibility. With such a complete documentation of the

breakdown process, it will be possible to isolate those parameters which

significantly affect the failure in apparatus.

6. NANOSECOND BREAKDOWN IN POWER SYSTEM DIELECTRICS

Task 05

Ronald H. McKnight, William E. Anderson, David L. Hillhouse,

and Charles Fenimore

Electrosystems Division

National Bureau of Standards

6.1 Introduction

The objectives of this effort are to develop measurement capabilities and

provide data in support of the DOE program concerned with the effects of fastrising electrical signals on electric power systems. Whereas the effects of

microsecond rise-time pulses are reasonably well understood, the effects of

nanosecond rise time pulses on power system dielectrics are not well known.

Appropriate model test systems will be developed along with the necessary

measurement techniques

.

There are several aspects to this effort which were investigated during

the year. The technical areas discussed are all related to the subtask

objectives and are interdependent. These areas are the development of test

53
facilities, including measurement systems; mathematical methods to

characterize measurement systems with primary emphasis on deconvolution

techniques; electro-optic measurements of fast transients to improve

measurement capability; and observations of streamer growth in dielectric

fluids for fast risetime voltages to verify that nanosecond and microsecond

processes differ.

6.2 Test Facilities

A coaxial test line has been modified so that it is suitable for both

high voltage and low voltage operation. This allows the use of a well

characterized signal on the test line, and presents an optimum method for

evaluating sensors, since the low voltage waveform is more repeatable than

would be expected for a spark gap pulser.

As a part of these investigations, a new flat plate sensor configuration,

assembled by using an adhesive -backed polyester dielectric and a spravable

conductive coating, was evaluated. Although the new configuration was easy to

install and appeared to have electrical characteristics similar to the plug

divider which has been extensively examined [49], detailed analysis indicated

that the dielectric constant of the material -adhesive combination varied

considerably over the frequency range of interest and that other dielectrics

will have to be chosen. The sensors to be used in the future will be based on

the gas dielectric (plug) divider developed in earlier work. A commercial

Rogowski coil has been installed in the line in the test section, which is

equipped with several ports for mounting instrumentation and will allow

comparisons between various types of current sensor. While the current

measurement is not explicitly part of the DOE program, it is necessary for

overall understanding of fast transient measurements and completes the

instrumentation of the coaxial test line.

The accuracy of a transient measurement is determined by the overall

measurement system which includes the sensor, the signal transmission system

and the data recorder. As part of the system evaluation, calibrations of the

waveform recorders were begun. The recorders used have approximately 500-MHz

bandwidth with sub -nanosecond rise times. Measurements made using well known

dc levels indicated anomalous system response and it was determined that the

units being used were defective. Because the measurements are made for submicrosecond pulses but the calibration is made using a 1-kHz square wave,

there remains a question concerning the validity of the calibration procedure

In the near future, a step generator developed at NBS [50] will be configured

for use as a calibration device.

Although designed to determine the step response of slower waveform

recorders, it appears that the generator may be useful as a calibrator (wi~hin

a given accuracy range) for the digitizers being used here. This calibration

procedure is crucial, since comparisons of different measurement techniques

can only be done by knowing the overall accuracy of each measurement

Estimates of accuracy are also necessary if critical evaluations of

mathematical methods of signal processing, such as deconvolution are to be

made

.

54
6.3 Mathematical Methods

One of the impediments encountered in comparing different sensors or characterizing a single sensor is the lack of equivalent circuits to be used

in modeling. A description in terms of frequency response is usually

incomplete, and is not practical for devices which are required to have

nanosecond response, such as those in use here which are not amenable to

analysis by use of microwave techniques. An approach to describing the sensor

in terms of a measured step response and the manipulation of measurements

using deconvolution methods was outlined in a previous report [51]. In this

method, the response of a given sensor to a step-like signal is determined

using the coaxial test line. By measuring the input signal independently,

step and impulse responses for a sensor can be calculated. Using this

response, a given input can be calculated by deconvolution. In mathematical

terms, if the system is linear, causal, and time invariant, the input signal

x(t) and the output signal y(t) are related by using the convolution integral

y(t) = JqX(s) g(t-s) ds, (63)

where g is the impulse response of the system [52] . Given 2 of the 3 quantities x, y or g, the equation can be solved for the third. Difficulties

encountered in solving this integral equation were discussed in [51], and

extensively in the open literature.

The methodology has been applied to a number of different sensors,

examples of which are presented here. Two separate sensors, a capacitive

sensor with a coaxial, air dielectric, low side and a resistive divider with

adjustable stray capacitance, were mounted in the coaxial test line.

Measurements were made using two different signals, each of which was sent

down the line and detected by the two sensors. The first of these was a steplike signal, which was independently recorded and used in conjunction with

eq (63) to determine the impulse response of each sensor. The second of these was a "test” signal, which was generated by sending the step signal from the

pulse generator through an impedance which was primarily inductive. The step

and test signals are shown in figure 23. The response of the capacitive

divider to these two signals is indicated in figure 24. The droop associated

with the limited low frequency response of the capacitive sensor is seen on

the response to the step, as is the distortion of the test signal (compare

with the directly recorded input in figure 23). An impulse response g was calculated for the capacitive divider using eq (63), where x is the directly

recorded input (step-like signal) and y the response of the sensor, mounted in

the test line, to the step- like signal. This impulse response was then used,

again in eq (63) , to calculate the input waveform x for the recorded test

waveform output y. The comparison of the input waveform calculated using the

deconvolution method and the directly measured input waveform is shown in

figure 25, where the waveforms have been offset in time to facilitate

comparison.

A similar procedure was followed for the resistive divider. Although not

shown, the response of the divider was adjusted so that there was an overshoot

on the divider response. This overshoot is obvious in figure 26, where the

measured input and output signals for the test waveform are shown. As with

the capacitive divider, the step signal and divider response were used to

55
Figure 23. Waveforms used in studies measured directly from pulse generator.

The test waveform does not replicate beyond 40 ns as indicated.

Figure 24. Output signals from a capacitive sensor for test line Input

signals shown in figure 1.

56
Figure 25. Comparison of deconvolved and input signals for the capacitive

sensor. Signals have been offset for comparison.

Figure 26. Comparison of the output signal from a resistor divider with the

input signal.

57
determine an impulse response, which was then used in the deconvolution

integral eq (63) to calculate the input signal. Calculated and measured

signals are compared in figure 27. In each of the cases presented here, the

deconvolved and directly measured signals show excellent agreement. Other

waveforms and theoretically generated functions are being compared to provide

a basis for estimating uncertainites generated by the deconvolution process.

Some aspects of this work were described in greater detail at a recent Pulse

Power Conference [53].

The use of discrete Fourier transform techniques [54-56] to deconvolve

the two signals y(t) and x(t) was also examined. If y(t) is related to x(t)

through eq (63) , then

Y(w) = G(w) X(w) , (64)

where Y(w) , G(w) and X(w) are the Fourier transforms of x, g and y. In

general the problem is to solve for g, (or x) , given the other two quantities.

This suggests that the deconvolution could be accomplished by Fourier

transforming y and x, calculating the ratio, and then back transforming.

In practice, the situation is different in that all the waveforms are expressed by their value at discrete periods of time (i.e. they have been

sampled). The question then is, if

y_(t) = g (t)*x (t) (65)

s s s

does

Y (u>) = G (w)X (w) , (66)

s s s

where Y is defined to be the discrete Fourier transform of the sampled

s waveform y^ and indicates the convolution operation. The answer to this

question is, in general, no. The relation does hold if the convolution

product is defined to be a circular convolution as opposed to the linear

convolution that is of interest. (A circular convolution assumes that all

waveforms repeat at cycles equal to the inverse of the sampling frequency and

the convolution is calculated by taking contributions from adjacent cycles ) In other words g (t) can be calculated but it has no physical significance

A method was described in the literature [56] to use the circular

convolution approach to perform the deconvolution. This method will work for

only a special class of waveforms and was found to be too restrictive zo he o: value for this work. Other approaches of treating the special problems of non- repetitive sampled data (which is the situation for all measurement

considered here) may be useful and are under consideration [54],

58
1.5

1.0

0.5

0

-0.5

Figure 27. Deconvolved and directly measured input signals as obtained using

the resistor divider. Signals have been offset for comparison.

59
6.4 Electro-Optic Methods

The use of electro-optic technology in measurements of physical

quantities is proliferating, with applications in many areas. NBS has

extensively studied and applied the Kerr effect to the measurement of

electrical quantities, including voltage pulses and space charge in

dielectrics, applications described in earlier publications [57-59]. More

recently, exploratory efforts have begun to examine selected electro-optic

techniques to measure transient electrical quantities, such as voltage and

current. Although only part of this work is supported by DOE, it is

worthwhile to describe briefly the work that is being done and its relevance

to nanosecond measurements

.

The literature describing measurements using fiber optic and electrooptic techniques is growing rapidly and review articles have appeared in both

U.S. and foreign journals [60-62]. The few references presented here are representative and are not intended to be comprehensive, even though they are limited to electrical measurement applications. Papers have appeared

describing power industry measurements [63], transient signal applications

[64], EMP measurements [65], electric field mapping [66] and completely

"photonic" systems in which signal transmission, sensors and recording devices

are based on the use of light [67].

A schematic representation of a measurement system which can be used to

measure voltages or currents is shown in figure 28. The important features

are (i) a light beam of known polarization incident on the electro-optic

medium; (ii) an optically active sensor, which is a system in which the

transmission of light is affected by a an external agent imposed on the

sensor; and (iii) a polarization analyzer. Suitable light sources, detectors,

and signal recording devices are also necessary. To illustrate the type of

systems that may be used, the sensor may be a Kerr cell, a solid material

exhibiting the Pockels or Faraday effects, or a single mode optical fiber.

For any given measurement, not all of the components shown will be necessary,

aijd the geometric relationship between the light beam, direction of the field,

and the electro-optic medium will be determined by the application as will the

polarization of the light beam.

The use of the Pockels effect is particularly attractive for a number of

applications, including measurements of steady state voltages, transient

voltages, and electric field mapping. An analysis of this system indicates

the salient features of the measurement. Referring to figure 28, if the

polarizers are crossed and oriented at ±45° to the optical axis of the

crystal, it can be shown that the system acts as a phase modulator and that

the normalized intensity transmitted through the second polarizer is

I/I

0 = sin2 r/2,

where I is the transmitted intensity, 1^ the incident intensity, and V the

relative retardation (phase shift) for the two components of polarized

passing through the crystal [68]. The phase shift depends on the materia,

used as the sensor, its shape and orientation, and ignoring natural

birefringence and effects due to other external parameters (such as

60
61
temperature), the electric field in the crystal. This is contrasted with the

Kerr cell, where the phase depends on the square of the field in the medium.

The intensity of the modulated light varies between one and zero, and so the

amplitude range required of the signal processing is known for any signal.

Clearly, what is recorded is not a direct replica of the electrical signal

producing the electric field, but an intensity which must be demodulated

numerically to obtain the actual waveform.

For appropriate configurations using the polarizers and quarter wave

plate, the Pockels effect sensor can be used to provide a direct measurement

of either steady state or transient phenomena, under the assumption that the

phase shift is small. For this mode of operation, the quarter wave plate

provides an "optical bias" so that the system is operating around the halfintensity point. A polarization analyzer at the output of the sensor may be

used to produce independently the two components of the polarized light

passing through the crystal, and a simple analysis shows that the two

intensities can be written as xyiQ = 0-5d±r

m

). (68)

where F^ = 7rE(t)/E^. Here E^ is the electric field required to produce 180°

retardation between the two polarization components and is a function of

crystal material, geometry, and orientation. If E varies sinusoidally, then

signal processing may be done directly to yield a measure of the field, or if

calibrated, the applied voltage [63]. The characteristics of the materials

used in these Pockels effect sensors are such that measurements may be made of

GHz signals [69]. Materials which appear to be of particular interest for

electrical measurements include the potassium dihydrogen phosphate family,

lithium niobate (LiNbO^) , lithium tantalate (LiTaO^) , bismuth germanate (BGO)

and bismuth silicate (BSO) , although others have been cataloged [70, 71].

A calculation was made using eq (67) and the waveform shown in figure 29

for a lithium tantalate crystal 1 mm x 1 mm x 10 mm, with the crossed

polarizer configuration described above. The trapezoidal waveshape in

figure 29 was maintained, but the amplitude was varied. The results for three

different amplitudes are shown in figure 30, which show clearly the intensi co- variation predicted by eq (67). In practice, of course, the signals would be

distorted by noise and would probably be a sampled, rather than a continuous

record. The actual waveform would be recovered by fitting eq (67) to the

data, with E(t) as a parameter.

Although the approach appears promising for a wide range of measurement s. there are problems to be solved. Thermal effects, long term stability,

calibration, and mechanical resonances in the crystals are just represent a r : examples. Clearly, a suitable choice of sensor material may minimize many of

the effects which may corrupt the overall integrity of the measurement

6.5 Streamer Growth in Dielectric Fluids under Fast Risetime Pulses

An existing test cell used in studies of dielectric liquids was modi:

and an outline of the cell is shown in figure 31 along with a small pulser

used to characterize the water resistor divider which is used to measure the

62
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HI 0)

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63
Figure 30. Normalized intensity of light for Pockels effect measurement

described in text and for waveshape shown in figure 29.

Raggedness in waveshape is due to coarseness of time grid u.-u-d rot calculations, a. E =0 . 5 Mv/m. b. E -1 Mv/m. c. E -2 Mv/m

.

m m m

64
Breakdown

cell s. j81 C/5 1 £I oI C/D l o ! o 0

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Configuration

used

to

determine

the

response

of

the

water

resistor

used

to

measure

the

voltage

waveform

applied

to

the

gap.

Response

is

shown

schematically

on

the

figure.
voltage applied to the cell. This pulser provides a step-like signal with a rise time under 10 ns. The response of the system, as configured, to the

pulser output is shown on the figure, and is clearly less than 10 ns. There

is some small droop in the voltage over the time scale indicated. A more detailed drawing of the entire assembly is given in Figure 32. The high

voltage pulse enters on a flexible high voltage cable. A sharpening gap, gap

1, breaks down in oil and provides a fast rising voltage pulse to the test

gap, gap 2. For the total assembly and the gap conditions used, the rise time

of the pulse at the test cell was 50 ns. The gap, with point-plane geometry

and a spacing of 2 mm, broke down in approximately 150 ns after application of

the negative- going pulse. High speed photographs of the breakdown indicated

an absence of the bush- like growth of ionization observed previously with

microsecond pulses [72]. An example of the observed streamer is shown in

figure 33. The images were taken from the photographic record and the time

and exposure information is as indicated. The breakdown observed for the

present waveform was mediated by a supersonic streamer with propagation speed

g estimated at 2x10 cm/s. By contrast, the bush- like structure grows at

subsonic speeds.

During measurements taken with the modified cell, it became apparent that

the method of charging the water capacitor using a high voltage pulse

transformer was inadequate because of insufficient stored energy. Because of

this, attempts to improve the operation of the modified cell were terminated,

and a conventional cell was used with a commercial pulse generator capable of

providing a nominal 100 ns flat top pulse with a rise time less than 100 ns. The actual rise time was approximately 80 ns and output voltages were less

than 300 kV. Examples of the framing camera exposures taken during these

measurements are shown in figure 34. The geometry was a point plane geometry

with a 1-cm gap, and the generator voltage was adjusted to provide different

breakdown voltages. In figure 34a, breakdown occured at 120 kV, and the speed

4

of the streamer growth was approximately 7.1 x 10 cm/s. The time between

exposures in this framing camera record was 500 ns. In contrast, the record

shown in figure 34b shows a streamer with speed of 3.4 x 10^ cm/s. Here the

time between exposures was 50 ns and the exposure time is 10 ns. In neither

case is there any evidence for the bush-like structures observed for slower

rise pulses. It is clear that not only the risetime, but also the magnitude

of the voltage is important. The interpretation of these results in terms of

applied waveform has not been completed, but there are significant differences

between these measurements and those made earlier using slower pulses. This

suggests that the development of physical processes involved in breakdown mav be different in nanosecond breakdown from that at slower time scales.

6 . 6 Summary

The development of the coaxial testline and the comparison of different

sensors suitable for measurements of nanosecond pulses provides both a facility for evaluating measurement techniques and intercomparing diiieren - sensors, and a system for future studies of fast breakdown in gaseous

dielectrics. Continued operation at low voltage measurements have indie.it*'d

problems associated with certain capacitive sensors, and different designs

will be used. Deconvolution methods offer both a method for character it : nr. individual sensors in a quantitative way and the opportunity for achieving

66
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68
a)

b)

Figure 34. Comparisons of framing camera pictures of streamer growth in

N-Hexane showing two different streamer speeds. a. Breakdown

voltage of 120 kV, with time between exposures of 500 ns. Order

of exposure is that shown in (fig. 33). b. Breakdown voltage of

240 kV, with 50 ns time between exposures. 7.05, 6.28 (±0.4)

69
optimum measurement accuracy from a given sensor. The use of electro -optic

measurement techniques for fast transients is being used extensively, and

offers many advantages, including improved accuracy and electrical isolation.

Initial measurements made using fast rise time pulses show differences in

streamer growth in dielectric liquids.

70
7 . REFERENCES

[ 1 ]

[ 2 ]

[3]

[4]

[5]

[ 6 ]

[7]

[ 8 ]

[9]

[ 10 ]

[ 11 ]

[ 12 ]

NBS Report NBSIR 83-2761, Development of Power System Measurements - - Quarterly Report, January 1 - March 31, 1983.

NBS Report NBSIR 84-2898, Development of Power System Measurements -

Quarterly Report, January 1 - March 31, 1984.

NBS Report NBSIR 85-3112, Development of Power System Measurements -

Quarterly Report, April 1 - June 30, 1984.

M. Misakian, Generation and measurement of dc electric fields with space

charge, J. Appl. Phys . , Vol. 52, pp . 3135-3143, 1981.

T.C. Fry, Two Problems In Potential Theory, Am. Math. Monthly, Vol. 39,

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perturbation above the aperture.

It is interesting to note that the perturbation along the z-axis of an initially uniform electric field by a conducting spheroidal boss on the

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greater than the radius

.

G.W. Penny and R.E. Matick, Potentials in DC Corona Fields, Trans. AIEE,

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A variable step-variable integration algorithm, PLOD, developed at NBS

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71
M. Yashima, T. Tsutsumi, and M. Akazaki, Electrification current and

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[16] R. J. Van Brunt, "Effects of H_0 on the Behavior of SF, Corona," Proc.

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[17] R. J. Van Brunt and M. Misakian, "Mechanisms for Inception of DC and

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o

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72
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.

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.

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' -

•

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.

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57, pp. 4843-4848, (1985).

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"Observations of Prebreakdown and Breakdown Phenomena in Liquid

Hydrocarbons II. Point-Place Geometry, 1981 Annual Report." Conference v. Electrical Insulation and Dielectric Phenomena, pp . 377-389 (Oct. 1°81

76
NBS-114A (rev. 2 -eo

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 86-3316

2. Performing Organ. Report No, 722.01

3. Publication Date March 1986

4. TITLE AND SUBTITLE

Research for Electric Energy Systems — An Annual Report

5. AUTHOR(S)

R. E. Hebner

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

Annual January 1, 1985

to December 3-1, 1985

9.

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

Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, S. W.

Washington, D.C. 20585

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FiPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant 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,"

Department of Energy Task Order Number 137, funded by the U.S. Department of Energy f s Office of Energy Systems Research and performed in the Electrosystems Division of the

U.S. National Bureau of Standards.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) breakdown; corona; deconvolution; electric field; electro-optics; magnetic fields;

SFg; space charge; transformer oil

14. NO. OF

PRINTED PAGES

87

15. Price

$11 . 95

13.

AVAILABILITY

|X | Unl imited

|

| For Official Distribution. Do Not Release to NTIS

[ | Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

[~X~| Order From National Technical Information Service (NTIS), Springfield, VA. 22161

u SCOMM-OC 6043-P80