NBS

PUBLICATIONS

NBSIR 81-2275

1 980 Annual Report: Optical

Measurements for Interfacial

Conduction and Breakdown

A11100

R. E. Hebner, Jr., and E. F. Kelley

Electrosystems Division

Center for Electronics and Electrical Engineering

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

J. E. Thompson and T. S. Sudarshan

University of South Carolina

T. B. Jones

Colorado State University

January 1981

Issued May 1981

Sponsored by:

100

, 056

31-2275

1531

Department of Energy

•ffice of Electric Energy Systems

2th Street & Pennsylvania Ave., N.W.

/ashington, DC 20461

MATtOMAli BVRKAl

OR ffTAVDARUS

UH*A*Y

JUL 2 0 1981

NBSIR 81-2275

* « « 1980 ANNAUL REPORT: OPTICAL

MEASUREMENTS FOR INTERFACIAL

CONDUCTION AND BREAKDOWN

R. E. Hebner, Jr., and E. F. Kelley

Electrosystems Division

Center for Electronics and Electrical Engineering

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

J. E. Thompson and T. S. Sudarshan

University of South Carolina

T. B. Jones

Colorado State University

January 1981

Issued May 1981

Sponsored by:

Department of Energy

Office of Electric Energy Systems

1 2th Street & Pennsylvania Ave., N.W.

Washington, DC 20461

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director
.V- KW"\S. f

y <' ,> *
TABLE OF CONTENTS

PAGE

Executive Summary 1

1. INTRODUCTION 3

2. RESEARCH PLAN 3

3. CALCULATION OF THE ELECTRIC FIELD DISTRIBUTION

DUE TO SURFACE CHARGE ON AN INTERFACE PARALLEL

TO THE APPLIED FIELD ..... 5

3.1 Introduction 5 3.2 Laplace' sEquat ion 6

4. EXPERIMENTAL ARRANGEMENTS AND RESULTS 12

4.1 Electro-Optical Measurements in Transformer

Oil 12

4.1.1 Introduction 12

4.1.2 Experimental Arrangement 16

4.1.3 Experimental Results 16

4.2 Breakdown Location in Composite Insulating

Systems 28

4.2.1 Introduction ' 28

4.2.2 Theoretical Background 28

4.2.3 Experimental Conditions 32

4.2.4 Experimental Results 32

4. 2. 4.1 Introduction 32

4. 2. 4. 2 Results Using Electrodes

Having a Radius of Curvature

of 1 .6 mm 34

4. 2. 4. 3 Results Using Electrodes

Having a Radius of Curvature

of 3.2 mm 34

4. 2. 4. 4 Results Using Electrodes

Having a Radius of Curvature

of 6 .4 mm 36

4. 2. 4. 5 Results Using Electrodes

Having a Radius of Curvature

of 12 .7 mm 39

4. 2. 4. 6 Conclusions 42

4.3 Prebreakdown Phenomena in Transformer Oil .... 43

4.3.1 Introduction 43

4.3.2 Experimental Conditions 43

4.3.3 Experimental Results 44

4.4 D i el ec trophoretic Phenomena 47
TABLE OF CONTENTS (cont.)

Page

4.4.1 Introduction 47

4.4.2 S e n s i t i v i ty 50

4.4.3 Dielectric Constant Measurements 59

4.4.4 Circle Aperture Electrode Geometry .... 60

4.4.5 Theoretical Consideration 64

4.5 Gas-Solid Interfaces 65

4.5.1 Introduction 65

4.5.2 Vacuum Interfacial Field

Measurements 65

4.5.3 Low Gas Pressure Surface Electric

Field Measurements 70

5 . SUMMARY 76

6. ACKNOWLEDGMENTS 77

7 . REFEREMCES 78

8. PUBLICATIONS AND TALKS 81
Executive Summary

The design of power system apparatus requires proper placement of the

solid insulation, which provides mechanical support as well as electrical

insulation, to minimize the electrical stress along the solid-liquid or

solid-gas interface. In transformers, for example, the design surface

stress is an order of magnitude lower than the partial discharge inception

stress of either the solid or the liquid component alone.

The development of more compact systems, therefore, requires materials

and/or configurations which permit higher surface stresses. The purpose

of this work has been to develop techniques to measure the phenomena which

lead to electrical breakdown along an interface. The new measurement techniques

which have been developed are the electro-optical measurements of electric

field distributions. Specifically, systems have been developed to measure

the electric field distribution in transformer oil using the Kerr effect and

to measure the surface charge accumulation at a gas-solid interface.

The measurements have been supplemented by calculations of the

distortion produced by selected interfacial surface charge distributions. The

calculations also serve to relate quantitatively the measured distortion of the

electric field to the magnitude of the charge which produced the distortion.

Measurements of the field distribution in transformer oil between

parallel-plates show little space charge distortion of the electric field

for the relatively pure oil used in these studies. To obtain a more complete

picture of interfacial phenomena, breakdown locations and voltages were

also determined. With relatively pure materials, there was no

statistically- significant preference for breakdown in the vicinity of

the interface.

At vacuum-solid interfaces, however, interfacial breakdown was observed

and surface charging of the interface was measured. As gas is introduced into
- 2

-

the system, the surface charge is removed so that no effect of surface

charging was detected at atmospheric pressure. At pressures above

atmospheric, there is evidence that surface charging may again become

important.

New measuring techniques, therefore, are being developed which provide

useful insight into the behavior of composite insulating systems. These

techniques are being developed using waveforms and materials which are

useful for power system apparatus. It is anticipated that the results of the

electrical, optical, and electro-optical measurements will yield a description

of interfacial phenomena which will be useful in the design and/or testing

of future insulating systems.
-3-

1. INTRODUCTION

The objectives of this investigation are to develop apparatus and

appropriate procedures for optical measurement of the interfacial electric

field distribution and space charge density in materials for electrical

power equipment and systems; to understand the interfacial processes in

specific insulation systems; and to demonstrate the applicability of the

instrumentation and developed procedure in systems involving liquid-solid,

gas-solid, and extruded dielectrics. This report will deal only with

progress in the studies of liquid-solid and gas-solid interfaces as the

work on extruded dielectrics has been completed and reported.

2. RESEARCH PLAN

The design of power system apparatus requires proper placement of the

solid insulation, which provides mechanical support as well as electrical

insulation, to minimize the electrical stress along the solid-liquid or

solid-gas interface. In transformers, for example, the design surface

stress is an order of magnitude lower than the partial discharge inception

stress of either the solid or the liquid component alone.

The development of more compact systems, therefore, requires materials

and/or configurations which permit higher surface stresses. So, it is

apparent that techniques are needed to measure the phenomena which lead to

electrical breakdown along an interface. The technical literature suggests

2 that at an interface the electric field is enhanced. On the time scales

measured and for the materials studied, however, subsequent research suggests

that the enhancement is insufficient to yield the presumed reduction in

3

the breakdown.
- 4 -

To provide a definitive description of the prebreakdown process, it is

necessary to measure the electric field distribution. Probes are

unsuitable for such measurements because of their poor spatial and

temporal resolution and because of the difficulties in developing a probe

which does not perturb the system under test. Optical measurement techniques

based on the Kerr effect and those based on the Pockels effect however, are

suitable for the measurement of interfacial fields.

It is necessary to correlate the electric field measurement with

breakdown or, perhaps more importantly, with prebreakdown phenomena at the

interface. The correlation requires accurate current and voltage measurements

before and during breakdown. It further requires that the location of the

breakdown and prebreakdown events be determined.

The results of the electrical, optical, and electro-optical measurements

yield a description of the interfacial phenomena which is anticipated will

be useful in the design and/or testing of future insulating systems.

To insure that the measurement techniques are useful for power system

apparatus, the development was focused on two types of interfaces. One was

an oil-solid interface and the other was a compressed gas-solid

interface. Recognizing that the specific properties of the insulating

material may have an impact on both the measurement technique and the

interfacial behavior, these two investigations were pursued in parallel.

The specific outputs of both are summarized in the following sections.
-5-

3. CALCULATION OF THE ELECTRIC FIELD DISTRIBUTION DUE TO SURFACE

CHARGE ON AN INTERFACE PARALLEL TO THE APPLIED FIELD

3. 1 Introduction

An optical measurement technique is used to measure insulator surface

electric fields as flashover is approached. The physical processes leading

to flashover may, however, depend on the temporal and spatial behavior of

interfacial surface charge. Analyses have, therefore, been performed to

determine the relationship between measured fields and existing surface

charge. The stated problem is to determine the surface charge distribution

which produces the measured surface electric fields. Electro-optical techniques

generally measure the magnitude of the components of the electric field which

are perpendicular to the light beam (x-axis):

E

2

- E

y

2 + E

z

2

. (1)

If the direction of the electric field were such that the components

Eyand E z could be determined, then any space charge (or surface charge)

could be calculated via Poisson's equation

= p/e . (2)

Assuming a surface charge, the field and the resulting electro-optical

pattern may be calculated. When the calculated interaction pattern

reproduces the experimentally observed pattern then the assumed surface

charge distribution is a reasonable approximation to the surface charge

produced in the experiment. It is, therefore, necessary to develop

methods of field determination from surface charge distributions.
- 6

-

3.2 Laplace's Equation

The geometry relevant to the experimental measurements and the

on-going analyses is shown in Fig. 1. It shows two plane electrodes,

one located at (y,z) = (y,0) and the other at (y,d). Two insulators, one

with permittivity and the other with permittivity z ^ » are shown. The

interface between these two materials is located in the y=0 plane. It

is assumed that the material with permittivity, z^ extends to y=°° and

that the other extends to y=-°°. The electrodes are assumed to be infinite

in extent (x is also a variable). The system extends to ±°° in the x direction.

The insulators fill the space between the electrodes located at z=Q and z=d.

Laplace's equation, which determines the potential for the regions

bounded by the electrode surfaces and interface shown in Fig. 1, is

with the following boundary conditions for the region of interest (y<0):

The potential is defined at both plates

V

2 $(y,z) = 0 , (3)

$(y,0) = 0 , $(y,d) = V ; (4)

the potential is bounded

$(±o°, z) f

°o ( 5 )

the tangential field components are continuous across the interface
Electrode

- 7

-

Figure

1.

Geometry

used

in

the

solution

to

Laplace's

equation.
- 8

-

3$(.y»z)

3z

3$(y»z)

<pz

9 y=o

( 6 )

and there is a discontinuity in the normal component of the displacement

vector

3$(.y,z)

1 3y

y=o

3$(y,z)

2 3y p(z) , ( 7 )

where p(z) is surface charge density on the insulator surface.

The last boundary condition deserves some explanation. Given any

charged surface, a Gaussian "pill box" may be constructed to show that the

discontinuity in the normal component of the displacement vector is equal

to the surface charge density. In terms of the electric field and the surface

charge of this problem, an equivalent statement is

e 2^2y

^ “ ei^]y(0 »z) - p(z) . (8)

This amounts to specifying the normal components of the gradient of the

potential along the interface. Equation (6) is simply a statement that

the tangential component of the electric field be continuous across the

interface:

E-j z

( 0 ,z) E2

Z (0 ,z) . ( 9 )
-9-

Thus, the potential is specified on both plates and bounded elsewhere and

the first derivatives of the potential are specified along the interface.

These are sufficient boundary conditions to determine the potential and

field anywhere between the plates.

Laplace's equation is solved by the method of separation of variables

where it is assumed that

(y.z) = Y(y)Z(z) . (10)

Since y and z are independent, Laplace's equation requires that

9 2Y _ _.2 V 9 2

7. _ 2.-, 9 - a Y 5 9 “ — cl Z (id

9y 9z

where a is an arbitrary constant. For the case of a=0 these equations

have the solution

Y = Ry + S, Z = Pz + Q , ( 12 )

where R, S, P, and Q are constants. Imposing the boundary conditions,

Eqs. (4), (5), and (6) require the solution

4>0 (y>z) = Vz/d , ( 13 )

which is the unperturbed potential, i.e., the potential in the case of

p=0. In a similar way, the solution for c^O is found to be
- 10 -

$>a

(y,z) = (Ae

laZ + Be"

1az

)(Ceay

+ De"ay

) , (14)

or, in terms of trigonometric functions,*

$a

(y,z) = (A' cosaz + B' sinaz)(Ceay

+ De~ay

) . (15)

The boundary conditions, Eqs. (4)-(6), impose the following restrictions

on the constants: $(y,0)=0 implies A'=0; $(y,d)=V for all y implies that

sinad=Q or that

a = nir/d , n = 1 ,2,3 . . . ; (16)

cj)(+°°,z) f °° implies C=0 for y>0; (-°°,z) f °° implies D=0 for y<0; and

Eq. (6) requires that B'D (for y>0) be equal to B'C for (y<0) for each n.

Calling this last product of constants C

n

, the possible potentials for

a^O are then

$

n (y»z) = C

(1 exp(-n7r y/d)sin(n-rr z/d), y>0

C

n exp(+n-rr y/d)sin(n7r z/d), y<0 . (17)

Since any linear combination of solutions is also a solution to Laplace's

equation, the general solution for these boundary conditions, Eqs. (4)-(6),

is

$(y,z) = (Vz/d) + E $ (y, z) . n=l

n

( 18 )
- 11 -

The final boundary condition specifies the normal gradient of the potential

oo p

s /(e

l + e2^ "

n

f-| C

n (mT/d)sin(nTrz/d) . (19)

Noting that

sin(mTz/d) sin(rmTz/d) dz = 0 for n f m

d/2 for n = m

* ( 20 )

the constants C

n may be determined from the surface charge density

C

n

= [2/ n7T ( e-j + e

2 )] p

s (z)sin(mrz/d) dz . (21)

This completes the determination of the potential in terms of the surface

charge density on the interface. The electric field is then given by

t = $ . (22)

For the above potential, the components of the electric field are

oo E = -(V/d) - E (n-rr/d) C exp(n7ry/d) cos(niTz/d) , (23)

z n=l

n

and
- 12 -

Ey

"

nl -

(n-rr/d) C

n exp(n7ry/d) sin(nTrz/d) . (24)

The magnitude of the field is calculated in terms of these components.

To illustrate the effects upon the field of surface charge distributions,

the selected distributions in Fig. 2 are used to determine the predictions of

Eqs . 23 and 24. For the purposes of this calculation, it is assumed that

-5 2

the maximum value of the surface charge p Qis 1 x 10 C/m , that

e-|=£o and that ££=21 £

q

. The calculated solutions for are shown in Fig. 3

and those for E

z are shown in Fig. 4. The values for E

z are calculated for

the applied voltage equal to zero. Equation 23, however, indicates that the

effect of the applied voltage is to add a constant to each value of Ez (or,

equivalently, offset the vertical scale in Fig. 4). There are, probably,

some variations in the calculated distribution which are due to the fact that

the series expansions are truncated, but the calculated distributions show

the anticipated structure.

Thus, a technique has been developed to predict the electric

field distortion resulting from a surface charge at the interface between

two dielectrics.

4. EXPERIMENTAL ARRANGEMENTS AND RESULTS

4.1. Electro-Optical Measurements in Transformer Oil

4.1.1. Introduction

This section reports the investigation of space charge distortion of

the electric field in commercially-pure transformer oil. The measurements

reported are Kerr effect measurements, so the electric field is determined

by measuring a change in the optical transmittance properties of the oil.

Results are presented from experiments using an impulse waveform as well

as from those in which 60-Hz voltage was used.
- 13 -

Figure 2. Assumed surface charge distributions. In the calculations,

it was assumed pQ

= 1 x 10" 5 C/m .
-14-

Lower Electrode Upper Electrode

Figure 3. The calculated electric field component E for the surface

charge distributions in Fig. 2 y
Ez

Figure 4. The calculated electric field component E for the surface

charge distributions in Fig. 2. z
- 16 -

4.1.2 Experimental Arrangement

The experimental apparatus is shown schematically in Fig. 5. The

detector permits measurements to be recorded simultaneously at a large

number of points in the y-z plane. Using a pulse generators

the detector can be activated only for selected intervals of time. For

example, during measurements using impulse voltages, the detector was activated

for 100 ns at the peak of an approximately rectangular pulse with a duration

of 6 ps. Using a 60-Hz waveform, the detector was activated for about 100 ns once

every other cycle. This mode of operation permitted observations to be

made of the average -- over a number of cycles -- electric field at a

preselected point in the cycle, e.g., positive peak, zero crossing, etc.

4.1.3. Experimental Results

Figure 6 shows six Kerr-effect intensity profiles for six different

„ : ‘J . . . .% •

'

,

' peak voltages of the impulses. For the paral lei -pi ate test system used, the

maximum transmittance, fringe n=l , occurred near 139 kV. The fringe number

n is given by

n = 2BV

2

£/d , (25)

where B is the Kerr coefficient of the transformer oil, V is the applied

voltage, l is the plate length and d is the plate separation. For the

electrode geometry used (d/£ = 0.02), the effect of the geometrical field

distortion at the ends of the electrodes contributes less than 1% to the

optical phase shift. The end field effect, therefore, was ignored.

The Kerr effect is described by the relationship

I = I m sw

2

U/2) , (26)
-17-

Laser

(b)

Figure 5. Test arrangement, electrode structure, and typical output

data. Polarizers are denoted by P, , P^, and P~ while NDF

is a neutral density filter.
- 18-

(siinh Auvanaav) aiisn3j.ni

Figure

6.

Kerr

effect

intensity

profiles

for

six

different

peak

impulse

voltages.
-19-

for crossed polarizers. Here I is the maximum transmitted intensity and

<p is the phase shift given by

<J> = 27rB£V

2

/d

2

. (27)

From Eqs. 26 and 27,

4>/2 - sin' 1 (I/IJ

1/2

- (irBl/d

2

)V

2

. (28)

A plot of sin"

1 (I/Im)

1 ^ vs. V2 should yield a straight line with a

2 slope of ttBZ/ d . Figure 7 shows such a plot of data taken under impulse

voltages. A linear fit (least squares) of the form

sin' 1 (I/Im

) 1/2 = aV

2 + B ,

was performed and yielded the values

a = (0.0854 ± 0.0015) x 10" 9V

-2

and

6

= -0.043 ± 0.017 . (31)

From the value of a and the known experimental parameters (£=0.30m, d=6.4mm),

-15 2

the Kerr coefficient B of the oil used was calculated to be 3.6 X 10 m/V .

It should be emphasized that this value may vary among different types

of transformer oils and is a function of both oil temperature and the wavelength of the incident light.

(29)

(30)
- 20-

m

CM

in

in o

CM Csi

me m(wi/I) t .NIS

Figure

7.

Plot

of

the

voltage

squared

vs.

a

function

of

the

measured

transmittance

yields

a

straight

line

as

predicted

by

Eq.

28.
- 21 -

The determination of the Kerr coefficient under impulse excitation

demonstrated that the measurement system is capable of recording the

electro-optic Kerr effect in transformer oil. The system was then used to

measure the electric field distribution under 60 Hz excitation. Figure

9 show two different comparisons of the intensity profile. Figure 8 is a

comparison of the shape of the transmittance profile with zero volts applied

with the shape of the profile at a positive peak (65.6 kV) of the 60-Hz waveform.

If there were significant space-charge distortion of the electric field, then

there would be an obvious difference between the two profiles. No such

distortion is observed.

A second comparison is shown in Fig. 9. In this case, a 60 kV

60-Hz waveform is applied and the profile is measured at a zero of the

voltage waveform. This profile is compared to the light beam profile with

no voltage applied. Again, no significant change in shape is apparent. This

type of comparison was found to be a good indication of space charge in

4 nitrobenzene because the space charge cannot respond to the instantaneous

value of the applied field at 60 Hz, i.e., the space charge distribution

remains approximately constant during any cycle of the waveform.

The sensitivity of these measurements can be quantified. Recall the

basic relationships for the Kerr effect

4>/2 = sin" 1 (I/Im

) 1/2 = -rrBilE 2

. (32)

Assume that the electric field can be represented by the sum of the

geometric field Eq and the field caused by space charge E , i.e..

( 33 )
- 22 -

AXISN3XNI aiZnVHUON

Figure

8.

Comparison

of

shape

of

transmittance

wi
,1

I

I

I

!j

i

i

in

SilND AHVHiiaaV) A1ISN31NI

Figure

9.

Intensity

with

no

applied

voltage

and

ata

zero

crossing

of

a

60-Hz

wave.
-24-

Now, for convenience, we can express

E

s - f < z > E 0

’ (34)

where f(z) is some arbitrary function. Then Eq. 33 can be expressed as

E

2 - Eq2 [1+2 f(z) + f2

(z)] . (35)

A simple expression for f(z), and one that was appropriate for previous

work in nitrobenzene^

- is

f ( z ) = 6z/

d , (36)

where 6 is a constant. Note that f(z) cannot be a constant because

the path integral of the field must give the voltage, i.e..

V

= L

.d/2 d/2

E dz (37) = f

< E0 + E

s

) dz . d/2 J-d/2

l E ol. then <5«1 , then from Eq. 35 the souare of

tten approximately as

E 2 = Eo

2

(1 + 26z/

d

)

# (38)

As a result, the shape of the intensity profile should exhibit a z- dependence,

(Eq. 32),
-25-

sin*

1 (I/Im

) 1/2

- itBS,E0

2

(1 + 26z/

d

) , (39)

or

I/I

m

sin

2

[T7B*.E

0

2

(l + 2«z/

d

)] . (40)

For the 60-Hz data presented in this section, the argument of the sine function

This equation indicates that the presence of the assumed space charge distribution

changes the slope of the intensity profile by 46. This implies, for example, that

a 5% change in the field due to space charge distortion would yield a 20% change

in the profile. In Figs. 8 and 9, it is estimated that a 10% difference in

amplitude between the two intensity profiles would be easily discernable. It is,

therefore, concluded that the field distortions due to space charge are less

than 2.5%.

As an example. Fig. 10 explicitly shows the relative electric field as a

function of position between the plates. In these plots, it can be seen

that the electric field is not distorted.

It should also be noted that the application of 60-Hz voltage to

transformer oil causes fluid motion and associated optical scattering.

Electrical and optical techniques were developed to compensate for this

scattering and Fig. 11 shows that negligible changes in the profile occur

during a five-minute run.

is small (I ^0.1 I ), so that

(41)
RELATIVE

ELECTRIC

FIELD,

E/EM

(ARBITRARY

UNITS)

-26~

Z =

Figure 10

-d/2 z=d/2

POSITION

. Relative electric field as a function of position for three

different 60-Hz voltage levels.
-27-

(siiNn Auvuuaav) aiisnbini

Figure

11.

Repeated

measurements

at

the

same

voltage

during

a

5-minute

test.
-28-

Final ly, Fig. 12 compares data taken under ac and under impulse

-15 2

waveforms. The ac data yield a Kerr coefficient of 3.1 x 10 m/V compared

-15 2

to the value obtained under impulse of 3.6 x 10 m/V . The differences

are somewhat larger than expected and may be due to temperature differences,

to a frequency dependence in the Kerr coefficient, or to a systematic

difficulty in detection. Measurements will be made to identify the cause

of this discrepancy.

4.2. Breakdown Location in Composite Insulating Systems

4.2.1. Introduction

The purpose of this portion of the investigation is to provide a more

comprehensive and more fundamental understanding of prebreakdown phenomena at

an oil-paper interface. Because of particular concern with power system

apparatus, the investigation has been limited to 60-Hz and impulse waveforms.

A significant feature of the experimental program was the use of

photographic techniques to record the location of the breakdown event in a

composite insulating system. Different electrode shapes were used to obtain

a variety of field distributions.

4.2.2. Theoretical Background

Much of the modern work on interfacial breakdown has been influenced

2

by the results of Wechsler and Riccitiello. Their work was apparently

motivated by the revision of an American Society for Testing Materials

standard test method for dielectric strength of solid materials. They

found, under 60-Hz voltages, that the breakdown generally occurred in the

liquid at or near the liquid-solid interface. The breakdown strength of

the combination was as much as 50% lower than the breakdown strength of the

oil alone.
CM

e

Q

m

cc

<

O

v>

at

o

<

m(wi/I) kNIS

Figure

12.

Comparison

of

60-Hz

and

impulse

measurements.
-30-

To explain these results, they proposed that the breakdown was due to

field enhancement caused by surface irregularities on the solid. An

idealized interfacial surface is shown in Fig. 13. They calculated that the

ratio of the breakdown strength of the composite system to breakdown

strength of the fluid alone is given by

V

B

/V

B

' • (1 + £.)• I z

sj

/d , (42)

where e. is the ratio of the relative permittivity of the liquid to that of

the solid, z . is a length element in the solid along the interface (as shown

^ J in Fig. 13), and d is the gap spacing.

An alternative explanation of the interfacial breakdown phenomena has

O C C also been proposed. 5 5 This model attributes the reduction of breakdown

voltage at the interface to electrohydrodynamic phenomena. The experimental

results which motivated this explanation were Kerr-effect measurements of the

steady-state field distribution in the vicinity of a liquid-solid interface,

current measurement in the vicinity of the interface, and fluid flow

measurements.

Measurements of the steady-state dc electric field showed that the field

near the interface differed by approximately 10% from the uniform field

value. The authors concluded that these steady-state distributions offer

little insight into the interfacial breakdown process.

The measured current density near the interface was significantly greater

than the current density through the bulk of the fluid. Separate measurements

were performed which suggested that the increase in current was due to

electrohydrodynamic motion of the fluid. A subsequent re-examination of

this work indicated that the magnitude of the interfacial current is strongly

dependent on the geometry of the electrode structure.
Paper

h^ON

-31-

Figure

13.

Idealized

model

of

an

oil-paper

interface.
-32-

The fluid flow pattern was recorded using conventional flow visualization

techniques. These measurements suggested that the fluid cavitation may

contribute to a reduction of the breakdown in the vicinity of the interface.

The investigations summarized above indicate that, although insight

has been gained, a comprehensive, coherent model of interfacial phenomena

has yet to emerge. This report describes a consistent set of measurements

which demonstrate the strengths and weaknesses of the present models. In

addition, the significance of selected additional measurements is discussed.

4.2.3. Experimental Conditions

The electrode-interface geometries under consideration are shown in

Fig. 14. The transformer oil used was commercial grade and was filtered

to remove particles greater than 20 ym. The paper, typical of that used in

transformers, was 0.062 mm thick. Care was taken to remove as much water

as possible from the paper prior to its use in the tests. The paper was

clamped by the split electrodes and held parallel to the field. The four

electrodes were designed to be identical in all respects except, of course,

for their differing radii of curvature. The four radii of curvature were

chosen to be 1.6 mm, 3.2 mm, 6.4 mm, and 12.7 mm. The entire electrode

structure is 25.4 mm in diameter. Tests were performed using both impulse

and 60-Hz voltage waveforms.

4.2.4. Experimental Results

4. 2. 4.1. Introduction

Results are reported using all of the electrode geometries shown in

Fig. 14. Most of the work concentrated on the two electrode structures

having the larger radii of curvature, because they were the most technically

significant.
. -33-

I!

v- j

r-

<

I d —^1

2.54 cm

Figure 14. Electrode structure used to support paper. In the lower left

is shown, in approximate scale, the four electrodes used.

rmA
-34-

4. 2. 4. 2. Results Using Electrodes Having a Radius of

Curvature of 1 . 6 mm

A preliminary set of measurements was made without an interface, to

characterize the behavior of the oil alone. A least-squares linear fit

indicated that, on the average, the breakdown strength increased by

approximately 0.4% per shot. For twenty breakdowns the mean field strength

at breakdown was 310 kV. This result repeated in a number of data

o runs and is qualitatively consistent with previously published data.

This increase in breakdown strength is presumably due to electrode conditioning.

Results both with, and without, an interface indicated that the

breakdown occurred preferentially near the edges of the electrodes for

this radius of curvature. It was assumed that there was sufficient field

enhancement in the vicinity of the electrode edges to dominate any interfacial

effect. No further studies, therefore, were performed using this electrode

system.

4. 2. 4. 3. Results Using Electrodes Having a Radius of

Curvature of 3.2 mm

In contrast to the data taken with the electrodes having a smaller

radius of curvature, there was no tendency for the breakdown sites to be

concentrated near the electrode edges. The photograph of the electrode surface

after a series of breakdowns in shown in Fig. 15a. This photograph shows the

relatively uniform distribution of discharge sites. A typical record of a

breakdown location is shown in Fig. 15b. The paper sample joins the

electrodes.

From these photographs it can be seen that there is no dominant preference

for breakdown at the interface. To isolate interfacial effects, it was

judged to be most efficient to concentrate the investigation on electrodes
35

Figure 15. Upper photo shows the electrode surface after a number of

breakdowns while the lower one shows a typical breakdown occurring

away from the interface.
-36

with even larger radii of curvature -- electrode systems in which the

highest field strength is in the vicinity of the interface.

Two additional observations were made using the 3.2 mm electrodes.

First the average breakdown field strength increased by about 20% as the

radius of curvature was increased from 1.6 to 3.2 mm. This observation is

consistent with the assumption that the data taken using the 1.6 mm

electrodes were influenced by field enhancement at the electrode edges.

Second, even though the oil was circulated and filtered, the test geometry

resulted in hydrodynamically "dead" spots in the vicinity of the interface.

After repeated breakdowns using the same interface, particulate contamination

accumulated in the interfacial region.

4. 2.4.4. Results Using Electrodes Having a Radius of

Curvature of 6.4 mm

A summary of the data taken under impulse conditions is shown in Table 1

It should be emphasized that in this work, the differences among the average

values is of the same order as the spread in readings under any one condition

It is difficult, therefore, to use these data to support the hypothesis that

there is a significant difference between the breakdown strengths with and

without an interface. This is particularly true if the data obtained using

contaminated paper samples are eliminated. Although these data apparently

2 contradict those presented by Wechsler and Riccitiello they are consistent

q with the more recent experimental work by Taylor. Taylor's data are

summarized in Table 2. In an attempt to summarize the influence of the

interface, Taylor calculated the "spacer efficiency" which was defined as

the ratio of the breakdown strength of the solid-liquid combination to the

breakdown strength of the liquid alone. Equation 42, which is based on the

model of Wechsler and Riccitiello, relates the efficiency to the properties
Table 1: Summary of Data for Uniform Field Breakdown

Experimental Conditions Breakdown Field (kV/cm) Breakdown Voltage

IkV)

Efficiency

No interface, oil alone 529 i 47 168 t 15 -•

Interface in place but breakdown

not at interface 554 ± 50 176 ± 16 **

Breakdown at interface

(all data) 482 t 63 152 ± 20 0.91 t 0.16

Breakdown at interface (exclude repeat breakdowns

at same location) 494 ± 54 157 + 17 0.93 ± 0.14

Breakdown at interface (exclude both repeated breakdowns and possible contaminated samples) 526 ± 38 167 ± 12 0.99 i 0.11

Notts: 1 . Uncertainties listed art ont standard deviation.

2. Tht efficiency Is the ratio ot the breakdown voltage at the Interface

to the breakdown voltage with no Interface in the system.
-38-

Table 2: Summary of Interfacial Breakdown Results Reported by Taylor

Experimental Conditions Field Strength

(kV/cm)

Efficiency

No interface, oil alone 480 ± 39 -

Oil-polycarbonate interface 454 ± 44 0.95 ± 0.13

Oil-pressboard interface 401 ± 57 0.84 ± 0.16

Oil-polyphenylene oxide interface 463 ± 33 0.96 ± 0.11

Notes: 1. The uncertainties listed are one standard deviation.

2. The efficiency is the ratio of breakdown field strength at the interface

to the breakdown field strength with no interface in the system.
-39-

of the liquid and the solid. In Tables 1 and 2, the efficiencies are

calculated along with their uncertainties. The uncertainty is taken as the

square root of the sum of the squares of the uncertainties in the two

quantities which are divided to calculate the efficiency.

Using the same electrode system, data were also taken using 60-Hz

voltage waveforms. As was the case with impulse voltages, however, the

data indicate no obvious enhancement of the breakdown probability in the

vicinity of the interface.

4. 2. 4. 5. Results Using Electrodes Having a Radius of

Curvature of 12.7 mm

In an attempt to drive the breakdown to the interface, a split spheresphere electrode geometry was employed. In such a system the maximum field

strength occurs at the interface. Because of practical difficulties in

assembling the electrode structure with the paper sample in place, a

misalignment of the electrode halves was frequently observed. This misalignment

was generally less than 2 °l of the gap spacing. It was assumed that this

misalignment would increase the probability of breakdown at the interface

because of field enhancement. The data, however, indicate the results were

largely insensitive to this parameter. The data are summarized in Table 3.

There is also selective data analysis in Table 3. Those values for

breakdown at the interface in which the breakdown occurred the first time

that voltage was applied were averaged separately. The motivation for this

was concern that breakdowns away from the interface would deposit particulate

debris on or near the interface. As can be seen in Table 3, the breakdown

voltage was slightly higher for interfacial breakdowns on virgin systems than

for non-virgin systems. The difference, however, was too small to be considered

significant with the limited amount of data that were taken. The 60-Hz

breakdown data are summarized in Table 4.
-40-

Table 3: Summary of Impulse Breakdown Data for Sphere-Sphere Electrodes

Experimental Conditions Breakdown Voltage

kV

Efficiency

No interface, oil alone 173 ± 19 --

Interface in place but breakdown not at interface 172 ± 20 —

Breakdown at interface

(all data) 170 ± 26 0.98 ± 0.19

Breakdown at interface

(virgin system) 173 ± 27 1.00 ± 0.19

Notes: 1. Uncertainties listed are one standard deviation.

2. The elticiency is the ratio of the breakdown voltage at the Interface

lo the breakdown voltage with no Interface in the system.
Table 4: Summary of 60-Hz Breakdown Data for Sphere-Sphere Electrodes

Experimental Conditions Breakdown Voltage

kV

Efficiency

Interface in place but breakdown

not at interface 108 ± 11

om

Breakdown at interface 111 ± 6 1.03 ± 0.12

Notes: 1. Uncertainties listed are one standard deviation.

2. Efficiency is the ratio of the voltage for breakdown at the

Interface to the voltage for breakdown not at the interface.
-42-

4. 2. 4. 6. Conclusions

It is widely assumed that in many situations the interface between the

solid and the liquid insulation is the point in a complex system at which

breakdown would occur. The data reported here do not support that conventional

wisdom. From these data, which were taken using a carefully prepared, paperoil interface structure, it was concluded that the breakdown will not

necessarily occur at the interface. Similarly, the breakdown voltage for

breakdown at the interface is not necessarily lower than the voltage at

which breakdowns occur away from the interface.

The work indicated, however, that it seemed a relatively simple

matter to force the breakdown to the interface and at the same time reduce

the breakdown voltage. It is likely that interfacial breakdown will occur

if the paper is not carefully dried or if many gaseous voids are left in

or on the paper. The influence of moisture and the influence of gaseous voids

are recognized in the process of manufacture of high voltage apparatus.

Procedures have been developed, and are being followed, to minimize the impact

of these factors on the finished piece of apparatus. If moisture and air

are the dominant restrictions to the increase of the breakdown strength of

practical apparatus, then future research and development is needed in this

area. The detection of the air and water content of the materials in the

devices must be improved and materials must be developed with lower sensitivity

to the deleterious effects of these quantities.

It is likely that additional factors must be considered. Experience

has demonstrated that breakdown voltages are distributed statistically.

It is, therefore, reasonable to assume that the breakdown voltage at the

interface and the breakdown voltage in the fluid are each statistically
-43-

9

distributed. In fact, Taylor s data support the hypothesis that these two

voltages are described by different distributions. If that is true, a critical

parameter is the normalization of the distributions of the breakdown probabilities

in the two cases. Although such a scaling parameter has also been suggested

by the data of Nelson, et al.,^ it must be emphasized that the existence of

such a parameter has not been verified experimentally. Carefully controlled

experiments are required to determine both the breakdown voltage and breakdown

location as the ratio of the interfacial surface area (or volume) to the

stressed oil volume is systematically varied. These tests will provide

information required to design model tests which simulate the behavior of

practical apparatus.

4.3. Prebreakdown Phenomena in Transformer Oil

4.3.1. Introduction

Both cathode and anode streamers have been observed in transformer

oil under divergent field conditions and under uniform field conditions.

The temporal evolution of these processes has, however, not been studied

using uniform applied fields. Measurements were, therefore, performed on

the temporal behavior of both cathode and anode processes. Under the

conditions of these experiments, the cathode streamer development was a

three-step process while the anode streamer was a two-step process. Two-step

cathode processes have been previously reported for divergent and for

1

uniform fields.

4.3.2. Experimental Conditions

For these measurements, a commercial grade of transformer oil was used

with a sphere-sphere electrode geometry. The spheres had a diameter of 2.5 cm

and were spaced so that the gap between them measured 0.32 cm. The streamer

propagation was recorded using an image converter camera having an exposure

time of 10 ns and a time of 50 ns between consecutive photographs. A
-44-

pul sed (pulse duration is approximately 6 ys) argon-ion laser was used for

illumination. In this discussion, events are classified as subsonic (or

supersonic) if their propagation velocity is less than (or greater than)

the speed of sound in the fluid. The speed of sound is assumed to be the

propagation velocity of the shock wave which emanates from the breakdown

event.

4.3.3. Experimental Results

Figure 16 shows two cathode phenomena in the first four frames. The

small "bushy" streamer coming from the surface of the cathode is a subsonic

event. Other photographs show that this type of event has an average

velocity of (2.4 ± 0.4) x 10

4 cm/sJ (The velocities listed are assumed to

be accurate to ±20.%.) In the same frames an event with a higher velocity

is also shown. This streamer has an average velocity of 1.5 x 10 cm/s

which is at or slightly greater than sonic speed. Some acceleration

of this streamer may be occurring. In the fifth frame a supersonic event

initiates and has bridged the gap by the seventh frame. This supersonic

C

streamer has an average velocity of 4 x 10 cm/s which is about 25 times

the sonic speed.

Figure 17 shows the analogous development of an anode streamer. A

supersonic, bush-like structure initiates at the anode (frame 5) and

5

grows at a rate of 4 x 10 cm/s which is about 2.5 times the sonic speed.

As the streamer propagates it shifts to a faster propagation mode and bridges

C

the gap in about 50 ns. This second anode event has a velocity of 4.5 x 10 cm/s

-- 30 times sonic speed.

These preliminary data demonstrate a number of significant features.

For example, these data suggest that, as is the case with divergent-field

electrode systems, only the cathode streamer has a subsonic propagation mode.
-45-

2 4 6 8 10

1 3 5 7 9

IMAGE—CONVERTER CAMERA

FRAMING SEQUENCE

Figure 16. Cathode streamers are shown in the upper photo while the lower

drawing is the framing sequence. Each frame is a 10 ns exposure

with 50 ns between frames.
-46-

Figure

17.

Typical

streamer

initiating

from

the

anode.
-47-

It is also clear that both anode and cathode processes can independently be

responsible for breakdown in uniform electric fields. Figure 18 shows

simultaneous development of both anode and cathode streamers, with the

anode streamer responsible for breakdown. Another interesting feature

of this photograph is the cylindrical shockwave emanating from the "frozen"

cathode streamer. From these data, it is inferred that the streamer propagated

much faster than sonic speed until it was stopped when an anode streamer

bridged the gap, thus reducing the potential difference between the electrodes.

Figure 19 shows the unusual situation in which the anode and cathode streamers

caused breakdown by meeting in the midgap region.

4.4. Pi electrophoretic Phenomena

4.4.1. Introduction

Basic research, intended to support the study of liquid-solid

interfaces, was performed in the area of dielectric constant measurement

using di electrophoretic (DEP) levitation. The PEP technique offers the

potential advantage that it can be used to measure directly the dielectric

constant of the small particles which commonly occur in practical apparatus.

The investigation was to refine the analysis of the sensitivity of the

technique for dielectric constant determinations, to determine effects

attributable to particle size and shape, to develop apparatus to measure

the dielectric constant and to perform the measurements.

With this approach, all measurements are made relative to a standard

liquid with a well-known dielectric constant. The cell is calibrated by

levitating a small gas bubble in the standard liquid at a fixed position.

Throughout this section (Section 4.4) the subscript "C" is used to indicate

parameters associated with the calibration fluid, process or measurement.

The dielectric constant of spherical particles is determined by levitating

them in the standard liquid. Similarly, the dielectric constant of a liquid
- 48-

Figure

18.

Streamers

initiating

from

both

anode

and

cathode

with

breakdown

caused

by

anode

streamer.
49

CL) -oOc(o

<u x:

+-> c EO cn

S_ aj 4- S_

4- Q.

CD fQ E O') ro i <D "O

5- *i—

+J E

cn

<3J ro .c

+-> -O C

-o

CD CH

CO c 3 •<- tO +-> CD CD

0) cn E c

•r- CD CD "O

-QO

JZ c -t-> 3 « O U

T3

-X (D

ro -c

O) +-> s- -Q EO

4— 5—

CD CD i— C

CL

O

ro "O X c

uj ro

cn

O)

s_3

cn •I'- Ll.
-50-

can be determined by levitating a bubble at the same position using the

calibration liquid and the unknown fluid alternately in the same test cell.

4.4.2. Sensitivity

The sensitivity analysis is essentially complete; the important

contributions to measurement uncertainty have been identified and sensitivity

functions have been obtained. The effect of positioning errors on the

measurement has also been investigated. Some knowledge of the spatial

dependence of the electric field is required to estimate the required

sensitivity functions. For analysis purposes, the analytical solution for

a charged ring mounted with its axis normal to a conducting ground plane

(Fig. 20) has been used as a model of the electric field distribution. The

solution provides the field intensity and various derivatives needed to

estimate the effects of positioning errors and the deviations from sphericality

for either a bubble or a particle. The validity of the charged ring/ground

plane model for the cusped electric field has been tested experimentally.

One important conclusion drawn from the analysis is that the uncertainty

in the dielectric constant measurement is a function of the position at which

the bubble or particle is levitated. For most typical situations, it is

best to levitate the bubble or particle near the release point -- the

release point separates the region along the symmetry axis in which levitation

is possible from that in which it is not.

A summary of the error analysis is found in Table 5. These

expressions relate the uncertainty |-~| in the measured dielectric constants

for liquids and solids to the uncertainties in the important fluid

parameters and physical measurements. Because the dielectrophoretic

levitation scheme is a relative measurement (relative to a standard), the
Figure

20.

Ring-plane

geometry

used

to

provide

an

approximate

analytical

model

for

the

cusped

electric

field.
-52-

uncertainties in the cell calibration exercise must be considered. This

cell calibration is quantified by a geometric term K(z), which is

1

defined by Kallio and Jones with the equation,

VE

2

= |

g K(z)V

2

z (43)

where E is the electric field, g is the gravitational acceleration, and

V is the voltage. In the calibration exercise, K(z) is determined at the

levitation point z p

by

K(z

p

) -

p 2c

' plc 1 (k

2c - 2K

lc>

£ 1C. I 1C, - K« | \T olc lc 2c ' c

(44)

where V

c is the voltage required to levitate an air bubble in the standard

dielectric liquid at z=z

d? p-j and p£ are the densities and and ^

the

respective dielectric constants of the liquid and air. The unknowns are

expressed as functions of K plus other parameters. Specifically, for liquids,

KV

2 £ k + 2Ap + /(KV 20 Ko + 2Ap) 2 + 4KV

2Ape k 9 2-2 22 0_2

( 45 )

1 2KV

and for solids

<i

(KV

2

£q<i - 2Ap)

KV

2

2 e + Ap

( 46 )
-53-

Table 5: Summary of sensitivity analysis

CALIBRATION

6K <SAp c 6V | c K AP

C

v

c

1

6V | r k k "I 6k. | f 2 k. +1 2 c +

2c— I _^c +

I lc V ]2k, + k. k -k [ K 1 2k, + K c

1 L lc 2c lc 2c J 2c 1 U lc 2c

lc

K, “ K. lc 2c

- 1 6k

lc lc

LIQUID DIELECTRIC CONSTANT MEASUREMENT

i 6 k, = 2S

h

4

2 2'

d Ep V

2 dz

2

h

3 dE2

CM > 1

dz

6z + S

li E

2

,2 o

h

3 dE^

V

2 dz

• 6(K lc' K 2c )2ghR

oc

Ap

c

• 6

i _ K

2

} ShR

o Ap

(2k ' +k ) a

lc 2c c (2kj + k

2

) a

+ S

k

i + (S 1 + 1)

6K

2 K 1 + S 6Ap K 1 + 2S 6V

K K

2 K Ap K V

-k

1 + (a + A) (a2 + 8a + 2) (a + 2) + (a2 + 8a+ 2)

SOLID SPHERICAL DIELECTRIC PARTICLE MEASUREMENT

K

2 (2k

i k

2

)

K

1^ K

1 “ <

1

)

6k 0 = 2S

h

4 d2E

o' V

2 dz 2

dEo

,,2 dz

6z

o + S K2 h K

.6(k, - k. ) ghR Ap

lc . 2c oc K c (2k

1c + K2c ) °c

4 (<i

- k

2 )o

5(2k

1 + k2

)

+ S f + (sk

2 + 1)

6k

i k 2 + S„

6Ap 1 6V + 2S

K

1 K Ap K | V

3a(K /k )

1 2 K K K " 2Ha ^ +1)

2 2

a = y - 1
-54-

Accurate determination of K is, therefore, crucial to overall measurement

6K

accuracy, and so Table 5 contains an expression for |—^|.

The various implications of the results in Table 5 are best illustrated

by an example. Below are listed the principal assumptions and results of

calculating the various sensitivity functions for (i) the calibration

exercise, i.e., experimental determination of K, (ii) measurement of a

typical liquid dielectric constant k-j

, and (iii) measurement of a typical

solid dielectric constant k^.

(i) Cell calibration (determination of K)

. test cell dimensions, h=l cm (c.f. Fig. 20)

. spatial resolution, ± 8 ym

. calibration system: 100 ym air bubble in carbon tetrachloride

liquid medium (CCl^): k-j c = 2.228 ± 0.0005

0 25 °C p lc = 1594.0 ± 0.05 kg/m3

a = 0.02695 N/m

air bubble : = 1.000590 ± 5 • 10’

0 25 °C, .

1 x 105 Pa p 2c = 1.185 ± 0.0005 kg/rrT ;

where a is the surface tension.

For the conditions summarized above, the uncertainty in K is

P|| = 0.05% + 2 |y-^| (47)

5V

c where |y—| is the uncertainty in the calibration voltage measurement.

(ii) Unknown Liquid Measurement

Here, an estimate of the uncertainty involved in measurement
-55-

of the dielectric constant of a common liquid dielectric is made.

In this example, a silicone oil is chosen which has a

published value of k-j = 2.75.

. same test cell

. spatial resolution (± 8 pm)

. measurement system: 100 ym air bubble in silicone oil

liquid medium (unknown): k-j ~ 2.75

@ 25 °C P] = 972 ±0.5 kq/m3

a = 0.0212 N/m

air bubble : same as calibration .

For the above assumed parameters and uncertainties, the uncertainty in

the unknown is determined to be

|^-| = 0.32% + 0.58 |^| + 1.16 |^y| . (48)

Using Eq. 47,

6k,

= 0.35% + 1.16 [

6V

i!£n (49)
-56-

ry

where |—J-| is the uncertainty in the voltage measurement on the unknown. The

0.35% error in Eq. 49 encompasses all uncertainties (bubble elongation, spatial

resolution, etc.), except for those associated with the voltage measurements.

Equation 49 can be used to examine the effect of voltage precision on the

overall uncertainty of k-. . Table 6 contains some of these results.

Table 6. Effect of Voltage Precision on | ^ K

1

1 6V| j c|

6k-,

1

1

1

1 v

I *

1 V

1 c

I L/* 1 ^1

1% 2.7%

0.51.5%

0.1% 0.6%

0.01%* 0.37%

*lower limit on rms voltage measurement precision
-57-

Obviously, very little benefit is gained in reducing the voltage error

from 0.1% to 0.01%; however, the lower limit of 0.01% for rms voltage

measurement may be achieved without too much expense. Thus, for liquid

measurement, overall accuracy is limited to about 0.4%. It may be shown

that the chief contribution to this value (0.4%) is spatial resolution.

6k-|

Smaller bubbles (~ 50 ym) might permit an additional reduction to |

1 ~ 0.2%,

K

1 but it is difficult to imagine further improvements.

(iii) Unknown Solid Measurement

In this example, solid polystyrene spheres (100 ym) are chosen

as the unknown particles. It is assumed that they are to be

levitated in capacitor-grade castor oil. The nominal dielectric

constant for polystyrene is k

= 2.3. The various assumptions,

etc., are summarized below.

. same test cell

. spatial resolution: ±8 ym

. measurement system: 100 ym polystyrene particles in caster oil

liquid medium (caster oil): = 4.74

0 25 °C P] = 958. ±0.5 kg/m3

solid particle (unknown): - 2.38

P2 = 1055. ±0.5 kg/m

3

The solid particles are assumed to exhibit prolate spheroidal deviations

from ideal sphericity. If y is defined to be the ratio of the semi-major

axis to the semi -minor axis, the uncertainty of becomes
-58-

0.37% + 0. 1 32(y-l )% + 0.827|~|

+

6ki

1.83

In those cases in which the densities p

1 and p

2 are quite close, in

measurements for example, Ap = |p 2

- P-| | is a significant parameter.

Eq. 47, we obtain

6k,

= 0.41% + 0. 1 3 | Y-1% I + 1 . 83 1—-| + 0.83|6Ap|

K

1 Ap

+ 1 . 65 [ | 6V

V

J *

Several factors conspire to limit the precision of k

2

measurement.

Table 7. Effect of Parameters on

6k,

1%

ovV

0.1% .01%*

2%

Y

- 1 - 10% 9.5% 6.4% 6.3%

Y

- 1 = 1% 8.3% 5.4% 5.1%

1 6k. I Y - 1 = 10% 7.7% 4.7% 4.4%

K

Y

- 1 = 1% 6.5% 3.5% 3.2%

** 0.1%

Y

- 1 = 10% 6.0% 3.1% 2.8%

Y - 1 = 1% 4.9% 1.9% 1.6%

* lower limit on rms voltage measurement precision

**reasonable limit on liquid dielectric measurement

precision using conventional capacitance cell methods.

(50)

sol id-liquid

Using

(51)

Values

it is
-59-

The results of Table 7 are instructive because they provide guidance for the

specification of the measurement precision required for solid dielectric

particles. For example, it can be inferred that voltage precision greater than

about 1.0% is unwarranted.

4.4.3. Dielectric Constant Measurements

A levitation cell was constructed and has been used to measure dielectric

constant for some readily available dielectric liquids. The standard liquid

has been chosen to be a silicone oil. The results are compared to

measurements made with a standard capacitance cell. The results are

tabulated below in Table 8. Data on liquid density are also provided,

because they are needed in the equation used to compute the unknown dielectric

constant.

Table 8. Comparison of Measured Dielectric Constant Results

and Published Values

Dielectric constant values Density

Fluid

Levitation

cell result

Capacitance

cell result

Published

value

Measured

result

Published

value

DC-702

silicone

oil

calibration 2.79 2.78 1.086 1.071

Castor oil

(DI grade)

4.79 - 4.81 4.60 4.67 0.963 0.958

Com Oil 3.22 3.20 3.1 0.918 0.914

Transformer

oil

2.25 2.23 0.887 0.893

These measurements of the dielectric constants of liquids demonstrate that

the system operates as intended.
-60-

4.4.4. Circle Aperture Electrode Geometry

The experimental work has indicated that an electrode geometry is

needed which is easy to fabricate in small sizes. The result of this quest

has been the "circle aperture" arrangement shown in Fig. 21. The geometry

consists of three parallel electrodes. One electrode, in the center features

a circular hole of diameter d. This aperture forms the axisymmetric cusped

electric field which permits levitation of particles or bubbles. The circular

aperture levitator behaves in most respects like the ring-plane geometry.

The locus of stable levitation equilibrium points exhibits a release point at the

intense electric field end, above which no levitation is possible. Under

certain conditions, the lower limit of the locus of equilibrium points is

defined by a field null. However, under some conditions, the lower limit is

discovered to be a point of marginal radial stability. As the voltage is

increased a levitated particle moves toward this point and when the particle

reaches the point, it becomes marginally stable to radial displacements. The

result is that the particle slowly drifts off to the side and is lost.

The electric field for the circular aperture is approximated by the

1

result used in electron optics theory

E

2 + E

F = — L +

z 2

(E? ' E l> -1

{tan (z/a) +

z/a,

(z/a)

2 + 1 (52)

where z = z/h and a^ = d/2h and h and d are defined in Fig. 22. E-j and E

2 are

the uniform electric fields for z < 0 and z > 0, respectively, far from the aperture.

Limited experimental measurements with the circular aperture geometry

have been obtained for the case E-j = 0. The results are shown in Fig. 22.
Figure

21.

Axisymmetric

circular

aperture

geometry.

The

grounded

electrode

needed

to

produce

E-.

is

not

shown.
- 62 -

CO

O 00 CD

^CM O

a>

i.3

+-> i.

QJQ.

*0

t.

f0

3

O

O r-

<+- -r- O

CL

03 S- E OM 4-> V) C ro O U

X3 C

fO *r- iCT> (/> a

•o r- r- JD

CJ -Q

•r- Z3 4- X> O i.

•l— *r— t. rO M

U «

a> g * 0) o p— • fa ro x II <a =a

"o

<u S £ «— +-> <a <u E E

S- O

o a> 2: 03

CVJ CM

<US-3

03

~ “! JU91PBJ0 Pl®!d pezHBUUON
-63-

The theory from Eq. 52 with E-j = 0 is plotted to show the success of the

ideal electrostatic field solution in predicting device behavior.

4.4.5. Theoretical Consideration

Theoretical work has been devoted to establishment of criteria

for identifying the locus of stable equilibrium points in axisymmetric

14 cusped electric fields. This work is discussed thoroughly elsewhere

1

and is merely summarized here. The field expansion of Holmes is used to

provide expressions for the electric field components correct to second

order in small displacements of a levitated particle from some equilibrium

point (r = 0, z = z ) on the axis. The expansions are:

r

E

z = V [a

Q + o^z 1 + (*2 (z'

2

- r^)] (53)

E r

= v [- “2 r - a

2 rz '

(54)

where z

1 = z - z , V is the applied voltage and the coefficients

r a0> a-j , (*2 are functions of the equilibrium point

a (z ) = tt-U- n

v p' Vn!

9 n

E

9z r=0,z=z.

( 55 )
-64-

Table 9. Defining Limits of Stable DEP Levitation Locus

release point: z zr UPPER

LIMIT

marginal radial stability point: z - z*

LOWER

LIMIT

electrostatic field null: z zh

The upper limit on the locus (at the high electric field end) is always

2 2 2 defined by the release point z = z r

» where 9 E

z /9z = 0. The lower point

can be either a point where the particle is marginally stable, that is

2 2 2 2

9 E /9r = 0, or a field null, where E =0. As a particle approaches

a point of marginal stability it reaches a location (r = 0, z = z) where

the particle drifts off to the side indicating that radial stabilization

is lost. If instead the particle approaches a field null, then the voltage
-65-

can be increased without bound (theoretically) and the particle will not

move from the null. Examples of both kinds of behavior have been observed

in the electrode geometries tested experimentally and the theoretical

criteria of Table 9 have successfully predicted this behavior. The ring-plane

geometry exhibits a field null. Particles are not lost radially, even for

very high voltages. The circular aperture geometry (with E-j =0), exhibits

radial instability which is predicted by the criterion in Table 9.

For the geometry shown in the inset of Fig. 22, the theory predicts a

marginal stability point at z/h = -0.28 at a voltage of V = 1.34 kV (rms),

while the experimental observed results are z/h = - 0.27 and V - 1.3 kV (±0.1).

4.5. Gas-Solid Interfaces

4.5.1. Introduction

Measurements have been performed to determine, using the Pockels^

6

effect, surface electric fields at vacuum-solid and at gas-solid interfaces.

4.5.2. Vacuum Interfacial Field Measurements

Vacuum-solid interfacial fields have been measured using the Pockels

effect in both KDP and LiNbO^. The results obtained using KDP have been

previously reported^

7 and show that vacuum interfacial surface charging does

occur. LiNbO^ has also been used as the electro-optical sensing and

insulating material because: (1) a higher surface hold-off field can be

attained (70 kV/cm compared to 25 kV/cm for KDP); (2) it is less susceptible

to surface tracking and changes due to flashover discharges (LiNbOg has a

higher melting temperature than KDP); and (3) it is not water soluble and

is, therefore, easier to clean and handle. The test cell is shown in Fig. 23.
Vacuum

Feedthrough - 66 - E£

t5

>*

(/>Eo

CO >2

Figure

23.

Vacuum

and

low

pressure

field

measurement

assembly.
-67-

Results obtained from vacuum-LiNbO^ interfacial field measurements imply

that (1) the surface field varies linearly from anode to cathode; (2) surface

charge is removed by the addition of gas to the system; and (3) the tendency

for the insulator surface to maintain charge in vacuum decreases as the insulator

is conditioned.

The linear variation of the interelectrode field is such that the field is

enhanced at the cathode, relative to the average field value. This has been

observed for 60-Hz, dc, and pulsed excitations. Typical data illustrating

this is shown in Fig. 24. Electric field non-uniformity is indicated by the

bent and tilted fringes. The observed fringe deviation, Az, at any position

(y,z) can be related to the field, E(y,z) by

E(y,z) = 1.79 x IQ

5 Az(y,z)/6z , (56)

where 6z is the undeviated fringe spacing. Figure 24a shows the

interferometric data and Fig. 24b shows the computed field distribution

obtained from the interferogram. The field is seen to be enhanced at the

cathode and reduced at the anode by approximately the same amounts,

relative to the average field value. This will subsequently be shown

to be consistent with the fields associated with spatially uniform

positive surface charge. These charges are postulated to arise

from field emission from cathode protrusions and secondary electrons

cascading along the insulator surface.

The observed, spatially-varying field distribution remains after the

excitation has been removed. This is due to surface charges which remain

on the insulator surface. It has been observed that these surface charges
- 68-

Representation Interferogram

(a) Interferometric data E(kV/cm)

(b) Field variation obtained from (a)

Fl9Ure 24 ‘ int6rfaCial fi *' d Nation ">r 25- kV
-69-

can be removed by the addition of low pressure nitrogen. The displaced

fringe behavior shown in Fig. 24a is indicative of charges residing on

the insulator surface. The charged surface fringe behavior remains

unchanged as N

2 gas is bled into the system until a pressure of 26.7 Pa

is reached. The fringes are observed to relax almost instantaneously at this

pressure, back to the fringe distribution which existed prior to excitation.

From this behavior, it is inferred that the deposited surface charge is

removed by the introduced gas when the gas pressure reaches a certain value.

The charge removal process is accompanied by a glow discharge at the time

of fringe relaxation.

It is postulated that the low pressure gas is ionized by the fields

arising from the deposited surface charge. The ionized gas provides a

discharge path and hence a removal mode for the surface charge. This

observed result is significant because it shows that surface charge is

deposited and resides on the insulator surface.

It should be noted that breakdown and charge removal can occur in

a similar manner for a highly stressed vacuum interface where the gas is

introduced by electron stimulated desorption from the insulator surface.

This would be consistent with the flashover model postulated by Anderson

1

and Brainard which proposes that the vacuum gap breakdown is due to

ionization of desorbed insulator surface gasses.

The tendency of the insulator surface to maintain charge has been

observed to diminish as the insulator is conditioned by applying a

near flashover value of voltage for an extended period of time. The

magnitude of the surface charge has also been observed to be less after

a conditioning period.
-70-

The reduced surface charge probably arises from the

reduction, by microdischarges, of the number of electron emission sites

on the cathode. It, however, appears that conditioning not only modifies

the source of charging electrons but also reduces the insulator surface

resistivity. It has been observed in this work, and the work of others,

that the flashover voltage of conditioned insulators exceeds that of

unconditioned insulators due to the removal of cathode electronemission sites or a change in the secondary emission characteristics;

further, reduced surface-charge retention time arising from a modification

of the insulator's surface resistivity from conditioning has also been

observed.

4.5.3. Low Gas Pressure Surface Electric Field Measurements

Measurements, obtained using KDP, have shown that no measurable,,

interfacial field nonuniformity occurs along KDP-gas interfaces in

5

1 x 10 Pa SFg. This implies that negligible surface charge is deposited

5 along KDP-gas interfaces for pressures up to 1 x 10 Pa.

Interferograms have also been obtained, using the test arrangement of

Fig. 23, for LiNbQ^-N^ and LiNb0

3

-SF

6

interfacial field measurements for

5

gas pressures of 1 x 10 Pa.

C Interferograms obtained for L i MbO^-N^ (1 x 10 Pa) are shown in

Fig. 25. A maximum peak field of 30 kV/cm was obtained before gap

failure. The fringes shown in Fig. 25 remain straight and indicate no

measurable surface charging. The breakdown was observed to occur, not
Figure

25.

Field

interferograms

for

LiNbCL-N

9

interfaces

and

60-Hz

ac

excitation.
-72-

along the insulator surface, but through the N

2 pas, approximately 1 cm away

from the solid-N^ interface. This observation is consistent with the low

1

pressure data of Laghari and Oureshi. They observed that the flashover path

moved towards the solid-gas interface as the gas pressure was increased.

Laghari and Qureshi observed that the spacer efficiency decreases from a

value of unity at atmospheric pressure to approximately 0.7 as the pressure

is increased to several atmospheres. He further observed that breakdown

occurred along the spacer-gas interface at high pressures but through the

gas, away from the insulator surface, at atmospheric pressure. This

suggests that the surface, and perhaps surface charging, becomes important

only at higher pressures.

Interferograms indicative of the LiNbO^-SF^ interfacial fields are

shown in Fig. 26. Field values as high as 72 kV/cm were used and the gas was

at atmospheric pressure. No flashover was observed even for these extreme

field values. The fringes remain straight and indicate that no measurable

LiNbOg-SFg interfacial charging occurred.

The interfacial fields along a KDP-gas interface have been measured

for pressures up to 4.1 x 10 Pa using the experimental arrangement of

Fig. 23. Typical interferograms are shown in Fig. 27. The interferogram

of 27a corresponds to no applied field. The interferogram of Fig. 27b

was taken at the positive peak of a 60-kV/cm excitation. It should be

noted that the image quality shown in Fig. 27 is lower than desired since

a damaged KDP crystal is being used. Unfortunately this is the

only remaining crystal. Careful analysis of the interferograms of 27a and b
*73*.

Figure

26.

Field

interferograms

for

Li

NbO^-SF^

interfaces

and

60-Hz

ac

excitation.
- 74 -

(a) No field

<b) With field, 80 kV/Cffi

s

Figure 27. With a pressure of 4.14 x 10^ Pa, the fringe pattern changes when a field is applied to a KDP-N

2 interface.
-75-

shows that the fringes of b are bent slightly with respect to the fringes

of a. This implies that a surface-charge- induced field nonuniformity is

present during the peak excitation. The deviation is clearly indicated

in Fig. 27 by the two lines tangent to the fringes in Figs. 27a and b. The

tangents are not parallel. This indicates that the fringes are not parallel.

The angle between the fringes is measured to be 3°.

The observed fringe shift is related to the electric field, for the

transverse Pockels effect, by

Az

6z

N

3

r,.,£ 0 63

2A

E , (57)

3 where N Q

and r^

3 are known electro-optical constants, £ is the crystal

length, and X is the probing laser wavelength. Solving Eq. 57 for the system

used shows that there is one fringe shift per 8 kV/cm. The data of Fig. 27

show that the fringes corresponding to excitation are deviated approximately

1 fringe, or 8 kV/cm for an applied field of 60 kV/cm. The fractional field

change from anode to cathode is, therefore, 8 kV/60 kV = 0.13. The

present data indicate a nonuniform electric field and hence surface charge.

It should, however, be noted that the data is presently judged

to be preliminary. Efforts are being made to improve the image auality

to facilitate more accurate fringe displacement measurements. It

is expected that surface charging will increase at higher electric field

values. Higher fields, however, cannot be obtained with the available

KDP due to track damage. The maximum field attainable without flashover

along the track damage is 60 kV. Higher field values will be employed in

upcoming measurements using more practical insulators and the Kerr effect

measurement technique.
-76-

5.

SUMMARY

Among the significant technical outputs of this work are :

1. Analyses have been performed to determine the relationship

between the measured field and the interfacial surface charge;

2. The electric field distribution in transformer oil has been

measured using the electro-optic Kerr effect under both

60-Hz and impulse voltage waveforms using paral lei -pi ate

electrodes;

3. For a carefully prepared, paper-oil interface the breakdown

voltage for breakdowns which occur at the interface is not

necessarily lower than the voltage at which breakdowns occur

away from the interface;

4. Under uniform field conditions in transformer oil, both anode

and cathode streamers exist with the cathode streamer development a three-step process and anode streamer development a

two-step process;

5. Dielectrophoretic levitation sensitivity has been analyzed

and apparatus has been developed for the study of dielectrophoretic

effects in selected systems;

6. A vacuum-solid interfacial field is non-uniform with the field

enhanced at the cathode and reduced at the anode;

7. Conditioning of the vacuum-solid interface studied reduces both

the surface charge and the surface charge retention;

8. No surface charging on the solid studied has been observed for

N

2 or SFg at a pressure of 8.3 X 10"^ Pa.
- 77 -

6. ACKNOWLEDGMENTS

The authors would like to acknowledge the contribution of J. Vora of the

U.S. Department of Energy through his interest in this program and his

many technical suggestions. In addition, they would like to thank

S.B. Kelley for help in the preparation of this report.
-78-

7. REFERENCES

1. H.P. Moser (privately published by EHV-Weidmann Lim., St. Johnsburg, VT,

1979), pp. 50-52.

2. See, For example, K. Wechsler, and M. Riccitiello, "Electric Breakdown

of a Parallel Solid and Liquid Dielectric System," Trans. AIEE,

Vol . 80, pp. 365-369 (1961).

3. E.A. Cherney, and J.D. Cross, "Electric Field Distortions at SolidLiquid Dielectric Interfaces," IEEE Trans, Elec. Insul., Vol. EI-9,

pp. 37-42 (1974).

4. E.C. Cassidy, R.E. Hebner, M. Zahn, and R.J. Sojka, "Kerr-Effect

Studies of an Insulating Liquid Under Varied High Voltage Conditions,"

IEEE Trans. Elect. Insul., Vol, EI-9, pp. 43-56 (1974).

5. J.D. Cross, and R. Tobazeon, "Electric Field Distortions Produced

by Solid Dielectric Spacers Separating Uniform Field Electrodes in

Nitrobenzene," IEEE Trans. Elect. Insul., Vol. EI-8, pp. 25-29 (1973).

6. A.E. Cherney, and J.D. Cross, "Electrical Breakdown at Solid-Liquid

Interfaces," IEEE Trans. Elect. Insul., Vol. EI-12, pp. 214-218 (1977).

7. F. Frey, and P. Atten, "Sol id Spacer Influence on the Liquid Motion

Induced by Unipolar Injection," J. Electrostatics, Vol. 5, pp. 145-155

(1978).
-79-

8. D. F. Binns, "Breakdown Between Bare Electrodes in Transformer Oil,"

Proceedings of the Sixth International Conference on Conduction and

Breakdown in Dielectric Liquids, Editions Frontieres, Dreux, France,

pp. 111-116 (1978).

9. R. J. Taylor, "Effect of Permittivity Matching on the Flashover of

Solid/Liquid Interfaces," Proc. IEE, Vol. 124, pp. 899-904 (1977).

10. J. K. Nelson, H. Sabuni, and P. B. McGrath, "Impulse Breakdown of

Oil-Paper Insulation in the Vicinity of a Transformer Oil Duct Spacer,"

Proc. Third International Conference on Dielectric Materials, Measurements

and Applications, IEE Conference Publication Number 177, pp. 14-17 (1979).

11. E. F. Kelley, and R. E. Hebner, "Measurement of Prebreakdown Electric

Fields in Liquid Insulants," 1978 Annual Report: Conference on

Electrical Insulation and Dielectric Phenomena (National Academy of

Sciences, 1978), pp. 206-212.

12. G. A. Kallio, and T. B. Jones, "Dielectric Constant Measurements

Using Dielectrophoretic Levitation," IEEE/IAS Conf . , Toronto, 1978

(paper #IAS 78:4B).

13. A. B. El-Kareh, and J. C. C. El-Kareh, Electron beams, lenses, and optics ,

(Academic Press, New York, 1970) Section 3.8.
-80-

14. T.B. Jones, "Cusped Electrostatic Fields for Dielectrophoretic

Levitation," submitted to publication J. Electrostatics.

15. L.M. Holmes, "Stability of Magnetic Levitation," J. Appl. Phys.,

Vol . 49, p. 3103 (1978).

16. J.A. Giordmaine, "Optics, Nonlinear," in Encyclopedia of Physics

ed. by R.G. Lerner and G.L. Triggo, Reading, Ma.: Addison-Wesley,

1981.

17. J.E. Thompson, D. Hyslop, and T.S. Sudarshan, "Electro-Optical

Measurement of Insulator Surface Preflashover Fields in Vacuum,"

1979 Annual Report: Conference on Electrical Insulation and

Dielectric Phenomena, (National Academy of Sciences, Washington,

D.C. ) pp. 334-344 (1979).

18. R.A. Anderson, and J.P. Brainard, "Surface-Flashover Model Based

on Electron-Stimulated Desorption," 8th International Symposium

on Discharges and Electrical Insulation in Vacuum, Albuquerque, NM

(1978).

19. J.R. Laghari, and A.H. Qureshi, "Surface Flashover Voltages in

SFg-N

2

Mixtures Under Uniform and Non-Uniform DC Fields," 1979

Annual Report: Conference on Electrical Insulation and Dielectric

Phenomena (National Academy of Sciences, Washington, D.C.)

pp. 268-279 (1979).
-81-

8. PUBLICATIONS AND TALKS

During FY 80, the following were prepared and/or presented or

publ ished.

E. F. Kelley, and R. E. Hebner, "Time Evolution of the Electric Field

Associated with Breakdown Phenomena in Liquids," 1979 Annual Report:

Conference on Electrical Insulation and Dielectric Phenomena (National

Academy of Sciences, Washington, D.C.) pp. 203-211, 1979.

J. E. Thompson, D. Hyslop, and T. S. Sudarshan, "Electro-Optical Measurement

of Insulator Surface Preflashover Fields in Vacuum," 1979 Annual Report:

Conference on Electrical Insulation and Dielectric Phenomena (National

Academy of Sciences, Washington, D.C.) pp. 334-344, 1979.

D. E. Cooper, T. C. Cheng, K. S. Kim, K. Kantak, and J. Sletback, "Kerr

Effect Measurements of Electric Fields in Crossl inked and Uncross! inked

Polyethylene," 1979 Annual Report: Conference on Electrical Insulation

and Dielectric Phenomena (National Academy of Sciences, Washington, D.C.)

p. 389, 1979.

R. E. Hebner, "Streamers in Liquids: Basic Phenomena and Practical

Applications," invited talk at 14th Electrical/Electronics Insulation

Conference, Boston, MA, 1979.

T. B. Jones, and J. J. McCarthy, "Electrode Geometries for Dielectrophoretic

Levitation," Submitted to J. Electrostatics, 1980.
-82-

T. B. Jones, "Cusped Electrostatic Fields for Dielectrophoretic Levitation,"

submitted to J. Electrostatics, 1980.

E. F. Kelley, "Streamers in Liquids: Electric Field Distributions and

Propagation Parameters," talk presented at the ASTM Symposium: Mechanism

of Dielectric Breakdown in Liquids, San Francisco, CA, 1980.

E. F. Kelley, and R. E. Hebner, "Breakdown Between Bare Electrodes with an

Oil-paper Interface," Nat. Bur. Stand., Washington, D.C. (U.S.) NBSIR 80-2071 (1980).

E. F. Kelley, and R. E. Hebner, "The electric field associated with prebreakdown

phenomena in nitrobenzene," accepted by the Journal of Applied Physics, 1980.

E. F. Kelley, and R. E. Hebner, "Prebreakdown phenomena between sphere-sphere

electrodes in transformer oil," accepted by Applied Physics Letters, 1980.

D. M. Hyslop, J. E. Thompson, and T. S. Sudarshan, "Electro-optical

Measurements of Solid Insulator Surface Fields and Surface Charging in

Vacuum," National Bureau of Standards Report GCR 80-203, 1980.

E. F. Kelley, and R. E. Hebner, "Electrical Breakdown in Composite Insulating

Systems: Liquid-Solid Interface Parallel to the Field," accepted by IEEE

Transactions on Electrical Insulation, 1980.
-83-

T.B. Jones and M.J. McCarthy, "Dielectric Measurements an Particles Using

Dielectrophoretic Levitation," 1980 Annual Report: Conference on Electrical

Insulation and Dielectric Phenomena (National Academy of Sciences, Washington

D.C.) pp. 82-93, 1980.
NBS-114A (REV. 2-80

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBSIR 81-2275

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

4. TITLE AND SUBTITLE

1980 Annual Report: Optical Measurements for Interfacial Conduction and

Breakdown

1

5. AUTHOR(S)

R.E. Hebner, E.F. Kelley, J.E. Thompson, T.S. Sudarshan, and T.B. Jones

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS Ift«-6010

DEPARTMENT OF COMMERCE 1. Type of Report & Period Covered

WASHINGTON, D.C. 20234 Annual 1980

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

Office of Electric Energy Systems

Department of Energy

12th Street and Pennsylvania Avenue, N.W.

Washington, D.C. 20461

10. SUPPLEMENTARY NOTES

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

11.

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

bibliography or literature survey, mention it here)

Techniques have been developed to measure the electric field distribution

along an interface between insulating materials. Specifically, systems

have been developed to measure the fields in transformer oil using the

Kerr effect and to measure the surface charge at a gas-solid interface

using the Pockels effect. Measurements are supplemented by calculations of

the distortion produced by selected interfacial charge distributions. The

relatively pure transformer oil used in these studies showed little space

charge distortion of the electric field. There also was no statistically

significant preference for breakdown in the vicinity of the interface. At

vacuum-solid interfaces, however, interfacial breakdown was observed and

surface charging was measured. As gas is introduced into the system, the

surface charge is removed so that no effect of surface charging was detected

at atmospheric pressure. At pressures above atmospheric, there is evidence

that surface charging may again become important.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

dielectrics; electro-optics; gases; high voltage; insulation; interfaces;

Kerr effect; liquids; Pockels effect; solids

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