Reference

NBS

Publications

NBSIR 80-2071

Allies istsvi

Breakdown Between Bare

Electrodes with an Oil-Paper

Interface

Edward F. Kelley and Robert E. Hebner, Jr.

Electrosystems Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

June 1980

-QC

100

.U56

80-2071

1980

sponsored by.

Electric Energy Systems Division

Department of Energy

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NBSIR 80-2071 C<- 'X

. Ui^

BREAKDOWN BETWEEN BARE

^ ELECTRODES WITH AN OIL-PAPER

II io

INTERFACE

Edward F. Kelley and Robert E. Hebner, Jr.

Electrosystems Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

June 1 980

Sponsored by;

Electric Energy Systems Division

Department of Energy

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick. Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch. Assistant Secretary for Productivity. Technology, and Innovation

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director
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TABLE OF CONTENTS

Surrmary 1

Introduction 2

Theoretical Background 2

Experimental Conditions *. 6

Experimental Results 9

Introduction 9

Results Using Electrodes Having a Radius of Curvature of 1.6 mm ... 12

Results Using Electrodes Having a Radius of Curvature of 3.2 rnn . . . 12

Results Using Electrodes Having a Radius of Curvature of 6.4 mm . . . 14

Results Using Electrodes Having a Radius of Curvature of 12.7 mm . . . 20

Conclusions 24

References 23
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Summary

This report describes experimental measurements of the location of

electrical breakdown in a composite insulating system. For these measurements,

a paper sample was mounted so that it connected the two electrodes. Electrode

structures ranging from plane-plane to sphere-sphere were used. The

electrode-paper system was tested in oil in an attempt to determine the

properties of an oil-paper interface.

The data indicated that in a carefully prepared system the breakdown

will not necessarily occur at the interface. In addition, it was found

that the breakdown voltages were not significantly lower for those breakdowns

which occurred at the interface than for those which did not.

It was noted that if the paper interface was not dried or if many

gaseous voids were left in or on the paper, the breakdown will regularly

occur at the interface and at a lower voltage.
- 2

-

I. INTRODUCTION

The purpose of this work 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

was 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.

The theoretical background for interfacial breakdown is reviewed in

Section II of this report. Section III describes the experimental conditions

used in this test and the results are presented in Section IV. Section V

summarizes the conclusions which were drawn from this investigation.

II. THEORETICAL BACKGROUND

Much of the modern work on interfacial breakdown has been influenced by

the results of Wechsler and Riccitiel lo. ^ Their work was apparently motivated

by the revision of an American Society for Testing Materials standard test

method for solid materials. This method, and their experiments, used tapered

pins as electrodes (about 7.5 cm long, 2.5 cm centers, 2 cm/m taper). The

pins were inserted through 0.3 cm thick solid samples and the test piece was

immersed in an oil tank for measurements. It was 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.
-3-

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

field enhancement in the fluid at the interface caused by surface irregularities

on the solid. An idealized interfacial surface is shown in Fig. 1. The

potential difference, V, is assumed to be given by the expression

V

= E- *Iz • + E, *Zz-,

.

(

1 )

where is the field strength in the solid, E-j is the electric field

strength in the liquid, z^ is a length in the solid and z-j is a length in

the liquid. Neglecting space charge and conduction

E3 = (c,/C3)E, ( 2 )

where and are the permittivities of the liquid and sol id, respectively.

Combining Eqs. 1 and 2, the breakdown voltage, Vg, is related to the electric

fields by

(3)

where, for convenience of notation, the summation indices are implied. If

no solid were present, we note that
- 4 -

JT $ 01Ao

Q.Q0. i
c

• w

where d is the spacing between the electrodes.

( 4 )

So the ratio of the breakdown strength of the composite system to breakdown

strength of the fluid alone is given by

V''b' = <1 " ( 5 )

An alternative explanation of interfacial breakdown has also been

2-4

proposed. This model attributes the reduction of breakdown voltage at

0 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.

j Measurements of the steady-state 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.

il

IIr The measured interfacial current density was significantly greater than the

I'

j! current density through the bulk of the fluid. Separate measurements were performed

t

I which suggested that the increase in current was due to electrohydrodynamic

k
- 6

-

motion of the fluid. A subsequent re-examination of this v^ork indicated that

the magnitude of the interfacial current is strongly dependent on the geometry

5 of the electrode structure.

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.

III. EXPERIMENTAL CONDITIONS

The electrode-interface geometries under consideration are shown in Fig. 2.

The transformer oil used was a commercial grade and was filtered to remove particle

greater than 20 ym. The paper, typical of that used in transformers , v;as

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 differina 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.

The 60-Hz tests were performed using the circuit shov^n in Fig. 3. A motor- i
J

r-

^ ^

1

d/32

d/8 —

d/4 LrO

r = d/2

2.54 cm

FIGURE 2. Split electrode geometry for mounting paper samples. The four

different electrode sets are shown in approximate scale.
-8-

^|l

1

n

JUOjI"

o o

zr

>

o

(NCM

O

CO
-9-

controlled variable transformer is used to power a steo-up transformer rated

at 100 kV. The output voltage is measured using a capacitor divider havino

a compressed gas capacitor as the high impedance element and an active lowimpedance element. The divider output is the input to a peak-detect-and-hold

circuit which provides a dc voltage proportional to the peak of the input

waveform.

The output from a pulse transformer is used to trigger a pulse generator

when breakdown occurs. The output from the pulse generator, in turn, is used

to open the power input to the circuit at the first current zero following the

breakdown. By limiting the postbreakdown current in this manner, damage to the

electrode surface is reduced, but in no way eliminated.

The impulse voltage circuit is shown in Fig. 4. The impulse is obtained

using a conventional ^arx generator. The external load on the generator is

chosen to insure that the waveshape is relatively independent of the electrode

geometry or the fluid under test. The voltage is measured using a resistive

divider which has a response time of about 20 ns.^ The typical applied pulse

shape is shov/n in Fig. 5. This pulse shape is approximately the same as that

used by Taylor^ in an earlier study of interfacial breakdown.

IV. EXPERIMENTAL RESULTS

IV- 1 Introduction

Results are reported using all of the electrode geometries shown in Fig. 2.

Most of the work concentrated on the two electrode structures having the larger

radii of curvature, because they were the most technically significant.
Current

Limiting

Resistor

V

- 10-

(U

Q.

O

O

O

CO

FIGURE

4.

Impulse

circuit.
Normalized

Voltage

Normalized

Voltage

- 11 -

FIGURE 5. Applied pulse shape for impulse studies.
- 12 -

IV-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. For twenty breakdowns the mean

field strength at breakdown was 310 kV. A least-squares linear fit

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

approximately 0.4% per shot. This result repeated in a number of data runs

O and is qualitatively consistent with previously published data. This increase

in breakdown strength is presumably due to electrode conditioning.

In this specific experiment, one effect of conditioning was to roughen

the electrode surface. Figure 6 is a photograph of the electrode surface

after a number of breakdowns has occurred. Although the effect of the breakdown

is to create pronounced asperities, the breakdown voltage increased as the

number of breakdowns increased. Because the conditioning was a small effect,

and because the details of the process were probably specific to this

particular set of experimental conditions, no further study of the effect was

attempted.

I

I Results both with, and without, an interface indicated that the breakdown !

I occurred preferentially near the edges of the electrodes for this radius of ,1

curvature. It was assumed that there was sufficient field enhancement in the ^

i' vicinity of the electrode edges to dominate any interfacial effect. No further

studies, therefore, were performed using this electrode system.

IV- 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
- 13 -

FIGURE 6 The electrode surface, after a number of breakdowns, exhibit
-14-

series of breakdowns is shown in Fig. 7a. This photograph shows the relatively

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

location is shov/n in Fig. 7b. 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 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 hydrodynamical ly "dead"

spots in the vicinity of the interface. After repeated breakdowns using the

same interface, particulate contamination accumulated in the interfacial

regi on.

IV-4 Results Using Electrodes Having a Radius of Curvature of 6.4 mm

Data taken under impulse conditions are shown in Fig. 8. In this figure,

three separate regions are indicated. These regions denote different methods

of handling the interface. The interfaces for regions A and B had been vacuum

baked several weeks prior to their use and were kept in a closed bottle.

Because they had not been stored under vacuum and because they were handled in

air, there is a good chance that they were contaminated by water before use.
FIGURE 7a.

(b)

FIGURE 7b.

- 15 -

Facing electrode surfaces after a series of breakdowns.

Typical breakdown photograph. The grounded electrode is

at the bottom of the photograph; the high voltage electrode

is at the top. The paper sample connects the electrodes.
o o o o o o o o

o o o o o o o

CO LO CO CN T“

o

CO

o

lO

o

o

CO

o

<N

O

0)

n

E

3

C

O

•a

ccQ)

CO

<T3 O 4J

O) OJ M -O O

C i-o

O) M—

i.

(T3 4-> X

to O) c -•-> o

•1- OJ CD OI O) oo

s_

CD • CD SS- CD ^ c

f— c

fO

-a OJE

OJ 03

•r* to

CD E x:

S- +J oc+- c

•r“ • r— c

-o

CD

<T3 r—

-o C E

r” 03 J=

CDO -*-> ra o

<+- c • r—

CD CD

+-> E CD c 03 t—

• r— JD

to 03 c CD 1—

ro r—

B- E

+J E •I— 03 O to o

OJ

4-> E M

f— CD • p— s Q. E

03 03 -o CL E

c £= 03 CD E

JC to

+-> +J CD

•r— +J E

2 03 03

to 4-> 03 c +J 2 CD 03 o +J TC o 03U CD

03 •p— to

CD -o CD

S- c sz

C33 •r*

CX) UJ

cc; a6e}|0A ‘S (luo/am) P|9!d 3!JP9|3 umop>)eajg
-17-

The papers used in region C were vacuum baked and were kept under vacuum to

assure all water removal; they contacted air only as they v/ere loaded into the

cell. The method of loading the cell with liquid was also different in each

region. In region A the liquid was pumped into the cell at atmospheric

pressure. In region B the cell was evacuated and the liquid was pumped in

under vacuum. Under these conditions, the outgassing of the oil produced many

small bubbles which were difficult to remove from the interface. In region C

the paper was coated with oil before being mounted in the cell. The paper was

outgassed in the cell. The cell was then repressurized with dry nitrogen and

the oil was pumped in at atmospheric pressure. The last process, when combined

with careful drying of the paper, produced interfaces which were resistant to

breakdown. These procedures were intended to simulate the methods used in the

manufacture of power system apparatus.

A summary of the data presented in Fig. 8 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 contradict those

presented by Wechsler and Riccitiello^ they are consistent 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.
TABLE

1:

SUMMARY

OF

DATA

IN

UNIFORM

FIELD

BREAKDOWN

-18- co

•r*

+-> (Q

•I—>

OJ aasfO ac

(C +J

CO

OJco

O)

s-fC T3

0)

00

OJ

4->EE

O)uc

CM

(/) <U -Mo i

The

efficiency

is

the

ratio

of

the

breakdown

voltage

at

the

interface

to

the

breakdown

voltage

with

no

interface

in

the

system.
TABLE

2;

SUMMARY

OF

INTERFACIAL

BREAKDOWN

RESULTS

REPORTED

BY

TAYLOR II!, - 19 -

<D

CJ

fC

<+-s_

<u

+->c

•r— • E

OI O) x: 4-1

4-> to +J to

<T3 OJ

4-> +J

O’. d c c cu •!- o s-

•r— -M o; +J to u

(O (T3 •p* c M- > r— S_

QJ O) CD ~o •>- -M TD

4-

C

iC

(O 3 o o o c c -a

rO -M (T3 +->

(/) O) -i0)

S- S

XI c JZ o 4- 4J

<v o cr. d

SO CD (C •r- S_ T3

4-> -M

IT3 to

O) s_

+-> o to CD 1—

•r- -C CD +J -rto 4-

to

O) •I- c

•r" 2 4-> >1 o C CJ -O

•f"

C -5^ fC CD CO

4-> •r- CD

SU S_

O) •r- XI u 4- c 4- CD 3 CD x:

oi +J

CD x: o

t— h- -M - CM

to

O)

+-> o
F

- 20-

Equation 5, which is based on the model of Wechsler and Riccitiello"* , relates

the efficiency to the properties 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 two quantities which are divided to calculate the efficiency.

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

waveforms. Fig. 9. The 60-Hz voltage supplied to the interfacial system

increases approximately linearly with time at a rate of 3.6 kV/s up to a

maximum of about 116 kV. On several occasions the system withstood the full

voltage for a few seconds before the breakdown occurred. In addition, because

of a problem with the vacuum system, it is possible that a few of the interfaces

| were contaminated by water. As v/as the case with impulse voltages, however, the i

I data indicate no obvious enhancement of the breakdown probability in the vicinity ij

of the interface. Moreover, except for the first two and the last data points,

j for which water contamination is suspected, repeated breakdowns were required 1

before the breakdown occurred in the vicinity of the interface.

IV-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 sphere-sphere

I 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% 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 shown in Fig. 10 and are summarized in Table 3.
400

- 21 -

2

0)

•LL

O

O

0)

§

o

"O

czCD

u. GG

E

o

h- max

100kV

a; 300

O)

CO

<

LU o3 200

100

o

o o O

o

50kV

60 Hz

(114kV max)

• Breakdown at Interface

O Breakdown not at

Interface

I I I I J L i J L 0 10

Breakdown Number

d = 2.54 cm

d/8

d/4

FIGURE 9. Breakdov/ns at, and away from, the interface for 60 Hz stress

with uniform field.
700

22 -

00 0) nE3CSo

ID

(00) w. CQ

FIGURE

10.

Breakdowns

with

and

without

interfaces

for

sphere-sphere

electrode

geometry

using

carefully I

prepared,

dry

interfaces.

Peak

voltage

value

is

205

kV.

These

data a

re

sum

marized

in

Table
TABLE

3:

SUMMARY

OF

IMPULSE

BREAKDOWN

DATA

FOR

SPHERE-SPHERE

ELECTRODES

-23-

O)a

fO 4-

5-

QJ

CD SZ

-M • E +-> CD 03 4-> CO

CD >, CD CO

fC

+-> CD 1— ^ sz O +> o > • r— C

+->

C *1-

(T3 s

•r—

O <D > T3 CJ

(D ^ 03 -o 03 c+- -a

CD SS- CD s- JQ +>

(O SZ "O CD •!- sz SZ

(T3 -M

O

4-> SZ

CO <+-

O) o x:

4-> SZ O T- o •r- 3

0) +J03 CD

s- S- CD

rC 03 -o

CD +-> ^ 1—

OJ +->

O +J >

CO CO

•I- sz 1

B

CO >> o u -a

0) sz zs

•rTM CD 03

+-> •I- CD SZ O i-

•r“ •r- SZ

rO H4-> 4- CD

%. CD SZ

CD +-> O CD

SZ -C

O

ZD 1— +->

c^J

CO

OJ +Jo
-24-

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 Fig. 11. The first breakdown (without an

interface) was anomalously low. It was assumed that this breakdown was due to

some uncontrolled contaminant and so this data point is not included in the data

summary given in Table 4.

V. 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 carefullyprepared, paper-oil 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.
400

-25-

300 -

o

05

*-» (0 0

o

LLi >

d

5 ?

200

1 >

§— 100

gs

<D 0)

IS

^

-Max

<100kV

0

o

o

o

60 Hz

<50kV {'^USkV Max.)

0 Breakdown At Interface

O Breakdown Not

At Interface

I I I

I l I l__! I I 5 10

Breakdown Number

g=d/8

r = d/2

d= 2.54cm

Breakdov/n at, and away from tho intorfacG for 60 Hz strGSS with

sphere-sphere electrode geometry. The first point recorded

reflects an unusually low breakdown voltage and is ignored in

comparisons made in Table 4.
TABLh

4:

SUMMARY

OF

60-Hz

BREAKDOWN

DATA

FOR

SPHERE-SPHERE

ELECTRODES

-26- I!

d) OJu

+J (T3 <+- +J s- (C O) 4J c c

't— o

-o a; c fC +> o (V

•r— S- +j -M J3 03

S- 4-> > o o

0) 4-

C

•c

0)C o C-. S

s_ (C c

(T3M 03

03 1— ^ C O 03

fC > (1) +-> JCO O! J3

J3

cu +J S- c O c 4- 4- O

O) 0) i. O CO

(O •r- rc

-t-J +-> T3 (O 1—

0! SO

+-> >

00 0)

•r- J3 0)

4-> ^+J

CO CO

O) •rO • r— 4J

-M >v C O OJ c o

03 OJ 03

4-> 4-

S_

O iOJ •I- OJ u 4- +J _c 4- C —^ LlJ tC\J

CO

<V MO 4Iji
- 27 -

9

The influence of moisture and 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 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

9

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

systematical ly varied. These tests will provide information required to design

model tests which simulate the behavior of practical apparatus.
-28-

VI. REFERENCES

1. K. Wechsler and M. Riccitiello, "Electric Breakdown of a Parallel Solid

and Liquid Dielectric System," Trans. AIEE, Vol . 80, pp. 365-369 (1961).

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

Solid Dielectric Spacers Separating Uniform Field Electrodes in

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

3. E. A. Cherney and J. D. Cross, "Electric Field Distortions at Solid-Liquid

Dielectric Interfaces," IEEE Trans. Elec. Insul., Vol. EI-9, pp. 37-^2 (1974)

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

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

5. F. Frey and P. Atten, "Solid Spacer Influence on the Liquid Motion Induced

by Unipolar Injection," J. Electrostatics, Vol. 5, pp. 145-155 (1978).

6. R. E. Hebner, D. L. Hillhouse and R. A. Bullock, "Evaluation of a

Multimegavolt Impulse Measurement System," National Bureau of Standards

Report No. MBSIR 79-1933 (1979).

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

Solid/Liquid Interfaces," Proc. lEE, Vol. 124, po. 899-90-1 (1977).

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

Proceedings of the Sixth International Conference on Conduction and

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

pp. 111-116 (1978).
-29-

9. J. K. Nelson, H. Sabuni and P. B. .Mc^^rath, "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, lEE Conference Publication Number 177, pp. 14-17 (1979).
NBS-114A (REV. 8-76)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 80-2071

%^v*t. 4iiw»sion lio;

'"S'' -N >

• - '

V'; ; ;v; Z , ''' /

4. TITLE AND SUBTITLE

Breakdown Between Bare Electrodes with an Oil-Paper

Interface

5. Publication Date

Code

7. AUTHOR(S)

Edward F. Kelley and Robert E. Hebner, Jr.

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

!€. IHoi^ct/Ta^t/Wor*t IJnit No.

11. Contract/Grant No. EA-77-01-6010

#A057-EES

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

Electric Energy Systems Division

Department of Energy

12th Street and Pennsylvania Ave.

Washington, D.C. 20461

13. Type of Report & Period Covered INTERIM

6/79 - 3/80

Code ..

15. SUPPLEMENTARY NOTES

i

^1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less tactual surrunary of moat significant information. If document includes a significant bibliography or

literature survey, mention it here,)

This report describes experimental measurements of the location of

electrical breakdown in a composite insulating system. For these measurements

a paper sample was mounted so that it connected the two electrodes. Electrode

structures ranging from plane-plane to sphere-sphere were used. The

electrode-paper system was tested in oil in an attempt to determine the

properties of an oil-paper interface.

The data indicated that in a carefully prepared system the breakdown

will not necessarily occur at the interface. In addition, it was found

that the breakdown voltages were not significantly lower for those breakdowns

which occurred at the interface than for those which did not.

It was noted that if the paper interface was not dried or if many

gaseous voids were left in or on the paper, the breakdown will regularly

occur at the interface and at a lower voltage.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Breakdown; electrical breakdown; impulse testing; interfacial breakdov/n,

oil-paper interfaces; transformer oil

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