OCT XI

NESIR 83-2755

NAT L INST. OF STAND & TECH

A 1 1 1 0 b OBbTlD

Development of Power System

Measurements -- Quarterly Report

October 1 , 1 982 to

December 31, 1982

U.S DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Enginee r ing

Electrosystems Division

Washington, DC 20234

September 1 983

Prepared for Department of Energy

Division of Electric Enetgy Systems

oo Independence Avenue, SW

QC- ihington, DC 20585

100

,U56

83-2755

1933

or rr*i«DAjLua

NBSIR 83-2755 Quo

DEVELOPMENT OF POWER SYSTEM

MEASUREMENTS - QUARTERLY REPORT

OCTOBER 1, 1982 TO

DECEMBER 31, 1982

R. E. Hebner, Editor

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

September 1 983

Prepared for Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Foreword

This report summarizes the progress on five technical investigations

during the first quarter of FY 1983. Although reasonable efforts have

been made to ensure the reliability of the data presented, it must be

emphasized that this is an interim report so that further experimentation

and analysis may be performed before the conclusions from any of these

investigations are formally published. It is therefore possible that some

of the observations presented in this report will be modified, expanded,

or clarified by our subsequent research.
TABLE OF CONTENTS

Page

Foreword i i i

LIST OF FIGURES v

LIST OF TABLES vi

Abstract 1

1. INTRODUCTION 1

2. DC FIELDS AND ION MEASUREMENTS

Subtask No. A018 1

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. A053 6

4. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

Subtask No. A057 14

5. ACTIVE INSULATORS FOR INSULATION AND SURGE SUPPRESSION

Subtask No. A057 18

6. INVESTIGATION OF INSULATOR SURFACE FLASHOVER IN GAS

Subtask No. A057 21

7. REFERENCES 34
LIST OF FIGURES

Page

Figure 1. Schematic view of apparatus used to generate 60 Hz electric

and magnetic fields. Circularly polarized magnetic fields

can be obtained with the orthogonally oriented Helmholtz

coils 4

Figure 2(a) Waveforms 90 degrees out of phase 5

Figure 2(b) Waveforms 90 degrees out of phase 5

Figure 2(c) Displacement produced by 4.5 degree shift in phase is

readily observed 5

Figure 3. Measured concentration of SOF 2 In moles as a function of

accumulated energy dissipated by a positive-dc corona

discharge in SFg for the indicated discharge power levels

and gas pressures 11

Figure 4. Single-ion chromatograms for ions with mass-to-charge ratio

(m/e) of 86 (in a.u.) corresponding to SFg degraded by

corona discharges for the indicated gas pressures 13

Figure 5. Circuit diagram showing capacitive isolation of pulse

generator from dc. Isolation is accomplished via four

capacitors in series with a total capacitance of about 1 nF.

The current limiting dc resistor is 800 ko. Voltage is

applied to the closed cell via feed-throughs in the tank's

top. The cell has its own oil circulation system (not shown)

outside the tank 16

Figure 6. Data showing the usefulness of the impulse-enhancement of

lower dc voltages in making the Kerr effect more observable.

One third the maximum signal is shown compared to dc alone,

pulse alone, and pulse plus dc 17

Figure 7. Equivalent circuit model for lightning striking on transmission tower 19

Figure 8. Equivalent circuit model for lightning striking on conductor 20

Figure 9. The effect of tower resistance and lightning current on maximum active insulator current 22

Figure 10. The effect of tower resistance on acceptable lightning

current for limited active insulator capacity 23

Figure 11. Substation voltage level under a simulated lightning at the

middle of the transmission line 24

Figure 12. Maximum station arrestor current under a simulated lightning

strike at the middle of the transmission line 25

v
LIST OF FIGURES

Page

Figure 13. Pre-f 1 ashover currents, vacuum, 1.27 cm PMMA insulator.

60 Hz signal is voltage, other is current 28

Figure 14. Preflashover currents, point-plane (2.86 cm), nitrogen,

5 psig, 3.48 kV/div, 1 ms/div 30

Figure 15. Preflashover currents, bridged gap, nitrogen, 5 psig,

d = 0.2 cm, 3.78 kV/div, 1 ms/div 30

Figure 16. Pd inception (upper) and extinction voltage, d is nitrogen

gap size, PMMA insulation 0.635 cm in nitrogen 31

Figure 17. Inception, breakdown, and extinction voltages, bridged and

unbridged gap in nitrogen 31

Figure 18. Interference pattern indicative of electric field

distribution, no interface 33

Figure 19. Electro-optical field measurement insulator arrangement ... 33

LIST OF TABLES

Page

Table 1. Measured production rates for SOF 2 in SFg at the different

indicated gas pressures and discharge power 8

Table 2. Measured production rates for SO 2 F 2 in SFg at the different

indicated gas pressures and discharge power 9

Table 3. Ratios of production rates for SOF 2 and SO 2

F 2 at the

indicated gas pressures and discharge power 10

vi
UEVELUPMENT OF POWER SYSTEM MEASUKEMENTS -- QUARTERLY REPURT

October 1, 1982 to December 31, 1982

R. E. Hebner, Editor

This report documents the progress on five technical

investigations sponsored by the Department of Energy and

performed by the Electrosystems Division, the national Bureau

of Standards. The work described covers the period from

October 1, 1982 to December 31, 1982. The report emphasizes the

calibration of instruments designed to measure the bU-Hz electric

and magnetic fields in biological exposure facilities, the

measurement of the rate of decomposition of SF 5 in positive

dc-corona discharges, and in the measurement of space charge in

transformer oil between 1UU°C and lbU°C.

Key words: cables; composite insulation; dc fields; high voltage;

incipient fault; insulation; liquid breakdown; SF 5 ; space charge;

transformer oi 1 .

1. INTRODUCTION

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

the National Bureau of Standards, the Electrosystems Uivision, nBS, has been

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

This support has been concentrated in the following areas — the measurement

of electric and magnetic fields, the measurement of partial discharge phenomena,

and the measurement of interfacial electrostatic field distributions and of

space charge density. The technical progress made during the quarter

October 1, 1982 to December 31, 1982 is summarized in this report.

2. DC FIELDS AND ION MEASUREMENTS

Subtask No. AU18

The objectives of this investigation are to develop methods to evaluate

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

the electric field, conducti vi ty , the space charge density, and current density

in the vicinity of high-voltage dc transmission lines and in apparatus designed

to simulate the transmission line environment; to provide electrical measurement

support of OUE-funded efforts to determine the effects of ac fields on biological

systems and to provide similar support for biological studies which will be

funded by the State of New York.

Uuring the current reporting period, an apparatus for simultaneously

generating a bU-Hz rotating magnetic field and a bU-Fiz vertical electric field

was assembled. The apparatus consists of two Helmholtz coils oriented orthogonally

relative to one another and placed in the NBS parallel plate system. A scnematic

view of the system is shown in figure 1 . Dne of the magnetic field components

is aligned with the vertical electric field. As a result, the rotating magnetic

field is in a vertical plane. This apparatus was developed in support of the

New York State-funded biological studies in which animals will be simultaneously

1
exposed to a rotating magnetic field and vertical electric field. Several in

vitro and one human study are also planned. A requirement for the animal and

human studies is that the electric field and magnetic field both be aligned

vertically when the electric field reaches its maximum value. When this condition

is satisfied, it can be shown that the Lorentz force on charges moving within

the body can, under certain conditions, reach its maximum magnitude.

The NBS apparatus was used to determine how accurately one can measure,

using a dual beam oscilloscope, the phase relations between the two opthogonal

magnetic field components, which must be in quadrature, and between the vertical

magnetic field and the electric field, which must be in phase. If it can be shown

that the phase relations between the various field components can be determined

to within a few degrees, researchers can establish the proper phase relations

without the use of costly phase measuring i nstrurrentati on . Included in the NBS

apparatus was a phase shifter, which could vary the phase difference between

the sinusoidal signals that were amplified and used to energize the Helmholtz

coils and parallel plates. Also part of the apparatus was a phase meter which

could measure the phase difference between two signals from sensors in the field.

When determining phase relations between the different field components,

the phase shifts introduced by the field-sensing transducers should be noted.

The sensors used to observe the magnetic field waveforms were two "pick-up

coils" which are electrically shielded loops of many turns. The sensor for

observing the electric field waveform (not shown in fig. 1) was a flat plate.

Currents induced in the plate by the electric field were measured with a currentto-voltage operational amplifier circuit. The voltages from the magnetic

field sensors will be shifted in phase by -yu° because of Faraday's Law. That

is, tne electromotive force (EMF) from the sensing coil is given by

EMF = -N(d<j>/dt) = -(Nwtt cos ut) x (coil area),

where the flux 4> is given by

<}) = (B sin oat ) x (coil area),

and N and B are the number of turns of wire in the sensor and B is peak value

of the flux density. No further significant phase changes occur in the circuit

with the oscilloscope and the phase meter because the output impedance of the

sensor coil is very small compared to the input impedances of the oscilloscope

and phase meter.

Similarly, the induced current from the plate sensor is 9L)° phase with

respect to the electric field. This current is typically detected with electronic

circuitry which can also affect the phase relationships. For example, a current-to-vol tage inverting operational amplifier circuit with resistive

feedback will shift the phase of the current waveform by 1BU°. The output

voltage from the amplifier will thus be -9u° out of phase with the electric

field.

Because the parallel plates are also in the magnetic field, eddy currents

are produced in the plates and the magnetic field from the eddy currents will

perturb the field that produces it. Because of Lenz's Law, the field from the

eddy currents will retard the Helmholtz coil field and decrease the maximum

value (i.e., superposition of fields.) However, this perturbation of the

2
magnetic field will decrease as some function of distance away from the parallel

plates or eddy currents. The implication of this effect for the New York

State biological studies is that when measurements of the circularly polarized

magnetic field are made, the measurements should be performed at the location

where the test animals will be placed (usually the bottom plate of the parallel

plate system) and not, for example, midway between the plates. Similarly,

when the phase relation between the vertical magnetic field and electric field

are examined, it should be done near the animal location.

All of the above remarks regarding the phases of the field probe signals

and perturbation of the ambient field by eddy currents have been confirmed

experimentally with the apparatus shown in figure 1.

To determine experimentally how accurately an oscilloscope can measure

phase differences between two sinusoidal signals, the case of two sine waves

90° out of phase was examined. Figure 2(a) shows half a cycle of two sinusoidal

signals centered on an oscilloscope screen. The phase difference between the

two signals was simultaneously measured with the phase meter and equaled 90°,

confirming the visual observation. Greater precision can be obtained with the

oscilloscope measurement, however, by expanding the vertical scale of one of

the waveforms as shown in figure 2(b). The zero crossings of the expanded

waveform are symmetrical ly located with respect to the zero crossing of the

unexpanded signal. Figure 2(c) shows the effect on the oscilloscope display

of changing the phase between the signals by 4.5°. This difference can be

calculated with fair accuracy by noting the shift of ~ 0.025 percent (1/40)

of the expanded waveform along the horizontal axis. This measurement technique

assumes that overdriving the oscilloscope amplifier does not introduce asymmetry

in the amplified waveform.

The electric field mill which was tested during the previous reporting

period was returned to the manufacturer for several modifications. Measurements

at NBS had revealed that surface charging of some exposed circuit board near

the stator of the field mill resulted in field strength values that were too

low. Upon being informed of our test results, the manufacturer has redesigned

the field mill sensing head, eliminating the exposed dielectric surface. This

change as well as several other adjustments were performed on our unit under

warranty. The field mill has been returned to NBS and recent measurements

indicate that the device is functioning properly in the presence of space charge.

NBS hosted a two-day Workshop on Measurement of the Electrical Environment

Near High-Voltage DC Transmission Lines in October. The meeting brought together

individuals interested in the state-of-the-art electric field and ion-related

measurements near HVDC transmission lines and in biological exposure systems.

Measurement problems which require further study were identified and include:

measurements of ion density above and at ground level, measurement of ion

mobility spectra, and- identification of ion species. An increased interest in

measurements off the right-of-way was also shown.

The first revision of an existing IEC draft standard on power frequency

electric field measurements was completed at NBS and distributed to members of

a newly formed IEC working group for comments. This latest version of the

draft includes changes that were agreed to by the parent IEC technical committee

(TC42) as well as some observer proximity effect data measured several years

ago by NBS staff.

3
KDh

Figure 1. Schematic view of apparatus used to generate 60 Hz electric and

magnetic fields. Circularly polarized magnetic fields can be

obtained with the orthogonally oriented Helmholtz coils.
5

Figure

2(c)

Displacement

produced

by

4.5

degree

shift

in

phase

is

readily

observed
During the next quarter, problems associated with ion density measurements

above and at ground level will continue to be studied. Additional measurements

are planned with an ion counter operating above the ground plane. The first

measurements with an ion counter operating in the ground plane are also planned.

Preliminary theoretical calculations suggest some possible-measurement problems

when the ion counter is used in the ground plane of a parallel plate or coaxial

cylinder apparatus. Several visits to laboratories funded by New York State

are expected during the next quarter (none occurred during the last quarter)

and some on-site field measurements will likely be made. Comments from the

newly formed IEC working group members who are preparing a draft standard for

power frequency electric field measurements will be sent to NBS during the

next quarter and the second revision of the draft will be written at NBS.

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

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. A053

The objective of this project is to develop diagnostic techniques to

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

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

fundamental information and data needed to improve test design and performance

evaluation criteria. The investigation of partial discharges (corona) in

gaseous dielectrics is emphasized. This phenomenon gives rise to degradation

of the gas under high electrical stress which leads to breakdown. Measurement

of partial discharge inception in highly nonuniform fields may prove to be a preferred method to determine dielectric strength of electronegative gases.

The planned activities for FY83 include:

1) Preparation of conference and archival papers on the effect of radiation

in enhancing electric discharge initiation near a positively stressed electrode

in compressed SFg and O 2 ;

2) Extension of our previous measurements [1] of the production rates of

oxyfluorides in SFg corona discharges to include negative discharges as well

as other gas pressures and discharge power levels, and preparation of the

results for publication in an archival paper;

3) Identification of the predominant gaseous decomposition products from

corona in one or more of the following gas mixtures: SFc + No, SFc + CO?,

and SFg + c-C 4

F 8 + CHF 3 ;

4) Evaluation of a thin-film, aluminum oxide hygrometer probe for calibration

of a gas chromatograph-mass spectrometer (GC/MS) used to measure trace quantities

of water vapor in SFg and other gaseous dielectrics, and extension of our previous measurements on the effects of trace levels of H 2 O on electron

avalanche growth and corona discharge characteristics in SFg; and

5) Design and construction of a drift tube-mass spectrometer system to

evaluate measurements proposed to identify and characterize corona-generated

ion species in air, SFg, and other gas dielectrics.

6
Activity 1) was completed during this past quarter. A paper describing

the results of our measurements and theoretical analysis was presented at the

35th Annual Gaseous Electronics Conference and a manuscript was prepared for

submission to the Journal of Applied Physics. Considerable time was also

devoted to activity 2). The results of this work are highlighted in this

report. Concerning activity 4), a thin-film aluminum oxide hygrometer system

for measurement of water vapor content in gaseous dielectrics was received

and subjected to preliminary tests.

The computer software, which was developed to extract molecular species

production rates from GS/MS data, underwent a thorough evaluation under a wide

range of data conditions. Some improvements were made which allow a more

rapid and reliable analysis of data. The results reported were obtained with

the use of this software.

Our earlier measurements [1] of the production rates of SOF2 and SO 2F 2

from positive corona discharges in SF 5 have been extended to other pressures

and discharge power levels. Because of improvements in the analytical techniques

used and inclusion of correction factors for discharge power variations, the

data presented here are considered to be more reliable than those previously

reported [1]. However, it should be cautioned that a thorough error analysis

, has not yet been applied and, therefore, even these data must be considered

preliminary in the sense that no estimated uncertainties are given. We feel,

nevertheless, that the data are sufficiently reliable to reveal certain trends.

Examples of production rate results obtained from recent analysis of data

are given in Tables 1-3. Tables 1 and 2 apply, respecti vely, to the

oxyfluorides SOF 2 (thionylfluoride) and SO 2

F

2 (sulfurylfluoride). Given in

these tables are the production rates of those species in nanomoles per joule

at the different indicated gas pressures and discharge power levels, and for

accumulated energies, U , dissipated in the discharge of 10 kj and 15 kj.

These tables also give the maximum observed production rates at the highest

accumulated energy dissipated and the exponential fitting parameter e with

which the production can be described using the assumed form

which appears to be adequate to describe the production over the range of

energies used in the measurements. Here C is the concentration in moles

and A is a constant. The parameters e and A are determined from a least squares

fit of the form (1) to each set of experimental data (Cm(t), U ( t ) } , as illustrated in figure 3. Here C m

(t) and U ( t ) are, respecti vely, the measured

concentrations and accumulated energies at times t.

For our experimental conditions where the gas volume is 4.0 liters and

the temperature is 300°K, the measured concentrations C m

in moles are given by

C = AU 1+e

, ( 1 )

Cm

= 1 .625X10" 9 P Cr

(qm

/q r

), ( 2 )

7
Table 1. Measured production rates for SOF 2 in SF6 at the different

indicated gas pressures and discharge power.

Gas

pressure

(kPa)

Ave. discharge

power

(mW)

Ion

monitored

(m/e)

116 804 86

200 335 67

200 335 86

200 777 67

200 777 86

300 198 86

300 430 86

300 945 86

Measured production rates

(n moles/J)*

(U = 10 kJ) (U = 15 kJ) (max.

)

£

0.4 0.5 1.3 0.590

4.2 4.9 5.0 0.347

4.3 5.0 5.1 0.335

2.6 3.3 4.1 0.582

2.5 3.4 4.7 0.785

3.3 4.9 6.0 1.018

2.0 2.6 3.9 0.653

1.0 1.4 3.8 0.817

*1 n mole = 10” 9 moles
Table 2. Measured production rates for SO 2

F 2 in SF 5 at the different

indicated gas pressures and discharge power.

Gas

pressure

(kPa

)

Ave. discharge

power

(mW

)

Ion

monitored

(m/e)

Measured production rates

(n moles/J)*

(U = 10 k J ) (U = lb kJ) (max.

)

£

116 804 83 0.6 0.7 2.0 0.667

201) 335 83 3.2 3.7 3.7 0.326

200 335 102 3.4 4.1 4.2 0.335

200 777 83 1.7 2.0 2.5 0.460

200 777 102 1.7 2.0 2.5 0.485

000 198 83 1.8 2.3 2.6 0. 658

300 43U 83 1.6 1.9 2.4 0.362

300 945 83 0.7 0.9 2.1 0.664

*1 n mole = 10“ 9 moles

9
Table 3. Ratios of production rates for SOF 2 and SO 2

F 2 at the indicated

gas pressures and discharge power.

Gas

pressure

(kPa)

Discharge

power

(mill

)

r(S0F 2 ) /r ( SU 2

F 2 ) (0 = lb kJ)

116 604 0.73

200 33b 1.36

777 1.67

300 198 2.13

430 1.39

945 1.45

10
0 1 2 3x10 4

ENERGY DISSIPATED (JOULES)

Figure 3. Measured concentration of SOF2 in moles as a function of accumulated

energy dissipated by a positive-dc corona discharge in SFg for the

indicated discharge power levels and gas pressures.

I I

Hi

11
where P is the gas pressure in kPa, Cr is the reference concentration in moles

from a standard gas sample, and (qm/qr) Is the ratio of the GC/MS response

for the unknown sample to that of the reference sample. The values of qm

and

q r are obtained from a computer analysis of the GC/MS single-ion chromatograms

such as shown in figure 4. The analysis takes into consideration interference

from adjacent peaks and can give q m

and q r in terms of either peak area or peak height. In general, peak height and area results agree favorably; however,

the latter is considered more reliable under conditions where variations in

peak shape are significant during a given experiment.

The results reported here were obtained from measurements in which the

average corona discharge current I was held constant. Treating the discharge

as a dc-current source, the accumulated energy dissipated, U(t), in the discharge

at any time t is then given by

t

U(t) = I / V(t')dt', (3)

o

where V(t') is the instantaneous voltage drop across the corona cell.

The rates quoted in the tables are derived from the fits according to eq (1)

using the relationship

r = dC/dU = (1 + e)AU £

. (4)

It should be cautioned that there is no fundamental basis for believing that

eqs (1) and (4) provide an adequate description for arbitrarily large dissipation

energies and, therefore, extrapolations much above the maxima reported here

may not be valid. In fact, it appears from the data analyzed thus far that at

high enough U, the production rates approach a constant value (see, for example,

fig. 1 ), and at that point a simple linear fit to the data may be justified.

From the results presented here, certain trends are evident. First, there

appear to be no dramatic changes in the production rates with either pressure

or discharge power. All of the results for both SOF2 and SO 2

F 2 appear to

lie roughly within the range of 0.5 to 6.0 n moles/J, which agrees favorably

with our previously reported estimates as well as with those of Castonguay [1].

However, for both species, there appears to be a gradual decrease in production

rates with increasing discharge power. Between 200 and 300 kPa there appears

to be no significant change in the pattern of decomposition as manifested in

ratios of the SOF2 to SO 2F 2 concentrations given in Table 3. Below 200 kPa

the data show a reversal in the relative production rates of SOF 2 and SO 2F 2

. Further evidence of a change at lower pressures is provided by a comparison of

the chromatograms for the different indicated pressures given in figure 4, which

shows a noticeable change in the relative production of SOF 4 compared to SOF 2.

Further measurements are needed to substantiate these trends and thus it

would be premature at this stage to attempt an explanation. It is perhaps

worth noting, however, that formation of both SOF 4 and SO^F 2 Is believed to

depend on the presence of free oxygen in the discharge [1]. Also, SOF 4 can convert to SO 2 F 2 through the reaction

12
RELATIVE

GC/MS

RESPONSE

Figure 4

m/e = 86

iu

f i

i i

f i

i i

! k

sof2

sof4 i [

ih

a

i L

i 1

f 1

r i i !

I L

! \

f !

4

l1 1 1;. i ==j I *:.! : i

•»

k = i -i ! - kkJ : =.i = ,= JLM . 7S ,» - «- « n=_

=

: ===

4 f'vi f UN

SF,

i* .

i ^

i i i i.

L

n,

r i

; i

! i

r L

4

1 4 4

I 4

! !

Ui,

nit1i.

i /

j

= _/

^ =r Vi

t i

; i r ;

i J

I * J r

i

.

P=116 kPa

1 4 3 ;

• ! ! 5 . rL =4 j . 4. “=~^ J l: rL i "= >A ; >k -== - \f

^U= i 4 ”4

iit

•«!U.

1H

r %

i k.

! “

i h

V. i . : ! IMA k L . . i .

. =jj ~4ry..j4 IIJ-0 ,144 . =?=• - ^y^=== yy

= a

4

4. , ==

*a! J

;i k ?d «y i1 44 4 I

4. ? 1.7 «4

k

:J~

Lt." L ii : : 1 : 1% I = L

!

L? ~=~y « = -- P = 300 kPa

0 1.0 2.0 3.0 4.0 5.0

RETENTION TIME (min)

Single-ion chromatograms for ions with mass-to-charge ratio (m/e) of

86 (in a.u. ) corresponding to SFg degraded by corona discharges for

the indicated gas pressures.

13
SOF 4 + H 2O SO 2

F 2 + 2HF

.

One, therefore, expects that any discharge parameter which can affect the

relative availability of oxygen or water vapor in the discharge region will

also affect the relative production of the various observed oxyfluoride species.

During the next quarter, we plan to continue the measurements of oxyfluoride

production rates from corona in SF 5 as described in this report. The measurements

will be extended to include not only other gas pressures and discharge power

levels, but also negative corona to check on possible polarity effects. More

attention will also be given to error analysis and determination of the uncertainties

in the measured production rates. The objective is to bring this series of

measurements to a conclusion so that preparation of an archival paper describing

these results can begin. Further testing of the thin-film aluminum oxide

hygrometer probe method for calibration of the GC/MS system for quantitative

water vapor measurements in SF 5 should also be carried out in the next quarter.

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

4. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION

AND BREAKDOWN IN INSULATING SYSTEMS

Subtask No. A057

The objectives of this investigation are to develop apparatus and appropriate

procedures for the optical measurements of interfacial electric field and

space-charge density in materials for electric power equipment and systems, to

understand the interfacial prebreakdown and breakdown processes in specific

insulating systems, and to demonstrate the applicability of the developed

instrumentation and the procedures in the development and design of future

systems.

Last quarter the electric field between parallel plates with a pressboard

spacer separating the plates was investigated. Since the electrical strength

of that interfacial system was much lower than without an interface, electrical

breakdown would occur before reasonable electro-optical measurements could be

performed. For example, if the cell constant V m were 170 kV and typical

breakdown voltages were around 30 kV, then the maximum transmittance I /

1

m through the cell due to the Kerr effect, given by

I/I m

= si n

2 [( Tr/ 2 )(V/V m ) 2

], (5)

is I/I^ = 2.4 x 10

“

3

. This amount of light is of the same order as the

transmi ttance due to imperfections in the optics used to monitor the Kerr

effect or depolarization of the light from particles or thermal turbulence.

Usually the transmittance must be more than 10

“ 2 before field measurements

can be made. The difficulty was handled by doping the transformer oil with

nitrobenzene to enhance the Kerr effect. The results showed that there were

no differences in the space-charge distribution along the interface than away

from the interface with the application of dc. Thus, the electric field was

not changed (at least macroscopi cal ly ) by the interface.

14

1
During this reporting period a method was developed to perform field

measurements in non-doped transformer oil at relatively low voltages -- much

lower than the typical breakdown voltages for interfacial systems. A comparison

of the field near the interface with the field away from the interface was then maae under a variety of conditions of water contamination and temperature.

In all cases the electric field between the plates was not affected by the

presence of the interface.

The increase in sensitivity was obtained by adding an impulse to the

steady-state voltage. It is known that the impulse strength of a liquid is

greater (often by a factor of two or more) than the dc or 6u-Hz ac strength.

Because larger impulse voltages can be applied to the cell, the electro-optical

transmittance will be larger and, thus, easier to measure, see eq (b). The

impulse generator is capacitively coupled to the cell so that an impulse can

be applied on top of the steady-state voltages (see fig. 5). This technique

exploits the fact that in insulating liquids the macroscopic space-charge

development occurs on a millisecond time scale [3,4]. Thus the space charge

distribution will result from the steady-state voltage and the microsecond

duration pulse will have no affect on the space charge except to enhance its

detectabi 1 i ty

.

To see the advantage of using an impulse voltage in this way, consider an example of our cell. At room temperature, the cell constant is 17U kV. Without

an impulse, a 10 kV dc voltage would produce a transmittance, according to

eq (1), of I kj/I m

= 3.0 x 10“ 5 which cannot normally be detected since

both turbulence and imperfections in the optics dominate such a signal. With

a 90 kV impulse the transmittance becomes Igo/I m

= 0.18. If the 90 kV impulse

is added to a 10 k V dc voltage, the 100 kV result produces a transmittance

of Iiuu/^m = 0.27. The benefit is shown by the increase of the 10 kV dc

signal with the impulse over the 10 kV dc signal without the impulse

(Iluo - I 9U)/I 10 = 3000 ,

that is, the 90 kV impulse has rendered a 10 kV dc signal 3000 times more

observable. (See figure 6 for an example of data.)

With this sensitive system, no space charge was observed at room temperature

or at 120°C. Even upon the addition of an estimated 40 ppm of water, no space

charge was evident which would produce more than a 10% modification of the

field throughout the gap. It must be kept in mind that because of the presence

of an interface [made of polytetraf luoroethy lene ( PTFE ) ] , the cell could not

withstand 30 kV dc for long periods of time. Hence it was not possible to

explore the field intensities which were previously used to observe space

charge in pure transformer oil.

A reasonable hypothesis is suggested from previous observations of space

charge in nitrobenzene: space charge may have an onset field which, in this

case, is larger than the breakdown field with an interface present. This

would explain why space charge could not be observed at the relatively low

fields used with the interface but were observed earlier in transformer oil

(without an interface) at higher field strengths. This hypothesis will be

explored next quarter where no interface will be present and various levels of
Figure 5. Circuit diagram showing the application of an impulse generator to

enhance the transmittance ratio of the coil. Isolation from the

dc supply is accomplished via four capacitors in series with a total capacitance of about 1 nF. The current limiting dc resistor

is 800 kQ. Voltage is applied to the closed cell via feed-throughs

in the tank's top. The cell has its own oil circulation system

(not shown) outside the tank.

16
;oN

"O

II CL<oo

t;wo

CL

NI

17

Figure

6.

Data

showing

the

usefulness

of

the

impul

se-enhancement

of

lower

dc

voltages

in

making

the

Kerr

effect

more

observable.

One

third

the

maximum

signal

is

shown

compared

to

dc

alone,

pulse

alone,

and

pulse

plus

dc.
contaminants will be investigated. Additionally, a careful determination of

the Kerr coefficient of transformer oil will be made as a function of temperature

and wavelength. Finally, a pressboard interface will be installed and its

effect upon the field will be determined.

For further information, contact Dr. E. F. Kelley (301) 921-3121.

5. ACTIVE INSULATORS FOR INSULATION ANU SURGE SUPPRESSION

Subtask No. AU57

The objective of this investigation is to evaluate the practicality of

using active insulators, i.e., insulators made from materials whose resistance

is a function of the voltage across the device, to minimize the effects of

switching and lightning surges. The theoretical aspect consists of the development

of a mathematical model of an electric power system wh.ich can be used to predict

the effects of active insulators. (The experimental aspect consists of the

development of a facility to fabricate insulation materials suitable for use

in an active insulator.) This work is being performed by the Department of

Electrical Engineering, University of Southern California, under a grant from

the National Bureau of Standards.

The single-phase mathematical models for the lightning transient performance

of active insulators has been completed. The equivalent model circuit for

lightning striking on a tower is shown in figure 7, and the resultant nonlinear

equation after substitution is

[K Mt)]“ +_A_ U,(t)

1+A «R-(-

where A = 1/Z + 1/Z

B l+A«R t

0 ,

3 = I n,n-l(T ’ T/m ) + 1 n,n+l (t-x/m) + 2 Is(t) Rt/z ,

and the nodal voltage is

U

n (t ) = U A

(t) + {[< U A (t)] a + Is(t ) }*Rt. ..

( 6 )

( 7 )

The equivalent circuit for lightning striking directly on the conductor

is shown in figure 8, and the nonlinear equation after substitution is

where A =

B =

The nodal

CK U A(t )] a

+ A_ U.(t) + _B

1+A »R-j- 1+A'R-t

1/Z + 1/Z + 1/Z

I n,n-l (t-x/m) - I$(t) .

voltage is

U

n (t) = U A

(t) + [K U A

(t)] a *R t

.

0 , (3)

18
in,n-l <t) Un( t) i a n . j (t)

Figure 7. Equivalent circuit model for lightning striking on transmission tower.

19
In.n-l(t) Un(t) In.n+Kt)

Figure 8. Equivalent circuit model for lightning striking on conductor.

20
The lightning current source is mathematically simulated as

I s (t) = I

ra (e*at - eat

) , (9)

where a]_ = 0.0175 x 10" 6 and = 2.2 x 10" 6

.

To maximize the influence of operating voltage, it is assumed that the polarity

of operating voltage is opposite to the lightning current, and at its maximum.

The results of the study presented here are based on lightning striking on a tower located at the middle of the line, even though the computer program is

capable of selecting any striking point.

Figure 9 shows the relationship between lightning current I s and maximum

current passing through active insulators with different tower resistances.

Results demonstrate that the maximum current passing through active insulators

is approximately proportional to the lightning current and to the tower resistance.

For active insulators, which have a limited maximum current capability, the

maximum lightning current as a function of tower resistance is shown in figure 10.

Within these limits, the active insulators will be able to maintain adequate

operation of the transmission system.

The major functions of active insulators are to protect substation equipment

under overvoltage conditions and to reduce the duties of station arresters. The

maximum overvoltage at substation terminal is shown in figure 11, for tower

resistances of up to 30 G, while the overvoltage is maintained below 350 k

V

(1.75 p.u.). The maximum current passing through the station arrester is less

than 1.2 kA for lightning currents of up to 200 kA and tower resistance of

30 n (fig. 12).

These results indicate that with only four active insulator strings between

the location of the lightning strike location and the substation, active insulators

can provide substantial protection to the station equipment. Since the voltage

across the insulator string is maintained below 600 RV (3.0 p.u.), the back

flashover risk is greatly reduced.

The material research facility has been producing much better quality

samples in the last two months. Materials with dielectric constants of less

than 3.0, volume resistivity of higher than 10 14 ft-cm and breakdown voltage

of higher than 17 kV/mm have been fabricated and tested. The material composition

and processing procedures will be finalized. The study on single-phase transient

performance of active insulators will continue in the next quarter. A three-phase

mathematical model is currently under development and will be completed. The

nonlinear element necessary for producing prototype active insulators will be

procured. The design of prototype active insulator unit will be conducted

according to the available nonlinear element shape.

For further information, contact Dr. T. C. Cheng (213) 743-6933.

6. INVESTIGATION OF INSULATOR SURFACE FLASHOVER IN GAS

Subtask No. A057

The weakest component of a gas-insulated system is typically the qas solid

interface. The basic mechanisms leading to the interfacial breakdown (flashover)

21
Maximum

Active

Insulator

Current

Is-150kA

The effect of tower resistance and lightning current on maximum active insulator current.

22

Figure 9.
LIGHTNING

CURRENT

SOURCE

IS

Figure 10. The effect of tower resistance on acceptable lightning

current for limited active insulator capacity.

23
ubuCation

Volta

Figure 11. Substation voltage level under a simulated lightning at the

middle of the transmission.

24
Lightning Current Source

Figure 12. Maximum station arrestor current under a simulated lightning strike

at the middle of the transmission line.

25
and the role of design parameters in the breakdown process are not well enough

understood to predict the ultimate system performance levels and/or to affect

optimum material and geometry design selections. Research is therefore being

conducted to investigate the primary mechanisms associated with insulator

surface flashover in practical insulating gas. An additional objective is to

determine the role of various geometric and material parameters on the basic

flashover mechanisms and the ultimate insulator system performance.

It has been observed that the electrical strength of an electrode arrangement,

bridged with a solid/gas interface, is less than that of the same electrodes

separated only by insulating gas. It can be postulated that the reduction of

insulation strength is associated with primary emission and electron multiplication

processes which occur near and along the surface of the bridging insulator.

Specific tasks, consistent with the above objectives, have therefore been

initiated to investigate surface currents and electric fields associated with

a gas/solid interface. Specific tasks, conducted using 60 Hz ac, include:

(1) Measurement of insulator surface currents associated with pre-f 1 ashover

events,

(2) Electro-optical measurement of interfacial fields associated with

pre-breakdown and flashover, and

(3) Development of analog and computer-based numerical techniques for the

calculation of insulator surface fields.

This work is being performed by the Department of Electrical and Computer

Engineering, University of South Carolina, under a grant from the National

Bureau of Standards.

Solid/Gas Interfacial Current Measurements

Insulator surface currents occurring before flashover have been measured.

It can be postulated that these currents are associated with insulator surface

charging and/or partial discharges along the insulator surface. Insulator

surface charging has been previously observed in vacuum insulated systems and

results in local enhancement of cathode fields and a reduction of insulation

strength [5-7]. Partial discharges along the insulator can result from

surface charge enhanced fields (on the insulator) or primary events initiated

near the electrodes.

Surface currents have been determined by measuring voltage across a resistor

placed in series with gas-insulated electrodes. The experimental arrangement

consists of two electrodes placed in a high pressure vessel and a current

viewing resistor (CVR). Various electrode geometries, with and without a bridging solid insulator, can be used (including point-plane, rod-plane, and

plane-plane) with electrode separations, up to a few centimeters. The pressure

vessel permits the use of SFg, N 2 , or other gases and has a maximum operating

pressure of approximately 1.38 MPa. The voltage across the CVR is determined

using a bridge network which effectively eliminates ac displacement current

signals. Excitation voltage is obtained utilizing a 100 kV, 60 Hz, ac transformer,

which is partial discharge free up to 40 kV.

26
Interfacial currents have been measured using the described system for

unbridged point -plane and plane-plane and bridged plane-plane geometries in

vacuum and nitrogen. Vacuum and unbridged gap data have been obtained to provide

a basis of comparison for the bridged gap performance in gas.

Bridged Vacuum Gap

Typical results observed for a 1.27 cm vacuum gap, bridged with a polymethylmethacrylate (PMMA) insulator, are shown in figure 13, which shows the

gap excitation voltage and the surface current as observed using the CVR. The

general behavior observed is listed below:

(1) Small current "spikes," shown in figure 13(a), occur just before the voltage

peaks for relatively low voltages. It should be noted the short current

pulse shapes are not necessarily indicative of the actual current pulse

temporal performance but are determined by the electrical (impulsed)

response of the measurement system.

(2) As the voltage increases the observed fast current pulses occur earlier

during the excitation. Additionally, a slower current component begins to

form. The fast pulses are observed to occur in addition to this slower

component. The slower current component peak occurs prior to the excitation

voltage maximum. This performance is shown in figure 13(b).

(3) As the voltage is further increased, the slower current component dominates.

As the voltage is further increased, a second, slow current component forms

which is in phase with the excitation. This performance is shown in figure 13(c).

(4) At even higher voltage values, the slow current component, which is approximately

in phase with the excitation, dominates. This performance is shown in

figure 13(d). It should be noted that the current behavior shown is not

symmetrical but is polarity dependent, apparently due to unintentional

electrode or insulator differences.

It is postulated that the current performance indicated by the data of

figure 14 is due to the following mechanisms:

(1) At relatively low voltages, the surface currents are due to short duration

partial discharges near the cathode.

(2) At higher voltage levels, more substantial currents are observed due to

increased cathode electron emission near the insulator/cathode junction.

The emitted electrons strike the insulator and cause positive charging of

the insulator due to the greater than unity secondary emission coefficient

of PMMA. The charging currents continue to flow until the insulator reaches

a steady-state charge value and accepts no more charge. The peak of the

surface charging current, therefore, does not occur at the same time as the voltage peak.

I (3) At higher voltage levels, field emission currents, which do not flow along

i the insulator, begin to dominate. Field emission current corresponds to

the current which is approximately in phase with the excitation. It has

been previously observed that these in phase currents obey Fowler-Norahei

predictions [8]. This implies that the electron interactions with the

insulator surface are minimal for these currents.

27
is

voltage,

other

is

current.
(4) It can be further postulated that flashover occurs in vacuum due to electron

avalanches, near the insulator surface, in gas which has been desorbed from

the insulator by the incident and cascading electrons.

Unbridged Gas -Filled Gap

Pre-fl ashover, current measurements have also been conducted using a pointplane geometry (2.8 cm separation) in nitrogen. Typical results are shown in

figure A which shows the excitation voltage and the CVR response. These data

indicate that partial discharges (pd) occur near the peak of the excitation.

Sufficient data have been obtained to determine that the pd inception ana extinction voltages are linear functions of the gas pressure. It has been

found that pd inception and extinction occur at 16 kV and 14.8 k V, respectively,

at a pressure of 448 kPa. The data, in addition to indicating the behavior of

an unbridged gap for comparison purposes, do indicate satisfactory operation

of the diagnostic system.

Bridged 8ap in Nitrogen

Pre-flashover current measurements have also been obtained for a bridged

gap in nitrogen. The electrode arrangement consisted of a 5 cm diameter

lower electrode and a 7.28 cm diameter upper electrode. A 1.27 cm diameter,

U.b3b cm high, cylindrical PMMA insulator was placed between the electrodes.

The electrodes, in this arrangement, were separated by the insulator and a nitrogen gap of length d, between the insulator top and the upper electrode.

The pd inception voltage was measured, using the CVR, for d values from

0 to U.5 cm.

Typical current pulses are shown in figure 16 for d = U.2 cm. The current

pulses are positive. The negative undershoot is due to the detection circuit

response. The observed pulses are short and are, again, indicative of the

measurement circuit impulse response. Only the response amplitude and time of

occurrence are considered as significant data. The upper electrode is the

anode for the data shown. Similar current pulses are observed, but with a greater repetition rate, when the upper electrode is the cathode. The pulse

repetition rate decreases substantially when d is decreased to zero.

The data of figures 16, 16, and 17 and considerable additional data of

this same type imply that, for an electrode arrangement with an insulator present;

(1) The pd rate is considerably lower than the pd rate observed for point -plane,

unbridged gap (fig. 14).

(2) The pd rate decreases to a small value when d goes to zero, correspondi ng

to electrodes with an insulator bridging the entire gap.

(3) The difference between the inception and breakdown voltage is less for

d = U than for non-zero values of d.

(4) The inception, extinction, and breakdown voltage are linear functions

of p for all values of d.

29
Figure 14. Pre-f 1 ashover currents, point-plane (2.86 cm), nitrogen, 5 psig,

3.48 kV/div, 1 ms/div.

Figure 15. Pre-f 1 ashover currents, bridged gap, nitrogen, 5 psiq,

d = 0.2 cm, 3.78 kV/div, 1 ms/div.

30
=5mm

31

Figure

16.

Pd

inception

(upper)

and

Figure

17.

Inception,

breakdown,

and

extinction

voltage,

d

is

extinction

voltages,

bridged

nitrogen

gap

size,

PMMA

and

unbridged

gap

in

nitrogen,

insulation

0.635

cm

in

nitrogen.
(5) The data imply that surface currents do flow. The relationship between

the amplitude and repetition rate of the currents and observed inception

and breakdown voltages are presently beiny determined.

Electro-Optical Surface Field Measurements

Insulator surface charging can be postulated to be a mechanism associated

with the flashover process. The current measurements reported above can be

interpreted as currents flowing alony the insulator surface which can contribute

to surface charging. An electro-optical surface field measurement technique has

previously been reported which directly measures the interfacial electric fields

at the gas/solid interface [5, 6, 7, 9]. Results obtained using this technique

have also previously been reported, primarily for vacuun/solid interfaces.

Measurements have most recently been obtained for a gas/solid interface.

However, for long pulsed or 60 Hz ac excitations a problem has been encountered

using the nitrobenzene sensing liquid. Typical results obtained using an unbridged gap are shown in figure 18. This figure shows an interference pattern

using a 1 ms risetime, long pulse excitation. This figure and similar additional

data show a density perturbation which occurs in the nitrobenzene for times

longer than approximately 1 ms. This density perturbation obscures the interference

pattern being used to determine the interfacial fields and is probably due to

charge injection, from the electrodes, into the nitrobenzene [3,4],

The observed perturbation is undesirable. Attempts have been made to eliminate

the perturbation by removing water from the nitrobenzene using a molecular sei ve and

a reci rculati on pump system. Nitrobenzene resistivities on the order of 10 12 -cm have been produced. However, the perturbation problem continued to exist.

More recently, the test cell arrangement -used has been modified such that no charge injection from the electrodes can occur. The test arrangement is shown in

figure 19 and consists of a glass tube of rectangular cross section which contains

the nitrobenzene without liquid contact with the electrodes. Tests are currently

being conducted to determine if charge injection and resulting liquid density

perturbati ons occur with this arrangement.

Electric Field Predictor and Calculation Techniques

Both analog and digital electric field calculation techniques are being

developed. It is important to develop these techniques to support the experimental

measurements of the surface electric fields. An electrolytic tank has been

constructed and is now operational. With this arrangement, the electric fields

associated with various electrode-i nsulator configurations can be determined and

compared with those electro-opti cal ly measured. A computer solution to Laplace's

equation, appropriate to the experimental configurations being used, has also been

developed. With this digital technique, the fields associated with various electrode

and insulator geometries and various surface charge distributions can now be

predicted and the results compared with the electro-optical results to be obtained.

For further information, contact Dr. J. E. Thompson, (8U3) 777-73U4.

32
no field with field shewing

density perturbation

at cathode (1.1 ms)

Figure 18. Interference pattern indicative of electric field distribution,

no interface.

glass tube

electrode

Figure 19. Electro-optical field measurement insulator arrangement.

33
7. REFERENCES

[1] R. J. Van Brunt, and D. A. Leep, "Corona-Induced Decomposition of SFg,"

Proc. 3rd Int'1 Symp. on Gaseous Dielectrics, Gaseous Dielectrics III,

pp. 402-409, 1982.

[2] R. J. Van Brunt, "Effects of H 2 O on the Behavior of SF5 Corona," Proc.

7th Int. Conf. on Gas Discharges and Their Applications, London,

pp. 255-258, 1982.

[3] R. E. Hebner, R. A. Malewski, and E. C. Cassidy, "Optical Methods of

Electrical Measurement at High Voltage Levels," Proc. IEEE, Vo 1 65,

pp. 1524-1548, 1977.

[4] E. J. Hopfinger and J. P. Gosse, "Charge Transport by Self-Generated

Turbulence in Insulating Liquids Submitted to Unipolar Injection," Phys.

Fluids, Vo 1 . 14, pp. 1671-1682, 1971.

[5] J. E. Thompson, J. Lin, K. Mikkelson, and M. Kristiansen, "Optical

Measurements of Insulator Surface Charging," Appl . Phys. Lett., Vol 37,

pp. 574-376, 1980.

[ 6 ] J. E. Thompson, J. Lin, K. Mikkelson, and M. Kristiansen, "Investigations of

Fast Insulator Surface Flashover," IEEE Trans. Plasma Science, Vol PS-3,

pp. 191-197, 1980.

[7] R. Lee, H. Rhinehart, J. E. Thompson, and T. S. Sudarshan, "Predi scharge

Current Measurements and Optical Surface Field Measurements Associated

with Insulator Surface Flashover," in Gaseous Electronics III , edited by

Loucas G. Christophorou, Pergamon Press, pp. 349-355, 1982.

[ 8 ] R. Lee, T. S. Sudarshan, J. E. Thompson, and R. L. Boxman, "Predischarge

Current Measurements in Vacuum Gaps Bridged with Plexiglas Insulators,"

Proc. 10th Int. Symp. on Discharges and Electrical Insulation in

Vacuum, Columbia, SC, pp. 288-296, 1982.

[9] J. E. Thompson, M. Kristiansen, and M. 0. Hagler, "Optical Measurements

of High Electric and Magnetic Fields," IEEE Trans. Instrum, and Meas.,

Vol IM-25 , pp. 1-7, 1976.

34
NUi-IIW IRIv. i-eil U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 83-2755

2. Performing Organ. Report No. 3. Publ i cation D ate

September 1983

4. TITLE AND SUBTITLE

Development of Power System Measurements - October 1, 1982 to December 31, 1982

- Quarterly Report

5. AUTHOR(S)

R. E. Hebner, Editor

5* PERFORMING ORGANIZATION (If joint or other than NBS, see in stru ction s) NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPO^SORING^RGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, DC 20585

10. SUPPLEMENTARY NOTES

[

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

11. ABSTRACT A OCT D A /-T (A/ A 200-word inn A or less i c _ r . ... . ... factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

This report documents the progress on five technical

investigations sponsored by the Department of Energy and

performed by the Electrosystems Division, the National Bureau

of Standards. The work described covers the period from

October 1, iy82 to December 31, 1982. The report emphasizes the

calibration of instruments designed to measure the 6 U-Hz electric

and magnetic fields in biological exposure facilities, the

measurement of the rate of decomposition of SF 5 in positive

dc-corona discharges, and in the measurement of space charge in

transformer oil between 1UU°C and 15U°C.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by cabl es , compos i te insulation; dc fields; high voltage; incipient fault;

insulation; liquid breakdown; SF 5 ; space charge; transformer oil.

semicolon s)

13. AVAILABILITY

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m Order From National Technical Information Service (NTIS). Springfield, VA. 22161

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