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NB55IR 82-2528

Development of Power System

Measurements -- Quarterly Report

January 1, 1982 to

March 31, 1982

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

April 1982

Issued June 1 982

“ Prepared for: Department of Energy

Division of Electric Energy Systems _ QC )00 Independence Avenue, SW

ashington, DC 20585

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<52-2523

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NBSIR 82-2528

DEVELOPMENT OF POWER SYSTEM

MEASUREMENTS -- QUARTERLY REPORT

JANUARY 1, 1982 TO

MARCH 31, 1982

R. E. Hebner, Editor

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

April 1982

Issued June 1 982

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
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Foreword

This report summarizes the progress on four technical investigations

during the second quarter of FY 1982. 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 iii

LIST OF FIGURES v

LIST OF TABLES v

Abstract 1

1. INTRODUCTION 1

2. DC FIELDS AND ION MEASUREMENTS

Subtask No. A018 1

3. INCIPIENT FAULT DETECTION/LOCATION

Subtask No. A063 4

4. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. A053 6

5. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

Subtask No. A057 12

6. REFERENCES 15

i V
LIST OF FIGURES

Page

Figure 1. Single ion chromatograms of SF 5 samples for mass-to-charge

ratio of 86 8

Figure 2. Measured concentration, C, of SO 2

F 2 in SF 5 at 200 kPa as

a function of energy dissipated, U, in corona discharges

at the indicated power levels. Also shown are fits to

the data 10

Figure 3. Measured average positive-dc -corona current versus applied voltage for pure (open symbols) and H 20 -contaminated

(closed symbols) SF 5 at a pressure of 300 kPa 11

Figure 4. Measured relative number of discharge pulses versus their charge for pure and H20 -contaminated SF5 at

300 kPa and applied voltage of 22.0 kV 11

LIST OF TABLES

Table 1. Error sources for parallel plate apparatus 3

Table 2. Space charge density vs thermal history 14

V

DEVELOPMENT OF POWER SYSTEM MEASUREMENTS -- QUARTERLY REPORT

January 1, 1982 to March 31, 1982

R. E. Hebner, Editor

This report documents the progress on four 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

January 1 , 1 982 to March 31, 1982. The report emphasizes the

calibration of instruments designed to measure the 60-Hz electric

field in biological exposure facilities, selected errors inherent

in the use of time-domain refl ectometry to determine the rf

characteri sties of power cables, the measurement of the rate of

decomposition of SF5 in positive dc-corona discharges, and in the

measurement of space charge in transformer oil between 1 00°C and

150°C.

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

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

transformer oil

1. INTRODUCTION

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

and the National Bureau of Standards, the Electrosystems Division, NBS,

has been providing technical support for DOE's research on electric energy

systems. This support has been concentrated in four areas -- the measurement

of electric fields, the measurement of electromagnetic properties of solid

insulating materials and cables, the measurement of partial discharge phenomena

in gaseous dielectrics, and the measurement of interfacial electrostatic field

distributions and of space charge density. The technical progress made during

the quarter January 1, 1982 to March 31, 1982 is summarized in this report.

2. DC FIELDS AND ION MEASUREMENTS

Subtask No. AOl 8

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, conductivity, 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; and to

provide electrical measurement support of DOE-funded efforts to determine

the effects of ac fields on biological systems.

The performance of four electric field meters used for measuring 60-Hz

electric fields in biological exposure chambers was examined at NBS during

the current reporting period. A parallel plate apparatus 1 in x 1 m x 0.24 m spacing was assembled for the tests to generate a nearly uniform 60-Hz

1
electric field. Voltages for the plates were obtained using two

transformers which were configured to act as a single center-tapped

transformer (center-tap grounded). An oscillator-amplifier combination

energized the primary windings. The sources of error for the parallel

plate apparatus can be classified into two categories, geometrical and

electrical. Table 1 lists the various error sources in each category

for these tests.

Parallel plates with the above dimensions will establish an electric

field along the electrode axis which is within 0.01% theoretical uniform

field value obtained by dividing the voltage difference by the plate

spacing [1]. The use of a center-tapped transformer reduces to a negligible level perturbations of the field between the plates (along

electrode axis) by nearby ground planes (e.g., walls, floor, etc.) [2].

The nearest ground plane was approximately 1 m from the parallel plates.

Measurements of the parallel plate spacing at six or more representative

locations on different days indicated an average value of 24.18 cm with an uncertainty of less than ±0.1 cm. These deviations are the basis for the

uncertainty estimate of ±0.5% in table 1.

The rms values of the applied voltages were determined using

electrostatic voltmeters which were calibrated by applying a known dc

high voltage to the meters. The uncertainty associated with this

calibration was ±0.5%.

Because the frequency response of the meters was not measured, it was important to insure that the harmonic distortion of the voltage

waveform was small. Using a capacitive voltage divider and spectrum

analyzer, it was determined that the power frequency harmonics in the

voltages applied to the parallel plates were 50 db or more below the

magnitude of the fundamental for electric fields less than 100 kV/m and

40 db or more for electric fields less than 180 kV/m. These measurements

are the basis for the uncertainty estimate, due to harmonic content, in

tabl e 1 .

Two of the meters had dipole sensors and reportedly had a true rms detector (the frequency response of the detector was not examined during

the tests). Therefore, in principle, the rms value of the electric field

should have been displayed even in the presence of harmonic distortion.

These meters, which were about 4 cm high, were supported between the

plates with a dielectric rod. The parallel plate spacing, noted above, is

sufficiently large to prevent interactions between the field probe and the

surface charge distributions on the plates [2]; i.e., the meter responded

as if it were inserted into a uniform field.

The remaining two meters consisted of flat discs with guard bands

and were connected to their respective circuits with shielded cables.

2
Table 1. Error sources for parallel plate apparatus

I, Geometrical

(a) Ratio of plate dimension and spacing

(b) Parallel plate spacing

(c) Field perturbation by nearby objects

(d) Field perturbation due to perturbation

of surface charge distribution

Estimated uncertainty

Less than ±0.01 % in

electric field value

along electrode axis.

Less than ±0.5% in

el ectric fiel d val ue.

Less than ±0.1% in

el ectric fiel d val ue.

Negl i gibl e.

II. Electrical

(a) Voltage

(b) Frequency

(c) Harmonic content

Less than ±0.5%.

Synchronized with

line frequency.

Less than ±0.3% for

electric fields less than

100 kV/m and ±1.0% for

fields between 100 kV/m

and 180 kV/m.

3
The frequency response of these meters was not examined. However, both

detectors contained an amplification stage which would weight signals due

to their harmonic content. Thus a third harmonic content in the electric

field of about 0.3% for the tests performed with the disc probes could

produce a ±0.9% contribution to the meter reading.

A monopolar dc line is being configured in a high bay area for use in

investigating off-ground measurements of ion density. Preliminary

measurements are expected to begin during the next quarter. This effort

will expand upon the exploratory measurements made last year. An optical

link will be used for data acquisition, since the ion counter and related

apparatus will be operated at voltages as high as 30 kV above ground.

One of the NBS staff members has been invited to participate in the

development of an International Electrotechnical Commission (lEC) standard

for the measurement of electric fields near ac transmission lines. He will

be a member of the U.S. delegation to the lEC General Meeting in Brazil this

June. With support by DOE and NBS, a U.S. (i.e., lEEE/ANSI) standard was developed and has been in force for the past few years. Because member

nations are strongly urged to follow the recommendations of lEC standards,

it is very desirable that the U.S. actively participate in the development

of the international document to insure that the U.S. and lEC standards are technically compatible and to minimize any unwarranted disruption of present

U.S. measurement techniques.

A NBS staff member will participate in the presentation of a workshop

on measurement techniques for characterizing 60-Hz electric fields in small

parallel plate biological exposure systems at the annual meeting of the

Bioelectromagnetics Society in June. The workshop is sponsored by the

lEEE/Power Engineering Society Working Group on the Biological Effects of

Power-Frequency Electric and Magnetic Fields.

A NBS staff member received a Certificate of Appreciation of the

lEEE/PES Corona and Field Effects Subcommittee for services rendered as Chairman of AC Fields Measurements Task Force.

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

3. INCIPIENT FAULT DETECTION/LOCATION

Subtask No. A063

The objective of this program is to identify and, insofar as practical,

remove technical barriers to the development of a successful incipient fault

detector/locator for underground power transmission use. NBS responsibility

includes conducting an experimental program that will aid the development of

an incipent fault detection/location system by measuring the rf characteristics

of power cables and evaluating the frequency content of partial discharge pulses

4
emanating from incipient fault sites in a cable dielectric. The measurement

program at NBS is essentially completed. The limitations of time-domain

refl ectometry (TOR) techniques for detecting incipient fault sites in transmission cables have been determined. The radi o-f requency (rf) responses of

these cables have also been measured. One item remains to be completed:

to determine the effect of sampl i ng-rate drift on these types of measurements.

During this past quarter, we have attempted to gain further understandi ng

on the effects sampling-rate drift can have on the measured cable transfer

functions. Our measurements involve using TOR techniques to determine the

rf attenuation of various types of power cable. The TDR pulse is detected

with a digital -processing oscilloscope and transmitted to a computer for

processing. Basically two measurements are required. These could be the

input pulse to the cable and the resulting reflected or transmitted

pulse. They could also be the output pulses from two identical, but

different length, cables. The problem is that these two necessary measurements are not taken at the same time. Errors can result, therefore, from

sampling-rate drift in the sampling oscilloscope.

Last quarter's report demonstrated theoretically that a sampling drift

in the time-domain data will result in a similar drift in the frequency

domain. Therefore, the experimenter observing such drift in the transfer

function can, presumably, make an allowance for it. Specifically, for

measurements on power cables, this drift will cause an apparent roll-off or gain in the measured transfer function. The experimenter can multiply

the transfer function by some appropriate correction function. This was

the subject of a paper presented recently [3]. The problem with this

approach is that it requires some a priori knowledge of the transfer

function in order to make the correction (i.e., is the transfer function

really supposed to be flat over a particular frequency range?). Also,

the experimenter must be sure that, in correcting the function to remove

the effect of sampling-rate drift, other errors are not introduced.

With these things in mind, the effort this quarter has been in

correcting for sampling-rate drift in the time domain. The typical TDR

waveform represents the reflections of the pulse from points along the

length of the cable from the output of the TDR unit to the cable end.

In our experiments, typically the first quarter of the waveform should

always be the same representing the impedance of the TDR output, the

cable from the TDR unit to the power cable under test, and the impedance

transition to the power cable. This means that the first 100 or so points of the 512-point sampled waveform should be identical for a particular measurement. We assumed that expanding or contracting the

time scale of the second of a pair of measurements until the first 100

points did agree with the first would remove the sampl i ng-rate drift without any arbitrary assumption as to what the transfer function of the cable

should be. While in principle this should work, there are problems.

5
First of all, besides a drift in the sampling rate, there are also

drifts in the vertical amplification, the horizontal screen position,

and the vertical screen position. The latter is no problem because

in the software the first sampled point of the waveform is defined as zero. The effect of vertical amplification drift on the measured transfer

function has been found to be negligible but horizontal screen position

drift is important.

Therefore, software has been written to correct for the horizontal screen drift by "sliding" one of the waveforms left or right in integer

steps with respect to the other waveform until the differences between

the waveforms are minimized. After this has been completed, the samplingrate drift correction is determined in a similar manner. While this

approach works excellently with analytically generated waveforms, the

results with real TDR waveforms are less satisfactory. The calculated

sampling-rate drift correction is very sensitive to how well the two

waveforms are aligned horizontally. A misalignment by one point in 512 can make a large difference (~100%) in the calculated sampling-rate

drift correction. This, in effect, makes the current approach without some modification of limited value and additional work is required.

The work on sampling-rate drift will be completed next quarter and will

be published. The completion of the final report during this same time

period will conclude this project.

Plans for next quarter are to finish the above investigation and

complete the final report.

For further information, contact Dr. W. E. Anderson, (301) 921-3121.

4. 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 and may lead 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.

Planned activities for FY82 include: 1) completion of an archival paper

on basic mechanism of corona inception in SF 5 under ac and dc conditions;

2 ) investigation of the wavelength dependence of photon-induced positive corona

inception in SF5 ; 3) continuation of measurements of power dependence of the rates

of oxyfluoride and water vapor production from corona discharges in SFs and

report the results in a conference and an archival paper; 4) exploration of the

6
feasibility of new methods to measure field-enhanced colli si onal detachment

of negative ions in SF 5 and other electronegative gases; 5) improvement of

the accuracy of quantitative analysis of trace gases such as H 2 O in SF5 and

other gaseous dielectrics using a gas chromatograph/mass spectrometer

(GC/MS), and to extend the measurements of the effects of trace H 2 O on corona characteristics and relative dielectric strength of the gas; and

6 ) completion of an archival paper on compilation and evaluation of electron

swarm data in molecular electronegative gases. In addition, we are completing an investigation remaining from FY-81, namely, the measurement

of light emission from corona pulses in SF 5 and correlation of results with

electrical detection of corona pulses.

Progress was made during the reporting quarter on all of the above

activities except 1) and 6 ) which are completed. A paper entitled "Mechanisms

for Inception of DC and 60-Hz AC Corona in SF5

" by Van Brunt and Misakian

appears in the April 1982 issue of the IEEE Transactions on Electrical

Insul ation.

A paper entitled "A Survey of Electron Swarm Data in Electro-Negative

Gases" is undergoing review for publication in the Journal of Physical

and Chemical Reference Data.

As part of activity 2), a paper entitled "Corona-Induced Decomposition of

SF 5

" by Van Brunt and Leep was presented at the Third International Symposium

on Gaseous Dielectrics held in Knoxville on March 10, 1982.

Work conducted during this reporting period on activity 5) resulted in

preparation of a paper entitled "Effects of H2 O on the Behavior of SF5 Corona"

for presentation at the forthcoming Seventh International Conference on Gas

Discharges and Their Applications.

Preliminary measurements were made of the wavelength dependence of

photon-initiated positive corona discharges in SF 5 , activity 2). The results

appear to indicate that in pure SFs discharge pulses, i.e., electron avalanches

or streamers, will not be initiated unless there is radiation present with a wavelength shorter than 350 nm. Further investigations of this effect are being conducted and the results will presumably be presented in our next

quarterly report. In this report we highlight some of the experimental

results obtained as part of activities 3) and 5).

New measurements were performed on the rate of decomposition of compressed,

gaseous SF5 in positive-dc -corona discharges. The data from these and earlier

measurements were analyzed by an improved method involving determination of

peak heights from single-ion chromatograms. Examples of chromatograms obtained

in the quantitative analysis of oxyfluoride content in corona-degraded SF5 are shown in figure 1. In this figure, a chromatogram from a degraded SF 5 sample is

compared with a chromatogram from a standard gas sample containing 125 ppm of

both SOF2 and SO2

F

2 in SF5 at the same pressure. The degraded gas shows the

presence of SOF4 not included in the preparation of the standard sample.

7
INTENSITY

m/e = 86

SFe

SOF4

U SO2F2

! i T

SOF 2

DEGRADED

i SFe SAMPLE

%

! CALIBRATIDN SAMPLE

[ 125 ppm SOF2

1 AND SD2 F2

0 2 4 6

RETENTION TIME (minj

Figure 1. Single ion chromatograms of SF 5 samples for mass-to-charge

ratio of 86 . The chromatogram for a corona degraded SFs gas

sample at 200 kPa is compared with a chromatogram for a standard

sample at the same pressure containing 125 ppm of SOF2 and SO2

F2

in SF 5

.

8
Quantitative analysis is carried out by comparing the ratio of peak height for

the m/e = 86 peak correspondi ng to the gas of interest (SOF 2 in fig. 1 ) to the

peak height for m/e = 19 {F+ from SFg) for the degraded sample vnth that for

the standard sample. That is, the concentration in the degraded sample [S0 F 2 ]^^ is given by

[S0F2lx = {(A86/Ai9)x/(^86/^19)c}1^50^2^C>

where A86 and A^g are respectively the peak heights of the relevant m/e = 86 and

m/e = 19 peaks, allowing for background subtraction, and the subscripts x and

C correspond respectively to the unknown and standard gas samples.

Examples of results obtained by applying this analysis to the determination

of SO2F

2 concentrations in SF5 as a function of energy dissipated by the discharge for three different discharge power levels are shown in figure 2. Contrary

to the tentative conclusions drawn from some of our preliminary data, the results

given here indicate that production rates for the oxyfl uorides, i.e., SOF 2 , SO 2

F 2 , and SOF 4 , expressed in moles per joule of energy dissipated, exhibit a dependence

on discharge power. The production rates for these species tend to decrease as power increases as can be seen by comparing the three curves shown in figure 2 . This trend is most pronounced in the case of SO2

F2

. Moreover, as indicated by

the fits to the data in figure 2 , the production rates are not constant at a particular power level. The concentration, C, is related to the energy

dissipated, U, by the expression

C = a U3,

where a and 3 are constants. The fact that 3>1 indicates that the production

rate, dC/dU, increases with increasing energy dissipated, i.e., initially the

decomposition products build up more slowly with time. This may be due to

absorption of these gases on the walls of the discharge cell. More work on the effect of surface absorption is necessary.

More measurements were performed to determine the effect of trace

levels of water vapor on the electrical characteristics of positi ve-dc -corona

in pressurized SF5

. The water was introduced by thermal desorption from a hot wire surface, and its presence was monitored with a gas chromatograph/mass

spectrometer. Maximum H 2O concentrations attained were estimated to be below

300 ppm. The results showed that small quantities of H2 O enhance the corona current and corona pulse count rate by more than an order of magnitude. This

is illustrated by the data for average current shown in figure 3. Consistent

with earlier results [4] obtained in our laboratory, the presence of H 2O also

causes a dramatic change in the charge level distribution of the corona pulses.

This is indicated by the data in figure 4. The change is associated with the

disappearance of burst pulses when H 2O is introduced, and it is speculated that

this effect is associated with formation of ion clusters like SF 5+{H20 )p which

reduce the mean mobility of ions in the interelectrode gap, thereby allowing a more rapid accumulation of space charge [5]. A measurement of the pulse-heightdistribution of corona pulses may prove to be a very sensitive indicator of

H 2 O contamination in SF5

.

9
ENERGY DISSIPATED |kJ

)

Figure 2. Measured concentration, C, of SO2

F 2 in SF5 at 200 kPa as a function

of energy dissipated, U, in corona discharges at the indicated

power levels. Also shown are fits to the data.

10
o

LUcs

ccooo

(sdujej iN^utino VN0U03 imm

11

Figure

3.

Measured

average

posi

ti

ve-dc-corona

current

versus

applied

voltage

for

pure

(open

symbols)

Figure

4.

Measured

relative

number

of

discharge

pulses

and H

20

-contaminated

(closed

symbols)

SF5

versus

their

charge

for

pure

and H

20

-contami

nate

at

a

pressure

of

300

kPa.

SF6

at

300

kPa

and

applied

voltage

of

22.0

kV.
Although the introduction of H 2 O in small quantitites greatly enhances

corona, it appears to degrade the overall dielectric strength of SF5 only

slightly. This was made most evident from comparisons of the measured inception

voltages for negative corona. Our measurements yielded negative corona inceptions

for ii 20 -contai ni ng SF 5 that were less than 5% below those for pure SF 5

. The

inception results which we obtained, therefore, suggest that the sparking

potentials for pure and H20 -contami nated SF5 in uniform fields should not differ

appreciably. However, the enhancement of corona when H2 O is present could

conceivably influence nonuniform field breakdown in the region where corona stabilization occurs. The effect of H 2 O may, in fact, be to increase the breakdown voltage for corona stabilized breakdown in SF 5 . More investigation of the

effect of H 2 O on nonuniform field breakdown in SF 5 seems desirable.

During the next quarter, a conference paper dealing with the above-mentioned

effects of H 2 O on SF5 corona will be submitted for inclusion in the Proceedings

of the Seventh International Conference on Gas Discharges and Their Applications,

to be held in London, August 31 to September 3, 1982. Our measurements on the

power dependence of gaseous by-product production rates from corona in SF5 will

be extended to higher discharge powers to determine power dependence and to

examine possible effects of absorption on the walls of the discharge cell. It is expected that improvements will also be made to speed up the analysis of data

from the gas chromatograph/mass spectrometer and to perform calibrations for

quantitative analysis more efficiently and more reliably. Measurements of effects

of changing polarity on corona-induced decomposition will also be considered,

together with extensions to other gas mixtures, especially SF 5 -N 2 mixtures.

Special emphasis will be given during the next quarter to investigating photoninitiated positive point-plane discharges in SF6 and possibly also O2 -N2 mixtures

at relatively high pressures (>100 kPa). The proposed investigations will be

carried out using both a tunable dye laser and a filtered mercury discharge lamp.

The purpose of this activity is to investigate the possible role of photodetachment

of negative ions in initiating and sustaining high-voltage corona discharges in

irradiated gaps.

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

5. 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 measurement 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.

A new system to measure the electric field in transformer oil using

the electro-optic Kerr effect has been constructed and tested as a portion

of this investigation. This new system is designed to perform experiments

above 100°C. Measurements were made over the range of 100°C to 150°C. In this range, space charge was observed at dc and to a lesser extent at ac.

12
During this reporting period, a paper [6] describing earlier work was prepared, accepted for presentation at the IEEE Power Engineering Society's

1982 Summer Power Meeting, and for publication in the IEEE Transactions

on Power Apparatus and Systems. This previous work generally emphasized

measurements below 100°C. At those temperatures, the space charge was undetectably small .

The new NBS test system is a thermally insulated aluminum cube,

approximately 60 cm on a side, with an acetal -pi astic top and is filled

with silicone fluid. The stainless steel test cell, filled with transformer

oil, is placed in the silicone fluid bath and the silicone fluid is

heated using resistance heaters on the outside of the tank as well as immersion heaters in the fluid. The stainless steel cell is equipped

with an oil circulation system, including a copper heat exchange coil,

to minimize any temperature difference between the silicone fluid and

the oil under test.

As before, no space charge was detected at room temperature. At

elevated temperatures , field distortions due to space charge were evident.

The data taken under direct voltage are summarized in table 2. Negative

space charge was observed near the anode producing a more intense electric

field near the anode. The system was initially heated to 125°C. At

this temperature, the space charge density In the vicinity of the anode

was -14 nC/cm^. As a consistency check, the polarity of the high-voltage

power supply was reversed. Space charge of about the same density again

formed at the anode which was now the opposite electrode. For a period

of three days, the system was maintained at 100°C or above and measurements

were taken at temperatures up to 150°C.

The system was returned to room temperature and no space charge was detected. The system was then reheated and measurements of the space

charge density between 100°C and 150°C were again recorded. As shown in

table 2, the space charge density at 125°C was approximately the same

for this set of data as it was for the previous measurements.

The system was then cooled to 43°C and reheated. At 127°C, the field

distortion was significantly smaller--the space charge density was calculated

to be about -5 nC/cm^. Presumably the high temperature operation or the

temperature cycling was reducing the space charge density. Since it was suspected the space charge density was influenced by the density of water

in the cell , 30 ml of transformer oil which was saturated with water at room temperature was added to the approximately 3 liters of oil at 127°C. It is

estimated that this process increased the water content by about 1 ppm. The

addition of this amount of water increased the space charge to -15 nC/cm^.

Measurements were also made using 60-Hz alternating voltage. In general , the space charge density was less with ac than with dc.

Further measurements will be required, however, to characterize adequately

the 60-Hz behavior. Accurate measurements of the space charge density

require a determination of the functional dependence of the Kerr coefficient

on temperature and on the wavelength of the illuminating light. These

13
Table 2. Space charge density vs thermal history

Temperature

(°C)

Space charge density

near anode

(nC/cm^

)

Comments

125 -14 Temperature maintained above

100°C for three-day period.

Increased to 150°C before

allowing system to cool

I to room temperature (24°C).

106 - 5 Temperatures maintained above

100®C for two-day period.

113 - 9 After 150°C data, system

cooled to 43°C overnight.

1 25 -15

1 50 - 7

1 27 - 5 The smaller reading was taken

just before water-contaminated

-15 oil was added; the larger

reading shows the effects of

adding the water-contaminated

oil sample.

14
functional dependences were estimated for the present data, so it is

possible that the value of the space charge densities could be in error

by as much as ±40%. It should be emphasized, however, that comparisons

at the same temperature should agree within a few percent as should

comparisons over the temperature range of interest in this work

(106°C - 150°C).

These measurements will be extended during the next quarter. The

temperature and wavelength dependence of the Kerr coefficient will be

measured so that absolute measurements of the space charge can be made.

Additional data as a function of water concentration will be taken under

ac excitation to compare the space charge density under ac with that

under dc excitation. Finally, we anticipate the initiation of measurements

using an oil-paper or oil -pressboard system. These measurements, which

will be an extension of previous work [7], should provide more fundamental

insight into the processes responsible for the results of the model study

presented in [8]. The latter study at the General Electric Company,

sponsored by the Electric Power Research Institute, suggested that the

dielectric strength of power transformers may deteriorate in the temperature

range from 100°C to 150°C. That project tested model structures to

determine the effect of operation under overloaded conditions and concluded

that insulated conductors deteriorate moderately over the 100°C to 150°C

temperature range. The deterioration was accompanied by evolution of

visible gas bubbles containing water vapor at conductor temperatures as low as 140°C. The results of those tests prompted the present investigations

above 100°C.

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

6. REFERENCES

[1] W. R. Smythe, Static and Dynamic Electricity (McGraw-Hill, New York, 1950),

2nd ed. , p. 103.

[2] F. R. Kotter and M. Misakian, AC Transmission Line Field Measurements,

U.S. Department of Energy, Energy Technology Report HCP/T-6010/E1, 1977.

[3] J. D. Ramboz, A. R. Ondrejka, and W. E. Anderson, Sampling-Rate Drift

Problems in Transfer Function Analysis of Electrical Power Cables,

Proceedings of the Waveform Recorder Seminar, Boulder, CO (in press).

[4] R. J. Van Brunt, M. Misakian, D. A. Leep, K. J. Moy, and E. C. Beaty,

1980 Annual Report: Technical Assistance for Future Insulation Systems

Research, Nat. Bur. Stand. (U.S.), NBSIR 82-2242 (1981).

[5] R. J. Van Brunt and D. A. Leep, Characterization of Point-Plane Corona

Pulses in SFs, J. Appl . Phys., Vol . 52, pp. 6588-6600 (1981).

15
[6] E. F. Kelley and R. E. Hebner, Electro-Optic Measurement of the

Electric Field Distribution in Transformer Oil, IEEE Trans. Power

Appar. Syst. (in press).

[7] E. F. Kelley and R. E. Hebner, Electrical Breakdown in Composite

Insulating Systems: Liquid-Solid Interface Parallel to the Field,

IEEE Trans. Elec. Insul., Vol . EI-16, pp. 297-303, 1981.

[8] W. J. McNutt, 6. H. Kaufmann, A. P. Vitols, and J. D. MacDonald,

Short-Time Failure Mode Considerations Associated with Power Transformer

Overloading, IEEE Trans. Power Appar. Syst., Vol. PAS-99, pp. 1186-1197,

1980.

16
NBS-114A <REV. 2-80

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See In structions)

1. PUBLICATION OR

REPORT NO.

NBSIR 82-2528

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

June 1982

4. TITLE AND SUBTITLE

DEVELOPMENT OF POWER SYSTEM MEASUREMENTS — QUARTERLY REPORT

January 1, 1982 to March 31, 1982

5. AUTHOR(S)

R. E. Hebner, Editor

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. T.O. No. 137

8. Type of Report & Period Covered

Quarterly

1/1/82 - 3/31/82

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

Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, S.W.

Washington, D.C. 20585

10. SUPPLEMENTARY NOTES

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

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

bi bliography or literature survey, mention it here)

This report documents the progress on four 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

January 1, 1 982 to March 31 , 1982. The report emphasizes the

calibration of instruments designed to measure the 60-Hz electric

field in biological exposure facilities, selected errors inherent

in the use of time-domain refl ectometry to determine the rf

characteri sties of power cables, the measurement of the rate of

decomposition of SF5 in positive dc-corona discharges, and in the

measurement of space charge in transformer oil between 1 00°C and

150°C.

12 . KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) cables; composite insulation; dc fields; high voltage; incipient fault; insulation;

liquid breakdown; SF^; space charge; transformer oil.

13. AVAILABILITY

Unlimited For Official Distribution, Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

[0^ Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

20

15. Price

$ 6.00

USCOMM-DC 6043-PB0


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