NBSIR 84-2809

Development of Power

Measurements -- Quarterly Report

April 1, 1983 to June 30, 1983

U S. DEPARTMENT OF COMMERCE

National Bureau cf Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

February 1 9S4

Prepared fo r

: Decartment of Energy

revision of Electric Energy Systems

(QC 00 independence Avenue, SW

ashington, DC 20585

.usy

-eM -28oK

iW

NBSIR 84-2809

DEVELOPMENT OF POWER SYSTEM

MEASUREMENTS - QUARTERLY REPORT

APRIL 1, 1983 TO JUNE 30, 1983

R. E. Hebner, Editor

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

February 1 984

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 four technical investigations during

the third quarter of FY 83. 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 vi

Abstract 1

1. INTRODUCTION 1

2. ELECTRIC FIELD MEASUREMENTS

Subtasks Nos. 01 and 02 1

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. 03 2

4. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

Subtask No. 04 14

5. ACTIVE INSULATORS FOR INSULATION AND SURGE SUPPRESSION

Subtask No. 04 16

6. INVESTIGATION OF INSULATOR SURFACE FLASHOVER IN GAS

Subtask No. 04 28

7. REFERENCES 33

IV
LIST OF FIGURES

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Page

Measurement of nonuniform electric field with miniature

NBS spherical probe (a) and large commercial probes with

rectangular geometries (b) and (c) 3

Recording of discharge current versus time for positive

and negative corona in SF 6 at a pressure of 200 kPa.

The average current in both cases is 8.0 yA 6

Measured concentrations of S0F 2 versus (a) net charge

transported in the discharge gap, and (b) net energy

dissipated in the discharge for negative corona in 200 kPa SF 6 at the indicated discharge powers and currents 7

Measured concentrations of S0F 2 versus (a) net charge

transported in the discharge gap, and (b) net energy

dissipated in the discharge for positive corona in 200 kPa SF 6 at the indicated discharge powers and currents 8

Measured concentrations of S0 2 F 2 versus (a) net charge

transported in the discharge gap, and (b) net energy

dissipated in the discharge for negative corona in 200 kPa SF 6 at the indicated discharge powers and currents 9

Measured concentrations of S0 2 F 2 versus (a) net charge

transported in the discharge gap, and (b) net energy

dissipated in the discharge for positive corona in 200 kPa SF 6 at the indicated discharge powers and currents 10

Measured concentrations of S0 2 F 2 versus (a) net charge

transported in the discharge gap, and (b) net energy

dissipated in the discharge for positive corona in 300 kPa SF 6 at the indicated discharge powers and currents 11

Maximum field enhancement in percent as a function of

applied dc field for various temperatures. The field

is enhanced nearest the anode indicating a negative

space-charge density. Measurable field nonuniformities

could not be found for temperatures below 50°C before

breakdown field levels were reached near 90 kV/cm.

Data taken were insufficient to determine the cause of the

apparent leveling off of the enhancement for the 122°C

measurements, however, the data point marked with "x"

indicates a reduction of space charge may have occurred

overnight. Because of the "noise" in the data and

imperfections in the optics, resolution was limited, and

reliable field measurements could only be made for fields

over 40 kV/cm 17

The overvoltage waveform during condition 1 19

The overvoltage waveform during condition 2 20

v
LIST OF FIGURES (cont.)

Page

Figure 11. The overvoltage waveform during condition 3 21

Figure 12. The distribution of maximum overvoltage along the line

during condition 3 22

Figure 13. The distribution of maximum overvoltage along the line

during condition 2 23

Figure 14. The distribution of maximum overvoltage along the line during

condition 1 24

Figure 15. The effect of the number of nonlinear insulators installed on the distribution of maximum overvoltage during condition 3 ... 25

Figure 16. The effect of the number of nonlinear insulators installed on the distribution of maximum overvoltage during condition 2 ... 26

Figure 17. Breakdown voltages for various cone angles 29

Figure 18. Insulator geometries tested 31

Figure 19. Breakdown, inception, and extinction voltage,

2 rnn gap with void 32

LIST OF TABLES

Page

Table 1. Measured production rates for S0F 2 in SF 6 at the different

indicated gas pressures, discharge powers, and polarities .... 12

Table 2. Measured production rates for S0 2 F 2 in SF 6 at the different

indicated gas pressures, discharge powers, and polarities .... 12

Table 3. Electrical and mechanical properties of polymer concrete

samples 27

vi
DEVELOPMENT OF POWER SYSTEM MEASUREMENTS — QUARTERLY REPORT

April 1, 1983 to June 30, 1983

R. E. Hebner, Editor

This report documents the progress on five technical

investigations sponsored by the Department of Energy. Three were

performed by the Electrosystems Division, the National Bureau of

Standards, the fourth by the Department of Electrical Engineering of

the University of Southern California, and the fifth by the College of

Engineering at the University of South Carolina. The work described

covers the period from April 1, 1983 to June 30, 1983. The report

emphasizes the calibration of instruments designed to measure the

60-Hz electric field in biological exposure facilities, the effect of

water on SF 6 corona discharges, the measurement of failure mechanisms

in liquid/solid and gas/solid insulating systems, and the development

and behavior of active insulators.

Key words: composite insulation; electric fields; high voltage;

insulation; liquid breakdown; SF 6 ; space charge; transformer oil.

1. INTRODUCTION

Under an interagency agreement between the U.S. Department of Energy (DOE)

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

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

systems. This support is concentrated in three areas — the measurement of

electric fields, 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 April 1, 1983 to June 30, 1983 is summarized in this report. Some

of this work is being performed at the University of Southern California and the

University of South Carolina under grants from NBS.

2. ELECTRIC FIELD MEASUREMENTS

Subtask Nos. 01 and 02

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, air conductivity, the space charge density, and ion 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 for DOE-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.

During the current reporting period the performance of free-body dipole

electric fieldmeters, which had been calibrated for use in nearly uniform power

frequency electric fields, was examined under nonuniform field conditions.

Measurements of nonuniform electric fields produced with a sphere-plane

electrode system were found to support the theory that free-body probes

calibrated in a uniform field could be used to measure significantly nonuniform
fields with small error. Figure 1 shows that measurements of a nonuniform

electric field obtained with commercial field probes with rectangular

geometries, (b) and (c), agree with those obtained with a miniature spherical

probe (a) developed at NBS within experimental uncertainty (less than ±3% for

all measurements).

Free-body probes such as those tested have been used for measuring the

electric field strength near ground level in the vicinity of transmission lines

where the field changes slowly as a function of position, as well as in

substations and homes where fields are less uniform. The theory and test

results have recently been submitted to the Institute for Electronics and

Electrical Engineering (IEEE) Transactions on Electrical Insulation for

publication.

NBS staff members visited the University of Texas Health Science Center in

San Antonio, Texas and the University of Wisconsin-Parkside to perform

measurements of 60-Hz magnetic fields and observe procedures employed by

researchers for the measurement of several related electrical quantities. The

investigators at both universities are performing in vitro biological studies of

cells exposed to 60-Hz electric and magnetic fields. The studies are funded by

the Department of Health of the State of New York.

The second revision of an International Electrotechnical Commission (IEC)

draft standard for the measurement of power frequency electric fields was completed at NBS and distributed to members of Working Group 6 of TC42 for

further comments. Copies of the draft were also distributed to members of the

IEEE HVTT Subcommittee and IEEE PES Working Group on AC Fields, to obtain

additional comments from the U.S technical community.

During the next quarter a draft of a primer for measurement of several

electrical parameters which must be controlled during biological studies with

60-Hz electric and magnetic fields will be written. At least one visit will be

made to a laboratory where 60-Hz biological studies are being performed with

support from New York State. Comments on the latest revision of the IEC draft

standard for measurement of 60-Hz electric fields will be received during the

next quarter from members of IEC TC42 Working Group 6. If a consensus is near

on the contents of the draft standard, a meeting of the working group will be

planned to prepare a final draft. If time permits, some tests associated with

the performance of ion counters operating in the ground plane will be made in

the NBS parallel plate structure. These measurements were originally planned

for the current reporting period.

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

National Bureau of Standards, Washington, DC.

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. 03

The objectives of this project are the development of 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.

2
0.8 0.9 1.0 1.1 DISTANCE FROM SPHERE CENTER (m)

Figure 1. Measurement of nonuniform electric field with miniature NBS spherical

probe (a) and large commercial probes with rectangular geometries (b)

and (c).

3
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 SF 6 and 0 2 ;

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

oxyfluorides in SF 6 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: SF 6 + N 2 , SF 6 + C0 2 , and

SF 6 + 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 SF 6 and other gaseous dielectrics, and

extension of our previous measurements on the effects of trace levels of H 2 0 on electron avalanche growth and corona discharge characteristics in SF 6 ; and

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

evaluate proposed measurements to identify and characterize corona-generated ion

species in air, SF 6 , and other gas dielectrics.

Activity 1 has been completed and during the past quarter an archival paper

describing the results of this work has been published [2]. Also during this

quarter, the results of our previous survey and critical evaluation of electron

transport data in electronegative gases were published in an archival paper [3].

Additional measurements and analyses of data were carried out as part of

activities 2 and 4. The results obtained in activity 2 will be highlighted in

this report. For activity 4 preliminary results were obtained on the

correlation between the responses of thin-film, aluminum oxide hygrometer probes

and the gas chromatograph-mass spectrometer (GC/MS) for detection of trace

levels of water vapor in static SF 6 gas. Recent measurements in our laboratory

indicate that the GC/MS is much more sensitive than the hygrometer probe for

detection of H 2 0 at concentrations below about 10 ppmv (parts per million by

volume). At low H 2 0 concentrations (<50 ppmv ), the hygrometer probe was found

to have potentially serious errors consistent with earlier observations due to

its relatively long response time and possible hysteresis [4]. Despite these

difficulties the hygrometer probe appears to provide satisfactory calibration

points for measurement of water vapor with the GC/MS at concentrations above

about 10 ppmv

. More detailed information about these measurements will be

given in our next quarterly report.

Measurement of the production rates of the oxyfluorides S0F 2 and S0 2 F

2 from

negative corona discharges in pressurized SF 6 have been extended to include the

higher discharge power levels of 0.82 W, 2.22 W, and 4.29 W. The results are consistent with the trends discussed in our previous quarterly reports. It was observed also that even at these fairly high power levels the surfaces of the

4
point electrodes, after being subjected to continuous negative corona for many

hours, exhibited very little in the way of deposits or pitting. This is in

sharp contrast to the situation which generally prevails for positive corona discharges where extensive deposits and damage at the electrode tip are evident

within a relatively short time (-1 h) when the discharge power approaches, or exceeds a level of about 1 W. This effect of polarity on electrode damage is

perhaps to be expected on the basis of the known behavior of SF 6 corona [5],

The positive discharge current exhibits much more fluctuation than the negative

discharge (see fig. 2) and tends to retain a pulsed characteristic even at relatively high currents; whereas, the negative discharge rapidly stabilizes

and tends to behave like a steady glow discharge as the current increases. In

the positive case the point electrode tip is evidently subjected to impact by

many fast bursts of electrons from avalanches, or streamers formed in the gas.

For the negative case, on the other hand, it is expected [5,6] that a space

charge cloud develops around the point electrode thereby grading the field and

reducing the energy of impacting ions. This appears to be consistent with the

relatively stable current seen. It thus appears likely that the point electrode

is heated more under positive discharge conditions, and the electrode surface

possibly plays a greater role in the discharge chemistry. This effect may have

some relevance to the observed polarity dependence for the ratio of S0F 2 and

S0 2 F 2 production rates (see tables 1 and 2). At present we can only speculate,

but it is possible that free oxygen is more readily generated near the hot

surface of the positive point; and since free oxygen is needed for S0 2 F

2 production in the discharge [1,7-9], it might be expected, as observed, that

this species is more readily produced in the positive case.

The GC/MS data have been reanalyzed to derive production rates for S0F 2 and

S0 2 F 2 expressed in terms of moles generated per unit of charge transported

across the discharge gap, i.e., in mol es-per-coulomb . The results of this

analysis are tabulated in tables 1 and 2, respectively, for S0F 2 and S0 2 F 2

. Indicated in the tables are the corresponding discharge currents, power

dissipations, and polarities as well as the gas pressures.

Examples of the data from which these rates were derived are shown in

figures 3a-7a in which the measured concentrations of S0F 2 or S0 2 F

2 in nanomoles

are plotted versus the total accumulated charge Q transported across the

discharge gap in coulombs. The net charge is computed from the discharge

current I (held constant in these experiments) and the time t, using Q

= It.

The tabulated rates were obtained from fits to the GC/MS data of the form

C = B + AQ 1+e

, (1)

where A, B, and e are constants, and C is the concentration of the species of

interest. From this the production rates are expressed as ^

= (1 + e)AQ

e

. (2)

The parameter B corresponds to initial concentration, usually below the limit of

detection, and the parameter e indicates the deviation from linearity. The

solid lines in figures 3a-7a correspond to fits of the form given by eg (1).

5
Figure 2. Recording of discharge current corona in SF 6 at a pressure of cases is 8.0 uA.

versus time for positive and negative

200 kPa. The average current in both

6
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so

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cc<

z

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Figure

4.

Measured

concentrations

of

S0F2

versus

(a)

net

charge

transported

in

the

discharge

gap,

and

(b)

net

energy

dissipated

in

the

discharge

for

positive

corona

in

200

kPa

SFg

at

the

indicated

discharge

powers

and

currents.
Q

Ul< ” CLM

<AQ

>

a

cc

Uiz

Ul

o

CO

w

cu

in

a

uiy-x

o

a.

UiZ

<

Z

H*

Uia

x

<

z

o

ino

(S9(OUJ rt) N 0llVa±N30 N 00 z d

z OS

9

Figure

5.

Measured

concentrations

of

S02F

2

versus

(a)

net

charge

transported

in

the

discharge

gap,

and

(b)

net

energy

dissipated

in

the

discharge

for

negative

corona

in

200

kPa

SFg

at

the

indicated

discharge

powers

and

currents.
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**- c

73 73

<V <V u 3 i/i 1- s_ s_ O 73 QJ

CO

OO

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a) s_ <V JZ 73 03 4-1 .C

su

73

C CO

U 73

73

•P (U 73

<u 4_> ai C 73 w

Q. 73

73 CO •»— '—• CO 73 — C

CO 73 3

CO >, O)

S- 03 J3

(U

L *J > OJC 4-1 cm a) /a

CM 4-> CO O (U IL

<S)

C C/3

<4- ' 73 0 3 0.

COC 73 o o e o •— 73 C\J

73 » S3

1- Q-

— 4-1 73 S3 03 73

O) S3 u aj o C 03 S_ o s- o U 73 U

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73 a) CO i. JJC O 3 z *4 a. u

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a>

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03

10
SOaF

a

CONCENTRATION

(

(I

moles)

SOaF

a

CONCENTRATION

(Mmoles)

0 10 20 30 40 50

ENERGY DISSIPATED (kJ)

Figure 7. Measured concentrations of S0 2 F 2 versus (a) net charge transported in

the discharge gap, and (b) net energy dissipated in the discharge for

positive corona in 300 kPa SF s at the indicated discharge powers and

currents.

11
Table 1. Measured production rates for S0F 2 in SF 6 at the

different indicated gas pressures, discharge powers, and polarities

Polarity

Gas

Pressure

(kPa)

Average

Power

(mW)

Current

(uA)

Measured Production Rates

(n moles/C)

Q

= 0.3 C Q

= 0.4 i

Positive 116 804 20.0 18.2 21.6

200 54 1.5 172 198

200 335 8.0 189 221

200 777 16.0 159 200

300 198 4.0 204 298

300 430 8.0 142 172

300 945 16.0 89 109

Negative 200 230 8.0 25.4 27.8

200 586 16.0 23.0 24.6

200 821 20.0 21.5 22.0

200 2215 40.0 19.0 20.2

200 4290 64.0 18.7 20.0

Table 2. Measured production rates for

different indicated gas pressures, discharge

S0 2 F 2 in SF 6 at the

powers, and polarities.

Gas

Polarity Pressure

(kPa)

Average

Power

(mW)

Current

(uA)

Measured Production Rates

(n moles/C)

Q

= 0.3 C Q

= 0.4

Positive 116 804 20.0 24.0 28.5

200 54 1.5 162 188

200 154 4.0 176 208

200 335 8.0 136 158

200 777 16.0 94 109

300 198 4.0 105 125

300 430 8.0 100 112

300 945 16.0 60 71

Negati ve 200 230 8.0 5.4 7.4

200 586 16.0 10.1 11.9

200 821 20.0 11.3 11.4

200 2215 40.0 9.7 10.3

200 4290 64.0 11.0 10.7

12
The production rates per-unit-time can be found using

dC _ r dC

dt

" 1 dQ * ( 3 )

or, if the production rates are expressed in terms of moles-per-unit of energy U dissipated in the discharge, then

dC

dt p(t)

dC

dU (4)

where p(t) is the discharge power, which was, in general, slightly time

dependent for the experiments reported here.

Figures 3b-7b show plots of the concentration C versus the energy U from

which the production rates expressed in terms of moles-per-unit of energy

dissipated given in our previous quarterly report were obtained. It is

significant to point out that the production rates given by (dC/dQ) show much

less variation with discharge power (or equivalently current) than rates defined

as (dC/dU). In fact, as figures 3b-7b indicate, (dC/dU) tends on the average to

decrease with increasing power, whereas (dC/dQ) shows in most cases only a small

variation with current, which in view of the uncertainties in the measurements

is of doubtful significance. Therefore, we can conclude from the measurements

performed to date that the time-rates of production, (dC/dt), of the species

S0F 2 and S0 2 F 2 in both positive and negative corona discharges are nearly

directly proportional to the discharge current I.

This conclusion is supported by a direct comparison of the C versus Q data

in figures 3a-7a with the C versus U data in figures 3b-7b. It should be noted

that although the trend suggesting that (dC/dt) « I is evident for both positive

and negative discharges, the results for the negative case must be considered

more reliable because, as the data in figure 2 would indicate, the average

discharge current is more accurately measured and controlled during the negative

discharge. The data in figure 2 correspond to a time interval of 1.5 h for 8 yA

discharges in SF 6 at a pressure of 200 kPa for positive and negative polarities.

If the formation of the observed stable oxyfluoride species involves an electron-induced dissociation reaction such as

e + SF 6 * SF 4 + 2F + e (5)

as the initial step, then it can be argued that their production rates should be

nearly proportional to I, since the rate of initial dissociations will be

proportional to the number of electrons which pass through the gas in a given

time, provided that they possess sufficinet energy to induce dissociation. Of course it can be objected that, if the discharge current is varied for a given

gas pressure and gap configuration, it is necessary to change the electric field

in the gap as well, and thus also, the mean translational kinetic energy of the

electros. This would in turn affect the probability for the occurence of

reactions such as that of eq (5) above. Although admittedly this could give

rise to an increase (or decrease) in (dC/dQ ) with discharge current, there is reason to believe that this effect should be relatively small. It is known [10]

that at high field-to-gas density ratios, E/N, where ionization in the gas

becomes quite probable, typical electron excitation coefficients, including

dissociation and dissassociative excitation, show only slow variations with

E/N. This is because the cross sections for processes such as eq (5) above

13
usually tend to level off and eventually slowly decrease with increasing mean electron energy. This consideration, together with the expectation that as current increases so will the space charge in the gap which tends to moderate

E/N near the point electrode, leads one to suspect that variations in the

production rates (dC/dQ ) with discharge power will be generally small. It should also be kept in mind that the production of oxyfluorides is also

influenced by external factors such as the concentrations of oxygen and H 2 0, and

these may be affected by the discharge power level.

The results presented here suggest that it may be preferable to express

production rates of discharge-generated gaseous by-products in terms of

concentration-per-unit of charge transported, rather than per unit of energy

dissipated. This choice is also more convenent from a practical point-of-view,

since discharge current is more easily and accurately measured than power

dissipated. Also, it allows a more meaningful application of the data to

situations where there may be time varying voltages, e.g., 60 Hz ac conditions.

During the next quarter, more measurements will be made of oxyfluoride

production rates from corona in SF 6 and SF 6 /H 2 0 mixtures. The emphasis will be

on attempting to understand the effects of water vapor, and the factors that

influence the equilibrium concentration of water vapor during a corona discharge. Measurements will be performed to evaluate the response time of a thin-film aluminum oxide hygrometer probe for measurement of H2 0 in SF 6 and to

determine the uncertainties in the use of this device to calibrate the GC/MS for

H 2 0 measurements. It is expected that the first quantitative determination will

be the H 20 consumption or production during discharge operation in SF 6

.

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

National Bureau of Standards, Washington, DC.

4. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

Subtask No. 04

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 pre-breakdown and breakdown of processes

in specific insulating systems, and to demonstrate the applicability of the

developed instrumentation and the methodology in the development and design of

future systems.

During the previous quarter, measurements were made of the Kerr coefficient

of transformer oil as a function of the wavelength of the probing light. The

determination was made over the visible spectrum and for two temperatures, 24°C

and 97°C. The results demonstrated that the variation was small, so that there

was no need to pursue that topic further for the present.

Using dc voltage and a pressboard interface parallel to the field, for

example, it is observed that breakdown occurs around 40 kV/cm, whereas without

an interface, the strength of the oil alone is around 90 kV/cm. This situation

suggests the question: Is the lowered breakdown voltage of the interfacial

system due to space charge field enhancements which are present and cannot

easily be measured, or is space charge not detected because it is insufficient

14
to modify the electric field? The question of onset of space charge and the

temperature dependence of the nonuniformi ty of the electric field (field

enhancement) is settled this quarter to a great extent. Further, some light is

shed on the possible origins of space charge.

A sample of transformer oil is subjected to the field between parallel

plates so that the spatial dependence of the electric field can be measured

using the electro-optical Kerr effect. If space charge is not present, then the

field between the plates is uniform. Let the uniform field strength be denoted

E ave

= V/d where V is the applied voltage and d is the plate separation (in

this case d = 0.64 cm). As discussed in earlier reports, the light transmitted

through the cell due to the Kerr effect is given by

I(z)/Im = sin 2 (-rr/2)[E(z)/E m] 2

, (6)

where z denotes the distance from anode to cathode, E is the electric field,

Em = Vm/d where Vm is the cell constant (voltage for full transmittance),

I m is the incident intensity and I is the transmitted intensity. Because the

Kerr effect in transformer oil is small, the following approximation is quite

accurate:

I(z)/I m = (ir/2) 2 [E(z)/Em]

tf . (7)

If there is no space charge, then an amount of light I ave will be transmitted

through the cell which corresponds to the average field E ave . If the incident

intensity I m is uniform then I ave will be uniform. Usually, it is rather

simple to arrange for a relatively uniform incident light intensity, at least

for the accuracy needed here — about 5%.

The field nonuniformity (or enhancement as it is sometimes called) which

arises from space charge in the oil is best character!' zed by examining the

relative field. That is, rather than determining the ratio E(z)/Em , the ratio

of the field to what it would be if space charge were not present is more meaningful, E(z)/E ave . Such a representation of the field aids one to think

in terms of percent field enhancements. Using eq (7) the relative field is

given by

E(z)/E ave = [I(z)/I ave ](l/4). (8)

By integrating eq (8) I ave can be related to an integral of the light

intensity as

(I ave )d/4) = (1/d) / [I(z)]l/4dz, (9)

which is just the average of the fourth root of the light intensity profile

between the plates. This method works well only if the incident light intensity

is relatively uniform. A much more extensive analysis is necessary whenever the

incident light is not sufficiently uniform, for then I m (z) must be measured

for each I(z).

Throughout this investigation dc voltages are applied. A determination of

the relative field is made for each polarity. The space charge is negative and

tends to increase its concentration from nearly zero^in the vicinity of the

cathode to its maximum value (of order 200-300 pC/cm 3

) next to the anode. The

15
maximum relative field nonuniformity is measured for each polarity and the

average used is to characterize the field nonuniformity at a given voltage and

temperature. A plot of the maximum relative field nonuniformity as a function

of temperature and applied field is shown in figure 8.

There are two aspects of the data which should be highlighted because of

their potential significance. First, the space charge content appears to have

an onset which decreases as the temperature increases. This would explain why

no field enhancements were observed in the vicinity of an interface during

previous investigations — the breakdown field strength was not sufficiently

strong to initiate space charge. Second, there may be a saturation of space

charge at the higher temperatures and fields. However, because of the

uncertainty of these data, further research must be carried out to test this

hypothesis.

In an attempt to identify the origin of the space charge, two additional

sets of measurements were performed. First, to test the hypothesis that the

space charge is due to water, the water was removed from the oil. Specifically,

oil at 122°C, in which the field distortion was approximately 30%, was filtered

through activated alumina. The filtered oil exhibited no field distortion. If

water is then added to the oil, however, the field distortion remains negligibly

small. So, water alone is probably not the origin of the space charge.

The second hypothesis tested was the suggestion that the space charge was the result of particulate contamination. This was done by adding 5 mg of carbon

particles (with a distribution of diameters which was probably peaked in the

1-10 urn range) to 2.8 liters of oil. Again, no space charge was produced.

It is anticipated that much of the construction of the interfacial

breakdown cell will be finished during the next quarter. Its purpose is to

provide a means to change the interface in a high-temperature cell without

opening the cell. The resultant breakdown information -- with and without an interface — as a function of temperature will be useful in modeling phenomena

in practical systems, especially if the space charge in the oil can be monitored

and controlled.

The International Conference on Interfacial Phenomena will be held in late

September. All authors have been contacted and a tentative agenda will be

developed early next quarter.

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

National Bureau of Standards, Washington, DC.

5. ACTIVE INSULATORS FOR INSULATION AND SURGE SUPPRESSION

Subtask No. 04

This section of the quarterly report describes the work performed by the

Electrical Engineering Department of the University of Southern California under

a grant from NBS to evaluate the concept of active insulators. These insulators

are to be built with a nonlinear voltage dependent resistance, which is designed

to reduce the effects of switching and lightning surges.

The project consists of two parts: first, a theoretical part to develop a mathematical model to predict the behavior of active insulators on a

16
Figure 8. Maximum field enhancement in percent as a function of applied dc

field for various temperatures. The field is enhanced nearest the

anode indicating a negative space-charge density. Measurable field

nonuniformities could not be found for temperatures below 50°C before

breakdown field levels were reached near 90 kV/cm. Data taken were insufficient to determine the cause of the apparent leveling off of

the enhancement for the 122°C measurements, however, the data point

marked with "x" indicates a reduction of space charge may have

occurred overnight. Because of the "noise" in the data and

imperfections in the optics, resolution was limited, and reliable

field measurements could only be made for fields over 40 kV/cm.

17
transmission line; and second, an experimental part to install a materials

research facility to fabricate and evaluate simple prototype insulators.

During this quarter the simulation program based on the algorithm described

in the previous quarterly report was implemented on a computer to model

three-phase systems under conditions of switching surges. The materials

laboratory setup was completed and the formulations tested showed significant

improvement, as described below.

The model developed to represent a three-phase transmission line during

transients was described in the previous progress report. The model has been

validated by comparing calculation results with those reported in the

1 iterature.

In this study a 240 km long, 345 kV transmission line was simulated under

three switching conditions:

1. Energizing an open line.

2. Energizing an open line with one phase shorted to ground at the

receiving end.

3. Re-energizing a line with one phase shorted to ground at the receiving

end.

Figures 9, 10, and 11 show the maximum voltage waveforms for the three

conditions, respectively. In each figure, part (a) is the voltage waveform

calculated at the sending end; part (b), at the midpoint of the line; and part

(c), at the receiving end. The solid line refers to the case without nonlinear

insulators on the line and the dotted line refers to the case for which 10

nonlinear insulators were evenly distributed along the same line (per phase).

The above figures clearly indicate that the installation of nonlinear

insulators reduces the overvoltage magnitudes for all three conditions.

Figures 12, 13, and 14 illustrate how the maximum overvoltages are reduced

and how the voltage is more evenly distributed along the entire length of the

line when nonlinear insulators are used. These figures also show the strong

effect the parameter a of the nonlinear impedance material has on the resulting

overvoltages.

The effect of the number of installed nonlinear insulators on the

overvoltage magnitudes and distribution can be seen in figures 15 and 16. If a total of only 30 nonlinear insulators are installed (one per phase every 24 km

in this study), an almost uniform distribution can be obtained for the 240 km

line.

Power dissipation during the transient by the nonlinear insulators is a primary consideration in the application of these devices, and it was also

investigated. It was found that for the case of energizing the system with one phase-to-ground, the energy which a nonlinear insulator unit must handle is

54 kWs, or 24% of the energy a station arrester must dissipate on a line without

nonlinear insulators. During re-energization, a nonlinear insulator unit

18
Figure 9. The overvoltage waveform during condition 1.

19
Figure 10. The overvoltage waveform during condition 2.

20
Figure 11. The overvoltage waveform during condition 3.

21
Distance Along the Line

(Arbitrary Units)

Figure 12. The distribution of maximum overvoltage along the line durinq condition 3.

22
Distance Along the Line

(Arbitrary Units)

Figure 13. The distribution of maximum overvoltage along the line during

condition 2.

23
Distance Along the Line

(Arbitrary Units)

Figure 14. The distribution of maximum overvoltage along the line during

condition 1.

24
2 4 6 8 10

Distance Along the Line

(Arbitrary Units)

Figure 15. The effect of the number of nonlinear insulators installed on the

distribution of maximum overvoltage during condition 3.

25
800

Distance Along the Line

(Arbitrary Units)

Figure 16. The effect of the number of nonlinear insulators installed on the

distribution of maximum overvoltage during condition 2.

26
handles 130 kWs, or 20% of the energy a station arrester must dissipate when

only conventional insulators are used. Figures 10 and 11 graphically illustrate

how the thermal duty of a station arrester is reduced with nonlinear insulators.

From this theoretical study, three preliminary conclusions of practical

importance can be derived:

1. Nonlinear insulators can effectively limit overvoltage magnitudes

along the entire length of transmission line, requiring the

installation of relatively few of these special devices (at towers

every 24 km for the 240 km line in this study).

2. The energy which the nonlinear insulators must dissipate is in

accordance with the present zinc-oxide technology, at least on a short-term basis.

3. With nonlinear insulators the insulation level of the system can be

reduced to a point where transient overvoltages are no longer a principal consideration in this regard.

During this period, discs, rods, and tensile test samples were prepared in

the materials research facility. Electrical and mechanical tests show that the

mechanical properties of the polymer concrete could be improved without

degrading the electrical characteri sties. The test results obtained appear in

table 3.

Future work in this area consists mainly in developing molding and coating

techniques to be able to fabricate simple nonlinear insulators. These prototype

insulators will be used in the experimental evaluation of the nonlinear

insulator concept.

Table 3. Electrical and mechanical properties of polymer concrete samples.

Average Best

Loss Tan 6 - 0.04 0.017

Dielectric constant ~ 3.5 1.7

Volume resistivity (ohm/cm) - 0.6 x 10 15 1.15 x 10

15

Dielectric breakdown kV/mm >16.0 -20.0

Compression strength Pa -1.8 x 10 8 2.14 x 10

8

Tensile strength Pa - 2.2 x 10 7 2.44 x 10

7

For further information contact Dr

University of Southern California, Los

'. T. C. Cheng, (213)

Angeles, CA.

743-6938,

27
6. INVESTIGATION OF INSULATOR SURFACE FLASHOVER IN GAS

Subtask No. 04

The objectives of this research, which is being carried out at the

University of South Carolina, are to determine the basic mechanisms leading to

insulator surface flashover in gas and to determine the roles of various,

controllable parameters in the flashover process. It has been previously

postulated that insulator surface charging, surface fields, and surface currents

are important in flashover production. Diagnostics have therefore been

developed and have been used to measure insulator surface currents and fields,

prior to breakdown, for different materials and geometries. Additionally,

analytical techniques are being developed to analyze and assist in the

understanding of the data produced. Accomplishments made this quarter and tasks

to be performed during the next quarter are presented in this report.

Accomplishments made during this quarter include: (1) measurement of

breakdown voltage and current, in gas along polymethylmethacrylate (PMMA)

insulators of various shapes, surface conditions, and geometries; (2) continued

modification of the electro-optical experimental arrangement; (3) a field

analysis computer code obtained from NBS has been made operational; and (4)

models are being fabricated to enable use of the electrolytic tank for field

analyses.

The breakdown voltage and surface currents have been measured along PMMA

insulators, using previously developed diagnostics. Results show the effect of

insulator surface angle A, surface roughness, and voids (along the insulator and

at the triple junction). Figure 16 shows the insulator breakdown (flashover)

voltage, Vbd, in nitrogen, as a function of pressure p, for various angles.

The electrode separation is maintained at 5 mm. The angle A is defined to be

the angle between the applied field (normal to the electrode surface) and a normal to the insulator surface. It can be observed that: (1) Vbd increases

approximately linearly with p; (2) Vbd (90 degrees) > Vbd (60 degrees) > Vbd (15 degrees), i.e., Vbd decreases as the angle decreases and the

insulator becomes "flatter."

It should be noted that this behavior is very different from that observed

for pulsed voltages in vacuum, where Vbd increases markedly as A varies from

90 degrees to 45 degrees. It should be further noted that Vbd increases for

larger A even though the effective path length along the insulator increases

with larger A. It can be postulated that the observed decrease in Vbd with A

is due to the increase in the surface fields in gas at the acute angle between

the insulator and electrode surfaces, for A less than 90 degrees, and/or the

increased insulator surface area and triple junction length for smaller A

values. The observed performance is significant, since it implies that high

pressure gas insulators should be designed with large A values.

Similar data have been obtained using SF 6 instead of nitrogen. The results

show a performance similar to that of figure 17 except all voltages are higher

by a factor of 2.5.

Regarding the data of figure 17, it has also been observed that Vbd (A)

for a rough surface (sandblasted) is slightly less than for a polished surface.

The performance is, however, more repeatable and predictable. Additionally, it

has been observed that the flashover along polished PMMA is constricted and

28
5 mm bridged

PRESSURE (10s Pa)

Figure 17. Breakdown voltages for various cone angles.

29
ultimately causes a track. The discharge is, however, diffuse along the

roughened surface. This last observation implies that roughened insulator

lifetime may exceed that of polished (or smooth surfaced) insulators.

It has been further observed that the flashover luminosity, during

conduction, is away from the insulator for 1.73 x 10 5 < p < 4.5 x 10

5 Pa and

along the surface for p > 4.5 x 10 5 Pa.

Surface currents (partial discharges), i, have also been measured as a function of A and p. These results show that no measurable current flows for

p < 5.5 x 10

5 Pa. Small surface currents are, however, measured for high

pressures.

Breakdown data and surface current data have also been obtained to

determine the effect of triple junction voids and voids (or imperfections) along

the insulator surface. The geometries test are shown in figure 18.

The values of Vbd, as a function of p, have been determined for the

geometries of figure 18(a) and (c) and are shown in figure 19, together with

values for the surface current inception voltage, V-j, the current extension

voltage, V e , and the breakdown voltage for an unbridged gap. The data show

that: (1) Vbd, Vi, and Ve are all linearly increasing functions of p;

(2) Vbd for the unabridged and bridged gap are approximately equal, up to

1 x 10° Pa; (3) Vbd» with the void (as shown in figure 18(c)), is lower than

Vbd without the void; and (4) Vi and V e are both linearly increasing

functions of p.

The values of Vbd for all the geometries of figure 18 have also been

determined for vacuum and 1.73 x 10 5 Pa nitrogen, for comparison purposes.

These data are indicated in figure 18, where Vbdv and Vbdn are the breakdown

voltages in vacuum and nitrogen, respectively. It should be noted that:

(1) Vbdv > vbdn» Tor geometries; (2) Vbdv 1S approximately the same for all geometries except that of figure 18(d); the presence of a total gap

between the insulator and the electrode, therefore, considerably degrades (by a factor of 2) the hold-off performance; (3) the presence of a perturbation on the

insulator does not degrade Vbdv or VbdnJ (4) the presence of a partial

gap, as in figure 18(c), substantially degrades Vbdv but not Vbdn* This

implies that the gas in the void, where there is considerable field enhancement,

due to the lower gas permittivity, is ionized by a lower, total applied

voltage. It further implies that field emission in the high field, vacuum gap

region does not greatly lower Vbdv* Particularly, these results show that

voids at the triple function substantially decrease Vbd Tor gas and should

certainly be eliminated in high voltage designs.

It should be noted that non-zero surface current has been measured in

nitrogen for the geometries of figure 18(c) and (d). This implies that the

presence of a void, near the electrode, produces partial discharges in the gas

void. It is further postulated that the partial discharges grow and ultimately

lead to complete insulation flashover.

It should be noted, additionally, that for all bridged gap data, V-j is

considerably less than Vbd Tor vacuum filling but that Vi - Vbd Tor gas.

This implies that the development of the pre-breakdown processes begins at

30
(a) (b)

0.6mm

T

1mm

bdv =38 kV W37 kV Vbdv =34 kV ^bdv

bdrT 24kV W22 kV V

bdrT 15kV V

bdn

(d)

= 17 kV

= 1 1 .2 kV

Figure 18. Insulator geometries tested.

31
PEAK

VOLTAGE

(kV)

1

1

50 —

40 —

30 —

20 —

10 —

2.4 3.8 5.2 6.6 7.9 9.3

PRESSURE (10 5 Pa)

Figure 19. Breakdown, inception, and extinction voltage, 2 mm gap with void.

32
rather low voltages for vacuum but high voltages, essentially equal to V^,

for nitrogen filled gaps.

Problems continue to be encountered in the electro-optical field

measurement experiment. The system was completed this quarter. However, during

final assembly the glass test vessel began to leak nitrobenzene around the

window. Analysis has shown that the adhesive used to attach the window had not

been properly cured. The test vessel is now being repaired, reassembled, and

will soon be reinstalled. It is anticipated that electro-optical measurements

of the insulator surface fields in nitrogen will be produced early during the

next quarter.

A copy of the electric field analysis computer program (PLTMG), developed

by Randolf Bank (now at the University of California at San Diego), has been

obtained from NBS. The program has been entered into the University of South

Carolina computer system, debugged, and interfaced with the University of South

Carolina output graphics. Various example electrode, insulator geometries have

been solved using the program. The program is now working well and is capable

of solving geometries with one dielectric. Solution of this type of problem is

relevant to insulator surface flashover, where a dielectric interface exists.

The electrolytic tank has been previously constructed and used. PMMA

models are being machined for use in the tank. The electric fields, for the

experimental geometries being used, will be determined using this analog

technique and compared with the computer solutions.

Tasks to be completed next quarter include:

1. Electro-optical measurement of insulator surface fields for nitrogen

and SF 6 gas.

2. Surface current and breakdown voltage measurements for nitrogen and SF 6 gas interfaces. Specifically, Vbd and V-j will be determined using

60 Hz ac for various insulator materials, insulator shapes, and triple

junction constructions for both gases.

3. The field distributions for the geometries used in 1 and 2 above will

be calculated using the computer program acquired from NBS and the

electrolytic tank.

For further information, contact Dr. T. S. Sudarshan, (803) 777-7302,

University of South Carolina, Columbia, SC.

7. REFERENCES

1. R. J. Van Brunt and D. A. Leep, Corona Induced Decomposition of SF 6 , Gaseous

Dielectrics III, Proc. of the 3rd Int'l. Symp. on Gaseous Dielectrics, Ed.

by L. G. Christophorou, Pergamon Press, New York, 1982, pp. 402-409.

2. R. J. Van Brunt and M. Misakian, Role of Photodetachment in Initiation of

Electric Discharges in SF 6 and 0 2 , J. Appl. Phys., Vol. 54, pp. 3074-3079,

1983.

33
3. J. W. Gallagher, E. C. Beaty, J. Dutton, and L. C. Pitchford, An Annotated

Compilation and Appraisal of Electron Swarm Data in Electronegative Gases,

J. Phys. Chem. Ref. Data, Vol. 12, pp. 109-152, 1983.

4. S. Hasegawa, L. Greenspan, J. W. White, and A. Wexler, A Laboratory Study of

Some Performance Characteristics of an Aluminum Oxide Humidity Sensor, Nat.

Bur. Stand. (U.S.) Tech. Note 824 (1974).

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

in SF 6 , J. Appl . Phys. Vol. 52, pp. 6588-6600, 1981.

6. M. Goldman and A. Goldman, Corona Discharges, Gaseous Electronics - Electrical Discharges, Vol. 1, Ed. by M. N. Hirsh and H. J. Oskam, Academic

Press, pp. 219-290, 1978.

7. W. Becher and J. Massonne, Contribution to the Decomposition of Sulfur

Hexafluoride in Electric Arcs, Elektrotech. Z., Vol. A91, pp. 605-610, 1970.

8. C. Boudene, J. L. Cluet, G. Keib, and G. Wind, Identification and Study of

Some Properties of Compounds Resulting from the Decomposition of SF 6 Under

the Effect of Electrical Arcing in Circuit Breakers, Revue Generale De

L'Electricite (RGE) Numero Special, pp. 45-78, 1974.

9. B. Siegel and P. Breisacher, Oxidation of Sulfur Hexafluoride, J. Inorg.

Nuclear Chem., Vol. 31, pp. 675-683, 1969.

10. J. Dutton, A Survey of Electron Swarm Data, J. Phys. Chem. Ref. Data,

Vol. 4, pp. 577-856, 1975.

34
NBS-H4A (rev. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

PUBLICATION OR

REPORT NO.

NBSIR 84-2809

2. Performing Organ. Report No 3. Publication Date February 1984

4. TITLE AND SUBTITLE

DEVELOPMENT OF POWER SYSTEM MEASUREMENTS — QUARTERLY REPORT

APRIL 1 , 1983 to JUNE 30, 1983

5. AUTHOR(S)

R. E. Hebner, Editor

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

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION 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

~~ TM

~| 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 significan t bibliography or literature survey, mention it here)

This report documents the progress on five technical

investigations sponsored by the Department of . Energy. Three were

performed by the Electrosystems Division, the National Bureau of

Standards, the fourth by the Department of Electrical Engineering of

the University of Southern California, and the fifth by the College of

Engineering at the University of South Carolina. The work described

covers the period from April 1, 1983 to June 30, 1983. The report

emphasizes the calibration of instruments designed to measure the

60-Hz electric field in biological exposure facilities, the effect of

water on SF 6 corona discharges, the measurement of failure mechanisms

in liquid/solid and gas/solid insulating systems, and the development

and behavior of active insulators.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) composite insulation; electric fields; high voltage; insulation; liquid breakdown;

SFg*, space charge; transformer oil

13.

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