Development of Power System

Measurements -- Quarterly Report

July 1, 1983 to September 30, 1983

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosvstems Division

Washington, DC 20234

February 1 984

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prepared for: Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

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NBSIR 84-2818

DEVELOPMENT OF POWER SYSTEM

MEASUREMENTS - QUARTERLY REPORT

JULY 1, 1983 TO SEPTEMBER 30, 1983

'KATIOMAL BUnCAU OF STAJJDAP.OS

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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
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Foreword

This report summarizes the progress on three technical investigations

during the fourth 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 NEASUREMENTS

Subtasks No. 01 and 02 1

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. 03 2

4. OPTICAL ^€ASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

Subtask No. 04 13

5. REFERENCES 20

iv
LIST OF FIGURES

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Page

Measured production of SOF 2 , SO 2 F 2 , H 2 O, and SO 2 in a continuous negative, point -plane corona discharge with

a current of 25 uA in SF 0 at 114 kPa 5

Measured production of SOF 2 , SO 2 F 2 , and H 2 O in a continuous negative, point-plane corona discharge

with a current of 40 pA in SFs at 200 kPa 6

Measured production of SOF 2 , SO 2 F 2 , and H

2 O in a continuous positive, point-plane corona discharge with

a current of 8 pA in SFs at 300 kPa. (The

concentrations for H 2O are only approximate) 9

Measured production of SOF 2 , SO 2F 2 , and H 2O in a continuous positive, point -plane corona discharge with

a current of 16 pA in SFe at 300 kPa. (The

concentrations for H 2O are only approximate) 10

Comparison GC/MS and hygrometer probe responses for

detection of trace water vapor in SF 0 at 200 kPa 12

Typical response curve for a thin-film aluminum oxide

hygrometer corresponding to a sudden increase in water

vapor concentration. Vertical arrow indicates time at which

water vapor concentration is increased . 14

The average breakdown field and maximum field

enhancement vs temperature. The average breakdown

field Eb is defined as the breakdown voltage

divided by the parallel plate separation. Under

the application of dc voltage the maximum field

enhancement is found at the anode due to

heteropolar space charge accumulation in its

vicinity. The maximum field enhancement

uncertainty is estimated to be ±3% 16

The relative field as a function of position at different

times for the two types of oil used. The applied voltage

is dc, and is 55 kV for type 2 oil, 50 kV for type 1 oil;

the gap spacing is 0.635 cm. Type 2 oil shows

essentially no change in time, however, type 1 oil shows

great changes in a relatively short time. All data are taken at 120'’C. The solid lines show the field when the

high voltage electrode is positive and the dashed lines

show the field when the high voltage electrode is

negative 18

V
LIST OF FIGURES (cont.)

Page

Figure 9. Relative electric field between parallel plates near and

away from a pressboard-oi 1 interface parallel to the

field. The inset shows the view from the detector looking

into the light beam. The light travels parallel to the

long plates between which a long pressboard interface is

placed. The detector sees the edge of the system and

monitors the electric field in the two regions specified.

The detection system is only sensitive to extended

perturbations of the electric field. Small changes in the

local electric field around a void or a particle would not

be noticed. The small fluctuations in the field profiles

arise from the localized nonuniformities in the light beam

as well as "waves" of small thermal gradients in the heated

oil tank enclosing the cell. These measurements are averages made over 10 s to 20 s intervals. Dashed lines

denote the field away from the interface while the solid

lines denote the field near the interface 19

LIST OF TABLES

Table 1. Reaction scheme 8

VI
DEVELOPMENT OF POWER SYSTEM MEASUREMENTS - QUARTERLY REPORT

July 1, 1983 to September 30, 1983

R. E. Hebner, Editor

This report documents the progress on three 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 July 1, 1983 to

September 30, 1983. The report emphasizes the measurement of the

60-Hz electric and magnetic field in biological exposure facilities,

the measurement of water vapor, the production rates of oxyfluorides

in Sp0 corona discharges, and in the measurement of space charge in

transformer oil.

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

voltage; incipient fault; insulation; liquid breakdown; SFe; 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 the following areas — the measurement of

electric 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 July 1, 1983 to

September 30, 1983 is summarized in this report.

2. ELECTRIC FIELD MEASUREMENTS

Subtasks 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, 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; 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

are funded by the State of New York.

During the current reporting period, an NBS staff member made a site visit

to the University of Utah in Salt Lake City to perform measurements of 60-Hz

magnetic fields and observe procedures employed by researchers for the

measurement of several related electrical parameters. The investigators are performing in vitro biological studies of cells exposed to 60-Hz electric and

magnetic fields. The studies are funded by the State of New York Department of

Health. Report of the measurements and observations has been forwarded to the

State of New York and Department of Energy.

The third revision of an lEC 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 discussion at the time of the working group meeting
which is being planned for the month of November. Consensus is near on the

draft document and preparation of a final draft may be possible after one more

revision.

During the next quarter it is anticipated that preparation of the text for

a technical note identifying electrical parameters which must be measured and

controlled during biological experiments with 60-Hz electric and magnetic fields

will be completed and ready for internal review. NBS staff will attend meetings

of (1) an EPRI-sponsored instrumentation measurements field day in October at the Bonneville Power Administration to compare the performance of the BPA

personnel electric field monitor and a vest-type monitor developed under

contract to EPRI; (2) the DOE contractors review meeting during November in

Kansas City during which a presentation on dc field and air ion measurements

will be given; (3) an lEC working group meeting which will prepare a draft

standard for power frequency electric field measurements. The meeting will take

place in November near London.

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

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. 03

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 SF 0 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 0 + N 2 , SF 0 + CO 2 , and

SF 0 + 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 0 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 SF 0 ; and

2
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, SF 0 , and other gas dielectrics.

Activity 1 has been completed and an archival paper describing the results

of this work has been published [2]. During the past quarter focus was on the

tasks necessary to bring activities 2 and 4 to completion. Some of the results

obtained as part of these efforts are highlighted in this report. Because of

the priorities assigned to these tasks and the unanticipated problems

encountered in completing them, it has been necessary to postpone beginning work

on tasks 3 and 5 until FY84.

Priority was given to task 2 because of its relevance to the design of

meaningful chemical diagnostics tests which could be used to evaluate

performance of practical SF 0 insulated systems. The need here, of course, is

for quantitative data on decomposition-species production rates that could be

used to interpret information from gas analysis in a way which might reveal the

nature of any internal electrical activity (incipient faults). The production

rates that we have measured for the oxyfluorides, SOF 2 and SO 2 F 2 , in SF 0 can, in

principle, be used to determine how long corona, or partial discharges, would

have to persist in a system at particular levels in order to yield observable,

known concentrations for these species. The data on production rates (see our previous two quarterly reports) are expressed in terms of either moles per unit

of energy dissipated in the discharge, or moles per unit of charge transported

in the gas during the discharge. If one knows the internal discharge power

dissipation or current, then one can predict from our data the production rates

expressed in terms of absolute quantities produced per unit of time. Conversely

if one knows the production per unit-of-time, one can infer something about the

level of discharge activity within the system, assuming of course that the

volume, pressure, and gas content of the system are also known.

In order that our results might be more useful in the design of tests to

evaluate practical system performance, it was decided during the past year that more data should be taken on production rates for a wider range of gas, gap, and

discharge conditions than had originally been anticipated. Before our results

can be applied to the interpretation of tests on high-voltage, gas-insulated

apparatus, one needs to know how the rates of production of discharge-generated

contaminants depend on the range of operating conditions likely to be

encountered. In the present report, we show data recently obtained on the

simultaneous variations in the absolute concentrations of SOF 2 , SO 2F 2 , SO 2 , and

H 2 O during a continuous, negative, dc corona discharge in gaseous SF 0 at

different pressures. Abstracts of papers describing some of these results have

also been submitted for presentation at the Thirty-Sixth Gaseous Electronics

Conference, October 11-14, 1983, and the Fourth International Symposium on Gaseous Dielectrics, April 29 - May 3, 1984.

More emphasis has also been given to activity 4 because of the persistent

uncertainties and controversies associated with the establishment of meaningful

criteria for standards of insulating gas purity, particularly as apply to water

vapor contamination in compressed SF 0 now widely used in practical systems. Our

measurements as part of both activities 2 and 4 have been directed at questions

concerning: 1) the role of water vapor in the production of decomposition

species such as the oxyfluorides and hydrogen fluoride which represent permanent

damage to the gas insulation; 2) the effects of water vapor upon the electrical

3
performance of gaseous dielectrics, e.g., the dielectric strength and intensity

of corona; and 3) the problems associated with the measurements of trace levels

of water vapor, particularly when electric discharges are present. It is hoped

that the results of our investigations can be used to help establish the

acceptable levels of water contamination for different high-voltage

applications, and suggest improved methods for monitoring water vapor in

practical systems. Our results on corona characteristics and statistics of

electron avalanche growth in SF 6 /H 2O mixtures [3] already suggest that the

presence of water vapor is much less tolerable in applications where SFg is used

as a fast, pulsed-power switching medium than in applications where it serves primarily as an insulant.

In addition to the activities listed above, during the past quarter a draft

of a computerized bibliography on electrical breakdown data for gases was prepared and distributed to members of the IEEE Insulation Society Technical

Committee S-32-11 on Gaseous Dielectrics for comments and suggested additions.

The relevant references have been entered into a computer database with a code

giving the types of gases considered and data presented. It is intended that

this bibliography will serve as a guide to current information available in the

open literature on electrical breakdown in gases. The gases included are those

considered most relevant to electrical insulation applications.

The bibliography will be discussed at the next meeting of the committee

which will occur October 19, 1983 during the 1983 Conference on Electrical

Insulation and Dielectric Phenomena. It is expected that during the next

quarter all the information necessary to complete the bibliography will have

been received, and a complete version with extensive cross referencing will be

prepared for publication in a special NBS report. The intention is also to

publish an abbreviated versiorv in the IEEE Transactions on Electrical

Insul ation.

As part of our technical work, our previous measurements of the production

rates for SOF 2 and SO 2 F 2 in SFg negative corona have been extended to lower gas

pressures. The data were found to be consistent with the trends discussed in our earlier reports and will not be shown here. Examples of simultaneous

production curves for SOF 2 , SO 2F 2 , SO 2 , and H 2O are displayed in figures 1 and 2 for negative corona discharges in SFg at pressures of 114 and 200 kPa,

respectively. Shown are plots of measured absolute concentrations in u-moles

versus net charge, Q, transported across the discharge gap in coulombs. This

latter quantity is given by

Q =jh(t')df , (1)

where I(t') is the instantaneous discharge current measured at time t'. These

figures show our first quantitative data on H 2 O and SO 2

.

The production rate for SO 2 is found to be more than an order-of-magnitude

smaller than the production rates for either SOF 2 or SO 2 F 2. This observation is

consistent with the expected predominant mechanism for SO 2 formation, namely, a slow hydrolysis of SOF 2 via the reaction

SOF 2 + H 2O > SO 2 + 2HF, (2)

4
80

Figure 1. Measured production of SOF 2 , SO 2 F2 , H 2O, and SO 2 in a continuous

negative, point-plane corona discharge with a current of 25 uA in SFc

at 114 kPa.

5
Figure 2. Measured production of SOF 2 , SO 2 F 2 , and H 2 O in a continuous negative,

point-plane corona discharge with a current of 40 uA in SFc at

200 kPa.

6
(also see Table 1). The quantitative analysis for SO2 was performed by the same method as used for SOF2 and SO2F2. For the H2O measurements, the gaschromatograph mass-spectrometer (GC/MS) was calibrated by comparing its response

to that of a previously calibrated thin-film aluminum oxide hygrometer probe

under equilibrium conditions as will be discussed in more detail below.

The vertical arrows in figures 1 and 2 indicate times when the discharge

was turned off for periods of 16 to 20 h. It is seen that during these periods

the concentration of water vapor in the discharge cell generally tended to

increase, whereas the concentrations of the oxyfluoride species remained

essentially unchanged. It should be noted that this observation also applies to

the species SOF4, even though its absolute level of concentration could not be

measured. This same behavior was found to occur under a wide range of discharge

conditions. Figures 3 and 4 show similar data for positive corona at different

discharge currents and gas pressures. In these figures, however, the

concentrations for H2O can only be considered approximate since no calibration

of the GC/MS was performed for this species.

The results for H2O indicate that water vapor is consumed during discharge

activity and its equilibrium value in the cell is diminished. This is expected

on the basis of the predominant reactions that are likely to occur in the gas

both inside and outside of the discharge as given in Table 1. The hydrolysis of

SF4 in particular is known [4] to be quite rapid, and probably accounts for most

of the H2O consumption and the observed build-up of SOF2. The fact that the H2O

content increases during periods when the cell is left undisturbed suggests that

the hydrolysis of SF4 is fairly complete within relatively short times after its

production. The lack of significant change in the amounts of SOF2, SO2F2, and

SOF4 during these same periods also indicates that the predominant reactions

leading to these occur during, or shortly after, discharge activity. Moreover,

this observation coupled with the measured slow rate of SO2 production implies

that the reactions of SOF2 and SOF4 with H2O are very slow (see Table 1).

A consideration of the possible important reactions which could occur in

the discharge and subsequently in the gas around the discharge, as given in

Table 1, would suggest that formation of thermal ized SO2F2 from the primary SFg

dissociation species, i.e., lower valence sulfur fluorides and free radicals,

would necessarily require the presence of O2. Of course, it could also be

formed in the discharge by reactions involving the other oxyf luorides, e.g..

SOF2 + 0H+ > SO2F2 + H+ (3)

SOF2 + 0 SO2F2 (4)

SOF4 + H2O+ > S02F2'^ + 2HF . (5)

These, however, are secondary reactions, and their predominance as formation

mechanisms would require significant deviations from linearity in the production

curves. As shown in figures 1-4, the production curves for both SOF2 and

SO2F2 eventually become quite linear, which is equivalent to saying that the

production rates approach constant values. Initially, there are nonlinearities

which could be accounted for by the times required for the decomposition species

to attain equilibrium with the walls of the cell. The fact that the production

curves eventually approach linearity is consistent with an assumption that

7
Table 1 . Reaction Scheme

IN DISCHARGE:

e + SFg - Sp0* + e

e + Sp0 - Sp0 + P + e

e + Sp0 SPi^ + 2P + e

e + Sp0 SP3 + 3P + e

e + Sp0 SP2 + 4P + e

e + Sp0 > SP5 + P + 2 e

e + Sp0 -* SP5- + P

IN GAS:

SP4 <- H2O -> SOP2 + 2 HP (fast)

SP^ f O2 S02P2 F2 ?

SFe* + H2O > SOP4 HK 2 HP ?

SOP2 + H2O SO2 + 2 HP (slow)

S0P4 + H2O SO2P2 + 2 HP (slow)

SP2 f O2 SO2P2 ?

SP0* + 0 ^ SOP4 + f^2 ?

8
Figure 3. Measured production of SOF 2 , SO 2 F 2 , and H 2 O in a continuous positive,

point-plane corona discharge with a current of 8 uA in SFs at 300

kPa. (The concentrations for H 2 O are only approximate).

9
Figure 4. Measured production of SOF 2 , SO 2 F 2 , and H 2 O in a continuous positive,

point-plane corona discharge with a current of 16 uA in SFc at

300 kPa. (The concentrations for H 2 O are only approximate).

10
reactions involving the primary dissociation products of SFg predominate in the

formation of oxyf luorides.

The presence of O2 in the gas has been detected, and our measurements

indicate a slow build-up of this contaminant during discharge activity. Because

of problems associated with air contamination we have not attempted a quantitative analysis for this species. Presumably oxygen can be released from

the stainless steel electrodes as well as being formed in the discharge. It is

worth noting that the increase in production rates for SOF2 and SO2F2 in going

from negative to positive corona is consistent with the observation (see our previous report) that the point electrode is heated and deteriorated more under

positive polarity thus implying a higher oxygen release.

As was noted above, thin-film aluminum oxide hygrometer probes were used to

calibrate the GC/MS system for quantitative measurements of water vapor content

in SFg. Commercially available probes were used which had previously been

calibrated under equilibrium conditions. In order to use these probes for

calibration, it is essential to insure that an equilibrium is established

between the probe and the surrounding gas. In a static gas not subjected to

turbulence or circulation, the time necessary to achieve equilibrium might be

quite long.

During the past quarter, tests were performed to evaluate the response

times of the hygrometer probes to variations in water vapor content, i.e., the

times necessary for the system plus probe to reach equilibrium after SF0 gas

with a known water vapor content was introduced. These measurements were performed by making a direct comparison of probe response to GC/MS response.

For this purpose the response of the GC/MS may be considered instantaneous.

Data from one such comparison is shown in figure 5. In this test

relatively dry SFg with less than 50 ppm H2O was introduced into a vessel at an absolute pressure of 200 kPa, and after about an hour simultaneous readings were made from the hygrometer and the GC/MS. The GC/MS in this case was operated in

the single-ion monitoring mode to look at the predominant ions from electron

bombardment of H2O, namely H20''' (ion 18) and O'*' (ion 16). Its calibration

was established before the beginning of the test. It is seen from figure 5 that

the concentration of water vapor in the cell as determined by the GC/MS

gradually increased to an equilibrium level of about 200 ppm in a time of

roughly 16 h. During this time the probe reading was found to be too low, and

only after about 22 h did it begin to give a true reading of water vapor

content.

This test indicates that the hygrometer probe has a long response time for

low levels of moisture (below 300 ppm). This probe, therefore, would not be

suitable as an accurate monitor of trace moisture content in situations where

fairly rapid variations are expected such as might happen during electrical

discharge activity. Of course, there are other reasons for excluding such

probes in this case, especially for gases like SF5, in which, as shown above,

highly reactive and polar gaseous decomposition species like HF are generated.

These species can be absorbed by the probe and attack the probe material, thus

adversely affecting its performance. Care was taken so that the probes used to

calibrate the GC/MS for the measurements described here were never exposed to

gas which had been decomposed in electrical discharges.

11
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It should finally be mentioned that the thin-film aluminum oxide probes

which we tested tend to exhibit an initial fast response to sudden increases in

water vapor content. This type of behavior has previously been noted by

others [5]. An example of such behavior is shown in figure 6 which corresponds

to a test in which 4 yl of liquid H 2 O were injected into a 4 1 vessel under vacuum followed by the introduction of SFg at a pressure of 200 kPa. The

initial response of the probe indicated a moisture level which was considerably

higher than the maximum predicted on the basis of the quantity of liquid

introduced. In this case, the system (probe plus vessel) seemed to require more

than 5 h to reach equilibrium. Results that we have obtained thus far suggest

that the response times of the probes increase with decreasing water vapor

concentration. Further tests will be required to verify this trend.

During the next quarter, at least one manuscript, describing our results on production of oxyfluorides in SFg corona discharges, will be prepared and

submitted for publication. The first draft of a complete bibliography on electrical breakdown data in gases will be compiled and submitted for review to

members of the IEEE Technical Committee on Gaseous Dielectrics. It is

anticipated that this bibliography will also be prepared for publication in the

form of a special NBS report.

More measurements will be performed to evaluate the response time and

accuracy of the thin-film aluminum oxide hygrometer method for measuring water

vapor content in SF 0

. Measurements on the production rates of oxyfluorides in

SF 0 and SF 0 /H 2 O mixtures should be completed and, in this connection, a new method for calibrating the GC/MS for quantitative determination of S0Fi+

concentrations in SF 0 will be examined. It is also anticipated that preliminary

measurements can be performed to identify the predominant stable gaseous

decomposition species generated by corona discharges in SF 0 /N 2 mixtures.

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. 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 prebreakdown and breakdown processes in

specified insulating systems, and to demonstrate the applicability of the

developed instrumentation and the procedures in the development and design of

future systems.

The results reported last quarter showed that the field enhancement due to

space charge in transformer oil increased with increasing field and increasing

temperature. The maximum enhancement was on the order of 30% over the

geometrical field Ey = V/d, where V is the applied voltage to parallel plates

with plate-separation d. The space charge is largest next to the anode having a magnitude of order -30 nC/cm^ — heteropolar space charge. The data suggests

that the space charge was not dependent upon the presence of water or conducting

carbon particles, but was dependent upon the chemical nature of the oil.

13
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14
The Conference on Interfacial Phenomena in Practical Insulating Systems was held this quarter (September 19-20, 1983) at NBS. Approximately 40 people

participated in the conference with representatives from government, university,

and industry coming from several countries. In addition to organizing the

conference, the NBS contribution consisted of two papers: "Interfacial

Phenomena — An Overview" by R. E. Hebner, and "The Measurement of the Electric

Field in the Vicinity of an Oi 1 -Pressboard Interface Parallel to the Field" by

E. F. Kelley and R. E. Hebner. Several conclusions were drawn at the

conference. One was that there is not one simple phenomena which can explain

the interfacial breakdown problems for all composite insulation systems.

Another was that it is not reasonable to perform terminal measurements upon a system without accompanying optical observations. Almost all the papers

presented optical observations to clarify the processes under study.

Three important results obtained this quarter are:

1. A correlation is observed between the lowering of the breakdown voltage

of transformer oil and the space charge induced field enhancement.

2. Different oils have different space-charge characteristics. Two oils

can exhibit substantially different field enhancements under the same conditions.

3. If a pressboard interface is placed between and in contact with

parallel plates so that its surface is perpendicular to the plates, no surface charge is detected on the oil -pressboard interface which

modifies the electric field in the vicinity of the interface by more

than 5%.

In all cases presented, the transformer-oil samples are prepared so that a minimum of water and oxygen remain in the oil. The procedure is to bubble dry

nitrogen through the oil for a minimum of 48 hours. A sintered glass filter is

used to produce fine bubbles which circulate through the oil. The pressboard is

cut to size and then baked at 125 “C in a vacuum oven. After a period of not

less than 48 hours, degassed, dehydrated, deoxygenated oil is added under vacuum

so that the oil covers the pressboard. The oven is then allowed to cool, and

the pressboard remains under vacuum until used in the experiment. In all cases,

the electrodes are parallel plates with separation d which is 1 cm or less. At one point, two different brands of commercially available transformer oil are compared. These will be referred to as type 1 or type 2 oil.

The average breakdown field (Eb = Vs/d) of type 1 oil as a function of

temperature is presented in figure 7. Included on this plot is the maximum

relative field enhancement due to space charge as a function of temperature.

Recall that the fields reported are measured relative to what they would be

without space charge in the oil ~ the geometric, unperturbed field Ey = V/d.

The maximum field is produced at the anode due to heteropolar space charge with

the application of dc fields. The maximum relative field enhancement is then

Eanode/^us the field enhancement is measured at fields which are approximately 90% of the breakdown field. These results are preliminary and it

is expected that further measurements will provide smaller error bars and

increased accuracy in the field measurements. A trend is seen in these data.

As the temperature increases, the field enhancement increases and the breakdown

field decreases. Whether or not the lowering of the breakdown field is due to

15
TEMPERATURE (®C)

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Figure 7. The average breakdown field and maximum field enhancement vs temperature. The average breakdown field E3

is defined as the

breakdown voltage divided by the parallel plate separation. Under

the application of dc voltage the maximum field enhancement is found

at the anode due to heteropolar space charge accumulation in its

vicinity. The maximum field enhancement uncertainty is estimated to

be ±3%.

16
the field enhancement is not established by these data. Only by removing the

space charge from the oil can the effects of the enhancement be distinguished

from the effects of temperature by itself. These additional measurements will

be attempted in the future.

Figure 8 shows the temporal development of the electric field in the two

types of oil studied. The data are taken at 120°C and each polarity of dc

voltage is applied. Type 1 oil initially showed less than a 10% field

enhancement due to space charge, but after 18 hours field enhancement of about

25% is observed. In the type 2 oil, the field enhancement remains 10% or less even after four days. Although the influence of the field enhancement upon the

breakdown voltage remains to be more firmly established, if a strong correlation

is found (as expected) then the measurement of the field enhancement may be an important parameter in future specifications of liquids used in high-voltage

apparatus.

To study interfacial phenomena, a pressboard interface was held between

parallel plates and in contact with each plate so that its surface is parallel

to the geometrical field direction (see inset of figure 9). The electro-optical

detector monitors the electric field between the plates in two places: near the

surface of the interface and about one gap length away from the surface of the

interface. Figure 9 shows the comparison of the relative electric field at both

positions for positive applied voltages. The case for negative voltage presents

a similar picture with the enhancement following the anode for dc voltages.

Note that for a large range of dc voltages, the field enhancement near the

interface is the same as the field enhancement away from the interface to within

the ±3% precision of the measurement. This measurement shows that the

interfacial surface confines insufficient surface charge to produce macroscopic

changes in the electric field distribution in the liquid. That is, field

enhancement arising from surface charges on the pressboard-oil interface

parallel to the field cannot be invoked to explain the lowering of the breakdown

voltage of such a system. This statement assumes that the breakdown voltage is

directly related to the field found at the electrode surfaces. It may,

therefore, be wise to consider microscopic conditions and contaminants as offering a more probable cause for failure of interfaces parallel to the field

in pressboard-oi 1 systems. The case for the surface of the interface

perpendicular to the field will be investigated at a later date.

There is a confounding factor in the electro-optical detection system which

must be minimized before field measurements can be completed for the

pressboard-oil system with the interface perpendicular to the field direction.

Measurements must be made over an extended region of the inter-electrode area. The difficulty is that the windows presently used are birefringent. Normally

for most Kerr-effect measurements, the small birefringence would be ignorable.

But since the accuracy of these measurements depends upon low-level, lightintensity measurements, a nonuniform birefringence makes measurements very

difficult. Thus, one of the important changes needed is to obtain better

windows for the tank and cell. Then measurements of interfacial fields can be

made with an estimated sensitivity of 5 kV/cm. Provided such windows can be

obtained, the case with the pressboard perpendicular to the field will be

investigated.

Measurements of the effects of particles on streamer propagation will be

completed next quarter with better documentation of the particulate content and

range of particle sizes. The results will be compiled in a monograph.

17
POSITION, z

Figure 8. The relative field as a function of position at different times for

the two types of oil used. The applied voltage is dc, and is 55 kV

for type 2 oil, 50 kV for type 1 oil; the gap spacing is 0.635 cm. Type 2 oil shows essentially no change in time, however, type 1 oil

shows great changes in a relatively short time. All data are taken

at 120‘’C. The solid lines show the field when the high voltage

electrode is positive and the dashed lines show the field when the

high voltage electrode is negative.

18
TYPE 1 OIL, T=125 »C

Figure 9. Relative electric field between parallel plates near and away from a pressboard-oi 1 interface parallel to the field. The inset shows the

view from the detector looking into the light beam. The light

travels parallel to the long plates between which a long pressboard

interface is placed. The detector sees the edge of the system and

monitors the electric field in the two regions specified. The

detection system is only sensitive to extended perturbations of the

electric field. Small changes in the local electric field around a void or a particle would not be noticed. The small fluctuations in

the field profiles arise from the localized nonuniformities in the

light beam as well as "waves" of small thermal gradients in the

heated oil tank enclosing the cell. These measurements are averages

made over 10 s to 20 s intervals. Dashed lines denote the field away

from the interface while the solid lines denote the field near the

interface.

19
Further measurements of the breakdown voltage and field enhancement near breakdown as functions of temperature will be attempted. Efforts will be made

to remove the space-charge from the oil in order to separate space-charge

effects (field enhancement) from purely temperature effects.

The temperature dependence of the breakdown voltage with and without a paper interface held parallel to the field will be completed. Such data will

help to isolate the controlling factors in such interfacial breakdown.

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

5. REFERENCES

1.

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

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

Dielectrics, Ed. by L. 6 . 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 SFg and 62 ," J. Appl, Phys., Vol. 54, pp. 3074-3079,

(1983).

3. R. J. Van Brunt, "Effects of H 2 O on the Behavior of SFg Corona," Proc. of

the 7th Int'l Conf. on Gas Discharges and Their Applications, Peter

Peregrinus (1982), pp. 255-258.

4. I. Sauers, H. W. Ellis, L. C. Frees, and L. G. Christophorou, "Studies of

Spark Decomposition Products of SFg and SFg/Perf luorocarbon Mixtures,"

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.

5. 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).

20
\

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

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

SHEET (See instructions) NBSIR 84-2818 February 1984

4, TITLE AND SUBTITLE

DEVELOPMENT OF POWER SYSTEM MEASUREMENTS — QUARTERLY REPORT

JULY 1, 1983 to SEPTEMBER 30, 1983

5. AUTHOR(S)

R. E. Hebner, Editor

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) 7. Contract/Grant No.

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8.

Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fStreet. City, State. ZIP)

Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, DC 20585

10.

SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Softv\rare 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 proqress on three 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 July 1, 1983 to

September 30, 1983. The report emphasizes the measurement of the

60-Hz electric and magnetic field in biological exposure facilities,

the measurement of water vapor, the production rates of oxyfluorides

in SFg corona discharges, and in the measurement of space charge in

transformer oil.

12.

KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) cables; composite insulation; electric fields; high voltage; incipient fault;

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

13. AVAILABILITY

^ I Uni imited

I I For Official Distribution. Do Not Release to NTIS

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

^31 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

27

15. Price

$ 8.50

USCOMM-DC 6043-P80
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