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NBSIR 84-2861

Development of Power System

Measurements -- Quarterly Report

October 1, 1983 to

December 31, 1983

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

May 1 984

Prepared for: Department of Energy

Division of Electric Energy Systems

* ‘300 independence Avenue, SW

-Qf— sshington, DC 20585

100

. U56

84-2861

1334

NBSIR 84-2861

DEVELOPMENT OF POWER SYSTEM

MEASUREMENTS - QUARTERLY REPORT

OCTOBER 1, 1983 TO

R. E. Hebner, Editor

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

May 1984

Prepared for: Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, DC 20585

DECEMBER 31, 1983 £

/

-

O

z Q*

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

C)

hi

Foreword

This report summarizes the progress of three technical investigations

during the first quarter of FY 84. 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 AND MAGNETIC FIELD MEASUREMENTS

Subtasks Nos. 01 and 02 1

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. 03 6

4. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

Subtask No. 04 12

5. REFERENCES 19
LIST OF FIGURES

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Page

Normalized electric field at the surfaces of and midway

between semi -inf ini te parallel plates 3

Reduction in field strength at surface of grounded

plate due to presence of nearby ground plane 4

Perturbation of electric field strength at probe

surface due to interaction between probe and surface

charge distribution on energized plate 4

Relative concentrations of S0F 4 versus net

charge transported across the gap for a negative

corona discharge in SF 6 at a pressure of 300 kPa for

a current of 16 uA. Shown are the concentrations of

S0F 4 measured at m/e = 86 relative to both the

concentration of SF 6 at m/e = 19 and S0F 2 at m/e = 86. The vertical arrows indicate times when

the discharge was turned off for more than 16 hours 8

Relative concentrations of S0F 4 versus net charge

transported across the gap for a negative corona discharge in SF 6 at a pressure of 300 kPa for a current of 21 uA. Shown are the concentrations of

S0F 4 measured at m/e = 86 relative to both the

concentration of SF 6 at m/e = 19 and S0F 2 at m/e = 86. The vertical arrows indicate times when the

discharge was turned off for more than 16 hours 9

Concentration of water vapor in 300 kPa SF 6 versus

time after sample preparation for two different samples

indicated by (§,a) and (o,a) respectively. Shown is a comparison between the GC/MS measurement and the

hygrometer probe reading 11

Split electrode system employed to clamp paper in a uniform field. Only the split electrodes are shown.

The support structure and controlling mechanism are not

shown. The paper is supplied in a long strip which has

been notched for indexing. A rectangular cutout is

provided at set lengths to permit determining the

breakdown voltage without an interface present. The

strip to the right of the electrode shows the results of

a shockwave tearing of the interfacial material versus

the appearance of a breakdown which occurred along the

interface. See the text for discussion 1-

Arrangement for separating the electrode system to

allow the introduction of a new interface 16

v
LIST OF TABLES

Page

Table 1. Normalized electric field strength midway between plates

and at plate surfaces 3

Table 2. Summary of preliminary results 17

vi
DEVELOPMENT OF POWER SYSTEM MEASUREMENTS — QUARTERLY REPORT

October 1, 1983 to December 31, 1983

R. E. Hebner, Editor

This report documents the progress of three technical

investigations sponsored by the Department of Energy and performed by

or under a grant from the Electrosystems Division, the National Bureau

of Standards. The work described covers the period October 1, 1983 to

December 31, 1983. This report emphasizes the errors associated with

measurements of electric and magnetic fields, the properties of corona

in compressed SF 6 gas, and the measurement of interfacial phenomena in

transformer oil.

Key words: electric fields; gaseous insulation; interfaces; liquid

insulation; magnetic fields; partial discharges; SF 6 ; solid

insulation; 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 concentrated on the measurement of electric and magnetic fields, the

measurement of interfacial phenomena, and of partial discharge phenomena. The

technical progress made during the quarter October 1, 1983 to December 31, 1983

is summarized in this report.

2. ELECTRIC AND MAGNETIC 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, 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 being funded by the State of New York.

During the current reporting period, measurements were made to determine

the dimensions of a parallel plate system which would be suitable for

calibrations of flat probes frequently used to measure the 60-Hz electric field

strength in biological exposure systems. Additional measurements were made to

test the adequacy of the NBS parallel plate system as a cal ibrational apparatus

for aspirator-type ion counters operating in the ground plane. This report

summarizes the calibration of flat probes for 60-Hz for electric field

measurement. Ion counter calibration will be described in a subsequent report.

While information is available for calibration of free-body type electric

field strength meters used for measurements above the ground plane [1], no guidance currently exists for flat (square or circular) probes wnich are used

for measurements on the surface of the ground plane of a biological exposure

system.

1
Regions of nearly uniform electric field strength of known magnitude can be

produced for calibration purposes with a parallel plate system provided that the

spacing of the plates, relative to the plate dimensions, is sufficiently small.

The uniform field value E0

is given by V/t where V is the applied potential

difference between the plates and t is the plate spacing. The magnitudes of the

normalized field E/E 0 at the plate surfaces and in the midplane of

semi -inf inite parallel plates can be theoretically determined [2], and are plotted as a function of normalized distance x/t from the plate edge in

figure 1; numerical values are given in table 1.

The numerical values in table 1 show that the departure from field

uniformity due to fringing field effects is 0.1 percent at a distance of one plate spacing from the edge. For finite square plates, the fringing field

effects from four edges can be estimated by superposition when the effect from

one edge is less than 0.2 percent. These results are valid in the absence of

perturbations due to nearby grounded objects, interaction between the probe

located on the bottom plate and the surface charge distribution on the top

plate, and the influence of the high voltage leads to the plates. These

possible sources of perturbation of the field were examined for the case of one plate referenced to ground potential, and this report summarizes the result of

that examination. It is noted that field perturbations due to nearby ground

planes and high voltage leads may be reduced when both plates are energized with

a center-tapped transformer.

Influence of Nearby Ground Planes . From figure 1, it can be seen that the

electric field strength at the surfaces of the parallel plates increases as the

distance to the edge decreases. However, when one plate is grounded, as would

be the case during calibration of a flat probe, nearby ground planes such as the

floor, ceiling or walls will perturb the field between the plates. In general,

the presence of ground planes leads to the reduction in field strength at the

grounded plate and an increase at the surface of the energized plate.

Corresponding changes occur in the field strength at intermediate points between

the plates. The effect of nearby ground planes on the field strength at the

surface of a grounded plate of a parallel plate system 1 m x 1 m x 0.18 m spacing was examined with a flat probe with dimensions 8.5 cm x 8.5 cm x 0.16 cm [3]. A guard band 0.5 cm wide reduced the field-sensing area to

7.5 cm x 7.5 cm. The parallel plates were made from 0.95 cm thick aluminum

plate with surfaces that had been milled flat in order to better define the

parallel plate spacing. The top plate was energized with an autotransformertransformer combination. A line conditioner was also used to stabilize the line

voltage.

Measurements of the 60-Hz electric field strength as a function of

distance d between the field probe edge and one of the edges of the bottom plate

were made in the presence of a vertical ground plane two plate spacings (0.36 m)

away from the edge of the parallel plates. The inset in figure 2 shows the

geometry of the parallel plates, the probe, and the vertical ground plane. The

electric field strength was near 3.33 kV/m during these measurements. Figure 2 also shows the change in field strength, in percent, as the distance (d)

normalized by the plate spacing (t) is varied. For example, the decrease in

field strength, which is largely [4] due to the vertical ground plane near one edge of the parallel plates, is near 0.64% when d is equal to one plate

spacing. In the absence of nearby perturbing ground planes, the field strength

would not decrease (see fig. 1).

2
Table 1. Normalized electric field strength midway

between plates and at plate surfaces

Midway Between Plates Plate Surface

x/t m m o x/t E/E0 0.0698 0.837 0.0185 2.449

0.1621 0.894 0.0829 1.414

0.2965 0.949 0.1230 1.265

0.4177 0.975 0.1624 1.183

0.6821 0.995 0.2431 1.095

0.7934 0.997 0.4376 1.025

1.0000 . 0.999 0.6861 1.005

0.7954 1.002

1.0000 1.001

Plate edges

\

(V) Semi-infinite plates

E0 = V/t

0 x/t

NORMALIZED DISTANCE

FROM EDGE (x/t)

igure 1. Normalized electric field at the surfaces of and midway :etween

semi-infinite oarallel plates.

3
NORMALIZED DISTANCE

TO PLATE EDGE (d/t)

Figure 2. Reduction in field strength at surface of grounded plate due to

presence of nearby ground plane.

PARALLEL PLATE SPACING

* PROBE SIDE DIMENSION

Figure 3. Perturbation of electric field strength at probe surface due to

interaction between probe and surface charge distribution on energized plate.

4
The edge effect on field strength due to four edges of a parallel plate

system in which the distance between the probe and each plate edge is the same

can be estimated by the principle of superposition (the presence of vertical

ground planes two plate spacings away from each side of the parallel plates is

assumed). The measured reductions in field strength due to a single edge and

vertical ground plane in figure 2 are multiplied by four and these results are also shown in figure 2 with the curve designated with x4.

The results assuming superposition of edge effects were checked

experimentally at one point with a smaller parallel plate system

46 cm x 46 cm x 18 cm spacing and constructed with the same type of milled

aluminum as used for the larger parallel plates. Each edge of the probe

(8.5 cm x 8.5 cm), placed at the center of the bottom grounded plate, was approximately one plate spacing away from the nearest edge of the bottom plate.

A vertical ground plane was located two plate spacings away from each edge of

the parallel plates. The measured field strength was found to be 2.6% less than

the uniform field value determined with the 1 m x 1 m x 0.18 m parallel plate

system. This reduction is plotted in figure 2 and is in very good agreement

with the predicted value assuming superposition of edge effects. The size of

the data point reflects the uncertainty associated with the measured value. It

is noted that the 2.6% field strength reduction was found after correction for a small perturbation due to the influence of the high voltage lead described

below.

In the absence of other perturbations, the superposition results in

figure 2 indicate that a calibration field that is within 0.25% of the uniform

field value (V/t) can be produced if the distance between the flat probe (edges)

and the edges of the bottom plate is twice the paral lel-pl ate spacing and if

there are no ground planes closer than two plate spacings away. This result

also assumes that the probe has a narrow guard band (i.e., width is — 6% of

side dimension). A larger guard band or ground planes further than two plate

spacings should reduce the influence of the parallel plate edge effects.

Finally, it is noted that the choice was made to use two plate spacings as the

distance between the parallel plates and nearby ground planes (and not a larger

distance) because it appears to be a condition that can be readily met in most

laboratories.

Interaction of Probe With Surface Charge Distribution on Top Plate . Because of the thickness of the flat probes, the electric field in the vicinity

of the probe will be perturbed. If the upper (energized) plate of a parallel

plate system which is being used for calibration of the probe is sufficiently

close, the surface charge distribution on the upper plate will be rearranged by

this perturbation. The resultant effect is an increase in field strength at the

surface of the probe [5].

Measurements of the same nearly uniform electric field V/t were performed

with a 8.5 cm x 8.5 cm x 0.3 cm thick probe in the 1 m x 1 m parallel plate

system with different plate spacing to determine when the interaction between

the probe and the upper plate became negligible. Figure 3 shows the field

strength measurements normalized by the field strength observed when the spac'nn

was 0.18 m (taken to be the uniform value E 0 ) as a function of plate spacing

which is normalized by the side dimension of the probe (8.5 cm). The error bars

shown in figure 3 reflect uncertainties in the measurement of the voltage

(<0.05%) and the parallel plate spacing (<0.3% for 5 cm spacing and less for

larger spacing). The data in figure 3 show that the interaction between the top

5
plate and a probe with a thickness that is 3.5% of its side dimension leads to a field enhancement at the probe surface that is less than 0.25% when the parallel

plate spacing is one and one-half times the probe's side dimension. Similar

measurements with a probe of half the thickness led to enhanced fields at the

probe surface which were about half as large as those shown in figure 3.

Influence of High Voltage Lead on Field Between Plates . The possibility of

the calibration field being affected by the presence of the high voltage lead to

the upper plate of the parallel plate system was examined for a number of

parallel plate spacings. The procedure used to determine an effect was

to compare field strength measurements obtained with a flat probe

8.5 cm x 8.5 cm x 0.16 cm thick when one high voltage lead was connected to the

upper plate with measurements obtained with two leads connected. The

connections to the upper plate were made at adjacent corners and the leads were

routed approximately horizontally to the transformer, maintaining approximately

equal distance between the probe (at the center of the bottom plate) and leads.

If there was no difference between measurements, the influence from the leads was considered negligible.

Field strength measurements obtained when the distance between the probe

edges and plate edges was 1.25 times the plate spacing indicated that the high

voltage lead contribution to the field, at the location of the probe, was about

0.1%. No contribution to the field could be detected (within 0.04%) when the

distance between the probe and plate edges was twice the plate spacing.

In summary, a calibration electric field which is within 0.5% of the

uniform field value V/t can be established for a ground referenced flat probe

using a parallel plate system, provided that the spacing of the plates is at

least 1.5 times the side dimension of the probe and the distance between the

probe (edges) and plate edges is no less than two plate spacings. These results

are valid for a square probe with 6% guard band width and a thickness that is

3.5% of the probe's side dimension. All nearby ground planes should be at least

two plate spacings away from the parallel plates.

During the next quarter, the latest revision of the IEC draft standard will

be completed and distributed to members of IEC TC42 Working Group 6. A

technical note describing electrical parameters which must be controlled during

biological experiments will undergo internal review. At least one site visit

will be made to measure 60-Hz electric and magnetic fields in a biological

exposure system. Additional measurements will be performed with an aspirator-type ion counter operating in the ground plane of the parallel plate

system.

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 scope of the project encompasses the following

6
investigations: 1) measurement and calculation of electric discharge inception

in compressed electronegative gases; 2) measurement, calculation, and

compilation of fundamental data on electron transport and electrical breakdown

in gases; 3) measurement of absolute electric discharge-induced decomposition

rates in gaseous dielectrics; and 4) examination of the influence of

contaminants like water vapor on the performance of compressed gas-insulated

systems. Emphasis is given in these investigations to the development and

evaluation of new measurement techniques.

During the past reporting period emphasis was given to activities 1, 3,

and 4. Some of the progress made in these activities is highlighted in this

report.

As a part of activity 3, about 500 archival publications dating back to

1950 were examined and evaluated according to their appropri ateness for

inclusion in the bibliography on electrical breakdown data in qases. Some

assistance in this effort was provided by members of the IEEE Insulation Society

Technical Committee S -32- 11 on Gaseous Dielectrics. A preliminary version of

the bibliography was presented and discussed at the last meeting of this

committee which took place during the 1983 Conference on Electrical Insulation

and Dielectric Phenomena. At present about 470 papers have been determined to

be suitable for inclusion, i.e., they contain theoretical or experimental data

on electrical breakdown in gases considered to be relevant to electrical

insulation technology. These papers are now being listed by computer in a cross referencing format that will facilitate the location of particular kinds of data

which are available for the different types of gases or gas mixtures that have

been studied. The computerized form of the bibliography has been designed so that it can be easily updated and periodically reprinted.

Concerning activity 1 above, a draft of a manuscript entitled "Production

Rates for Discharge Generated S0F 2 , S0 2

F 2 and S0 2 in SF 6 and SF 6 /H 2 0 Mixtures"

by R. J. Van 8runt, T. C. Lazo, and W. E. Anderson has been prepared for

submission to the Fourth International Symposium on Gaseous Dielectrics,

Knoxville, April -May, 1984. This paper presents the results of our measurements

of the absolute production rates for S0F 2 , S0 2 F 2 , S0 2 , and H

2 0 generated by

point-plane corona discharges under a wide ranae of conditions in SF 6 containing

trace levels of water vapor.

Related to this work, our previous measurements of decomposition species

production rates for negative corona discharges in SFg were extended to a higher

pressure of 300 kPa (~3 atm) for several different discharge power levels.

The results for S0F 2 , S0 2 F 2 , and H 2 0 production were consistent with the trends

that we previously reported (see previous quarterly report) for other gas

pressures and discharge conditions.

The gas chromatograph-mass spectrometer data for degradation of SF 6 in

negative point-plane discharges at several qas pressures were analyzed to obtain

information on the relative production of thionyl tetraf 1 uori de (S0F 4

1. T he

ratios of the peak heights of the S0F 4 and S0F 2 features in the sinqle-ion

chromatograms for a mass-to-charge ratio (m/e) of 86, and the ratios of S0F a at m/e = 86 to SFg at m/e = 19 are shown in figures 4 and 5. These ratios inc’-* • the concentrations of S0F 4 relative to S0F 2 and SFg respectively as a fund'

of net charge transported across the qap. The slopes of the [S0F 4 WCSFsJis

curves are directly proportional to the production rates for S0F 4 n mol es-per-coulomb . The vertical arrows indicate times when the discharae was

7
Figure 4. Relative concentrations of S0F 4 versus net charge transported

across the gap for a negative corona discharge in SF 5 at a pressure

of 300 kPa for a current of 16 uA. Shown are the concentrations of

S0F 4 measured at m/e = 36 relative to both the concentration of SF s at m/e = 19 and S0F 2 at m/e = 86. The vertical arrows indicate

times when the discharge was turned off for more than 16 hours.

8
RELATIVE

SOF4

CONCENTRATION

Figure 5. Relative concentrations of SOF 4. versus net charge transported

across the gap for a negative corona discharge in SF- at a oressure

of 300 kPa for a current of 21 uA. Shown are the concentrations of

SOF^ measured at m/e = 36 relative to both the concentration of Zr at m/e = 19 and S0F 2 at m/e = 36. The vertical arrows indicate

times when the discharge was turned off for more than 16 hour:.

9
extinguished for periods of 16 h or more. The concentrations of SOF are seen

to change very little during these times whereas the H 2 0 concentrations were observed to increase (see our previous quarterly report). This observation

indicates that the gas phase hydrolysis of S0F 4 is very slow, and thus it can be

concluded that the reaction

H2O + SOF4 + SO2F2 + HF

is not the major source of S0 2 F 2 production as has previously been

suggested [6-8].

Related to activity 4, a new method of preparing samples of pressurized SF 6 containing known trace quantities of water vapor in a discharge was tested. By

this method 1 - 10 ji£ of liquid H 20 were injected into the cell under vacuum followed by introduction of SF 6 to the desired pressure. Changes in the

absolute water vapor content immediately after this preparation were monitored

with the GC/MS which was initially calibrated by direct comparison with the

readings from a thin-film aluminum oxide hygrometer probe. The results of the

measurements as shown in figure 6 indicate that the water vapor content reached

equilibrium in the cell within 20-30 minutes following the initial preparation,

and thereafter varied by less than ±10% for the next 2 to 6 hours. The

hygrometer probe however gave erroneously high readings during this time (see

fig. 6), and appeared to require more than 8 hours to reach equilibrium with the

gas and thereby attain agreement with the GC/MS measurements. The hygrometer

probe is thus not capable of providing reliable measurements of water vapor

content during short term sample preparation. The method investigated here

appears to be much more efficient, reliable, and versatile than the hot wire

technique previously used [9] to prepare H 20 contaminated SF 6 samples. It is

expected that this method will be employed in future investigations of the

effect of trace levels of H 20 on electron avalanche and streamer development in

SF 6 discharges.

Planned activities for the next quarter include the following:

1) A manuscript describing our results on production of oxyfluorides in SF 6 corona discharges will be submitted to the Proceedings of the Fourth

International Symposium on Gaseous Dielectrics;

2) A bibliography on electrical breakdown data in insulating gases will be

completed and prepared for publication as an NBS report;

3) The effects of controlled low levels of oxygen contamination on oxyfluoride production rates in SF 6 discharges will be measured at selected

gas pressures; and

4) Standard samples of S0F 4 gas will be prepared and the gas

chromatograph-mass spectrometer system will be calibrated for quantitative

measurements on this species. All previously acquired data on SOF4

production in SF 6 discharges will also be reanalyzed to obtain absolute

production rates for this species.

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

10
140 -tb

E

a

a

z

o

I-

<

CE

100

ob

- o

S 60

o

z

o

o

0

<N 1

20

H2O in 300 kPa SF0

°V

GC/MS

Hygrometer probe reading

o o o o o o

5 A

5 a a ^

i 20 40 60 80 100 120

TIME (min)

-*>

1310

Figure 6. Concentration of water vapor in 300 kPa SF 5 versus time after

sample preparation for two different samDles indicated by and

(*,a) respecti vely. Shown is a comoarison between the GC

/

MS

measurement and the hygrometer probe reading.

11
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 processes in

specific insulating systems, and to demonstrate the applicability of the

developed instrumentation and the procedures in the development and design of

future systems.

The underlying goal of this research is to understand the factors which

contribute to the failure of liquid-solid high-voltage systems -- in particular

transformer oil and paper or pressboard systems. There are two areas of

interest:

1. Fundamental properties of a liquid-solid system which include:

The electric field distribution between conductors and in

the vicinity of an interface, and

The prebreakdown processes in pure liquids and in the vicinity

of an interface.

2. The influence of contaminants, particles, voids, and chemical

additives upon the electrical strength of the liquid-solid system.

Since it is impossible to prepare a perfectly clean system, it is not really

possible to draw a hard dividing line between these two areas of interest, but

experiments can be designed to concentrate on specific areas of interest.

To support this investigation, two experimental systems have been

developed:

1. Optical Field Measurement (OFM) systems [10] to determine the

electric field in transformer-oi 1 systems with and without

interfaces, and with and without the addition of specific

contaminants.

2. Electrical Breakdown Visualization (EBV) system [11] to provide

high-speed photography of the time-evolution of the entire

breakdown process, both pre- and post-breakdown.

Previous quarterly reports have documented the effects of space charge in

transformer oil alone and with a pressboard interface parallel to the field

direction using the OFM system. The more significant results have been the

measurement of the temperature dependence of space charge in transformer oil and

the demonstration that macroscopic field distortions arising from surface

charging of the interface parallel to the field do not occur to within the

accuracies of the experiment. These results serve to clarify the role of space

charge and surface charge in the failure of practical insulation systems.

12
With the EBV system, liquids have been investigated and a general picture

of the electrical breakdown process has been obtained. Notable results have

been the documentation of multiple modes of streamer propagation and the

documentation of the effects of the addition of el ectron-trapping chemical

impurities on streamer structure. These results were obtained with liquids

using both the point-plane and quasi -uniform electrode geometries.

Concerning electrical breakdown in the vicinity of a paper interface

parallel to the electric field and between plane electrodes, it has been

established that electrical breakdown does not necessarily occur at the

interface in the oil-paper system, and the presence of the paper does not

necessarily lower the breakdown voltage. Such results were achieved using

carefully prepared paper samples and careful liquid-handling procedures, and the

data were taken at room temperature.

The present investigation is attempting to determine if the same phenomena

which occurred during the room temperature investigation are observed at operating temperatures of practical high-voltage apparatus. In order to carry

out this investigation, it was necessary to construct a cell which would permit

one to insert new interfacial material between the electrodes without opening or touching the cell. At room temperature it is a simple matter to open the cell

and insert a new interface, but at temperatures of 125°C and higher one cannot

handle the cell. The main effort this quarter was to construct and operate a cell which would permit the changing of an interface from outside the cell (and

outside the heated tank which contained the cell under silicone oil for

electrical insulation and to maintain a uniform temperature) . This report

describes the test cell and summarizes preliminary results obtained using it.

The cell body is made of polytetraf luoroethylene (PTFE) which provides

electrical insulation, is stable at temperatures above 150°C, and has sufficient

flexibility to withstand the shockwaves associated with repeated breakdowns.

Large windows permit visual observations of the cell's interior, the breakdown

location, and the condition of the interface. Because PTFE becomes soft at the

intended operating temperatures, the windows and other flat surfaces are sealed

with silicone rings with circular cross section rather than harder rings which

could dent the hot PTFE. The flanges which hold the windows and other flat PTFE

plates against the cell are secured with nylon screws. PTFE screws were tried

but would break after extended periods of time. Because the nylon screws are

not under much stress, their tendency to become brittle over long times at high

temperatures under oil should not be a problem.

PTFE pipe-to-tubing fittings are sealed with PTFE tape in the body of the

cell, and are used for electrical and mechanical access to the interior of the

cell. Metal fittings cannot be used because of the difference in the thermal

expansion between the metal and the PTFE. Leaks will invariably develop unless

the fittings are made of PTFE. The tubing end of the fittings are sealed with a silicone ferrule which provides sufficient flexibility to seal metal tubes and

shafts throughout the range of temperatures used, yet allow mechanical rotation

of a shaft without leaking. Mechanical coupling from outside the cell to the

exterior of the enclosing heating tank is provided by a nylon or acetal -pi ast 1 c chain-sprocket system with a metallic shaft with u-joints or metallic flexible

shaft.

The structure which holds the electrodes is a frame made from a mach’naD'*-,

glass-ceramic which has an expansion coefficient comparable to that of met: 1

13
(glass-ceramic: 1.2 x 10“ /°C stainless steel: 2 x 10“ 5 /°C). With an electrode

gap of about 2.8 mm, because of the respective dimensions of the component

materials, a change of 100°C produces less than a 0.3% change in the gap

spacing. The uniform field in the gap is produced by stainless steel rod ends

with rounded edges providing a cyl indr i cal ly symmetric plane-plane electrode

geometry with a diameter of 2.5 cm. The electrode system is split in half along

its axis of symmetry. The halves are separated to accept a new paper sample,

then clamped together under spring pressure during voltage application. See

figure 7 for details of the electrode-paper system.

The electrodes are separated using a cam which is controlled by a stainless-steel shaft from outside the cell; see figure 8. The cam moves a hinged plate, the cam-plate, to which are attached two spring-loaded

glass-ceramic pieces which attach to the electrode halves. These movable

electrode halves are aligned by two rods in each of their bases which allow

separation of the electrode halves but will not allow any misalignment of the

electrodes when they are clamped back together. When the cam is turned, the

cam-plate separates the electrodes sliding the halves along the alignment rods.

The stationary halves of the electrode system are attached to the glass-ceramic

frame.

The paper is supplied in long strips 1.3 cm wide wound on a spool within

the cell. At approximately one meter intervals, a rectangular hole is cut in

the paper strip so that breakdown voltages can be determined without an interface present. Every 3.8 cm a small notch is cut out of the side of the

paper strip to provide indexing marks so that one can be sure to advance the

paper far enough to provide a new interface between the electrodes. When the

electrodes are separated, rotation of the take-up spool advances the paper strip

providing a new interface. The take-up spool is controlled via a stainless-steel shaft from outside the cell. At each temperature, several

values of the breakdown voltage (Vg) will be determined without an interface,

then a minimum of twenty interfaces can be used to determine the breakdown

voltage with the interfacial paper present, and then at the next rectangular

cut-out several more values of Vg may be recorded without an interface

present. In this way, the data for as many as ten different temperatures may be

obtained before the cell has to be opened and a new spool of paper installed.

The paper strip was prepared with rectangular cut-outs and notches and then vacuum baked at 125°C for over 48 hours. As the oven cooled, degassed and

dehydrated transformer oil was added under vacuum completely submerging the

paper sample under the oil. The paper remained in the oil under vacuum until

loaded into the cell — a minimum of 48 hours later.

The cell system worked properly at room temperature, but at 125°C minor

problems were encountered whicn prevented continuous use of the paper-advancing

mechanism. Simple modifications are in progress which will correct the

deficiencies. The cell should be fully operational next quarter. Despite the

inconvenience, some data were obtained at elevated temperatures.

The most notable feature of these preliminary results is that the breakdown

does not necessarily occur at the interface even at high temperatures. The

applied voltage is 60-Hz ac, and the results are summarized in table 2. During

the experiments, liquid was circulated through the cell and then through a particle filter to remove particulate breakdown by-products.

14
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16

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Table 2. Summary of prel imi nary results

Temperature

(°C)

No. of

Samples or Breakdowns

(a)

V B(kV)

Condition/Result of Tests

23 11 96(4) Breakdown NOT at interface

23 5 78(14) Breakdown at interface

23 11 90(10) NO interface present

115 14 78(14) NO interface present

125 5 81(8) Breakdown NOT at interface

125 1 51 Breakdown at interface

(a) Average voltage with standard deviation in parentheses.

Aside from the fact that the interface would not necessarily break down,

there are several other observations to be made from these data. When an interface was present and the breakdown occurred at the interface, the

interfacial breakdown voltage was usually lower than when the breakdown occurred

away from the interface or when an interface was not present. When the

breakdown did not occur at the interface, the breakdown voltage with the

interface present was not significantly different from that with no interface

present. It must be stressed that these are preliminary data which demonstrate

the success of the cell design; more data are needed before firm conclusions can

be drawn.

Whenever the interface is involved in the breakdown process, that fact is

obvious from visual observations after the breakdown unless the interfacial

material is completely torn away by the post-breakdown shockwave — a rare occurrence. Often the interface is torn by the post breakdown shockwave;

however, such a tear is clearly distinguishable from a breakdown which touches

the interface. Where the breakdown channel -- the hi gh-temperature

post-breakdown plasma channel — touches the paper interface, the interface is

burned away. After the breakdown has occurred, the path of the breakdown

channel along the interfacial paper is visible as a narrow paperless path which

has relatively sharp edges and is quite well defined. Occasionally, when the

interface is involved in the breakdown, the breakdown does not confine itself

entirely to the interface. Either the prebreakdown streamer starts at the

interface and moves away from it, or it starts away from the interface and moves

to touch it. In either case, a complete breakdown channel of burned-away paper

is not seen to connect the plates. Even when the paper is only partially burned

away or if the breakdown channel touches the paper at any point, it is

considered to be a breakdown at the interface in table 2.

In view of the method of handling and preparing of both the oil and the

paper, the fact that the interface will not necessarily break down is not too

surprising. If the water were successfully removed from the paper and there were no voids in the paper because of using degassed oil, then low-field

17
initiation sites arising from such contaminants would not be present even at elevated temperatures. Field enhancement from surface charginq of the interface

is not expected in view of the field measurements made last quarter where the

field along an interface in hot transformer oil was nearly the same as the field

in the bulk liquid between parallel plates. Thus, unless the solid material

provides contaminants in the form of voids, conducting particles, or voids from

vaporization of water or other low-boiling-point liquid, then it is hypothesized

that the breakdown will not necessarily occur at the interface. Further effort

is needed to determine if this hypothesis is correct. The effort next quarter

will concentrate in the following areas:

1. Determination of the temperature dependence of the breakdown

voltage of transformer oil with and without a paper interface

parallel to the field direction. Repeat these measurements

for paper which has not been dehydrated but is merely surrounded

by degassed oil so that all voids will be removed from the paper.

It is anticipated that as the temperature approaches the boiling

point of water, the breakdown will tend to favor the interface more than when the paper is dehydrated.

2. Preparation of a monograph on the effects of particles on the

prebreakdown streamers in liquids.

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

18
5. REFERENCES

[1] IEEE Recommended Practices for Measurement of Electric and Magnetic Fields

from AC Power Lines, IEEE Std. 644-1979 (IEEE, Inc. New York, 1979).

[2] P. M. Morse and H. Feshbach, Methods of Theoretical Physics, Part II (McGraw-Hill, New York, 1953) p. 1245. ”TM

[3] For a description of flat field sensing probes and their use, see

W. T. Kaune, A Prototype System for Exposing Small Laboratory Animals to

60-Hz Vertical Fields: Electrical Measurements, in Biological Effects of

Extremely Low Frequency Electromagnetic Fields, (Proceedings of the

Eighteenth Annual Hanford Life Sciences Symposium at Richland, Washington,

1978), Editors R. D. Phillips, M. T. Gillis, W. T. Kaune, and

D. D. Mahlum, available from NTIS, Springfield, Virainia 22161 as C0NF-781016.

[4] The next nearest ground plane to the edge of the parallel plates was the

floor, a distance of 0.95 m away.

[5] This effect is discussed further in the report by F. R. Kotter and

M. Misakian, AC Transmission Line Field Measurements, NBS Report NBSIR

77-1311, Re-issued 1981, p. 49ff.

[6] R. J. Van Brunt and D. A. Leep, "Corona Induced Decomposition of SF ,"

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

Dielectrics, Ed. by L. G. Chri strophorou, Pergamon Press, New York (1982)

pp. 402-409.

[7] C. Boudene, J. Cluet, G. Keib, and G. Wind, "Identification and Study of

Some Properties of Compounds Resulting from the Decomposition of SF unde 1" the Electrical Arcing in Circuit Breakers," Revue Generale de

1

1 E 1 ect r i cite (RGE), pp. 45-78 (1974).

[8] W. Becher and J. Massonne, "Contributions to the Decomposition of Sulfur

Hexafluoride in Electric Arcs and Sparks," Elektrotech. 2., Vol. A91, pp.

605-610 (1970).

[9] R. J. Van Brunt, "Effects of H 2 0 on the Behavior of SF 6 Corona," Proc. of

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

Pereginus, London, pp. 255-258 (1982).

[10] E. F. Kelley and R. E. Hebner, "Electro-optic Measurement of the Electric

Field Distribution in Transformer Oil," IEEE Trans. Power Appar. Sys.,

vol. PAS-102, pp. 2092-2097, 1983.

[11] R. E. Hebner, E. F. Kelley, E. 0. Forster, G. J. FitzPatrick, "Observat ' on of Prebreakdown and Breakdown Phenomena in Liquid Hydrocarbons,

J. Electrostatics, vol. 12, pp. 265-283, 1983.

[12] E. F. Kelley and R. E. Hebner, "Electrical Br eakdown in Composite

Insulating Systems: Liquid-Solid Interface Parallel to the F'eld," Ii".

Trans, on Elec. Insul., vol. El -16 , pp. 297-303, 1981.

19
NBS-I 14A irev. 2>8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET ( See instructions)

PUBLICATION OR

REPORT NO.

NBSIR 84-2861

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

4. TITLE AND SUBTITLE

DEVELOPMENT OF POWER SYSTEM MEASUREMENTS — QUARTERLY REPORT

October 1, 1983 to December 31, 1983

5. AUTHOR(S)

R. E. Hebner, Supervisory Physicist

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Con tract/ 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 (Street. City. State, ZIP)

Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, S.W.

Washington, DC 20585

10.

SUPPLEMENTARY NOTES

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

11.

ABSTRACT A 200-word or less factual summary of most si gnifi cant in formation. If document includes a si gnificant

bibliography or literature survey, mention it here) This report documents the progress of three technical investigations sponsored by the

Department of Energy and performed by or under a grant from the Electrosystems

Division, the National Bureau of Standards. The work described covers the period

October 1, 1983 to December 31, 1983. This report emphasizes the errors associated

with measurements of electric and magnetic fields, the properties of corona in

compressed SFg gas, and the measurement of interfacial phenomena in transformer oil.

12. KEY WORDS ( Six to twelve entries ; aiphaceticai order; capitalize only proper names; and separate <ev words by semicolon s) electric fields; gaseous insulation; interfaces; liquid insulation; magnetic fields;

partial discharges; SF-; solid insulation; transformer oil

13. availability 14. NO. OF

X Unlimited

For Official Distribution. Do Not Release to NTIS

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