NBS

PUBLICATIONS Reference

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NBSIR 84-2898

Development of Power System

Measurements -- Quarterly Report

January 1, 1984 to March 31, 1984

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Gaithersburg, MD 20899

July 1984

Prepared for: Department of Energy

r»i»/ision of Electric Energy Systems

“QC ) Independence Avenue, SW

100 lington, DC 20585

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NBSIR 84-2898

DEVELOPMENT OF POWER SYSTEM

MEASUREMENTS - QUARTERLY REPORT

JANUARY 1, 1984 TO MARCH 31, 1984

R. E. Hebner, Editor

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Gaithersburg, MD 20899

July 1984

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 second 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

Abstract 1

1. INTRODUCTION 1

2. ELECTRIC AND MAGNETIC FIELD MEASUREMENTS

Subtasks Nos. 01 and 02 1

2.1 Operation of Aspirator-Type Ion Counters in Ground Plane 2

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. 03 5

4. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

Subtask No. 04 15

4.1 Temperature Dependence of Breakdown Voltage of Transformer Oil

With and Without a Paper Interface 17

4.2 Space and Surface Charge Associated With a Pressboard Interface

Perpendicul ar to the Field 19

4.3 Electrical Breakdown in Liquids -- Streamer Initiation 19

4.4 Plans for Next Quarter 21

5.

REFERENCES 21
LIST OF FIGURES

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Page

Ion counter location in parallel plate system.

System details have been eliminated for clarity

and drawing is not to scale 3

Top view of geometry used for air speed

measurements. Scans along the x direction were

taken at the 5 y positions indicated. Discrete

measurements were taken at selected locations.

The probe was positioned at heights of 1.5, 3.5,

and 5.5 cm above the inlet 3

Air speed measurements taken for two different

volumetric flow rates as indicated. The probe was

3.5 cm above the inlet and the scan was along a y

position of 5.5 cm (see fig. 2). Discrete

measurements were taken as indicated. The

vertical scale is non-linear (see text). Inlet

size was 14.2 x 11 cm. .... 4

Air speed measurements for two different ion

counter inlet geometries. The probe was 1.5 cm above the inlet and the scan was along the y

position of 5.5 cm (see fig. 2). The large

opening was 14.2 x 11 cm, and the smaller

9 X 8.3 cm, and the relative locations of the

openings are shown on the figure. For these

measurements the volumetric flow rate was about

0.017 m^/s. The vertical scale is non-linear (see

text) 6

Measured concentrations of SOF 2 versus net charge

transported in a point-plane negative corona discharge in SFg and SF 6 /O 2 mixtures at the

indicated discharge power levels. The absolute

gas pressure is 200 kPa (- 2 atm) and the

discharge current is 40 uA for all data 9

Measured concentrations of SO 2 F 2 versus net charge

transported in a point-plane negative corona discharge in SFg and SF 0 /O 2 mixtures at the

indicated discharge power levels. The absolute

gas pressure is 200 kPa (- 2 atm) and the

discharge current is 40 uA for all data 10

Measured relative concentration of gaseous carbon

monoxide versus net charge transported in a point-plane negative corona discharge in an SFe/10% O 2 mixture. The absolute gas pressure is

200 kPa and the discharge current is 40 uA 11

V
LIST OF FIGURES (cont.)

Page

Figure 8. Measured concentration of carbon dioxide versus

net charge transported in a point-plane negative

corona discharge in an SF6/10% O 2 mixture. The

absolute gas pressure is 200 kPa and the discharge

current is 40 uA 12

Figure 9. Breakdown voltage of transformer oil with and

without a paper interface bridging the

uniform-field gap under the application of 60 Hz

ac voltage. At each temperature, ten breakdowns

are made without an interface present in the gap

and then twenty interfaces are employed after

which ten more breakdowns are made without an interface present. When interfaces are present, a new interface is used after each breakdown. The

numbers beneath the error bars are the number of

breakdowns used to determine each point. The error bars are the standard deviations associated

with each group of measurements 18

Figure 10. The field on each side of a pressboard interface

placed perpendicular to the uniform field. 60 kV was applied, the gap was 1 cm. The inset shows

the entire view of the electrode geometry. The

electro-optical scanner measures the field in the

oil only. The field on each side of the

pressboard interface can be compared with the

field between the plates where the interface is

not present — Ey, the uniform field. The

important feature to note is that the field on each side of the pressboard is roughly 20% lower

than the uniform field. Any nonuniformity in the

field which may be indicated cannot be taken too

seriously in view of the problems with dirt and

cellulous "hairs" on the surface of the pressboard

which serve to either obscure the light or produce

a slight intensification of the light due to

depolarized scattering. Generally speaking, the

field measurements at the midpoints of the regions

between the pressboard and the plates are probably

quite accurate, within ±5% of their true value —

the overall measurement accuracy of the

measurement system 20

VI
DEVELOPMENT OF POWER SYSTEM MEASUREMENTS - QUARTERLY REPORT

January 1, 1984 to March 31, 1984

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 January 1, 1984 to

March 31, 1984. The report emphasizes the performance of ion counters

like those used to measure the ions near dc transmission lines, the

production rates of oxyfluorides in SF 5 corona discharges, and the

measurement of space charge associated with a pressboard interface in

transformer oil.

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

voltage; incipient fault; insulation; liquid breakdown; SF g; 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 January 1, 1984 to

March 31, 1984 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, 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, measurements were made of the air flow

outside an ion counter inlet for different opening geometries and volumetric

flow rates. This information will be used to assess the usefulness of the NBS

parallel plate system in investigating the effects of external electric fields

on ion counter operation and as a facility to calibrate ion counters. The

results are summarized in this report. In addition, measurements were performed

related to the fringing field effects of finite parallel plates near vertical

ground planes. These measurements are relevant for calibrating flat electric

field probes in a parallel plate apparatus. The results of the measurements

have been combined with previously reported measurements into a draft manuscript

which was sent to the journal. Bioelectromagnetics, to be considered for

publication.
2.1 Operation of Aspirator-Type Ion Counters in Ground Plane

Previous results have been reported which show that operation of ion

counters in regions of high external field result in measurements which depend

on various operating parameters such as ion counter potential, air speed at the

counter inlet and inlet geometry [1,2]. It appears that the source of these

results is the influence of both the electric field and the air flow in

determining the trajectories of the ions near the inlet of the ion counter. Air

speeds at the inlet are of the order of 1 m/s, which for reference corresponds

to a, wind speed of about 2 mph. In an unperturbed electric field of 10 kV/m, an

ion will move with a drift speed of 1-2 m/s. If the ion counter is located

above the ground plane and is grounded, the actual field at the inlet of the ion

counter could be several times the unperturbed field, depending on the geometry

and orientation of the counter. These problems are much less important in the

situation normally encountered in atmospheric physics, where the electric field

in fair weather is less than 1 kV/m. It also appears that it is best to operate

an ion counter so that the inlet is located in the ground plane, although this

introduces a number of operational problems of non-electrical origin.

Investigations have been initated to determine the feasibility of using the

NBS parallel plate system [3,4] to: (a) study in a quantitative way the effects

of external electric fields on ion counter operation, and (b) calibrate ion

counters against ion densities determined from the relation J = pKE, where J, p, and E can be determined; here J is the current density, p is the charge density,

and K is the average mobility. Because the ion counter is an aspiration device,

there is an air flow outside the instrument. This air flow perturbs the region

near the inlet and adds an additional term to the equations describing ion

transport near the counter [4]. The extent to which this air flow perturbs the

normal operation of the parallel plate system must be determined. A second

perturbation is considered in figure 1, which indicates schematically the

installation of the ion counter. The inlet is in the ground plane of the

parallel plate system, and must be open to allow air flow. This opening

significantly modifies the electric field near the ion counter inlet [4].

To estimate the extent to which the air flow into the ion counter perturbs

the electrical conditions existing in the parallel plate system, a set of

measurements have been made for different openings into and volumetric flow

rates through the ion counter. The approximate sizes of the openings used are

14 X 11 cm, 8x9 cm, and 5x5 cm. A wide range of air speeds and flow

conditions can be obtained with existing equipment. The measurement arrangement

is shown schematically in figure 2. Scans were made across the opening at five

different locations and for elevations above the plane of the opening of 1.5 cm,

3.5 cm, and 5.5 cm. An example of scans taken at a distance of 3.5 cm above the

plane and for two different flow rates is given in figure 3. During these

measurements, the axis of the probe was oriented parallel to the plane

containing the inlet of the ion counter. Discrete data points are an average

taken at fixed locations as indicated and the speed values shown are obtained

from a calibration chart for the probe used in these measurements. The

continuous records have not been corrected for the nonlinear calibration of the

probe in this low speed region. The air speeds indicated are not the actual

speeds, since the flow is three-dimensional and the hot film probe responds only

to velocity components perpendicular to the probe. Knowledge of the full air

velocity field would not be of use at this point. The fluctuations in the

continuous record are due in part to the motion of the probe during the scan.

2
Screen

Figure 1. Ion counter location in parallel plate system. System details have

been eliminated for clarity and drawing is not to scale.

Figure 2. Top view of geometry used for air speed measurements. Scans along

the X direction were taken at the 5 y positions indicated. Discrete

measurements were taken at selected locations. The probe was positioned at heights of 1.5, 3.5, and 5.5 cm above the inlet.

3
HOT FILM SENSOR LOCATION, cm

Figure 3. Air speed measurements taken for two different volumetric flow rates

as indicated. The probe was 3.5 cm above the inlet and the scan was along a y position of 5.5 cm (see fig. 2). Discrete measurements

were taken as indicated. The vertical scale is non-linear (see

text). Inlet size was 14.2 x 11 cm.

4
The error bars on the discrete points indicate the standard deviation for a set

of five measurements taken at each point. Air speed values below 0.2 m/s are only approximate since they lie outside the calibration range for the probe.

The speed of a small ion in a 10 kV/m field is approximately 1 m/s, so it is

clear that the air flow field significantly perturbs the conditions in the

parallel plate system, even for the lowest flow rate considered.

Figure 4 compares the air speed for two different size openings. The data

presented in figures 3 and 4 are representative of the set taken to characterize

the flow region outside the ion counter inlet.

In earlier unreported work, a small commercial ion counter was installed as shown in figure 1. A field mill was located 10.2 cm from the counter opening

and current-sensing elements were installed next to the ion counter opening.

With no flow through the ion counter, the current to the sensors next to the

opening increased by about 10% when the opening to the counter was uncovered.

This increase is caused by the perturbation in the electric field due to the

opening and is comparable to the theoretical prediction reported

previously [4]. A coarse (2 cm x 2 cm mesh) screen covering the hole reduced

the change to about 2%. Turning on the air flow through the counter caused the

current in the sensors to increase by an additional 2%. The changes in electric

field strength recorded by the field mill for any of the conditions was less

than 1%. For these measurements the electric field was about 3.4 kV/m. The air

speed at the center of the inlet at a distance of 3.5 cm was 0.34 m/s.

The next step in this effort will be to install the laboratory ion counter

in the parallel plate system and do a systematic series of measurements to

determine the extent to which the ion counter perturbs the system operation.

This will be done for different flow rates and ion counter operating

conditions. This data base should allow us to make conclusions about the

usefulness of this approach to study ion counter operation in the presence of

external fields. It does not appear that the system can be modeled without

extensive effort, but consideration will be given to this.

Additional experiments will also be done using the monopolar line used to

study the off-ground operation of ion counters reported in [4]. For this set of

measurements, an elevated ground plane has been constructed to allow the ion

counter inlet to be placed in the ground plane. Values of space charge density

determined directly from the ion counter will be compared to those obtained from

separate J and E measurements. This investigation will supplement the more

fundamental work being done with the parallel plate system.

For further information contact Or. 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

investigations: 1) measurement and calculation of electric discharge inception

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

5
HOT FILM SENSOR LOCATION, cm

Figure 4. Air speed measurements for two different ion counter inlet

geometries. The probe was 1.5 cm above the inlet and the scan was along the y position of 5.5 cm (see fig. 2). The large opening was 14.2 X 11 cm, and the smaller 9 x 8.3 cm, and the relative locations

of the openings are shown on the figure. For these measurements the

volumetric flow rate was about 0.017 m /s. The vertical scale is

non-linear (see text).

6
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.

The planned activities for FY84 include:

1) Preparation of conference and archival papers presenting results of our measurements on the production rates for the oxyfluorides S0F 2 » SO 2F 2 and

SOFh, and SO 2 and H 2O generated by corona discharges in SF s or SFe/HzO

mixtures;

2) Extension of the measurements on by-product production rates in gas

discharges to the mixtures, SFs+ Ozj SFs+ N 2> SF e + CO 2» and

SFe + c-Ci+F 8 + CHF 3;

3) Compilation of a computerized bibliography of electrical breakdown data

in insulating gases and publication of this bibliography in an NBS report;

4) Investigation of the effects of trace quantities of water vapor on the

enhancement of electron avalanche growth and corona discharge development

in SFe.

During the past reporting period emphasis was given to activities 1, 2, and 3.

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

As part of activity 1, a paper entitled "Production Rates for Discharge

Generated SOF 2, S0 2F 2> and SO 2 1n SFg and SF 6/H 2P Mixtures" by R. J. Van Brunt,

T. C. Lazo, and W. E. Anderson has been accepted for publication in the

Proceedings of the Fourth International Symposiim on Gaseous Dielectrics,

Knoxville, Tennessee, April - May, 1984. This paper presents the results of our measurements of the absolute production rates for SOF 2, S0 2p 2> SO 2» and H 2P

generated by point-plane corona discharges under a wide range of conditions in

SFg containing trace levels of water vapor.

Concerning activity 3, a bibliography of data on electrical breakdown in

gases has been completed and is available as an NBS Technical Note [5]. The

bibliography contains 1) a list of archival papers and books that present data

on electrical breakdown in gases and which have been published since 1950; 2) a computer generated index indicating the references that give particular types of

data for each gas; 3) an author index; and 4) a list of relevant technical

conferences.

The citations given in the bibliography contain experimental or theoretical

data on breakdown which include: 1) sparking potentials; 2) breakdown voltages;

3) critical fields, or field-to-gas density ratios; 4) corona inception

voltages; 5) voltage-time characteristics ; 6) relative and absolute dielectric

strengths; and 7) breakdown probabilities. The types of data which were considered include those which apply to: 1) uniform and nonuniform electric

fields; 2) ac, dc, and impulse voltages; and 3) possible special effects

associated with particles, surfaces, interfaces, and corona. The bibliography

is intended to serve as a guide in locating data on breakdown which are most

7
relevant to particular applications. All gases likely to be encountered in

electrical insulation applications are included. Any omissions are due either

to a failure to locate relevant references or to a complete lack of data.

Copies of the bibliography are available from the National Bureau of Standards,

Electrosystems Division, Gaithersburg, MD 20899 and from the Government Printing

Office.

The emphasis of our effort during the past quarter was on activity 2.

Measurements were made of the charge and energy rates of production of stable,

gaseous, oxygenated species from negative, point-plane corona discharges in SFs

containing O2 at levels from 0.5 to 10 percent. Absolute production rates were measured for SOF 2> SO2F2J SOF 4, C0 2> CO, and H2P for a total gas pressure of

200 kPa ('2 atm) and at a constant discharge current of 40 vA. Examples of some of the production data are shown in figures 5-8. Except for the data in

figure 7 for carbon monoxide, the results plotted in these figures correspond to

absolute concentrations in u moles versus net charge transported in the

discharge gap in coulombs as given by Q

= It, where I is the discharge current

and t the time at which a measurement of gas composition was made. For CO, only

relative concentrations in arbitrary units were determined.

It is seen in figure 5 that the introduction of oxygen into SF e results in

a net decrease in the SOF2 production rate, whereas from figure 6 the SO2F2

production is only slightly affected by oxygen. The production rates, it should

be noted, are given by the slopes of the curves that have been fit to the data.

The error bars indicate the maximiiTi uncertainties associated with variations in

the response of the gas chromatograph-mass spectrometer (GC/MS) used to perform

the quantitative gas analysis. It has been determined that these variations are due primarily to chromatograph-column, membrane, and ion source conditioning

effects whereby the transmission (or efficiency) of the column-membrane

combination for passage of certain species depends on its past history, in

particular the number and frequency of previous injections containing the same

gases. For the column used in these measurements, the effect was found to be

most noticeable for the more polar gases like H2P and SOF 2* The conditioning of

the ion source in the mass spectrometer is a temperature-dependent effect

related to the time that the filament in the electron gun remains on. The

effect can be reduced by preheating the ion source prior to injecting a gas

sampl e.

The reduction of about 45 percent in the charge-rate of SOF 2 production at

40 uA brought about by introducing 3.0 percent O2 is somewhat surprising and

possibly indicates that O2 has a significant effect on the dissociation rate of

SFe through modification of the electron energy distribution in the discharge.

If one assumes, for simplicity, that the production of SOF 2 can be accounted for

predominantly by the gas phase reactions

e + SF0'^SF4+2F + e, (1)

followed by

SF4 + H2O ^ SOF2 + 2 HF , (2)

then, because reaction (2) is known to proceed at an extremely high rate [6],

SOF2 production will be governed by reaction (1), i.e., the electron induced

dissociation rate for SF e leading to SF 4 fragments. The dissociation rate v of
SOF2

CONCENTRATION

(m

Moles)

0 1.0 2.0 3.0

NET CHARGE TRANSPORTED (C)

Figure 5. Measured concentrations of SOF 2 versus net charge transported in a point-plane negative corona discharge in SF^ and SFg /02 mixtures at the indi cated discharge power levels. The absolute gas pressure is

200 kPa (• 2 atm) and the discharge current is 40 uA for all

data.

9
SO2F2

CONCENTRATION

(

p

Moles)

Figure 6. Measured concentrations of SO 2 F 2 versus net charge transported in a point-plane negative corona discharge in SF- and SF(3/02 mixtures at the indicated discharge power levels. The absolute gas pressure is

200 kPa (~ 2 atm) and the discharge current is 40 yA for all

data.

TO
RELATIVE

CO

CONCENTRATION

0 1 2 3 4 5 6

NET CHARGE TRANSPORTED (C)

Figure 7. Measured relative concentration of gaseous carbon monoxide versus

net charge transported in a point-plane negative corona discharge

in an SF^/IO^ O 2 mixture. The absolute gas pressure is 200 kPa and

the discharge current is 40 uA.

11
C02

CONCENTRATION

(p

Moles)

NET CHARGE TRANSPORTED (C)

Figure 8 . Measured concentration of carbon dioxide versus net charge

transported in a point-plane negative corona discharge in an

$85 / 10% O 2 mixture. The absolute gas pressure is 200 kPa and the

discharge current is 40 yA.

12
SFe corresponding to SF^ production is related to the spatially dependent

electron energy distribution function f(e, x) by the expression

N

V = .1/2 A Jo

n

x) a^( e)d e d^x ( 3 )

where N is the SFe gas number density, m, e, and x are the electron mass,

kinetic energy, and position respectively in the discharge volume V, ngCx) is

the spatially dependent electron density, and cTp(e) is the cross section for

excitation to the nth antibonding state of SFe which yields an SF 4 fragment. It

is conceivable that the well known [7] resonantly enhanced inelastic scattering

of electrons from O 2 in the energy range of 0.3 to 1.4 eV gives rise to a significant lowering of the mean electron energy as given by f ( e, x) sufficient

to affect the rate v. It would be desirable to verify this by attempting to

solve the Boltzmann Equation for f ( e, x) with SF e/O 2 niixtures.

The fact that the SO 2F 2 production is relatively unaffected by the

introduction of O 2 and even shows a possible increase between the 3 and 10

percent O 2 mixtures is consistent with the proposed reaction scheme for its

formation. In particular, formation of SO 2F 2 in the gas phase necessarily

involves reactions of O 2 with the lower valence sulfur fluorides and free

radicals from SFe dissociation such as

SF 3 + O 2 SO 2F 2 F (4)

SF 2 + O 2 SO 2F 2 • (5)

Since SO 2 F 2 formation in the gas phase from direct two-body encounters requires

the presence of O 2 , it is to be expected that as the O 2 concentration is

increased, free radicals will be more readily removed from the gas by the oxygen

thus inhibiting recombination which could form SFi+ as well as SFg and yielding a higher relative SO 2F 2 rate of production as compared to SOF 2 which, according to

reactions ( 1 ) and ( 2 ) above, need not require O 2 for its formation.

This is consistent with previous investigations of decomposition in SF 6/O 2 mixtures resulting from laser-induced multiphoton processes [8] and spark or arc discharges [9]. It is also consistent with measurements on processes relevant

to plasma etching of silicon and silicon oxide in SF g/O 2 mixtures [10].

The presence of O 2 also seems to affect the production of SOF 4 . A

preliminary analysis of the results for this species suggests that the effect in

this case is like that for SOF 2 .

With the introduction of 02 * the production of CO was found to be

insignificant as indicated by the data in figure 7. The CO which was observed

appeared to be a contaminant of the O 2 gas used and exhibited very little change

in concentration during the course of the experiment. The production of CO 2 however was dramatically increased by the presence of 02 * Its production rate

in pure SFs was too low to measure. Figure 8 shows its production in the

SF6/10% O 2 mixture. It is seen that the CO 2 production rate is initially quite

high and then diminishes in time, or equivalently with charge transported. This

is in contrast to the production rates for the oxyfluorides which are either

13
constant or exhibit a slight increase with time. The behavior of the CO 2 production with time appears to be consistent with a formation mechanism

involving direct reaction of O 2 with carbon at the hot surface of the stainless

steel point electrode. As the carbon from the electrode surface is depleted, it can be expected that the production of CO 2 will decline as observed.

Another significant difference between the discharge-induced decomposition

of SFe with and without O 2 appeared in the rate of solid sulfur deposition on the planar anode surface. Sulfur deposits had previously been observed in pure

SFe at 200 kPa only for negative corona discharges at currents in excess of

60 uA. The deposits which have been observed are roughly uniform within a circle of about 1.2 cm diameter centered about a perpendicular line from the

point electrode. This pattern suggests a condensation of S“ ions originating in

the discharge region. The sulfur ions would tend to follow the circularly

symmetric field lines from the point thus giving the circular pattern observed.

No attempt has yet been made to measure the amount of sulfur deposited;

therefore no rates of deposition could be estimated. Nevertheless, there

appeared to be no noticeable sulfur deposition from pure SF 6 for negative corona

at 40 liA after 24 hours of operation. When O 2 was introduced at the 3 percent

level, a significant and easily recognized sulfur deposit appeared at 40 vA after a comparable discharge duration. Deposits also appeared in the mixture

containing 10 percent O 2 at the same current level. This observation tends to

confirm the suggestion made above that O 2 inhibits the recombination of free

radicals resulting from dissociation of SF e* In this case the O 2 effectively

removes free fluorine radial s to form the oxyfl uorides, thereby preventing

recombination with S" ions. The S* ions, because they have a higher mean drift

velocity in the direction of the field, will tend to exit the active discharge

region more readily and are consequently more likely to survive and be deposited

than neutral fragments like free sulfur atoms. The fact that S" ions are formed

is in itself a significant observation, for it indicates that there exists

sufficient energy within the discharge to effectively completely dissociate SFe

molecules. It is doubtful, however, that this occurs in a single step. The

formation of S" probably results from electron induced dissociation of free

radical fragments like SF 2 and SF generated by previous electron collision

processes in the gas. Again, removal of these by O 2 should reduce the rate of

recombination thus yielding a higher probability for dissociative attachment of

free radial s leading to S“ formation.

The data on H 2O formation in the mixtures containing O 2 appeared to be

similar to the data for pure SF e. However, from measurements made before and

after the discharge was extinguished for a prolonged time (> 16 h) there

appeared to be less change in the H 2O content for the oxygen containing gases.

This suggests that the discharge in these mixtures is less effective in

suppressing the equilibrium level of water vapor than is the case in pure SF e- This observation is consistent with the decrease in SOF 2 production which occurs

when O 2 is added since formation of this species via reaction (2) could account

for the consumption of H 2O in the gas phase.

With assistance from Dr. Darryl DesMarteau of Clemson University, samples

of pure SOF4 gas were prepared in the past quarter by direct reaction of SOF 2 with F 2 in a high pressure vessel at elevated temperatures. This gas can be

used to prepare standard samples to calibrate the GC/MS system for quantitative

measurements of SOF4 production in gas discharges. Test samples containing

known trace quantities of SOF 4 in SF e have already been prepared and

successfully analyzed with the GC/MS. A calibration method was devised

14
employing a determination of the relative GC/MS responses to SOF 4 and SOF 2 at a particular mass-to-charge ratio corresponding to the SOF 2

^ ion (mass 86 ). This

method will enable analysis of all previous GC/MS data on SOF 4 to determine its

absolute production rate in SFs corona discharges under a wide range of

conditions. Preliminary analysis of some existing data has revealed that SOF 4 is generated at a rate that is comparable to that of the other oxyfluoride

species SOF 2 and SO 2F 2 under most conditions. The results of our analysis of

SOF 4 production should be highlighted in our next report.

Planned activities for the next quarter include the following:

1) An archival paper describing all of the results on production of stable,

oxygenated, gaseous species from corona discharges in SFe as well as giving

detailed information about the analytical techniques employed will be

prepared.

2) Further measurements will be made on the influence of O 2 on decomposition rates of SFe in point-plane corona discharges. The

measurements may also be extended in the next quarter to SF e/N 2 niixtures.

3) Analysis of all previously acquired data on SOF 4 production in SFe

corona discharges will be completed in order to determine the absolute

production rates for this species.

4) New procedures for minimizing errors associated with GC/MS column and

ion source conditioning effects in quantitative analysis of gases will be

investigated. The purpose of this is to improve the accuracy and

reliability of our measurements on discharge-generated gas species

production rates.

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 the 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 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 the liquid and liquid-solid system:

a. The electric field between conductors and in the vicinity of

interfaces,

b. Breakdown strengths of selected interfacial systems and liquids.

15
c. The characteristics of the prebreakdown streamers in liquids and in

the vicinity of interfaces.

2. The influence of contaminants, particles, voids, and additives upon the

electrical strength of the liquid alone and the liquid-solid system.

Since it is impossible to clean a system perfectly, it is not possible to

draw a clear dividing line between these two areas of interest, but experiments

can be conducted which concentrate on specific areas of interest. To perform

this study, two experimental systems have been developed:

1. Optical Field Measurement (OFM) system to determine the electric field

in transformer-oil with and without interfaces and with and without the

addition of specific contaminants. This apparatus is capable of

regulating the temperature of the oil under study.

2. Electrical Breakdown Photography (EBP) system to provide high-speed

photographs of the time-evolution of the entire breakdown process, both

pre- and post-breakdown.

The OFM system supports electro-optical field measurements in liquids as well as temperature-dependent breakdown studies. The EBP system is generally used to

document streamer propagation characteristics in liquids.

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 outstanding results have been the

documentation 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

uncertainty of the experiment. These results clarify the role of space charge

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

this quarter extends these results to include an interface perpendicul ar to the

field.

With the EBP system many liquids have been investigated and a general

picture of the electrical breakdown process has been obtained. These methods

can now be extended to investigate a broader range of experimental conditions.

Notable results have been the documentation of multiple modes of streamer

propagation and the documentation of the effects of the addition of electrontrapping chemical impurities on streamer structure [11,12]. These results were obtained with liquids alone using both the point-plane and quasi-uniform

electrode geometries. The work this quarter focuses on the initiation of the

streamer.

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 [13], Such results were achieved using

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

data were taken at room temperature. This is not to say that in practical

systems the interface will not break down at a lower voltage. The effects of

the voids or particles which might be found in practical systems were not

16
systematically investigated. Last quarter a cell was developed to permit the

extension of this work to higher temperatures. This quarter's work corrected

several deficiencies of that cell and obtained the first complete set of data

for a range of temperatures.

4.1 Temperature Dependence of Breakdown Voltage of Transformer Oil

With and Without a Paper Interface

A paper interface is placed between uniform field electrodes touching both

electrodes and aligned with the direction of the field, and the assembly is

placed in transformer oil. As described in the previous quarterly report, the

interface can be changed from outside the cell so that high-temperature data may

be obtained rapidly. This cell required several modifications this quarter

before sustained high-temperature operation could be accomplished.

The paper was carefully dehydrated by baking in a vacuLin oven and the

transformer oil was dehydrated by bubbling dry nitrogen through it after which

it was degassed in a vacuum. The temperatures used in this were 27°C, 76°C,

100°C, and 125®C. The results are presented in figure 9. When an interface was present the breakdown did not necessarily occur at the interface and the

interfacial breakdown voltage was not necessarily lower — the same results as were obtained previously when the study was initially conducted several years

ago at room temperature. It is interesting to note that previous work [14]

showed a variation of the breakdown voltage with temperature while no similar

tendency is shown by these data. The reason for the difference will be

addressed in future experiments, but there is a plausible explanation for the

difference if one is willing to assume that the breakdown voltage can be

influenced by the electric field distortion induced by space charge. Previous

work has shown a tendency for greater field distortion under dc than under ac voltage and for the distortion to increase with increasing temperature. In light of these results, one would expect to record the largest effect under

direct voltage at elevated temperatures. This expectation is consistent with

our measurements. An additional confounding factor arises because it has been

observed that the amount of space charge formed may vary significantly from

sample to sample of oil [15]. Thus, if one uses different samples to

investigate waveform and/or temperature effects, care should be taken to assure

that the levels of space charge are comparable in all of the samples.

It is important to note that these results do not suggest that an interface

parallel to the field is always just as strong as the oil but they suggest that

it can be. The paper used in these experiments is carefully handled to

eliminate the possibility of voids and water as much as possible. In the near future, further experimentation will employ oil and paper which will not be so carefully prepared. It is anticipated that under less favorable conditions the

interface will be found to be weaker than the oil.

There is also a possible systematic effect due to the temperature

dependence of the oil's viscosity. At room temperature the oil seems to be more

likely to be contaminated with particles which inevitably are always present

despite the cleaning methods. However, at higher temperatures where the oil

exhibits lower viscosity, the ability of the oil to support larger particles is

impaired. Additionally, the effectiveness of the pump increases so that more

liquid is filtered per unit time during the course of the experiment. This

potential problem with dirt may explain the difference in the data at room

17
110

z

o

Q

<

111 ccm

<

HICL

50

40

30

20

10

0

• - BREAKDOWN NOT AT INTERFACE

A - BREAKDOWN AT INTERFACE

- NO INTERFACE PRESENT

DISCRETE TEMPERATURES

Figure 9. Breakdown voltage of transformer oil with and without a paper

interface bridging the uniform-field gap under the application of

60 Hz ac voltage. At each temperature, ten breakdowns are made

without an interface present in the gap and then twenty interfaces

are employed after which ten more breakdowns are made without an interface present. When interfaces are present, a new interface is

used after each breakdown. The numbers beneath the error bars are the number of breakdowns used to determine each point. The error

bars are the standard deviations associated with each group of

measurements.

18
temperature compared with the data taken at higher temperatures . The room temperature data exhibits the "expected" behavior in that the breakdowns

occurring at the interface occurred at the Tower voltage. Even so, not all the

breakdowns occurred at the interface. It is expected that better data will be

obtained if the temperature cycling time can be reduced as is planned in a forthcoming revision of the apparatus.

4.2 Space and Surface Charge Associated With a Pressboard

Interface Perpendicular to the Field

In a previous research effort, the field in the vicinity of an oi 1 -pressboard interface was investigated with the interface parallel to the

uniform field direction. It was determined that there was no measurable field

enhancement due to surface charging of the interface. This quarter, the

interface was placed perpendicular to the applied uniform field.

Electro-optical field measurements were made and revealed that the field on either side of the interface was reduced from what it would be if the interface

were not present, see figure 10. The positive 60 kV applied voltage over the

1 cm gap was from 60% to 80% of the breakdown voltage of the interfacial system.

Since the relative permittivity of oil is about 2.2 and for dehydrated

pressboard it is 4.4, one would expect to see the field stronger in the oil

outside the pressboard. This is not what is observed. Figure 10 shows that the

field in the oil on either side of the pressboard is roughly 20% lower than the

uniform field value Ey. This proves that there is surface charging of both

sides of the interface and that the field inside of the pressboard is larger

than Ej. Given the geometry and field shown in figure 10, calculations showed

a minimum surface charge on the order of 40 nC/cm^ with a positive surface

charge on the surface of the pressboard nearest the anode and a negative surface

charge on the surface of the pressboard nearest the cathode. The resulting

field within the solid pressboard appears to be about four times larger than it

would be without the surface charge.

At elevated temperatures, 107“C and 125“C, space charge field enhancement

became apparent in the liquid — again, it was negative space charge near the

anode which is typical for the oil used. The presence of the space charge did

not seem to modify the above observed surf ace-charging phenomena, because the

field on each side of the interface was substantially reduced from what it would

have been if the interface were not present. Because of measurement

difficulties associated with the higher temperatures, the possible temperature

dependence of the magnitude of the surface charge has not been measured yet,

4.3 Electrical Breakdown in Liquids — Streamer Initiation

There has been some informal debate concerning the role of the local field

in the initiation and development of streamers in liquids. During discussions

at conferences, some have speculated that the origin of the streamer is as independent upon surface features of the metal or even sub-surface features as

it is upon the field at the surface. Experiments were initiated this quarter to

address this issue. The work was done in collaboration with E. 0. Forster and

G. J. FitzPatrick of Exxon Research and Engineering Company.

The needle-plane electrode geometry in cyclohexane was analytically modeled

by a hyperboloid above a plane to determine the field strength along the metal

19
Figure 10. The field on each side of a pressboard interface placed perpendicul ar to the uniform field. 60 kV was applied, the gap was 1 cm. The

inset shows the entire view of the electrode geometry. The

electro-optical scanner measures the field in the oil only. The

field on each side of the pressboard interface can be compared with

the field between the plates where the interface is not present —

Ey, the uniform field. The important feature to note is that the

field on each side of the pressboard is roughly 20% lower than the

uniform field. Any nonuniformity in the field which may be indicated

cannot be taken too seriously in view of the problems with dirt and

cellulous "hairs" on the surface of the pressboard which serve to

either obscure the light or produce a slight intensification of the

light due to depolarized scattering. Generally speaking, the field

measurements at the midpoints of the regions between the pressboard

and the plates are probably quite accurate, within ±5% of their true

value — the overall measurement accuracy of the measurement system.

20
surface of the tip. The data obtained from the experiment suggested that the

first streamer initiated only in the highest field region near the point of the

tip. After the streamer initiates it is hypothesized that the streamer grades

the field at the tip so that a high-field region occurs further down the shaft

of the needle. Computer modeling of this possibility is presently being

explored by the team at Exxon. It is anticipated that the results of these

studies will show that the location of streamer initiation is dominated by the

local field intensity at the metal surface. It is of interest to note that with

a gap of 1 cm and tip radius of 25 urn the initiation fields at the tip were on the order of 4 MV/cm for the approximately 3 vs rise-time pulses used.

4.4 Plans for Next Quarter

A thorough study of the effects of temperature, space charge, additives,

and electrode material upon the breakdown strength of transformer oil will be

continued. Additionally, to successfully complete the study of the problem of

the interfacial breakdown of a paper interface parallel to the field, an experimental arrangement needs to be developed to permit the rapid changing of

the temperature of the experimental system. Presently, more than three days is

required to take the present system to 150®C and back to room temperature. It

is anticipated that the new apparatus will permit the same cycle to be reduced

to less than three hours — neglecting the time needed to take data, of course. Completion of this new apparatus is scheduled for the next quarter.

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

5. REFERENCES

[1] R. H. McKnight, F. R. Kotter, M. Misakian, and J. N. Hagler, 1981 Annual

Report: Electric and Magnetic Field Measurements, Nat. Bur. Stand. (U.S.),

NBSIR 82-2527 (1982).

[2] R. H. McKnight and P. M. Fulcomer, Operation of Ion Counters Near High

Voltage DC Transmission Lines, presented at the 4th International Symposium

on High-Voltage Engineering, Athens, Greece, Sept. 1983.

[3] M. Misakian, Generation and Measurement of DC Electric Fields with Space

Charge, J. Appl. Phys., Vol. 52, pp. 3135-3144, 1981.

[4] "Development of Power System Measurements — Quarterly Report, January 1 - March 31, 1983," R. Hebner, ed., Nat. Bur. Stand. (U.S.) NBSIR 83-2761,

(1983).

[5] R. J. Van Brunt and W. E. Anderson, "Bibliography of Data on Electrical

Breakdown in Gases," NBS Tech Note 1185 (1984).

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

Spark Decomposition Products of SF e and SF s Perfl uorocarbon Mixtures,"

Gaseous Dielectrics III (L. G. Christophorou, ed.) Pergamon Press, New York

(1982) pp. 387-401.

[7] G. J. Schulz, "Resonances in Electron Impact on Diatomic Molecules," Rev.

Mod. Phys., Vol. 45, pp. 423-486 (1973).

21
[8] I. 0. Leipunskii, A. K. Lyubimova, A. I. Nadeikin, and V. L. Talroze,

"Investigation of the Process of Formation of Oxygen-Containing Products as

a result of multiphoton Dissociation of SFe by Radiation from a High-Power

Pulsed CO 2 Laser," Sov. J. Quantum Electron., Vol. 12, pp. 413-417 (1982).

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

Some Properties of Compounds Resulting from the Decomposition of SF 0 under

the Electrical Arcing in Circuit Breakers," Revue Generale de I'Electricite

(RGE), pp. 45-78 (1974).

[10] R. d'Agostino and D. L. Flamm, "Plasma Etching of Si and SiO£ in SFg-02

Mixtures," J. Appl. Phys., Vol. 52, pp. 162-167 (1981).

[11] G. J. FitzPatrick, E. 0. Forster, E. F. Kelley, and R. E. Hebner, "Effects

of Chemical Impurities on Prebreakdown Events in Toluene," 1982 Annual

Report Conference on Electrical Insulation and Dielectric Phenomena,

Oct. 17-21, 1982, IEEE Service Center, Piscataway, NJ, pp. 464-472, 1982.

[12] E. F. Kelley, R. E. Hebner, G. J. FitzPatrick, and E. 0. Forster, "Effects

of Aromatic Impurities on the Positive Streamer Growth in Marcol 70," IEEE

International Symposium on Electrical Insulation, in press.

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

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

Trans, on Elec. Insul. EI-16, pp. 297-303 (1981).

[14] "Development of Power System Measurements — Quarterly Report, July 1 - September 30, 1983," R. Hebner, ed., Nat. Bur. Stand. (U.S.) NBSIR 84-2818,

(1984).

[15] E. F. Kelley and R. E. Hebner, "Measurements of the Electric Field in the

Vicinity of an Oi 1 -Pressboard Interface Parallel to the Field," Conf.

Record of 1983 Interfacial Phenomena in Practical Insulating Systems,

Sept. 19-20, 1983, IEEE 83 CH1946-3, IEEE Service Center, Piscataway, NJ,

pp. 19-22, 1983.

22
NBS-1 MA (REV. 2 -aci

4. TITLE and subtitle

Development of Power System Measurements -- Quarterly Report -- January 1, 1984 to March 31, 1984

5. AUTHOR((^) Hebner, Editor

6. PERFORMING ORGANIZATION (If joint or other than MBS. see instructions) 7. Contract/Grant No.

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8. Type of Report & Period Covered

9. Ojf^^NIZATION 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

1 1 Document describes a computer program; SF-185, FlPS Software Summary, is attached.

U.S. OEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

NBSIR 84-2898

2. Performing Organ. Report No, 3. PubI ication Date July 1984

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

bi bliography or literature survey, mention it here)

This report documents the progress on three technical investigations sponsored

by the Department of Energy and performed by the El ectrosystems Division, the

National Bureau of Standards. The work described covers the period from

January 1, 1984 to March 31, 1984. The report emphasizes the performance

of ion counters like those used to measure the ions near dc transmission lines,

the production rates of oxyfluorides in SF 5 corona discharges, and the

measurement of space charge associated with a pressboard interface in

transformer oil.

12 . KEY, WORDS (Six to twelve entries.; alpbabeticaLorde^r: capiulizs only proper names- and separate key wnrds.Sy cables; composite insulation; e I ectfic fields; Tifgh voTtage; incipient fduTt;

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

I colon '

13. AVAILABILITY

r-Xn Uni imited

I

I For Official Distribucion. Do Not Release to NTIS

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

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

14. NO. OF

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