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I NBSIR 83-2705

Development of Power System

Measurements -- Quarterly Report

July 1 , 1 982 to

September 30, 1982

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

May 1983

Prepared for:

n ''"''rtment of Energy

on of Electric Energy Systems

100 Independence Avenue, SW

. U 5 6 ington, DC 20585

83-2705

1983

Research Information Center

-National Bureau of Standards * ^

Gaithersburg, Maryland 20899 fTO

,

%

7,

NBSIR 83-2705

/W3 DEVELOPMENT OF POWER SYSTEM

MEASUREMENTS - QUARTERLY REPORT

JULY 1, 1982 TO

SEPTEMBER 30, 1982

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 1983

Prepared for: Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, DC 20585

z>

Q

C

Q*

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Foreword

This report summarizes the progress on four technical investigations

during the fourth quarter of FY 1982. Although reasonable efforts have

been made to ensure the reliability of the data presented, it must be

emphasized that this is an interim report so that further experimentation

and analysis may be performed before the conclusions from any of these

investigations are formally published. It is therefore possible that some

of the observations presented in this report will be modified, expanded,

or clarified by our subsequent research.
TABLE OF CONTENTS

Page

Foreword iii

LIST OF FIGURES v

LIST OF TABLES v

Abstract 1

1. INTRODUCTION 1

2. DC FIELDS AND ION MEASUREMENTS

Subtask No. A018 1

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. A053 5

4. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

Subtask No. A057 13

5. ACTIVE INSULATORS FOR INSULATION AND SURGE SUPPRESSION

Subtask No. A057 14

6. REFERENCES 18
LIST OF FIGURES

Page

Figure 1. Schematic view of parallel plate apparatus and rotor-stator

assembly of field mill 3

Figure 2. Measured discharge pulse rate versus applied voltage for

positive, point-plane corona in SFg at a pressure of

300 kPa (~3 atm) for the different indicated conditions

of gap irradiation 8

Figure 3. Relative difference in electron avalanche counts for light

beam on (N^) and light beam off (N 2 ) measured for two

different indicated conditions of gap irradiation for a positive point electrode in O2 at a pressure of 200 kPa (~2 atm).

The error bars indicate the statistical uncertainties in the

measurements 11

Figure 4. Equivalent circuit for single phase transmission line with an active insulator 15

Figure 5. Equivalent circuit for a voltage source with breaker resistance

Rg and a source inductance L 16

LIST OF TABLES

Page

Table 1. Error sources for parallel plate apparatus 4

v

DEVELOPMENT OF POWER SYSTEM MEASUREMENTS — QUARTERLY REPORT

July 1, 1982 to September 30, 1982

R. E. Hebner, Editor

This report documents the progress on four technical

investigations sponsored by the Department of Energy. Three were performed by the Electrosystems Division, the National Bureau of

Standards and the fourth by the Department of Electrical Engineering

of the University of Southern California. The work described covers

the period from July 1, 1982 to September 30, 1982. The report

emphasizes the calibration of instruments designed to measure

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

determination of the role of photodetachment of SFg corona discharges,

the measurement of failure mechanisms in liquid/solid insulating

systems, and the development and behavior of active insulators.

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

incipient fault; insulation; liquid breakdown; SFg; space charge;

transformer oi 1

.

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 is concentrated in three areas — the measurement

of electric fields, the measurement of partial discharge phenomena in

gaseous dielectrics, and the measurement of interfacial electrostatic field

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

the quarter July 1, 1982 to September 30, 1982 is summarized in this report.

2. DC FIELDS AND ION MEASUREMENTS

Subtask No. A018

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 of 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.

To support ac measurements, a magnetic field probe has been obtained

and its evaluation was initiated. The evaluation of dc instrumentation

has focused on the off-ground operation of ion counters and on electric

field meters. In applications near high voltage dc lines, it is very

convenient to operate an ion counter in such a manner that the instrument

protrudes above the ground plane. In this configuration, the investigator

has the option of grounding the counter, insulating the counter and

1
allowing it to assume the potential determined by the experimental conditions,

or biasing the counter to a preselected value. To determine the effect of

counter potential on counter response, a model dc transmission line was constructed as was a fiber optic system to couple the counter output to

grounded recording equipment. Preliminary results have been obtained and

this investigation is continuing.

A new electric field meter was received and tested during this reporting

period. In spite of some damage sustained during shipment, the field meter

functioned well enough to be evaluated using the NBS parallel plate apparatus.

A field-mill -type field probe was sought because earlier studies at NBS [1]

demonstrated that this type of probe, when operated in the presence of

large ion current densities, will have smaller measurement errors than a vi brati ng-plate-type probe. The evaluation of the recently arrived device

demonstrates, however, that field mills can malfunction in the presence of

large space-charge currents, if not properly designed.

The experimental setup for the tests is shown schematically in the

inset of figure 1. Details on the operation of the parallel plate apparatus

are given elsewhere [1] and are not necessary for the discussion presented

here. Also shown in figure 1 is a sketch of a portion of the field mill

sensing head which is mounted in a cylindrical metal housing. The grounded

rotor periodically interrupts the electric field lines impinging on the

sensing electrode, or stator, producing a time-varying induced current to

ground. The signal voltage across resistor R is then amplified and the

signal is applied to a phase sensitive detector. The output signal from

the phase sensitive detector, monitored with a digital voltmeter, was found to be a linear function of the electric field strength between the

parallel plates in the absence of space charge.

Measurements of electric fields containing positive space charge were

also made with the newly acquired field mill and compared with simultaneous

measurements obtained with a second field mill. The performance of the second

field mill has previously been characterized and is referred to as the "NBS

field mill" in table 1. While less portable and having signal stability

problems, it is able to measure electric fields without significant errors

due to the presence of space charge currents [1], The ratios of the field

strength with and without positive space charge at fixed voltages applied

to the parallel plates were recorded and are presented in table 1(a).

When the ion current density J becomes limited by space charge, as was the case during the present tests, the electric field is enhanced in our apparatus by a little more than 50%. The results in table 1(a) show that

the recently acquired field mill indicates field values that are too low.

When the electric field was turned off, a change in the zero-field reading

was also observed for the new field meter. This negative offset decayed over many minutes until the original zero-field signal was restored. Correlated

with this decay was the reduction in amplitude of a small, approximately

sinusoidal, signal from the stator. The stator signal waveform was observed

via a buffered output following the first stage of amplification. This

correlation suggested that the offset was not due to a deficiency in the

electronics which analyze the "output signal," as shown in figure 1. It was more likely due to the properties of the basic field mill assembly itself.

2
GROUNDED

ROTOR

UJ

5 £E

°

Z UJ L.

C/O LU ill

3

Figure

1.

Schematic

view

of

parallel

plate

apparatus

and

rotor-stator

assembly

of

field

mill.
The long decay constant suggested that the exposed circuit board on which

the stator was mounted (fig. 1) was charging up when the system contained space

charge. To test this hypothesis, the circuit board was covered with grounded

aluminum foil and the field measurements were repeated. The disagreement

between the two field mill readings as well as the charging effect were both

eliminated. Table 1(b) shows the results of several measurements. The

third entry is for the case when the chopping frequency on the new field

mill is reduced from 100 to 86 Hz. The scatter in the NBS field mill data

reflects the previously noted instability of this device.

Table 1. Error sources for parallel plate apparatus

(a) Uncovered insulator

aral lei

spacing

plate

(m)

Parallel plate

voltage (kV)

J (xl0 -6A/m 2

)

New field mi 11 E(Jji0)/E(J=0)NBS field mil

E(J^0)/E(J=0)

0.12 1.2 1.16 1.52 1.56

0.10 1.0 1.46 1.40 1.55

(b) Covered insulator

0.10 1.0 1.51 1.57 1.55

0.10 1.0 1.51 1.57 1.56

0.10 1.0 1.51 1 . 57 a 1.54

a Chopping frequency changed from 100 to 86 Hz.

While the problem of low field readings in the presence of large space

charge currents has been eliminated, a unique explanation for the effect

has not been found. Some ion current can be expected to flow from the

circuit board surface onto the stator and cause a negative contribution to

the total signal (see below). However, this mechanism is not expected to

produce effects that are as large as ±10% because of the small area of

the circuit board. It may be that some ion current is also continuously

deflected onto the stator by the positive charge on the circuit board.

However, the magnitude of this focusing effect, also thought to be small,

has not been estimated.

Another source of error occurs because of the ion current impinging

on the stator and the long time constant (RC) of the stator circuit.

While signals due to the chopped conduction current are in quadrature to

the induced current signal and largely eliminated by the phase sensitive

detector, the long time constant results in the stator acquiring a small

positive voltage (the RC time constant is approximately 0.5 s and the

chopping frequency is approximately 90 Hz). Thus, a small periodic,

local electric field is established between the stator and grounded rotor

which is opposite in sign to the external field. Examination of the phase

relation of the signal from the local field to that from the external field

indicates that the local negative field will contribute to the total signal.

4

!
The maximum amount of this contribution can be estimated by calculating the

voltage difference across the input resistor for a given current impinging

on the stator. Knowing the voltage of the exposed stator, the approximate

magnitude of the local field between the rotor and stator, when the rotor covers the stator, can be calculated. As already indicated, the signal

due to the local field is estimated to be less than 0.7% of that due to the

external field. This small systematic error can be reduced by increasing

the spacing between the rotor and stator.

During the next quarter, the newly acquired field mill will be retested

after two modifications are completed. One will be to increase the rotor-stator

spacing from 1 mm to 3 mm and the other will be to coat the circuit board with

a conducting surface connected to ground.

In addition to the measurements of field meter performance, the

evaluation of ion counter measurements is expected to continue. It is

anticipated that the off ground measurements using the model transmission

line will be completed. The feasibility of using the parallel plate system

to study the effects of field penetration and ion current on the operation

of the counter in the ground plane in the absence of external air flow will

be examined. Similarly, the feasibility of studying the effects of external

air flow and ion current in the wind tunnel in the absence of an external

field will also be evaluated. We intend for the output of this study to be

appropriate guidance in the use of ion counters to determine the ion density

in the vicinity of dc transmission lines. To bring together those individuals

with a concern for ion measurements or dc field measurements, NBS is hosting

a two-day workshop on the measurements of the electrical enviornment near high-voltage dc transmission lines on October 27 and 28, 1982. The workshop is

intended to provide a forum to discuss the capabilities and limitations of

present measurement techniques and to identify future measurement requirements.

In the area of ac measurements, it is expected that two activities will

be emphasized. The present schedule calls for the first on-site measurement

of 60-Hz electric and magnetic fields to be performed in an exposure system

developed for New York State during the coming quarter. In addition, NBS

intends to prepare a draft International Electrotechnical Commission standard

on the measurement of power frequency electric fields and to distribute

this draft to the newly formed working group for comment.

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

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No. A053

The objective of this project is to develop diagnostic techniques to

monitor, identify, and predict degradation in future compressed gas electrical

insulating systems under normal operating conditions. The focus is on the

fundamental information and data needed to improve test, design, and performance

evaluation criteria. The investigation of partial discharges (corona) in gaseous

dielectrics is emphasized. This phenomenon gives rise to degradation of the gas

under high electrical stress which leads to breakdown. Measurement of partial

discharge inception in highly nonuniform fields may prove to be a preferred

method to determine dielectric strength of electronegative gases.

5
Planned activities for FY83 include: 1) completion of an archival paper

on basic mechanisms of corona inception in SFg under 60 Hz ac and dc conditions;

2 ) investigation of the wavelength dependence of photon-induced positivecorona inception in SFg; 3) measurements of power dependence of the rates of

oxyfluoride and water vapor production from corona discharges in SFg, the

results of which are to be reported in conference and archival papers;

4) exploration of the feasibility of new methods to measure field-enhanced

colli si onal detachment of negative ions in SFg and other electronegative gases;

5) improvement in the accuracy of quantitative analysis of trace gases such

as H2 O in SF6 and other gaseous dielectrics using a gas chromatograph/mass

spectrometer, and extend the measurements on the effects of trace H 2 O on corona characteristics and relative dielectric strength of the gas; and

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

swarm data in molecular electronegative gases.

Activities 1) and 6 ) were completed in previous quarters. During the

past reporting period, measurements were performed which extended activity

2) to include the gas O2

. This was necessary in order to verify the conclusions

reached from the measurements performed on SF5 which were reported in our previous quarterly report [2], Activity 2) has now been completed. The final

results and conclusions obtained from this investigation have been prepared

for presentation at the Thirty-Fifth Annual Gaseous Electronics Conference to

be held in Dallas, Texas, October 19-22, 1982, and will be highlighted in this

report.

Progress was also made on activities 3) and 5). A paper entitled "Effects

of H 2 O on the Behavior of SF 5 Corona," by R. J. Van Brunt [3] was presented at

the Seventh International Conference on Gas Discharges and Their Applications

held in London, August 31 - September 3, 1982. The data presented in this

paper showed that small quantities of water vapor in SFg can significantly

influence the electron avalanche growth and level of corona around a positively

stressed electrode. In order to better understand these results, it is necessary

to measure the H2 O content in SFg. During the past quarter, a system employing

a thin film aluminum oxide hygrometer probe for use in calibrating an existing

gas chromatograph-mass spectrometer (GC/MS) for quantitative analysis of

H2 O in SFg was designed and the necessary components were ordered.

Concerning activity 3), computer programs were written which can be used

for rapid analysis of data from the GC/MS system. More measurements were performed [4] to determine the pressure dependence of corona-generated oxyfluoride

production rates in SFg. There are no reasons a priori to expect large variations.

Although the data accumulated thus far must still be considered preliminary, there

appear to be no indications of large variations in the production rates with

pressure.

Earlier results show that photodetachment, e.g.,

hv + SFg + SFg + e

makes a negligible contribution to the release of avalanche or coronainitiating electrons in irradiated point-plane gaps containing pure SFg to

which dc voltages are applied. It was suggested that the enhancement of

6
avalanche rates that often results from irradiation of the gap is due to

release of photoelectrons from the cathode by scattered radiation, which in

the case of stainless steel electrodes in SFg requires wavelengths shorter

than 350 nm. These photoelectrons quickly attach to SFg molecules to form

stable negative ions which can then give rise to electron release through

collisional detachment in the region, near the point electrode, where the

ionization coefficient a exceeds the attachment coefficient i> Additional

data were obtained for both SFg and O2 during the past reporting period

which lend further support to this interpretation.

Examples of results obtained for SFg are shown in figure 2. Again

stainless steel point-plane electrodes were used with a poi nt-to-pl ane gap

spacing of 1.33 cm and point radius of curvature of 0.04 mm. The gap

configuration is nearly identical to one previously used in the measurement

of corona discharge inception [5] and corona pulse characteristics [6] in

pressurized SFg. The gas pressure was 300 kPa and the gap was irradiated

with uv-radiation directed either at the cathode (plane electrode) or the anode

(point electrode) for the wavelengths indicated. The results shown in figure 2

for the case where there was no radiation and when the anode was irradiated for

295 nm < \ < 365 nm are similar to those previously reported [5,6]. For these

cases, the discharge below about 21 kV appears in the form of single electron

avalanches. At 21 kV, there is an abrupt increase in pulse rate, and at this

point large "streamer"-type pulses appear and the discharge becomes completely

self sustaining, i.e., it is no longer significantly influenced by the radiation.

These results are in sharp contrast to those obtained when the cathode

is irradiated. For the latter, there is a more abrupt increase in the

avalanche rate near threshold, similar to the behavior observed for negative

corona [5,6], By irradiating the cathode, the inception voltage, in fact, comes closer to the theoretically expected value based on the streamer

criterion, and the previously reported [5] large difference between positive

and negative corona inception is nearly eliminated. As for the results

previously reported [2], irradiation of either the anode or cathode required

wavelengths shorter than 350 nm to produce the effects shown. Irradiation

of the cathode with 254 nm radiation from a Hg-discharge lamp was found to

produce the most dramatic, observed enhancement in the avalanche rate. There was some evidence that at these higher avalanche rates, formation of the much

larger streamer-type pulses is suppressed. The effect here might be similar

to that reported for negative corona [6] and should be investigated further.

Since there is a lack of information about the relevant cross sections

for photodetachment and collisional detachment processes in SFg, it is

somewhat difficult to argue that photodetachment should be relatively

unimportant compared to collisional detachment as observed. However, in

the case of O2 , the relevant cross sections have been either measured or calculated, and one can show rather convincingly, as previously argued [7],

that unless very intense radiation sources are used, the probability of

electron release by photodetachment is orders-of-magni tude smaller than

that for collisional detachment.

To show this, one must assume some arbitrary flux of negative ions

moving toward the point electrode into a region of length z where a > t> This means that within z the electrical f i el d-to-neutral gas density ratio,

E/N, exceeds the critical value ( E/N

)

c at which a = ni Thus if an electron

7
APPLIED VOLTAGE (kV)

Figure 2. Measured discharge pulse rate versus applied voltage for

positive, point-plane corona in SF5 at a pressure of

300 kPa (~3 atm) for the different indicated conditions

of gap i rradi ati on

.

8
is released, there is a finite probability that it will initiate an avalanche

The probabilities that a negative ion will detach its extra electron, either

by encountering a photon of sufficient energy or by colliding at sufficient

energy with a neutral molecule, will depend in general on the value of E/N

at any given point x in the discharge gap. We consider then the ratio Wp )C

of the probabilities for photodetachment-to-col 1 i si onal detachment of an arbitrary ion moving through the distance i. It can be shown that, to a good approximation, this ratio is given by the expression

l a p (x)$(x) / dx/(yE(x))

.,- 0 . (1)

{ 1-expC-N / ( 6d/N)dx]

}

o

In this equation, <jp(x) is the photodetachment cross section at

wavelength x, $(x) is the incident photon flux, y is the ion mobility,

and 5d is the collisional detachment coefficient, i.e., the number of

detachments per unit length. In general, 6d increases quite rapidly with

E/N above a "threshold" value ( E/N ) d at which a significant number of ions

acquire enough kinetic energy from the interaction with the electric field

between collisions with neutral molecules to allow detachment to occur,

i.e., a kinetic energy which roughly exceeds twice the negative ion electron

affinity [5], The dependence of 6d on x is derived from the dependence

of E/N on x.

The detachment coefficient can be derived either from the collision

cross sections ac or from the detachment rate coefficients k. Using

cross section data, the proper expression is:

00 S d /N =1 / v ac (v)f (v)dv , (2)

v o

where f(v) gives the distribution of ion velocities v for which 7 is the mean velocity. For detachment rates, we use the formula

5 d /N = k/v . (3)

In deriving equation (1) above, we have also assumed that

v = y E (4)

and that

£ a

p (x)$(x) / dx/(yE(x ) ) « 1 , (5)

o

which is reasonable provided the flux $(x) is not excessively large.

9
For the pressures of interest here (P > 50 kPa), the predominant

negative ions in O

2 are likely to O

2

" and 0-> [7], In the following

discussion, consideration is given only to 02 “, although it can be shown

that a similar conclusion is reached if one considers 03 ". For the present

calculation, a gas pressure of 100 kPa (~1 atm) will be assumed at a uniform temperature of 300 K. Since it is apparent from inspection of

equation (1) that Wp jC will decrease with increasing E/N (assuming, of

course, that 6 <j increases with E/N), it is sufficient for purposes of

this argument to consider an extreme case where E/N = (E/N) c over the

entire length In this way, we obtain a kind of upper limit on W

p c , since any E/N values higher than ( E/N) c will only tend to diminish Wp jC *

Although to obtain the experimental results on O 2 , described below,

measurements were made using laser and discharge light sources at several

wavelengths and intensities, we shall select a case here where the beam was known to be at an optimum intensity correspondi ng to a flux $ = 8 x 10 19

photons/(cm2 s ) at x = 595 nm. At this wavelength, the O

2

” photodetachment

cross section is known to be near its maximum value of ~1.5 x 10" 18 cm 2

[ 8 ].

The critical E/N for O

2 is approximately 1.2 x 10" 1 9 V 2 [9], and the

mobility y of O

2

" is nearly constant over a range of E/N which includes

this value [10]. For the present calculation, it was assumed that

y

= 2.0 cm 2 /(V/s). Data are also available on the cross sections and

rates for colli si onal detachment of 0o~ from which values for 5^ can be

determined [7,9,11]. In this calculation, a constant value of 5 x 10 18 cm 2 was assumed for 5d/N. The length a of the active region was taken to be

0.05 cm. Although this is much larger than the expected size of the active

region in our experiment, it should be noted that the result obtained from

equation (1) is quite insensitive to the assumed value for 1 .

Substituting the above values into equation (1) gives the result

Wp

c

= 3.5 x 10

“

5

, which means that for radiation intensities achievable,

less than one electron avalanche in 10 4 is likely to be initiated by a photodetachment event. Moreover, because of the i nhomogeniety of the electric

field, E/N will exceed ( E/N) c over most of the distance a, so that W

p c will be even smaller than estimated here. On the basis of this calculation,

therefore, one concludes that electron avalanche pulses initiated in O2 via photodetachment cannot be easily observed. Even if sufficient radiation

intensity were available, it would be difficult to observe photodetachment

in O

2

. This is because at the high laser beam intensities required for

photodetachment to compete favorably with collisional detachment, other

multiphoton ionization processes are likely to become important during times

that the gaps are exposed.

Attempts to observe electron avalanches resulting from photodetachment

of negative ions in O 2 have yielded the expected null results similar to

those found for SF5 [1]. Examples of typical results are shown in figure 3. In this figure, N

3 and N

2 are respectively the accumulated avalanche counts

in-phase and out-of -phase with the chopped light beam used to induce

photodetachment near the tip of the positive point electrode. The error

bars represent the calculated statistical uncertainties in the measurement,

namely, ±2 o, where a = (N^ + N

2 )" 1/2 . Since all the data points shown

lie within the interval ±2 <j, one must conclude that there were no statistically significant signals that could be attributed to photodetachment.

Because the equality of the time intervals for measurement of N 3 and N

2 could

10
(n-,

- n

2

)/(n

1

+

n

)2

+ 0.02

+ 0.01

0

- 0.01

0.02

0.01

0

-0.01

- 0.02

1 r

]

1 1 02 AT 200kPa _

> h T 5 z

—

? T a jl 2

A = 595nm _

— i >

i > i T S

L

It 5

295nm<A<365nm

1 1 1 13.0 14.0 15.0

APPLIED VOLTAGE (kV)

Figure 3. Relative difference in electron avalanche counts for light

beam on (N]_) and light beam off (N 2 ) measured for two

different indicated conditions of gap irradiation for a positive point electrode in O 2 at a pressure of 200 kPa (~2 atm).

The error bars indicate the statistical uncertainties in the

measurements

.

11
not be assured to better than ±0 . 1%, it would not have been possible to

observe fractional differences smaller than this even for arbitrarily large

Nj and N 2

. It should be noted that the data shown correspond to O 2 at a pressure of 200 kPa (~2 atm) in a gap where the electrode spacing was

1.5 cm and the point electrode radius of curvature was 0.45 mm. Similar

results were found at other gas pressures from 50 to 300 kPa.

The results of this investigation for O2 support our previous

interpretation [1] of the data for SFg. Since there is no reason to expect

the relevant cross sections for SFg to be drastically different from those

for O2 , it is reasonable to expect that photodetachment should also play a relatively minor role in the initiation of avalanches for SFg. Photodetachment

could dominate, and possibly be observed under conditions near discharge

inception where (E/N)d > (E/N) c , i.e., where 6d ~ 0 for electric fields

sufficiently high to allow ionization to occur. The limited data available [13]

on 6 d/N for the process 1

SFg + SFg - SFg + SFg + e ,

however, would suggest that ( E/N

)

cj < ( E/N

)

c

. Thus, unless there is a predominance of other negative ion species having lower 6^, such as clusters SFg (SFg)n, n = 1,2,..., there is no basis for expecting that

conditions exist for which photodetachment would dominate in SFg.

Again, as emphasized previously [2], these measurements have provided

us with a better understanding about the fundamental processes that occur in

irradiated discharge gaps. This information is useful from the poi nt-of-vi ew of designing breakdown tests for gaseous insulation. If, for example, one wants to enhance the breakdown probability using radiation, the data presented

here indicate that there are preferred methods of irradiation.

During the next quarter our activities will be focused into three main

areas. First, an archival paper will be prepared in which the results of our investigation of photon enhanced electron avalanche initiation around a positive point electrode in SFg and O2 will be described. Secondly, in an effort to bring activity 3) to completion, more measurements will be

performed to determine the pressure and power dependencies of the production

rates for SOF2 , S 02 ,F2 , SOF4 and H2 O from corona discharges in SFg. The

measurements will also be extended to include negative corona to determine

if there are any polarity dependent effects. Finally, a hygrometer probe

will be installed and tested in our corona discharge cell for measurement

of trace water vapor content in SFg. The probe will initially be used to

calibrate the gas chromatograph-mass spectrometer so that the measurements

in activity 3) as well as our previous investigations on effects of H 2 O on SFg corona [2] can be placed on a more quantitative basis. Work may also

begin on the design of a system to measure negative ion mobilities and

collisional detachment coefficients at high gas pressures.

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

12
4. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION

AND BREAKDOWN IN INSULATING SYSTEMS

Subtask No. A057

The objectives of this investigation are to develop apparatus and

appropriate procedures for the optical measurements of interfacial electric

field and space-charge density in materials for electric power equipment

and systems, to understand the interfacial prebreakdown and breakdown

processes in specific insulating systems, and to demonstrate the applicability

of the developed instrumentation and the procedures in the development and

design of future systems.

Recent effort has been focused on the development of consistent

description of the interfacial breakdown process. Previous results from

this project [14] have shown that breakdown does not necessarily occur

along or at a liquid/solid interface for a paper interface parallel to the

field. Initial measurements using pressboard, however, did show a tendency

toward interfacial breakdown. Electro-optical measurements were performed to

identify any electric field enhancement in the vicinity of the interface [2],

This is an important measurement because measurements in France [15] and in

Sweden [16] suggest that space charge effects may dominate the processes at

the interface. Electro-optical observations showed that the enhancement of

the field in the vicinity of the interface was undetectable, i.e., any

enhancement was less than a few percent of the field strength in the fluid

at points well removed from the interfacial region. This result led to the

current working hypothesis that any interfacial effect in this system is not

due to macroscopic field enhancement but is due to the properties of the

solid spacer or to the electrode-interface-liquid triple junction.

It is presently assumed, but not yet verified, that the triple junction

can be constructed in such a manner that it does not contribute to any

lowering of the breakdown strength. This was accomplished in the previous

work on paper interfaces. The properties of the solid spacer that have been

identified as possibly being significant include serving as a source for

bubbles or as an area in which the particle density is larger. The particle

density can be higher either because microscopic particles are trapped on the

spacer and are removed when the field is applied or because the electrohydrodynamic flow can cause particles to be deposited in the vicinity of the

interface [15].

NBS investigations into the processes of breakdown in insulating liquids

suggest that bubbles and particles may have similar effects on the breakdown

process. To isolate the effects of particles, aluminum, carbon, alumina,

and cellulose particles were introduced into such liquids as transformer oil,

hydrocarbon fluids, and water, and the breakdown process was photographed.

The general result from these studies is that a relatively large density

of particles stimulates the transition from the slower modes of streamer

propagation to the faster modes [17]. This has the effect of shortening

the time to breakdown. The process is reversible in that filtering the

fluid retards the transition of slow modes of streamer propagation to

faster modes.

13
Based on these results, it seems probable that bubbles at the pressboard

liquid interface act as conducting particles at high field strength and

because they are not readily mobile at the interface, they would enhance the

breakdown probability along the interface. This hypothesis will be evaluated

by replacing the pressboard interface. Careful attention will be paid to

handling the interfacial material and a new liquid-handling system will be

developed to allow degassing of the oil (as well as water removal) during the course of the experiment in order to dissolve any gaseous breakdown by-products.

In order to investigate the properties of paper interfaces at high

temperatures, a new cell will be designated permitting a changing of the

interface without opening the cell. All the above experiments will provide

us with information so that the field modifications due to an interface

alone may be measured electro-optical ly.

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

5. ACTIVE INSULATORS FOR INSULATION AND SURGE SUPPRESSION

Subtask No. A057

The objective of this investigation is to evaluate the practicality

of using active insulators, i.e., insulators made from materials whose

resistance is a function of the voltage across the device, to minimize

the effects of switching and lightning surges. The theoretical aspect

consists of the development of a mathematical model for a single phase

electric power system which can be used to predict the effects of active

insulators. The experimental activity is the development of a materials

research facility to fabricate insulation materials suitable for use in an active insulator. This work is being performed by the Department of

Electrical Engineering, University of Southern California, under a grant

from the National Bureau of Standards.

The single-phase transmission line mathematical model with active

insulator string was developed based on Bergeron's method. The equivalent

circuit for such a transmission line is shown in figure 4 for line segments

and figure 5 for a voltage source with breaker resistance R^ and source inductance L. The modified surge impedance V is equal to Z (line surge

impedance) plus R (line segment resistance), and the line loss factor is:

Z - R/2

S = . (6)

Z + R/2

A series of voltage and current equations were derived with only one unknown variable and the rest of the equation consisted of either line

constants and/or known variables. The nodal voltage equation was nonlinear

and was solved using Newton-Ral phson 1 s iteration method. Since the nonlinear

equation would have more than one solution, the initial value for the

iteration was critical in order to obtain a reasonable solution. Thus far,

the best initial value was the nodal voltage at t-At, where At was the

incremental time step.

14
'n- 1 ,nW ’n,n— 1 (*) 'n,n+1 (t) ’n+ 1 ,n^)

n - 1 - —* -«

Figure 4. Equivalent circuit for single phase transmission line with an active insulator.

15
,2 (t-T)

Figure 5. Equivalent circuit for a voltage source with breaker resistance

R(3 and a source inductance L.

16
The analysis was carried out using computer programs based on this model.

The transmission line was divided equally into nine sections. The sending

end and receiving end arresters were assumed to have the same current-voltage

characteristics as the active insulator strings. For comparison purposes,

the station arresters were also included under all conditions. The overvoltages

under switching conditions were studied under two different switching sequences,

namely, (1) switch closed at maximum voltage level and (2) switch closed at

maximum voltage level with subsequent switch open at current zero level and

closed at next current zero level. Results showed that under the first

switching sequence the maximum overvoltages on the transmission line were

1.4 p.u. (p.u. = per unit, i.e., 1.4 p.u. is 1.4 times the normal operating

voltage) with eight active insulator strings evenly positioned between

the two terminals. The maximum overvoltage was 1.75 p.u. without any active

insulators. The overvoltage was about the same throughout the line when

active insulators were employed. Without them, however, line segments

between the terminals sustained considerably higher overvoltage. The peak

current passing through arresters was 350 amperes without active insulators

as compared to about 40 amperes with active insulators. Under the closeopen-reclose sequence, results showed that the maximum overvoltages were

1.45 p.u. with active insulators and 2.25 p.u. without active insulators.

Both of these maximum overvoltages occurred after reclosure. These results

indicate that active insulators could reduce the transient overvoltage

considerably and the thermal ratings of station arresters could be greatly

reduced.

The material research facility has been set up with most of the necessary

equipment installed. Plate-type samples were fabricated for electrical testing.

Electrical tests showed that the material produced had a dielectric constant of

less than 4.0 and volume resistivity of greater than 10 13 n-cm.

Plans for the next quarter include extension of the single-phase transient

model to include lightning transients, for lightning strikes either on the

conductor or on the tower. The three-phase model will be developed next for

switching transient conditions. In addition, the materials fabrication

facility will be completed and the procedure and formulations will be improved

to reach comparable values as other insulating materials become available.

For further information, contact Dr. T. C. Cheng, (213) 743-6938.

17
6. REFERENCES

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

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

[2] Development of Power System Measurements — Quarterly Report,

April 1, 1982 to June 30, 1982, R. E. Hebner, Editor, Nat. Bur.

Stand. (U.S.), NBSIR 82-2586 (Oct. 1982).

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

7th Int. Conf. on Gas Discharges and Their Applications, Peter Peregrinus

Ltd., pp. 255-258, 1982.

[4] R. J. Van Brunt and D. A. Leep, Corona-Induced Decomposition of SFg,

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

Dielectrics, ed. by L. G. Chri stophorou , Pergamon Press, pp. 402-409, 1982.

[5] R. J. Van Brunt and M. Misakian, Mechanisms for Inception of DC and

60-Hz AC Corona in SFg, IEEE Trans. Elec. Insul., Vol. E

I - 1 7 , No. 2,

pp. 106-120, Apr. 1982.

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

Pulses in SFg, J. Appl. Phys. Vol. 52, No. 11, pp. 6588-6600, Nov. 1981.

[7] W. F. Schmidt and R. J. Van Brunt, Comments on the Effect of Electron

Detachment in Initiating Breakdown in Gaseous Dielectrics, Gaseous

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

ed. by L. G. Chri stophorou , Pergamon Press, pp. 561-563, 1982.

[8] D. W. Goodson, R. J. Corbin, and L. Frommhold, Electron Avalanches in

Oxygen: Detachment from Diatomic ion O2 , Phys. Rev. A, Vol. 9, No. 5,

pp. 1049-2059, May 1974.

[9] P. C. Cosby, R. A. Bennett, J. R. Peterson, and J. T. Moseley,

Photodissociation and Photodetachment of Molecular Negative Ions. II.

Ions Formed in Oxygen, J. Chem. Phys., Vol. 63, No. 4, pp. 1612-1620,

August 1975.

[10] J. W. Gallagher, E. C. Beaty, J. Dutton, and L. C. Pitchford, A Survey

of Electron Swarm Data in Electronegative Gases, J. Chem. and Phys.

Ref. Data (in press).

[11] L. G. McKnight, Drift Velocities and Interactions of Negative Ions in

Oxygen, Phys. Rev. A, Vol. 2, No. 3, pp. 762-770, Sept. 1970.

[12] J. A. Rees, Fundamental Processes in the Electrical Breakdown of Gases,

Electrical Breakdown of Gases, ed. by J. M. Meek and J. D. Craggs,

John Wiley and Sons, pp. 1-128, 1978.

[13] B. C. O'Neill and J. D. Craggs, Collisional Detachment of Electrons

in Sulphur Hexafluoride, J. Phys. B: Atom. Molec. Phys., Vol. 6,

pp. 2634-2640, Dec. 1973.

18
[14] E. F. Kelley and R. E. Hebner, Electrical Breakdown in Composite

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

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

[15] J. D. Cross, Breakdown at Liquid/Solid Interfaces and the Role of EHD

in Liquid Breakdown, 1982 Conference on Electrical Insulation and

Dielectric Phenomena (in press).

[16] U. Gafvert, Oil and Particle Motion Close to Electrode Surfaces,

1982 Conference on Electrical Insulation and Dielectric Phenomena

(in press).

[17] E. F. Kelley and R. E. Hebner, Prebreakdown Phenomena Between

Sphere-Sphere Electrodes in Transformer Oil, Appl . Phys. Lett.,

Vol. 38, pp. 231-233, 1981.

19
.

,
U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REKflf%3-2705 May 1983

4. TITLE AND SUBTITLE

Development of Power System Measurements -- Quarterly Report

July 1, 1982 to September 30, 1982

5. AUTHOR(S)

R. E. Hebner, Editor

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

DEPARTMENT OF COMMERCE 8 WASHINGTON, D.C. 20234

9.

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

T.O. No. 137

. Type of Report & Period Covered

Quarterly

7/1/82 - 9/30/82

Department of Energy

Division of Electric Energy Systems

1000 Independence Ave., S.W.

Washington, DC 20585

10.

SUPPLEMENTARY NOTES

’

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

11.

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

bibliography or literature survey, mention it here)

This report documents the progress on four technical

investigations sponsored by the Department of Energy. Three were

performed by the Electrosystems Division, the National Bureau of

Standards and the fourth by the Department of Electrical Engineering

of the University of Southern California. The work described covers

the period from July 1 , 1982 to September 30, 1982. The report

emphasizes the calibration of instruments designed to measure

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

determination of the role of photodetachment of SFg corona discharges,

the measurement of failure mechanisms in liquid/solid insulating

systems, and the development and behavior of active insulators.

12.

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

insulation; liquid breakdown; SFg; space charge; transformer oil.

13.

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