A NBSIR 82-2586

HlDt 03T455

Development of Power System

Measurements -- Quarterly Report

April 1, 1982 to June 30, 1982

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Eiectrosystems Division

Washington, DC 20234

October 1 982

Prepared for: Department of Energy

Division of Electric Energy Systems

QC Independence Avenue, S.W.

2(JJ ington, DC 20585

U56

62-2566

1562

i

NBSIR 82-2586

DEVELOPMENT OF POWER SYSTEM

MEASUREMENTS - QUARTERLY REPORT

APRIL 1, 1982 TO JUNE 30, 1982

NATIONAl BUREAU Qt STANDARDS

UjBAjtr

NOV 4 1982

Q C ioo

R. E. Hebner, Editor

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

October 1 982

Prepared for: Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, S.W.

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
>. ' ‘ * '

• <U^.< -J
Foreword

This report is intended to summarize the progress on four technical

investigations during the third 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

Abstract 1

1. INTRODUCTION 1

2. DC FIELDS AND ION MEASUREMENTS

Project No: A018 1

3. INCIPIENT FAULT DETECTI ON/ LOCATI ON

Project No: A063 5

4. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Project No: A053

5. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

Project No: A057 15

6. REFERENCES 20
LIST OF FIGURES

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Page

Measured space charge density for three different flow

rates as a function of ion counter voltage (relative to

ground). Data are for flow rates of 16.5 x 10

“ 3 m

3 /s - ;

4.4 x 10 - 3 m

3 /s - 0; 2.5 x 10 “ 3 m

3 /s - • 3

Comparison of space charge density measurements for

front and top inlets to the ion counter. The flow

rate was 16.5 x 10

" 3 m

3 /s. Top opening

front opening - • 4

Experimental arrangement for investigating the effect

of radiation on discharge initiation in SF5

9

Voltage dependence of fractional differences between

electron avalanche count rates recorded by the in-phase

and out-of-phase counters for a chopped uv beam irradiating

the tip of the point electrode and for no irradiation.

The wavelength band of the radiation was from 295 to

575 nm; the gas pressure was 200 kPa; and the point

radius of curvature and gap spacing were 0.71 mm and

12.9 mm, respectively 11

Voltage dependence of count rates recorded by the

in-phase and out-of-phase counters for filtered

and unfiltered, chopped uv beams irradiating

the cathode 13

Electrode geometry with pressboard interface as viewed from the detector. This is an end view of

the ~30 cm plates with a pressboard interface

extending 28 cm of the length. The notch in the

electrode serves to produce almost zero field region.

The light intensity as a function of position along

different tracks A, B, and C can be measured by the

detector and compared with one another to obtain

relative electro-optical field measurements 16

Computer calculation of the equi potenti al s for a notch

in a parallel -pi ate electrode system assuming no space charge. The field is seen to be unaffected

along the plate opposite the notch and essentially

zero at the interior of the notch. The inset shows

higher resolution of equi potential s where the dotted

equi potentials are the ones shown in the main drawing 17

v
LIST OF FIGURES (cont)

Page

Figure 8. Comparison of electric field next to the interface and

away from the interface in nitrobenzene dopped-transf ormer

oil. Although there is an obvious, negati ve-spacecharge-i nduced field distortion which is spread somewhat

uniformly throughout the gap, the field distortion is

not different near the interface from what it is

away from the interface (within 10%) 19

vi
DEVELOPMENT OF POWER SYSTEM MEASUREMENTS — QUARTERLY REPORT

April 1, 1982 TO June 30, 1982

R. E. Hebner, Editor

This report documents the progress on four 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 April 1, 1982 to June 30, 1982.

This report emphasizes the errors associated with measurements of

space charge near dc transmission lines, the measurement of rf

attenuation in distribution cables, the characteristics of positive

and negative corona in compressed SFg gas, and the measurement of the

space charge density in transformer oil subjected to 60-Hz excitation.

Key words: cables; dc fields; high voltage; incipient fault;

insulation; SFg; 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 four areas — the

measurement of electric fields, the measurement of the electromagnetic

properties of solid insulating materials and cables, the measurement of

interfacial electrostatic field distributions, and of space charge density.

The technical progress made during the quarter April 1 to June 30, 1982

is summarized in this report.

2. DC FIELDS AND ION MEASUREMENTS

Project No: A018

The objectives of this effort are to investigate devices and

measurement techniques which may be used to characterize the electrical

environment near high-voltage dc (HVDC) transmission lines, to evaluate

methods being used for calibrating such devices, and to develop and

establish calibration facilities at NBS which will permit independent

verification of the accuracy of user calibrations.

By developing appropriate methods of instrument calibration, it will

be possible to compare measurements taken in the vicinity of a high-voltage

transmission line with measurements taken in biological exposure facilities.

Such a comparison will be necessary in any formal or informal risk assessment.

During this quarter, primary effort has been directed at the problem of

measuring space charge densities near a HVDC transmission line by using an ion counter. These studies are a continuation of those done previously [1 J

.

1
The purpose of these studies is to determine the effect of the large electric

fields existing near a HVDC transmission line on measurements of ion density.

Ion density is measured by using an ion counter, which is an aspiration

device. Air containing ions is drawn into the ion counter and ions are precipitated onto collector plates by an internal electric field. Near the

inlet of the ion counter, ions are subjected to competing forces: the

hydrodynamic forces of the air flowing into the counter and the electric

forces due to the external electric field. If the speed of the ions due to

the electric field is much larger than that due to the air flow, then it can

be expected that ions are driven to the counter walls, rather than entering

the air stream. In these studies, the effects of varying the air speed at

the inlet and the potential of the ion counter assembly were investigated.

(The potential referred to here is that of the overall counter system, not

the voltage that exists inside the counter to remove the ions from the air

stream)

.

A monopolar line 2 m high and 6 m long was erected in an open space

in a high bay area to minimize the effect of surrounding objects. An

ion counter and associated electronics were arranged so that the potential

of the counter could be varied from +25 kV to -25 kV. In addition, an extension was placed on the counter with both front- and top-facing

openings. For the tests reported here, the ion counter inlet was located

under the center of the line and the top of the counter was approximately

0.43 m above ground. The flow rate through the ion counter could be

varied by approximately a factor of 10. By using this system, it is

possible to investigate the effects of ion counter voltage, geometry,

and flow rate on indicated ion density. Here geometry refers both to the

ion counter position and the location of the inlet.

Electric field and ion current density were measured under the monopolar

line. It was found that both electric field E and current density J were uniform to within ~±10 percent as the measurement position was varied in a direction parallel to the line. In the transverse direction, measurements

showed that both E and J peaked under the line as expected. For a line

voltage of -60 kV, the electric field under the line was approximately -30 kV/m.

For a fixed line voltage, ion counter flow rate, and geometry, the spacecharge density was determined as a function of the ion counter voltage.

These measurements were repeated for different flow rates, geometries, and

line voltages.

An example of the results obtained, figure 1, shows the measured

space-charge density p as a function of the voltage of the ion counter

assembly for three different flow rates. Clearly, the space-charge density

depends on the flow rate. Also, the indicated ion density drops

significantly as the voltage approaches -20 kV. These data were obtained

with an inlet on the top of the ion counter housing and -60 kV on the line.

The effects of inlet location are shown in figure 2. Plotted here are the measured ion densities as a function of ion counter voltage for a top

opening and a front opening. The air flow was the same for the two measurements

and line voltage was -60 kV.

c
Ion Counter Voltage (kV)

Figure 1. Measured space charge density for three different flow rates

as a function of ion counter voltage (relative to ground).

Data are for flow rates of 16.5 x 10”° m^/s - ;

4.4 x 10“3 m^/s - 0; 2.5 x 10"3 m^/s - •.

3
Figure 2. Comparison of space charge density measurements for front and

top inlets to the ion counter. The flow rate was 16.5 x 10"3 nr/s.

Top opening - ; front opening - •.

4
Space potential near the counter inlet was measured using an electrostatic

voltmeter. It varied from 10.2-13.5 kV over the vertical extent of the

front opening. By contrast, the ion counter assembly assumed a potential of

-20 kV when allowed to float above ground under the same conditions. These

observations are consistent with the theory of charging of a particle in a uniform field with a unipolar ion flux which predicts an isolated object will

equilibrate at a potential greater than the unperturbed midplane potential [2],

Two questions arise from this (and previous) work. The first has to do

with attempting to bias an ion counter to space potential to minimize measurement errors. The second, and more important one for present day interest,

concerns the flow dependence of the measured space charge density. Estimates

of losses in the ion counter assembly due to space charge effects give results

which are too low to explain the observations. It appears more likely that

the strong electric fields near the ion counter cause a loss of ions to the

counter inlet. This effect becomes less important as the hydrodynamic forces

become more important, i.e., the flow rate increases.

Many questions have been raised concerning ion density measurements

under a HVDC transmission line as a result of this work. Future work will

concentrate on resolving the cause of the flow rate dependence and the

geometry effect. Some computer modeling will be done, although it will

be necessary to use a greatly simplified model of the actual physical

situation.

For further information, contact Dr. R. H. McKnight, (301) 921-3121.

3. INCIPIENT FAULT DETECTI ON/ LOCATION

Project No: A063

The objective of this program is to identify and, insofar as practical,

remove technical barriers to the development of a successful incipient fault

detector/1 ocator for underground power transmission use. NBS responsibility

includes conducting an experimental program which will aid the development of an incipient fault detection/location system by measuring the rf characteristics

of power cables and evaluating the frequency content of partial discharge pulses

emanating from incipient fault sites in a cable dielectric. The measurement

program at NBS is now completed. The limitations of time-domain ref 1 ectometry

(TDR) techniques for detecting incipient fault sites in transmission cables

have been determined. The radio frequency (rf) responses of these cables

have also been measured. This quarter the studies of the effect of sampling

rate drift on transfer function measurements have been completed.

In order to measure the transfer function of a power cable using TDR

techniques, two waveforms are required (e.g., the input and output waveforms

or the output waveforms from two different lengths of cables). In our measurements, the two time-domain waveforms (shaped like step functions)

are Fourier transformed to the frequency domain and their ratio yields the

transfer function. In our early efforts, we noted that when making very

precise measurements (i.e., ±0.1 dB), the measured transfer function

often showed a steady gain or roll-off with frequency. The fact that a gain implied more energy was leaving the cable than entering it suggested

5
that something was wrong with the measurement approach. The problem was traced to sampling rate drift in the digitizing oscilloscope. If the

sweep speed changes between the measurement of the two waveforms, then

the time interval between successive samples will change. Such a drift

in sampling rate can be shown to yield a similar drift in the transformed

frequency domain transfer function.

One approach to solving this problem is to apply a correction in the

frequency domain. That is, if the measured transfer function shows a rolloff or gain with frequency, multiply it by a suitable frequency-dependent

correcti on-f actor to remove the effect. In order to do this, one must

be certain that the transfer function is supposed to be flat. One must,

therefore, have prior knowledge of the transfer function before the

measurement. For that reason, this approach seems less than sati sf actory

.

As described last quarter, our approach has been to correct the data in

the time domain. In our measurements, the first 100 or so points of the

sampled time-domain waveforms should be the same for both measured waveforms

since these data points represent the measured response of the cables and

connectors between the TOR unit and the cable being measured. If no sampling

rate drift has occurred, these points will be identical. By contracting or expanding the sampling interval of the second waveform with the aid of linear

or cubic interpolation, the first 100 points of the second waveform should

align with the first 100 points of the first waveform. There is one major

complication, however. The two waveforms are often horizontally displaced

with respect to one another (i.e., the "step" does not occur at the same horizontal position on the oscilloscope screen). Our approach was to do

a least squares fit of the two waveforms by first sliding the second

waveform left or right until the correct horizontal displacement was determined, and then doing a least squares fit by expanding or contracting

the sampling interval of the second waveform to agree with the first.

Unf ortunately , we found that these two operations are not independent

(i.e., the best fit sampling drift correction is dependent on the horizontal

displacement correction and vice versa).

In order to do a least squares fit with two dependent variables, we implemented a version of the Marquart algorithm available on the main NBS

computer. This software worked very well in quickly finding correction

parameters yielding a better fit than our previous approach. The resulting

transfer functions from these calculated correction parameters appeared to be

free of the effect of sampling rate drift. Unfortunately, the signal -to-noise

ratio was decreased for the transfer functions derived from the corrected

waveforms. This may be a result of the interpolation scheme used to create

the corrected waveforms, although we attempted to make the interpolation as mathematically smooth as possible.

We have shown that sampling rate drift can affect the results of precision

transfer function measurements in a predictable manner. This by itself is an important contribution. We have, also, demonstrated that this drift may

be corrected for in the time domain without the a priori knowledge which is

required for a f reqnency-domai n correction. Our results indicate, however,

that a perfect correction for sampling rate drift is difficult if not impossible

(i.e., the corrected waveform has a significantly lower signal -to-noi se ratio).

6
This suggests that the experimenter should attempt to avoid the problem if at

all possible as opposed to attempting to correct for it in software. This

might be accomplished by using an oscilloscope with a more stable sweep

speed or using a stable time source to calibrate the oscilloscope sweep

speed for each acquired waveform.

For further information, contact Dr. W. E. Anderson, (301) 921-3121.

4. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Project 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 and may

lead 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.

Planned activities for FY 82 include: 1) completion of an archival

paper on basic mechanism of corona inception in SFg under ac and dc

conditions; 2) investigation of the wavelength dependence of photon-induced

positive corona inception in SFg; 3) continuation of measurement of power

dependence of the rates of oxyfluoride and water vapor production from

corona discharges in SFg and report the results in a conference and an archival paper; 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 of the accuracy of quantitative

analysis of trace gases such as H2 O in SFg and other gaseous dielectrics

using a gas chromatograph/mass spectrometer (GC/MS), and to extend the

measurements of the effects of trace H2 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 essentially brought

activity 2) to completion. The results obtained from these measurements

will be highlighted in this report.

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

"Effects of H2 O on the Behavior of SFg Corona," by R. J. Van Brunt was accepted for inclusion in the Proceedings of the Seventh International

Conference on Gas Discharges and Their Applications, which was held in

London, August 31 - September 3, 1982. Information was acquired on various

standard and common methods used to quantitatively measure moisture content

in static SFg gas samples. It was decided that a calibrated, thin film,

aluminum oxide hygrometer probe should be purchased which can be used

simultaneously with the present gas chromatograph/mass spectrometer and

7
serve as a secondary standard for calibration of the GC/MS for cases where

H2 O is the only contaminant present in SF5

. The GC/MS could then be

used to evaluate the performance of the hygrometer probe when other gaseous

polar contaminants are present, such as generated during an arc or corona

di scharge.

A means of interfacing the NBS GC/MS data processor system to external

computers was established during the past reporting period. This will

allow more efficient and rapid handling of GC/MS data acquired from

quantitative measurements of corona-induced end decomposition products

in gaseous dielectrics. It also gives more flexibility to the data

analysis procedure. Development of new software for GC/MS data analysis

is currently in progress.

Major emphasis during this quarter was given to performing measurements

relevant to activity 2 ) on investigation of the wavelength dependence of

photon-enhanced electron avalanche initiation around a positive point

electrode in pressurized SFg. Previous measurements performed in our laboratory have shown [3,4] that initiation of electron avalanches near inception of positive point-plane corona in SF 5 can be dramatically

enhanced by exposing the region near the tip of the point electrode to

radiation from an Hg-discharge lamp in the range of 295 to 750 nm. It was speculated that this enhancement was due to an increase in the rate of

electron release near the point by the mechanism of photodetachment from

negative ions, e.g.,

hv + SFg“ > SFg + e.

An increase in the probability of discharge initiation for positive dc and

impulse breakdown in SF 5 when uv-radiation is directed at the more highly

stressed electrode has also been observed by others [4].

The experiment described here was designed to determine the relative

importance of photodetachment as an electron release mechanism near a positive point electrode in S Fg and other electronegative gases. Up to

the present time results have been obtained in our laboratory only for SF5

.

A diagram of the experimental setup is shown in ^igure 3. The plane electrode

(cathode) could be uniformly irradiated with uv-radiation from an Hg-discharge

lamp, the output of which was predominantly at a wavelength of 253.7 nm. This

radiation was used to control the negative ion density in the i nterel ectrode

gap through release of low-energy photoelectrons that immediately attach to

SFg molecules in the gas.

The region near, and including, the tip of the point electrode could

be irradiated with a chopped and collimated beam from either a tunable dye

laser with a range from 570 to 630 nm or a high-power, broadband Hg-discharge

lamp with output in the range of 750 to 295 nm. The latter source was used to produce the effect previously reported [3,4], although in those

experiments, no attempt had been made to insure that the beam was sufficiently

collimated to avoid hitting surfaces other than the point electrode, e.g.,

the cathode surface. The purpose of the beam directed at the point was to

induce photodetachment. With the use of the chopper and detection scheme

shown in figure 3, it was possible to measure the difference between the

8
CHOPPER WHEEL

UV LAMP

OR LASER

Figure 3. Experimental arrangement for investigating the effect of radiation

on discharge initiation in SFg.

9
rates of electron avalanche formation for irradiated and unirradiated

conditions near the point. The count rates for the irradiated and

unirradiated point electrode were recorded respectively by the in-phase

and out-of-phase counters indicated in figure 3.

When the Hg-lamp was used, it was possible to restrict the band of

wavelength of the incident radiation with optical filters. Otherwise, the

radiation was simply transmitted through a quartz window. Both light sources could be blocked in order to check the background avalanche rates.

It was also possible to direct the collimated beam to locations in the gap

other than the tip of the point electrode. All avalanches with a charge

level in excess of 0.02 pC were detected and recorded by the counters.

The trigger level of the oscilloscope served as a common discrimi nator for

both counters. Care was taken to eliminate phase sensitive noise, e.g.,

pickup from the chopper motor. Polished stainless steel electrodes were

used and measurements were performed for gap spacings of 12.9, 12 . 2 , and

13.5 mm, with corresponding point electrode radii of curvature of 0.720,

0.475, and 0.043 mm, respectively.

The results for SFg at pressures in the range between 100 and 300 kPa

indicate that photodetachment of negative ions plays a very minor role in

the initiation of electron avalanches near a positive electrode. This

process, therefore, cannot be invoked to explain the photo-enhancement of

electron avalanche rates previously reported [3,4]. In all cases studied,

less than 5 percent of the observed avalanches could be attributed to an interaction of the light beam with the gas in the immediate vicinity of

the positive electrode. In most cases, no statistically significant signals

were observed. Weak evidence of photodetachment was sometimes seen, but

only at voltages close to the threshold for avalanche formation and only

for wavelengths shorter than 350 nm. An example of a typical result is shown

in figure 4. Plotted in this figure are the fractional differences in the

count rates recorded by the in-phase and out-of-phase counters as a function

of applied voltage for unfiltered uv-radiation, i.e., (N]_ - N 2 ) /

(

N 2 ),

where and N

2 are the accumulated counts from the in-phase and out-of-phase

counters, respectively. The error bars represent the calculated uncertainties,

namely, ±2 a, where a = (Nj + N

2 ) _i

. Points above the axis that lie

outside the error bars are considered to be statistically significant. Also

shown by the open symbols are data obtained when the incident beam was blocked.

Because the cross sections for photodetachment of negative ions are known to be relatively small (< 10~1° cm^) [5], it is perhaps not too

surprising that the contributions from this process were found to be negligible.

Although the cross sections for photodetachment of SFg“ and its clusters

with other molecules, in particular SF5 , are not known at the present time,

there is no a priori reason to expect the cross sections for these molecular

ions to differ appreciably from those of other species for which data exist.

Moreover, recent calculations [7] of the relative energies of SF^" and SF^

electronic states have yielded a photodetachment threshold of 3.0 eV for

SFg“. If this is true, then one would not expect to observe photodetachment

for incident radiation with wavelengths shorter than 400 nm. For the

unfiltered radiation source used, about 47 percent of the radiative intensity

is due to wavelengths shorter than this value, and most of this is concentrated

in the range 355-365 nm. For wavelengths shorter than 350 nm the maximum

10
|Ni-N

2

)/(N,+N )2

JL 1 I I I I L

31 32 33 34

APPLIED VOLTAGE (kV)

Figure 4. Voltage dependence of fractional differences between electron

avalanche count rates recorded by the in-phase and out-of -phase

counters for a chopped uv beam irradiating the tip of the point

electrode and for no irradiation. The wavelength band of the

radiation was from 295 to 575 nm; the gas pressure was 200 kPa;

and the point radius of curvature and gap spacing were 0.71 mm and

1.29mm, respectively.

11
obtainable intensity is less than 140 mW/steradian. It thus appears that

there is insufficient photon flux in the range below 400 nm from the light source used here to observe a significant effect due to photodetachment.

The previously observed effect of uv-radiation on electron avalanche

enhancement in SFg was probably the result of photoelectron release from

the cathode surface by scattered radiation. As previously noted, these

electrons are expected to be of low energy (<0.5 eV) and will therefore

readily attach to gas molecules to form negative ions. These ions will

drift into the high field region near the point electrode where they may

then acquire sufficient translational kinetic energy to allow detachment

to occur via collisions with gas molecules. It therefore appears that

when radiation is present at the intensities used in these experiments, the

dominant electron release mechanism for avalanche initiation is still

collisional detachment.

If, as seems to be the case here, the effect of the radiation is merely

to increase the negative ion density in the gap, the optimum enhancement

in the avalanche rate should be achieved by directing the chopped uv-beam

at the cathode surface rather than at the point. Some measurements were

performed for a configuration where the gap was irradiated only by a collimated

beam directed at the cathode. An example of results obtained from such an experiment are shown in figure 5. These indicate that if an optical filter

is used which eliminates wavelengths shorter than 350 nm, no avalanching

is observed to take place over the range of voltages shown. Thus, photons

with energies in excess of 3.5 eV are apparently needed for photoelectron

ejection from the stainless steel electrode.

It is also seen from figure 5 that when photons with an energy between

3.5 and 4.2 eV are allowed to impinge on the cathode, electron avalanches

appear at voltages below 30 kV only when the uv-light beam is on, i.e.,

only during the in-phase condition. Above 30 kV avalanches also begin to

appear during the out-of-phase condition, i.e., when the uv-beam is cut off.

For this particular measurement there was a delay of about 1.0 ms between

the time that the uv-beam was cut off and the time that the out-of-phase

counter was gated on. It appears that when the number of negative ions

created in avalanches during the uv exposure becomes sufficiently high,

there is reasonable probability that some ions will remain in the gap to

initiate avalanches 1 ms or more after the beam is turned off. This is

not unexpected since our previous calculations [4] based on known ion

mobilities have shown that ion transit times can be up to ~1 ms for gap

field conditions such as existed for this measurement. Furthermore, some reduction in the electric field in the gap which will cause an increase in

ion residence times can be expected if the ion space charge is allowed to

build up during avalanching. On the other hand, the effect observed here

may be at least in part due to interactions of positive ions or metastables

with the cathode which cause release of secondary electrons even if the

radiation is turned off. Further studies of these "delayed" avalanches

might be of interest because of the information that can possibly be extracted

about ion transit times as well as times required for ion space charge to

clear the gap.

The results reported here indicate that in the measurement of pointplane breakdown in SFg or other highly electronegative gases, a greater

12
Fiqure 5. Voltage dependence of count rates recorded by the in-phase and out

of-phase counters for filtered and unfiltered, chopped uv beams

irradiating the cathode.

13
enhancement in the breakdown probability can be achieved by irradiating

the cathode rather than the point electrode as is now the common practice.

This suggestion of course only applies to situations such as existed for

the measurements discussed here where the radiation is not of sufficient

energy to cause ionization of the gas.

During the next quarter, the measurements of the effect of uv-radiation

on discharge initiation in SFg will be completed and extended to include

O2 and O2/N2 mixtures. Again, for these gases an attempt will be made

to determine the possible importance of photodetachment in initiation of

avalanches in an irradiated gap. The photodetachment cross sections and

thresholds are well known for negative ions from oxygen. Thus, there is some hope in this case of predicting the contribution to avalanche formation

from photodetachment. Some of the measurements on SFg may be repeated

with a more intense uv source. More measurements will be performed to

determine the discharge power dependence of gaseous by-product production

rates in SFg corona. The development of software for analyzing GC/MS

data should be completed. This will allow a more rapid and efficient

handling of data. Improvements in the methods of calibrating the GC/MS

will be explored. A thin film aluminum oxide hygrometer probe will be

acquired for use in performing in-situ absolute calibrations of the GC/MS

response to trace water vapor content in SFg. By this method, it should

be possible to calibrate the GC/MS for H2O concentrations down to 10 ppm.

It is expected that the results of measurements performed here to

investigate effects of irradiation on discharge initiation will prove useful

in designing tests to measure the dielectric strengths of electronegative

gases used for insulating high-voltage systems. To achieve conditions of

gap irradiation which optimize the probability for breakdown requires a better understanding of the mechanisms responsible for discharge initiation

when radiation is present.

The effects of contaminants, such as water vapor and air, on the electrical

properties of gaseous dielectrics are also not well understood, particularly

for conditions where these are present only at trace levels. These effects

need to be better understood from a fundamental poi nt-of-view before one can decide on meaningful standards or requirements for gas purity under

various practical operating conditions. This question is quite important

in the case of H2O contamination of SFg which is now commonly used in

insulating systems. Water vapor which can be introduced during gas handling

even under the most careful procedures is very difficult to avoid. Although

H2O vapor is known to affect the static, uniform field dielectric strength

of SFg only slightly, there is evidence that it affects impulse breakdown,

the intensity of corona, and the rate of gaseous decomposition during a discharge. The influence of water on the statistics of discharge initiation

in gaseous dielectrics needs to be further investigated as a function of

controlled water concentration in order to isolate and understand its

effects on time-to-breakdown under impulse conditions and on the growth of

electron avalanches and streamers in nonuniform fields. Modeling of avalanche

growth in water-contaminated SFg appears feasible and should be considered

for the future. Methods for measuring trace levels of water in SFg and

other gaseous dielectrics need to be evaluated, particularly under conditions

where other polar contaminant species may be present.

14
Improvements need to be made in techniques for analyzing quantitatively

the presence of gaseous contaminants in gas insulation at trace levels. This

is important for the experiments designed to measure the rates of production

of decomposition products under a wide range of discharge conditions. More

reliable quantitative data are needed on these rates before tests can be

designed which use gas analysis as a diagnostic technique for fault detection

in practical gas insulated power systems.

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

5. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION

AND BREAKDOWN IN INSULATING SYSTEMS

Project 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 applicablity of the

instrumentation developed and the procedures in the development and design

of future systems.

Previous electro-optical measurements of the electric field in transformer

oil were confined to uniform fields without the presence of an interface. Such

data provided an understanding of space charge in transformer oil. It was found that space charge was present in quantities which would change the uniform

field by more than 10 percent only when the temperature of the oil exceeded 100°C.

This quarter's activities centered on examining the electric field in the

presence of an interface. Figure 6 shows the electrode geometry and a view

of the detector. The scanning characteristics of the two-dimensional detector

permit the isolated viewing of different portions of the i nterelectrode region.

In this way, electro-optical electric field measurements as a function of

position may be made for different locations in the gap. One electrode is

notched along its length parallel to the light beam in order to provide a nearly zero-field region. This notch provides a means to check the accuracy

of the measurement system.

In figure 7, we show the results of an equi potenti al calculation [8] for

the notched, paral 1 el -pi ate electrode system. Symmetry is assumed about the

center of the notch. The results of this plot show the field reduction at

the full depth of the notch. Experimentally, the electro-optical observations

of such a low-field region will not show a change in light intensity as the

electric field increases in the gap.

For the measurements reported here, the interface material was 3.2 mm

(1/8 in) thick pressboard which was soaked in 120°C transformer oil overnight.

Two thicknesses were used to separate the plates by 6.4 mm. The cell was heated

to 106°C and dc voltage was applied. Upon the application of voltage, bubbles

were observed to be emitted by the pressboard — but only when voltage was applied.

Because of these bubbles, the cell would not withstand 20 kV initially. After

a few days, the bubbling stopped and 30 kV dc could be applied for short

periods. Unfortunately, 30 kV is really a minimum voltage for obtaining

15
NOTCH

Figure 6. Electrode geometry with pressboard interface as viewed from the

detector. This is an end view of the ~30 cm plates with a pressboard interface extending 28 cm of the length. The notch

in the electrode serves to produce almost zero field region.

The light intensity as a function of position along different

tracks A, B, and C can be measured by the detector and compared

with one another to obtain relative electro-optical field

measurements.

16
y

Figure 7. Computer calculation of the equipotenti al s for a notch in a parallel-plate electrode system assuming no space charge. The

field is seen to be unaffected along the plate opposite the

notch and essentially zero at the interior of the notch. The

inset shows higher resolution of equipotential s where the dotted

equi potenti al s are the ones shown in the main drawing.

17
accurate field measurements with a 6 mm gap (50 kV/cm) because of the small

Kerr effect in transformer oil. For larger fields, it will be necessary

to develop better i nterf ace-preparation and liquid-handling methods.

To enhance the observability of the Kerr effect, the transformer oil was

doped with nitrobenzene. Electro-optical observations were made along a strip directly above the interface and a strip about one gap spacing above

the interface — tracks A and B in figure 6. The relative electric field as

a function of position between the electrodes for these two strips is shown

in figure 3. To within 10 percent of the average field, the fields have the same

spatial distribution. Based on our previous calculations [9], it is possible

to show that a 10 percent field shift would represent a uniform surface charge

of about 3 nC/cm^.

During the next quarter, these measurements will be refined in the

following ways. First, the usefulness of the notched (zero field) area in

improving the accuracy of the electric field measurements will be evaluated

quantitatively. Second, the effect of various techniques to prepare test

samples will be evaluated to determine the relationship between sample

preparation and electric field distortion. Finally, the optical system

will be improved to permit more accurate and more precise field measurements.

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

18
RELATIVE

FIELD,

E/Em

NITROBENZENE-DOPED TRANSFORMER OIL

Figure 3. Comparison of electric field next to the interface and away from

the interface in nitrobenzene dopped-transf ormer oil. Although

there is an obvious, negat i ve-space-charge-i nduced field distortion

which is spread somewhat uniformly throughout the gap, the field

distortion is not different near the interface from what it is

away from the interface (within 10%).

19
6 . REFERENCES

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

Report: Electric and Magnetic Field Measurements , Nat. Bur. Stand.

(U.S.), NBSIR 82-2527, 48 pages, July 1982.

[2] H. J. White, Industrial Electrostatic Precipitation, Addison Wesley,

Reading, Mass. 1963, p. 126 ff.

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

Pulses in SF5 , J. Appl . Phys. , Vol. 52, No. 11, pp. 6588-6600, Nov. 1981.

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

60-Hz AC Corona in SFg, IEEE Trans. Elec. Insul., Vol. EI-17, No. 2,

pp. 106-120, Apr. 1982.

[5] Y. A. Sahar, A. H. Qureshi, and N. H. Malik, Impulse Corona in Positive

Rod-Plane Gaps Filled with SFg Gas Mixtures, Proc. 1982, IEEE Int'l Symp.

on Elec. Insul., pp. 197-201, June 1982.

[ 6 ] B. M. Smirnov, Negative Ions, translated by S. Chomet and edited by

H. S. W. Massey, McGraw Hill, pp. 148-168, 1982.

[7] P. J. Hay, The Relative Energies of SFg" and SFg as a Function of

Geometry, J. Chem. Phys., Vol. 76, No. 1, pp. 502-504 , Jan. 1982.

[ 8 ] Fortran subrouting PLTMG developed by R. Bank and A. Sherman of the

University of Texas, and amplified and modified by J. Blue and C. Wilson

of MBS.

[9] E. F. Kelley and R. E. Hebner, 1981 Annual Report: Optical Measurements

for Interfacial Conduction and Breakdown in Insulating Systems, Nat. Bur.

Stand. (U.S.), NBSIR (in press).

20
NBS-114A (REV. 2*60 ) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC OATA

SHEET (See i nstructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 82-2586

2. Performing Organ. Report No, 3. Publication Date October 1982

4. TITLE AND SUBTITLE

Development of Power System Measurements — Quarterly Report

April 1, 1982 to June 30, 1982

5. AUTHOR(S)

R. E. Hebner, Editor

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

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

T.O. No. 137

8. Type of Report & Period Covered

4/1/82 - 6/30/82

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

Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, S.W.

Washington, DC 20585

10. SUPPLEMENTARY NOTES

’ j

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

bi bl iography or literature survey, mention it here)

This report documents the progress on four 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 April 1, 1982 to

June 30, 1982. This report emphasizes the errors associated with measurements of

space charge near dc transmission lines, the measurement of rf attenuation in

distribution cables, the characteristics of positive and negative corona in compressed

SFg gas, and the measurement of the space charge density in transformer oil subjected

to 60-Hz excitation.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

cables; dc fields; high voltage; incipient fault; insulation; SF

fi ; space charge;

transformer oil. D

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