Reference

NBS

Publications

NAT L INST. OF STAND & TECH

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NBSIR 83-2761

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Development of Power System

Measurements -- Quarterly Report

January 1, 1983 to

March 31, 1983

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Eiectrosystems Division

Washington. DC 20234

October 1983

Prepared for: Department of Energy

_ qq _on of Electric Energy Systems

1 nn Independence Avenue, SW

•WU mgtcn, DC

,1156

83-2761

1983

ATIOWAl BTJTCAq

OF STANDAKD8 UBRABT NBSIR 83-2761 P6-

Oc/oo

DEVELOPMENT OF POWER SYSTEM

MEASUREMENTS - QUARTERLY REPORT

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 1 983

Prepared for: Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, SW

Washington, DC 20585

JANUARY 1, 1983 TO

MARCH 31, 1983

o

Q*

rT

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Foreword

This report is intended to summarize the progress of five technical

investigations during the second quarter of FY 1 983. Although reasonable

efforts have been made to ensure the reliability of the data presented,

/ it must be emphasized that this is an interim report so that further

experimentation and analysis may be performed before the conclusions from

any of these investigations are formally published. It is therefore

possible that some of the observations presented in this report will be

modified, expanded or clarified by our subsequent research.
TABLE OF CONTENTS

Page

Foreword iii

LIST OF FIGURES v

LIST OF TABLES vi

Abstract 1

1. INTRODUCTION I

2. ELECTRIC AND MAGNETIC FIELD MEASUREMENTS

Subtask Nos: 01 and 02 1

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No: 03 4

4. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

Subtask No: 04

b. ACTIVE INSULATORS

Subtask No: 04 11

6. INVESTIGATION OF INSULATOR SURFACE FLASHOVER IN GAS

Subtask No: 04 13

7. REFERENCES 20
LIST OF FIGURES

Page

Figure 1. Variation of vertical electric field strength between

parallel plates with ion counter connected to one plate.

The diameter of the ion counter aperture is taken to be

half the parallel plate spacing. Perturbations in the

plane of the aperture are in excess of 40 percent 3

Figure 2. Flow pattern for two-dimensional orifice in flat plate.

Relative velocities are indicated at representative

points along the streamlines 3

Figure 3. View of electrode system looking into the light beam

from the position of the detector. The upper half of

the area between the plates is covered by a neutral

density filter to reduce the amount of light to avoid

overloading the detector. The lower half of the area between the plates is covered with the analyzing polarizer

to exhibit the Kerr effect. This method of making

Kerr-effect measurements provides a means to measure

the incoming light intensity at the same time the Kerr

effect is observed, eliminating possible errors due to

light source intensity changes from shot to shot. The

wide, thin rectangles show the regions which' were scanned by the detector 9

Figure 4. Kerr coefficient of transformer oil for the visible

spectrum at 24°C and 97°C. These data were all taken

at the peak of a 120 kV pulse. The precision of these

data are indicated by the two points at 425 nm for room temperature oil and at 625 nm for the 97°C oil. The

solid data points are all taken at the same voltage.

The points marked "A" are taken over a range of pulse

voltage — from 72 kV to 120 kV peak 10

Figure 5. Maximum current passing through active insulators

decreases as tower resistance increases 12

Figure 6. Flow chart for implementing three-phase model 14

Figure 7. Plots showing the E field distribution for different

dielectric mismatches 17

Figure 8. Plots showing the E field distribution for different

insulator angles 17

Figure 9. Electric field distribution between electrodes with

permittivity interface (£2/^1 = 9),

Ejvjax = 3.76 V/ cm/volt applied 18

v
LIST OF TABLES

Table 1

Tab! e 2

Page

Measured production rates for SOF2 in SFg at the

different indicated gas pressures, discharge powers,

and pol ari ti es 6

. Measured production rates for SO2F2 in SF5 at the

different indicated gas pressures, discharge powers,

and pol ari ti es 6
DEVELOPMENT OF POWER SYSTEM MEASUREMENTS — QUARTERLY REPORT

January 1, 1983 to March 31, 1983

R. E. Hebner, Editor

This report documents the progress on five technical investigations

sponsored by the Department of Energy and performed by or under a grant from the Electrosystems Division, the National Bureau of

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

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

measurements of electric and magnetic fields, the characteristics

of corona in compressed SF 5 gas, and the measurement of the space

charge density in transformer oil, the development of active

insulators, and interfacial phenomena.

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

insulation; magnetic fields; partial discharges; SF 5 ; solid insulation;

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 has concentrated on the measurement of electric and magnetic

fields, the measurement of interfacial phenomena, and of partial discharge

phenomena. The technical progress made during the quarter January 1, 1983

to March 31, 1983 is summarized in this report.

2. ELECTRIC AND MAGNETIC FIELD MEASUREMENTS

Subtask Nos: 01 and 02

The objectives of this investigation are to develop methods to evaluate

and calibrate instruments which are used, or are being developed, to measure

the electric field, conductivity, the space charge density, and current density

in the vicinity of high-voltage transmission lines and in apparatus designed

to simulate the transmission line environment and to provide electrical

measurement support to other investigators’ efforts to determine the effects

of fields on biological systems.

During the current reporting period, the development of a standard ion

source for experimentally calibrating aspi rati on-type ion counters has been

considered theoretically. Operators of aspi rator-type ion counters normally

use the rel ati onshi

p

P = I/* , (1)

where p is the ion density, I is the measured ion current, and $ is the air

flow, to determine the ion density for air ions with mobilities greater than

a certain critical value. Experimental attempts to check the validity of

this relation have involved the use of ion sources with coaxial cylinder

geometries [1]. Ions which are produced by corona on a center wire conductor

1
are measured with an ion counter mounted to the side of the cylindrical outer

conductor. By measuring the electric field E, and current density J, at the

surface of the outer cylinder, and assuming a value for the ion mobility K,

the charge density is calculated from the relation

P = J/KE (2)

and compared with the result obtained with eq (1). With this approach the

assumption is made that the electric field across the opening, created in the

side of the cylinder for the ion counter, is unchanged by removal of the

conducting surface. The validity of this assumption is examined theoretically

for a parallel plate ion source geometry with obvious implications for the

coaxial cylinder case. The parallel plate geometry is considered as part of an effort to evaluate the feasibility of using the NBS parallel plate apparatus

as a standard ion source for calibration purposes.

The geometry for the problem is shown with a sketch in figure 1. A circular

aperture is created in the top plate of an infinite parallel plate system; the

diameter of the aperture is made equal to half the parallel plate spacing. The

top plate is grounded and a voltage is applied to the bottom plate to produce

an electric field between the plates. Cylindrical symmetry about the y-axis

is assumed. The field calculation is performed without considering the presence

of space charge and thus provides only a qualitative view of the field conditions

between the plates. The influence of conductors normally in the ion counter

as well as fringing fields from the ion counter are also not considered.

Using a finite element code, resident on NBS computers, field strength

values at representative points between the plates were calculated. Plotted

in figure 1 are values of the vertical field strength as a function of distance

above the bottom plate (y) and the parameter x (fig. 1). For this model

calculation it can be seen that perturbations of over 4U% occur across the

entrance to the ion counter. At a point midway between the parallel plates, a distance equal to the diameter of the aperture, the perturbation is as large

as about 2%. While the above results are obtained with a number of simplifying

assumptions, they do raise questions regarding the validity of the previously

described procedure for experimentally checking the performance of ion counters

using eq (2 )

A second question associated with the use of eq (2) which merits attention

is how the air flow, due to the operation of the aspiration-type ion counter,

influences the movement of air ions as they traverse the distance between the

parallel plates or the interelectrode region for the coaxial cylinder case. Use of eq (2) assumes that the ions move with a drift velocity v^, given by

the product of ion mobility and electric field strength KE. However, because

of the air flow through the ion counter, the velocity of the ion near the

entrance of the ion counter may be as large as the sum of the drift velocity

and air velocity. An approximate view of the situation under non-turbul ant

flow conditions is shown in figure 2. The sampling of streamlines in figure 2 represents the solution to the two-dimensional orifice problem using a conformal mapping technique [2]. The velocities shown along the streamlines

at representative points are relative to the uniform flow velocity (taken to

be unity) well below the plane of the orifice. While only a two-dimensional

treatment of the problem, the calculated contraction of the jet, after passing

through the orifice, agrees closely with the experimental value obtained with

2
Figure 1. Variation of

vertical electric field

strength between parallel

plates with ion counter

connected to one plate.

The diameter of the ion

counter aperture is taken

to be half the parallel

plate spacing. Perturbati ons

in the plane of the aperture

are in excess of 40 percent.

DISTANCE ABOVE BOTTOM PLATE (y)

Figure 2. Flow pattern for

two-dimensional orifice in

flat plate. Relative

velocities are indicated at

representative points along

the streamlines.

3
three-dimensional models. Other approximations to the actual problem include

the assumption of a non-viscous fluid, neglecting the effects of the wall of

the ion counter connected to the underside of the plate, and obstructions to

the flow in the ion counter.

To place the results of the calculation into perspective, it should be

noted that the air speed at the entrance of commercial ion counters can be

as high as 1 m/s, which is comparable to drift velocities of positive air ions

(about 1.4 m/s) in a field of about 10 kV/m [3], Therefore, while only an approximation, the information in figure 2 suggests that the drift velocity of

ions approaching an ion counter operating in a ground plane of a parallel

plate system can be significantly perturbed while still at a distance comparable

to the dimension of the ion counter opening.

The effects of an aperture on the electric field strength and air flow on the drift velocity of the ions near the entrance to an ion counter will be

examined further during the next quarter.

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

3. TECHNICAL ASSISTANCE FOR FUTURE INSULATION SYSTEMS RESEARCH

Subtask No: 03

The objective of this project is to develop diagnostic techniques to

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

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

fundamental information and data needed to improve test design and performance

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

gaseous dielectrics is emphasized. This phenomenon gives rise to degradation

of the gas under high electrical stress which leads to breakdown. Measurement

of partial discharge inception in highly nonuniform fields may prove to be a preferred method to determine dielectric strength of electronegative gases.

The planned activities for FY83 include:

1) Preparation of conference and archival papers on the effect of radiation

in enhancing electric discharge initiation near a positively stressed electrode

in compressed SF 5 and O 2 ;

2) Extension of our previous measurements of the production rates of

oxyfluorides in SFg corona discharges to include negative discharges as well

as other gas pressures and discharge power levels, and preparation of the

results for publication in an archival paper;

3) Identification of the predominant gaseous decomposition products from corona in one or more of the following gas mixtures: SFg + No, SFg + CO?,

and SF 6 + c-C 4F 8

+ CHF 3 ;

4) Evaluation of a thin-film, aluminum oxide hygrometer probe for

calibration of a gas chromatograph-mass spectrometer (GC/MS) used to measure

trace quantities of water vapor in SFg and other gaseous dielectrics, and

extension of our previous measurements on the effects of trace levels of H 2 O on electron avalanche growth and corona discharge characteri sti cs in SFg;

4
5) Design and construction of a drift tube-mass spectrometer system to

evaluate measurements proposed to identify and characterize corona-generated

ion species in air, SFg, and other gas dielectrics.

Activity 1 has been completed, and during the past quarter a paper describing

the results of our measurements and theoretical analysis was accepted for

publication in the Journal of Applied Physics. Most of the work conducted

during the past reporting period was related to activity 2 , and the results

of this work are highlighted below.

Measurements of the production rates of the oxyfluorides SOF 2 , S 02 F 2 , and SOF 4

in SF 5 point-plane corona discharges have been extended to negative polarities as well as to other gas pressures and discharge power levels. Results which have been

analyzed to date for SOF 2 and SO 2

F

2 are summarized in tables 1 and 2, respectively.

Indicated in these tables are the polarities of the discharge (the polarity of

the voltage applied to the more highly stressed electrode), the gas pressures,

the average discharge power, and the corresponding production rates expressed

in nanomoles per joule at the indicated accumulated energies U dissipated in

the discharge. The rates given are extracted from fits to data from gas analysis

using a gas chromatograph-mass spectrometer (GC/MS) which were of the form

C = B + AU 1+e

, (3)

where C is the oxyfluoride concentration in moles and A, B, and e are fitting

parameters which are assumed to be constant for each set of discharge conditions.

The production rates are found from the expression

dC/dU = A (1 + e )U e

. (4)

The concentrations C are calculated from the calibrated GC/MS single ion

responses using the method given in our previous quarterly report. In the cases where more than one ion was used to determine the concentration for a given species, the results for the different ions were averaged. For example,

some of the results in table 1 for SOF 2 represent averages of data from ions

with mass-to-charge ratios oj 86 and £7 corresponding respectively to the

predominant ion species SOF^ and SOF observed in the mass spectra.

The results given in the tables exhibit the trends described in our previous

report; namely, for positive corona the production rates for both SOF 2 and

SO 2

F 2 at a given gas pressure tend to decrease with increasing discharge

power, and at the lowest pressure (116 kPa) there is an enhancement in the

production of SO 2

F 2 relative to SOF 2

. In addition, results obtained during

the past quarter, as shown in the tables, indicate a significant polarity

effect. Not only are the production rates for both species seen to drop in

going from positive to negative polarity, but also the ratio of SOF 2 to SO 2

F 2

production increases by more than a factor of two.

In view of the fact that these data must still be regarded as preliminary,

it would be premature at this stage to speculate on the causes of the polarity

effect. There is reason to believe, however, that a large change in production

rates resulting from switching of polarities may be an indication of the relative

importance of ion-molecule reactions in the formation of the final neutral

decomposition species. It is clear that more data are needed on the pressure

and power dependence of oxyfluoride production from negative corona in SF 5

.

5
Table 1. Measured production rates for SOF 2 in SF6 at the different

indicated gas pressures, discharge powers, and polarities.

Gas Ave. Measured production rates

Pol ari ty pressure power (n*mol /J

)

(kPa

)

(mW

)

(U = 5 kJ) (0 = 10 kJ) (U = 15 kJ)

Posi ti ve 116 804 0.24 0.37 0.48

200 54 3.39 4.54 --

200 335 2.83 3.95 • 4.78

200 777 1.60 2.52 3.31

300 198 1.71 2.98 4.12

300 430 1.29 1.95 3.49

300 945 0.75 1.13 1.44

Negati ve 200 230 0.76 0.93 1.05

200 586 0.55 0.63 0.68

Table 2. Measured production rates for S0?F? in SFfi at the different

l ndi cated gas pressures, discharge powers, and polarities.

Gas Ave. Measured production rates

Pol ari ty pressure Power (n »mol /J

)

(kPa) (mW

)

(U = 5 kJ) (U = 10 kJ

)

(U = 15 kJ)

Posi ti ve 116 804 0.30 0.49 0.65

200 54 3.09 4.25 —

200 154 2.84 4.15 —

200 335 2.25 2.93 3.43

200 111 1.23 1.67 2.00

300 198 1.14 1.68 2.10

300 430 1.33 1.59 1.76

300 945 0.57 0.80 0.97

Negati ve 200 230 0.11 0.22 0.35

200 586 0.26 0.30 0.33

6
Further tests were carried out during this quarter to determine the major

sources of error associated with these measurements. Sources that have been

identified and considered include: 1 ) fluctuations and drift in discharge

power (this is more a problem with positive than with negative corona, and

most significant at higher power levels); 2 ) fluctuations in chromatograph

background levels; 3) drift and nonlinearities in GC/MS response; and 4) errors

associated with volume and gas pressure determinations and surface absorption

during calibration sample preparations. Of these, at present the drift in GC/MS

response appears for most experiments to be the largest source of uncertainty.

The uncertainty due to this effect is estimated to be less than ±25% of the

rates given in the tables. Although some further adjustments in the rates may

be necessary, we believe that the numbers given in this report are correct to

within an overall uncertainty of ±35%.

During the next quarter, we plan to continue the measurements of oxyfluoride

production rates from corona in SFq as described in this report. In particular,

more measurements will be carried out for negative corona discharges at different

gas pressures and discharge power levels. Some measurements for positive

polarity will be repeated to assess reproducibility, and to verify our estimates

of uncertainties. Further consideration will be given to the possible sources

of error in the measurement of production rates, and a search will be made for

a set of GC/MS operating conditions which will minimize uncertainties due to

oscillations of the background level in the chromatograph output which has become

a problem for measurements performed at certain gas pressures. An attempt will

be made to calibrate the response of the GC/MS to trace water vapor content in

SF 5 using a thin-film aluminum oxide hygrometer probe.

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

4. OPTICAL MEASUREMENTS FOR INTERFACIAL CONDUCTION AND BREAKDOWN

IN INSULATING SYSTEMS

, Subtask No: 04

The objectives of this investigation are to develop apparatus and appropriate

procedures for the optical measurements of interfacial electric field and spacecharge 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.

Last quarter a method was developed to perform field measurements in

transformer oil which rendered low voltage, steady-state fields electrooptically visible. The method consisted of capacitively coupling an impulse to

the electrode upon which a steady-state voltage was present. This method

increased the observable Kerr effect by several orders of magnitude. The cell was installed with a polytetrafl uoroethyl ene (PTFE) interface. In this

arrangement, it was not possible to observe any field distortions near the

interface even when water was added to the system and the temperature was increased to 120°C. To explain these observations it was hypothesized that the

space charge displayed an onset which could not be reached before the interface

would break down. The present quarter's activities examined this question

further but the primary accomplishment has been further measurements of the

Kerr coefficient as a function of temperature and light wavelength.

7
During the present quarter we were able to provide preliminary measurements

of the electro-optical Kerr coefficient for the visible spectrum at room temperature (24°C) and at 97°C. A xenon flashtube light source was used with a monochrometer to provide the wavelength-selected light with an approximate

uncertainty of ±15 nm. In order to provide simultaneous measurements of both

the unpolarized light intensity and the light transmitted through the cell

because of the Kerr effect, half of the image of the electrodes was covered by

a neutral density filter, and the other half was covered by the analyzing

polarizer (see fig. 3).

This method of observing both the incident light intensity and the Kerr

effect at the same time is necessary whenever the light source does not produce

the same intensity from shot to shot (as in the case of our xenon source).

Under these circumstances, the detector was operated in the two-track scan mode

to obtain profiles of the light between the plates in both regions of interest --

in the regions of the neutral density filter and of the analyzing polarizer

(see fi g. 3).

It is sufficient to note that data taken in this way provide two intensity

measurements: I is the intensity due to the Kerr effect, and I m

is the incident

light intensity. The Kerr effect is then expressed by

I/I m

= si

n

2

( (tt/ 2) (V/Vm ) 2

) , (5)

where the lowest voltage necessary to produce maximum transmitted intensity is

V m which is related to the Kerr# coeff i ci ent B and the cell parameters, i.e.,

the plate spacing d, and the plate length 1, by

B = d2/(21V m

2) . (6)

The cell constant is obtained from eq (5)

V m

= V[(2/ir)sin” 1 ((I/I m

) 1/2 )]"l / 2, (7)

where V is the applied voltage.

The Kerr coefficient as a function of wavelength for two temperatures is

shown in figure 4. The detector system made a 1 ys recording at the peak of a 12U kV pulse. The data show a decrease in the Kerr coefficient for higher

temperatures which is largest in the orange area of the visible spectrum. The

variation of the Kerr effect with wavelength is larger -- changing about 30% over the visible spectrum — for room temperature oil, than at the higher

temperature.

The precision of these data is around ±10%, however, the accuracy is

less certain, probably ±25%. the reason for the lack of accuracy is most

likely the documented optical activity of the tank windows. This manifests

itself in a voltage dependent Kerr coefficient (see fig. 4). Optical activity

of windows would affect the lower voltage data more than at higher voltages.

We chose to operate at 120 kV peak which would occasionally break down the gap.

Further cautions in handling this type of a system come from the monochrometer

A grating will cause a small amount of scattering of light at wavelengths other

3
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than what is selected. This scattered light becomes important at the ends of

the visible spectrum where either the detector is less sensitive, the liquid

becomes more absorbing, or the light source lacks intensity. For this reason,

the data reported here are most reliable in the region 650 nm to 450 nm.

These results demonstrate that a measurement system can be made and

calibrated to provide measurements of the Kerr coefficient as a function of

temperature and wavelength. Although the present system suffers from an absolute

calibration due to unaccounted optical activity of the windows, these data show

that there are no fundamental obstacles which would prevent the construction of

a system capable of accurate measurements should there be a need for such

accuracy in the future. It is sufficient here to note the trends and identify

possible problems.

Attempts will be made during the upcoming quarter to complete the

measurements to determine space charge onset phenomena for steady-state voltages.

Some progress was made this quarter, but additional measurements are required.

It is anticipated that this question, along with the question about the water

content affecting space charge, will be answered.

The cell -tank system for the examination of the effects of temperature

upon the strength of a paper interface parallel to the field will be designed

and built next quarter. This system will permit the rapid replacement or removal of the interfacial material after each breakdown, thus, the effects due

to opening the system to replace the interface will be minimized. The system

will permit an accurate comparison of the electrical st rength .wi th and without

the interface present.

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

5. ACTIVE INSULATORS

Subtask No: 04

As a continuing effort to study the effects of active insulator assembly

on power transmission lines under lightning, the mathematical model developed

in the previous quarter was implemented to consider cases in which lightning

strikes a transmission line conductor directly. As a worst case scenario, the

operating voltage was assumed to have the same polarity as the lightning strike

and have its maximum peak voltage; this would introduce the maximum stress on the insulator assembly. The lightning current was rated at 50 kA and struck at

the middle of the transmission line. Results indicated that the maximum current

passing through an active insulator assembly decreased linearly as the tower

resistance increased (see fig. 5). Regardless of tower resistance, the maximum

current passing through the substation arrester was 1.2 kA. On the other hand,

without active insulators, the maximum current was found to be 38 kA. Even

though active insulator assemblies could reduce the duty of station arresters

significantly, the large current passing through these insulators poses a serious threat to the integrity of the insulator assembly itself. Appropriate

measures to protect the insulators are essential, such as ground wires which

should be provided to protect the transmission line from direct hits.

The second phase of this project is to develop a model for a three-phase

system. The effects on adjacent phases can also be investigated. The basic

11
Max.

Current

in

Active

Insulators

12
model is similar to the single-phase model: instead of single values, the

parameters are expressed as vectors and matrices. The Bergeron model for the

three-phase system can first be expressed in the modal domain. Using phasemodal transformati on, the nodal voltage is evaluated in the phase domain. The

general transformation of all parameters and variables is expressed as:

[Z']Ji = [T ][Z 1 ]

1 CT

]

1 (8)

CF]* = [T][F]

m

[T]-i (9)

n , n-1 (t-T ) ]<j, = [T][I n>

-i (t-T )

]

m

(1U) ^ n-1, n (t-t) ](j, = [T][i n -i jn (t-x)]m

(llj

CU

n (t-t)]+ = [T][U

n (t-x)]

m . (12)

In all parameters and variables, the modal domain values are expressed as sequence values.

The propagation time t is a vector, since the propagation time for each

sequence is different as line parameter sequence varies. For the calculation of

node voltage with active insulator assembly, the resultant voltage equation is

[(KU

n (t))«] t+

2[Z']-l[U

n (t)] t+

[i;

>n . 1 (t-T)]

t

+ [I

n , n+1 (t“ T ) J(j> = ^ • (1^)

Using Newton's iteration method the approximated solution for eq (13) can be

obtained. A simplified flow chart for implementing this model is shown in

figure 6.

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

6. INVESTIGATION OF INSULATOR SURFACE FLASHOVER IN GAS

Subtask No: 04

The objective of this investigation is to determine and describe the basic

processes leading to the flashover of insulator surfaces in vacuum and pressurized

gas. Specific objectives include: (1) measurement of insulator surface currents

to determine the role of geometry and material parameters on the surface currents

and the role of surface currents in insulator surface charging and flashover;

(2) electro-optical measurement of insulator/gas and insulator/vacuum interfacial

electrical fields prior to flashover; and (3) development of analog and numerical

techniques to predict interfacial electric fields for applicable geometries and

established surface charge distributions.

Surface currents have been measured this quarter in vacuum and nitrogen,

along polymethylmethacrylate, PMMA, insulators. Results are similar to those

reported last quarter. Additional measurements have, however, been obtained

this quarter for additional pressures and electrode separations. Additionally,

analyses have been performed to improve the understanding of the observed

current behavior.

13
Figure 6. Flow chart for implementing three-phase model.

14
Surface currents have been measured in vacuum and nitrogen. The observed

surface current behavior in vacuum differs from the behavior in gas. It has

been previously reported, and also observed this quarter, that surface currents

in vacuum, along 60 Hz excited PMMA insulators, have three components: (1) a low amplitude, randomly occurring component. This component is observed at

relatively low voltages and occurs in the form of short duration pulses, near the excitation peak; (2) a longer duration component. This component begins to

occur at higher voltages (above a threshold). The peak of this component

occurs, in time, before the voltage peak and, in many cases, extinguishes before

the voltage peak; and (3) a third component arises at higher voltages. This

component is large in amplitude and is approximately in phase with the excitation.

Based upon this observed behavior, it is being postulated that: component (1)

is due to partial discharges across the gap, either away from or on the insulator

surface; component (2) is due only to current along the insulator and is the

current responsible for charging the insulator; and component (3) is due to

field emission current flowing from the cathode to the anode, away from the

insulator surface.

Analyses have been performed this quarter to relate the observed surface

behavior to the postulated mechanisms. It has been determined that the time

integral of current component (2) yields a value of charge equal to 1 x 10" 5 C/m2 . This value is comparable to charge values previously measured using the electrooptical field measurement technique. This implies that current component (2)

is associated with insulator surface charging. Analyses have also been performed

on the component (3) current data. The observed current/voltage relationship

of this component has been determined and compared to the relationship predicted

for field emission by the Fowl er-Nordhei m equation. Agreement is good for the

limited data analyzed. These preliminary analysis results imply that component (3)

current is therefore due to current flowing between the cathode and the anode

with negligible interaction with the insulator surface.

Additional PMMA/nitrogen interfacial current measurements have been made

this quarter. The observed surface current behavior in gas is different than

that observed in vacuum. The results show that the current flows in short

pulses occurring during the excitation. Relatively long duration (type (2))

current components have not been observed. The observed emission has a definite

voltage threshold. This threshold voltage is very close to the voltage necessary

for insulator surface flashover in nitrogen.

The observed temporal shape of these short duration surface current pulses,

observed in vacuum and gas, is indicative of the impulses' response of the

measurement system and is therefore not significant. However, the response

amplitude and repetition period are significant and indicative of preflashover

(possibly surface charging) event size and period of occurrence.

A multi-channel analyzer (MCA) has therefore recently been obtained to

enable computer data acquisition and analysis regarding the pulse amplitude,

frequency, and number. Time has been spent this quarter designing pulse shaping

and computer interfacing circuitry. The system is essentially complete and

consists of a pulse shaper, a computer, and graphics output. The system is

now operational but has not been used to acquire or process surface data.

Completion of a necessary computer link should permit use of this system next

quarter. The objective of the planned measurements is to quantify the relationships

15
between size and frequency of surface current pulses and various insulator

characteristics (such as geometry and material) in both vacuum and gas.

The surface fields, associated with the measured surface currents, have

been previously el ectro-opti cal ly measured and reported. This quarter

modifications to the experimental arrangement have continued to reduce charge

injection into the electro-optical sensing liquid and resulting charging of the

liquid for 60 Hz excitation. It is anticipated that these modifications will

be complete and initial measurements made by the first of May.

The measurement techniques described above measure surface currents and

electric fields associated with insulator surface charging. It is postulated

that this surface charging reduces the insulator hold-off strength due to local

electric field enhancement established by the surface charge. Analog and

digital analysis techniques have been developed to predict the insulator fields

associated with the surface charge.

An electrolytic tank has been constructed to predict surface fields

associated with various surface charge distributions likely to occur. The tank

construction has been completed and consists of a large PMMA container, a step

motor driven probe, and a computer data acquisition and manipulation system.

The entire system is capable of determining the electric field components

associated with various insulator shapes and permittivities. The results are produced in the form of three dimensional plots of electric field E, as a function of position along and away from a dielectric interface. Results have

been obtained for insulator cone frustums having angles 0 of 30°, 45°, and 60°

(0 is the angle between the insulator and electrode surface) and for relative

permittivities e

r

, of 3, 9, and 35. Results are shown in figures 7 and 8

and are indicative of other results also obtained for geometrical and permittivity

changes. The electrode and interface locations are indicated in figure 7(a).

The results show the spatial variation of the electric field and the value of

the maximum field E^x* The results show that E^ax increases for increases in

both 0 and e r

. It has also been determined that, with this geometry, the

field at one electrode is decreased, while the field at the other electrode is

increased. This analysis technique is presently being used to determine the

effect of uncharged insulator shape and permittivity on triple junction fields.

Triple junction fields play an important role in insulator charging processes.

A computer based, finite difference, numerical technique has also been

developed to determine the surface fields associated with various geometries

and surface charge. Typical results are shown in figure 9 for 0 = 45° and e r = 9.

These results compare favorably with those obtained using the electrolytic

tank. These results are also being used to determine the effect of various

geometries and surface charge on the i nter-el ectrode fields.

In summary, surface current measurements have been made in vacuum and

nitrogen using the previously reported electrical diagnostic system. For

vacuum, the results have been analyzed and related to previously, electrooptically, measured surface charge values and voltage/current performance

predicted from field emission equations. The results imply that surface charging

and gap field emission are both occurring, but have different 60 Hz voltage

thresholds. Gas data presently imply that surface currents are of short duration

and sporatic. A computer-aided data acquisition and analysis system is being

implemented to permit more systematic analysis of the data. Analog and numerical

16
0.0775

V/cn/V E

mAV =

0.1077 /

\kR

may "

°-

1253

Figure

8.

Plots

showing

the E

field

distribution

for

different

insulator

angles.
Figure 9. Electric field distribution between electrodes with permittivity

interface = S), E^x = 3.76 V/ cm/volt applied.

18
techniques have been developed and have been used to calculate electric fields

associated with various insulator material and geometry. These results are being used to predict the role of insulator shape, permittivity, and surface

charge on breakdown, and to assist in the understanding of the experimental

surface charge and field data obtained.

Tasks to be accomplished next quarter include:

1) The MCA will be used to acquire and process surface current measurements

in vacuum and nitrogen. The current behavior, as a function of voltage, insulator

material, and insulator geometry, will be determined. Measurements will also

be done in SFg.

2) The electro-optical diagnostic system will be completed and preliminary

glass/gas interfacial measurements made. It is anticipated that no charge

injection will occur into the sensing liquid. Field nonuniformities will then

be due only to surface charge and geometry effects.

3) Numerical and analog analyses will continue to relate surface fields

to surface charge. The capability is being developed to determine what surface

charge is associated with the el ect ro-opti cal ly measured surface field.

For further information contact Dr. J. E. Thompson, (803) 777-7304.

19
7. REFERENCES

[1] T. D. Bracken and B. C. Furumasu, Field and Ion Current Measurements in

Regions of High Charge Density Near Direct Current Transmission Lines,

p. 544, Proceedings of the Conference on Cl oud Physics and Atmospheric

Electricity , (Ameri can Meterol ogi cal Society, Boston, 19/8). For a description of related studies conducted at the National Bureau of Standards,

see Report NBSIR 81-2267, 198U Annual Report to the Department of Energy

and Report NBSIR 82-2527, 1981 Annual Report to the Department of Energy.

[2] L. V. Bewley, Two-Dimensional Fields in Electrical Engineering, p. 146,

(Dover, New York, 1963).

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

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

20
NBS-114A (rev. 2 -ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 83-2761

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

September 1983

4. TITLE AND SUBTITLE

Development of Power System Measurements -- Quarterly Report

January 1, 1983 to March 31, 1983

5. AUTHOR(S)

R. E. Hebner, Supervisory Physicist

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9.^PO^SpRI^K^gRGANIZATION NAME AND COMPLETE ADDRESS ( Street. City, State, ZIP)

Department of Energy

Division of Electric Energy Systems

1000 Independence Avenue, S. W. Washington, DC 20585

10.

SUPPLEMENTARY NOTES

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

11.

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

bi bliography or literature survey, mention it here)

This report documents the progress on five technical investigations

sponsored by the Department of Energy and performed by or under a grant from the Electrosystems Division, the National Bureau of Standards.

The work described covers the period January 1, 1983 to March 31, 1983.

This report emphasizes the errors associated with measurements of electric

and magnetic fields, the characteristics of corona in compressed SFg gas, and

the measurement of the space charge density in transformer oil, the

0

development of active insulators, and interfacial phenomena.

12.

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

electric fields; gaseous insulation; interfaces; liquid insulation;

magnetic fields; partial discharges; SF^; solid insulation; transformer oil

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

| X | Unlimited

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

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