NBS

Publications ’Reference

NBSIR 82-2527 NAT'L INST. OF STAND & TECH

ailDb 2STflM3 I 1 Annual Report: Electric and

Magnetic Field Measurements

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

July 1982

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Prepared for: Department of Energy

Division of Electric Energy Systems

Washington, DC 20585

NBSIR 82-2527

»•* 1981 ANNUAL REPORT: ELECTRIC AND

MAGNETIC FIELD MEASUREMENTS

R. H. McKnight, F. R. Kotter, M. Misakian, and J. N. Hagler

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

July 1982

Prepared for; Department of Energy

Division of Electric Energy Systems

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

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TABLE OF CONTENTS

Page

LIST OF FIGURES v

Summary . viii

Abstract 1

1. INTRODUCTION 2

2. PARALLEL PLATE APPARATUS 3'

3. STUDY OF ERRORS ASSOCIATED WITH MEASUREMENTS

OF VERTICAL CURRENT DENSITY 3

4. PRODUCTION OF SPACE CHARGE IN THE LABORATORY . 5

A. Introduction 5 B. Approach 5 C. Results 7

D. Discussion and Conclusions 11

5. INVESTIGATION OF SOURCES IN ERROR IN SPACE CHARGE DENSITY

MEASUREMENTS USING ION COUNTERS 12

A. Introduction 12

B. Approach ..... 12

C. Results 12

D. Discussion and Conclusions 16

6. ABOVE GROUND OPERATION OF ION COUNTERS 19

A. Introduction 19

B. Experimental Approach 19

C. Results. 21

0. Discussion and Conclusions 21

7. MEASUREMENT OF NET SPACE CHARGE DENSITY

USING AIR FILTRATION METHODS 25

A. Introduction 25

B. Approach 25

C. Results 28

D. Discussion and Conclusions 28
TABLE OF CONTENTS (cont.)

8. MEASUREMENTS OF MOBILITY SPECTRUM

USING RETARDING FIELD METHOD 30

A. Introduction 30

B. Approach 30

C. Results 32

D. Discussion and Conclusion 32

9. DISCUSSIONS AND CONCLUSIONS 37

10. REFERENCES 38

i V
LIST OF FIGURES

Page

Figure 1. Errors in Wilson plate current density measurements as a function of guard ring width and elevation above the ground

plane. The solid curves are results of measurements with

alternating current. The ^E/Eg parameter (see main text)

values for the dc data plotted are as follows: + - 0.004;

A - 0.014; ? - 0.024, § - 0.004 4

Figure 2. Top view of permanent low-speed air flow facility to

produce space charge 6

Figure 3. Net space charge measurements made at midplane at locations

II, III, and IV. Relative positions of test section inlet

walls are shown. Source configuration was 1 corona wire

(position indicated) and coarse ground planes (see text). ... 8

Figure 4. Net space charge measurements made at midplane at locations

II, III, and IV. Source configuration was 3 corona wires

(positions indicated) and coarse ground planes (see text). ... 9

Figure 5. Net space charge measurements made at midplane at locations

II, III, and IV. Source- configuration was 9 corona wires

(as indicated) and window screen grbund planes. ...» 10

igure 6. Ion counter output current as a function of polarizing

voltage for different plate configurations. The

displacement between the leading edges of the collection and

polarizing plates is x. Positive values of x correspond

to a situation where the polarizing plates protrude in

front of the collector plates. For x = 0, the leading

edges were flush. Because the ion density is independent

of polarizing voltage over the range plotted, the ion

counter current should be constant 13

Figure 7. (a) Two dimensional geometry used in finite element

calculations, (b) Computer-generated high level

triangulation for geometry (a). Inside the crosshatched

area, the triangul ation was too dense to be displayed 14

Figure 8, (a) Boundary conditions used in finite element

calculation, (b) Computer generated equipotential

curves, plotted in units ofU = 0.1 15

Figure 9. (a) Boundary conditions and geometry used for a model calculation in which the polarizing plates

protrude in front of the collector plates.

(b) Computer generated equipotential curves, plotted

in units of U = 0.1 17

C-?

V
LIST OF FIGURES (cont.)

Page

F 1 gu re

Figure

F i gu re

F i gu re

Figure

Figure

Fi gure

Figure

Figure

Fi gure

10. Trajectories of ions entering the ion counter at

different initial locations (as shown). The

geometry is that of figure 9 18

11. End view of experimental arrangement for studies of offground ion counter operation. Corona wire diameter was

254 urn. The electrometer and ion counter were isolated

from ground. The wire length was approximately 6 m 20

12. Space charge density determined for different flow rates

as a function of ion counter potential. Corona wire was operated at -80 kV. No corrections were applied to data

(see text) 22

13. Conduction current to ion counter as function of ion

counter potential. Polarizing plates were grounded and air

flow through counter was zero. Corona wire was at -80 kV.. . 23

14. Space charge density determined for different flow rates

and ion counter potential. These results are those of

Figure 12 corrected for leakage current (see text and

fig. 13) 24

15. Experimental configuration for measurements in which the

ion counter is used to measure transmitted charge density.. . 26

16. Experimental configuration for measurements in which a filter is used to measure transmitted charge density 27

17. Summary of HEPA filter transmission measurements. Shown

for comparison are the results of Moore et al . Paltridge

and the factory measured transmission (OOP test). Filterfilter measurements —

0

- positive ions, o - negative ions.

Filter-ion counter measurements—+ - positive ions,

x - negative ions. Filter-ion counter measurements

(alpha source) —V - positive ions, v - negative ions 29

18. (a) Schematic of retarding field analyzer. The actual

assembly was located near III in fig. 2. Screen material

was aluminum window screen, (b) Representation of

transmission function G(k,ko) 1^or retarding field

analyzer. Shaded portion of p(k) would be transmitted. ... 33

19. Space charge density detected downstream from analyzer

(fig. 18). Shown are results for both positive and

negative ions 34

VI
LIST OF FIGURES (cont.)

Page

Figure 20

Figure 21

. Mobility distribution p(k) for negative ions taken on two different days. Results shown are obtained from raw data as described in the text. No smoothing techniques

were employed.

. A comparison of mobility distribution for positive and

negative ions taken on different days, but for similar

flow and electrical conditions. Data are unsmoothed. . . .

vi i

. 35

. 36
Summary

The NBS program is concerned with developing methods for evaluating

and calibrating instrumentation for use in measuring the electric field and

various ion-related electrical quantities in the vicinity of high-voltage

direct current (HVDC) transmission lines and in apparatus designed to

simulate the transmission line environment.

The laboratory investigation of errors associated with above-ground

operation of sensors for measuring vertical current density has been

completed and a report prepared describing the results. Significant errors

were observed for both unguarded and guarded sensors, ranging from 4 to 25%

for the guarded plates and 10 to 35% for unguarded plates for a wide range

of geometrical parameters. Preliminary results from a field day held

in October 1981 are in agreement with the laboratory results.

A new low-speed air flow facility has been constructed. Using multiwire

planar corona discharge ion sources, ion densities from 1.6 x lO^/cm^

to 1.4 X 10^/cm^ have been measured using an absolute filter technique.

The space charge density is uniform to ±5% over most of the cross section

of the l:est volume, but decreases in magnitude by about 20% in passing

through the test volume. This loss appears to be due to ion-ion interactions.

Losses at the inlet of a parallel plate ion counter due to fringing

fields have been determined using an ion counter with variable geometry.

The inlet structure has been modeled using a two-dimensional finite element

code and a program has been written to calculate the trajectories of ions

moving in the counter inlet. The calculations predict a greater loss

than actually observed but are in qualitative agreement with experimental

results.

The above-ground operation of an ion counter has been investigated

using a monopolar line and an experimental configuration in which the

potential of the ion counter relative to the local space potential

could be varied. The initial results show that the ion densities

measured using the counter are relatively independent of counter potential

where ions are strongly attracted to the counter. However, as the potential

nears that of the average space potential near the counter, the indicated

ion density drops significantly. These results represent only a limited

set of measurements, but indicate problems associated with above-ground

measurements of ion density.

vi i i
The use of a high -efficiency particulate air (HEPA) or "absolute" filter

in measuring net space charge density was investigated. The transmission of

charge in an airstream through this type of filter has been determined

to be less than 0.1% for a wide range of flow and ion density conditions.

This result indicates that a HEPA filter may be useful in calibrating

ion counters when ions of only one sign are present. In this case, the net

space charge density and ion density are equivalent.

A retarding field scheme has been considered for use in measuring

the mobility spectrum of the ions in the low-speed air-flow facility.

Tlie results of a limited number of measurements show that the method may

be a useful one for this application but that a number of problems need

further investigation, including the effects of space charge. Since the

data reduction involves numerical derivatives, quality data is required

and the use of signal averaging using a computer-controlled apparatus

is indicated.

IX

1981 ANNUAL REPORT: ELECTRIC AND

MAGNETIC FIELD MEASUREMENTS

R. H. McKnight, F. R. Kotter,

M. Misakian, and J. N. Hagler

Abstract

The NBS program is concerned with developing methods for

evaluating and calibrating instrumentation for use in measuring

the electric field and various ion-related electrical quantities

in the vicinity of high voltage direct current (HVDC) transmission

lines and in apparatus designed to simulate the transmission

line environment.

The laboratory investigation of errors associated with

above-ground operation of sensors for measuring vertical current

density has been completed. Significant errors were observed

for both unguarded and guarded sensors, ranging from 4 to 25%

for the guarded plates and 10 to 35% for unguarded plates for a wide range of geometrical parameters. Preliminary results from

a field day held in October 1981 are in agreement with the

laboratory results.

A new low-speed air-flow facility has been constructed.

Using multiwire planar corona discharge ion sources, ion densities

from 1.6 X 10^/cm^ to 1.4 x 10^/cm^ have been measured using an absolute filter technique.

Losses at the inlet of a parallel plate ion counter due to

fringing fields have been determined using an ion counter with

variable geometry. Calculations based on a two-dimensional

finite element code predict a greater loss than actually observed,

but are in qualitative agreement with experimental results.

The above-ground operation of a parallel plate has been

investigated using a monopolar line. The initial results obtained

show that the ion densities measured using the counter are strongly

dependent on ion counter potential. These results represent only

a limited set of measurements, but indicate problems associated

with above-ground measurements of ion density.

The transmission of charge in an air stream through a high

efficiency particulate air (HEPA) filter has been determined to

be less than 0.1% for a wide range of flow and ion density

conditions. This result indicates that a HEPA filter may be

useful in calibrating ion counters when the net space charge

density and ion density are equivalent.

A retarding field scheme has been considered for use in

measuring the mobility spectrum of the ions in the low-speed air

flow facility. The results of a limited number of measurements

show that the method may be a useful one for this application

but that a number of problems need further investigation, including

the effects of space charge.

1
I. INTRODUCTION

A determination of the electrical environment near high voltage

transmission lines is important both to the line designer and to those

utilities concerned with possible biological effects of the line.

Adequate instrumentation and measurement procedures exist for

characterizi ng ac lines [1]^, but the situation is in a much more

rudimentary state for high voltage dc (HVDC) transmission lines.

The quantities primarily of interest are summarized symbolically

in the equation J = pKE where J (a/m^) is the current density, p (C/m^)

the space charge density units, K (m^/vs) a weighted ion mobility, and

E (V/m) the electric field. This equation would appear significantly

more complex if a suitable expression for K were written out explicitly.

Other electrical quantities which also are of interest include the

air conductivity and net space charge density. Related parameters

are the mobility spectrum p(K) and the ion species present near

the ground. Measurements of p(K) and identification of ion species

are difficult tasks but may be necessary in the future. Concern

with charge transport away from the line may require measurements of

aerosol production and dispersal.

In previous reports we have discussed the adaptation of various

instruments and measurement techniques developed by atmospheric

electricity researchers to the HVDC measurement problem [2,3]. One

important result of the NBS effort has been the development of

facilities designed to provide, in the laboratory, a means of evaluating

instruments under conditions similar to those found near a HVDC line.

As discussed below, some laboratory results have also been directly

compared with similar measurements made under a full-scale test line.

The parallel plate facility which was described in detail in

earlier publications [4,5] has been used to complete an investigation

of errors associated with vertical current density measurements. The

permanent air flow system designed to produce a test volume containing

space charge has been completed and evaluated.

In the following discussion of the FY81 effort, subtasks are described individually with each section self-contained including

discussion.

Three reports published recently describe in detail some of the

effort discussed below. These reports are:

(1) Measurement of Ion Current Density at Ground Level in

the Vicinity of High Voltage DC Transmission Lines,

R. H. McKnight, F. R. Kotter, and M. Misakian,

NBSIR 81-2410, Dec. 1981.

^ Numbers in brackets refer to the literature references listed at the

end of this report.

2
(2) The Measurement of Met Space Charge Density Using Air

Filtration Methods, R. H. McKnight, NBSIR 82-2486,

April 1982.

(3) A Facility to Produce a Uniform Space Charge for

Evaluating Ion Measuring Instruments, R. H. McKnight

and F. R. Kotter, MBSIR 82-2517, June 1982.

2. PARALLEL PLATE APPARATUS

The development of a parallel plate apparatus for generation of

dc fields with space charge [4] has been published in the Journal of

Applied Physics [5]. This apparatus was used in the study of errors

associated with measurements of vertical current density described below.

3. STUDY OF ERRORS ASSOCIATED WITH MEASUREMENTS

OF VERTICAL CURRENT DENSITY

Conducting sensors, known as Wilson plates, are used to measure

the vertical current to ground near HVDC transmssion lines. These

sensors, which take the form of sensing elements surrounded by a guard band or ring, are usually operated several centimeters above

ground to avoid problems with moisture, insects, and nearby foliage.

This mode of operation introduces a source of error into the measurement

since a grounded metal object located above the ground plane will

cause a local enhancement of the electric field. In the FY80 annual

report [2], our efforts to determine the enhancement factors for

various sensors were described in detail. The approach to the problem

will be briefly reiterated here. The enhancement of the electric

field is geometrical (ignoring space charge effects) so that for a given sensor configuration, the enhancement factor may be determined

by making ac measurements of displacement currents. It would be

expected that these same enhancement factors would be observed for

measurements made using dc fields plus ions produced by the

parallel plate system.

During the present reporting period, experiments were completed

which allow a comparison between ac and dc results. It was found

that the dc measurements yielded enhancement factors which depended

on the electric field and space charge density conditions existing

in the parallel plate apparatus as well as on the electric field.

However, for conditions like those expected under a HVDC transmission

line [6], there was agreement between the ac and dc measurements.

The results of the investigation are summarized in figure 1, which

displays both ac results in the form of continuous curves and dc

fields plus ions results which are shown as discrete data points.

The data are plotted as a percentage error, for different sensor

spacings above ground. The dc results are in terms of a parameter

AE/Eg = |Eg - E]_| /Eg whero Eg is the value of the field at the

ground plane and E] is the value of the field 1 cm above the ground

plane. The value of E] is calculated from the theoretical description

3
Measurement

Error

(Percent)

0 10 20 30 40 50

Normalized Wilson Plate Dimension (a/d)

Figure 1. Errors in Wilson plate current density measurements as a function of guard ring width and elevation above the ground

plane. The solid curves are results of measurements with

alternating current. The AE/Eg parameter (see main text)

values for the dc data plotted are as follows: + - 0.004;

k - 0.014; T - 0.024 , I - 0.004 .

4
of the parallel plate facility [4]. The parameter AE/Eg is an estimate of the normalized change in the electric field near the

sensor due to space charge. It is clear from the figure that the

percentage error is not independent of the dc operating conditions,

but that there is good agreement between ac and dc data for small

parameter values.

During a field day held at Project UHV the first week in

October 1981, an extensive series of measurements was made under an operating test line. The preliminary results of these measurements,

which were made using Wilson plates actually employed in the field,

are in good agreement with the laboratory results.

A detailed description of the investigation of errors associated

with Wilson plate measurements has been prepared as a report [6] and

has been submitted to DOE.

4. PRODUCTION OF SPACE CHARGE IN THE LABORATORY

A. Introduction

A new low-speed air-flow facility has been constructed for

evaluating instruments which measure ion-related electrical quantities.

This facility replaces an earlier system which was used fn developing

measurement techniques and ion sources. The earlier system and

related research have been described in previous reports [2,3]. The

new facility is illustrated schematically in figure 2. The aerodynamic

design has been improved substantially. Provision has been made for

incorporating high efficiency particulate air filters at the inlet.

A variable-speed fan completes the system. For the work described below,

the three-speed fan which was used earlier produced the air flow.

B. Approach

To evaluate the new facility, a number of investigations have

been completed. Measurement locations are indicated in figure 2 (II,

III, and IV). At each location, access holes permitted probes to be

inserted at varying distances at different positions above the floor

of the system.

A hot film anemometer was used to characterize the system air

flow. The measurements indicate a substantial improvement over the

earlier system. Turbulence levels are significantly lower, and the

air speed in the test section is uniform except near the walls. No

further efforts were made to improve the air flow, since the space

charge produced in the system using an optimum source design was

very uniform, as is shown later.

5
h*- 2.44 m +i—1.22 m -4— 1.22 m

Test Section Transition Inlet

Figure 2. Top view of permanent low-speed air flow facility to

produce space charge.

6
C. Results

The absolute filter system described in earlier reports [2,3]

was used to measure the net space charge in the flow facility. The

investigations reported here were made to determine, in the present

system, the operational features of ion source configurations developed

previously. These sources utilize a planar geometry in which a series of vertically oriented wires are placed midway between two

wire-mesh ground planes. Provision has been made to allow variation

of ground plane mesh size, number and diameter of corona wires, and

wire-to-ground plane spacing. In all of the results discussed here,

the wire was 64 urn stainless steel and the wi re-to-ground-plane

spacing was 10 cm. Parameters which were varied in the studies are listed below:

(a) Number of corona wires (1, 3, and 9),

(b) Screen mesh size (5 cm x 5 cm and window screen),

(c) Corona voltage (7, 10, 15, and 20 kV),

(d) Air speed at the source (0.5, 0.7, and 0.95 m/s).

Not all permutations were examined, although for each source

configuration all three air speeds were used. For each source, the

absolute filter system was used in a scanning mode to determine the

net space charge density at locations II, III, and IV (^ig. 2) for

five different positions above the floor of the flow system.

Representative results of these measurements are displayed on figures

3, 4, and 5. These figures show the net space charge density measured

at midplane at the three locations of interest. Indicated on the

drawings are the relative positions of the inlet and test section

walls as well as the locations of the corona wires. All results

shown were for a corona voltage of 10 kV, air speed at the source

of 0.5 m/s, and no inlet filters.

In figures 3 and 4, the ground plane was the coarse 5 cm x 5 cm structure and there were one and three corona wires respectively. The

source in figure 5 used nine corona wires and "window screen" ground

planes. This particular configuration was determined earlier to be

optimum in terms of the uniformity of space charge density produced

in the test section.

A number of common features appear in all figures. There is a substantial decrease in net space charge density in going from location

IV to location II (that is, farther downstream). Also, as the ions

move downstream, the distribution becomes more uniform. The peaks

due to the individual wires are observed at all locations for the oneand three-wire sources. The nine-wire source produces the most uniform net

space charge density in the test section.

It is interesting to note the decrease in space charge density

measured at IV as the number of corona wires is increased. This is

probably due to the influence of the individual wires on their nearest

7
Figure 3

DISTANCE FROM WALL (ml

Net space charge

II, III, and IV.

walls are shown.

measurements made at midplane at locations

Relative positions of test section inlet

Source configuration was 1 corona wire

(position indicated) and coarse ground planes (see text)

8
NET

SPACE

CHARGE

DENSITY

tx

108

C/m

Figure 4 .

DISTANCE FROM INLET WALL (m|

Net space charge measurements made at midplane at locations

II, III, and IV. Source configuration was 3 corona wires

(positions indicated) and coarse ground planes (see text).

9
F i gu re 5 .

DISTANCE FROM INLET WALL (m)

Net space charge measurements made at midplane at locations

II, III, and IV. Source configuration was 9 corona wires

(as indicated) and window screen ground planes.

10
neighbors. Calculations done for electrostatic precipitators indicate

a lowering of the field around a corona wire as other wires are placed

near it [7]. This will also affect the spatial distribution of the

ions produced in the corona discharge. The increase near the walls

seen for the nine-wire source reflects the influence of the walls and

the source support structure on the outer corona wires.

The sources examined produced substantially different net space

charge near the source, but in the test section only a 20%

variation from source to source was seen.

Because the new air flow facility was constructed of insulating

materials (plywood and hardboard) there was some concern over charging

of walls during operation. To ascertain the effects of such charging,

the transition section and inlet nearest the source were lined with

aluminum foil. The only observable change was a slight increase in

net space charge in the test section.

0. Discussion and Conclusions

The results of the measurements discussed here, which are similar

to those made previously, may be summarized as follows:

(a) The net space charge measured, in the system ranged

from a maximum of 2.5 x 10"° C/m^ to 2.2 x 10"^ C/m^.

Assuming singly charged ions, this is a range of ion

densities from 1.6 x lO^/cm^ to 1.4 x lO^/cm^.

(b) Changes in corona current as large as a factor of 10 result

in only a 20% change in the measured net space charge.

(c) The net space charge density decreases by as much as a factor

of three in going from location IV to location II (fig. 2).

(d) The optimum source configuration for producing a spatially

uniform space charge is a multi-wire source with window

screen ground planes.

The system is being used to investigate ion counters and net

space charge measuring devices. A number of improvements and

modifications are planned to optimize the system. A variable speed

fan has been installed. Other changes include the construction of a retarding-field array to control the ion density, and development of

a source to be located at the outlet of the transition section.

This last task will allow much higher ion densities to be introduced

into the test section.

11
5. INVESTIGATION OF SOURCES OF ERROR IN SRACE CHARGE DENSITY

MEASUREMENTS USING ION COUNTERS

A. Introduction

The measurement of homopolar space charge density is most commonly

done by using aspiration devices called ion counters. These instruments

can be subject to large systematic errors due to improper design or operation. In earlier reports, we describe one such source of error

which results from the effect of fringing fields at the inlet of the

counter [2]. If the plates to which voltage is applied (the polarizing

plates) are not recessed sufficiently behind the collector plates, the

fringing fields to the wall of the ion counter enclosure can cause

mobile ions to be precipitated to the wall. This loss, which depends

on geometry, polarizing voltage, and the volumetric air flow through

the system, can be significant as is seen in figure 6. Shown are current voltage curves for a counter which was constructed to allow

the geometry of the plate structure to be changed. If there are no voltage dependent losses at the inlet due to fringing fields, then

the current decreases as the polarizing potential increases. It is seen in figure 6 that there are losses even when the polarizing

plates are recessed one plate spacing behind the collector plates.

B. Approach

An ab-initio analysis of the motion of ions near the ion counter

inlet is not possible because of the complicated geometry. However,

insight into the loss mechanisms can be gained by using a twodimensional model and applying numerical analysis methods. A I'inite

element code, resident on NBS computers, has been used to calculate

the electric potential distributions and electric field at the inlet

of the ion counter. The geometry assumed for the model calculation

and the finite element triangul ation resulting from the higher level

code adaptation of the problem are shown in figure 7. In figure 8,

the equipotential values are displayed. A solution was generated for

one-half the geometry, since the reflection symmetry inherent in the

problem was used to specify the appropriate boundary condition on the

midplane. This saves computer space and computation time. An additional

simplifying feature involves the assumption that the entrance to the

counter is not open, but closed and grounded. The effect of this

assumption will be determined in later calculations.

C. Results

The calculations were done for a polarizing potential of one unit

and the equipotentials are drawn in units of 0.1 of the polarizing

potential. For this example, in which the polarizing plates are recessed one plate spacing, there is little field leakage. In contrast.

12
ION

COUNTER

CURRENT,

AMPERES

1 0

1.0 2.0 5.0 10 20 50 100 300

POLARIZING VOLTAGE

Figure 6. Ion counter output current as a function of polarizing

voltage for different plate configurations. The

displacement between the leading edges of the collection and

polarizing plates is x. Positive values of x correspond

to a situation where the polarizing plates protrude in

front of the collector plates. For x = 0, the leading

edges were flush. Because the ion density is independent

of polarizing voltage over the range plotted, the ion

counter current should be constant.
Fi gure 7

.

b.

(a) Two dimensional geometry used in finite element

calculations, (b) Computer-generated high level

triangulation for geometry (a). Inside the crosshatched

area, the triangulation was too dense to be displayed.

14
Figure 8. (a) Boundary conditions used in finite element

calculation, (b) Computer generated equi potent ial

curves, plotted in units of U = 0.1.
figure 9 displays the results of a the polarizing plates protrude one collector plates. Here the extent

Compare, for example, the location

figures 8 and 9.

calculation for a geometry where

plate spacing in front of the

of the fringing fields is significant,

of the 0.1 equipotentials on

While such plots are informative, they do not relate directly to

the problem of losses at the inlet. In order to provide quantitative

results, a program has been written to calculate the trajectories of

ions carried into the ion counter by an air stream. The electric

field as calculated using the finite element program and an appropriate

air flow velocity field, are stored as arrays. In the program, an ion

enters the counter at a particular point. The velocity at that (and

any other) point is determined by adding vectorial ly the flow velocity

vector and the electrical velocity vector:

'' =

’'flow + = Vfio„ + kE

where k is the ion mobility and E is the electric

Using this velocity, the ion is transported for a and, at the new loction, the process is repeated,

trajectory for the ion is estimated.

field at the point,

predetermi ned time

In this way a

Sample trajectories for the geometry of figure 9 are shown in

figure 10. The ion mobility was taken to be 1.4 cm2/V*s, the

polarizing voltage 100 V and the air flow velocity was 42.4 cm/s

into the counter. From such calculations it is possible to estimate

the percentage of ions lost at the inlet for the model geometries

shown. The calculated losses were larger than the measured losses.

In the calculation, the air flow was assumed to be constant while in

the ion counter, the flow converges to the active collection region.

This increased flow rate would decrease the losses. In general, the

calculations were in qualitative agreement with the experiment.

D. Discussion and Conclusions

It is somewhat more difficult to relate these calculations

directly to experimental measurements. Primarily, this is the result

of trying to model a complicated three-dimensional problem with a two-dimensional calculation. Also, in the actual ion counter the

air flow is not uniform and there is some convergence of the air

stream, since the air is restricted to flow only through the

active volume of the counter. In figure 10, this would be the region

inside the plate structure. This convergence implies there is a velocity component perpendicular to the counter axis. While we can measure the air flow speed using an anemometer, we do not have the

capability of measuring velocity components.

The next step in this research will be to optimize the field

calculations and to use a more realistic air flow field. Estimates

will be made of the inward velocity component and the effect of

16
u = 0.0

u = 0.0

“ u = 1.0

”U = 0.0 —-—u = 1.0 du

dn

0.0

7

a

b

Figure 9. (a) Boundary conditions and geometry used for a model calculation in which the polarizing plates

protrude in front of the collector plates.

(b) Computer generated equi potential curves, plotted

in units of U = 0 .1 .
DISTANCE FROM COUNTER WALL

Figure 10. Trajectories of ions entering the ion counter at

different initial locations (as shown). The

geometry is that of figure 9.

18
changes in mobility and air speed on the trajectories will be

investigated. By doing this, we would like to obtain a better

correlation between theory and experiments.

6. ABOVE GROUND OPERATION OF ION COUNTERS

A. Introduction

Direct measurements of the space charge densities p"**, p" near

HVDC transmission lines are required to provide information necessary

to characterize the electrical environment. A value of p can be

inferred by using related measurements, since J = pKE where J is

current density, K is a suitably weighted average mobility and E is

the electric field. One can solve for p if J and E are known from

measurements and an estimate is made of K. However, a direct measurement is preferable and provides information which can be used to

obtain K from the equation above.

A number of sources of error can cause uncertainty in ion counter

measurements [8]. One important concern is the effect of operating

an ion counter above the ground plane in the high electric field

region found near a HVDC line. In particular, the question of what

conditions must exist to allow an accurate measurement of p to be

made must be answered. A critical parameter is the potential of the

ion counter relative to the space potential around the counter. A

cursory examination of the problem suggests that the space charge

density determined may depend on whether ions are attracted to or are repelled from the counter. This in turn depends on the electrical

potential of the counter. This question was addressed by Schwann

[9] who concluded that for any electrical condition in which ions are attracted to the counter, an accurate measurement will be obtained.

This is the case, for example, when the ion counter is operated above

ground level, is at ground potential, and is detecting ions which

are drawn to the counter. There are a limited number of experimental

results available which address this problem, primarily of conductivity

measurements aboard aircraft where self-chargi ng is of concern [10, 11].

The results of these earlier experiments are not directly applicable

to the present problem because of differences in geometry and

measurement technique.

B. Experimental Approach

At NBS we have completed a preliminary series of experiments the

results of which suggest that there are serious problems associated

with making off-ground space charge density measurements. A short

monopolar line was erected in the laboratory as shown in figure 11.

An ion counter and electrometer which were isolated from electrical

ground were located as indicated. The electrical potential of the

measuring system (called the system bias in following discussions)

19
Corona Wire

(50-80 kV)

2.2 m

Electrometer |on

1—1 Counter

1 I

Figure 11. End view of experimental arrangement for studies of

off-ground ion counter operation. Corona wire diameter

was 254 ym. The electrometer and ion counter were

isolated from ground. The wire length was approximately

6 m.

20
could be varied up to 30 kV. Measurements were made for both positive

and negative line voltages of 50 and 80 kV. Two different ion counters

were used, but only the results obtained with a counter in which the

air flow could be varied are reported. Similar results were obtained

for both counters.

The lab in which the experiment was conducted contained various

other pieces of equipment. As a result, it would not be expected that

the electrical conditions under the line would be uniform along the

line. All measurements reported were taken with fixed geometry. The

room temperature was 25 ®C and relative humidity 40%. Total uncertainty

in ^low and current measurements is estimated to be less than 10%

except for the 3.3 x 10 "^ ni^/s flow condition, which is slightly

outside the flowmeter calibration range. Since these were preliminary

measurements, no attempt was made to improve measurement accuracy.

The ion counter polarizing voltage was 218 V which assured that all small

ions entering the counter would be collected for all ^low conditions.

C. Results

For a given line operating condition and ion counter flow rate, the

counter bias was varied over the range ±15 kV, a range which included

the ’space potential ne^r the ion counter. The results of one series

of measurements is shown in figure 12, which is a plot of space charge

density as a function of bias voltage for different counter flow

rates. What is shown is a strong dependence on bias voltage and lack

of consistency among different flow rate measurements.

The cause of the discrepancies among these measurements was found to be field penetration into the opening of the ion counter.

This field penetration resulted in a conduction current to the

collector plates for conditions with no air flow and no polarizing

voltage on the counter. This current is shown in figure 13 as a function of bias voltage. The current goes to zero at a voltage of

11 kV, which is near the space potential measured using a radioactive

source and an electrostatic voltmeter [12]. This conductive current

was considered as a systematic error and was subtracted from the

current obtained under normal operating conditions.

In figure 14, the corrected results are shown for three different 'low

rates. The agreement between the two high flow rates is excellent,

while the difference between these two and the third may be due to an error in flow measurement mentioned earlier.

D, Discussion and Conclusions

The results show that for bias conditions where ions are strongly

attracted to the counter (bias voltage 0 - +15 kV), the calculated p is

approximately constant. However, as the bias voltage approaches the

average space potential, the calculated p drops dramatically. This

result strongly suggests that the actual value of p is unknown.
15.0

Figure 1

Counter Potential (kV)

. Space charge density determined for different flow rates

as a function of ion counter potential. Corona wire was operated at -80 kV . No corrections were applied to data

(see text)

.

22
Ion

Counter

Current

(x

10

Amperes)

-13-11 -9 -6 -3 0 5 10 15

Ion Counter Potential (kV)

Figure 13. Conduction current to ion counter as function of ion

counter potential. Polarizing plates were grounded and

air flow through counter was zero. Corona wire was at

-80 kV.
Counter Potential (kV)

Figure 14. Space charge density determined for different flow rates

and ion counter potential. These results are those of

Figure 12 corrected for leakage current (see text and

fig. 13).

24
Earlier authors [9-11] infer that the flat region is one where there

is a reliable measurement. This conclusion is not substantiated

by the present measurements, since the counter at space potential

should represent a minimum perturbation to the system. It would be

expected that for this condition an accurate measurement of p would

result.

One possible explanation for this observation is that for the

large electric fields at the counter (approximately 30 kV/m) electrical

forces dominate and when ions are not attracted directly to the

counter inlet, the aerodynamic forces are too small to cause ion flow

into the counter. Inlet air speeds are 0.5 - 1.5 m/s just inside the

inlet, while at 30 kV/m an ion with a mobility of 1 x 10"'^m2/V*s

will have a speed due to electrical forces of 3 m/s. Clearly the

net flow near the inlet is complicated and will depend on the

orientation of the counter opening relative to the local field direction

as well as the flow rate into the counter. In the aircraft measurements

cited previously [10,11], fields as high as 50 kV/m near the conductivity

device inlet were observed, but these were for situations where ions

were attracted to the aircraft. When the aircraft was not charged, there

was no electric field near the inlet of the Gerdien tube. These experimental conditions are not analogous to the present situation.

As a result of these preliminary measurements, an unavoidable

conclusion is that accurate off -ground measurements of space charge

densities cannot be made until there is a more complete understanding

of the effect of ion counter bias on the measurement. In particular,

it may be undesirable to operate an ion counter at space potential

rather than in a grounded condition. A number of relevant experiments

have been considered and will be part of any future work in this area.

7. MEASUREMENT OF NET SPACE CHARGE DENSITY

USING AIR FILTRATION METHODS

A. Introduction

The use of air filtration techniques to measure net space charge

is more than 30 years old [13]. Various filter materials have been

used, but the most efficient filters are known as high efficiency

particulate air (HEPA) filters. These filters, which have very high

efficiencies for removing small particles from an air stream, have

also been shown to be very efficient at removing charge from an air

stream [14-16]. A series of measurements has been completed at

NBS which indicate a HEPA filter transmits less than 0.1% of the

incident charge for a wide range of charge densities and flow rates.

B. Approach

In the filtration method, an air stream containing ions is drawn

through a filter. As indicated above, HEPA filters remove more than

99.9 % of the ions from the air stream. The resultant current I to

ground from the electrically isolated filter can be measured. By

knowing the volumetric flow rate (}> the net space charge density p

25
FILTER UNDER TEST

Figure 15. Experimental configuration for measurements in which the

ion counter is used to measure transmitted charge density.
FILTER TO DETECT

Figure 16. Experimental configuration for measurements in which a filter is used to measure transmitted charge density.

27
can be calculated from p = The measurements described below

were designed to measure the fraction of charge transmitted through

the filter as a function of various experimental variables.

These studies were made using the low speed air flow facility

described in section 4, The two experimental configurations used

in the measurements are shown in figures 15 and 16. In each case, the

filter under test was followed by a device to detect the transmitted

space charge. This second device was either a second filter or an ion counter. The use of two different measurement techniques allowed

a check for any possible systematic errors. The measurement process

was somewhat circular. The first set of measurements involved the

ion counter as a detector and these measurements showed that

substantially less than 1 % of the incident charge was transmitted

by the HEPA filter. The HEPA filter was then used as a detecting

filter to measure the transmission of other (non-HEPA) filters. One

of these other filters was found to transmit less than lOfj of the

incident charge for the conditions of interest. This filter was then used as a detecting filter and the HEPA transmission remeasured.

Ions were extracted from the test volume of the low speed air

flow facility as indicated in figure 2. The volumetric flow rate was measured using a commercial flowmeter. Measurements were made for

positive and negative ions together, for conditions where ions of only

one sign were present. A related set of measurements involved the

use of an alpha particle source located near the inlet to the filter

assembly. In this mode of operation large numbers of positive and

negative ions were present in nearly equal numbers.

The ion counter was operated with a polarizing potential which

was large enough to ensure collection of all ions with mobilities

greater than 0.005 x 10"^ m2/V*s for all flow conditions. No means were available to measure the mobility spectrum p(K), but I-V curves

made with the ion counter indicated more that 90% of the ions had

mobilities greater than 0.5 x 10"^ m2/V*s.

C. Results

The results of measurements on the HEPA filter are summarized in

figure 17. It is clear that for all data presented the transmission

is substantially less than 0.1%. The cause of the differences

between the transmissions measured using the ion counter and filter

as a detector is unknown, but may be due to mobility dependent response

of the filters and the ion counter.

0. Discussion and Conclusions

The HEPA filter has been shown to be highly efficient at removing

charge from an air stream. This implies that for conditions where

space charge is predominately made up of ions of one polarity, a direct comparison can be made between the net space charge density

determined by a filter and the polar space charge density measured by

an ion counter. By using a source of space charge such as that

28
Uigurs 17. Sunmary of HEPA filter transmission measurements. Shown

for comparison are the results of Moore et al . Paltridge

and the factory measured transmission (DOP test). Filcer

filter measurements--# - positive ions, o - negative ions

Filter-ion counter measurements---!- - positive ions,

X - negative ions. Filter-ion counter measurements

(alpha source)--^- positive ions, 7 - negative ions.

29
produced by the low speed air flow facility, the HEPA filter can be

used to calibrate ion counters. This can be done by directly comparing

measurements of net space charge density made using the two instruments

when ions of only one sign are present. Other sources of error, such as ion losses in ducting, would also have to be determined.

8. MEASUREMENTS OF MOBILITY SPECTRUM

USING RETARDING FIELD METHOD

A. Introduction

The ions making up a given space charge p have a distribution of

mobility values p(K). These mobilities reflect the different ion

species present in the space charge and are determined by the "age"

of the ions, the gas mixture of which the ions are a part, etc.

Techniques for measuring p(K) for small ions using classical air

flow methods are described in the literature [17,18] and extensively

analyzed in the monograph by Tammet [8]. Similar measurements of the

mobility of charged aerosols are used to infer a size distribution.

Mobility measurements are of interest in that they provide information

about a collection of ions. There is, however, no way of extracting

direct information about the identification of ionic species from

mobility data and so the most useful studies of ions involve mobility

measurements combined with mass spectrometric identification of ions.

Knowledge of p(K) is useful and necessary if there is to be a complete understanding of devices used to measure space charge

densities. For example, the response of an ion counter to a space

charge sample depends on the mobility spectrum of the space charge.

This may also be the case for certain filtration methods used to

measure net space charge. As a result, consideration has been given

to different approaches to measuring the mobility spectrum of ions

produced in the low speed air flow facility discussed earlier.

Classical methods developed by atmospheric electricity scientists

involve the use of cylindrical Gerdien tubes with either a continuous

or divided geometry [18]. In each case, an I-V curve is generated

for the device under constant flow conditions. Numerical derivatives

are required to obtain p(K) from the I-V curve; two for the continuous

Gerdien tube and one for the divided Gerdien tube. This in itself presents

difficulties but a more fundamental problem is caused by the presence

of the space charge inside the instrument. The self-field due to the

space charge sufficiently modifies the applied field so that the simple

electrostatic analysis ignoring space charge is i nappropri ate. As a result of this, an alternative approach was taken which was particularly

appropriate for the air flow system, but which is also susceptible to

space charge effects. Only differential measuring techniques appear to

be relatively immune to space charge effects.

B. Approach

A retarding field technique similar to that used in measurements

of electron energies was adopted. The approach is illustrated in

figure 18(a). The ions are carried downstream in the flow facility

30
at constant speed by the air stream. They pass through a grounded

screen and enter a uniform -^'eld region. 3y apolying an adjustaole

voltage (V) to the center screen, the fraction of the ions to be

prevented from passing through the retarding *’eld region can be var'ed

In the region between the plates, the net speed will be v - kE,

where v is the air speed, 1< the ion mobility and E the electric field.

Only ions with kE<v will pass through the screen. "Hie limiting

mobility is

'<0 = V/E.

This is a simplified discussion which ignores such ef"^ects as ’'eld

penetration through the screens, turbulence, diffusion space charge

influence, etc. These ions which do pass through the ^ilter are detected downstream with a HEPA filter assembly ('^’g. 2).

Tnis technique can be analyzed mathematically as shown below.

Assume a distribution of soace charge density o{K) where

o(k)dk = oq,

where Oq 's the total space charge density. the ions are oassed

through some ’-egion wh^ch has a filter characterisit’ c 3(k), then

only some fraction oq transmitted,

(’<o) =1 f

G(k,ko)p(k)dk.

In the following discussion, pq assumed to be ecual to 1.

If we have a retarding field analyzer, the idealized form o^ 3(k,!<Q)

is shown in figure 18(b). With this form of G, all ions w^th k<kQ will

pass through the filter where kQ = v/E, v = air soeed and E 's the

ODDOsing electric f'eld. The electric field is calculated -^ron E = //d

where V is the ootential of the retarding screen and d ’s the screen-to

screen spacing.

-or this form of 3, can be calculated as

c =

I 3{k,kQ)o(k)dk = / p(k)dk.

"^en, taking the derivative of p gives do’/dk^ = p (ko).

change of variable is reouired to relate the f'aw data to the relat'cnships above, since what is reouired is a derivative with resoect to V,

the '^etard'ng voltage. Using the follow'ng ’e " ationsh' o.

31
kQ = v/E = v/(V/d) = vd/V

we can write dk^ = -(vd)/V^ dV

and finally obtain

dp/dK^ = (-V^/vd) dp/dV .

C. Results

Examples of data obtained using this technique are shown in

figure 19. Although the curves appear smooth, there is uncertainty

in each point and the curves are not very reproducible. Using these

data and calculating a numerical derivative, values for p(k) can be

determined. The results of two different measurements for negative

ions taken on different days are shown in figure 20. The variations

introduced by the numerical derivatives are obvious, as is a large

high mobility tail which appears to be an artifact of the measurement

technique, possibly due to space charge effects.

In figure 21 are shown mobility spectra for positive and negative

ions taken on different days, but with the same flow and electrical

conditions (except for polarity).

Some attempts were made to use various smoothing techniques to

improve the spectra obtained. Although some "peaks" disappeared, the

overal 1- results were not satisfying. Better raw data are required if

the technique is to prove satisfactory.

D. Discussion and Conclusion

During this preliminary work, it became clear that computer-aided

data acquisition would be necessary. Tnis would allow a series of scans through the retarding voltage which would result in significant

averaging. This procedure would certainly improve the raw data and

smooth the numerical derivative. A more serious problem appears to

be one alluded to earlier in the discussion of the use of Gerdien

tubes - namely, the perturbation of the applied field by the space

charge. A recent paper considers this phenomena in a discussion of

electron energy measurements using retarding field methods [19].

It appe ikely that reliable measurements of p(k) for ion

densities 10 /v.m or greater may have to be done using differential

instruments, where the space charge inside the device is limited.

Such devices are described in the literature [20]. An air blast

32
Flow Facilty

Wall

V=o V = Vq V=o

(a)

(b)

Figure 18. (a) Schematic of retarding field analyzer. The actual

assembly was located near III in fig. 2. Screen material

was aluminum window screen, (b) Representation of

transmission function G(k,ko) retarding field

analyzer. Shaded portion of p(k) would be transmitted.
Retarding Voltage (Volts)

Figure 19. Space charge density detected downstream from analyzer

(fig. 18). Shown are results for both positive and

negative ions.
p

(K)

(Abritrary

Units)

Kg X 10"^ (m^/Vs)

Figure 20. Mobility distribution p(k) for negative ions taken on two different days. Results shown are obtained from raw data as described in the text. No smoothing techniques

were employed.
Figure 21. A comparison of mobility distribution for positive and

negative ions taken on different days, but for similar

flow and electrical conditions. Data are unsmoothed.
system has been adopted for use at Project UHV and used to measure

mobility spectra for different sources [21]. This measurement problem,

in the present context, remains unsolved and will require considerable

future effort.

9. DISCUSSIONS AND CONCLUSIONS

The individual efforts described in this report show continuing

progress toward the goals of the project. Completion of the

investigation of errors associated with above-ground operation of

sensors for measuring vertical current density has provided guidelines

for substantially improving the accuracy of these measurements.

Considerable effort has been expended in studying errors associated

with measurements of net space charge and ion density. Part of this

effort has been the construction and evaluation of a facility to

produce a test volume containing space charge. The use of this

facility in investigating air filtration methods for the measurement

of net space charge density has led to a potentially useful scheme

for calibrating ion counters. If the space charge is made up of ions

of predominately one polarity, then ion density and net space charge

density are equivalent. In this case, a measurement of net space

charge density can be made using a filtration method and can be

directly compared with that made using an ion counter. The uncertainties

in this procedure are associated with the filtration method and are known or can be estimated.

Two sources of error directly associated with the operation of

ion counters have also been investigated. The study concerned with

losses at the counter outlet due to fringing fields has been completed.

The second and more severe problem is associated with operation of an ion counter above the ground plane. The work discussed in this report

has shown clearly that these off-ground measurements are affected by

the ion counter potential.

In the future, further investigations of the electrical environment

around a HVDC transmission line will require mobility measurements

and an identification of ionic species present near the ground.

Knowledge of the mobility spectrum of ions is also required in the

laboratory when evaluating ion counter performance. A preliminary

evaluation of a retarding field method for measuring the mobility

spectrum of ions in the low speed air flow facility has been described.

Tnis method is specialized and is not of general usefulness. An

increase in emphasis on ion identification and mobility measurements

is considered essential to an increased understanding of the complex

electrical environment near a HVDC transmission line and should form

an important part of any laboratory effort in this area.

37
10. REFERENCES

[1] IEEE Standard 644, Recommended Practices for Measurement of

Electric and Magnetic Fields from AC Power Lines.

[2] Annual Report to Department of Energy, Report No. 81-2267,

October 1, 1979 - September 30, 1980.

[3] Quarterly Report to Department of Energy, Report No. 81-2334,

April 1 - June 30, 1981.

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

With Space Charge, Nat. Bur. Stand. (U.S.), NBSIR 80-2177 (1980).

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

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

[6] R. H. Mcknight, F. R. Kotter, and M. Misakian, Measurement of Ion

Current Density at Ground Level in the Vicinity of High Voltage

DC Transmission Lines, Nat. Bur. Stand. (U.S.), NBSIR 81-2410 (1981)

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

,

Reading, Mass., pp. 95-115, 1963.

[8]

- H. F. Tammet, The Aspiration Method for the Determination of

Atmospheric-Ion Spectra, Scientific Notes of Tartu State University,

Issue 195 (translated from Russian), 1970. Available from U.S.

Dept, of Commerce, Clearinghouse for Federal Scientific and Technica

Information, No. TT68-50499.

[9] W. F. G. Swann, On Certain Matters Relating to the Theory of

Atmospheric-Electric Measurements, Terr. Mag. 19, pp. 205-218, 1914.

[10] S. C. Coroniti, A. J. Parzcale, R. C. Callahan, and R. Pattern,

Effect of Aircraft Charge on Airborne Conductivity Measurements,

Jour. Geophys. Res. 57, pp. 197-205, 1952.

[11] J. H. Kraakevik, The Airborne Measurement of Atmospheric

Conductivity, Jour. Geophys. Res. 63, pp. 161-169, 1958.

[12] J. A. Chalmers, Atmospheric Electricity, Pergamon Press,

Oxford, Eng. (2nd Ed., 1967), pp. 218-219.

[13] J. Zeleny, On the Ratio of the Velocities of the Two Ions

Produced in Gases by Roentgen Radiation; and on Some Related

Phenomena, Phil. Mag. 1898, p. 120.

38
REFERENCES (cont.)

[14] R. B. Bent, The Testing of Apparatus for Ground Fair Weather

Space Charge Measurements, J. of Atm. and Terr. Phy. 25, 1964,

p. 313.

[15] G. W. Paltridge, The Retention of Atmospheric Radioactivity by

Fibrous Filters, J. Geo. Res. 1967, p, 1269.

[16] C. B. Moore, B. Vonnegut and F, J. Mallabon, Airborne Filters

for the Measurement of Atmospheric Space Charge, J. Geo. Res., 56,

1961, p. 3219.

[17] 0. Blackwood, The Existence of Homogeneous Groups of Large Ions,

Phys. Rev. _16.> 1920, p. 85.

[18] E. C. Whipple, Jr., An Improved Technique "^or Obtaining

Atmospheric Mobility Distributions, J. Geo. Res. 55, 1960,

p. 3679.

[19] R. N. Varney, Falsification by Space Charge of Electron Beam

Energy Measurements, Am. J. Phys. 49, 1981, p. 425.

[20] M. Knoll, J. Eichmerer and R. W. Schon, Properties, Measurement

and Bioclimatic Action of "Small" Mul timol ecul ar Atmospheric Ions

j_n Advances in Elec, and Electron. Phy., Marton, L. ed, 19, 1964,

p. 178.

[21] G. B. Johnson, Characterization of Ions Produced for HVDC

Exposure Systems, abstract of paper presented at 3rd Annual

Conference of the Bioelectromagnetics Society, August 9-12,

1981.

39

NBS«n4A tREv. 2 *ac) U.S. OEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 82-2527

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

July 1982

4. TITLE and subtitle

1981 ANNUAL REPORT: ELECTRIC AND MAGNETIC FIELD MEASUREMENTS

5. AUTHOR(S)

R. H. McKnight, F. R. Kotter, M. Misakian, and J. N. Hagler

S. performing organization (If joint or other than NBS, see in stru ction s) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report 8. Period Covered

WASHINGTON, D.C. 20234

9.

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

Department of Energy

Division of Electric Energy Systems

Washington, D.C. 20585

10.

SUPPLEMENTARY NOTES

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

11.

ABSTRACT I A lOO-^r/ord or less factual summary of most si gnificant information. If document includes a si gnifi cant bibliograohy or literature survey, mention it here)

The NBS program is concerned with developing methods for evaluating and calibrating instrumentation for use in measuring

the electric field and various ion-related electrical quantities

in the vicinity of high voltage direct current (HVDC) transmission

lines and in apparatus designed to simulate the transmission

line environment.

The laboratory investigation of errors associated with above-ground operation of sensors for measuring vertical current density has been completed. Significant errors were observed

for both unguarded and guarded sensors, ranging from 4 to 25%

for the guarded plates and 10 to 35% for unguarded plates for a wide range of geometrical parameters. Preliminary results from

a field day held in October 1981 are in agreement with the laboratory results.

A new low-speed air-flow facility has been constructed. Using multiwire planar corona discharge ion sources, ion densities

from 1 .6 X 105/cm4 to 1 .4 x lO^/cm^ have been measured using an absolute filter technique.

Losses at the inlet of a parallel plate ion counter due to

fringing fields have been determined using an ion counter with

variable geometry. Calculations based on a two-dimensional

finite element code predict a greater loss than actually observed,

but are in qualitative agreement with experimental results.

The above-ground operation of a parallel plate has been investigated using a monopolar line. The initial results obtained

show that the ion densities measured using the counter are strongly

dependent on ion counter potential. These results represent only

a limited set of measurements, but indicate problems associated

with above-ground measurements of ion density.

The transmission of charge in an air stream through a high

efficiency particulate air (HEPA) filter has been determined to

be less than 0.1% for a wide range of flow and ion density conditions. This result indicates that a HEPA filter may be useful in calibrating ion counters when the net space charge

density and ion density are equivalent.

A retarding field scheme has been considered for use in measuring the mobility spectrum of the ions in the low-speed air

flew facility. The results of a limited number of measurements

show that the method may be a useful one for this application

but that a number of problems need further investigation, including

the effects of space charge.

12.

<EY WORDS (Six to twelve entries; alohabetical order; capitalize only proper names; and separate key words by semicolons)

current density measurements; high efficiency particulate air filter; high voltage

dc transmission lines; ion counter; ion density; net space charge density.

13.

AVAILABILITY

i^Unl imited

i~

For Official Distribution. Do Not Release to NTIS

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

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