EPA-520/2-76-008
AN EXAMINATION OF ELECTRIC FIELDS
UNDER EHV OVERHEAD POWER
TRANSMISSION LINES
:*,,
•SSS?
mm
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Radiation Programs
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AN EXAMINATION OF ELECTRIC FIELDS UNDER
EHV OVERHEAD POWER TRANSMISSION LINES
R. A. Tell, J. C. Nelson, D. L. Lambdin, T. Whit Athey,
N. N. Hankin, and D. E. Janes, Jr.
April 1977
This work was supported, in part, by the
Nuclear Regulatory Commission, Division
of Reactor Licensing, under Interagency
Agreement AT(49-24)-0125 [EPA-IAG-R5-0717]
U.S. Environmental Protection Agency
Office of Radiation Programs
Electromagnetic Radiation Analysis Branch
9100 Brookville Road
Silver Spring, Maryland 20910
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DISCLAIMER
This report has been reviewed by the Office of Radiation Programs,
U.S. Environmental Protection Agency, and approved for publication. Men-
tion of trade names or commercial products does not constitute endorsement
or recommendation for their use.
ii
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PREFACE
The Office of Radiation Programs of the Environmental Protection Agency
carries out a national program designed to evaluate population exposure to
ionizing and nonionizing radiation, and to promote development of controls
necessary to protect the public health and safety. This report describes
the measurement of electric field strengths under extra-high voltage trans-
mission lines and compares the measured values with analytical predictions.
Readers of this report are encouraged to inform the Office of Radiation
Programs of any omissions or errors.
W.D. Rowe, Ph.D.
Deputy Assistant Administrator
for Radiation Programs (AW-458)
ill
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TABLE OF CONTENTS
Page
DISCLAIMER 11
PREFACE Ill
TABLE OF CONTENTS v
LIST OF TABLES vl
LIST OF FIGURES vll
SUMMARY AND CONCLUSIONS 1
INTRODUCTION 2
ANALYTICAL EVALUATION OF FIELD STRENGTHS 2
FIELD MEASUREMENT TECHNIQUE AND RESULTS 16
MEASUREMENT OF SHORT CIRCUIT CURRENT AND PERCEPTION 27
REFERENCES 39
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LIST OF TABLES
Table 1.
Table 2.
Table 3.
Table A.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Page
Specifications for Transmission Lines Used in Measurements 4
Minimum Line Clearances and Expected Field Strengths 16
Specifications of Monroe Electronics Model 238A-1 Portable
Differential AC Electric Fieldmeter 17
765 kV Marysville Line Measured Clearances and Voltages 23
Measured and Calculated Field Strengths at Several Line
Heights 24
Measured Electric Field Strength as a Function of Height
Above Ground 26
345 kV Twin Branch Double Circuit Line Clearances and
Voltages 26
Body Currents While Shorting Van to Ground 32
Isc for Three Individuals as a Function of the Electric
Field Strength - Individuals Standing on Hot Line Blankets 35
IBC for Three Individuals as a Function of the Electric
Field Strength - Individuals Standing Directly on Earth
Ground
Human Perception and Annoyance Tests for Transient Spark
Discharge
Table 11.
Table 12. Human Perception and Annoyance Tests: Umbrella Discharge
35
36
38
vi
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LIST OF FIGURES
Page
Figure 1 Electric field strength profile for 765 kV line 5
Figure 2 Variation in field strength as a function of height above
ground for a 765 kV line 6
Figure 3 Electric field strength profile for 345 kV double circuit
line 8
Figure 4 Variation in field strength as a function of height above
ground for a 345 kV double circuit line 9
Figure 5 Electric field strength profile for 345 kV single circuit
line 10
Figure 6 Variation in field strength as a function of height above
ground for a 345 kV single circuit line 11
Figure 7 Electric field strength profile for 500 kV single circuit
line 12
Figure 8 Variation in field strength as a function of height above
ground for a 500 kV single circuit line 13
Figure 9 Ex, Ey, and |E~| from 765 kV power lines at 6 ft. above
ground 14
Figure 10 Ex, Ey, and |1"| from 765 kV power lines at 12 ft. above
ground 15
Figure 11 Phase configuration of 765 and 345 kV lines used for
measurements 19
Figure 12 Photograph of field measurements technique 20
Figure 13 Photograph of 765 kV line measurement site 21
Figure 14 Measured and calculated Ey for Marysville 765 kV line 22
Figure 15 Measured and calculated Ey for Twin Branch 345 kV double
circuit line 25
Figure 16 Measured and calculated Ey for Doubs-Conastone 500 kV
single circuit line 28
Figure 17 Measurement of I for Chevrolet car 29
SG
vii
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Page
Figure 18 Measurement of I8C for EPA van 30
Figure 19 Measurement of electric field above roof of van 31
Figure 20 Measurement of Isc for an individual 33
Figure 21 Measurement of converged electric field strength above
individual's head 34
Figure 22 Perception testing with an umbrella 37
viii
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AN EXAMINATION OF ELECTRIC FIELDS UNDER
EHV OVERHEAD POWER TRANSMISSION LINES
SUMMARY AND CONCLUSIONS
This report contains measured data on electric field strengths near ground
level under 765 kV and 510 kV single circuit and 345 kV double circuit over-
head power transmission lines, values of short circuit current originating on
vehicles situated in relatively intense electric fields, and estimates of
perception and annoyance for humans resulting from transient spark discharge.
Calculated data is given for the electric field strength beneath 765 kV single
circuit, 500 kV single circuit, and 345 kV single and double circuit lines.
The maximum measured electric field strength beneath the 765 kV line was
approximately 10 kV/m at 3 feet above ground, occurring just outside the outer
phase conductors. This value rises to about 12.5 kV/m at 6 feet above ground.
The field strength decreases to 1 kV/m at approximately 170 feet from the
center of the right-of-way (ROW) and to 100 V/m at approximately 360 feet
from the ROW center. The maximum measured electric field strength beneath the
510 kV line was approximately 5 kV/m at about 3 feet above ground, occurring just
outside the outer phase conductors. The field strength decreased to 1 kV/m at
approximately 120 feet from the ROW center and to 200 V/m at approximately
200 feet from the ROW center. The maximum measured electric field strength
beneath the 345 kV double circuit line was about 4 kV/m at the center of the ROW,
and decreased to 1 kV/m and 100 V/m at distances of 69 and 160 feet from the ROW
center, respectively.
Short circuit currents of 2 mA were measured from the EPA radiation
analysis van and 0.6 mA from an intermediate sized automobile, when they were
located in a position where the unperturbed field was about 9 kV/m.
A 100 foot long, simulated wire fence produced a short circuit current of 0.9 mA
to ground when aligned parallel to an outside conductor and subjected to a
field strength of about 10 kV/m.
An average value of induced body current of about 18 yA/kV/m was deter-
mined for several individuals located in the electric field beneath the
765 kV line. For humans insulated from ground, the threshold for annoyance
from transient spark discharge was about 6 kV/m. This is the maximum field
strength at 3 feet above ground under the outer phase conductors of a
765 kV line with 60 feet of clearance (see Figure 1).
Based on this study we conclude that under normal conditions electric
fields strengths under EHV overhead power transmission lines can be calculated
with sufficient accuracy to preclude the necessity of field measurements.
Exceptions might occur where the terrain is non-uniform or where geographical
or man-made anomalies might significantly perturb the field. In addition it
would appear that annoyance due to spark discharge is not likely to occur at
distances greater than 100 feet from the center of the power line right-of-way,
and then only for power lines that are 500 kV or greater.
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INTRODUCTION
This report contains information on several physical parameters of
environmental interest in the near vicinity of extra-high-voltage (EHV)
overhead power transmission lines. The work consists of analytical analyses
and subsequent field measurements for a number of different power line con-
figurations. The study consisted of two phases: (a) an analytical evalua-
tion of the electric field strengths in the vicinity of typical 345, 500, and
765 kV transmission lines, and (b) field measurements to determine actual
field strength values under a 345 kV double circuit line and under 510 kV and
765 kV single circuit lines. The second phase included measurements of short
circuit current (I8C) to ground from several people, a car, a van, a farm
tractor, and a simulated wire fence. Data were also obtained on transient
spark discharge from people immersed in electric fields in an attempt to
define thresholds for both perception and annoyance. This work was supported,
in part, by the Nuclear Regulatory Commission through an interagency agreement,
AT(49-24)-0125, [EPA-IAG-R5-0717].
Of particular interest for this study was the opportunity to make field
measurements under a normally operating 765 kV line. An ideal measurement
site was identified, with the assistance of the American Electric Power
Company [1], near the Indiana and Michigan Electric Company's (I&M) Dumont
Substation, approximately 20 miles south of South Bend, Indiana. I and M is
a subsidiary of AEP. This particular location was chosen because of the flat
terrain under the available transmission lines, the number of 765 kV lines
available (four 765 kV lines enter the Dumont station and, in the event of an
outage on one line, measurements could have been shifted to another 765 kV
line), and the close proximity to the measurement site of the substation,
ensuring that the operating voltage on the lines being used for the field
measurements was accurately known. Field measurements were conducted during
the period 12-14 May 1975.
ANALYTICAL EVALUATION OF FIELD STRENGTHS
A number of methods have been used to obtain theoretical values for the
electric field strength beneath overhead transmission lines [2]. The method
used to calculate electric field strengths in this report is one developed
by Mr. John Walker of the Bonneville Power Administration, Portland, Oregon
[3], The procedure is Implemented by a computer program which takes as input
the electrical and geometric characteristics of the line. This data includes
the height of the line conductors above ground, the number and geometry of the
subconductors (if more than one is used) for each phase of the line, the line
to neutral voltage on each conductor, the diameter of each subconductor, the
phase spacing for the line, and the coordinates of the desired calculation
point (i.e., the point at which one wishes to compute the field strength).
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3
This calculation is based on the fundamental field equation
E = where (1)
2ire0r
E is field strength in volts/meter
q is charge per unit length in coulombs/meter
e0 is permittivity of air = 8.85xlO~12 farads/meter
r is distance from the charge in meters.
When the geometry in the cross-section of interest is known, the only unknown
in the above equation is q, the charge on the conductor. The charge can be
calculated from
q = CV where (2)
C is capacitance per unit length in farads/meter
V is voltage impressed on the conductor in volts.
The procedure incorporates the method of images where a set of equal and
opposite charges are placed directly below the earth surface the same distance
the conductors were above the earth. The cross-section of line charges now
consists of charges representing the conductors and opposite charges on the
conductor images in the earth to produce a line of zero potential at the
earth's surface. The field strength due to the energized conductor can then
be computed at any point. This program was modified to run on the IBM 370
computer available to EPA and was subsequently modified to determine values
for the horizontal and vertical components of the field strength in addition
to the total magnitude.
Several line types and configurations were analyzed with this program.
Calculations were made for a number of line clearances and measurement
heights for a 765 kV single circuit line and a 345 kV double circuit line,
each with nominal physical specifications equal to the lines located at the
measurement site and, in addition, for a 345 kV single circuit line and a 500
kV single circuit line. Typical line specifications were obtained from AEP
and the EHV Transmission Line Reference Book [4]. Table 1 summarizes the
particular line configurations chosen for this study which are assumed to be
representative of possible configurations found at various locations in the
United States. The transmission line, which is suspended between two towers,
hangs in a catenary curve so that the height above ground varies from a
maximum at the towers to a minimum somewhere between the towers. For
simplicity line height will be expressed as the minimum vertical distance
between the ground surface and the line. Typical tower heights range from
approximately 50 to 100 feet.
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Table 1
Specifications for Transmission Lines Used in Measurements
345 kV 345 kV 500 kV 765 kV
Nominal Specifications DEL Single Single Single
Phase Spacing (ft.) 22.5 25 35 44
No. of Subconductors 2224
Diameter of Subconductors (in.) 1.165 1.108 1.821 1.165
Spacing of Subconductors (in.) 18 18 18 18
The computed values of the magnitude of the electric field strength, E,
are plotted vs. the distance from the center of the line right-of-way (ROW)
for the 765 kV line configuration in Figure 1. This graphical display shows
only half of the total lateral profile of the electric field strength since
the field is symmetrical about the center of the ROW. Figure 1 shows the
magnitude of the field strength at a distance of 3 feet above ground for line
clearances of 30, 40, 50, 60, and 70 feet respectively. Actual line installa-
tions always exceed 40 feet in this country, 40 feet being the minimum design
clearance used for 765 kV technology. The family of curves is intended to
illustrate the effect of line clearance on the value of the field strength
beneath the line. Peak values are seen to vary from 17.8 kV/m to 4.8 kV/m
for line clearances varying from 30 feet to 70 feet respectively. In all
cases of line clearance, the field strength is down to about 1 kV/m at a
distance of approximately 170 feet from the center of the ROW. From this
point the field diminishes very slowly until it reaches a value of 100 V/m
at a horizontal distance of 330 feet. Commonly found minimum clearances for
presently operating 765 kV lines in this country cover the range of 40 to 60
feet which implies an unperturbed maximum, field strength range of 11.8 to
6.3 kV/m.
Figure 2 is a plot of the variation in the electric field strength at
various heights above ground for line clearances of 30, 40, 50, 60, and 70
feet for the same 765 kV line. It is evident that the change in field
strength over moderate changes in heights above ground is small. This holds
true until the height above ground begins to approach the line clearance. As
an example, for a line clearance of 40 feet, the field strength changes less
than 20 percent for all heights from ground level to 14 feet above ground.
This slight variation with height above ground will be evident in some of the
experimental data presented later. As the line height above ground increases,
its influence on the field strength near the ground decreases, where, for
example in the 70 foot clearance case, the change in field strength is less
than 11 percent for the same 14 foot variation in height above ground.
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ELECTRIC FIELD STRENGTH PROFILE FOR A 765 kV SINGLE CIRCUIT LINE
40'
FIELD STRENGTH VALUES ARE COMPUTED
AT A POINT 3 FT. ABOVE GROUND FOR
VARIOUS LINE CLEARANCES
25
50
75
100 125 150 175 200
DISTANCE FROM CENTER OF ROW |FT.|
225 250
27S
Figure 1 Electric field strength profile for 765 kV line (Note that two ordlnate
scales are used, the left for distances to 125 ft., the right for dis-
tanges greater than 125 ft.)
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36
34
32
30
28
26
24
J 22
*»
2 18
Ul
fc 16
3
uj 14
. u.
12
10
8
6
4
2
0
VARIATION IN FIELD STRENGTH AS A FUNCTION OF HEIGHT ABOVE GROUND
FOR A 766 kV SINGLE CIRCUIT LINE
, 30' LINE CLEARANCE
8 10 12
HEIGHT ABOVE GROUND (FTJ
14
18
18
J
20
Figure 2 Variation in field strength as a function of height above ground for a
765 kV line
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Similar field strength plots are provided in Figure 3 and 4 for a
500 kV line. Maximum gradients vary from 9.9 kV/m at a clearance of 30 feet
to 4.3 kV/m with a 50 foot clearance. Here the gradient is down to 1 kV/m at
about 115 feet from the ROW center and 100 V/m at 220 feet. Again the varia-
tion of gradient with height above ground is not rapid, as long as the line
has sufficient clearance.
Figures 5 and 6 contain similar information for a 345 kV double circuit
line. Here the low line clearance of only 20 feet produces a well defined
peak in the vicinity of the conductors. Again the variation of electric
field strength with height above ground, in cases of realistic clearances, is
slight. The maximum field strength lies in a range of 11.4 to 4.0 kV/m for
clearances from 20 to 50 feet respectively. The field strength is down to
1 kV/m at approximately 80 feet from the ROW center and 100 V/m at 350 feet.
Data for a single circuit 345 kV line are given in Figure 7 and 8. It
was noted [4] that a number of lines of this type have minimum clearances in
the neighborhood of 25 feet. Such a clearance would imply a field gradient
of 7.8 kV/m under the conductor at 3 feet above ground. This is comparable
to the field strength produced at the same height above ground under a 765 kV
line with a nominal clearances of 55 feet (interpolated from Figure 1). The
maximum gradient values range from 11.1 to 3.6 kV/m for the 345 kV single
circuit line with corresponding clearances ranging from 20 to 40 feet. The
field strength is down to 1 kV/m at approximately 80 feet from the ROW center
and down to 100 V/m at approximately 160 feet.
From reference 4 minimum clearances for 345, 500, and 765 kV lines were
identified and corresponding calculated electric field strength values are
summarized in Table 2. It is noted that the field strengths associated with
345 kV lines can approach those found near 765 kV lines.
Additionally, it appears that the unperturbed electric field strength
does not vary appreciably over the average height of a man standing on earth,
or about 6 feet. In the case of a 765 kV line with a 40 foot clearance, this
variation amounts to about 3-5 percent maximum. It should be noted that this
is with respect to the unperturbed electric field before the individual is
introduced into the field. Perturbations caused by the conductive character
of humans can cause relatively large increases in the immediate surface gra-
dient by concentrating the field [5].
In all of the above graphs, the electric field strength has been specified
in terms of the resultant field strength or its magnitude. In another cal-
culation, the vertical and horizontal components of the electric field were
determined. Figures 9 and 10 show how these components vary at two different
heights above ground for a 765 kV line with a clearance of 40 feet. The
interesting aspect to these plots is the increase in the absolute value of
the horizontal component as the height above ground increases. As the height
approaches the line, the influence of the adjacent horizontally spaced phases
becomes significant, causing the horizontal component of the gradient to be a
substantial fraction of the magnitude. However at heights above ground of
less than 6 feet, the horizontal component is essentially negligible.
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ELECTRIC FIELD STRENGTH PROFILE FOR A SOOkV SINGLE CIRCUIT LINE
FIELD STRENGTH VALUES ARE COMPUTED
AT A POINT 3 FT. ABOVE GROUND FOR
VARIOUS LINE CLEARANCES
25
50
75
100 125 150 175 200
DISTANCE FROM CENTER OF ROW |FT.|
225 250
275
Figure 3 Electric field strength profile for 500 kV single circuit line (Note
that two ordinate scales are used, the left for distances less than
100 ft., the right for distances greater than 100 ft.)
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VARIATION IN FIELD STRENGTH AS A FUNCTION OF HEIGHT ABOVE GROUND
FOR A 500 kV SINGLE CIRCUIT LINE
8 10 12
HEIGHT ABOVE GROUND (FT.)
30' CLEARANCE
16
18
J
20
Figure 4 Variation in field strength as a function of height above ground for a
500 kV single circuit line
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10
10
i
ELECTRIC FIELD STRENGTH PROFILE FOR A 345kV DOUBLE CIRCUIT LINE
CONDUCTOR
ItCATIONS
FIELD STRENTH VALUES ARE COMPUTED
AT A POINT 3 FT. ABOVE GROUND FOR
VARIOUS LINE CLEARANCES
.7
25 SO 75 110 125 150 175 200
DISTANCE FROM CENTER OF ROW |FT.|
225
250
27S
310
Figure 5 Electric field strength profile for 345 kV double circuit line
(Note that two ordinate scales are used, the left for distances
less than 100 ft., the right for distances greater than 100 ft.)
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11
VARIATION IN FIELD STRENGTH AS A FUNCTION OF HEIGHT ABOVE GROUND
FOR A 345 kV DOUBLE CIRCUIT LINE
36
34
32
30
28
26
24
g 18
| 16
5! w
12
10
8
6
4
2
20'CLEARANCE
8 10 12
HEIGHT ABOVE GROUND (FT.)
14
16
18
20
Figure 6 Variation in field strength as a function of height above
ground for a 345 kV double circuit line
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12
ELECTRIC FIELD STRENGTH PROFILE FOR A 345kV SINGLE CIRCUIT LINE
FIELD STRENGTH VALUES ARE COMPUTED
AT A POINT 3 FT. ABOVE GROUND FOR
VARIOUS LINE CLEARANCES
.4 _
.3
25
50
100 125 150 175 200
DISTANCE FROM CENTER OF ROW |FT.|
Figure 7 Electric field strength profile for 345 kV single circuit line (Note
that two ordinate scales are used, the left for distances less than
100 ft., the right for distances greater than 100 ft.)
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1 3
VARIATION IN FIELD STRENGTH AS A FUNCTION OF HEIGHT ABOVE GROUND
FOR A 345 kVSINGLE CIRCUIT LINE
36
34
32
30
28
26
24
I 22
2
cc
6"
ui 14
a.
12
10
8
6
4
2
0
20' CLEARANCE
B 10 12
HEIGHT ABOVE GROUND (FT.)
14
16
18 20
Figure 8 Variation in field strength as a function of height above
ground for a 345 kV single circuit line
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14
Ex, Ey AND IEI FROM 765 kV POWER LINES AT 6 FT. ABOVE GROUND
12.0
11.0
10.0
9.0
8.0
a
g 6.0!
oc
£ 5.0
a
Si 4.0
3.0
2.0
1.0
X /'
^CONDUCTOR'
BUNDLES
I
50 X
m
5
H
40
20
O
•n
o
m
m
39
8
0 10 20 30 40 50 60 70 80 90 100' 110 120 130 140
DISTANCE FROM CENTER OF ROW (FT.)
a
10 3
a ~
150
Figure 9 Ex, Ey, and JEJ from 765 kV power lines at 6 ft. above ground
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15
Ex, Ey, AND IEI FROM 765 kV POWERLINES AT 12 FT. ABOVE GROUND
-160
0 10 20 30 40 60 60 70 80 90 100 110 120 130
DISTANCE FROM CENTER OF ROW (FT.)
Figure 10 Ex, Ey, and |E| from 765 kV power lines at 12 ft. above
ground
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16
Table 2
Minimum Line Clearances and Expected Field Strengths
Typical Minimum Clearance Maximum Field Strength*
Type of Line (ft.) (kV/m)
345 kV single 25-35 4.5-8
circuit
345 kV double 30-40 5-7
circuit
500 kV single 40 6-7
circuit
765 kV single 40-50 8-12
circuit
*At three feet above ground.
FIELD MEASUREMENT TECHNIQUE AND RESULTS
Measurements of electric field strength were accomplished using a Monroe
Electronics model 238 A-l portable differential AC electric field meter. This
meter is a small, battery operated instrument designed for use in general
physical and biological testing for electric field intensity levels in the
vicinity of high voltage AC transmission equipment. Plates attached to the
top and bottom of the instrument constitute the sensing electrodes with the
axis of sensitivity normal to the plates. A four foot long handle is used
to reduce field distortion created by the presence of the individual performing
the measurement. Table 3 provides a summary of the more pertinent specifica-
tions for the instrument.
The sensitive electrodes consist of two parallel plane conductors
separated by the metal instrument case. These electrodes are exposed to the
AC field to be measured and are oriented with their plane perpendicular to
the desired incident AC electric field component. These electrodes are
connected to the inputs of a differential preamplifier. The preamplifier
output is a voltage proportional to the RMS value of the differential AC
electric field incident upon the sensitive electrodes, and is independent of
the frequency of the incident AC field.
The calibration of this meter is based on the "square meter panel"
method of measuring AC electric field strength at ground level below an
extra high voltage transmission line. The "square meter panel" is a one
square meter metallic panel surrounded by and insulated from a guard ring.
The center panel is slightly less than one square meter, since fringing in
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17
Table 3
Specifications of Monroe Electronics Model 238A-1
Portable Differential AC Electric Fieldmeter
Range: Full scale meter ranges of 5, 10, and 25 kV/m
Accuracy: +5% of full scale
Minimum Detectability: ^100 V/m
Frequency Response: Independent of frequency for normal power transmission
frequencies (50 Hz to 1,000 Hz)
Stability: Instability as a function of time, temperature, and battery
voltage is included in the overall accuracy specification
Battery: Type - Standard 9 volt transistor radio battery (Eveready No. 216
or equivalent) - one required
Life - 100 hours minimum
Physical Dimensions: 4 1/4" deep x 6 1/2" wide x 5 3/4" high not including
handle
Readout: Analog meter movement
the air gap divides the displacement flux equally, making the effective area
of the center panel equivalent to the center of the gap which encloses one
square meter. The guard ring extends one half meter beyond the center panel.
Virtual electric ground should be less than 10 cm below the square meter
panel. The panel is then positioned below an extra high voltage (300-500 kV)
single phase transmission line at least 30 feet above ground and connected
to the true earth through a microammeter. The guard ring is connected
directly to true earth ground. The magnitude of the 60 Hz phasor RMS voltage
gradient is obtained by
E » 0.3I(kV/m) where I is measured in microamperes.
The meter is then positioned directly over but not touching the center panel.
When making measurements under power lines, the operator should be at least
one meter away from the field meter.
A model M100 optical tapemeasure rangefinder, manufactured by Ranging,
Inc., was used to optically determine the various line heights at the time
of measurement in the field. This particular instrument covers the range of
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18
6 to 150 feet. Due to a measurement error of several percent, all distances
determined with this device were measured 10 times. The mean value of these
10 readings was then checked for accuracy by setting the range finder to
this mean value and measuring the actual distance. A typical error of +3
percent was determined for the instrument.
The procedure for determining the field strength lateral profile was to
establish a point on the line which was taken as the origin of a coordinate
system from which measurements were taken along a perpendicular direction to
the transmission line. A surveyors transit was utilized to assist in these
mappings and determining the point of minimum sag for each line. For each
power line measured, the lateral profile was determined as nearly as possible
along a line perpendicular to the powerline at the point of minimum sag. This
was done since (a) it was of interest to determine the maximum field strengths
possible, and (b) it was presumed that the results obtained would more nearly
equal the calculated values of electric field strength since the computer
model assumes the transmission lines to be infinite in extent and perfectly
horizontal. In reality, the line hangs as a catenary between adjacent towers.
After staking out the profile line, a 100 foot nonmetallic tape measure was
used to measure lateral distance and measurements of vertical and horizontal
field gradients were taken every three feet from the ROW center. When the
field strength reached approximately 1 kV/m, readings were taken at 10 foot
intervals. Figure 11 presents a sketch showing the geometric layout and
phasing of the 765 kV Marysville line and the 345 kV Twinbranch line used for
field measurements. A picture of the measurement process is seen in Figure 12.
The ground surface condition under the 765 kV line (see Figure 13) was freshly
disced soil, while the ground under the 345 kV line had grass cover and was
fairly hard. The soil was a clay which becomes very hard when dry and very
sticky when wet.
Figure 14 shows the results of the field measurements taken under the
765 kV Marysville line near the Dumont Substation. Measured data are plotted
as discrete points while the calculated values of field strength are shown by
the solid curve. Substation personnel provided accurate voltage measurements
for the time of the measurements. This data and the measured line clearances,
given in Table 4, were used as input data to the computer program to compute
a theoretical value for the gradient. Specific phase to neutral voltages
were used in the program and account partially for the lower gradient observed
under the phase 1 side of the ROW. Additionally the phase 1 bundle was
slightly lower than the other outside phase by 0.6 foot. Maximum discrepancy
between theoretical and measured values of field strength was about 8 percent,
this occurring on the phase 1 side of the line.
Measured values of field strength (vertical component) had a maximum
value of 9.9 kV/m approximately 10 feet beyond the outer most conductor of
phase 3. The measured field was down to 1 kV/m at 170 feet from the ROW
center and down to 100 V/m at 360 feet on the phase 3 side of the ROW. The
field was down to 1 kV/m and 100 V/m at 170 and 335 feet respectively on the
phase 1 side of the ROW.
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19
•M-
PHASE CONFIGURATION OF 765 kV MARYSVILLE LINE
( NOT TO SCALE )
-*.i.
"•"•'
•50.1'-
•485'-
PHASE CONFIGURATION OF 345 kV TWIN BRANCH LINE
•43'-
23.5'
•73'-
21.5'
43'
Figure 11 Phase configuration of 765 and 345 kV lines used for
measurements
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20
Figure 12 Photograph of field measurements technique
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21
r
:l^v
Figure 13 Photograph of 765 kV line measurement site
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MEASURED AND CALCULATED Ey FOR MARYSVILLE 765 kV LINE
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• MEASURED VALUES
SOLID LINE IS CALCULATED VALUE
NOMINAL LINE HEIGHT =46 FEET
240 230 220 210 200 190 180 170 160 150 140 130 120 130 140 150 160 170 180 190 200 210 220 230 240
DISTANCE FROM CENTER OF ROW
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23
Table 4
765 kV Marysville Line Measured Clearances and Voltages
Phase to Neutral Voltage (kV) Clearance (feet)*
Phase 1 443.125 48.9
Phase 2 438.750 46.9
Phase 3 445.000 48.3
Average Phase to Phase Voltage = 766.1
*Height above ground to center of each phase bundle.
As a check on the computer program, field strength measurements were
conducted at three different positions longitudinally along the 765 kV line
where .the line clearances were significantly different. Subsequently the
theoretical values were obtained and compared with the measured values.
Readings were taken directly beneath the phase 3 bundle without regard to
finding the absolute peak value of the field strength. This data is presented
in Table 5. These comparisons give a feel for the validity of using the
analytical approach at locations other than the point of minitmim line height.
New voltage readings were again taken at the time of these measurements and
incorporated in the analytical solutions. The results show that as the line
becomes more inclined with respect to the ground more discrepancy occurs
between measured and calculated values. The theoretical approach assumes
that the line conductors are horizontal and infinite in extent. However, at
points along a transmission line where the line sag is essentially symmetric
about the measurement point, the correlation between measured and calculated
electric field values is excellent.
An attempt was made to measure the horizontal component of the electric
field along the lateral profile and compare this data with the calculated
results. In general the agreement between the two methods was not as good
as expected; there appeared to be an anomaly in the results. A maximum
horizontal component of 1.2 kV/m was measured at approximately 18 feet from
the center of the ROW. This value is twice as large as the calculated value
and occurs at a point where the theory says a null would appear. There
appeared to be another broad peak in the neighborhood of 80 feet from the ROW
center of about 0.8 kV/m. This value is also about a factor of two above the
theoretical value but occurs generally in the correct area spatially. It is
not clear why the two methods diverged in this case but possibilities include:
(a) a calibration phenomena with the field meter in terras of its orientation
with respect to the ground and (b) the possibility of inaccurately holding
the field meter in a horizontal position. An error in orientation of
approximately 5° from true horizontal would be responsible for a possible
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Position
1
2
3
24
Table 5
Measured and Calculated Field Strength
at Several Line Heights
765 kV Marysville Line
Field Strength
(kV/m)
Line Height (ft)* Measured** Calculated
47.5
51.3
71.3
9.5
7.5
3.9
9.4
8.1
4.6
*Center phase.
**Values measured under outside phase.
contribution of about 10 percent from the vertical component and thus could
affect the horizontal readings enough to cause the above effect. A more
careful investigation of the horizontal component of field strength is
warranted to fully document this apparent discrepancy. At measurement heights
less than 6 feet above ground, the horizontal component does not significantly
contribute to the resultant field strength.
Measurements were made of the field strength at various heights above
ground up to a maximum height of 8 feet. This data is presented in Table 6
and confirms the rather slow increase in field strength as the measurement
height is increased. Data was taken at a lateral point relative to the
minimum line height of the 765 kV line where the field was a maximum at 3
feet above ground. Maximum variation was about 30 percent and probably
reflects the distortion of the local field by the observer as he bent down
for lower readings and reached upward for the highest readings.
A field strength profile for the 345 kV Twin Branch double circuit line
was taken using a similar approach. Figure 15 shows the spatial dependence
of the electric field with distance from the center of the ROW. A fence
running parallel to this line prevented measurements at extended distances on
one side. The data is plotted from 40 feet on one side to 180 feet on the
other. Maximum field strength at 3 feet above ground was observed to be 4.1
kV/m at the center of the ROW. The field strength values decreased to 1 kV/m
and 100 V/m at distances of 69 and 160 feet respectively. Theoretical values
of the field strength are plotted as a solid curve to show the comparison of
the methods, which is quite good, being typically within 4 percent. Table 7
gives the measured phase to neutral voltages on this line and the clearances
of the various phases used in the computer program. Only the height to the
lowest phase was actually measured; other line heights were assumed by adding
the phase spacings given in Figure 11 to this lowest line height.
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MEASURED AND CALCULATED Ey FOR TWIN BRANCH 346 kV DOUBLE CIRCUIT LINE
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• MEASURED VALUES
SOLID LINE IS CALCULATED VALUE
NOMINAL BOTTOM PHASE CLEARANCE =50 FT.
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40 30 20 10
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210
DISTANCE FROM CENTER OF ROW (FT.)
Figure 15 Measured and calculated Ey for Twin Branch 345 kV double circuit line, at
3 ft. above ground.
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26
Table 6
Measured Electric Field Strength as a Function
of Height Above Ground
Height Above Ground (ft) Field Strength (kV/m)*
1 10.1
2 10.4
3 10.0
4 10.1
5 11.0
6 12.5
7 13.0
8 13.0
Data taken under 765 kV Marysville Line.
*Value determined for vertical component of gradient.
Table 7
345 kV Twin Branch Double Circuit Line
Clearances and Voltages
Phase to Neutral Voltage (kV) Clearance (ft)
Phase 1 205.5 50.4
Phase 2 206.1 71.9
Phase 3 206.1 95.4
Average Phase to Phase Voltage = 356.6 kV.
An electric field strength profile for a 500 kV single circuit
line was obtained at another location. The line, operated by the
Potomac Edison Company, extends east from Doubs, MD which is south
of Frederick, MD. The line was a single three-phase circuit with each
bundle consisting of two conductors of 1.8 inch diameter, spaced
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27
18 inches apart and oriented horizontally, each bundle being spaced 35 feet
apart. Two shield wires spaced 54 feet apart were located at a height of
72.8 feet. The voltage on the line at the time of the measurements, November
2, 1974, was 510 kV according to substation logs. The right-of-way was 200
feet wide, 100 feet on either side of the center line, at the measurement
site.
Measurements were made with the Monroe meter over nearly flat terrain
essentially as described above and the vertical line heights were determined
with an optical range finder. Figure 16 presents the measured data as points
and the analytical values as a solid curve. The maximum variation between
measured and computed values is of the ordsr of 5 percent.
MEASUREMENTS OF SHORT CIRCUIT CURRENT AND PERCEPTION
Short circuit currents to ground (Isc) were determined using two
different vehicles immersed in a fairly intense field. A 1972 Chevrolet
Malibu (intermediate sized) was driven to a point under one of the outer
conductors of the 765 kV Marysville line where the field strength at three
feet above ground, when the vehicle was not present, was about 9 kV/m (see
Figure 17). When the car was present under the line, the converged electric
field was observed to be 16-17 kV/m depending on position when the meter was
placed approximately 12 inches above the roof. Measurements inside the
vehicle showed essentially a zero gradient except very near the windshield.
Current measurements were made with the vehicle situated directly on the
earth and when driven onto a set of hot-line blankets to insure good insula-
tion of the car from ground. Isc was measured as 80 yA when the vehicle was
in contact with the earth and 620 uA when insulated from earth by the
blankets. A copper ground rod was driven into the earth to provide a good
ground connection for the current measurement. A Triplet model 601 Type 2
solid state VOM was used to measure the current.
The large EPA electromagnetic radiation analysis van (see Figure 18),
a 27 foot fiberglass body vehicle with steel ribbing, was similarly tested
in the normally ambient 9 kV/m field. Electric fields of 25 kV/m could be
found by carrying the survey meter up the rear ladder of the van and holding
it a foot above the van roof (Figure 19). The height of the van roof is
about 10 feet above ground. With the van in normal contact with the earth,
Isc was 2.0 mA and when driven onto the hot-line blankets did not noticeably
change, indicating that the van tires are very good insulators as opposed to
the tires on the automobile.
The transient spark discharge was very uncomfortable when an observer
standing on the ground touched the van chassis. Table 8 gives values of body
currents while different individuals stood on a hot-line blanket and shorted
the van to ground through themselves to the ground rod. In this situation
the initial transient discharge effect was quite startling and the continual
current flow thereafter was very uncomfortable. A typical value of 1,6 mA
was observed. A 15 kft resistor was used to short the van to ground and a
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ELECTRIC FIELD STRENGTH PROFILE
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510 W DOUBS-CONASTON E
POTOMAC EDISON COMPANY
SOLID LINE IS CALCULATED VALUE
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CONDUCTOR LOCATIONS
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LINE HEIGHT 46.3 H
MEASUREMENT HEIGHT 1 r»eiei
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:
160
'120
BO '40 0 40
DISTANCE FROf* CENTER PHASF ill,
80
"MJ
Figure 16 Measured and calculated Ey for Daubs-Conastone 500 kV single circuit line
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Figure 17 Measurement of Isc for Chevrolet car
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Figure 18 Measurement of l~r for EPA van
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31
Figure 19 Measurement of electric field above roof of van
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Table 8
Body Currents While Shorting Van to Ground
Subj ect Body Current mA
A 1.6
B 1.6
C 1.4
D 1.6
current of 1.8 mA was observed as compared to 2.0 mA Isc for the van directly
connected to earth. This measurement implies that the impedance of man is
somewhere in the vicinity of 15 kfi but more careful measurements are called
for to accurately determine the true impedance.
A number of measurements of Isc for individuals immersed in various field
intensities were made with the person standing on a hot line blanket at a
position beneath the 765 kV line where the unperturbed field gradient had
been determined just prior to the tests (see Figure 20). The data obtained
from this series of measurements is reported in Table 9. The height of each
person (a total of three) is also shown. From each set of data an average
normalized body current, Isc/E, was computed and listed. This parameter
expressed in yA/(kV/m) is seen to be approximately 18 uA/(kV/m) for individuals
insulated from ground by standing on a hot line blanket. Similar measurements
were taken with the individuals standing directly on the earth ground, and
this data is seen in Table 10. The normalized body current in this case is
about 17 yA/(kV/m). These measurements are in general agreement with others
reported in the literature [4] and are a function of the person's height and
shoe resistivity. No determinations were made of the distribution of this
current in the body. When the field meter was held 9 inches above one person's
head (see Figure 21), while he was standing in an unperturbed field of 10
kV/m, the field strength meter read 25 kV/m. This reveals the convergent
properties of the field in the neighborhood of the conductive body.
Another interesting test was conducted to determine a person's threshold
for perception and annoyance of transient spark discharge. Perception in
these tests was defined as the person's ability to tell that a shock effect
was occurring as he touched a ground rod driven into the earth. Annoyance
was defined, subjectively, as a condition which the observer would not like
to occur again. The results are tabulated in Table 11 where a coarse sub-
jective rating scheme was used as follows: a (-) sign means not perceived, a
(o) means perception was noted, and a (+) sign means the feeling had reached
an annoying level.
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33
Figure 20 Measurement of Isc for an individual
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34
i •
Figure 21 Measurement of converged electric field strength above
individual's head
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35
Table 9
Isc for Three Individuals as a Function of the Field Gradient
Individuals Standing on Hot Line Blankets
Subj ect
A
B
C
Average
Isc(pA)
E(kV/m)
Height
(ft-in)
6-3
6-1
6-0
Electric Field Gradient (kV/m)
2
38
38
35
18.5
4
76
73
68
18.1
6
110
110
100
17.8
8
140
140
135
17.3
10
190
180
170
18.0
Table 10
Isc for Three Individuals as a Function of the Field Grandient
Individuals Standing Directly on Earth Ground
Subject
A
B
C
Average
Isc(wA)
E(kV/m)
Height
(ft-in)
6-3
6-1
6-0
Electric Field Gradient (kV/m)
2
36
36
33
17.5
4
72
71
68
17.6
6
110
100
95
16.9
8
140
130
120
16.3
10
160
160
150
15.7
The tests were made by first using the tip of one's index finger to
touch a round head screw located near the top of each ground rod. Next the
individual lightly brushed the back of his hand against the screw head as a
perhaps more sensitive indicator. The results of these subjective tests
indicated that in almost all cases persons could perceive the spark discharge
produced between their body and ground when immersed in fields of 4 kV/m or
more, if they were well insulated from ground. Generally, at field intensities
of 8 kV/m or greater, the sensation was considered extremely annoying.
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36
Subject
A
B
C
D
Table 11
Human Perception and Annoyance Tests
Transient Spark Discharge
Subjective Perception/Annoyance Rating E(kV/m)
2
o/-
o/-
o/-
o/-
4
o/-
o/o
o/o
o/o
6 8
O/O +/O
+/+ -H-/-H-
+/+ -H-/-H-
-f/+ -H-/-H-
10
+ /+
•H-/-H-
-H-/-H-
++/-H-
Rating symbols
Back of.
Hand /Tip of
Finger
(-) no perception
(o) perception
(+) annoyance
(++) extremely annoyed
A final test of perception was made using the so-called umbrella test
wherein a person walks continually into a higher electric field strength
holding the umbrella by the insulating handle and repeatedly touches the
button just above the umbrella handle to allow spark discharge. The umbrella,
being charged as a capacitor in the field, can create substantial reflex
reactions, in contrast to the above tests, due to transient current flow
through the person's thumb (see Figure 22). In these tests either the spark
discharge was graded as OK, which meant tolerable, or NO which meant that the
spark discharge was very uncomfortable and not desired again. This data is
given in Table 12. Again a general threshold of about 6 kV/m was found to
demarcate the levels below which individuals would accept repeated shocks
from the umbrella and that level beyond which shocks were clearly disturbing
and not desirable.
A test was made of the short circuit current available from a simulated
wire fence strung parallel to the 765 kV overhead line outer phase bundle and
approximately 3 feet above ground. The simulated fence consisted of 102.5
feet of copper antenna wire placed at a height corresponding to an unperturbed
field gradient of about 10.2 kV/m. A measurement of 0.88 mA was made for I8C
to a ground rod. The spark discharge effect was very vivid, and painful,
while the continuous current flow after the initial discharge was also
unpleasant.
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Figure 22 Perception testing with an umbrella
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38
Table 12
Umbrella Perception Transient Spark Discharge
Electric Field Strength (kV/m)
2
OK
OK
OK
OK
4
OK
OK
OK
OK
6
NO
OK
NO
NO
8
NO
NO
NO
*_
10
NO
NO
NO
*_
Subj ect
A
B
C
D
Perception Rating - OK means subject could tolerate spark discharge on a
repeated basis.
NO means subject would definitely not like to tolerate repeated discharge.
*Subject would not enter higher field strengths.
Near the completion of these tests a farm tractor was made available for
measuring short circuit current under the 765 kV line. A value of 0.6 mA was
measured but the shock effect was not nearly as severe as that created by the
simulated fence which had a similar Isc value. It is possible that the
impedance of the tractor was low with respect to a human and consequently
relatively little current was being conducted through the body. No body
current measurements were taken to substantiate this observation.
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39
REFERENCES
1. Shih, C.H., American Electric Power Service Corporation, 2 Broadway, New
York, NY 10004, personal communication.
2. Deno, D.W., "Calculating Electrostatic Effects of Overhead Transmission
Lines," IEEE Trans, PAS-93, p, 1458, 1974.
3. Bracken, T.D. (Ed.) In Proceedings of an electrostatic and electro-
magnetic measurements program held in conjunction with the IEEE Working
Group on E/S and E/M Effects at the Bonneville Power Administration, Port-
land, OR, 9-11 July 1974.
4. Project EHV, General Electric Company (Author and Editor), EHV Trans-
mission Line Reference Book, published by Edison Electric Institute, New
York, NY, 1968.
5. Schneider, K.H. „ et al., "Displacement Currents to the Human Body Caused
by Dielectric Field Under Overhead Lines," CIGRE Report 36-04, 1974.
• u.e. OOVIRIHEHT PMNTXHO omcx i i»77 o-j4i-oi7/4s
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