&EPA
United States
Environmental Protection
Agency
Office of
Radiation Programs
Washington DC 20460
Technical Note
ORP/EAD 80-1
Radiation
Electric Fields Under
Power Lines
(Supplement to "An Examination
of Electric Fields Under EHV
Overhead Power Transmission
Lines")
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ELECTRIC FIELDS UNDER POWER LINES
(Supplement to An Examination of Electric Fields
Under EHV Overhead Power Transmission Lines)
Marc Wiqdor
March 1980
OFFICE OF RADIATION PROGRAMS - SILVER SPRING
ELECTROMAGNETIC RADIATION ANALYSIS BRANCH
ENVIRONMENTAL ANALYSIS DIVISION
U.S. ENVIRONMENTAL PROTECTION AGENCY
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.
Mention 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 U.S. 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 examines magnetic field
strengths and compares electric field strength measurement
techniques under extra-high-voltage overhead power transmission
lines. Readers of this report are encouraged to inform the Office
of Radiation Programs of any omissions or errors. Comments or
requests for further information are also invited.
Floyd L. Galpin
Director
Environmental Analysis Division
Office of Radiation Programs
m
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TABLE OF CONTENTS
LIST OF FIGURES v
LIST OF TABLES v
INTRODUCTION 1
ANALYTICAL EVALUATION OF FIELD STRENGTHS 1
REFERENCES 17
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LIST OF FIGURES
Number Page
1 Design Specifications of BGE 115-kV and 230-kV 4
Towers
2 Electric Field Strength Profile for a 115-kV 5
Double Circuit Line
3 Electric Field Strength Profile for a 230-kV 6
Double Circuit Line
4 Electric Field Strength Profile for a 115-kV 8
Double Circuit Line
5 Electric Field Strength Profile for a 230-kV 9
Double Circuit Line
6 Variation in Field Strength as a Function of 10
Height Above Ground for a 115-kV Double Circuit
Line
7 Variation in Field Strength as a Function of 11
Height Above Ground for a 230-kV Double Circuit
Line
8 Variation of Multiple Line Electric Field Profile 14
with Phase
9 Comparison of Electric Field Strengths Due to a 15
Single 500-kV Single Circuit Line to Three 500-kV
Single Circuit Lines
10 Difference in Field Strengths Plotted in 16
Figure 9 as a Function of the Distance from the
Edge of the ROW
LIST OF TABLES
Number Page
1 Specifications for 115-kV and 230-kV Double 3
Circuit Transmission Lines
2 Specifications for 500-kV Single Circuit 13
Transmission Lines
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INTRODUCTION
In 1976, the Environmental Protection Agency published a report
on the electric fields produced by extra-high-voltage (EHV) overhead
transmission lines [1]. In that report, an analytical study was
made of the electric fields due to 345-kV, 500-kV, and 765-kV trans-
mission lines and the results compared to actual measurements on
several typical transmission lines. The measured and calculated
values of electric field strength agreed very well. Since the
publication of that report, we have received inquiries about the
electric fields produced by lower voltage overhead transmission lines.
The first part of this report extends the earlier analysis [1] to
include 115-kV and 230-kV double circuit transmission lines. The
second part of this report examines the electric field strengths
due to multiple 500-kV transmission lines. The results of measurements
of magnetic field strength produced by 500-kV transmission lines have
been published elsewhere [2J.
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 [3]
The method used to calculate electric field strengths in this report
is one developed by Mr. John Walker of the Bonneville Power Admin-
istration, Portland, Oregon [4]. 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 subcon-
ductors (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). This calculation is based on the
fundamental field equation
E = q/2ne0r, where (1)
E is field strength in volts/meter
q is charge per unit length in coulombs/meter
EO is permittivity of air = 8.85xlO~12 farads/meter
r° is distance from the charge in meters.
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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's
surface at 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. The predictive
results of this program were compared to actual field measurements as
reported in the previously mentioned EPA study [1]. The excellent
agreement between the two methods supports the use of this predictive
model as an analytical tool.
Specifications for the 115-kV and 230-kV double circuit lines
were supplied by the Baltimore Gas and Electric Company [5]. Both
newer and older tower designs were supplied with the information
listed in Table 1 and illustrated in Figure 1. For each of these
designs, there is only one subconductor per phase.
Figures 2 and 3 show the electric field strengths under worst
case conditions for the 115-kV and 230-kV towers respectively.
Under these conditions, the electric field strengths are larger
than what would occur under normal use. The field strengths were
computed for a lowest line clearance of 30 feet. This is the
minimum design clearance, yet the lowest line clearance under
normal operating conditions is 45 feet and since the wire hangs
in a catenary curve between two towers, the average height of the
lowest wire is approximately 60 feet.
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Table 1
Specifications for 115-kV and 230-kV Double Circuit Transmission Lines
New Old New Old
115kV 115kV 230kV 230kV
Configuration I II II II
Dimensions (ft)
Horizontal
GW1-GW2 ,N/A 26.5 12 20
A-A1 16 23 23 35
B-B1 21 30 25 48
C-C1 17 23 23 40
Vertical
GW-A 12 11 17 21
A-B 10 13 18 23.5
B-C 10 13 14 23.5
Diameter (in)
Subconductor 1.504 1.504 1.735 1.735
Groundwire .385 .385 .385 .385
Number of Subconductors in Bundle: 1 for all lines
Relative Phase of Bundles: A=B'
C=A'
B=C'
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FIGURE 1.
DESIGN SPECIFICATIONS OF BGE 115kV AND 230kV
TOWERS. (NOT TO SCALE)
CONFIGURATION I
CONFIGURATION II
GW
o
Ao
o A'
Bo
OB'
GW1
GW2
Ao
Bo
OA'
OB'
C'
C'
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Figure 2. Electric Field Strength Profile for a 115 kV Double Circuit Line
Field strength values are computed at a
point 3 ft. above ground for a minimum
clearance of 30 ft. The tower designs are
those of Baltimore Gas and Electric.
— —— Old design
Modern design
60 -50 -40
.1
-110 -100 -90
-30 -20 -10 0 10 20 30
Distance from Center of ROW (ft.)
70 80 90 100 110
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Figure 3 Electric Field Strength Profile for a 230 kV Double Circuit Line
Field strength values are computed at a
point 3 ft. above ground for a minimum
line clearance of 30 ft. The tower designs
are those of Baltimore Gas and Electric.
«_ Old design
New design
.25 -
-110 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10
80
90 100 110
Distance from Center of ROW (ft.)
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As can be seen from these two figures, the field strengths for
the older towers are in general higher with the result that the
right-of-way (ROW) widths are wider. The asymmetry in the figures
is due to the asymmetry in the phase relationships of the conductors.
At a height of 3 feet above ground, the electric field strength is
under 1 kV/m everywhere for the newer design 115-kV lines and under
1 kV/m at distances greater than 20 feet from the center of the ROW
for the older design. Similarly, the 1 kV/m distances for the newer
and older design 230-kV double circuit lines are 36 and 50 feet
respectively. The ROW widths on these figures are represented by
the vertical lines on the horizontal axis.
The variation of the electric field strength profiles with the
height of the clearance for the old design of the 115-kV and 230-kV
double circuit lines are shown in Figures 4 and 5. The old design
towers were chosen for this display because of the generally higher
fields, thus maintaining a worst case analysis. The results here
follow the same pattern for double circuit lines that emerged in
earlier analysis [1]. The electric field decreases everywhere at
ground level as the line clearance increases, whereas for single
circuit lines and for large distances from the center of the ROW,
the reverse is true.
For the 115-kV line, the peak field ranges from 1.05 kV/m to
0.46 kV/m as the line clearance varies from 30 feet to 50 feet.
For the 230-kV line, the peak field ranges from 2.94 kV/m to
1.24 kV/m as the line clearance again varies from 30 feet to
50 feet. This peak occurs at a distance of 20 feet from the center
of the ROW for the 230-kV double circuit line while in the case
of the 115-kV transmission line, the position of the peak varies
between the center of the ROW and a point 12 feet from the center.
Figures 6 and 7 show the variation of the peak electric field
strength at various heights above ground for line clearances of
30, 35, 40, and 45 feet for the same 115-kV and 230-kV double
circuit lines. It is clear that for moderate changes in the height
above ground, there is only a small change in the electric field,
as long as the height above ground does not approach the line
clearance, as evidenced by the case of the 30 foot line clearance.
As the line height above ground increases, the influence of the
test height on the field strength near the ground decreases, as
shown by the increasingly flatter curves.
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Figure 4. Electric Field Strength Profile for a 115 kV Double Circuit Line
CO
E
>
£
"01
c
o>
right scale
conductor locations
50
Field strength values are computed at a
point 3 feet above ground for various
line clearances
I
I
I
I
25 50 75 100 125 150 175 200
Distance from Center of ROW
225
250
275
.18
.17
.16
.15
.14
.13
.12
.11
.1
.09
.08
.07
.06
.05
.04
.03
.02
.01
300
2.
a
(D
<
i
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Figure 5.
Electric Field Strength Profile for a 230 kV Double Circuit Line
3
2.8
2.6
2.4
2.2
2
'••
£ 1-6
01
I 1.4
cfl
T3
~Z 1.2
1
.8
.6
.4
.2
left scale
30'
Field strength values are computed at a
height 3 feet above ground for various line
clearances
.18 2-
.16 2
.14
.12
.1
.08
.06
.04
.02
<
3
_L
25 50 75 100 125 150 175 200
Distance from Center of ROW
225
250
275
300
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Figure 6. Variation in Field Strength as a Function of Height Above Ground for a 115 kV Double Circuit Line
3|
I
>
c
0)
ka
=*=«
(fl
m
il
8 10 12
Height Above Ground (ft.)
14
16
18
20
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Figure 7. Variation in Field Strength as a Function of Height Above Ground for a 230 kV Double Circuit Line
8 10 12
Height Above Ground (ft.)
14
16
18
20
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When examining the electric field profiles for multiple
transmission lines, or even for double circuit lines, it is
imperative to know the relative phase of each of the conductors.
An example is the 500-kV single circuit line characterized in
Table 2. The specifications for this line were supplied by
Arkansas Power and Light [6]. When two of these 500-kV lines
are placed side by side, in parallel and spaced 140 feet apart,
the electric field profile can look like either of the two curves
shown in Figure 8, depending on the relative phases of the
conductors. Outside of the right-of-way, the profiles are almost
identical but inside the ROW, the fields are markedly different.
At the center of the right-of-way, the field can be as large as
5.55 kV/m or as low as 1.75 kV/m depending on the phase design.
Note that for the upper curve, the phases are mirror images of
each other, not so for the lower curve.
Multiple lines can usually be found emanating from power plants.
In Arkansas, for example, three 500-kV single circuit lines, in
parallel, spaced 140 feet apart and with a right-of-way that extends
to 90 feet beyond the outside towers, can be found leaving a nuclear
power plant and traversing near rural properties. Each line is
identical and also described in Table 2. The electric field profile,
along with a comparison profile of a single 500-kV single circuit
line, is illustrated in Figure 9. The relative phase of each
conductor bundle is denoted at the top of the figure. Clearly,
the three lines together can produce higher fields inside the
right-of-way, but only 27% higher at the peaks. Outside their
respective right-of-ways, the fields due to both configurations
are almost identical, as seen in Figure 10, which is a plot of the
difference between the field strength due to the three lines and
the field strength due to the one line as a function of the distance
from the edge of their respective right-of-ways. As can be seen
from this figure, the differences are indeed minor - the largest
difference which occurs at the right-of-way edge, is less than 3%.
In fact, on one side of the ROW, the field due to the single line
is greater than the field due to the three lines for distances
from the edge of the ROW up to about 260 feet.
12
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Table 2
Specifications for 500-kV Single Circuit Transmission Lines
Conductors
Height (ft) 50
Horizontal Positions (ft) -30.25, 0, +30.25
Diameter (in) 1.165
Number of Subconductors 3
Subconductor Spacing (in) 18
Groundwires
Height (ft) 95
Horizontal Positions (ft) -20.25, +20.25
Diameter (in) .433
13
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Figure 8. Variation of Multiple Line Electric Field Profile with Phase
E
>
0)
2
o>
Phase Relationship
A= C'
B = B'
C = A
C'
Field strength values are computed
for a point 3 feet above ground
Relative Conductor Phases
ABC ABC
Conductor Locations
1 -
-450 -400 -350 -300 -250 -200 -150 -100
Distance from Center of ROW (ft.)
50 100 150 200 250 300 350 400 450
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Figure9. Comparison of Electric Field Strengths Due to a Single 500 kV Single Circuit Line to Three 500 kV Single Circuit Lines
Relative Phase
E
>
cn
V)
2
Bundle locations for three
500 kV single circuit lines
Bundle locations for one
500 kV single circuit line
^— ^— Three 500 kV single
circuit lines
^—^ One 500 kV single
circuit line
Field strength are computed at a
point 3 ft. above ground
-450 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150
Distance from Center of ROW (ft.)
200 250 300 350 400 450
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Figure 10. Difference in Field Strengths Plotted in Figure 9 as
a Function of the Distance from the Edge of the ROW
.06
I
.05
\
\
h \
.04
.03
v
^
v
| .02
55
"S
.£
u,
.£ .01
o
-.01
-.02
-.03
_L
_L
50 100 150 200 250 300 350 400
Distance from Edge of ROW
450
500
550
600
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REFERENCES
1. Tell, R.A., J.C. Nelson, D.L. Lambdin, T.W. Athey, N.N. Hankin
and D.E. Janes, "An Examination of Electric Fields Under EHV
Overhead Power Transmission Lines," EPA-520/2-76-008, U.S.
Environmental Protection Agency, Silver Spring, MD, April 1977.
2. Lambdin, D.L., "A Comparison of Measurement Techniques to
Determine Electric Fields and Magnetic Flux Under EHV Overhead
Power Transmission Lines," ORP/EAD 78-1, U.S. Environmental
Protection Agency, Las Vegas, NV, March 1978.
3. Deno, D.W., "Calculating Electrostatic Effects of Overhead
Transmission Lines," IEEE Trans. PAS-93, p. 1458, 1974.
4. Bracken, T.D. (Ed.) In: Proceedings of an electrostatic and
electromagnetic measurements program held in conjunction with
the IEEE Working Group on E/S and E/M Effects at the Bonneville
Power Administration, Portland, Oregon, 9-11 July 1974.
5. Nabet, G. and J. Reynolds, Personal Communication, Baltimore Gas
and Electric Company, Baltimore, MD, 1979.
6. Reeter, D., Personal Communication, Arkansas Power and Light,
Arkansas, 1979.
17
- U.S. GOVERNMENT PRINTING OFFICE : 1980-31-1--J.32/34
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