Download EMF Effects from URD Systems

EMF EFFECTS FROM URD SYSTEMS
by
Eugene G. Preston, P.E.
City of Austin Electric Utility
Austin, Texas
presented at
American Public Power Association
Engineering & Operations Workshop
Washington, D.C.
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March 15, 1989
EMF EFFECTS FROM URD SYSTEMS
by
Eugene G. Preston, P.E.
City of Austin of Electric Utility
Austin, Texas
ABSTRACT
The widely held belief that putting a power line underground will eliminate all electric (E)
and magnetic (M) fields above ground has been found to be in error. Although this is true for E
fields, recent experiments by the City of Austin and others [1,2] are showing that under some
circumstances the M fields from underground circuits can exceed M field levels from overhead
distribution circuits by a factor of ten and are about as strong as M field levels directly below
overhead transmission lines. The earth being almost completely transparent to M fields, combined
with the many paths by which a neutral current can return to its source, sets up the conditions
needed to generate the higher than expected M fields.
We explore this topic in further detail under the following sections: 1) a historical
perspective to show why this subject may become increasingly important to the electric utility
industry; 2) the discovery at the City of Austin of URD (underground residential distribution)
circuits with significant magnetic fields; 3) the research and experiments conducted at the City to
gain an understanding of the physical process; 4) a simplified explanation of the physical process
that produces URD M fields; and 5) a computer model developed at the City of Austin that can
capture the physical effects that have been observed.
EMF EFFECTS FROM URD SYSTEMS
by
Eugene G. Preston, P.E.
City of Austin Electric Utility
Austin, Texas
HISTORICAL PERSPECTIVE
In 1979 Nancy Wertheimer and Ed Leeper published a paper [3] linking childhood leukemia in Denver with
60 Hz (Hertz) M (magnetic) fields from overhead distribution lines. Initially the Wertheimer study was not taken
seriously because: 1) extensive research for years by many investigators had produced no cancer linkages to ELF
(extremely low frequency) E or M fields; 2) the statistical sample from which the conclusions were based was
small; 3) no actual M fields were ever measured; 4) the experiment was not repeatable in other cities; and 5)
numerous other confounding factors were present that were not taken into account. Shortly after the
Wertheimer paper was published, Fulton and others repeated the study in Rhode Island with nil results [4]. This
created a battleground between Fulton and Wertheimer on the many technical parameters used in the studies with
each claiming the other had biased the results [4,5].
The Wertheimer study took on new importance for the electric industry when Houston Light and Power
lost a case to the Klein Independent School District in 1985 in which HL&P had condemned land for the rightof-way in 1981 and constructed and energized a 345 kilovolt (kV) transmission line in 1984. In this case, the
KISD was able to convince a jury, using the Wertheimer study as its basis, that the M fields from the line would
pose a health threat to the students. HL&P unsuccessfully argued that M fields from the line were no greater
than from the wiring and electrical devices already within the school. As a result of the jury's decision, the judge
ordered the line to be taken out of service. Since then, HL&P has rerouted the portion of the line near the school
and has been allowed to re-energize it.
In the 1980's the possibility of cancers being caused by low frequency M fields was a hot topic for
investigators. A study in Stockholm Sweden by Tomenius showed a small increase in leukemia for fields
between 3 to 4 milligauss (mG) [6]. Above 4 mG the correlation was statistically insignificant. Other
investigators had findings. These findings (+ means a correlation was found, - means none was found) included
Milham in Washington state (+) [7], Wright in Los Angeles (+) [8], McDowall in England and Wales (one + out
of several -) [9], Coleman in southeast England (+) [10], Wiklund in Sweden (- for telecommunications workers)
[11], and Vagero and Olin in Sweden (+) [12]. Additional details on these studies can be found in [13]. Quoting
from [13]: "Overall, 8 of 10 epidemiological studies that have sought to find an association between cancer
incidence and residential or occupational exposure to ELF fields from electric power sources have obtained
positive results." However, [13] also points out that these studies have many of the deficiencies that were noted
previously for the Wertheimer study [3].
David Savitz was commissioned by an independent scientific advisory body as a part of the 765 kV New
York Power Lines Project to repeat the Wertheimer study and correct the shortcomings that were in the original
study. The results were reported in [14] and supported the findings by Wertheimer although the correlation
between childhood leukemia and cancer was lower in the Savitz study. By the summer of 1987, the results of
this study were widely reported and played a major role in the cancellation of several 345 kV lines that were to
be built in the Austin area. Unlike the Wertheimer study that made no M field measurements, specific M field
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levels were being discussed by Savitz and others. These newer studies implicate 60 Hz M fields that are stronger
than roughly 1 mG. M field measurements performed by the City of Austin show that 1 mG is about the
minimum background level on average in most environments.
John Peters at the University of Southern California School of Medicine has been searching for cancer
causing agents for a number of years using a data base of about 20 million people in the Los Angeles area. His
past studies have linked childhood cancers to: 1) the use of household pesticides; 2) the burning of incense; and
3) fathers working in businesses that use chlorinated chemicals. He mentioned these results at a recent EPRI
(Electric Power Research Institute) seminar in Minneapolis [15]. He is presently working with EPRI on the
inclusion of 60 Hz M fields in his ongoing studies. This study is expected to be completed by the end of 1989
and may help resolve some of the uncertainty surrounding this issue.
DISCOVERY
The City of Austin first became aware that URD circuits can have relatively high M fields through an
accidental discovery in 1987. At that time, the City had recently purchased a new M field meter and was in the
process of measuring M fields under transmission lines in the Austin area. While driving home on a Sunday
afternoon with the new meter turned on, an almost full scale reading was observed by the author upon entering
his URD subdivision. There were no power lines in sight. The peak reading was found to be about 25 mG 1
meter above ground at one point in the street. This is about the same magnetic field strength a moderately
loaded transmission line would produce. Overhead distribution lines normally are much less.
The author's home was found to have a surprisingly high M field level also. The meter gave a reading of
nearly 10 mG in the author's bedroom. A URD feeder creating the field was found to be located about 4 feet just
outside the bedroom exterior wall. Directly over the feeder the field was about 13 mG. This level was slowly
changing with time as the feeder load varied. The feeder route crossed the street, went past the house, and
continued to the pad mounted transformer in the back yard. Figure 1 shows the general layout.
F
ig. 1. URD Feeder Near House
These M field levels were much higher than
expected.
There should have been almost
complete cancellation of the M field from the
concentric neutral current and the primary
conductor current since there was effectively zero
separation distance. An attempt was made to
explain the field strength using classical formulas
[16]. However, the calculated fields from this
approach were considerably less than those
observed. An early assumption was made that
there must be an open neutral somewhere along
the feeder. Additional field data was needed in
order to understand what was happening;
therefore, two field experiments were planned and
executed to gain this knowledge.
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FIELD EXPERIMENTS
The first data collection exercise was
performed at the location shown in Figure 1
from 2 to 3 p.m. on October 26, 1987. The
purpose was to collect magnetic field data
and current readings on individual
conductors. An M field cross section was
made along the line labeled as "H Field" in
Figure 1. The resulting M field strengths
were taken 1 meter above ground. The
actual M field readings were adjusted to
reflect a constant current of 20 amperes from
the actual range of 5 to 8 amperes as
measured on "Feeder A" in Figure 1. The resulting M field strength is plotted in Figure 2.
As the distance was increased from the feeder, the M field intensity dropped off more slowly than was
initially anticipated. This is explained by noting that for an overhead line with currents that sum to zero, the
magnetic field roughly drops off at a rate inversely proportional to the distance squared. For an isolated single
conductor, the M field drops off at a rate inversely proportional to distance to the first power.
Also of intense interest was the proportion of current in the primary and neutral conductors. Taking
current readings at the transformer shown in Figure 1 on Feeder A showed the primary had 7.1 A, the neutral 3.8
A, and the vector current sum that was obtained by clipping the ammeter around both conductors was a
surprising 5.8 A. This combination of currents could only occur if 1) the ammeters were not properly working
or 2) the neutral current was less than 180 degrees out of phase from the primary conductor current. More than
one ammeter was used to check the readings, and they all were in reasonable agreement. The information gained
from these measurements is that the neutral current was quite different than expected. The currents associated
with Feeder B were also measured with the surprising results of 5.8 A primary, 2.4 A neutral, and 6.8 A vector
sum for the primary and neutral combined. Unfortunately, the ground rod current could not be measured since
the ground rod was not accessible. Again, the conclusion from this data is that the primary and neutral currents
were highly unbalanced and causes the higher magnetic fields.
Ground rod currents were suspected of being significant in the observed current imbalances. Intuitively, it
was believed that the frequent use of closely spaced ground rods (one at every 200 feet on average) could
provide a means for having much of the current return through the earth. With that thought in mind, the second
experiment was directed toward collecting data that would verify a model built around formulas from the EPRI
Red Book [16]. The objective was to measure ground rod resistances to verify a model. What was discovered
in the experiment shed new light on what was causing imbalanced primary and neutral currents. It was not due
to open neutrals or an earth return through many ground rods.
Figure 3 shows the layout of the experiment conducted on January 7, 1988. The drawing shows a
residential subdivision in northeast Austin that did not yet have houses constructed on the lots. A section of
URD cable feeding pad mounted transformers at 1, 2, 3, and 4 was isolated from the rest of the system. A
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floating ground variable voltage power supply at S provided power to drive the several circuit configurations that
would be tested. The transformers 1 through 4 were disconnected from the feeder since the tests were conducted
to measure only feeder currents and ground rod currents and voltages. An oscilloscope was set up to show the
angle between current and voltage at the supply end. The distance between transformers 1 and 4 is 380 feet.
Earth conductivity was high since the clay soil was saturated with water and a low earth resistance was expected.
The feeder cable was installed in a plastic duct.
Fig. 3. A New Residential Underground Feeder System (No Houses)
The original intent of the second experiment was to measure the electrical properties of the earth given that
ground rods are closely spaced. The following basic measurements were planned:
1)
2)
3)
Measure neutral open loop voltage while driving the primary conductor
Measure primary conductor open loop voltage while driving the neutral conductor
Drive the feeder and measure all closed loop currents on the primary conductor, neutral, and ground
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4)
5)
rod
Disconnect all the ground rods and verify that the M fields cancel while driving a floating feeder
Drive the neutral with all current returning through the earth between two ground rods and measure
the current, voltage, and phase angle between them
After spending several hours making measurements on 1, 2, and 3, the magnetic field cancellation test on 4
failed to work. There was a field present. A closer look at the circuit turned up a telephone circuit that
paralleled the feeder but was separated from the feeder by a few feet. However, the grounds of the telephone
and the power circuit were connected together at several points. These connections were hard to spot because
the telephone circuit neutral was connected to a small copper wire that came out of the side of the transformer
pad and was usually covered by grass or earth. This ground wire was specifically put there to allow other
services to connect their ground wire to the electric system ground although the pad mounted transformer is
closed and locked. This practice allows all the grounds of other services full access to the power circuit ground
system without the need to have personnel from the electric utility present to open the transformer.
This discovery suddenly hit home. The data that had been collected for several hours up to that point was
nearly worthless. When measurement 5 produced an impedance of 6 ohms rather than the expected value of 40
ohms, the culprit had been exposed. This measurement was expected to have the highest impedance of any of
the measurements because the circuit was supposed to include the resistances of the ground rods and earth.
However, the telephone cable neutral provided a short cut parallel current return path that bypassed the earth and
lowered the overall loop resistance.
The conclusion from the conduct of the second experiment was that the neutral system being
interconnected at many points provides many paths for the current to return to its source in contrast to the
primary conductor that has only one path. This results in a highly imbalanced system as seen in the first
experiment.
SIMPLIFIED MODEL
Figure 4 shows a typical URD system in which the neutrals are highly interconnected. The power cable
neutral is tied to a ground rod, a water pipe, and the neutrals of telephone and cable TV lines. Figure 4 shows
how the currents typically might flow. There is only one path for the current from the riser to the pad mounted
transformer and this is shown as 10 amperes. Many paths are possible for the neutral return current. In this
example, the neutral return current is 5 amperes, the cable TV and telephone is 1 ampere, the earth 1 ampere,
and the water pipe 3 amperes. Each of these currents produces a magnetic field. Fields around the water pipe
are shown cutting through the house. A field is also shown around the unbalanced feeder. All these currents are
close to the earth's surface and produce fairly high magnetic fields from relatively small currents.
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The magnetic field strength around any conductor is easily calculated once the current is known. The M field
intensity in amperes per meter is simply the current (amperes) in the conductor divided by the circumference
around an imaginary circular loop (meters). Multiplying this value by 12.5 will give the M field strength in
milligauss. If microtesla units are desired, use a 1.25 factor rather than the 12.5 factor for mG. The equation for
calculating this is:
H = (12.5)(I)/(2 pi R) milligauss
(1)
where:
I = current in amperes
R = distance from wire in meters
pi = 3.14159
NETWORK MODEL
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An electric network model has been developed at the City of Austin that calculates the current distribution
in all the conductors put into the model. Multiple neutral current paths are modeled as well as earth currents.
The model is able to predict the unusual current distributions seen in the first experiment. The model assumes
that none of the neutrals are at zero potential. Small ground rise potentials occur at each ground rod and result
in the multipath dispersion of the neutral current. The zero voltage reference is some remote point deep within
the earth. Figure 5 shows a schematic representation of the City of Austin model and how it is set up and used.
F
ig. 5. URD Feeder Model
The source of power will typically be a riser
point into the underground subdivision. The
assigned as node 1, and the rest of the nodes are
consecutively going away from node 1. The
locations are assigned as nodes:
o load points
o grounding points
o turning points
o any test points
pole entry
source is
numbered
following
For each node, the X,Y location measured in inches from a
drawing is entered along with the drawing scale so that
feeder lengths and impedances are automatically calculated.
The solution procedure begins with the earth model. An earth resistance matrix is formed for all nodes
having a ground rod attached to that node. As shown in Figure 5, these are the nodes numbered n+1 through 2n.
The model allows floating grounds by entering a ground rod with a zero diameter. For floating grounds the
program simply enters a 1 on the matrix diagonal and 0's on the same row and column. This resistance matrix is
dense. There are no zero off diagonal matrix terms except for the floating grounds.
The following concept is used to calculate the earth matrix terms. A current of 1 ampere is injected into a
ground rod. The resistance of the ground rod is then the voltage at this node. This voltage is entered as a
diagonal term in the matrix. The voltage falls off as an inverse function of distance going away from the rod with
the injection current. The voltages on all the other ground rods is easily calculated from the separation distances
of each rod from the rod with the injection. These voltages are the terms entered into the matrix. There is no
simple circuit model using only lumped circuit elements. All the matrix terms are calculated by consecutively
injecting the 1 ampere in each ground rod and then calculating the matrix terms on the row and column
associated with the injection node. Reference [16] gives equations for ground rod resistances.
The earth resistance matrix is inverted to create an earth admittance matrix. The remaining circuit
components are added using conventional nodal admittance equations. The loads are represented as fixed
impedances. Initially the source is an injection of 1 ampere current into node 1 and out of node n+1. After the
matrix is solved, the solution voltages are scaled to the desired phase to ground voltage at the source. This is the
voltage between node 1 and n+1, not the absolute voltage of node 1. This is required so that the ground rise
potential at each ground node is captured. This model has a remote earth voltage reference of zero volts. The
circuit ground rods have potentials different from zero. The sum of injection currents in the model must be zero
in order to obtain results with any meaning since the remote earth is not really accessible in the real world.
Equation (2) (derived by the author) shows the inductance formula used to calculate a feeder's self and
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mutual inductances between the primary conductor, its concentric neutral, and the earth. It takes into account
earth currents due to the electric field gradient within the earth. However it does not include the mutual
inductance of earth current M fields from several feeders taken simultaneously. These were felt to be
insignificant for this problem because of the short distances between ground rods and the shallow depth of the
earth currents.
The mutual inductance between the primary conductor and its concentric neutral can be found using (2) and
is the same value as the self inductance of the concentric neutral. Equation (2) does not rely on an image
conductor, but rather assumes earth return currents arising from the underground electric field gradients. The
ground rod separation distance d should be greater than the length of the ground rods.
(mu)(d)
L = ------- ln(d/g) henries
(2)(pi)
(2)
where:
d = length of a feeder in meters
g = primary or neutral GMR in meters
mu = magnetic permeability of earth
pi = 3.14159
Equation (2) will produce slightly different inductances than the Carson formula [16] used for overhead
lines. For instance, using (2) with a 2 kcmil copper conductor will produce the same inductance as the Carson
formula at 3000 feet (900 m), 80 percent of the Carson formula at 200 feet (60 m), and 50 percent for only 5 feet
(1.5 m) between ground rods. Equation (2) recognizes that earth currents are not as deep for short runs between
earth grounds as they are for cross country overhead transmission lines.
Fig. 6. 19-Node Feeder
Figure 6 shows a 19 node feeder that was used to test the City's
URD feeder model and to illustrate the model's results. This feeder
example is the entire length of feeder used to serve the house shown in
Figure 1. The "Feeder A" segment in Figure 1 is between nodes 10 and
11. The pad mounted transformer at node 11 is the XFMR in Figure 1.
Several cases were run. In each of these cases the load levels were
held constant so that the total feeder current at the riser was about 60
amperes. The grounding configurations were changed from case to case
to see what current imbalances would occur in the system.
The first case assumed that the feeder was isolated. No other
neutral paths were possible other than through the earth and the
concentric neutral of the radial feeder. In this run, the neutrals at nodes
9 and 13 were connected together as they are in the real world. The
computer model gave a balanced system everywhere except between
nodes 9 through 13 which showed a net primary and neutral current of
22 amperes which is highly unbalanced. Magnetic field measurements
and current readings taken on the actual circuit for the segment between
nodes 10 and 11 are in reasonably good agreement with the model when
the loads are scaled down to the conditions occurring at the time the first
experiment was performed. The model captured the circulating current around the block caused by tying the
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neutrals together. This example shows that this practice of looping a feeder around a city block and tying the
neutrals together will result in large current imbalance and much higher magnetic fields.
The second run was the same as the first except the neutral connection between nodes 9 and 13 was
opened. The current imbalance between nodes 9 and 13 was reduced from 22 amperes to 1.1 ampere. This
smaller imbalance is due to the ground rod currents alone since there are no other return paths other than the
neutral of the feeder and the earth.
The third run considers the possibility of an open neutral conductor. To illustrate the effect of opening a
neutral, the circuit was modified by opening the neutral between nodes 7 and 8. This section of feeder crosses
under a creek bed and is a likely point for water filling the plastic conduit. Also, the 15 kV class cable with a 2
kcmil main conductor and unjacketed 16 strands of #14 copper wire has a good possibility of being highly
corroded. The open neutral model showed very large current imbalances throughout the feeder system. Net
currents ranged from 11 to 40 amperes for all the feeder segments. In the simulation, the ground rods are forced
to carry the full current that would have flowed on the neutral that was opened. The relatively high rod earth
resistance pushes the ground rod potentials to hundreds of volts. This condition is not possible in the real world
because there are numerous metallic neutral current paths such as CATV, telephone, and water pipes that will
bypass the need to force these currents through the earth. An open neutral URD system should function very
well but will have highly imbalanced currents throughout the feeder.
The fourth and final run included a separate neutral conductor path from node 1 to node 19. The model
predicted that 17 amperes would flow over this path and substantially unbalance the system. The common
practice of solidly connecting grounds of utilities such as telephone, CATV, water, and gas insures that multiple
return paths will upset the balance of currents in URD feeders and result in larger magnetic fields.
CONCLUSIONS
Underground power circuits can have magnetic fields comparable to those of overhead transmission lines.
These fields arise from a current imbalance between the primary and neutral conductors of a feeder. The
experiments performed and the model circuits presented in this paper demonstrate that the current imbalances in
feeders are due to: 1) looping a feeder and tying the feeder neutrals together; 2) having an open circuit on a
feeder neutral; and 3) connecting grounds of telephone lines, water pipes, and other services to the electric
system. In particular, the common practice of solidly connecting grounds from telephone lines, gas lines, water
pipes, etc. insures that multiple return paths will upset the balance of currents in URD feeders and result in larger
magnetic fields.
The discovery that URD circuits can have significant magnetic fields is important in the sense of being
aware that they exist. The systematic engineering approach taken here to develop experiments and a model that
captures the physical effects might be useful at some point in the future if the designing of low magnetic field
URD systems becomes prudent.
References
[1] P. Heroux, "60-Hz Electric and Magnetic Fields Generated by a Distribution Network,"
Bioelectromagnetics, 8:135-148 (1987).
[2] B. White, "Update on Electric and Magnetic Field Topics," presented at the Forty-first Annual Power
Distribution Conference, Austin, Texas, 1987
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[3] N. Wertheimer, E. Leeper, "Electrical Wiring Configurations and Childhood Cancer," American Journal of
Epidemiology, 109, 273, 1979.
[4] J. P. Fulton, S. Cobb, L. Preble, L. Leone, E. Forman, "Electrical Wiring Configurations and Childhood
Leukemia in Rhode Island," American Journal of Epidemiology, 111, 292, 1980.
[5] N. Wertheimer, E. Leeper, "Electrical Wiring Configurations and Childhood Leukemia in Rhode Island,"
American Journal of Epidemiology, 111, 461, 1980.
[6] L. Tomenius, L. Hellstrom, B. Enander, "Electrical Constructions and 50 Hz Magnetic Field at the
Dwellings of Tumor Cases (0-18 years of age) in the County of Stockholm," a paper presented at the Inst.
Symp. Occup. Health Saf. Min. Tunnelling, Prague, June 21-25, 1982.
[7] S. Milham Jr., "Mortality From Leukemia in Workers Exposed to Electric and Magnetic Fields," New
England Journal of Medicine, 307, 249, 1982.
[8] W.E. Wright, J.M. Peters, T.M. Mack, "Leukemia in Workers Exposed to Electric and Magnetic Fields,"
Lancet, 2(8308), 1160, 1982.
[9] M.E. McDowall, "Leukemia Mortality in Electrical Workers in England and Wales," Lancet, 1(8318), 246,
1983.
[10] M. Coleman, J. Bell, R. Skeet, "Leukemia Incidence in Electrical Workers," Lancet, 1(8332), 982, 1983.
[11] K. Wiklund, J. Einhorn, G. Eklund, "An Application of the Swedish Cancer-Environment Registry.
Leukemia Among Telephone Operators at the Telecommunications Administration in Sweden," Int. J.
Epidemiol., 10, 373, 1981.
[12] D. Vagero, R. Olin, "Incidence of Cancer in the Electronics Industry: Using the New Swedish Cancer
Environment Registry as a Screening Instrument," Br. J. Ind. Med., 40, 188, 1983.
[13] C. Polk, E. Postow, Handbook of Biological Effects of Electro-magnetic Fields, CRC Press, Inc. Boca
Raton, Fl., 1987, pp 208-225.
[14] A. Ahlbom, E.N. Albert, A.C. Fraser-Smith, A.J. Grodzinsky, M.T. Marron, A.O. Martin, M.A. Persinger,
M.L. Shelanski, E.R. Wolpow, "Biological Effects of Power Lines: New York State Power Lines Project,
Scientific Advisory Panel, Final Report," New York State Department of Health, July 1987.
[15] J. Peters, "Community Exposures to Electric and Magnetic Fields and Possible Health Effects", presented at
the EPRI sponsored seminar "Epidemiological Studies of Electromagnetic Field Exposure", Minneapolis,
Minn., 1987
[16] Transmission Line Reference Book 345 kV and Above/Second Edition, EPRI Tech. Rept. EL-2500,
Electric Power Research Institute, Palo Alto, Calif., 1982. pp. 133, 355, 415, 556-560.
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