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Modeling Magnetic Fields Near Transmission Lines |
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| Introduction
If long-term measurements of existing lines are not practical, or if a
new line is not yet in operation, the MF3D modeling tool
is an attractive alternative for characterizing transmission system
magnetic field levels.
A good theoretical assessment of magnetic fields in the vicinity of a
transmission right-of-way will consider a variety of key issues. For
example, the parameters that contribute to the resulting magnetic field
include the magnitude and phase angle of electric current flow on
transmission conductors, the spacing between conductors and the distance
from the conductors to an area of interest.
Although at first glance these parameters seem straightforward, their
determination often accounts for a significant amount of effort in
developing a model. For example, questions will arise as to the phase
angle of electric currents and the variability of current magnitudes with
respect to the time of day, day of week and week of the year. Accurate
dimensional data for transmission conductor geometry is usually available,
but one still needs to determine who can readily provide this information.
Maps showing the scaled distances of transmission right-of-way boundaries
and property development sites are also necessary and may require time to
prepare.
Modeling Approach
Once the key model parameters are determined, MF3D
computes magnetic flux density using a Biot-Savart algorithm with
three-dimensional conductor models. The active conductors in one or more
transmission circuits are modeled through the development of a data input
file. Since the MF3D models are created in a three
dimensional environment, the effects of conductor sag, turning towers,
line transposition and changes in conductor configuration can all be taken
into account.
After building the three-dimensional conductor data file, the current
parameters are entered, including magnitude and phase angle for each
conductor. To simplify the modeling process and expedite the method of
running the model for different loading conditions, a symbolic algebraic
expression can be assigned to each conductor segment. This allows the user
to specify in a separate "symbol file" the definition of current
magnitude and phase angle for each conductor segment. Finally, the
computed results can be displayed in linear or planar plots and saved as
ASCII text in an output file.
As shown in the example above, the magnetic flux density can be
computed to illustrate the field on the right-of-way and the attenuation
of field beyond the edge of right-of-ways. If the annual
time-weighted-average electric current flows were used in the model, then
the results would show the TWA magnetic field. In this manner, current
year conditions can be approximated and future year conditions can be
predicted.
Choice of Results
A number of different magnetic flux density attributes are available in
the computed output. In addition to the scalar (rms resultant) value shown
above, MF3D can provide individual components (x, y, and
z) for magnitude and phase angle of the magnetic field, maximum and
minimum components (magnitude and phase), and axial ratio (ratio of the
minimum to the maximum component).
When plotting planar profile, the viewing angle can be adjusted with
simple keystrokes to suit the user's needs.
Installation / Operation
MF3D is written in ANSI-standard C programming
language. The menu-based program runs on PC-compatible computers. The
following platform specifications are recommended:
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Making
a reasonable assessment of the magnetic field environment near a
transmission right-of-way for property developers and others who have
questions about EMF levels can involve a number of issues. For example,
when an on-site magnetic field assessment is performed, the measurements
reflect the magnetic fields at the actual time of recording and do not
necessarily result in possible maximum or minimum conditions. Nor would
spot measurements be expected to yield a time-weighted-average (TWA) value
for the magnetic field.
The input file is constructed in
ASCII text using the text editor provided with MF3D or
with a word processor. The MF3D text editor includes
familiar functions such as copy, cut and paste which facilitate rapid
model development.