THE ELECTRIC AND MAGNETIC FIELDS AROUND UHV LINES
Complex patterns of electric and magnetic fields surround a transmission line. At each point in space around the lines, these fields change in magnitude and direction 60 times each second. For 60-Hz overhead lines, the electric and magnetic fields do not depend strongly on one another. For example, a line that is energized to its operating voltage will have very much the same electric field, whether or not current flows. The magnetic field increases in proportion to the current transmitted. Close to the conductor surface, the electric fields are nearly radial, whereas the magnetic fields are nearly concentric around the conductor. A complete description of the fields requires complex mathematics. The figures described below help portray the fields typical of 60-Hz 765-kV lines. Of primary interest at the hearings were the fields near ground level.
Figure II.1 illustrates the field pattern around a 765-kV line at one instant in time. Only the fields in a hypothetical plane between two towers are shown. A grid of points makes the display clearer. The arrows indicate the direction and magnitude of the electric field at each of the grid points. Because the voltage on each conductor varies sinusoidally 60 times per second, the total pattern is repeated many times per second.
Figure II.2 shows the fields one quarter cycle later. At the center of the span, the electric fields lie primarily in a plane perpendicular to the line. Away from midspan, the fields have a small horizontal component parallel to the line.
Figure II.3 shows the peak value of the electric field measured at ground level along a hypothetical line at midspan perpendicular to the transmission line, and the field measured along a hypothetical line about l m above the ground. The peak electric fields increase as one approaches the maximum at the conductor surface.
Figure II.4 shows the peak value of the magnetic field measured along the same hypothetical line used in Figure II.3.
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