ETS-Lindgren HI-3638 ELV/VLF Electric Field Meter - Manual (Page 26)

26 Power Frequency Fields www.ets-lindgren.com
The previous illustration portrays an electric field environment surrounding a
typical power transmission line. It illustrates a single-circuit, three phase
power line consisting of three separate electrical conductors, each having an
impressed voltage which is 120 degrees out of phase with its neighboring
conductors. A shield wire may be present above the three phases of the line; this
grounded wire acts as a preferred point for lightning strikes which could, if
unprotected, strike the current carrying conductors, potentially damaging and
removing the line from service for repairs. A double circuit line would consist of
two sets of the three phase conductors.
Electric and magnetic fields produced by the power line originate because of the
voltages impressed on the conductors and the magnitude of current (electricity)
flowing through the conductors. The previous illustration depicts the approximate
spatial orientation of these fields; electric field lines are shown to be directed
such that they terminate at perpendicular angles to the surface of the earth and
magnetic field lines are shown as lines encircling the conductors. At any
particular point in space, the field can be determined by the superposition of the
fields associated with each conductor; because the voltage and current of each
conductor is out of phase with that in any of the others, and the conductors have
some finite spacing between them, the resulting electric and magnetic fields are
calculated on the basis of the vector sum of fields caused by each of the
three conductors. At some points the fields can constructively add together
causing a relatively elevated field strength. At other points the fields from the
conductors may destructively add leading to minima in the fields. Thus,
power line fields can have rather complex spatial distributions about the line.