In our main page on the phasing of a double-circuit power line, we explain how an arrangement called transposed phasing produces lower fields to the sides of the power line than the alternative untransposed phasing. This is because the magnetic field produced by each circuit is, roughly, a dipole, and with transposed phasing the two dipoles are in opposite directions, leading to greater cancellation of the fields. (See some drawings of the actual field lines)
This effect varies with the ground clearance of the conductors (see also how the ground clearance affects the field directly). Well to the sides of the power line, it is always true that transposed phasing gives lower fields, whatever the clearance. But between the two circuits, a different effect comes into play.
With transposed phasing, when you cross under one of the circuits to go from "outside" to "inside" the conductors, the vertical component of the dipole field from those conductors goes through zero and reverses direction. So the two vertical components from the two circuits now reinforce each other instead of cancelling. This is more significant at lower clearances. So at the lowest clearances, transposed phasing actually gives higher fields than untransposed at ground level between the two circuits.
Similarly, with untransposed phasing, to the sides of the line the two dipoles add, but between the conductors, they partially cancel. This leads to a "dip" in the field on the centreline, half way between the two conductors.
(Actually, whatever the phasing, if the clearance is low enough this dip will appear. That is because if you are close enough to the actual conductors of one circuit, the field you experience basically comes just from that circuit.)
These effects are illustrated in the following graph.
- At a relatively high clearance, 20 m, the transposed phasing is lower than the untransposed everywhere
- At a relatively low clearance, 8 m, the untransposed phasing dips between the two circuits and the transposed phasing gives the higher field in this region
- At intermediate clearances, 12 m, typical of actual power lines, the two are about the same between the circuits
- To the sides of the line, transposed phasing is always lower.
Note: these graphs are calculated for L12 lines with 500 A in each circuit. In real power lines, the currents are not always equal both between circuits and within a circuit, and these reduce the effectiveness of the transposed phasing. See more detail on exactly how fast fields fall with distance under these different conditions.
