How the balance between the currents in the two circuits affects the field produced by an overhead line
The actual field produced by an overhead line depends on several factors. This page illustrates this for one standard line, a 400 kV L12 transmission line with typical loads. Our detailed calculations of fields all specify the conditions they were calculated for.
A typical transmission line has two circuits, one each side of the pylons, each with three bundles of conductors or “phases”. The magnetic field depends not just on how big the currents are that flow in these conductors but also on how well balanced the different currents are, that is, how nearly equal they are – are the currents in each of the two circuits equal or not? The related factor - are the currents in the three phases in each circuit equal or not? - is dealt with here.
How it works
The importance of the balance of current between the two circuits depends on the phasing of the line. With "untransposed" phasing , the magnetic fields from the two circuits reinforce each other, and it doesn't matter much whether they are equal or not. But with "transposed" phasing, the magnetic fields from the two circuits partially cancel each other. If the two currents are equal, the cancellation is good, and the magnetic field is reduced, especially at larger distances. But if the two currents are not equal, the cancellation is less good, and the magnetic field at a distance is higher.
This is illustrated in the following graph where we take the same total current, 1000 A, but share it differently between the two circuits of the line.
The average National Grid transmission line does not have perfectly balanced currents in the two circuits. (Some lines, where the two circuits are connected in parallel between the same two points, actually do have almost perfect balance except for odd weeks when one circuit is switched out for maintenance, but for others, the two circuits supply different places and are rarely balanced.)
To allow for this, all the detailed calculations of magnetic fields here on this web site deliberately introduce some unbalance for transposed phasing - that is why the curve for "typical" magnetic field for 400 kV and 275 kV is not symmetrical, because the current on one side of the line is higher than on the other. Similarly, the figures in this table are an average over a number of actual National Grid lines so they include whatever unbalance was actually present during the year in question in those lines. Likewise, when SAGE calculated the average distance for fields to fall to a given level, this took account of unbalanced loads.