Transmission

Transmission lines

What is a transmission line?
A transmission line is a high-voltage overhead power line - the lines operated by National Grid.

In England and Wales there are 7000 km of overhead transmission lines at 275 kV and 400 kV. Virtually all of these lines carry two separate circuits, one each side of the towers, each with three wires or bundles of wires.

Click here for more details on what a transmission line looks like and what its parts are.

Examples of field lines produced by transmission lines

How do transmission lines produce fields?
The magnetic field produced by a current in a conductor falls with distance from the conductor. Where there is more than one current forming part of one or more electrical circuits, there is also partial cancellation between the magnetic fields produced by the individual currents, and that cancellation generally becomes better at greater distances. Overall, the magnetic field is highest at the point of closest approach to the conductors and falls quite rapidly with distance. Similarly, there is partial cancellation between the electric fields produced by the voltages on individual conductors, and the electric field is usually highest at the point of closest approach to the conductors and falls quite rapidly with distance.

Therefore transmission lines produce a magnetic field which peaks underneath the conductors and falls rapidly with distance either side, as illustrated in the figure below. More on how fields fall with distance.

What determines how much field they produce?
The actual field produced depends on several factors.

• It depends on the currents. The largest lines in use have a rating of over 4000 A per circuit, but the average current in a typical circuit is more like 700 A.

• It depends on the clearance of the line. The minimum ground clearance of a 400 kV line is 7.6 m, but it is rare for lines to be this low, and the ground-level field falls rapidly with the height of the line above ground. The maximum fields that are produced by a line occur directly underneath the line, underneath the lowest point of the conductors, which is usually towards the middle of each span. Actual conductor clearances above ground would generally be higher than this (and therefore the fields produced near ground level would be lower) for two main reasons. Firstly, for most of the length of a span, the conductor clearance is higher than it is at the lowest point. Secondly, the actual ground clearance of the conductors depends on their temperature. For the vast majority of the time they operate at less than their rated maximum temperature and therefore sag less, resulting in higher ground clearances.

• The field also depends on the relative phasing of the two circuits (see the figure below). A few lines have "untransposed" phasing, with the phases in the same order from top to bottom on the two sides of the towers. This produces a field which falls as the inverse square of distance from the line. However most lines have "transposed" phasing, with the opposite order of the phases on one side to the other. This introduces an extra degree of symmetry and extra cancellation between the fields from equal currents on the two sides; the resultant field falls more nearly as the inverse cube of distance, producing a much lower field at large distances from the line. This is illustrated below.

Magnetic fields can be calculated with considerable accuracy if the currents are known. The graph below shows the actual field measured from a 400 kV transmission line - the red dots - compared to the calculated field at the same instant of time.

How much field do they produce?
Taking account of all these factors, the steady-state maximum ground-level field beneath a transmission line is 100 microteslas (µT), but in practice fields are often below 10 µT. Similar considerations apply to electric fields (see below). The maximum unperturbed ground-level electric field beneath a 400 kV line is 11 kilovolt per metre (kV m^-1).

This table gives more details on fields produced by various overhead lines:

Typical ground-level UK field levels from overhead power lines

This is a summary. We also give a lot more detail on maximum and typical fields from various power lines in two separate (and complex) tables for electric and magnetic fields.

Examples of typical maximum fields from lines of different voltages

What is the average field from a line?
The following table gives more detail on the average magnetic field at various distances from a typical National Grid line. These figures were calculated from one year’s worth of recorded load data and are the average for a representative sample of 43 different lines.

How many people live near high-voltage power lines?
Although people living near high-voltage power lines are a group whose exposure is high and can often be calculated reasonably well, they are a small group. In the UK, 0.07% of homes are within 50 m of transmission lines and 0.21% within 100 m. Percentages in other countries seem to be comparable (USA 1.1% within 40 m; Denmark 0.43% within 75 m), with higher percentages partly reflecting a broader definition of “transmission”.

More detail on numbers of homes near power lines.

Averaged over the population, high-voltage power lines contribute only a small fraction of collective average exposure to EMFs, because so few people are exposed to them. The best estimate possible from the UK is that high-voltage power lines contribute 5% of total average population exposure.

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