"Phasing" refers to the order of the phases (the separate conductor bundles) of the two circuits of a double-circuit power line. See a full explanation with diagrams of how "transposed phasing" - one circuit reversed with respect to the other - produces lower magnetic fields to the side of the line (and sometimes underneath it as well), and how, when you consider the direction of current flow as well, you get to the concept of Optimum and Non Optimum phasing.
On this page we give the best available data on the extent of phasing in the UK.
The UK as a whole
As part of the UK policy of Optimum Phasing, the electricity industry agrees to report on the prevalence of optimum phasing at three-year intervals. The first report was in 2013, and the summary results table is reproduced here:
This shows that, across all lines at 132 kV and above (and also including 110 kV in Northern ireland), 68% of double-circuit lines have optimum phasing. The prevalence of optimum hasing is lower (50%) on 132 kV lines and higher (86%) on 275 and 400 kV lines.
More detail on National Grid system
The National Grid system in England and Wales was built from the 1960s onwards with a policy of transposed phasing where possible. The following table gives detailed statistics for National Grid in England and Wales.
| Phasing category | Sub-category and explanation | length of line | |
|---|---|---|---|
| km | % | ||
| Optimum | Transposed and current flows known or assumed to be in same direction |
6191 |
|
| Untransposed and current flows generally in opposite directions |
99 |
||
| total |
6290 |
87 | |
| Non Optimum | Single power line |
338 |
|
| Three power lines meeting at "T" point - not geometrically possible to have complete transposed phasing |
358 |
||
| total |
696 |
10 | |
| Not classifiable |
Direction of currents varies so not possible to classify. |
2 |
0.03 |
| Not applicable - single circuit | True single circuit - either built as such or second circuit not connected |
138 |
|
| Single National Grid circuit but other circuit is used by DNO |
64 |
||
| total |
202 |
3 | |
| Overall total |
7190 |
100 | |
Lengths are for "routes" - that is, the physical length of power line. The length of "circuits" would be twice as big.
How are these expected to change?
The UK has adopted a policy of Optimal Phasing. The details are spelled out in a Code of Practice.
According to this, electricity companies will consider changing lines which do not have optimum phasing when work is being undertaken on those lines that involves replacing the conductors. Lines will be changed when this is "reasonable", which generally means when it does not require extra structures to be built. So:
- Non Optimum lines that result from three lines meeting at a "T" point will often not be optimum because of the geometrical constraint, changing this would involve building a special phase transposition tower, this is not considered "reasonable", so these lines are unlikely to change.
- Non Optimum lines that don't involve a "T" point will be considered for changing. But changing them requires the order of the downleads at the ends to be changed, and there may of may not be space to achieve this without building extra structures.
- New lines will continue to be built with Transposed phasing where possible.
So we should expect to see the fraction of the system which has Optimum phasing increase over time, but maybe not by very much.