Replacing a 132 kV line with a 400 kV line

In the UK, when National Grid proposes a new 400 kV power line, one scenario is that it replaces an existing 132 kV line.We give more information on what effect this has on the fields on this page.  (Another scenario is that the 400 kV and 132 kV lines are built parallel to each other.)

What is the difference between a 132 kV and a 400 kV line?

A typical 132 kV line and a typical 400 kV line look pretty similar:

132-double-cct
132 kV

phot of double-circuit pylon
400 kV

 

They both have a lattice steel pylon with one "circuit" - group of three conductors or bundles of conductors - on each side (more on the parts of a power line).  The difference is that the 400 kV line is bigger - the pylons are typically 50 m tall, against 25-30 m for the 132 kV pylon.  But the distance between pylons is also larger for the 400 kV line:

400-spans-diagram

132-span-diagram

Other designs of line

132 kV lines are sometimes only have one circuit:

vertical-single-circuit

And sometimes they are carried on wood poles instead of steel pylons:

trident-scotland-150

And in future, 400 kV lines may be carried on the new T-Pylon:

photomontage of t-pylon

And how much bigger are the fields?

Because the 400 kV line is bigger, it does produce bigger fields.  This graph shows typical fields for the two voltages, though individual lines can vary a lot:

 graph comparing 132 and 400 kV fields

The peak field, directly under the line, is higher, but this depends a lot on the clearance of the line above ground.  The field also takes longer to fall away.  The distance to fall to 0.1 µT, typical of background fields in homes not near power lines, increases from 30-50 m to perhaps 100 m.