66 kV lines are not very common in the UK. They are often built
using one of the 132 kV line designs, so the magnetic field is
the same as for 132 kV. 132 kV overhead lines are usually carried
on lattice steel pylons, but smaller than used for 275 kV and
400 kV lines. Sometimes they are carried on wood poles.
The maximum field is produced by the largest design of line –
the L7 – when the ground clearance is the minimum allowed – 7.0
m – and the loads are the highest allowed – 1.4 kA in each circuit.
The field also depends on the phasing. 66 kV lines usually
have Untransposed (U) phasing.
Typical fields are lower than the maximum field because the clearance
is usually higher and the loads are usually lower. The two curves
shown here are for typical loads, the normal U phasing, and two
different line designs: a lattice steel pylon, L132 (the higher
field), and a wood-pole design (the lower field).
This table gives some actual field values for the same conditions.
| |
|
|
|
|
magnetic
field in µT at distance from centreline |
|
maximum under line |
10 m |
25 m |
50 m |
100 m |
| 66 kV |
largest lines |
L7
twin bundles
0.305 m
lynx
|
maximum |
clearance 7 m
phasing U
load 1.4/1.4 kA
|
30.445 |
20.532 |
5.553 |
1.528 |
0.392 |
| typical |
clearance 10 m
phasing U
load 0.13/0.13
|
1.848 |
1.359 |
0.468 |
0.138 |
0.036 |
| smaller lines |
L132
single conductors
0.4 sq in
|
maximum |
clearance 7 m
phasing U
load 1.2/1.2 kA
|
24.585 |
17.217 |
4.587 |
1.247 |
0.318 |
| typical |
clearance 10 m
phasing U
load 0.13/0.13 kA
|
1.731 |
1.317 |
0.451 |
0.132 |
0.034 |
| smallest wood-pole
design |
trident
150 m span
single conductors
lynx
|
maximum |
clearance 7 m
single circuit
load 0.7 kA
|
12.347 |
12.347 |
0.738 |
0.192 |
0.048 |
| typical |
clearance 10 m
single circuit
load 0.1 kA
|
1.764 |
0.385 |
0.099 |
0.027 |
0.007 |
Note:
1. All fields calculated at 1 m above ground level.
2. All fields are given to the same resolution for simplicity
of presentation (1 nT = 0.001 µT) but are not accurate to better
than a few percent.
3. Calculations ignore zero-sequence current. This means
values at larger distances are probably underestimates, but this
is unlikely to amount to more than a few percent and less closer
to the line.
4. The “maximum field under the line” is the largest field,
which is not necessarily on the route centreline; it is often
under one of the conductor bundles.
5. Sometimes, a 66 kV circuit could be carried on a line
designed for 275 kV or 400 kV. Then the magnetic fields could
be larger than shown here. Or it could be carried on a one-off
wood pole design, but the fields would probably be similar to
those shown here.
6. Calculated fields agree well with measured fields. more
