66 kV lines are not very common in the UK. They are often
built using one of the 132 kV line designs. 66 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. 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. The curve shown here is for the
normal U phasing, and a typical medium-size lattice steel pylon.
This table gives some actual field values for the same conditions.
| |
|
|
|
|
electric
field in V m-1 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
|
1808 |
456 |
91 |
40 |
12 |
|
smaller lines |
L132
single conductors
0.4 sq in
|
typical |
clearance 10 m
phasing U
|
890 |
345 |
43 |
30 |
9 |
Note:
1. All fields calculated at 1 m above ground level.
2. All electric fields are calculated for the nominal voltage.
In practice, voltages (and hence fields) may rise by a few percent.
3. All electric fields calculated here are unperturbed
values.
4. All fields are given to the same resolution for simplicity
of presentation (1 V/m) but are not accurate to better than
a few percent.
5. Calculations ignore zero-sequence voltages. 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.
6. 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.
7. Sometimes, a 66 kV circuit could be carried on a line
designed for 275 kV or 400 kV. Then the electric 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.
8. Calculated fields agree well with measured fields. more
