400 kV is the highest voltage in use in the UK.
The maximum field is produced by the largest design of line
– the L6 – when the ground clearance is the minimum allowed
– 7.6 m. The field also depends on the phasing. Untransposed
(U) is generally higher and is shown here.
Typical fields are lower than the maximum field because the
clearance is usually higher. Also, many lines are a slightly
smaller design – the L2. Both L6 and L2 are shown on the following
graph. The phasing here is transposed (T) as this is more common.
| Larger 400 kV pylon (L6): |
Smaller 400 kV pylon (L2): |
 |
 |
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 |
|
400 kV |
largest lines |
L6
quad bundles
0.305 m
zebra
|
maximum |
clearance 7.6 m
phasing U
|
11400 |
10130 |
774 |
446 |
156 |
|
typical |
clearance 13 m
phasing T
|
4244 |
4206 |
962 |
106 |
32 |
|
smaller lines |
L2
twin bundles
0.305 m
zebra
|
maximum |
clearance 7.6 m
phasing U
|
9897 |
6618 |
283 |
357 |
118 |
|
typical |
clearance 13 m
phasing T
|
3128 |
2964 |
469 |
56 |
27 |
|
typical design used
for new lines |
L12
twin bundles
0.5 m
araucaria
|
maximum |
clearance 7.6 m
phasing U
|
10642 |
8410 |
669 |
404 |
136 |
| clearance 7.6 m
phasing T
|
9384 |
7933 |
737 |
151 |
30 |
|
typical |
clearance 13 m
phasing T
|
3704 |
3595 |
732 |
98 |
26 |
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. In efforts to reduce aerodynamic problems, a small number
of 400 kV lines with quad bundles have had expanded bundles
fitted, eg 500 mm horizontally. This produces slightly higher
electric fields but is not included in these tables.
8. Calculated fields agree well with measured fields. more
