275 kV lines are built to the same designs as 400 kV lines and
can carry the same loads so they produce the same magnetic fields.
(Technically a 275 kV line can be 7.3 m instead of 7.6 m above
ground but that has been ignored here.)
The maximum field is produced by the largest design of line –
the L6 – when the ground clearance is the minimum allowed – 7.6
m – and the loads are the highest allowed – 4.7 kA in each circuit.
The field also depends on the phasing. Transposed (T) is
higher close to the line and Untransposed (U) is higher away from
the line. This graph shows both.
Typical fields are lower than the maximum field because the clearance
is usually higher and the loads are usually lower. Also, many
lines are a slightly smaller design – the L2. Both L6 and L2
are shown on the following graph. The phasing here is T as this
is more common. It is rare for the two circuits to carry exactly
the same load. That is why the magnetic field shown here is not
symmetrical.
see also fields calculated
from one year of actual load data
| Larger 275 kV pylon (L6): |
Smaller 275 kV pylon (L2): |
 |
 |
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 |
| 275 kV |
largest lines |
L6
quad bundles
0.305 m
zebra
|
maximum |
clearance 7.6 m
phasing U
load 4.7/4.7 kA
|
108.422 |
95.780 |
38.422 |
11.697 |
3.096 |
| typical |
clearance 13 m
phasing T
load 0.4/0.6 kA
|
5.783 |
5.247 |
2.194 |
0.578 |
0.119 |
| smaller lines |
L2
twin bundles
0.305 m
zebra
|
maximum |
clearance 7.6 m
phasing U
load 2.4/2.4 kA
|
54.142 |
46.300 |
16.283 |
4.865 |
1.278 |
| typical |
clearance 13 m
phasing T
load 0.4/0.6 kA
|
4.971 |
4.158 |
1.557 |
0.400 |
0.084 |
| typical design used
for new lines |
L12
twin bundles
0.5 m
araucaria
|
maximum |
clearance 7.6 m
phasing U
load 3.5/3.5 kA
|
81.942 |
72.818 |
22.103 |
8.148 |
2.145 |
| typical |
clearance 13 m
phasing T
load 0.4/0.6 kA
|
5.604 |
4.938 |
1.979 |
0.514 |
0.106 |
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. Calculated fields agree well with measured fields. more
