11 kV overhead lines: magnetic
field
see also:
11 kV lines are either carried on small lattice steel pylons or
on wood poles. There are many different variants of lines design
but they all produce roughly similar fields.
The maximum field shown here is produced by a lattice pylon design
when the ground clearance is the minimum allowed – 5.5 m – and the
loads are the highest allowed – not usually above 500 A in each
circuit but there may be exceptions. The field also depends on
the phasing. 11 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 (the higher field),
and a wood-pole design (the lower field).
typical 11 kV wood pole:
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 |
| 11 kV |
larger lines on steel pylons |
SL11
single conductors |
maximum |
clearance 5.5 m
phasing U
load 0.5/0.5 kA
|
9.456 |
7.664 |
1.490 |
0.364 |
0.090 |
| typical |
clearance 8 m
phasing U
load 0.1 kA
|
1.004 |
0.942 |
0.274 |
0.071 |
0.018 |
| smaller lines on wood poles |
wood pole
horizontal array
+-1.15 m |
maximum |
clearance 5.5 m
single circuit
load 0.2 kA
|
3.744 |
0.668 |
0.124 |
0.032 |
0.008 |
| typical |
clearance 8 m
single circuit
load 0.05 kA
|
0.399 |
0.134 |
0.030 |
0.008 |
0.002 |
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, an 11 kV circuit could be carried on a line designed
for a higher voltage. Then the magnetic fields could be larger
than shown here.
6. The dip in field on the centreline between the two circuits
happens when the two circuits are carrying the same load, and is
less pronounced if the loads are different.
7. Calculated fields agree well with measured fields. more
|
