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.
