33 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. The field also depends on the phasing. 33 kV lines
usually have Untransposed (U) phasing.
Typical fields are lower than the maximum field because the
clearance is usually higher. The two curves shown here are
for normal U phasing and two different line designs: a lattice
steel pylon (the higher field), and a wood-pole design (the
lower field).
| 33 kV double-circuit line: |
33 single-circuit wood pole: |
 |
 |
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 |
|
33 kV |
larger lines on steel pylons |
D
single conductors
r=9.8 mm
|
maximum |
clearance 5.5 m
phasing U
|
897 |
46 |
30 |
10 |
3 |
|
typical |
clearance 8 m
phasing U
|
543 |
81 |
21 |
9 |
3 |
|
smaller lines on wood poles |
wood pole
horizontal array
+-2 m
|
maximum |
clearance 5.5 m
single circuit
|
505 |
165 |
16 |
2 |
0 |
|
typical |
clearance 8 m
single circuit
|
241 |
147 |
21 |
3 |
0 |
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 33 kV circuit could be carried on a line
designed for a higher voltage. Then the electric fields could
be larger than shown here.
8.
Calculated fields agree well with measured fields. more
