33 kV

33 kV double-circuit line:                   33 single-circuit wood pole:
 thumbnail photo of 33 kV wood poles thumbnail photos of 33 kV wood poles

 

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.

Magnetic field

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 1 kA in each circuit but there may be exceptions.  The field also depends on the phasing. 33 kV lines usually have Untransposed (U) phasing.

 graph of maximum field 33 kV

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 an estimate of typical loads, 100 A, the normal U phasing, and two different line designs: a lattice steel pylon (the higher field), and a wood-pole design (the lower field).  In practice, when we measure fields from these lines, we find even lower fields, suggesting that 100 A may be an overestimate of typical loads.

graph of typical fields 33 kV

This table gives some actual field values for the same conditions.

     

magnetic field in µT at distance from centreline

maximum under line10 m25 m50 m100 m
33 kVlarger lines on steel pylonsD
single conductors
r=9.8 mm
maximumclearance 5.5 m
phasing U
load 1/1 kA
25.68610.7422.2740.5940.150
typicalclearance 8 m
phasing U
load 0.1 kA
1.5560.8220.2140.0580.015
smaller lines on wood poleswood pole
horizontal array
+-2 m
maximumclearance 5.5 m
single circuit
load .5 kA
14.7482.9610.5410.1380.035
typicalclearance 8 m
single circuit
load 0.1 kA
1.3250.4710.1030.0270.007

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, a 33 kV circuit could be carried on a line designed for a higher voltage.  Then the magnetic fields could be larger than shown here.

Electric field

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.

graph of maximum field 33 kV 

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).

graph of typical field 33 kV

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.

Underground cables

33 kV underground cables are usually a single cable: the three cores are twisted round each other in a single outer sheath. Because the cores are so close together and twisted, the fields they produce directly are very small.  Instead, the field comes from any net current in the sheath.  This is very variable and cannot be predicted accurately.

The following graph shows typical fields from the net current in a 33 kV cable.

graph of field from 33 kV underground cable 

Occasionally a 33 kV cable may have separate cores.  Then the field would be more like a 132 kV cable with separate cores.

Underground cables do not produce any external electric fields.

This table gives some actual field values for the same conditions.

    

magnetic field in µT at distance from centreline

0 m

5 m

10 m

20 m

33 kV

single cable

0.5 m depth

typical

1.00

0.29

0.15

0.07

Notes

1. All fields calculated at 1 m above ground level

2. All fields are given to the same resolution for simplicity of presentation (0.01 µT = 10 nT) but are not accurate to better than a few percent.

3. These calculations are for a single, isolated cable.  This is rare in practice.  Other nearby cables would modify the field.