400 kV

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 .

The maximum fields come from a design of line called the L6.  This was used extensively when the transmission system was built in the 1960s and 70s.  More recently, new 400 kV lines have often been built using a slightly smaller design of line, the L12.  This produces smaller fields than the L6.  We give here graphs showing the maximum fields from an L12 line at 400 kV.

see also an example of unusually heavily loaded lines.

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.

The data

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

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. 

The maximum field is produced by the largest design of line – the L6 – when the ground clearance is the minimum allowed – 7.6 m. The field also depends on the phasing. Untransposed (U) is generally higher and is shown here. (see also the maximum field produced by the slightly smaller L12 line.)

The maximum fields come from a design of line called the L6.  This was used extensively when the transmission system was built in the 1960s and 70s.  More recently, new 400 kV lines have often been built using a slightly smaller design of line, the L12.  This produces smaller fields than the L6.  We give here a graph showing the maximum fields from an L12 line at 400 kV.

Typical fields are lower than the maximum field because the clearance is usually higher. Also, many lines are a slightly smaller design – the L2.  Both L6 and L2 are shown on the following graph.  The phasing here is transposed (T) as this is more common.

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

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. In efforts to reduce aerodynamic problems, a small number of 400 kV lines with quad bundles have had expanded bundles fitted, e.g. 500 mm horizontally.  This produces slightly higher electric fields but is not included in these tables.