275 kV overhead lines: electric field

see also:

275 kV lines are built to the same designs as 400 kV lines.

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.

[graph: 27em]

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 T as this is more common.

Larger 275 kV pylon (L6):	Smaller 275 kV pylon (L2):

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
275 kV	largest lines	L6 quad bundles 0.305 m zebra	maximum	clearance 7.6 m phasing U	7838	6964	532	307	107
			typical	clearance 13 m phasing T	2918	2892	661	73	22
	smaller lines	L2 twin bundles 0.305 m zebra	maximum	clearance 7.6 m phasing U	6804	4550	195	245	81
			typical	clearance 13 m phasing T	2151	2038	322	39	19
	typical design used for new lines	L12 twin bundles 0.5 m araucaria	maximum	clearance 7.6 m phasing U	7316	5782	460	278	94
			typical	clearance 13 m phasing T	2547	2472	503	67	18

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. Calculated fields agree well with measured fields.  more