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SAGE Recommendations on phasing of lines.
More on SAGE.
Where overhead lines have two circuits, there is a choice about how they are
wired relative to each other, called the "phasing". One option, usually a
phasing known as "transposed", results in more cancellation between the two
circuits and lower fields.
More on this including graphs showing this
effect.
SAGE says:
"We recommend that electricity companies be encouraged to choose the optimal
phasing (usually transposed phasing) for all new lines, and also be encouraged
to convert existing lines where possible and justifiable."
And also
"We do not expect our recommendation to result in any change for the National
Grid"
This is because the National Grid (400 kV and 275 kV lines) was built from the
start with a policy of using transposed phasing where possible. About 90% is
already transposed. The remainder is mainly made up of single-circuit lines,
where phasing is not relevant, or situations where three lines meet at a "T"
point, where it is infeasible to achieve transposed phasing for all three
lines.
For 132 kV lines, SAGE collected data from electricity companies. Between 70%
and 90% (depending on the company) of lines are double-circuit as opposed to
single-circuit. Of these, between 70% and 90% are already transposed.
SAGE estimates that 12% of 132 kV lines, 2000 km, are not currently transposed
but reasonably could be.
SAGE estimates the cost of converting a line to transposed phasing as £60k for
275 kV and 400 kV lines or £10k - £30k for 132 kV lines, where it can be done
simply by changing the terminations at the end of the line. If it cannot be
done that way and requires a new pylon or other new work the cost rises.
SAGE says:
"We do not advocate a rigorous definition of what should be done, nor do we
believe it should be enshrined in regulation or should have a specific
timescale attached. Instead, we consider this will be most effective if
expressed as a general encouragement to electricity companies to take the
desired action where possible."
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