On Tue, 30 Apr 2013 10:59:37 +0100, Ian Jackson
wrote:

m pretty sure that somewhere down the line,
at the sub-stations, there is some tap-switching equipment which tries
to maintain a more-constant voltage at the customer.

There is automatic voltage control at the generator to keep the terminal voltage
near constant, terminal voltage being a predefined generator design parameter
and controlled to within a few % of that figure. It's typically somewhere in
the region of 15 - 25kV on the majority of UK coal/oil/nuke/gas generating
capacity) This is then stepped up to a transmission voltage (400/275/132kV)
The grid systems with one or two exceptions is always run at their nominal
voltage, the transformers between these three voltages are usually
autotransformers or in a few cases double wound. So the lower voltage networks
run at a strict ratio to the higher voltage ones with no adjustment possible.

The variable tapping takes place at the interface between the transmission
voltage and the distribution voltage and often on the secondary of a transformer
having 132kV on its primary. Metering of power leaving the grid system is
normally performed at these nodes. (Contrary to the norm are a small number of
400kV and 275kV to 66kV/33kV/11kV transformers on UK power networks)






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