So whilst we are on the subject of switching power lines, how do
different generating stations get themselves syncronised? I presume
two genny's are pumping power into the national grid at the same time,
and thus must be properly syncronised or else.....

IANAEE[*] but you are right that the whole grid must be synchronised or
Bad Things(tm) will happen. When a genset is brought online, it is first
spun up to speed and then synchronised with the grid by carefully
monitoring its output compared to the grid until there is no phase
variance. It is then switched into the grid. I believe this used to be
done manually but I am sure it all happens automatically now.

Generators are synchronous devices so once they are switched in to the
grid they stay synchronised. Any tendency for a particular genset to lose
synchronisation (perhaps caused by excessive load) would cause a breaker
to trip.

When one section of the grid becomes isolated from another then both parts
can lose synchronisation and that is why it can take some time to bring
everything back online.



* I Am Not An Electrical Engineer

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Alistair Riddell - BOFH
Microsoft - because god hates us