In message , MM
writes
On Tue, 08 Mar 2011 22:14:39 +0000, The Other Mike
wrote:

Maybe 20-30 years ago it dropped to minus 4 secs or so on one occasion
when there was a split system. It *might* have been October 1987.
Not sure if they got the clocks back to zero error at +24hours.

During winter on another occasion the frequency dropped to somewhere
in the mid to high 47's before a system split and load shedding gave a
high frequency in the north and midlands and a low one in the south.

It took a hell of a lot of work to get the two systems back in sync,
and as a consequence over the next couple of years they changed the
synchronising relays associated with a very large number of circuit
breakers across the 400kV and 275kV grid such that a second method of
automatic closure was possible dependent on slip angle and rate of
change of slip angle rather than just the slip angle. These could be
primed and left for 15mins so that if the system either side of the
breaker met the targets it would automatically close, previously the
only method was a 2 minute timer for 'perfect sync' or a manual
closure that ran the risk of exploding the breaker into copper and
porcelain shrapnel.

The long period of split running during this incident gave a clock
error such that various bits of the country were out of sync by a
second or more. None of this made the papers at the time

Crikey, I never realised just what technology is needed to provide a
country with an electricity grid!

A long time ago, one of my set lab experiments was to run up fairly
large three-phase generator, synchronise it with the mains, get it
generating power and do various test measurements. Fortunately, my
professional career was more concerned with valves and transistors, and
I never needed to do it again!
--
Ian