On Thu, 10 Mar 2011 17:09:04 +0000, The Other Mike wrote:


If the frequency still drops then load that is on a disconnection
agreement is removed in a controlled manner. This is usually
commercial heating / cooling / non critical processes.

Then there is voltage reduction in a number of stages to further
reduce load that isn't on a disconnection agreement (most domestic
load for instance)

Two stages, as I said earlier, 5% and then 10%. Carried out very easily
these days with the automated control systems in use. It's worth noting,
however, that the load reduction is usually carried out by the Distribution
companies through their control rooms.

If the frequency drops further then load gets shed in defined blocks
it could be an entire city, or an area , or a particularly large
industrial user - that load then stays off, there is no automatic
reconnection.

Unlikely to be a whole city, unless it's an emergency load shed. Normally
the rota disconnections are planned on an 11kv feeder by feeder basis,
again carried out these days by a simple command from a central control
room. I disagree with your comment that there's no automatic reconnection,
rota shedding is usually for 3 hour periods.

If we're talking emergency load shedding, that usually implies something
catastrophic has happened, eith on the generation or transmission systems.
I seem to recollect it happened some years ago during summer, when part of
the grid was out for maintenance and there was another failure on either a
275 or 400kv cable somewhere around north London meaning effectively that
the country was electricaslly split in two. Lasted for several hours if
memory serves correctly.

Let's also be fair though, for the conditions you mention we are talking an
absolute catastrophe. That we don't get therm is why control engineers
suffer high blood pressure.....

BTDTGTTS.

--
The Wanderer

I cannot teach anybody anything, I can only make them think. ~Socrates