On Thu, 10 Mar 2011 13:38:50 +0000 (GMT), "Dave Liquorice"
wrote:

On Thu, 10 Mar 2011 11:31:59 +0000, The Natural Philosopher wrote:

You will get far more familiar with this as more silly wind power is
added to the grid.

I'm not overly keen on this frequency based automatic load shedding
(as in switching off) either. Sounds like a recipe for grid
instabilty if you ask me. Frequency drops, lots of stuff drops off
line, Whe Hey! too much capacity frequency goes up, stuff comes back
online, frequency drops...

It doesn't work like that.

If the frequency drops then the first response is the governors on all
the generators on the system. They will always actively control the
energy input such that the frequency is maintained at the target of
50Hz.

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)

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.

If by this stage the frequency is still dropping then the generators
automatically disconnect to protect themselves.

The grid is then dead.

Then sometime later certain contracted for 'black start' sites
commence generation.

Starting with a diesel generator, to power the fuel pumps etc to run
up a gas turbine (kerosine fueled) which then runs the auxiliaries to
run the main generation at that particular site (usually coal but
could be gas or oil) You might get some hydro running too.

Once part of the grid is energised it is interconnected with other
generation and load, controlling generation and load such that the
frequency and voltage is within limits. Gradually everything is
reconnected.

What happens at each stage is defined in advance, as are the frequency
targets (beyond the published +/- 0.5Hz) and the voltage reductions.
Consumers on a disconnection agreement know that they could be
disconnected at little or no notice.


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