In message , Andy Champ
writes
On 30/04/2013 06:15, harry wrote:
On Apr 29, 11:46 pm, The Other Mike
wrote:

There was someone on another group in the past few months, maybe
uk.railway? I
think the poster was possibly German, and they were claiming that on
the German
grid, the system frequency during the loss of 1.5 - 2GW of
generation (this
was a one trip, one site event) with a system loading of 50GW was
about 0.05Hz.

Yes you read that right zero point zero five hertz. So that's a
larger loss than
the Sizewell B Longannet incident of 2008 that resulted in a
frequency of around
48.8Hz. No, I didn't believe it either!

Not got any measurements in Germany but the frequency moves about in
Italy about
the same as the UK.

--

Frequency, load and efficiency are separate issues.


I've never understood why a load on the grid _must_ affect frequency.
(and yet it does...)

A generator is shoving out a certain amount of power into a load. If
the load increases (reduces its resistance) either the power goes up,
or the voltage comes down.

I can see why the easiest way for the voltage to come down is for the
generator to slow down. The coils aren't going through the fields as
fast, so you get less power consumed. In the case of the grid, where
all the generators are synchronously locked, if they all slow down then
the grid frequency drops.

But why can't you just control the current into the field coils to keep
the frequency spot on, and let the volts go up and down?

When the load increases, the generator output voltage will decrease,
because of the resistance of the armature windings. However, this will
probably be a minor effect compared with the voltage drop on the power
distribution cables, and I'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. Unfortunately, this
will result in a further increase of the load on the generator.

Also, as the load on the generator increases, it will slow down, and the
guys at the power station will try to maintain its speed by (literally)
putting more coals on the fire in order to increase the drive power to
the shaft of the generator, and to apply more voltage to the field
windings (to increase its current, and thus maintain the output
voltage).

Unfortunately, if the load is so great that not enough coals can be put
on the fire quickly enough, something has to give - and there are only
two things that can happen. One is that the load has to be reduced (and
I guess that this is usually done by switching the customers to a slight
lower voltage - although in some cases, this may mean that some will
then try and draw more current to compensate). Alternatively, the output
voltage of the generator itself could be allowed to fall by reducing the
field current (although this might cause complications at the tap
switchers). The other alternative is to allow the generator to slow
down. In practice, in the home, we usually see both effects, ie that the
voltage is lower that we normally get, and frequency is less than 50Hz.
--
Ian