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Recharging the Grid with Electric Cars - Page 20

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Posted by Eeyore on April 6, 2009, 2:13 am

Tim Jackson wrote:

There's an audio analyser I used that registered mains frequency and THD. I
never saw it differ
from 50Hz by more than 0.1 Hz and that was the resolution of the counter.

If you want to 'send equipment to sleep' it's far more sensible to do it with
data over power.
This is how modern electricity meters work. They don't need to be manually read.


Posted by Alistair Gunn on April 6, 2009, 9:41 am
In alt.energy.renewable Tim Jackson twisted the electrons to say:

Don't even need to do that, National Grid helpfully put a graph of the
last 60 minutes on their website :-
These opinions might not even be mine ...
Let alone connected with my employer ...

Posted by Eeyore on April 6, 2009, 2:08 pm

Alistair Gunn wrote:

Interesting. That will help me with a technical issue.

What's amazing is that ALL the generators have to be frequency and phase
locked to do that.


Posted by Tim Jackson on April 6, 2009, 3:49 pm
 Eeyore wrote:

Of course they do. it's not rocket science.  In simple terms, ignoring
excitation, you connect an AC electrical machine to the grid it will
motor round at grid speed.  You want it to be a motor, you apply a load
to the shaft and it draws current, You want it to be a generator then
you drive the shaft in the direction of rotation and it generates.  Its
actual rotation is locked to all the others because they are connected
together - what you control is the torque.

Bringing on one generator, you have control over the unloaded frequency,
so you spin up, synchronise, connect, then open up the valves to wind up
the torque to start exporting.  Disconnecting, you wind down the torque
and drop the contactor when the current hits zero.

The grid collectively controls the frequency, which controls the speed
of all the generators, each individual generator controls its output
current by adjusting the input power.  When any generator or load
changes its current, lest say a load increase, first the grid voltages
fall slightly which increases the mechanical load presented by each of
the generators, so they all slow down a bit, which in turn signals their
local control systems to increase their power input, if possible.

Bringing two grids together to enable a power transfer is harder because
you've got to move the whole loaded grid into phase by fiddling the
target frequencies and get it to stay there long enough to lock it up.
Then two *target* frequencies have to be moved apart until the desired
flow is achieved, one grid "pushing" the other.  This does not mean the
actual frequencies move apart, obviously that's not physically possible,
it means that any given actual frequency produces a stronger signal to
the exporting grid to generate power than it does the importing one.


Posted by EskWIRED on April 7, 2009, 1:28 am

I was just visiting a client's hydro plant, and was asking him about the
goveror,  I had thought it was used to control RPM, which in turn affected
frequency.  He explained that the grid itself keeps the geerator in
frequency, so its interesting to see this discussed here,

Where can I find out more about his subject?

The whole problem with the world is that fools and fanatics are always so
certain of themselves, but wiser people so full of doubts.
    -- Bertrand Russell

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