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Recharging the Grid with Electric Cars - Page 23

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Posted by daestrom on April 7, 2009, 9:25 pm

Well, not exactly.  The *governors* on regulating units can respond faster
than that (governors can respond on the order of a second or less).  But
because the governors are simple proportional controls, they have to have a
fixed error to maintain the new output level.

Then the system operator and/or SCADA will slowly 'tell' the regulating
units to raise/lower their speed setpoints.  Or if a regulating unit is near
a limit, the operator and/or SCADA will have to contact a base-load unit and
ask them to raise/lower their output.  These operations are both
deliberately slower adjustments to avoid 'chasing their own tail' (hunting a
lot when load is ramping continuously up/down).

This is why the variations you usually see are on the order of 5 to 20


Posted by Don Kelly on April 8, 2009, 5:04 am
daestrom wrote:

Right on: I'd like to add the following:

Typically governors of units under regulation have a speed droop which
may be of the order of 5% from no load to full load if the machine was
running alone.   This and the no-load speed setting determine how load
is shared between interconnected machines. The droop is necessary as if
there is no such droop, the sharing of load becomes indeterminate  and
the fastest machine to respond will try to pick up all the load (and
even try to motor other machines). Hydro machines have an additional
temporary droop as because of  hydraulic considerations- -if you open
the gates to pick up load, there can be a temporary decrease in output
as it takes time to change the flow in a penstock.   In some cases this
constraint may result in full valve opening or closing taking minutes
where a steam valve can operate within a second in some cases.
In any case these governor settings may not be what is wanted so that an
external override (SCADA or other) is needed to re-adjust the load
sharing and frequency.
I recall one system which had a master clock which sent out control
signals every few seconds. Each machine had its own clock and a contact
arm. If the arm was between the signal contacts- nothing happened - but
if it was on a signal contact it got a signal to increase and decrease
the speed. Load changes could bias this - effectively it wasn't a load
frequency control but a load -time control. It was very ingenious and
worked well in the particular system with remotely controlled hydro
plants. This was discarded later on interconnection with the much larger
NW pool and conventional load frequency control was used.
As I recall, the frequency charts generally showed a band about 0.05 to
0.1 Hz wide.
In one system in Quebec, an excursion of 0.5Hz had to be reported as an
unusual incident

Don Kelly
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Posted by Eeyore on April 3, 2009, 3:15 am

Alistair Gunn wrote:

That's schedulable. Very easy to organise.

Hopeless record.


Posted by Alistair Gunn on April 3, 2009, 11:15 am
 In alt.energy.renewable Eeyore twisted the electrons to say:

You still need the back-up plant available.  Also Sizewell-B dropping
itself out of the UK's electricity supply (and thus taking 3% of supply
away) wasn't exactly predicted in the case I mention above.

Your prediction of supply from wind/solar doesn't need to be 100%
accurate, because your prediction of how much demand there's going to be
is never going to be 100% accurate either.
These opinions might not even be mine ...
Let alone connected with my employer ...

Posted by Eeyore on April 4, 2009, 3:28 am

Bill Carter wrote:


Huge uptime compared to either solar or wind. I'm not averse to using some wind
but the
economics of *PV* solar are just daft.


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