[time-nuts] Mains frequency
Jim Sanford
wb4gcs at wb4gcs.org
Wed Mar 12 23:53:09 UTC 2014
All:
Some crude approximations.
Generators that I know of do in fact have a negative slope of frequency
versus load. This is deliberate, to enable stable load sharing. On
small systems, you try to set the slopes proportional to load capacity
so that load sharing remains proportional in the face of a step increase
in load. The amount of load each machine carries is proportional to
capacity in these systems if their no-load frequencies are equal before
parallel. Once in parallel, the proportion can be adjusted in infinite
combinations by adjusting governor (frequency setting) on the two machines.
It gets much more complex in larger systems, but the fundamentals above
are a good start in understanding. With networked automation, what I
described above can be largely automated, as long as the system is stable.
As an aside, similar situation exists with voltage versus reactive
load. Increased reactive (usually inductive; large motors) load is seen
as higher line current at the generator output, requiring increased
excitation current in the generator field to overcome internal losses
and maintain the same terminal voltage. This is what initiated the 2003
blackout in parts of the US & Canada. A utility had a paucity of
reactive generation on a day with large reactive load, and one of its
generators tripped on over-excitation to prevent damage to the generator
and voltage regulator. This initiated the cascading events that left
many in the dark. (The Joint US/Canada task force on that event is a
/fascinating/ read!)
Relaxing frequency tolerance gives the system operators additional
freedom in managing their systems in the face of rapidly changing load
or generation. As the penetration of solar and, in particular wind,
increases, managing this is becoming more difficult, so additional
variation helps keep the grid on line. A 2007 US DOE report stated that
to be stable, the grid needs some percentage of excess generation
capacity over load, and stated at the time, the US had just UNDER that
amount of excess, and projected construction was much less than
projected load increase. That report predicted widespread and frequent
rotating blackouts in the US by 2010, which obviously didn't happen, due
to a /decrease/ in load, probably due to the combination of the economy
and energy conservation efforts.
Since then, large amounts of generation (primarily coal) has been shut
down, so I was not at all surprised by the request.
I missed the announcement that the request was withdrawn, and actually
thought it had been approved and enacted -- all my line-frequency based
clocks are now erratic and not very accurate.
Hope this helps.
Jim
wb4gcs at amsat.org
On 3/12/2014 3:23 PM, Hal Murray wrote:
>> So we know there are deviations in line freq. But it seems strange in this
>> era of very accurate and inexpensive freq references. How much is related to
>> the generation?
> Controlling the line frequency is a giant PLL, with horrible complications.
>
> The simple setup for a big generator is that if you add load, the generator
> will slow down slightly. You can feed it more fuel to get it back up to
> speed. I think that part is classic PLL theory. Given the inertia of the
> generator and time delay around the loop, you can predict the response to a
> simple change in load, watch for instabilities and such.
>
> In the real world, there are at least two levels of complications. The first
> is that you are doing it with many generators rather than one. When load is
> added, you have to decide which generator(s) will work harder.
>
> The other nasty complication is that you want to do it as cheaply as possible
> as well as follow all the rules from regulators.
>
> One of the complications from regulators is a requirement to make clocks that
> depend on the line frequency keep good time. There was a proposal a while
> ago to remove that constraint. I think it got dropped, but I could easily
> have missed an interesting announcement.
>
> ---------
>
> Has anybody collected data from a typical few-KW portable generator? It
> would be interesting to see if "interesting" things happen if you turn some
> lights on/off at the right frequency.
>
> Here is the Aurora video:
> Staged cyber attack reveals vulnerability in power grid
> http://www.youtube.com/watch?v=fJyWngDco3g
> (1 min)
>
>
>
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