[time-nuts] Mains frequency

Jim Sanford wb4gcs at wb4gcs.org
Wed Mar 12 19:53:09 EDT 2014

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.

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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