Frequency standard

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I am needing to generate about 1v p-to-p at 50Hz sine wave as accurately as
I can. Please don't suggest syncing to mains as that's the application -- to
calibrate an instrument to measure the actual variance of the mains from
time to time from 50 Hz precisely.

My current (ouch) thoughts are to get a 2 MHz crystal and divide down to
50Hz. Any comments on this idea or an alternative?



Re: Frequency standard

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  I wouldn't trust a 'plain' crystal on its own, it's not good enough to
compare against mains.

  There are several options.

  One "problem" that comes to mind is temperature stability.

  To work around that, there are temperature compensated crystal oscillators,
that are pretty good (and cost more), and oven temperature regulated crystal
oscillators that are better (and cost lots more).  I'm sure both are available
as modules, to make it easy to integrate into your design, but I can't think
of any off the top of my head.

  I also vaguely recall hearing of "pre-aged" crystals too, so they're less
likely to drift as far due to ageing than new crystals.  Basically, this means
it drifts less between each calibration interval (well, the first few anyway).


  Would be best for you to come down to numbers, that is, find out what
stability you *DO* need, and go shopping from there.
  You would need short term stability for this, for mains monitoring work,
this is vital.  Long term stability would be important if you need to go long
periods of time between calibrations.
--
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Re: Frequency standard

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Thanks for that.  I've been looking at 2meg crystals with accuracy of
30herz.  Wouldn't that give me plus/minus .06 Hz @ 50Hz?



Re: Frequency standard

"Suzy"

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** Huh ??

What sort of math is that ?

30 / 2exp6  times 50  =  0.00075



......   Phil



Re: Frequency standard

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0.075Hz, at the specified temperature.

MrT.



Re: Frequency standard

"Mr.Turd"
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** Get real.

You have quoted a percentage as a simple ratio.




.......  Phil



Re: Frequency standard

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2MHz divided down to 50Hz would give a possible inaccuracy drift of 0.0025%,
surely close enough???
What is the "real" purpose of this exercise in relationship to 50Hz?  Prove
the electricity provider is 'off' frequency?
If so, what is the point?  Sue them for being 0.000025Hz off?  Your mains
synchronised alarm clock was out so you missed your bus or train by a few
microseconds?  ROTFLMAO

 



Re: Frequency standard

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0.0025%,
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Prove

No point in doing calculations when you just can pick figures out of the air
eh?

MrT.



Re: Frequency standard

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I think you've missed the point Alan! The exact frequency of the mains gives
some indication of the demand/supply balance.  It is imposible to maintain
exactly 50 Hz unless supply and demand are perfectly balanced, which is a a
rare event. There are in fact legal limits and overall there is an attempt
by the managers of the grid to compensate so that clocks stay pretty well in
time over the long term.  However, it is still informative to see
fluctuations as demand peak and more generators are brought onto line, and
vice versa.



Re: Frequency standard
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Hi Suzy

The grid may be slow by two or three seconds during the day and they
usually catch up at night. We've just upgraded to a Siemens T3000
control system and we use a GPS for time keeping. I think Silicon Chip
published a project recently using a GPS as a frequency standard.

Elmo

Re: Frequency standard

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Yes, Elmo, that's right. But it's not the overall slowness during a typical
high load day that interests me (which as you say is made up during the
night). It is the constant fluctuations that illustrate changes in the
supply/demand balance, which promise to be more dramatic as consumption
grows and various political (state, national and international) effects are
felt. As Phil intimates, such variations as exist are (currently anyway)
very small and fractions of a Herz, but they are there nonetheless. The link
I posted gives a most interesting monitoring of the UK grid. Significant
loads *do* drag down grid speed, and surplus supply *does* ramp it up,
albeit by very small amounts. In fact there are legal limits for the
variation (someone here will no doubt know them) but legal limits are not
brick walls... What I have in mind is somewhat of a "Grid stress" indicator.
Some time in the future, it may serve to give early warning of rolling
blackouts.

I'll look up that Silicon Chip project. Certainly to make an accurate unit
would require a very good calibration sources.



Re: Frequency standard
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Rather than rely on Phil's postulations, you may as well have the
*actual* figures for for eastern mainland and even Tasmania.

http://www.nemmco.com.au/powersystemops/250-0071.pdf

That was for Nov 2007, but there are older months on line if you poke
around. Also, you can see the actual state loads over 5 minutes on line,
and spot price for energy.

http://www.nemmco.com.au/data/GRAPH_5NSW1.htm


The frequency of the power grid will drop significantly when there is a
generator failure, and will stay low until the boilers on other plant
can catch up. This takes some time due to physical thermal constraints
on the plant. If the frequency drops too low, automatic load shedding
will drop some loads. Significant industrial loads are often load shed
in this way (ie the pot lines at Aluminum smelters etc)

The frequency of the grid will rise if there is a fault which reduces
load. This is typically caused by transmission line failure. If the
frequency of the generator rises too high, it risks damage due to over
speed, and will shut down. If as steam turbine generator shuts down, it
can take significant time (hours) to restart the boiler / generator, and
get it back to full load.

David


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Much as I expected, David. But I am a bit surprised about the pot lines...
With my limited knowledge of metallurgy, I would have thought that switching
off the supply to a pot line causes all kinds of problems, and takes a long
time (and lots of energy) to clean up.



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The pot lines take *HUGE* amounts of energy, in Australia around 15% of
total supply. To refine 1 Tonne of Al, requires approx 15MWh of
electricity (which is why recycling Al is very cost effective).

For example, the Tomago Aluminium smelter in NSW, uses around 690MW of
power, and is supplied directly by Transgrid from its own 330KV
substation. The smelters also get heavily subsidised electricity, paying
around 1/3 of what other industries would be paying, (and much less than
you and I pay).

The pot lines can loose power for 4-5 hours without them solidifying. By
dropping and Al smelter load, the power system can quickly recover from
a loss of a generator. When replacement generation has ramped up, supply
can be restored. Their supply contracts do however require that power be
restored within a certain time, before the pot lines are in trouble, and
the maximum number of times they can be load shed etc. The pricing and
conditions was all very political to encourage (bribe?) the smelter
operators to build there smelters in the local area, and provide
employment etc.

David

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Have just been looking at those sites, David. Excellent. From the first one,
it appears that frequency excursions *are* useful indicators. I think that I
can cope with frequency to voltage conversion in my proposed monitor (which
will work via a plugpack) , but I remain a bit dubious about calibrating and
checking it on a long term basis. Clearly synchronising to the actual mains
is a no no! GPS synchronisation looks best at this stage. Any other ideas
anyone?  (not PA, back to the killfile to avoid boredom...)



Re: Frequency standard
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    Back in about 1983 when I was watching the mains change in phase
relative to my comparatively stable ceramic resonator reference, it used
to gradually advance then retard over a period of minutes. It looked
like a slow feedback loop at work to me. I didn't see any consistent
increase or reduction in frequency at different times of the day. You'd
need to do a lot of averaging over a long period to see a clear trend.
    Maybe things are different now ... but I'd have expected better
control of the network frequency these days, with improvements in
technology and communications in the network.


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    I'd have thought that was kind-of obvious. That's why I don't check
my frequency counter's accuracy by measuring its own crystal oscillator.


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    Sounds like a lot of effort to get a vague result which might or
might not give useful information. The last rolling blackouts I can
remember here in Sydney were in the 1970s, and that was due to strike
action, not hardware/capacity problems in the electricity network.

    Good luck.


Bob



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use a (digital) PLL to lock onto (and hence measure) the mains
frequency. standard xtals have similar % tolerances, but still around
the tens of PPM. For better frequency stability use a TCXO (Temp.
Compensated Xtal Osc) or even better an OCXO (Oven Compensated Xtal
Osc). these are available in brick form; $XO < $TCXO < $OCXO

The PLL loop filter can be sized to eliminate most of the noise -
assume, for example, that the grid frequency response is slower than
1Hz, anything faster is noise.

If you do it digitally, its trivial to change the loop dynamics e.g. the
loop filter. Heck, its a hop, skip & a jump from there to a ULF spectrum
analyser.

Cheers
Terry

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It's still not going to give you any useful indication of when the
power is going to fail at *your* place. So what's the point measuring
this?, if as I gather your point is to predict blackouts and such?
Unpredictable local storms, car crashes, transformer failures, and
nearby local load switching are going to potentially cause you more
blackouts and/or brownouts.

Dave.

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    Now that's putting it into perspective. :-)

    My little UPS from Rockby Electronics [free plug] has saved me from
losing data quite a few times during brief interruptions caused by
storms and car accidents bringing down power lines in the local area.


Bob




Re: Frequency standard
    One crude way to watch what the mains frequency's doing is to be in
an area of poor (analog) TV reception where there are high voltage power
lines nearby.
    Wait for a nice humid day when the insulators are crackling, then
watch a low VHF channel (Channel 2 is good) and watch what the
horizontal lines of RF noise from the insulators arcing are doing in the
picture. This is comparing the mains frequency against the very accurate
50Hz field rate timing of the TV station.
    If the noise lines are moving up the picture, the mains frequency's
above 50Hz and vice versa. I think I got that the right way around. :-)
    You could use a stop watch to measure how long it takes for the
interference to do a complete 'loop' and do some calculating of the
exact mains frequency.


Bob

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