[time-nuts] Mains frequency

Bill Dailey docdailey at gmail.com
Sat Nov 16 22:49:10 EST 2013


This resonates with me somewhat since I used to run nuclear power plants and operate the actual turbines.  It does seem that the time interval measurements have much more jitter than I would expect.  I suspect the thousands of turbines phase locked may introduce all kinds of very subtle variations.  I do know when you put a submarine turbine on shore power (grid). You no longer have to control speed... The grid does that for you.

Sent from my iPad

> On Nov 16, 2013, at 9:35 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> tvb wrote:
> 
>> I think we agree. Just to clarify...
>> 
>> I rely on no hardware and no software filters when I use a time-stamping counter such as a sub-nanosecond Pendulum CNT-9x or sub-microsecond picPET. An electrical zero-crossing happens when it happens. If you "filter" you're just trying to change history: spikes are spikes; noise is noise; history is history. Deal with it. Record it, don't filter it away.
> 
> Well, it depends on what one wants to investigate.  The "naked" history one captures with no filtering may not be the cleanest history available of the phenomenon under investigation.  Except in unusual circumstances, mains voltage is generated by massive rotating machinery -- so anything fast that happens on your incoming mains voltage is not a reflection of the grid frequency.  If what you want to know is the grid frequency over time (vector sum of the rotational velocity of the various generators on the grid, as seen from your location), a filtered and limited signal may (probably will) provide the best assessment.  Note that local zero crossings are only a proxy for grid frequency to begin with -- and not a very good one, specifically because of the high noise level.  Of course, you can always filter in software if you time-stamp each zero cross in all its naked glory, but removing the noise prior to time-stamping is often preferable to digitally processing a noisy capture.
> 
> Put another way, the massive rotating machinery that generates the mains voltage can only change the zero cross of the grid by a tiny amount from one cycle to the next.  If a data capture method shows cycle-to-cycle jitter that is significantly greater than this amount, the increase cannot be due to the generators, it can only be due to noise.  If one's interest is the grid frequency, removing this noise prior to time-stamping can only help.
> 
> Note that I'm not talking about a filter Q in the millions -- I'd probably be inclined to use a linear-phase filter with several Hz bandwidth, after a more rigorous analysis of the application.
> 
>> You can either focus on the signal, or the noise. That's two separate plots.
> 
> Agreed.  If you are investigating incidental noise on the mains rather than the grid frequency, then the signal you capture needs to be at least as broadband as the noise in which you are interested.
> 
> Since I do not use the actual local mains zero crossings for anything (other than electronically switching loads on at zero voltage and off at zero current, where absolute timing is irrelevant), I'm not sure why one might be interested in characterizing them.  OTOH, since I do have equipment that responds to the grid frequency, I can see practical utility in characterizing that.  Hence my suggestion to filter.
> 
> Best regards,
> 
> Charles
> 
> 
> 
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