[time-nuts] Mains frequency

Charles Steinmetz csteinmetz at yandex.com
Sat Nov 16 22:35:29 EST 2013


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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