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 <[email protected]> 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 > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
