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From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Newsgroups: sci.electronics
Subject: Re: Sidereal Time Clock
Message-ID: <9851@as0c.sei.cmu.edu>
Date: 3 Jan 91 16:27:58 GMT
References: <5711.277c4a63@nuhub.acs.northeastern.edu> <5972@mint17.UUCP>
Reply-To: rsd@sei.cmu.edu (Richard S D'Ippolito)
Organization: Software Engineering Institute, Pittsburgh, PA
Lines: 47


In article <5972@mint17.UUCP> wilner@motcid.UUCP (Corey S. Wilner) writes:

>From what I understand, using the 60Hz line as a reference for timing is not
>a good practice if you want any accuracy.  I have heard that cycles can be
>longer or shorter than 1/60th of a second and some cycles can be lost completely.

>Anyone care to comment on this with more substance other than my glaring
>generalities?!

Most, if not all, power companies are connected to a wide-area grid.  Any
generator on the grid which gets more than a few degrees out of phase with
the others will cause huge reactive circulating currents which will result
in lots of line losses.  Any generator which gets 180 degrees out will be a
sink and find itself the recipient of one heck of a lot of energy for a
short, but spectacular, while!  Utilities are very careful about their
physical plant, and have extensive protection systems to prevent the above.

Therefore, you will find that a whole grid may be ahead or behind "real" time
by several cycles and that sections of the grid may be slightly ahead or
behind that by very small fraction of a cycle.  Usually, but not always, by
the end of the day the difference is cleared by adjusting the frequency of
the grid so that the correct number of cycles (60*60*60*24) have occurred.
This is done according to allowed correction rates (how fast or slow they
can run the generators without causing other problems).

I did say "usually":  During long periods of extra heavy loads, the grid
generators slow down and the utility group may decide not to make up all of
the lost cycles by the end of the day, or the amount to be made up can't be
completed at the maximum correction rate.  Very light loads (a major feeder
goes out for a while) causes them to run fast, and again, the gain during
the transient may not be fully compensated for.

I've called my local electric utility and seem to remember that an error of
a second or so, i.e., 100 cycles lost or gained out of 5M (60Hz +/- 0.001Hz)
could occur in unusual circumstances, so if you need better than that, use a
separate reference.  Remember, the utility is charged with delivering
energy, not providing a frequency standard.

If you decide to call your utility, ask for someone in the engineering
department and tell the engineer you want to know the normal cycle loss or
gain at the end of the day and the maximum correction rates that they will
apply.  For short terms, only the correction rate would matter (for example,
in a 1-2 Hr camera exposure).  For longterm time keeping, forget it!


Rich