[time-nuts] Mains and TV 50Hz
bill at iaxs.net
Wed Oct 8 22:52:59 UTC 2008
High voltage power lines subject to lightning strikes have reclosing
breakers that allow the fault arc to extinguish before reclosing.
Last I knew (1968) the reclosing delay was 30 cycles or 1/2 second.
Since the breaker recloses into a system of thousands of generators,
there is no phase change, except from the damped oscillation in the
distribution network when the breaker recloses. Breakers are probably
designed to reclose at the zero volt crossing, approximately.
It's not correct to say that the daily error can be 10 seconds. You
may find that much error over part of the day, but power dispatchers,
who direct the production of the power plants, work all night to
bring the lost cycles back so that the 6 AM (or so) error is less than
See http://clockhistory.com/telechron/ for the early days of power
frequency regulation. Now central dispatch stations compare an
electronic cycle-counting clock against GPS time (or WWV) and
manually adjust the steam rates at different plants, perhaps with
a phone call.
How did the cycles get lost? Demand is not completely predictable.
Higher demand than available steam causes the generators to slow
down. The time constant of the system is too long to allow close
control of generator speed. Individual control is not possible in
an integrated synchronous system. HV DC tie lines are used to
isolate major systems. There were four in the US 20 years ago.
What you could do is get to the oscillator of a UPS and phase lock it
to the power line, holding the comparator output when there is no
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From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Hal Murray
Sent: Wednesday, October 08, 2008 3:18 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Mains and TV 50Hz
> On this latter point, let's say that a clock driven by the mains is
> correct to within 10 seconds/day (about 1e-4), but in the longer term
> accuracy is improved. To reach even 1ppm level, you need to accumulate
> time for 100 days. What chance is there of accumulating for 100 days
> without a power failure?
There are other issues besides power.
Will your OS stay up that long?
How many other things are you running on that box?
Will you get impatient with the software?
Will you need to rearrange that corner of your lab?
I've got one box that's been up for 50 days since my last power failure.
I'm pretty sure I could keep it up for a long time as long as I didn't
want to do much else on it.
That last power problem was out for several hours. Most of my problems
are minor glitches. A UPS could easily ride over them.
I think the interesting problem area would be filtering the input
Mechanical clocks have a flywheel. We'd need an analog filter on the
front end and/or a software filter.
What's the Q of a typical mains powered clock? What is the Q of a
(small) motor/generator set with a big flywheel?
Does anybody have any scope traces of a typical power glitch? How many
cycles get trashed? What's the typical duration? Does the phase of the
before-glitch signal match well with the phase of the after-glitch
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