Re: [LEAPSECS] Leap seconds in the European 50.0 Hz power grid

From: Markus Kuhn <>
Date: Thu, 30 Jan 2003 22:05:51 +0000

Steve Allen wrote on 2003-01-30 20:58 UTC:
> On Thu 2003-01-30T12:54:09 +0000, Markus Kuhn hath writ:
> > The UCPTE specification says that the grid phase vectors have to rotate on
> > long-term average exactly 50 * 60 * 60 * 24 times per UTC day.
> Obviously the grid frequency shift after leap seconds is annoying, and
> it is undoubtedly one of the reasons contributing to the notion of
> stopping leap seconds.

I doubt that this is really the case. UCPTE is happy if it can guarantee
that the grid time remains within 20 seconds of UTC. Leap seconds are
only a relatively minor reason for the power grid clock to deviate from
UTC temporarily. Remember that in a national or continental distribution
grid, power is transferred whenever there are phase differences between
parts of the grid. So if demand raises in one area, it will fall behind
in phase relative to the others and thereby it slowly pull the frequency
of the entire grid down until control loops detect this and compensate
the deviation from the target frequency by pulling rods a few
centimeters out of nuclear reactors all across the continent. First you
keep the short-term frequency constant, then you keep the voltage
constant, then you keep the power transfers in line with the contracts,
and only after you have fulfilled all these targets, you use what
degrees of freedom are left in the control space to keep the grid clock
synchronized, i.e the long-term frequency accurate.

> But the question arises as to why the spec
> can't easily be changed to indicate that it is per TAI day.

As long as UTC is as it is currently, you don't want to do this:

Firstly, there are zillions of clocks that use the power grid as their
reference oscillator, and you want them to run locked roughly to UTC,
because they are supposed to display local civilian time and not
something linked to TAI.

Secondly, in Europe, exact UTC-based civilian time was available for a
long time via LF transmitters such as DCF77, MSF, HBG, etc., not to
forget BBC-style beeps before news broadcasts and telephone speaking
clocks. TAI on the other hand has only relatively recently become
reasonably easily available automatically through GPS and NTP extensions
and would otherwise have to be manually looked up from tables. So TAI
was just far less practical, and in addition simply unknown to most

My point was that leap seconds are not a problem in the power grid and
for power-grid controlled clocks.

About power-grid controlled clocks:

Around 1990, West Berlin was temporarily connected to what was then the
East European grid into which East Germany was integrated, which did not
provide a grid time that was kept long-term aligned with UTC. Customers
in West Berlin started to complain that their clocks suddenly needed to
be adjusted regularly. If the average frequency for a week was only
49.95 Hz, your alarm clock would go 10 minutes late by the end of the
week, which is definitely noticeable, especially if the same clock
before never needed any adjustment between power outages. The problem
persisted until East Germany (and now also its neighbors) was integrated
into the UCPTE.

> My power company cannot supply me with a reliability of 0.99999997, so I can
> never see leap seconds from my household clocks. I don't really
> believe that other power companies achieve it either

Unfortunately, I can't confirm that my supplier here in Cambridge can
either. However, in the urban centers of Bavaria where I grew up, power
outages where certainly far less frequent than leap seconds. Of the few
we ever had there, most outages were announced a week in advance by mail
because of local network work. I am being told that the North American
power grid does not have a particularly good reputation among
Continental power distribution engineers, so you probabaly shouldn't
assume that its reliability represents a high standard in international
comparison. (E.g., even solar wind has been known to drive transformers
in the US/CA grid into catastrophic saturation and bring the entire grid
to a collapse, something that UCPTE regulations have prevented by
requiring the installation of capacitors that eliminate continental DC

> So what is the value obtained by a specification like this?

Grid-powered clocks that in practice do not have to be adjusted, for
example. Note that these were long around before DCF77 and GPS receivers
became low-cost items. Even though embedded DCF77 receivers/antennas now
cost less than 15 euros and GPS receivers less than ~50-100 euros, it
still doesn't beat costwise a few 10 Mohm resistors for a voltage
divider directly from the 230 volt line to the spare input pin of a
clock microcontroller.

Plus remember the remarks above that UTC was for a long time far more
easily available than TAI in Europe. Only *very* recent power plants
have GPS receivers in the control system and could therefore use TAI as
a reference in theory, if they wanted. (My brother happens to set up one
of these in Turkey at the moment and told me a bit about the GPS-driven
timing infrastructure that is used today in modern power plants).


Markus Kuhn, Computer Lab, Univ of Cambridge, GB | __oo_O..O_oo__
Received on Thu Jan 30 2003 - 14:06:04 PST

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