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 engineers. 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 loops). > 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 -- Markus Kuhn, Computer Lab, Univ of Cambridge, GB http://www.cl.cam.ac.uk/~mgk25/ | __oo_O..O_oo__