Re: Leap seconds in the European 50.0 Hz power grid
On Thu 2003-01-30T22:05:51 +, Markus Kuhn hath writ: > > 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: But I think that the further answer is this: Should it be decided that civil time shall track TAI, there will be no technical problem (and indeed, possibly great rejoicing) to change the specification. For the purposes of power grids: The current forms of UTC and TAI are both acceptable. Ongoing changes in the length of a second are not acceptable. Fractional leap seconds are not acceptable. Here I perceive pieces of a checklist of civil time strategies vs. technologies affected by them. This could be valuable. > Plus remember the remarks above that UTC was for a long time far more > easily available than TAI in Europe. I believe that is true almost everywhere, and is a root of the problem at hand. -- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 [EMAIL PROTECTED] Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E49 89 0E FE 26 B4 14 93
Re: Leap seconds in the European 50.0 Hz power grid
Markus Kuhn scripsit: > 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. What are the long-term guarantees? -- Even a refrigerator can conform to the XML John Cowan Infoset, as long as it has a door sticker [EMAIL PROTECTED] saying "No information items inside". http://www.reutershealth.com --Eve Maler http://www.ccil.org/~cowan
Re: Leap seconds in the European 50.0 Hz power grid
Steve Allen wrote on 2003-01-30 20:58 UTC: > On Thu 2003-01-30T12:54:09 +, 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 0.9997, 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 on
Re: Leap seconds in the European 50.0 Hz power grid
On Thu 2003-01-30T12:54:09 +, Markus Kuhn hath writ: > VERDIN phase tracking is perhaps a somewhat pathological case. True, but I know of someone who built a household clock to use it, and for someone living in a Navy base town during the early years of the Reagan era that seemed like a prudent harbinger. >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. But the question arises as to why the spec can't easily be changed to indicate that it is per TAI day. My power company cannot supply me with a reliability of 0.9997, so I can never see leap seconds from my household clocks. I don't really believe that other power companies achieve it either, so what is the value obtained by a specification like this? My power reliability is more like 0.999, and various folks in my region recently experienced outages lasting from hours to weeks. My recent outage was 8 hours. In order for a household device with battery backup and internal clock to keep phase with the grid while it was offline it would have needed an oscillator which would drift only 1 second in 20 days. Are there any battery-backed devices, let alone household ones, with internal clocks of this caliber which rely solely on power grid phase locks to keep SI time? -- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 [EMAIL PROTECTED] Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E49 89 0E FE 26 B4 14 93
Re: Leap seconds in the European 50.0 Hz power grid
Steve Allen wrote on 2003-01-29 20:53 UTC: > On Wed 2003-01-29T15:05:59 -0500, John Cowan hath writ: > > I was a little too clipped. If you know all the leap seconds, you can > > convert a Unix-style timestamp to UTC reliably; if you further know all > > the timezone changes, you can convert UTC to LCT reliably. > > I remain confused about why this "isolated system" cares whether it > keeps time as UTC or TAI. How does its time get set? How does its > time stay locked to SI seconds? > > Are you supposing that the system is able to keep SI seconds because > it has some sort of unshielded PLL which is tracking the carrier > signal from something like the US Navy's high powered VERDIN VLF > transmissions for submarines? (With their 50 baud message that > basically says "We're still here so don't launch" and if your clock > stops ticking, nothing really matters much anymore.) VERDIN phase tracking is perhaps a somewhat pathological case. Here is a more realistic source of standard frequency that can easily be tracked and is in practice tracked in lots of low-cost consumer electronics: Most of the European continent (excluding Britain and some East European Countries) runs a 50.0 Hz continent-wide phase-locked power grid known as the UCPTE grid (Union for the Coordination of Transmission of Electricity - the organization responsible for the reliable operation of the interconnected electricity network in Europe). 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. That is you get on average 50 * 86400. oscillations out of each power socket in Europe every day, even if you consider days that end in a leap second and are actually 86401 SI seconds long. Near an inserted leap second, they are actually reducing the power grid frequency in a coordinated way for a few minutes by up to 50 mHz in order to make sure that all the many clocks that use this 50 Hz standard frequency as their time reference remain in sync with UTC. You can observe this nicely with an oscilloscope if you have a stable reference signal to triger it independently. Power-grid coupled clocks will go 0.1% slower briefly all over Europe to resync with UTC after a leap second. Note that the power frequency deviates sometimes significantly from 50 Hz, but the PLL controllers contain an integrator and eleminates any long-term error relative to UTC this way. References: - http://europa.eu.int/comm/energy/en/elec_single_market/florence9/position_paper/ucte/policy1.pdf Section S.6.1. - http://www.verbund.at/at/apg/stromtransport/TOR%20E.pdf Section 3.1.1 (5) Another ubiquitously available high-quality reference frequency in Germany are the national TV broadcast sync signals, which are derived from caesium clocks in the basements of the broadcasting houses (at least ZDF does this). Their TV signals are not frequency adjusted to follow UTC, they stay at 50.00 Hz exactly. However this is not a problem for consumer electronics, because the teletext data in the vertical bank interval labels each TV frame with an ASCII encoded hh:mm:ss timestamp that tightly follows UTC. In practice, TV sets with radio controlled clocks simply evaluate the teletext time stamps when the receiver is switched on and run freely when it is off. Same for radio receivers that evaluate RDS time signals. They are not phase locking clocks to the TV signal, and even if they would, they could learn about the leap second from the teletext data (with a small delay as teletext lacks a leap second announcement). Markus -- Markus Kuhn, Computer Lab, Univ of Cambridge, GB http://www.cl.cam.ac.uk/~mgk25/ | __oo_O..O_oo__
