AC Power Line Frequency Accuracy Question - Page 1
Forum section: electronics / projects

author: rfdes
author_rank: Regular Contributor
timestamp: December 22, 2022, 02:27:02 pm
content: In the US, the 60Hz residential power frequency has poor short term accuracy but I understand the power companies regulate the frequency depending on system load. Is there a specification that power companies must abide to maintain a long term accuracy of 60Hz? If so, what is this frequency specification? Is there a similar requirement in countries that use 50Hz power?Thanks

author: BradC
author_rank: Super Contributor
timestamp: December 22, 2022, 02:37:02 pm
content: Quote from: rfdes on December 22, 2022, 02:27:02 pmI recall as a kid doing an insiders tour of our main local coal/gas fired plant (we have a very large but isolated grid). The aim of the operators was to be at most a second out over 24 hours and to make that back up over the next if required. They had an analogue synchronous clock on the wall alongside a "reliable timesource" and used to tweak over the day to keep things close, while doing the fine adjustment overnight.That was 30+ years ago. Now with the advent of renewables and the push to shut down the rotating machines my local measurements "aren't great" and logging at once per minute I see some wild local fluctuations across the day (like up to +/- 0.8 HZ) which would have been unheard of when I was a kid.Still, the voltage is less stable, the grid is less reliable and the cost of power is up, so they must be doing something right.

author: Brianf
author_rank: Regular Contributor
timestamp: December 22, 2022, 02:54:29 pm
content: In the UK the relevant law is "The Electricity Supply Regulations 1988" and the frequency tolerance is +/- 1%.

author: jonpaul
author_rank: Super Contributor
timestamp: December 22, 2022, 02:57:48 pm
content: As a retired EE ,the utilities have specifications and regulations depending on the area.Long term average is very accurate for old clocks with synchonus motors.Synchronisation is critical in AC D&T links and done nowadays bu computer.Short term F and v fluctuations dépend on plant outages, system loading ,faults.At the Con Edison NYC 9 November 1963 blackout as the various 360,230and 120 kV links went down, system frequency dropped to50..55 Hz and w et dar on underfrequench relay action.For voltage a typically spec can be 117v nominally range 108...128.The IEC, IEEE, FPC, NEA, might have a more precise F, V spec.Nowadays any exact frequency std is XO, or GPS SO.So the exact line frequency is not an issue.JonJon

author: RoGeorge
author_rank: Super Contributor
timestamp: December 22, 2022, 03:01:18 pm
content: There are no long term accuracy specs. If you want to use the power line frequency for keeping the time, that would be worst than a mechanical clock.

author: mawyatt
author_rank: Super Contributor
timestamp: December 22, 2022, 03:06:39 pm
content: Recall here is US the power grid used to be split into the East and West, reason was the grid became highly unstable having both E and W connected. Power was bilaterally exchanged between E and W by means of AC converted to DC, which was then synchronous converted back to AC on receiving side.Over a small range Power Grid Frequency and Power Demand are inversely related. Peak loads during the day would slow down (lower frequency) the grid and the overall goal was to speed up (higher frequency) at night when the load was lighter, this has the effect of making up for "time lost" during the day. The goal was to have the 24hr overall cycle be very accurate, thus 24hr/day*60 min/hr*60 sec/min*60 cycles/sec, or 5,184,000 cycles/day.Back in ~1973 we made an instrument that monitored the grid frequency. This was a highly stable crystal oscillator divided down to 60Hz, this was then phase compared to the narrow bandpass filtered mains line. The result was a phase shift that varied with time and updated 60 times per second. This was LP filtered then the time derivative taken which was displayed on a paper strip chart recorder as Delta Frequency from an ideal 60Hz with a full scale factor of +-0.05Hz. Remember looking at some charts and you could see the daily frequency variation, and even see when power generation came On or Off line with the requisite sudden slight increase or decrease in mains frequency.Best,

author: TimFox
author_rank: Super Contributor
timestamp: December 22, 2022, 03:10:49 pm
content: An interesting Canadian study of variation in the nominal 60 Hz line frequency:https://ve3oat.ca/frequency.htmlThe author notes that he could not find a formal specification from Hydro One, the local power utility.I was surprised that his measured mean error corresponds to a few seconds per day.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 22, 2022, 06:51:45 pm
content: Most of the USA is using AGC (Auto Generator Control)https://en.wikipedia.org/wiki/Automatic_generation_controlI have the hand held dashboard of MISO (Midcontinent Independent System Operator)but it does not have a real time frequency display ( If so, I can't find it)Here is the real time frequency display for Texas ERCOT, updated each minute.https://www.ercot.com/content/cdr/html/real_time_system_conditions.html

author: TheMG
author_rank: Frequent Contributor
timestamp: December 22, 2022, 07:47:49 pm
content: Here we are on a small isolated grid, with a mix of mainly hydro and diesel generation. Long term accuracy seems pretty good as mains-referenced clocks don't seem to drift much at all, but short term is pretty bad and can vary +/- 0.3Hz over just a few seconds as the load changes and the power plants adjust.But I have been to other small isolated communities that are powered entirely by diesel and where no attempt is made to achieve any kind of long term accuracy. Mains referenced clocks in these areas can drift more than a minute a day! Even in the short term, fluctuations of +/-2Hz are common.So I guess the answer, at least here in Canada, is it depends. Some areas there is no long term accuracy at all, and others there is. As for the short term stability, it seems that the larger the grid, the better it is.Doubt there's any regulation/requirement for utilities to maintain long term accuracy, otherwise quite a number of isolated areas with small grids would be in violation.Speaking of mains frequency referenced clocks, it's one thing that drives me crazy. RTC ICs and crystals are super cheap these days, as are small supercapacitors. Why do manufacturers of appliances still insist on using mains referenced clocks? We get relatively frequent short (few minutes) power outages, have to reset those damned flashing clocks on the microwave, oven, etc all the damn time!

author: wasedadoc
author_rank: Super Contributor
timestamp: December 22, 2022, 09:42:53 pm
content: Here in the UK power outages are infrequent and tend to be localised due to someone with a mechanical digger not consulting the supply company before starting to dig.The long term accuracy of our mains driven clocks is excellent.

author: tooki
author_rank: Super Contributor
timestamp: December 22, 2022, 09:49:58 pm
content: Quote from: RoGeorge on December 22, 2022, 03:01:18 pmThat is completely, utterly incorrect in every way.1. There are accuracy specs, here they are:https://www.mainsfrequency.com/frequ_info_en.phpThe home page of the same site lets you see the current frequency of the European grid in real time:https://www.mainsfrequency.com/And this page explains cumulative deviation of timing error and how it’s corrected:https://www.mainsfrequency.com/gridtime.php2. Mechanical clocks have low accuracy, period.AC line frequency has short term variability but extremely high long-term accuracy, which is why it is extremely common to use mains frequency as the timing reference for mains-powered clocks. (For example, in ovens.)Quartz clocks are the opposite: high short-term accuracy (since they’re very stable), but the small error they often have will accumulate ruthlessly over the long term.

author: TimFox
author_rank: Super Contributor
timestamp: December 22, 2022, 09:57:24 pm
content: I've been searching for an equivalent specification for North America, but have been unable to find it.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 22, 2022, 10:14:52 pm
content: Quote from: TimFox on December 22, 2022, 09:57:24 pmIt appears to be the long winded function in this document at Page 2 Section B Part R1https://www.wecc.org/Reliability/BAL-004-WECC-01%20BC.pdf

author: Thunderer
author_rank: Regular Contributor
timestamp: December 23, 2022, 03:03:26 am
content: Quote from: TimFox on December 22, 2022, 03:10:49 pmWell, the electrical utility or the power distributor has no obligation to guarantee any range of frequency to the customer, it makes no difference for the customer if it is 59 or 62Hz. Otherwise, it would be in the contract.Anyway, the information from the electrical utilities (power generation mostly) is not available for the public (the common Joe), as it is useless (see above, no contractual obligation). And the era of clocks synchronized with the electrical networks is long gone.But, just next to Ontario, in the french province of Canada, if you want to be part of the network, as a provider/power generator, you'd have to stick to max 1% frequency variation. It gives you an idea about tolerances. The only reason the frequency is monitored and controlled is the network stability, as pointed out by the OP in the first post.PS: As Canada has interconnections (via HVDC links) with US, once again, no need to care about the frequency.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 23, 2022, 11:13:15 am
content: In Western Australia, the 50Hz accuracy seems to be fairly good.When I worked at a TV studio, we had Composite video, & sometimes blanking & syncs piped around the tech areas.I found that, it was sometimes easier to, when looking at field rate signals, put the 'scope on "Line (Mains rate) trigger".The display would sit steady for hours with no discernible drift.It had been many years since the analog TV field rate was locked to the Mains, but there was very little difference.

author: TheMG
author_rank: Frequent Contributor
timestamp: December 23, 2022, 07:00:16 pm
content: Quote from: Thunderer on December 23, 2022, 03:03:26 amYou would think, but that is unfortunately not the case. Virtually every appliance (oven/ranges, microwaves, etc) that has a clock on it still synchronizes time with the incoming mains frequency. Even many mains-powered alarm clocks still sold today still use this.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 23, 2022, 07:23:46 pm
content: The only real problem with using mains signal for timekeeping is that you can't keep time while the power is out. A quartz crystal based RTC can keep running on a capacitor for hours or a coin cell for years. Of course plenty of appliances with quartz crystals still don't keep time during a power outage so it's kind of a moot point.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 23, 2022, 08:00:43 pm
content: FWIW, whenever I've connected an accurate counter to the line in Upstate NY, the frequency is within a few hundredths of a Hz, often better.

author: Vovk_Z
author_rank: Super Contributor
timestamp: December 23, 2022, 10:34:46 pm
content: IDK exact requrements but here in Ukraine (pre war) frequency drifted +-0.1 Hz typically. Often it was even +-0.05 Hz. So accuracy is about 0.1-0.2%.

author: ledtester
author_rank: Super Contributor
timestamp: December 24, 2022, 02:55:48 am
content: TwoThree things...1. As a kid I was told that the AC line frequency did exhibit variation but that the power company tried its best to maintain an average of 60 Hz because most consumer clocks depended on that to keep time.2. This Tom Scott video on how the variation in AC line frequency can be used to date recordings:The hidden background noise that can catch criminals -- Tom Scotthttps://youtu.be/e0elNU0iOMYThe UK National Grid has released historical frequency data at 1 second resolution:https://data.nationalgrideso.com/system/system-frequency-data3. There are a bunch of visual displays related to power line frequency here:https://fnetpublic.utk.edu/index.html(mainly for the US grid)and for the German grid:https://www.netzfrequenzmessung.de/https://gridradar.net/en/mains-frequency

author: tooki
author_rank: Super Contributor
timestamp: December 24, 2022, 10:34:26 am
content: Quote from: Thunderer on December 23, 2022, 03:03:26 amThat is not even remotely true. And it’s a good thing, because it’s a great source for long-term stability.

author: tooki
author_rank: Super Contributor
timestamp: December 24, 2022, 10:47:19 am
content: Quote from: TheMG on December 22, 2022, 07:47:49 pmInexpensive RTCs and crystals aren’tthataccurate.* 20ppm doesn’t sound like much, but over the course of a year, it adds up to over 10 minutes. That means you’d be needing to correct your clock by a minute nearly once a month.A mains-referenced clock won’t accumulate that amount of error, ever.More accurate RTCs are a lot more expensive. The excellent DS3231SN costs about $5 in quantities of 1000, making it a very expensive part. And its long term accuracy is still far worse than mains.What they really should do is integrate a very inexpensive low-power RTC to the microcontrollers used in these appliances, such that a simple electrolytic can hold enough charge to last, say, 10 minutes. (In fact, I think there are already MCUs out there with such RTCs. But the appliance makers have to decide to use them, and design a backup power circuit for it.)*Great example: In the days before computers synced with Internet time servers automatically, the RTCs in computers were notoriously inaccurate, running slow or fast and thus necessitating regular setting to be reasonably accurate. We never bothered fixing this in hardware because it became better to simply frequently sync with an ultra-accurate atomic time standard, especially as secure internet communication requires reasonably correct time settings for certificate validation.

author: mawyatt
author_rank: Super Contributor
timestamp: December 24, 2022, 01:54:36 pm
content: You would think the appliances that have a RTC would synch with the Mains and only "free run" when the mains were not active. This would keep the RTC long term stability equal to the mains which is very good, and the RTC could be lower grade (cheaper) since it only needs to "coast thru" the mains short term outages.Best,

author: tooki
author_rank: Super Contributor
timestamp: December 24, 2022, 02:15:54 pm
content: Quote from: mawyatt on December 24, 2022, 01:54:36 pmI think the issue is powering it when mains is unavailable.

author: TheMG
author_rank: Frequent Contributor
timestamp: December 24, 2022, 06:22:27 pm
content: Quote from: tooki on December 24, 2022, 02:15:54 pmNot really. Modern RTC can be powered by a simple capacitor for quite a while, anywhere from minutes to days depending on the size of the capacitor.

author: floobydust
author_rank: Super Contributor
timestamp: December 24, 2022, 07:24:45 pm
content: When I worked in power generation, I remember their were regular scheduled clock "correction days", the grid system operator would command them and the Control Room would change the generators' operating point. If I remember right, it was during low grid demand say at night time, they would slightly overspeed the generators to catch up.It was steam pressure verses rotor excitation for output voltage. Somehow they got frequency up without mains voltage going too high.My old MM5316 alarm clock uses mains as the ref, and a one-transistor 60Hz RC phase-shift oscillator during 9V battery backup. On mains it's quite accurate compared to another MM5369 (MM5314) running off an old TV colour burst crystal, I was checking calibration of it and noticed that VCC and room temperature move frequency around many ppm so it's pretty much impossible to make it exact but long-term the highs and lows average out.RTC's on a battery I found not great because the oscillator frequency shifts due to the drop in voltage, i.e. 3.3V to 3.0V with power on/off.The DS3231S is fantastic but stupid expensive and out of stock for at least a year now. I haven't tried it out over extreme temps or battery ops for long periods.

author: Salome2
author_rank: Regular Contributor
timestamp: December 24, 2022, 08:23:30 pm
content: Pimp up your DIY Nixie clocks accuracy.A simple way to achieve accurate time displayed:https://en.wikipedia.org/wiki/Standard_frequency_and_time_signal_servicePrecise time/date are broadcast on SW and LW bands and simple AM radio receivers in Europe can receive time from transmitters in Germany that covers most of Europe and from WWV and WWVH in the USA.Many AM radio time/date transmitters are found all over the globe:You can buy cheap table LCD clocks that get time/date via an AM radio receiver using just a loop antenna and a MCU/receiver blob to decode BCD time and date info received.UK transmits: The MSF signal is transmitted from Anthorn Radio Station in Cumbria by Babcock International, under contract to NPL. The signal covers the whole of the UK, and can also be received throughout most of northern and western Europe. It is monitored against the national time scale UTC(NPL) and corrected when necessary, ensuring that the transmitted time is always correct.https://en.wikipedia.org/wiki/DCF77#Reception_area.Germany uses this system to sync their railroad clocks but their LW transmitted signal can be received over 1000 miles away.Putin time is transmitted over several locations (at 14.996 MHz), including one transmitter near Moscow.Ukrainian time/date transmissions are a little irregular in broadcast at this time.USA:https://www.nist.gov/time-distribution/radio-station-wwv/wwv-and-wwvh-digital-time-code-and-broadcast-format]https:www.nist.gov/time-distribution/radio-station-wwv/wwv-and-wwvh-digital-time-code-and-broadcast-formatIt is possible to build a DIY broadcast time/date receiver with few components.

author: tooki
author_rank: Super Contributor
timestamp: December 25, 2022, 01:13:16 am
content: Quote from: TheMG on December 24, 2022, 06:22:27 pmYes, but that’s added cost that they’d just as soon dispense with. It’s not a difficult problem to solve, but it’s an issue in the sense that it’s something thatmustbe solved for the solution to work. The solution costs money so they don’t do it.

author: tooki
author_rank: Super Contributor
timestamp: December 25, 2022, 01:19:21 am
content: Quote from: Salome2 on December 24, 2022, 08:23:30 pmThing is, that often just doesn’t make sense anymore. A time signal service receiver is around $1.30 on AliExpress. For $3 you can get an entire ESP32 microcontroller board with WiFi, which can use NTP to sync the time with just a few lines of code. And it’s the entire MCU for the project. Plus, it syncs instantly (the old radio systems do not) and works even deep indoors if you have WiFi. The ESP32 is also smaller than the ferrite bar antenna needed for a time signal service.

author: CaptDon
author_rank: Super Contributor
timestamp: December 26, 2022, 03:07:00 pm
content: During the 13 years I worked in radio broadcasting in central Pennsylvania I can say the grid frequency accuracy was very very good!! Our syncronis motor wall clocks were within +/- .5 second of our WWV sync'd master station clock any time of the year (except during power outages of course). Most of our cartridge tape machines had line sync a.c. motors and we really never noticed any pitch shifts in music or voice. The central control for the grid was very intent on making up for a few cycles (total) lagging during the day by increasing waterflow (a.k.a. horsepower) in the hydro installations to push the grid slightly faster during light loads at night. The coal fired plants and the nukes liked to be at constant power so it was easier to adjust the outputs of the hydro units which included using excess power to fill 'Muddy Run Pumped Storage' facility. Overall the short term was good and the long term was nearly dead on!!!