How stable is the mains voltage? - Page 1
Forum section: electronics / beginners

author: Faringdon
author_rank: Super Contributor
timestamp: October 28, 2023, 11:24:39 pm
content: Hi,Supposing you have 50Hz mains. Suppose you know when every other zero cross occurs...eg, every 20ms.....then can you definetely say that the inbetween" zero crossings are exactly half way between the ones occurring at T=20ms?

author: ataradov
author_rank: Super Contributor
timestamp: October 28, 2023, 11:38:58 pm
content: This question is answered by a simple google search "frequency stability of power grid".

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 28, 2023, 11:39:27 pm
content: Your title is wrong. The correct question is how stable is mains frequency.Define that and you have your answer.

author: coppice
author_rank: Super Contributor
timestamp: October 28, 2023, 11:43:16 pm
content: Quote from: Faringdon on October 28, 2023, 11:24:39 pmYou may have more than 2 zero crossings per cycle. If there is serious high harmonic distortion you can get fast ripples on the sine wave big enough to cause multiple crossings. It takes pretty bad distortion to make that happen, as the main sine wave is changing at its fastest near the crossings, but it can happen.

author: nightfire
author_rank: Frequent Contributor
timestamp: October 29, 2023, 12:36:54 am
content: For reference, and maybe it is hopeful as a reference to others in the future that found this thread by usage of the search function:The quality of the mains grid never is intended to be a precision instrument. For practical purposes the carrier(s) define in their standards the allowable deviation from some middle value.In germany, we follow some european standards, sopme quick search (at this time, I am a bit lazy and don't look for maybe newer publications...) unearthed some paper from 2013 from german energy providers:In the EN 50160, there are the following corridors for service quality:Merkmal WertebereichNetzfrequenz 49,5 Hz bis 50,5 HzLangsame Spannungsänderung 230 V ± 10 % (=Slow changing voltage)Schnelle Spannungsänderung 230 V ± 5 % (rapidly changing voltage)(selten bis 10 %)For germany, here:https://www.netzfrequenzmessung.de/is a live display of the current net frequency.

author: ArdWar
author_rank: Frequent Contributor
timestamp: October 29, 2023, 01:44:00 am
content: It varies a lot between grids. Check your national standards and power company policy.Most will only mandate long term stability if at all, but rarely guarantee short term accuracy as frequency can be integral to the grid balancing scheme. Eg they might only guarantee that there will be exactly 4320000 cycle in a day but the frequency itself can drift from 49 to 51 Hz at any given moment.

author: WimWalther
author_rank: Regular Contributor
timestamp: October 29, 2023, 05:53:26 am
content: It depends on your stability criteria - short-term, long-term, etc.In most any first-world nation with a national power grid, the long-term stability is exceptionally good. This means, for example, if you have a clock that uses a synchronous motor (like the classic GE Telechron movement) or an IC time base synced to the line (digital clock / radio), you can expect it to hold within a second or two over the course of years.Short-term, over several hours, you might see a couple seconds error - but these errors are nulled out on a regular basis by occasionally varying the frequency for exactly this reason.As for cycle-to-cycle, I'm unsure, but I'd imagine it's good enough for GPS to be accurate within a couple of meters.. so plenty good enough for almost any use you're likely to need it for.

author: radiolistener
author_rank: Super Contributor
timestamp: October 29, 2023, 07:35:14 am
content: Quote from: Faringdon on October 28, 2023, 11:24:39 pmNo, you can't say this. it may vary depends on connected load and mains frequency stability. ±0.02 Hz frequency variations happens very often, sometimes it can be more, so you can estimate that 10 us variations is pretty common for mains even with no effect of high power reactive loads.

author: Synthtech
author_rank: Regular Contributor
timestamp: October 29, 2023, 08:30:21 am
content: Domestic solar feed-in systems might perhaps put noise and phase issues into local mains power supplies?

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 29, 2023, 10:08:33 am
content: Quote from: WattsThat on October 28, 2023, 11:39:27 pmI missed checking the author. Then everything made sense!*sigh*I'm assuming you're switching some devices, but you should specify the application of this.One thing is to drive a motor dimmer, a very different one is injecting 100KW into the mains from a huge battery bank, where a small drift might case hundreds of amps flowing in the wrong direction.Zero cross will be pretty accurate between few cycles, the mains voltage will be very low within 300us off zero cross (30V @ 230VAC), so switching the device slightly earlier/later won't cause a huge current spike as when hard-switching at main's Vmax.As long as you sync the zero cross every few cycles, it'll be fine. 10us variation will cause a negligible change in voltage.Main's won't suddenly drift by 10mS! Might be 50.2Hz-49.8Hz along the day (I don't remember the limits, but recall them being pretty tight), and the power station will compensate any small drift so at the end of the day the average frequency was 50.00Hz.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 29, 2023, 10:24:08 am
content: www.gridwatch.templar.co.ukBut being a bloody bot you should have already found this?

author: Siwastaja
author_rank: Super Contributor
timestamp: October 29, 2023, 10:43:57 am
content: Quote from: ataradov on October 28, 2023, 11:38:58 pmYes, and another relevant term to look for: jitter.

author: tggzzz
author_rank: Super Contributor
timestamp: October 29, 2023, 11:29:11 am
content: Quote from: Faringdon on October 28, 2023, 11:24:39 pmFirstly, don't confuse voltage (As per thread title "How stable is the mains voltage?" ) with frequency and time (As per the question body). That's a surprising mistake for someone that designs SMPSs do a living.Secondly, learn to use a search engine such as google.Finally, the answer will probably depend on whatever other "nearby" loads exist, especially if they are inductive or switched.Hence any design that relies on a specific answer to your question will probably be fragile and prone to "entertaining" behaviour in an installed system. Your alter-ego, treez, has a lot of issues with his designs, didn't he?

author: Jwillis
author_rank: Super Contributor
timestamp: October 30, 2023, 04:42:16 am
content: http://mainsfrequency.uk/fm-home"The GB mains frequency is nominally 50Hz. National Grid is obliged by its licence commitments to control the frequency within ±1% of 50Hz so it can fluctuate between 49.5Hz to 50.5Hz. However......"

author: SeanB
author_rank: Super Contributor
timestamp: October 30, 2023, 04:57:19 am
content: Well, here is the current snapshot of the last 24 hours by me. The 2 hour dropout at midnight was Eskom, welcoming SA being World Champions, and back to rotational load shedding again.They ran out of diesel for the full time running peaking plants, and pumped storage is dry.

author: alligatorblues
author_rank: Regular Contributor
timestamp: October 30, 2023, 05:58:18 am
content: When commercial electricity users turn huge loads on or off, or anything that uses power factor correction, and a slew of other loads on utility power, are responsible for 90% of the harmonic disruption in the power grid. Near where I grew up there was a building at which the major transmission lines went inside, and came out the other side.I found out years later from my uncle, who was a lone lineman in northern Michigan for 30 years, and then became a foreman for a team building transmission lines, that that building is a switch to reverse the flow of power between 2 large power plants.Running that switch on 96,000 V power lines has got to have a noticeable effect on the geometry of the power. But, America developed AC power transmission, and that was long ago. So, engineers have learned a few things to balance everything out. The frequency is the easiest to keep stable, because it's directly tied to the rotational speed of the generators. 3,600 rpm for fossil fuel plants, and 1,800 for nuclear plants.It is not difficult to control that precisely. Generators will lead in stabilizing frequency according to how much power they put on the grid.

author: Andy Chee
author_rank: Super Contributor
timestamp: October 30, 2023, 07:07:15 am
content: This guy analysed the stability of his local 120V 60Hz mains:http://www.leapsecond.com/pages/mains/The same principle can easily be applied to analyse your local grid.

author: ebastler
author_rank: Super Contributor
timestamp: October 30, 2023, 01:31:10 pm
content: Quote from: Faringdon on October 28, 2023, 11:24:39 pmIt seems to me that every reply so far has still missed the point of the question. Which is not so much about frequency stability (be it short or long-term), but aboutsymmetryof the AC waveform. Even for a perfectly stable 50 Hz signal, the alternate zero crossings do not have to be 10 ms apart if the waveform is asymmetrical.I have no idea whether anything is specified regarding symmetry in the mains network. I think it is not a good question to ask anyway, since the timing of detected positive vs. negative zero crossings will depend on implementation details of your detector.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 30, 2023, 01:48:11 pm
content: Quote from: ebastler on October 30, 2023, 01:31:10 pmTrue. I was closest by mentioning jitter but fair enough, symmetry is even more specific, it can be thought as a special case of jitter.And while anyone can easily google about the frequency stability of grid, that is something measured in time scale ofminutes(if you are lucky, maybe seconds), that was not the question.This treez guy is an interesting psychological experiment; the questions are actually pretty interesting and difficult, yet people jump, blinded by rage, into belittling the question without actually even understanding what was asked.

author: radiolistener
author_rank: Super Contributor
timestamp: October 30, 2023, 03:00:51 pm
content: Quote from: ebastler on October 30, 2023, 01:31:10 pmNo, I was talking about both - duty cycle and period. Duty cycle is affected by reactive load. And frequency stability also affects stability of duty cycle.Reactive load distorts sine wave and affects its symmetry. When frequency is changed it also affects duty cycle at least for one sine cycle. All this leads to unstable duty cycle and as result leads to asymmetric sine.Also when someone plugs some powerful load to the mains it leads to sparks and pulses in the mains. These pulses also affects stability of zero crossing period...

author: antenna
author_rank: Frequent Contributor
timestamp: October 30, 2023, 04:25:18 pm
content: I saw a forensics video once where mains frequency tracking is used to identify the time and date a particular video is created. Using the AC hum that can sometimes be detected in the audio track, they can tell exactly when the video was created. The frequency is always varying and that forms a nice fingerprint-like timestamp that is totally unique.

author: TimFox
author_rank: Super Contributor
timestamp: October 30, 2023, 04:43:23 pm
content: Six years ago, I posted this about a surprise I encountered in a design:https://www.eevblog.com/forum/projects/ac-line-fluctuations/msg1265692/#msg1265692Short explanation: I naïvely thought that, for a vacuum-tube audio project, I only needed to ensure a low value of 60 and 120 Hz ripple on the DC plate voltage, relying on filter capacitance but no regulation.For the actual audio signals, that was probably right, but when doing careful measurements I found them disrupted by lower frequency (below, say, 10 Hz) fluctuations on the DC level.I found that my AC outlet voltage was varying by more than I expected over that short a time interval.See older post for details: I had to add a reasonable voltage regulator circuit to get useful results.

author: langwadt
author_rank: Super Contributor
timestamp: October 30, 2023, 04:48:58 pm
content: Quote from: antenna on October 30, 2023, 04:25:18 pmalso to see if the recording was modified, like sudden jumps in phase where pieces was cut

author: pdenisowski
author_rank: Super Contributor
timestamp: October 30, 2023, 05:01:09 pm
content: Well, from a test and measurement point of view, the AC mains voltage provided by a "standard" wall outlet is often (at least in my experience) not stable enough for some types of testing.For example, if you want to run standby power tests as per EIC 62301, the voltage and frequency supplying the device under test must be within 1% of the nominal values and there is also a limit on the amount of harmonic distortion -- I think THD must be < 2% for the first 13 harmonics.It's not uncommon that running these kinds of measurements in the United States requires the use a "lab grade" AC power source in order to meet these requirements: you can't just plug the power analyzer into the wall. The usual culprit is ... surprise! ... THD, probably as a result of all the non-linear, non-PFC'ed devices being plugged into the grid these days.(Coincidentally, I just finished a video on this very topic -- standby power measurements -- which should go live on YouTube in the next couple of weeks)

author: tridac
author_rank: Regular Contributor
timestamp: October 30, 2023, 05:18:13 pm
content: Here in the UK, the frequency does vary a little, but has long term correction. Restored a couple of the old 1930's (Temco) synchronous motor clocks, one running in the lab for months, that is still within a few seconds of the radio clock next to it. ~Ñinety years between them. Short term variations, but corrected in the long term

author: soldar
author_rank: Super Contributor
timestamp: October 30, 2023, 06:42:58 pm
content: Quote from: pdenisowski on October 30, 2023, 05:01:09 pmI think pretty much in any country if you look at the mains on a 'scope you are going to see a very flattened top of the wave and not even close to a perfect sine. Lots of distortion there.Not to mention other types of distortion like spikes here and there.

author: soldar
author_rank: Super Contributor
timestamp: October 30, 2023, 06:46:11 pm
content: Quote from: tridac on October 30, 2023, 05:18:13 pmThe old clocks with sync motors kept time much better than the newer digital ones which used the grid as reference which, in many cases could run fast due to glitches and spikes in the signal being counted.

author: dietert1
author_rank: Super Contributor
timestamp: October 30, 2023, 07:43:19 pm
content: Should be easy to build an opamp bandpath filter that does something similar to the synchronous motor. There will be more parts yet no moving parts..Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: October 31, 2023, 07:03:45 am
content: The device on my bench has a comparator looking at mains voltage via a two stage clipping plus low pass filter, so here is the requested measurement. This is with 50 Hz mains and of course this measurement may not be representative.The scope gets triggered on the comparator output positive edge and shows the next negative edge (10 msec delay). One can see the jitter.The blue curve is a trend plot that shows 1000 past delay measurements. The exact 10 msec would be a constant line in the middle. So the average frequency was a little higher than ideal.The delay histogram has its peak shifted to lower delays, too. Jitter is about +/- 10 usec. There are about 10 or 20 outliers (not visible on screen).Regards, Dieter

author: tridac
author_rank: Regular Contributor
timestamp: October 31, 2023, 10:36:46 am
content: The accuracy of frequency of the mains is quite amazing, considering that the frequency depends on the rotational speed of thousands of tons of turbo alternators, all in sync. Variiations is load, input variations from renewables, the rotational inertia time constant of the system, probably measured in minutes, and the control system for that must be a pretty cool piece of design. The frequency does drop a little at peak load, but must be advanced a litlte later to ensure the second accuracy of clocks in the long term...

author: coppice
author_rank: Super Contributor
timestamp: October 31, 2023, 10:47:56 am
content: Quote from: tridac on October 31, 2023, 10:36:46 amThe frequency has to be kept accurate. If the generating sets start to advance or retard more than a very small amount from the network mean you can start to get instabilities in the whole network. Even when they let the frequency fall during periods of excessive load, the frequency has to ramp down in a slow and smooth manner.

author: tggzzz
author_rank: Super Contributor
timestamp: February 14, 2025, 10:32:04 am
content: Quote from: younhee0702 on February 14, 2025, 08:18:21 amSuggestion: necroposting to a thread started by Faringdon, and then pushing your forum is probably not a good idea. Better to avoid necroposting, and to be open by putting link to your forum in your .sig.Ditto in your other 4 responses.

author: ebastler
author_rank: Super Contributor
timestamp: February 14, 2025, 02:48:11 pm
content: Quote from: tggzzz on February 14, 2025, 10:32:04 amThanks for catching this one and reporting it. Now that the moderators have cleaned up the spam post, could you remove the spam link from your quote as well please? Still having it in there would defeat the purpose of the cleanup.

author: themadhippy
author_rank: Super Contributor
timestamp: February 14, 2025, 04:08:52 pm
content: QuoteMinor? looks like there at least 3600 seconds adrift

author: tggzzz
author_rank: Super Contributor
timestamp: February 14, 2025, 04:13:12 pm
content: Quote from: ebastler on February 14, 2025, 02:48:11 pmDone.

author: AussieBruce
author_rank: Regular Contributor
timestamp: February 15, 2025, 11:31:36 am
content: The only specific input I can provide is in relation to where I live, in suburban Western Australia, where the grid load peaks above 3GW regularly. I monitor the frequency constantly as part of tracking the states attempted transition towards renewable sources. Based on extensive data over more than a year, the frequency rarely deviates from target 50 Hz by more than 0.1% either way. Other large, stable grids are likely to perform similarly, if you're in a small community serviced with diesel generators the band could be a lot wider.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 15, 2025, 11:44:04 pm
content: Quote from: Faringdon on October 28, 2023, 11:24:39 pmNo, because there can happen sudden spurious transients due to shortcircuits etc so voltage waveform and thus zero crossing may be distoted.So 10 ms is only average value but not always exact.

author: SteveThackery
author_rank: Super Contributor
timestamp: February 16, 2025, 02:20:17 pm
content: The answer appears to be "no" for two reasons:1/ The mains waveform may be asymmetric, so the zero crossings may not be exactly 10ms apart in both directions.2/ There may be noise on the waveform, such that there may be multiple crossing events as the waveform changes from one polarity to the other.Is this something that a phase locked loop can help with? I believe it could track the mains waveform at the gross level whilst ignoring jitter and noise. Does anyone know if this is viable?

author: Terry Bites
author_rank: Super Contributor
timestamp: February 16, 2025, 03:23:34 pm
content: Suppose you google first.