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Old 16th Jun 2017, 1:07 pm   #21
russell_w_b
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Default Re: Mains frequency time error?

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Originally Posted by emeritus View Post
My understanding is that the permissible frequency tolerance is set by statute...
I wasn't suggesting the actual frequency tolerance has been altered - there're implications for large motor loads and generator turbines if this were so - but the requirement to pull the frequency into line over a 24 hour period might've had to change because of the inconsistency of supply and rapidity on which modern generating plant can go on circuit.
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Old 16th Jun 2017, 1:17 pm   #22
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Default Re: Mains frequency time error?

The UK national grid is required by law to keep the grid frequency within the limits of 49.5 cycles and 50.5 cycles. These limits being the average frequency over some very short interval, I think that it may be 10 seconds, but stand to be corrected as to the exact interval.

So a very brief excursion beyond these limits is acceptable provided that the short term average is within the range of 49.5 to 50.5.

Frequency excursions beyond the legal limit have occurred but are very rare.

The grid aim, so far as is reasonable, to keep the frequency within closer operational limits of 49.8 cycles up to 50.2. cycles.

In the short term, frequency fluctuations are handled by generating plant that is contracted to operate in "frequency sensitive mode" Such plant is contracted to provide say 450MW at exactly 50 cycles, but 500MW at 49.8 cycles and 400MW at 50.2 cycles.

Any serious drop in frequency results in the automatic starting of open cycle gas turbine plant, or diesel engines.

If all else fails, large areas of the country are automatically blacked out, being disconnected by frequency relays, at IIRC about 49 cycles.
Such events are very rare indeed but have occurred.
The last large scale automatic disconnection was caused by the almost simultaneous failure of two large generating units.
The grid are required to have sufficient margin to cope with any single failure, but two almost simultaneous failures are so rare that such events have to be accepted.
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Old 16th Jun 2017, 2:04 pm   #23
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Default Re: Mains frequency time error?

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I think that it may be 10 seconds, but stand to be corrected as to the exact interval.
The application notes I have Alsthom protection relays suggest a typical time delay of between 5 and 20 seconds so that any transient dips in frequency are factored in. A four-stage load-shedding system would allow (say) 20 seconds down to 49Hz but only 10 seconds down to 48.2Hz.
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Old 16th Jun 2017, 2:55 pm   #24
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Default Re: Mains frequency time error?

I'll make my telemetry data available sometime over the weekend and post a link, just in case anyone fancies a trawl through. No prizes for spotting anything interesting, though!
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Old 16th Jun 2017, 4:07 pm   #25
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Default Re: Mains frequency time error?

My mains frequency derived nixie clock is two minutes slow now, referenced to my PO36 Magnata which, referenced to NPL, is one second slow. And my fast/slow o meter has shown less than 50Hz for a while.
 
Old 16th Jun 2017, 4:11 pm   #26
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Default Re: Mains frequency time error?

I would think that for sheer national pride we should keep to the right number of cycles per day. Are we locked to European mains frequency or just take/give power via DC links?

(too big for an edit)
 
Old 16th Jun 2017, 4:16 pm   #27
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Default Re: Mains frequency time error?

I have been keeping a close eye on my synchronous time vs BST, it has improved from the initial -2 minutes, but is still around -1 minute compared to my usual synchronous time of 'zero' +/- 20secs.
If they are correcting it, they are taking their 'time'
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Old 16th Jun 2017, 4:22 pm   #28
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Default Re: Mains frequency time error?

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Originally Posted by merlinmaxwell View Post
Are we locked to European mains frequency or just take/give power via DC links?
As far as I know, continental links are DC for just this reason.
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Old 16th Jun 2017, 4:31 pm   #29
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Default Re: Mains frequency time error?

This is all very odd. I have a couple of synchronous clocks and neither are showing gross errors (i.e. more than a minute fast or slow). In fact, they appear to be spot on, though I've no way of measuring that precisely. I thought the entire grid had to run at the same frequency. Are we seeing grid splits, maybe because of PV generation in the sunny weather?
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Old 16th Jun 2017, 8:14 pm   #30
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Default Re: Mains frequency time error?

Links to mainland Europe are indeed DC so we are not tied to the continental grid frequency.

Under all normal circumstances all parts of the UK grid are interconnected and the frequency is therefore exactly the same at any point of the UK grid.

"System splits" whereby the grid is run as two or more sections have occurred and will no doubt occur again, but such events are rare and likely to be remarked upon.
Also, in the event of a system split, if one part was slow relative to the other, then efforts would be made to "catch up" by generating in the slow portion at say 50.2 cycles for a bit before synchronising and re-connecting the two portions.

If synchronous clocks are losing time and not recovering this, I would suggest that deep but very brief "dips" in the mains supply may be to blame.

If the mains voltage drops very briefly to say 100 volts, then one or even two cycles of the mains waveform may be of too low a voltage to be detected, the clock therefore only "sees" 49 or 48 cycles in that second and runs slow.
If such events are frequently repeated then significant time may be lost.

Another possibility might use of a generator by the power supplier, but that becomes less likely if the clocks are regularly running slow.
Such dips in the supply may be caused by faults or switching of large loads.
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Old 16th Jun 2017, 9:26 pm   #31
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Default Re: Mains frequency time error?

A synchronous motor is not quite the same as a stepper motor, so a missing cycle will not necessarily mean that 50Hz looks like 49Hz. Inertia will keep the motor running and hopefully it will carry on regardless. A long period of low voltage could lead to slippage, but then it is no longer acting as a synchronous motor.
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Old 17th Jun 2017, 6:31 pm   #32
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Default Re: Mains frequency time error?

I've looked into the grid frequency stability, and the 2 minutes loss on synchronous clocks is not excessive if the frequency drops to 49.9 cycles average over 24 hours. That will make 2 minutes error, so its not excessive by any means. If you want to check yourself go onto IET forum and you'll find a post about it by kellyselectric.
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Old 17th Jun 2017, 9:43 pm   #33
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Default Re: Mains frequency time error?

""kellyselectric

Posts: 86
Joined: 22 July 2016

Thank you for all your replies I think that the 2 minutes drift on the clocks is normal judging by the last post. I'm sure its just that the person who noticed it maybe didn't realize it was something that happened often and maybe just got a bit panicked for want of a better phrase""


Above is the final post on the IET forum regarding the error.
I certainly didn't get 'panicked' by the 2 minute loss, it is just something that doesn't happen, I am a clock enthusiast, and as such, errors like this don't go un-noticed, I appreciate frequency wobbles of <+\- 1 minute , but typically < 30 secs. are normal.
I guess we will never know what happened, maybe it was just a long run of 49.** Hz?
Anyway, an update as of now, they are about -48 secs
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Old 17th Jun 2017, 9:52 pm   #34
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Default Re: Mains frequency time error?

If it's being compensated for, presumably this must have been due to a low frequency event. Given that others haven't experienced similar effects, surely your area of the grid must still be isolated from the rest, otherwise everyone else's clocks would now be gaining.
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Old 17th Jun 2017, 11:02 pm   #35
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Default Re: Mains frequency time error?

I have now set up experimental access to my inverter telemetry data!

If you go to
http://solarmonitor.earthshod.co.uk/API/CSV/today/
This should open up today's results in LibreOffice Calc (or your favourite spreadsheet program, if different). You can specify a date using Japanese-style YYYYmmdd with no punctuation, such as
http://solarmonitor.earthshod.co.uk/API/CSV/20170617/

Each row is measured one minute apart. The frequency is in column G; if you make the spreadsheet editable, you can add =AVERAGE(G2:G__) below the bottom row, changing the row number as appropriate (there will be more rows on days with more sunlight).

Obviously, figures are available during the hours of daylight .....

A graph showing instantaneous and averaged power levels is available at
solarmonitor.earthshod.co.uk/

I might well add new API functionality in due course.
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Old 24th Jun 2017, 5:50 pm   #36
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Default Re: Mains frequency time error?

ISTR that power station clocks were checked against GMT and that generators were varied so that at 08:00 the clock was in synch with the Greenwich "pips" This may be one of these "urban myths" so correct me if I am wrong

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Old 24th Jun 2017, 10:21 pm   #37
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Default Re: Mains frequency time error?

How about this -
take the mains signal and square it up. Feed it into an RS232 serial port configured
for 600 baud. You will get one character input every mains cycle.

Then write a computer program to do the counting and compare with network time.
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