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Mains frequency

Just checked the dynamic demand site and the frequency was down to around49.7 cycles almost down to the lower legal limit never seen that before
  • Where did you see that? It doesn't seem to show up on GB Grid Frequency - I wonder if it was just a glitch somewhere in the internet, rather than real world.


    I did stumble on an interesting use for mains hum though - BBC News: The hum that helps to fight crime (an old story, but one I'd missed before)

      - Andy.
  • Hi Andy the sight I saw it on is called Dynamic Demand when you open it click on the little picture of a frequency meter and that takes you to the real thing. It doesn't work on my tablet but is fine on my fone

  • Dynamic Demand page

    Does need Java to be allowed (at least for that site) - hence iPad's etc won't work.


    and is still hovering low...

    would correlate with the Red 'average' ? line on the other GB Frequency site ?

  • The gridwatch site only updates very slowly and this number seems to be hugely smoothed. The DD site is interesting, and pretty much matches some other results I have with a similar resolution. Grid is slow this morning and there is coal in use as well. 41.35 GW is coming well towards max availability, all we need now is a power station problem!

  • Kelly Marie:

    Just checked the dynamic demand site and the frequency was down to around49.7 cycles almost down to the lower legal limit never seen that before




    Actually, 49.7Hz is not near the lower statutory limit, which is 49.5Hz, it is just under half way towards it. There are many reasons for a rise or drop in frequency. There is a requirement to keep clocks that derive their time from the mains within a certain tolerance of actual time over a 24 hour period. This means that the system frequency must average close to 50Hz over a 24 hour period. If the average is a bit high, then the system frequency will be deliberately held below 50Hz to allow the average to drop, and therefore the nation’s clocks to slow down to reach the correct time. The lowest frequency we recorded yesterday was 49.7993Hz, which is near enough 49.8Hz for most. 


    Stories about the UK grid falling apart are simply that - stories, often to be found in certain tabloid newspapers, without much evidence of reality. 


    Regards,


    Alan. 

  • Thanks all for your replies looking at the dynamic demand meter is a little misleading as it looks like the frequency is closer to the lower limit than it really is  49.5 to 49.7 cycles is only a tiny bit of movement which makes it look like things are worse than they are. I agree that newspapers seem to say that we're all doomed to sitting in the dark each winter which hasn't happened yet except when there's been a major fault somwhere  and that's rare of course dramatic headlines sell help them sell these daily rags  so that's that
  • I suspect that the frequency display on the dynamic demand site is not working correctly.

    For some weeks it has been displaying a generally lower frequency than that displayed from other sources.


    The DD meter is much quicker acting than most other sources, it is virtually real time rather than being smoothed or averaged over seconds or tens of seconds. Therefore on ANY ONE occasion, the DD meter might well be accurately displaying information that is not shown elsewhere.

    However, the frequency shown on Dynamic demand is too much lower, too often, and for too long to be compatible with actuality.


    Extreme load, sudden and unexpected increases in load, or sudden losses of generating/import capacity can all cause a drop in frequency. Such situations are usually handled fairly quickly with the frequency recovering in minutes, or very exceptionally in a few hours.


    The national grid aim to keep the frequency within normal operational limits of 49.8 cycles up to 50.2 cycles, brief excursions beyond these limits occur fairly regularly.

    The legal limits are rather wider at from 49.5 cycles up to 50.5 cycles, averaged over some short time. Excursions outside these legal limits are rare.


    Any severe drop in frequency triggers automatic starting of OCGT plant and calls for extra output from other plant. If that does not relieve matters, then areas are automatically blacked out, a rare event that last occurred in August 2019, and previously about ten years before that.
  • A lot of people think only old style clocks with synchronous motors are affected by the mains frequency. This is not the case, and it is surprising how many new electronic clocks, including alarm clocks, use a calculation based on the mains frequency rather than a highly accurate chrystal oscillator. I guess this is because the calculation results in a cheaper clock for the achieved accuracy. 


    Regards,


    Alan.
  • It is interesting - though I must say have not seen a mains derived electronic clock for years - quartz crystals are pence in bulk.

    Mechanical generators slow down and generate a  lower frequency as the load current rises, and we can use that to sychronise a generator into phase -lock with an existing supply - you get it nearly synchronised so the difference frequency is sub Hz, and close the breaker, and the current that flows either adds or subtracts some torque, and in turn speeds up or slows the rotor until things settle to exactly the same frequency, and a voltage and phase offset that represents the fraction of the load handled by that one generator relative to the average of the rest.

    There is an assumption in the way that the rules of the grid are written that all generators do this, but of course inverter derived things like solar farms, and most wind turbines (certainly those at the end of DC links...) have no such characteristic, the voltage droops as more current is taken, but the zero crossings occur where ever the electronics tells them it is time to invert the DC to do the other half cycle - Imagine if you will a big battery and a reversing switch operated at 100 times a second (as a full cycle is 2 reversals.)

    It is possible in software to alter the timing of the zero crossings, either in a phase shift way, i.e. a one time shift and then a constant period again, or in a frequency shift way (all cycles have the new period.) and right now we go to some effort to make these inverter sources mimic the behaviour of a traditional generator, but it is not clear if as we may soon approach a situation with more kit on the grid emulating rotating generators than actually are, we wish to continue.

    It may end up like the phone system with electronics at one end pretending to be a mechanical exchange, and stepping up to a really awkward 50V and 80V ringing   voltage, while at the other end we have phones containing electronics emulating a carbon microphone and a moving iron earpiece stepping down from this really awkward voltage to handle audio at lower levels internally. If we were starting again, this is not the system we would design.

    Traditionally with comms systems inefficiency like this is a secondary consideration after compatibility and usability,   it is not clear that it should be for power system like a national grid.





  • Former Community Member
    0 Former Community Member
    I've often thought about the impact of more and more invertor derived AC on the grid, and less and less conventional synchronous generation on the grid


    As we move more to the former, and less of the latter one has to wonder if at some point we need to introduce a central or regional frequency clock to kick all the invertors back into synch


    The man who owns that clock, owns the world !!


    Regards


    OMS