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Mains frequency

Kelly Marie Angel
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
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    Well round here there are fields of the damn things springing up on every 1000 sq meter scrap of  land that would be too rough to plough and cannot get planning permission for a housing estate . The sort of land that  would previously have been supporting sheep or pigs.  In some cases the sheep and the panels seem to co-exist, though I do not know how well the cable protection will last. Not seen pigs & panels yet - I guess they dig too much.
    The dept of Energy statistics  do suggest that as of dec 2019 the UK has solar panels installed of total name plate capacity a touch over 13GW nominal (and GB without NI a touch under, more like 12& 3/4 ) . In principle then if the whole UK were ever all sunny at once, we might muster a quarter of our daytime supply I suppose.

    Now given the actual metered generation history suggests about 10% of that on average over all days of the year (more on sunny ones ?) I am inclined to agree the gridwatch figures that imply 1.5GW today are probably an over estimate - sky is clear here with odd clouds, but I do not know how sunny it is in other parts of the country. That was not really my main point- the main point was a large fraction, maybe half, of the generation with inertia on the grid has gone since it was first designed.
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    Surely this article is joking. a 44 MW help when GW are connected is not going to do much at all. I don't know what the total consumption in SA was, but probably not all that much. This is only about 1000 cars worth of battery, and it saved the day? I suggest it was the industrial load shedding of aluminium heating which really did the trick.


    I would be very surprised if the solar in the UK was anything like you suggest Mike, on a nice clear sunny day in summer it could probably make the whole 40GW worth. Really? In winter the solar is a small fraction of a clear sun summer day, or perhaps they have shifted the cells to the equator somewhere. Note really bright sunlight on equator perhaps 300W output per m2, about 3000 m2 /MW, so we need about 450,000,000 sq metres or 45,000 hectares. In the UK now you would be lucky to get 50W, so 250,000 Hectares. Where are they, all 45,000 hectares of active surface for 40 GW! I rather doubt there are more than 50 Ha total.
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    Denis McMahon:





    Posted by Simon Barker on Feb 14, 2020 8:13 am



    Or build big battery banks around the country.  Spare power from renewables can be stored.  Then when the generations drops too low, the batteries can top the grid up for long enough to bring backup generators on line.




    On a grid system powered by solar power we would certainly lose the stability provided by inertia. However, this is hypothetical, since I cannot envisage such systems. Solar panels are not very clever at night. Also bear in mind that nuclear power, though not strictly renewable, is virtually carbon free and is going to be with us for a good while yet. It is going to be some time before we worry about losing the stability of inertia.

     



     

    Not hypothetical at all.  South Australia already has a huge battery bank, and it appears to be doing a good job. https://reneweconomy.com.au/how-the-tesla-big-battery-kept-the-lights-on-in-south-australia-20393/ 


    It isn't intended to run the whole grid for hours on end.  It is there to store enough power to handle any short-term peak load, or sudden losses of supply.
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    As I type this (10 40 AM ) a little over a third of our mains is now inverter derived, about 30% from wind, and 5% from undersea links - French nuclear probably, and if the gridwatch website is to be believed about 6% from solar.

    At the same time a lot of the spinning load  from large industrial plants, cooling systems and so forth is now more likely to be variable frequency drive than the plain induction motor that turns generator during a phase shift it would have been.

    I think it is quite credible that we have already lost half the inertia we had when the systems that control the grid were being designed, and the trend will continue - if we add a few GW of car charging load that will not be inertial either.

    The frequency shift thing has an elegant and fail -afe simplicity, in that the phase shifts automatically cause current to flow from places with an excess of generation towards those with more load, so maybe we should continue to emulate it with the larger inverters at least until a clear alternative is available. It also allows things like the dynamic demand circuit, perhaps optimized better, to give smart load shedding within the appliance without needing any network connection and all the time delay and unreliability that entails.
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    Frequency control by clock is not ideal, it takes quite a time to be one second out! In fact many generation systems away from the grid are controlled by frequency standards (generally quartz crystals) and a complex control system usually to be 50.00 Hz. However large load changes still cause deviations from ideal, as it takes time for increased fuel to recover the lost speed, even though this may be very small. Inverter supplies have to follow the connected mains frequency because they have only a very small power available compared to the grid, and so have a correspondingly tiny influence in speeding up all the rotating plant. Nuclear sources are basically steam plants so control as you describe. Steam plants have another advantage over other types in another way too. They have a boiler which contains a lot of steam acting as a big energy reserve, and boost if necessary while the heat input adjusts more slowly. If one had entirely inverter derived supplies the frequency could be set from a common standard, but the problem of using all the available output would have to be tackled as well, perhaps with some small local storage. The problem should improve when we build another 20 nuclear plants, to power electric transport, but don't hold your breath!
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    OMS:

    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




    There is a centralised clock - already used in fact. It's called Greenwich Mean Time, provided by the  National Physics Laboratory.


    Much has been said about the inertia of rotary generators. If the load on the grid increases, the generator speed decreases. The governors sense this and cause more steam to be injected until a new equilibrium is reached where the generation once again matches demand. The kinetic energy released as the generators lose speed provides the shortfall between turbine power generated and demand.


    We are now running at slightly reduced frequency - an offset compared with target frequency. Hence the grid "clock" running slightly slow compared with Greenwich time. As long as this situation persists, a time disparity, i.e. difference between grid time  and Greenwich time, builds up. The object is to eliminate this disparity. It is theoretically possible to feed back the disparity into the turbo-generator speed control system. In Control Engineering terms, integral action eliminates the offset.  


    In the days when I worked in the electricity supply industry (1960s), this level of automation was not applied, to my knowledge, though the technology was available then. Grid control engineers preferred to adjust things manually by calling power stations to deliver requested output. Apart from the consideration of  maximising usage of the most efficient plant, issues affecting the locality of individual power stations could make it impracticable to use them to try to regulate grid frequency automatically. Nowadays there is the additional complication of regulating the trading among the many private power providers. My guess is that there is still a lot of manual control - does anyone have up-to-date knowledge on this?


    Inverters have no inertia, and I am not sure it is desirable to design them to simulate inertia. Their output frequency is not affected by load, and can "lock-in" to an existing mains frequency. We could provide a control frequency by radio transmission. There are ways in which we could regulate power output if desired, though it would be sensible to divert excess power to battery storage.




    Posted by Simon Barker on Feb 14, 2020 8:13 am



    Or build big battery banks around the country.  Spare power from renewables can be stored.  Then when the generations drops too low, the batteries can top the grid up for long enough to bring backup generators on line.




    On a grid system powered by solar power we would certainly lose the stability provided by inertia. However, this is hypothetical, since I cannot envisage such systems. Solar panels are not very clever at night. Also bear in mind that nuclear power, though not strictly renewable, is virtually carbon free and is going to be with us for a good while yet. It is going to be some time before we worry about losing the stability of inertia.



     

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    I have made several meters to measure the mains frequency to various accuracy, but not using the period measuring method. There is no guarantee that all the mains cycles are identical for several reasons. Whilst a mechanically made sine waves may all look the same, tolerance on exact magnetic field strength and windings positions will make zero crossings (or any particular voltage levels) do vary slightly. In a large grid this is even more likely when one is looking for changes of parts per million as we are here. My units used a phase locked loop to multiply the mains frequency to a convenient figure (500 kHz) but these inherently have a frequency averaging effect over a significant number of cycles. They also use ALL cycles which again means that cycle  timing errors have much less effect. I could thus measure the 50Hz to 3 decimal places at 1 reading per second, or 2 decimal places at 10 readings per second, but as the loop bandwidth (effectively an averaging time) of 1 Hz (1 second) or so gave fairly stable readings to great accuracy on a smallish generator (100 kVA). These were used by the film industry to make sure that lighting flicker from discharge lighting did not cause varying exposure of film. Modern electronic cameras don't have this problem for a number of reasons, and one can see the effect live anyway, but of course one never knows what is actually on film until it is developed! I think this variability on DD is due to lack of averaging, and cycle by cycle timing differences, but these do not make any difference to normal usage of electricity.
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    Dynamic Demand is a not for profit organisation,  but the chap who is responsible for the meter is Joe Short at demandlogic.co.uk a modest sized company with offices in Hackney.
    Who they are   it rather sounds as if the frequency meter is more of a zero-crossing counter lash up, and the basis of a web site gimmick to draw attention to the sort of thing they can do, rather than supposed to be a precision instrument,

     The how explanation  

     they do not warrenty the accuracy


    Their big and quite elegant idea is that a simple frequency meter like the one behind the display on the website could be used either within smart appliances or as a stand alone to operate a contactor for load shedding, helping with grid stability.

    Right now there is no mechanism for rewarding people for fitting one, but the idea seems a potentially very interesting one - effectively using grid frequency as a near instant signal to manage demand at the load end in much the same way as it already acts as a balancing influence at the rotating generation end. Such an approach will always be faster than anything involving the internet, ADS routers and a some central server a hundred miles away.


    If it is just a zero crossing detector then on a noisy supply  it is quite likely to suffer a bit from AM to PM conversion, where voltage spikes or harmonic distortion to the waveforms  that push the centre of the sine wave up or down are detected as if they were  frequency effects, that move the zero crossing sideways. A cleaner but more involved method would be a phase sensitive detector.


    Rather fetching animation of making a

    square wave from harmonics


    a slight alteration of the relative phases and amplitudes  of the harmonics (also helps to visualise why you take a 'good' square wave, and pass it via something like an LC filter that suppresses or phase shifts some but not all of the higher frequencies and it becomes ripply.)

    changes the waveshape completely



      The rotating generators on the left represent the harmonics, and the resultant voltage waveform is on the right.


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    Alan Capon:

    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. 




     

    Also, think about time switches. There are plenty of these everywhere, many embedded in electronic devices such as central heater timers, video recorders, etc. I imagine that since most electronic ones are battery-maintained they are timed by quartz oscillators rather than the mains frequency. But there is still a big market for electro-mechanical time switches, which take their timing off mains frequency. They are popular because they are cheap, cheerful and easy to use, and do keep good time. (Well until there is a power failure; then things go to pot.)
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    Hi again I just took delivery of a new True RMS multimeter to replace my old one that is a bit knackered(like me) the new one has a frequency range so whilst playing with it I tried that out and according to it the mains frequency is actually really stable I looked at dynamic demand site and my meter at the same time and the readings did not agree so I'm thinking maybe the DD site has got problems   Maybe someone's plugged in a noisy  bit of machinery on the same mains as there meter my meter and grid watch seemed to agree

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