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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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    I do not believe that any centralised clock would help.

    The problem is that inverters have no inertia and can not help stabilise the grid frequency.

    A steam turbine has considerable inertia, and in case of a frequency drop will "try" to limit the drop in frequency. As the turbine slows, the governor detects this and supplies more steam, the resulting drop in boiler pressure will result in the fuel supply being increased.

    The stored rotational energy is supplied in milliseconds, and the energy stored by the steam in the boiler is applied within a second or so, the chemical energy stored in fuel bunker can be applied at an increased rate within a few minutes.


    None of this is possible with wind or solar generators and static inverters. They supply into grid whatever the wind or solar source can supply. There is no question of briefly supplying a bit more in response to a falling frequency.


    Short term grid storage, mechanical flywheels, or batteries for example will be needed, trial installations are already underway.
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    None of this is possible with wind or solar generators and static inverters. They supply into grid whatever the wind or solar source can supply. There is no question of briefly supplying a bit more in response to a falling frequency.



    It might be possible. Say an inverter, in normal circumstances, shoved only say 95% of the available power into the grid - sending the remainder into some local dump (resistive heater say). Then when a bit of extra power is required it could increase output (up to the amount normally dumped) pretty much instantly(*) - whether the need for more power is communicated by falling frequency or by some other method.   For wind, you probably don't even need to generate the extra electricity and dump it - but just tweak the angle of the blades slightly to catch a little less wind.


    Certainly at the moment, where the primary aim is to produce as much as possible from renewables, we probably don't want to adopt such an approach - so keep using inverter designs that try to export every mW that can - but if we were in the situation where almost all our power was from renewables and so most of the time we had a bit of excess generating capacity lying around, it might be workable.


    (*) - I can't claim the approach as a new invention - I've seems something along those lines in a old (probably 1920s) generator powered by a water wheel - with some very dodgy looking open frame resistances!


       - Andy.
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    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.
  • 0
    There is a lot more to this than just the inverters. It is an enormous control system problem, with many many parameters, some of which are unknown. An Inverter connected to a windmill must take all the available output and get it into the grid cycle by cycle. The blade angle has a control system to adjust the blade angle, but this is much too slow to deal with wind gusts immediately. If the grid load does not match the blade speed, they will speed up, but slowly due to the huge inertia. Gas turbines are also not known for their immediate response to more fuel (heat) and must be synchronous anyway. The same goes for steam turbines, but here the limiting factor is probably rotor inertia. There are also time delays due to distribution systems and consumers loads, particularly if these are large rotating machines. There is a huge problem with batteries, some of it due to the required inverters, as well as the chemical problem of short extremely large charge and discharge cycles, perhaps even at the inverter switching frequency, perhaps 20-50 kHz. Overall large amounts of wind power are a pretty bad idea, unless we have nice constant winds, which are somewhat unusual at many of the wind sites. Wind companies are not very interested in further development unless government makes large subsidies, overall costs are higher than expected and in many cases revenue lower. The person who solves this lot is quite likely to end up fairly rich, so who is volunteering?
  • 0
    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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    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.)
  • 0
    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.


  • 0
    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.
  • 0

    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.



     

  • 0
    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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