National Grid Frequency

Discussion in 'Infrastructure & Stations' started by a_c_skinner, 7 Oct 2019.

  1. GRALISTAIR

    GRALISTAIR Established Member

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    The rules of chemistry are that 1 mole of gas occupies 22.4 liters at STP (standard temperature and pressure). CO2 is 44 g/mol so 44 g occupies 22.4 litres.
    1 Tonne is 1000kg = 10^6 g so 10^6/44 = 509,000 liters of CO2
    200 tonnes = 100 million liters (rounded)
     
  2. GRALISTAIR

    GRALISTAIR Established Member

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    put another way

    CH4 (methane) + 2 O2 (oxygen) ---> CO2 + 2H20
    16 tonnes + 64 tonnes ---> 44 tonnes + 36 tonnes + energy

    So 200 tonnes of CO2 produced means 200/44 *16 = circa 77 tonnes of gas burned

    Back on the subject of railways even though all this is OT - with all those heavy trucks on the road, you can only realistically reduce C02 meaningfully by modal shift to an electrified freight railway. The Woodhead route would be proud - though its main load was coal to the Manchester area coal fired power stations -but the grid frequency would have been OK!!!! How is that for getting back on topic?
     
    Last edited: 8 Oct 2019
  3. DaveTM

    DaveTM Member

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    One grid instability that is going away with the loss of coal fired power stations is the furnace fans of those stations. I remember reading years ago that the fans that fed air to the coal fires were synchronous motors, so the fans ran at grid frequency. If grid frequency fell far enough, the fans would slow down, less steam would be raised, less steam would go to the turbines, less power would be generated, and the grid frequency would fall further. Of course, as with steam locomotives (getting back on topic?), there is a certain amount of boiler inertia so it would require the grid frequency to be below normal bounds for minutes rather than seconds before this feedback loop became a problem.

    A grid instability that is new though is switched mode power supplies (SMPS). All traditional resistive loads (incandescent lights, heaters like toasters or kettles, or the universal motors on old DC trains) consume less power when the grid voltage drops. All traditional inductive loads (synchronous and inductive electric motors like on older locomotives) consume less power when the grid frequency drops. So when the grid comes under pressure, the demand from these drops giving some respite to the grid. However modern goods (compact fluorescent and LED lightbulbs, flat screen TV, laptop charger, the traction packages on a 377) use switch mode power supplies which pull the same amount of power whatever the grid conditions. So if the grid stalls, they just pull harder.
     
  4. Flying Phil

    Flying Phil Established Member

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    Fascinating thread - but I also thought that the frequency was tightly regulated because so many devices (clocks and other such things) rely on that for timekeeping.
     
  5. DaveTM

    DaveTM Member

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    On the subject of Dinorwig power station (and similar pumped storage stations), and their contribution to grid stability, I understand Dinorwig was specified to be big enough to cope with a PWR nuclear power station going off line suddenly.
    At a massive coal power station like Drax, there are several boilers, several steam turbines, and several sets of pylons carrying power away from the plant. However they are grouped into individual grouped systems of about 600MW each (Drax has 6 of them), so one boiler feeds one turbine feeds one three-phase power line. If a fault develops somewhere in the power station only one 600MW system is lost to the grid and all the rest of the 600MW systems carry on regardless. On average the UK is using about 35GW, so loss of one generating set at any of the big power stations in the UK amounts to only about 1.7% of UK usage. If left uncorrected, grid frequency would suffer, but by having a few power stations running at less than 100% output the grid could cope by opening the valves at those stations.

    At the time that Sizewell B PWR was designed, it was found that a reactor size big enough to produce steam for about 1200MW of electricity was most efficient, so to avoid designing new turbines the decision was made to simply use two turbines of the existing design. However, the national grid now had to be able to cope with losing two generating sets simultaneously if for some reason the reactor had to be shut down suddenly. A sudden loss of 3.4% of grid capacity was thought to be too much for the existing spinning spare capacity to deal with, so Dinorweg was conceived. In the event that a PWR shut down suddenly, Dinorweg would switch on (or refrain from pumping) for an hour or two while a cold power station was gently coaxed into life.

    Sizewell was built, and then privatisation happened and the follow-on PWR reactors to the same design were sadly never built. Now, Dinorweg is still sometimes factored into the cost of Sizewell when people want to paint a picture of nuclear being expensive, whereas in reality it was already required for black start purposes and to deal with short term fluctuations dues to the likes of Coronation Street and deaths amongst royalty.
     
  6. Elecman

    Elecman Established Member

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    It has to disconnect the dc-ac inverter from the building wiring when the incoming mains supply is lost
     
  7. Bald Rick

    Bald Rick Established Member

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    A CCGT does, however, produce about half the CO2 per unit of electricity generated than coal. This is one of the key factors in U.K. electricity generation more than halving its carbon footprint over the past 20 years or so.
     
  8. DaveTM

    DaveTM Member

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    OK, this is probably going far enough off topic that the mods will have kittens, but...

    Frequency is tightly regulated for that reason amongst others, but my dad who worked in the industry tells me there was an occasion (I think in the 70s) when the UK national grid for a short period became separated into two separate grids because a number of power lines between north and south were lost. With coal and power stations concentrated in the north, and population concentrated in the south, the grid slowed in the south and sped up in the north.

    Load was shed and reserve generation was brought online in the south, while generation was tripped out in the north, and the frequencies of the two sub-grids were finally brought back into sync (but only after the south had got several seconds worth of cycles out of sync). When the two were at the same frequency and the phase was close enough, somewhere a switch was thrown bringing the powerlines back into service and the grids were connected back together (in my imagination I see Igor throwing an enoooorrrmous Frankenstein style knife switch). But then there was a realisation that the factory workers in the north would be woken up by their alarm clocks several seconds earlier than the bankers in London. Apparently serious thought was given to setting up generation the next night to make the southern grid sourcy and the northern grid sinky, then deliberately opening the same power lines as the night before to get the two back in time. In the end it went to the man at the ministry who decided that forever more clock radios in the north would wake the workers up a few seconds early.

    Interestingly, thanks to the US military, the whole clock sync thing is much less important these days. I can look at my smart watch and know where I am on Earth to within an error of around 10m, because it listens to the atomic clocks on the 30 or so GPS satellites up above, and as a side effect shows the correct time. My various computers at home including my TV, laptop, mobile phone, etc, are also synced via another US military invention (the NTP protocol) to various atomic clocks around the world. Very few clocks these days rely on a human, the speaking clock, and a 50Hz mains supply. But still not all clocks are in time; I usually depart a statioin based on the clock of the Mitrac computer on my 377; the other day I had a train where the clock was about 20 seconds fast...
     
  9. GRALISTAIR

    GRALISTAIR Established Member

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    During the 1984-1985 miners strike there was also imbalance - with the Isle of Grain Oil powered generators providing huge power source with the coal fired burners not burning coal. The north did indeed become the sink IIRC
     
  10. Bald Rick

    Bald Rick Established Member

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    Yep - I visited Pembroke Power station (oil) in the late summer of 84; I remember the supervisor showing us round saying that normally that time of year they would have 1 of their 4 units running, perhaps at half load. On that day 3 were on full load and one was at half.
     
  11. Belperpete

    Belperpete Member

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    Can't remember the last time I saw a stand-alone mains-powered domestic clock (although they are still common built into cookers). You used to be able to get special plugs and sockets for mains-powered clocks, with a 1A fuse. There were a couple of these sockets in my last house (although the only one actually in use fed a convector radiator).
     
  12. swt_passenger

    swt_passenger Veteran Member

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    I remember special sockets for mains clocks, in our house in Newcastle in the 60s, they were centrally placed on the mantelpieces in both main rooms, so as to be behind the clock. IIRC we only had one such clock, and after a power cut it had to be manually started, with a sort of thumb wheel on the back. I guess as a very simple single phase motor it wasn’t self starting?
     
  13. AM9

    AM9 Established Member

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    It would have been a synchronous motor. They also had a spring/lever mechanism that kicked the motor to change direction if it started rotating the wrong way. They had absolute accuracy as the rotation was locked to the mains frequency. That was one of the reasons why the mains had to have a certain number of cycles per 24hr period, even though the instantaneous frequency could vary by as much as +/- 2%. It was usually necessary to catch up, - especially in the winter, by running slightly above 50Hz in the small hours when demand was lower.
     
  14. DelW

    DelW Member

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    My parents also had such a clock, given to them as a wedding present in 1947. IIRC it could actually be started in either direction, although there was an arrow next to the starter knob to show which way to spin it. Sadly, when I inherited it, it wasn't working, and beyond the obvious checks on fuse and wires, and trying some light lubrication, there wasn't any obvious way to take the motor unit apart to try to find the fault.
     
  15. DaleCooper

    DaleCooper Established Member

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    A vital point has been missed. Many turntables use synchronous motors so a change in supply frequency will affect the pitch of the music being played.
     
  16. Ken H

    Ken H Established Member

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    Dinorwig isnt near the Ffestiniog railway. Its in Llanberis
    Ffestiniog Railway was diverted for the Ffestiniog power Station
    both pump storage. They were built as part of the nuclear program, Dinorwig to take spare power from valley, and Ffestiniog from Trawsfynydd.
    I am not aware of any other large pump storage plant in the UK.
    Nearby Maentwrog is a simple hydro plant, not pump storage.
     
  17. Ken H

    Ken H Established Member

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    Please can you enlarge on 'Black Start'? I think it refers to a restart from a total stutdown of the grid. I believe much power plant needs a mains supply to start up. So if the grid is dead they cant start. Is that right?

    Edit. And how much power do you need to start a big power station?
     
  18. Ken H

    Ken H Established Member

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    There is discussion above about synchronous and asynchronous generation.

    How do trains regenerating manage to make power at the right frequency for the grid?
    (I assume the kit at DC substations have to convert excess DC power to AC also.)
     
  19. GusB

    GusB Established Member

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    Cruachan and Foyers are also pumped storage, and I believe there are a few more schemes at various stages of planning.
     
  20. swt_passenger

    swt_passenger Veteran Member

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    Regeneration passes through a DC stage in the train, it is then converted back to AC at whatever exact frequency the OHLE needs at the time, and in phase with it.

    AIUI in the general case, regeneration in the DC area doesn’t usually pass back to the distribution system, because most of the older transformer rectifiers are not “reversible”, so the DC network relies on regenerated power being used by motoring trains elsewhere - it’s effectively all in parallel over wide areas...
     
  21. edwin_m

    edwin_m Veteran Member

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    The traction package of a modern train (for operation on AC electrification) has a variable voltage variable frequency inverter which generates AC for the motors at the correct voltage and frequency according to the current speed and power (or regenerative braking) demand. But it also has a similar inverter connected to the transformer, with the two being connected by the "DC link" (which is where the DC is fed in if the train is dual voltage). This inverter synchronises to the supply frequency during regenerative braking in the same way as the inverters on those electricity generators that don't have rotary plant. a

    As far as I'm aware, the substations on the DC traction supply systems in the UK are not able to put regenerated power back into the grid supply, as they have simple rectifiers rather than inverters. There's probably no reason why they couldn't have inverters and some new ones may now have them. But with rectifier substations the train can only regenerate if there is another train nearby that can use the generated power. If this isn't so then the train detects the resulting increase in line voltage, decides the supply is not receptive and diverts the regenerated current into on-board resistors instead. Trains on AC supplies also send the current to resistors if the line is not receptive, but this is rather less likely because with the lower voltage drops in the system the current can get much further to a train that needs it.
     
  22. Ken H

    Ken H Established Member

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    Thanks for your answer, and to SWT_Passenger.
    didn't substations need modifying before trains were allowed to regen on DC? If they dont regen into the grid, what was that mod? I think i read about DC trains running with regen turned off because of substation issues.
     
  23. Bald Rick

    Bald Rick Established Member

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    Yep, Cruachan is 400MW, Foyers is 300MW. Coire Glas near Spean Bridge is in final stages of planning, that will be 1500MW. The latter was consented for 600MW, but SSE have resubmitted. I don’t blame them either, it will be a nice little earner when all the new wind comes on line.
     
  24. Bald Rick

    Bald Rick Established Member

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    What follows is how it was explained to me. The substation mods were about fault identification. In simple terms, if there is a fault in the system - typically a short circuit of some sort - you want the system to detect it and trip out the supply. However if the fault occurs at the same time as a train is regenerating, then the extra power going into the system can mask the short circuit, the system doesn’t trip, and thus the short circuit keeps happening, with potentially damaging consequences for equipment and even people.

    The substation mods enabled detection to be improved, and also (I think) trips out both ends of a section if there is a problem.
     
  25. HSTEd

    HSTEd Established Member

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    That depends on the output of the station and the type of the station.
    A nuclear station requires house loads on order of ten percent of it's net output. But ti should be able to provide many of those from it's emergency generators if the regulator approves the use of them in the black start scenario.

    What would normally happen is all the circuit breakers connecting the transmission system to the distribution system would be opened, and then staitons with very low house loads would be started up to provide power to start other plants up.
    Once a large number of stations are ready the distribution systems can be reconnected stepwise.

    An obvious choice for black start is a hydro scheme, because a station service generator can potentially be started by having someone open a sluice gate with a hand crank.

    This process is obviously much less difficult and much faster if some partso f the grid remain intact, which is why some modern nuclear stations are designed to suddenly reject full power to the condenser directly, maintaining only enough flow through the steam generator to keep the plant's house loads operational.
    This means the station can stand ready to restore the grid as required.

    As the fuel costs of nuclear are very small and sometimes negligible, wasting power output for a few hours is a small price to pay for being able to rapidly restore service. Especially as many freshly shut down nuclear reactors are required by xenon poison out to remain shut down for up to three days if they cannot restart immediately.
     
  26. hwl

    hwl Established Member

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    On DC one of the big effects is that the voltage rises and older substations could cope with the higher max voltage than needs to be allowed to make DC regen sensible in addition to the protection / fault issues.
     
  27. tomuk

    tomuk Member

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    Most of the CEGB coal stations eg Ironbridge B, Rugeley B, etc had backup diesel and gas turbine generators to allow for black start. Rugeley for example had two 25MW Rolls Royce Olympus GT generators
     
  28. edwin_m

    edwin_m Veteran Member

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    That sounds plausible. I think there were also some concerns about a train regenerating while incorrectly running into a section that was isolated and earthed for maintenance, and feeding large amounts of current through the earthing straps or the person who was about to attach them. However I would have thought a unit bridging into the section while its tail end was still on the juice would create the same situation with no regeneration involved.
     
  29. Bletchleyite

    Bletchleyite Veteran Member

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    I don't think there are any UK 25kV EMUs that have two pantographs that would both be in use at the same time and a bus line between them. A third rail EMU could, though.
     
  30. marko2

    marko2 Member

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    There's a huge difference between lopping load-peaks intra-day and providing providing an alternative source of supply for calm winter days when PV solar and Wind. Pump storage schemes operate for a matter of hours at name-place capacity before the upper reservoir becomes empty. The latter problem is really only solved through geographic diversity of supply if it's renewable- otherwise it's gas turbine plant running only when necessary - and incurring the same maintenance costs to be amortised over less output.
     

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