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  1. #1
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    Well regulated mains to large cities

    How is the mains so stable in large cities like London?....surely it is because there are thousands of micro generators all over London, with regulating tap changers on their transformers?...otherwise there would be horrendous load regulation problems leading to low mains voltages.

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    Re: Well regulated mains to large cities

    I think it is on a much bigger scale.
    In my area they switch whole power stations on and off to keep the mains steady. We have a 50MW hydro electric generator about 30Km away and another 498MW generator about 70Km away but there is a 'reserve' of six 300MW hydro turbines which can be switched in individually at short notice on demand. They are clever, they use spare power from the grid during off-peak times to pump water from a low lake to a higher one then run it back through the turbines to the low lake to add capacity to the grid. See: https://www.youtube.com/watch?v=McByJeX2evM for an explanation. If you are ever in this area it is a "must see" and they will take you on a bus tour inside the mountain!

    Load requirements are calculated using complex algorithm and importantly a TV program schedule so the capacity can be increased during commercial breaks and at the end of big sports events when lots of people boil electric kettles! Sounds unrealistic but it is true, I've been in the network control stations and seen how they predict the demand and get ready for it. Don't forget that generally, the mains voltage doesn't drop much but the load on the generators slows them down and drops the mains frequency. To ensure synchronous motors and counters, in particularly in clocks, keep good time, they target a number of cycles per day (~43,2000,000 for 50Hz) and go over-frequency sometime to compensate for lost cycles.

    I'm probably wrong but I do not know of any 'tap changers' used in mains regulation in the UK.

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    Re: Well regulated mains to large cities

    In my area they switch whole power stations on and off to keep the mains steady...
    True but the power to your home comes from a local (relatively) transformer that is perhaps a 3-phase to 3-phase one supplying perhaps 10-20 nearby houses.

    Once our local distribution transformer went bad and we (about 100 houses) had no power for over 7 days.

    Our local university has a HV power connection and the load drops at night and the voltage goes up. During the day, the voltage fluctuates a lot. But I guess our network is badly planned.

    By the way,
    number of cycles per day (~43,2000,000 for 50Hz)
    , isn't that too much?


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    Re: Well regulated mains to large cities

    4,320,000.00



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    Re: Well regulated mains to large cities

    Sorry about the typo - it is indeed 4,320,000 cycles per day.

    I (in the UK) have never come across a regulator device in the mains supply, the local transformer serves about 100 properties but it is just a "3-phase in/3 phase & Neutral out" transformer with no control gear at all. I'm not sure what it's power rating is but an underground cable fault put it out of action for a week a couple of years ago and a 250KW Diesel generator was temporarily wired in its place to keep us going. The incoming three wires carry 11KV based on the information tag on the transformer itself and the outgoing voltage is a fairly stable 250V RMS, slightly higher than it is supposed to be but it is stable.

    Brian.
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    Re: Well regulated mains to large cities

    Hi,

    Here in Germany we have some HV distribution plants 3 phase 20kV usually. They do the voltage regulation on the HV side.
    Afaik there is no voltage regulation on the relatively small distribution transformers 20kV --> 400V.
    I think they are in the range from 50kVA to 1MVA.

    It seems 20kV wiring, transformers and 400V wiring is fairly stable...thus regulation of 20kV is sufficient.

    With my measurement device I can see that the voltage steps of the regulation are about 3V on the 230V (P-N) lines.

    Klaus
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    Re: Well regulated mains to large cities

    Thanks Klaus, especially for your great help on this subject some time ago aswell.
    I wonder when we have people charging electric cars all over the place we are going to see mains voltages plummeting...because i suspect that in UK etc we dont regulate the 20kV side....apart from in the way that Betwixt kindly describes.

    My old pal used to be a radiographer (take mediacl x-rays) in the 1970’s, and he describes how the mobile x-ray units (used on various hospital wards) had to be of capacitor discharge type due to the unreliability of the 1970’s uk mains voltage…..In the 1970’s, most power supplies in bits of kit were either inefficient transformers or linear regulators or both…and so the current draw on the uk mains generally was far more than it is today, and load regulation problems on UK mains was a big issue…with horrendous mains dips…..then SMPS came in and this problem disappeared……..but with car chargers coming in, I fear this problem is about to return.

    Thanks Betwixt, thats very interesting, I don’t mean to boast, but my father worked on the commissioning of Dinorwig power station so I am familiar with it to an extent. My father then worked as an inspector of UK companies supplying heavy electrical plant to the then UK run power network of UK (Central electricity generating board, CEGB). My father explained to me that the UK power network was sold overseas (now run by France) because small UK supplier companies would hugely rip-off the CEGB in selling it their plant…and the whole thing became unaffordable.
    My father described him going to inspect plant that simply didn’t work properly…when he pointed this out, and tried to stop or delay paying them their ridiculously high fees, he would sometimes find his car vandalised outside in their company car park.
    Last edited by treez; 8th September 2019 at 10:39.



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    Re: Well regulated mains to large cities

    Voltage regulation in Finland seems to be similar to German electric network. Voltage regulation information is not so easy to find, but google found several finnish language student thesis and I collected following information from those. Base distribution network is at 110kV level and local electric stations transform 110kV to 20kV or to 10kV. 20kV is used with overhead lines and 10kV is typically used in cable network. 110kV voltage regulation requirement, when average rms voltage is monitored in 10 minutes intervals:
    105-123 kV 95% of the time
    100-123 kV 100% of the time

    Typical measurement period is one week. This is what local electric (sub)station gets. There is on-load tap changer in the 110kV/20 kV or 110kV/10kV trafo and it is where local voltage regulation is done. There is no regulation done in 20kV/0,4kV (or in 10kV/0,4kV) trafo. Typical on-load tap changer has ± 9 x 1,67 % taps. This is for 20 kVsecondary, but we can assume similar functionality for 10kV secondary voltage.

    How many households or x/400V transformers are covered with one tap changer? I calculated average ballpark number for Helsinki. There are 23 "110kV electrical stations" (at least few years back) in Helsinki. If assuming, that there is one 110kV trafo per station and 660000 citizens in Helsinki, about 30000 citizen are covered with one tap changer.

    Helsinki utility company tend to use 31,5 MVA and 40 MVA transformers. If typical power rating of 10kV/0,4kV transformer is 500 kVA or 1 MVA, we can say that tens of those transformers are behind one tap changer. Following image shows (nominally) 230V rms at my home. Values are 1 minute averages. It is possible to see, that voltages goes higher late evening, and voltage is stepped lower. On the morning, voltage is stepped higher.

    Click image for larger version. 

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    This shows one of the tap changes, when rms values are calculated with 2 seconds interval.
    Click image for larger version. 

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