Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Total load using step-up transformer

  1. Aug 12, 2010 #1
    Hi all.
    I'm hoping someone can help me out on a (for me) tricky little issue.
    We are to use a communication module that we know will draw 5amp and must be supplied via a UPS. Tricky bit is that its 3.5km away so we need to step up the voltage at source and step down again at load. 400Vac is to be stepped up to 1000Vac (50Hz) and back down again at load. And now to the question, how do I calculate the load on the UPS with this setup?
    Grateful for help.
  2. jcsd
  3. Aug 12, 2010 #2


    User Avatar
    Science Advisor

    You would need to know how efficient the transformers are and the power factor of the load.

    Each transformer will need more power going into it than is coming out due to it not being 100 % efficient.

    So, you add up these power losses and add it to the input power of the whole setup.

    Then the UPS has to supply this power.
  4. Aug 12, 2010 #3
    in above add energy loss in 3.5km conductors, which will depend on load current. Transformers is a very efficient electrical machine with efficiency ~98% if operated near full load
  5. Aug 20, 2010 #4
    Thanks very much for your replies. All this ends up well above my head so I was hoping you could give me rough idea of what my load on UPS would be with the following setup.

    Step-up Tx: 230Vac to 1000Vac, rated 5kVA >98% efficiency.
    ~3000m of aluminium cable with 70mm2 cores.
    Step-down Tx: 1000Vac to 230Vac, rated 5kVA.
    Load of 5amp at pf~0.97

    I only really need to confirm that our UPS is man for the job, before installing this setup we still have about 5000W left (and still leaving 20% of full capacity spare).
    So question is really, will this draw more than 5000W (roughly) and definitely no more than 8000W.

    I'd be very grateful for your information gents.

  6. Aug 20, 2010 #5
    Aluminium has resistivity of 28.2 nΩ·m.
    So, L = 3000, A = 70 mm2 will have total resistance of just R = 1.2Ω. Even without voltage stepping only 5A will flow. For a 5 A ampere current the loss is just around 25 watts and voltage drop is a mere 6 volts; quite small for such a length.
    So, I don't see the need of voltage stepping at all if you are going to use 70 mm2 aluminium wire.

    If you are anyway going to use voltage stepping, then I don't see the need to use 70 mm2 wire.
    Last edited: Aug 20, 2010
  7. Aug 20, 2010 #6
    --Sorry, multiple posts. Deleted--
    -- See above post ---
    Last edited: Aug 20, 2010
  8. Aug 20, 2010 #7


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    It may need to be pointed out that, essentially, the load on the UPS (in VA) will be about the same as the VA used at the other end, irrespective of the step up and step down. There is only the issue of inefficiency and line resistance to take into account.
    The point of the step up step down is that the transmitted current will be less. In this case, the the transmitted current is reduced by a factor of 1/(voltage ratio) compared with the current taken by the remote equipment. Presumably someone did a sum, using the known resistance of the transmission cable, which indicated the need to transmit at 2.5 times the supply volts to reduce the effect of series resistance loss in the cable.
    Without knowing the actual gauge of the transmission cable, you can't calculate the actual losses but you may be able to rely on the fact that the high voltage transmission system will be 'adequate' so that you can work on the assumption that Power Out = Power In and then give yourself a 20% overhead. In any case, it's not a heavy load (only 2kW) so a UPS to cover a bit of margin shouldn't cost an arm and a leg.
    Mightn't it be better to put the UPS at the far end, in any case? (If it's at all practicable) The supply line could well be the most vulnerable part of the power supply chain.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook