Clarification on Transformers - High voltage transmission

Discussion in 'Homework Help' started by bobby19, Aug 18, 2007.

  1. bobby19

    Thread Starter Member

    Jun 3, 2007
    I seem to understand transformers very well, but one aspect of stepping up voltage for transmission is confusing me. I'm sure it is a very small oversight on my part.

    In an ordinary electric circuit, when the voltage is increased, keeping the load resistance the same, the amount of current increases as well. Why is it that when we step up voltage for transmission, we are not also increasing the amount of current and thus the (i^2)(R) power loss?

  2. recca02

    Senior Member

    Apr 2, 2007
    u r correct when u say voltage increase leads to current increase,
    that is what happens even in case of transformer ohm's law applies.

    what happens is the low voltage side sees this load as much more greater
    (it sees the resistance as R/SQ(transformation ratio) )
    so an even greater current flows in the low voltage side.

    to get the idea even clear remember that transformer is a constant power device it does not add to the power. the power always equals voltsXamperes on both sides.

    for a constant voltage applied at one side there is a constant voltage at other side the current is decided on the resistance (or load) connected which is transferred from one side to other with the transformation ratio. (the source side or primary side sees the load on sec side with transformation ratio sq)

    the va ratings of transformer are power that they can safely handle without damage to windings and cores. whatever power is taken by load on secondary side has to be supplied by the primary(source) ao the power = voltsXamperes remains same on both sides (of course there are losses as well).

    edit: there is other way to explain this ,that wud require delving into the theory of transformer on load and transformer on no load.
    remember that current is dependent on the load as well as voltage.
    a load on secondary side is seen as a much greater load on source side(in this case) and hence a larger current flows on the low voltage side to mantain a constant flux in the transformer core.
  3. techroomt

    Senior Member

    May 19, 2004
    i think the idea of stepping up voltage for transmission/distribution is as the voltage is stepped up, the current is stepped down. therefor applying a lower current to the I squared R power loss for the conductors carrying the power. then, at the receiveing end the voltage is stepped back down and the current back up
  4. recca02

    Senior Member

    Apr 2, 2007
    my previous reply was to explain how current on low voltage side is larger even with lower voltage.
    now for a given power/load to be delivered thru transmission wires that are several of hundred Km long the sq(I)*R drop shud be eliminated hence it is a
    gud idea to reduce the I.(as MR techroomt stated) .remember for a given power if voltage is increased current decreases.

    let me give you an example.

    say u have a requirement to pressurize water which requires u a power of 3MW

    u have a supply of some voltage based on V*I u will design a motor to handle I amount of current. the larger the V the smaller the I for same power req.

    this power is to be supplied by a power plant from 300km away which has wires of some resistance for transmission. now when the I current starts flowing thru this resistance they will dissipate I^2*R heat which is a total waste. since u cannot reduce the resistance u will like to reduce the current which is an even better idea.
    but since you can not have a motor built for handling very large voltages(insulation becomes a problem + other practical limitation and distribution is not done at high voltages) so we can isolate the transmission side and keep as high voltage side by the use of transformer which serves the purpose of keeping the voltage high on transmission side and low on load side.
    does that help?
  5. cb83


    Aug 9, 2007
    The voltage is stepped up for transmission and then stepped down again for use, so there are two transformers. The current when transmitted is not the same as the current in the load. The main voltage drop in the transmission circuit is across the coils which passes the energy (not current) to the secondary of the step down transformer. The larger the coil the larger the impedance. The primary coil of the step down transformer will have high impedance which limits the current which causes less voltage drop accross the transmission line.

    When using power calculations you have to take into account the impedance of the whole circuit, not just the resistance of the transmission line. The voltage drop accross the t-line is very small and the voltage drop accross the primary coil of the step down transformer is very large.