High voltage transmission

Discussion in 'General Electronics Chat' started by cheddy, Nov 24, 2007.

  1. cheddy

    Thread Starter Active Member

    Oct 19, 2007
    87
    0
    I am reading about Transformers and have a question about the specifics of reducing loss in a transmission line.

    The book says because engineers cant do much about the wire resistance or the power consumed by the loads they can adjust the voltage and thereby the current using P=EI. If the voltage was increased by a factor of 10 then the current could be reduced by a factor of 10 resulting in reduced loss.

    How is it possible to have for example

    100W = 10 V * 10 A

    and

    100W = 100V * 1 A

    If the resistance is the same in both? Doesn't that violate Ohms Law?
     
  2. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    Ohm's law says nothing about power. Can you explain why you believe it violates Ohm's law?
     
  3. cheddy

    Thread Starter Active Member

    Oct 19, 2007
    87
    0
    I thought you could only have 10 Amps with 10 Volts and 1 Ohm?

    How can you have 1 Amp with 100 Volts if the resistance is still 1 Ohm?
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    It's a formula based on real world observation.

    P=V*I This doesn't care what the resistance is it just tells you what the power is *after* you've gotten an answer for voltage and current.

    If P=1000 finding all values of V and I to solve the equation would be near infinite.

    If P=1000 and V=0.1 I=10,000
    if P=1000 and V=1 i=1000
    if P=1000 and V=3.1415929 i=318.310

    Simple algebra.
     
  5. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    The losses are due to P = I^2*R losses - you reduce the current by a factor of 10 the power loss reduces by the same fator for a constant TM line resistance, R.

    If you are wondering where P = I^2*R comes from, take P = VI and factor in Ohm's Law V= IR.

    Please note I use V where you use E.

    Dave
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I think you meant to say that if you reduce the current by a factor of 10, the power loss reduces by a factor of 100. ;)


     
  7. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    I have just got in after a night out (its 00.52 here!) ;) :D

    Yes you are correct - the power reduces by 10^2 because of the I^2.

    Dave
     
  8. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    yes that wud have violated ohm's law, but that is not what the case is.
    u never connect a 10V device to a 100 volts device.
    all u need is 100 watt power an if u are taking it from 100 volts outlet u'll design it for
    drawing 1A current.

    now suppose about 500MW power is to be transmitted thru a 100 ohms resistance of transmission what do u think is better idea:
    to transmit it at 10000V or 100V thru the transmission line.
    (it can will be stepped to lower voltage later).
    the secondary will determine how much power is going to be consumed based on which
    the amount of current in primary is decided and since the voltage ratio is higher on primary side(in distribution transformer) the current for same power is low.

    ur confusion will be removed if u transfer all the resistance from secondary to primary side.
    or u can transfer all resistance(transmission resistance) to secondary side u'll see the transmission line resistance gets reduced by a factor of (transformation ratio)^2
    as seen by secondary.
     
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