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Homework Help: Calculating power absorbed in transmission lines

  1. Aug 26, 2015 #1
    1. The problem statement, all variables and given/known data
    Calculate the power (Pso) absorbed from the supply by the transmission line on open circuit.

    Given parameters are:

    A1= 0.8698
    A2= 0.03542
    B1= 47.94Ω
    B2= 180.8Ω
    C1= 0S
    C2= 0.001349S
    D1= 0.8698
    D2= 0.03542

    I have calculated Vs and Is earlier in the question

    Vs= 77325 + j3149V
    Is= j119.93A

    2. Relevant equations

    3. The attempt at a solution

    I don't know where to start with this one to be honest and haven't got any real direction from my course notes. Can i use equation P=I^2*R? I suspect it's more complicated than that!

    Attached Files:

  2. jcsd
  3. Aug 26, 2015 #2


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    Education Advisor
    Gold Member

    Ok, you clearly have not given enough here to solve the problem. What do these constants A1 etc. refer to? They don't appear on your diagram.

    A transmission line can act like a big antenna. That means it has inductance and capacitance. And it can radiate power, and have current flowing even though there is no completed circuit. If you have not covered these things in class, and your text does not describe how to deal with it, you need to go to your instructor and get help.

    There should be a formula that tells you how to calculate this. It will include such things as the frequency of the applied voltage.
  4. Aug 26, 2015 #3
    Thanks DEvans,

    Sorry, i should have included this information on the original post.

    I have been given the open-circuit received voltage of 88.9kV and the only formulas i have for this question are:

    Vs= Vr (A1+jA2) + Ir (B1+jB2)
    Is= Vr (C1 + jC2) +Ir (D1 + jD2)

    'S' and 'R' are for sending end and receiving end. These are what i used to calculate Vs and Is earlier in the question, although i used zero as 'Ir' due to it being open circuit (thinking no current could flow) but from what you've said above i may need to look at those answers again.

    With regards A1 etc. i have no further information, this is all that is given on the assignment. Looks like i'll have to contact the tutor for this module.
  5. Aug 26, 2015 #4

    rude man

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    Homework Helper
    Gold Member

    I believe this problem has appeared earlier on PF homework help.

    @DEvens, these are the ABCD 2-port parameters for the xmsn line.

    If the OP got the right Vs and Is (post 1) then computing the power is straightforward.
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