Three phase

Discussion in 'Homework Help' started by IState21, Sep 20, 2010.

  1. IState21

    Thread Starter New Member

    Sep 20, 2010
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    The problem I am working on is:
    A 480 V, star connected, three phase ac generator supplies power to a delta connected load through a transmission line. The per phase impedeances of the load and of the transmission line are (16+j20) and (1+j20) respectively. Determine:
    (a) The line current
    (b) The magnitude of the line to line voltage across the load
    (c) The efficiency of the transmission

    For a I have I=480/(1+j20)=1.197-j23.94
    but should I instead add both impedances together and divide 480 by them?
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Yes, to find the line current you have to consider the total load plus line impedance - not just the line impedance. You may find it simpler to refer everything back to a 'per phase' representation. In that case calculate the generator line-to-neutral voltage and apply this to the line impedance plus the delta load impedance converted to its star equivalent. Remember the total impedance is a complex number addition. The subsequent complex division of the line-to-neutral voltage by the total line + load impedance will give you the line current.

    The rest is complex number manipulation.

    %Efficiency=(Real Load Power)/(Real Generator Power) expressed as a percentage.
     
  3. IState21

    Thread Starter New Member

    Sep 20, 2010
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    Thanks for the advice,

    What I did was:
    Vln=480/sqrt(3) <-30 = 277.13 <-30°
    Zy=ZΔ/3= (16+j20)/3 = 5.3+ j6.67 <== Not sure why it needs to be converted to ZΔ

    Ztotal= (5.3+j6.67)+(1+j20)= 6.3+j26.67

    Iline= (277.13 <-30°)/(6.3+j26.67) = (240-j138.57)/(6.3+j26.67) = -2.9 - j9.69

    This doesn't seem right to me though
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Your answers look fine.

    So how would you have otherwise determined the total load seen by the generator?
     
  5. IState21

    Thread Starter New Member

    Sep 20, 2010
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    Ahh makes sense, does the work that I showed look like I'm on the right track?
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Yes - it looks fine by me.
     
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