3phase double circuit trasmission line

Discussion in 'Homework Help' started by Jess_88, Mar 30, 2012.

  1. Jess_88

    Thread Starter Member

    Apr 29, 2011
    Hey guys :)

    I'm trying to work out this question, using an equivalent Pi circuit, but that fact that it is a "double circuit" is confusing me.

    Here is the background.
    A 3-Phase, 50Hz, 330kV, 360MW at 0.82 PF is supplied by:
    -330kV, 50Hz, Length (L) = 160km, 3-Phase double circuit.
    Each circuit has per-phase
    Resistance (R) = 120x10^-6 per-unit/km
    Reactance (X) = 990x10^-6 per-unit/km
    Shunt Susceptance to Neutral (jB) = 461.7x10^-3 per-unit/km
    Neglecting Shunt Conductance.

    All units have been given in Vbase = 330kV, Sb = 100MVA

    So what I think so far:
    Because its a balanced and double
    -both transmission line circuits will have the same voltage and current
    -Thus we can calculate perimeters of just one line
    But -The receiving current will be multiplied by two

    Can I do these things?
    Dose this effect the values used for each line impedance and the Shunt Susceptance?

    With the Pi network
    Z(line) = L x(R + jB)

    Do I need to convert this from phase resistance :confused:

    I understand I may also need to determine the load impedance and add this to the line impedance for the equivalent circuit.?

    What do you guys think?

    thanks heaps :)
    Last edited: Mar 30, 2012
  2. Jess_88

    Thread Starter Member

    Apr 29, 2011
    ok, so I have a little bit more direction i think.
    theta = cos^(-1)(PF)
    Ir(line-toline) = Pr/[(sqr(3)*Vr(line-to-line)*PF]

    Because it is a medium length
    A=D=(1 + (YZ)/2)--- p.u
    C = Y(1+(YZ)/4) ---S
    AD -BC = 1
    Vs = AVr + BIr
    Is = CVr + DIr

    From my calculations, AD - BC = 1 seem to hold true. So i'm pretty sure that part is right.
    But when I use
    Is = CVr + DIr, I seem to get 85.86<89.702 (kA)
    which seems way to big.

    Am a suposed to do anything because it is a double connection?
  3. Jess_88

    Thread Starter Member

    Apr 29, 2011
    Can anyone confirm weather the equations I'm using for this circuit are correct?