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Homework Help: 3 phase, star connected system

  1. Nov 14, 2012 #1
    1. Statement

    A star connected load of 3300 kVA at 18 kV and a power factor of 0,8 lagging is supplied by a three-phase transmission line having a resistance of 4Ω per phase and an inductive reactance of 7Ω per phase.

    a. the sending-end voltage
    b. the percentage regulation
    c. the efficiency of the line

    2. The attempt at a solution

    I've attached my attempt at this.
    i'm confused on what ''sending-end voltage'' is. also do i need to workout the impedance for this.

    Attached Files:

  2. jcsd
  3. Nov 15, 2012 #2
    There is a voltage drop on the transmission line due to its impedance. That means the 'sending voltage' (the voltage source attached at the other end of the transmission line) is different than the voltage at the load.
  4. Nov 15, 2012 #3
    So then, because it's star-connected, do I use the line voltage or the phase voltage to start with? Using VL = [itex]\sqrt{3}[/itex] × Vph, but I can only do that when IL = Iph?
  5. Nov 15, 2012 #4
    For (a), first i took the resistance and reactance to get impedance: Zphase = 4 + j7 = 8,062 |60,26° Ω

    then i use the rating: S = I × V (not sure if i should use VLine or Vphase)

    Iphase = [itex]\frac{3300 kVA}{18 kV / \sqrt{3}}[/itex] = 317,543 A or ILine = [itex]\frac{3300 kVA}{18 kV}[/itex] = 183,333 A

    then to find voltage drop: E = I × Z (which current do i use?)
    Last edited: Nov 15, 2012
  6. Nov 15, 2012 #5
    Heres my full attempt attached, just struggling on c. whether to use rated power or the true power.

    Attached Files:

  7. Nov 15, 2012 #6
    I'd use phase voltages and phase currents to find the voltage drop in a single supply line, then use that to find the phase voltage at the supply end.

    It has been a very long time since I have done anything with power so it would be better if someone else came along to check your work. I'd need to do a review first.
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