Per unit system - Calculating I from Power and Voltage.

Discussion in 'Homework Help' started by EEE123, May 18, 2016.

  1. EEE123

    Thread Starter New Member

    May 16, 2016
    2
    0
    Hi,

    I'm struggling with calculating the current from per unit voltage and power values. I can do it one way but get a different result when doing it another way. Here are the values I'm using:

    This is for 3 phase.

    VAbase = 1000kVA
    VAload = 750kVA
    Voltage base = 415V
    Line voltage = 415V
    Power factor = 0.96

    I need to find the per unit load current. The way I've done it first to get the correct answer is:



    Find the load current = I = (VAload)/((root3)*(Line voltage)) = (750kVA)/(root3)*415 = 1043.4

    Find base current = (1000kVA)/(root3)*415 = 1391.2

    Divide load current by base current = 0.75 p.u.

    Calculate angle from power factor = 16.26 degrees.
    So load current = (0.75<16.26) p.u. which is correct.

    However, using the load VA and the line voltage with the equation S=(root3)VI, with a line voltage as the reference V=1<0 p.u.,

    I = (S)/((root3)V)) = (0.75<16.26)/(root3)(1<0) = (0.433<16.26) p.u. which is wrong.

    The only thing I can think of is that you're not meant to divide by (root3) in the above equation, but surely you have to because its 3 phase?

    Thanks
     
  2. DGElder

    Member

    Apr 3, 2016
    344
    85
    Is this a Y-delta circuit, what's the topology?

    I don't know if you have supplied enough info to solve this unambiguously. I haven't solved these 3 phase problems in 40 years- so it might be me, but I am sure I can find the solution if you supply a drawing of the configuration that makes the problem statement clearer. It might become clearer to you as well.

    But taking a stab at some problems, in the second equation your load current should be base current/root(3) - if this is a Y-delta configuration. You assumed the base and load currents were the same - not true in Y-D configuration. Or your first base current equation is wrong because you multiplied V by root(3) which you don't do for Y config. Draw it out and show the currents and voltages - at least qualitatively.
     
    Last edited: May 18, 2016
Loading...