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

    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?

  2. DGElder


    Apr 3, 2016
    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