# 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
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
347
87
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