If in a balanced star load the Line voltage = root 3 x Phase voltage, then why when calculating power in a Power system does the Phase power P = Vphase x current x power factor (p.f) but the total power of the 3 lines = root 3 x Vline x current x power factor (p.f).

I can understand that the power lines may have came from a delta wound transformer, but I still can't understand why the total power of the 3 lines would equal root 3 times the line voltage?

Edited: 04 June 2012 at 11:48 PM by lamusio1

Hi lamusino1
I am not sure if I understand your question but let me give you ask you, did u understand the difference between Symetrical component and 3-phase system if No then try to understand the difference between them and that will clear your doubt

Hi thanks for the reply. Yes I understand symmetrical component however it still doesn't explain the problem.

Sorry perhaps my initial question isn't very clear. Here's a better example.

Q) An electricity supply system developes an earth fault at a feeder cable connected to a 6.6KV busbar. The apparent power at the fault point is 26.1MVA.
Determine the current at the fault point.

Now to do this I believe you would use: Fault current = (Fault MVA) / (root 3 x V Line). Substituting in the number this would be: 26.1(x10^6) / [(root 3) x 6.6(x10^3)] = 2283 Amps.

The part I'm getting confused at is, if 6.6KV is the Line Voltage at the feeder cables why would you multiply the line voltage by root 3 when it is not a phase voltage?

The power in each phase is phase volts x current x power factor.
Therefore the power in a three phase system = 3 x phase volts x current x power factor
But line volts = root 3 x phase volts
Therefore
The power in a three phase system = root 3 x line volts x current x power factor

Best wishes
John

Thank you John, much appreciated.