Transformer short circuit fault calculation clarification

[lavalin]

Hi,

I'm trying to perform a short circuit fault calculation on this transformer and load I have when a 3 phase bolted short (worse case) occurs.

The transformer specs are below:
Z(transformer)=5%, Z(cable)= .00306 + j.0027, Delta/Star 6.6kV/400V

I can understand calculating the short circuit available current at the load:
Ifault(load) = Vphase/Z(transformer)
where Z(transformer) is the impedance of one winding, and the voltage across it would be 400/1.73 ~=230V
Ifault(load) = 230/.05
= 4.6kA

Now calculating the fault down at the load is where I get a little confused:
The example I'm following states that the load current fault for a 3 phase bolted short is:
Ifault(load) = Vphase/(Z(transformer) + Z(cable))

My only guess/understanding of why they use Vphase is that Vphase represents the voltage drop from the neutral (mid point) of the WYE transformer to the end of 1 phase? I've drawn a diagram to help clarify what I mean, its hosted at:
http://img83.imageshack.us/my.php?image=61884112bb8.jpg

Can someone please explain to me why they use Vphase in the load fault calculation?

 

[m.s.j]

If = V_ph / Z_tr(ohm)

Z_tr(ohm) = %Z / Z_b

Z_b = V²/S

%Z : Transformer per unit impedance
V : Transformer Rated voltage (V)
S : Transformer Rated power (VA)


--------------------------------------
Creative thinking is enjoyable, Then think about your surrounding things and other thought products. http://electrical-riddles.com

 

[lavalin]

Thanks for your reply. My question was however clarification on why they use phase voltage when they're calculating at the load end, total fault current that the transformer can deliver.

 

[Enthalpy]

Hi! I would say:

They use the phase voltage because this fits with the winding impedance, and the load is in star connection as well. This gives you the current in a secondary winding.

Within this logic, you could equally well compute everything with line voltages and equivalent delta connections. It would be consistent, but less direct and less usable.