- #1
lavalin
- 6
- 0
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?
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?