# Electrical Engineering: Calculate the fault level for a 3Ø fault

1. Sep 18, 2012

### sayeen

1. The problem statement, all variables and given/known data
Calculate the fault level for a 3Ø fault at F
With bus section CB closed
With bus section CB open

2. Relevant equations
The circuit diagram can be found here
http://s1167.photobucket.com/albums/q629/sayeen1/?action=view&current=fault_level.png

3. The attempt at a solution
The solution of this question is provided as follows

Sbase = 0.5 MVA

(Xfault)Source = [Sbase/(Sfault)] = 0.5/250 = 0.002 p.u

Zbase = [(Vbase)2/(Sbase)] = [(11kV)2/(0.5 MVA)] = 242 Ω

Rcable = [(Rcable)Ω/(Zbase)] = 0.252/242 = 0.001 p.u

Xcable = [(Xcable)Ω/(Zbase)] = 0.0817/242 = 0.00034 p.u

With bus section closed:
*This is the part that i dont understand, how did they get "j0.0273"?*

(Xfault)Total = 0.001 + j0.0273 → (Xfault)Total = 0.0273 p.u → Sfault = 0.5 MVA / 0.0273 p.u = 18.3 MVA

With bus section open:
*This is the part that i also dont understand, how did they get "j0.0523"?*

(Xfault)Total = 0.001 + j0.0523 → (Xfault)Total = 0.0523 p.u → Sfault = 0.5 MVA / 0.0523 p.u = 9.56 MVA