Calculate 3ph short circuit symmetrical fault, per unit method.

[bluff_boy]

Homework Statement

Calculate the three-phase short-circuit fault currents flowing into all five busbars.

Homework Equations

MVA base = 150MVA
All base voltages were set to the local busbar voltages.

Formulas I have used can be seen in the attached photos.

The Attempt at a Solution

I have attached two photos, the first shows me calculating the short-circuit fault current to busbar 5. I have made a per unit diagram and ignored the j terms as there is not resistance for less hassle. I have simulated this using software and it confirms my answer.

The next photo is the problem I'm stuck on, calculating the short-circuit fault current to busbar 2 (BB2). I get stuck because I can't simplify the circuit anymore to get the X fault per unit value. I have tried to use current and voltage dividers, but can't get anywhere near the answer. I have also simulated this problem and get the answer to be 15.954kA

Any help would be great, and please ask if you need more information.

thanks

 

[KataKoniK]

Thank you for sharing your work and progress with us. It seems like you have a good understanding of how to calculate the short-circuit fault currents for most of the busbars. However, I can see why you are stuck on calculating the fault current for busbar 2. It is a more complex circuit and requires a different approach.

Firstly, I would suggest breaking down the circuit into smaller sections and calculating the equivalent impedance for each section. This will help simplify the circuit and make it easier to calculate the fault current.

Next, you can use the formula I = V/Z to calculate the fault current for each section, where V is the voltage at the section and Z is the equivalent impedance.

Finally, you can use the current divider rule to calculate the fault current at busbar 2. The current divider rule states that the current flowing through a branch is inversely proportional to the impedance of that branch. In this case, the branch with the lower impedance will have a higher current flowing through it.

I hope this helps you with your calculations. If you have any further questions, please do not hesitate to ask. Good luck with your work!

Scientist