It's been a while since I used per-unit calculation so I am embarrassing to say that I am drawing a blank when I ran into this. I requested the utility for available fault current at the transformer I was give this value.

250MVA transformer (138kv to 38kV), Z=11% with 100MVA. They told me to use infinite bus for my calculation.

So, I just need to find the new Z for 250MVA transformer right? Is this formula correct to find new Z?

Z (new) = Z (old) x ( S (base)/S (new) ) = 11 x ( 100/250 ) = 4.4%

Is this right? It seem Z is too low for this size transformer. I guess I can go back to my old power system book and verify but I thought asking here would be easier and more accurate. Thanks,

250MVA transformer (138kv to 38kV), Z=11% with 100MVA. They told me to use infinite bus for my calculation.

So, I just need to find the new Z for 250MVA transformer right? Is this formula correct to find new Z?

Z (new) = Z (old) x ( S (base)/S (new) ) = 11 x ( 100/250 ) = 4.4%

Is this right? It seem Z is too low for this size transformer. I guess I can go back to my old power system book and verify but I thought asking here would be easier and more accurate. Thanks,

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