Originally Posted by

**elec_eng**
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,

Infinite bus just means that the amount of fault current through the transformer will only be limited by the transformer impedance itself. It appears what the utility gave you is the impedance of the system either on a 100MVA base or they are saying 100MVAsc; can't tell from the post. However, they are telling you to base your design on an infinite bus.

Thus a 250MVA transformer with impedance of say 8.5% (based on HV KVBIL of 650 and LV KVBIL of 150) will have an MVAsc = 250MVA/0.085 = 2941MVAsc

@ 38kV that is around 45kAsc.

However, those numbers may present a problem for 38kV rated equipment. 45KA is out of the range of 38kV gear either vacuum, dead tank or SF6. the limit is 40KA, also the maximum current rating is going to be 3000A but that require fan cooling.

To limit the current on infinite bus system, the maximum amount of fault current is going to need to be around 38kA (a little margin) thus you need an impedance on the transformer of around 10 to 10.5%. But that doesn't solve the issue of max continuous current which would limit the transformer MVA to around 165MVA or even less depending on manufacturer.

**"Just because you're paranoid, doesn't mean they're not out to get you"**

## Bookmarks