Transformer Sizing 2

[Energyfit24381]

Good Evening,

Im assisting our senior engineering group on designing a substation that will be powered by engine/gen sets. Im hitting a wall on designing the switchgear and associated transformers. Whats a reliable way to size my switchgear based on the generator data? And whats a reliable way to size the step up transformers? The alternators are:

13.8kV
4900 Kva (which is around 205 amps if im converting right)

I will have (10) of these on a bus so i need at least a 3000 amp bus (including auxiliaries). From here i will be feeding a step up transformer to get to 34.5kV. How do i size a transformer for this application? Also, what should my bus kAIC rating be rated and how do i calculate?

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-

 

[wcaseyharman]

Is the facility connected to a larger grid at all?
If not, the fault current will be solely due to the gens. You should be able use the subtransient generator reactance to calculate fault current, the total of which will be 10 times (1/xd”) in per unit (that’s assuming gen is unloaded, it’ll be a little when loaded, so I’d probably do 1.1/xd” for margin)

If connected to the grid you will need to know the grid available fault current, which will of course add to the generator fault current.

Typically in the power stations I work at the transformer is at the same or higher kVA than the sum of the generators. That’s a reasonable starting assumption.

 

[Energyfit24381]

Thanks wcasey! That confirms alot and yes we will be connecting to the grid. They have not responded on fault current just yet.

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-

 

[bacon4life]

The grid contribution will be limited by the impedance of the transformer. I have not seen a transformer in that size range smaller than 7% impedance.

If the plant has a low load factor, sizing the transformer ONAF2 rating to match the total generation might be appropriate. If the plant has a very high load factor, it may make sense to size the transformer significantly larger, perhaps matching the ONAN rating.

 

[Energyfit24381]

@wcasey using your formula above if the subtransient reactance(xd”) is 0.140 p.u. then the formula is 10(1/.140)
(1/.140) = 7.14
10(7.14) = 71.4

Would this number need to be multiplied by the rated current to get fault value?

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-