TRANSFORMER SIZE

[NAZ55]

I have following information on 120/240V loads

MOTOR STARTING + CONTINUOUS LOADS
Single phase – 120 kVA
Three phase – 51 kVA

MOTOR RUNNING + CONTINUOUS LOADS
Single phase – 104 kVA
Three phase – 12 kVA

AFTER 10 SEC (CONTINUOUS LOADS ONLY) KVA
Single phase – 97 kVA
Three phase – 6 kVA

I would like to size a transformer, which could handle this load. Assuming a unity power factor, which transformer size do you think is appropriate?

I am leaning towards a 300 kVA delta secondary padmount transformer.
Could possibly use a 225 kVA. Any suggestions? ( Primary is a 24kVY)

 

[davidbeach]

Watch out for single pad mount transformers. They will probably have a very low single phase rating. Your 300kVA transformer might be good for 50 or 60 kVA of single phase load. If you want lots of single phase and a little bit of 3-phase you're better off with a bank of single phase transformers or converting your 240V loads to 208V and going with a wye secondary.

 

[waross]

The maximum single phase loading on a three transformer bank is 2/3 of the total rating, or twice the rating of the lowest rated transformer assuming that the PU impedances and X/R ratios are equal.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[NAZ55]

Bill,

That was my understanding that the mid tapped winding will carry 2/3 of the single phase load (2/3*120 =80kVA) and 1/3rd of the three phase (1/3*51 = 17kVA).
The other two windinging will take 1/3rd of both single (1/3*120 = 40kVA) and three phase loading (17kVA). However, now I am a little confused from Davidbeach's comment that a 300kVA will only serve a 50 or 60 kVA single phase load. David, where did you come up with this number?
I have not worked with padmount transformers before, but think that it would on the same principle as a bank of three single phase equal impedance transformers. Is that not correct?

 

[davidbeach]

What I remember from catalogs. Could have changed, may be transformers designed for higher single phase loading. Best bet is to verify with the manufacturer.