POWER SYSTEMS STUDY - TWO TRANSFORMER RATINGS (AA/FA)

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PE (always learning)

PE (always learning)

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Professional Engineer
Hey everyone,

I'm doing a power systems study for a data center and the substation transformer has two KVA ratings based on the cooling system being used. When using ambient air for cooling the transformer is rated at 750 kVA and has and impedance of 5.99%. When using forced air for cooling, the transformer has a rating of 1000 kVA. I'm assuming it would make sense to use the worst case scenario of 1000 kVA for my study, but does the impedance stay the same when using this higher kVA rating? The transformer nameplate only lists the impedance for the 750 kVA rating.

Best Regards
 
Thanks, that's kind of what I was figuring, just wanted to clarify because the transformer nameplate lists the transformers impedance at 750 kVA. Just curious, but when doing a power systems study, do you use both transformer impedances for each cooling scenario or do you just stick with the nominal kVA?
 
You should always input the Base kVA, the Base kV, and the Base impedance. The software will make the appropriate adjustments if you decide to use the fan cooled rating.
 
As mentioned by Jim, if you are using software such as SKM, you put in the base ratings and tell the software it is an AA/FA transformer, and it will apply the multiplier to get you to 1MVA when needed.

If you are working the calcs by hands, generally you use the base ratings for impedance related stuff, but you can use the FA rating when it makes sense like to determine adequate ratings for the transformer.
 
The % impedance will change with the kVA rating. In other words, the actual impedance and fault current stays the same. Some nameplates will list the impedance for the different load ratings but there is really no point.

The higher rating is for increased heat dissipation so you can handle higher continuous loads. It does not change the actual impedance. If the impedance stays the same, then the % impedance will have to change as you change the reference rating. Higher kVA means a lower % impedance and vice-versa.
 
ron said:
As mentioned by Jim, if you are using software such as SKM, you put in the base ratings and tell the software it is an AA/FA transformer, and it will apply the multiplier to get you to 1MVA when needed.

If you are working the calcs by hands, generally you use the base ratings for impedance related stuff, but you can use the FA rating when it makes sense like to determine adequate ratings for the transformer.
Click to expand...
Alternately, you could just use the FA, FOA, etc rating with the adjusted impedance. Makes no real difference as long as you plan to have the fans and/or pumps operational.
 
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