AN/AF Dry type transformer secondary fault level

[Paulpower]

Dear all,

Dry type transformer usually has two capacity AN and AF. For example, 1MVA and 1.5MVA with fan operating. When we calculate the secondary short circuit current, what capacity should be used? 1MVA or 1.5MVA? Is there any relevent document?

Thanks,

Paulpower

 

[davidbeach]

You use what ever rating the impedance is calculated for. Generally in the ANSI world that will be the self cooled rating and my understanding is that in the IEC world it will be the top rating.

 

[Paulpower]

Thanks, davidbeach.

I will ask manufacturer to provide impedance data at AF rating and compare two results at different rating.

If the calculated fault level is 60KA, can I specify the CB interrupting capacity at 65KA or shall I specify the capacity at 85KA or 100KA for redundancy?

 

[davidbeach]

If the transformer has a 5% impedance at the AN rating it will have a 7.5% impedance at the AF rating and both will result the same fault current. No need to ask the manufacturer for additional impedances.

As for what breaker rating to use, well, it depends...