Auto-Transformer Magnetizing Current

[majesus]

Auto-Transformer Magnetizing Current

For 3phase Auto-transformers used in Step-up/down voltage application, is the magnetizing inrush current approximately the same as a two winding 3phase transformer?


I use the following guidelines for inrush current on two winding XFMRs:

2 X Ipri @ 100 seconds Cold-Load Inrush
3x Ipri @ 10 seconds Cold-Load Inrush
10 x Ipri@ 1 second Cold-Load Inrush
12 x Ipri @ .1 second Magnetizing Inrush
25 x Ipri @ .01 second Magnetizing Inrush


Source: from Copper Power SYstem, Protective equipment Update (APRIL 1992)

http://www.cooperpower.com/library/pdf/91027.pdf

Is the magnetizing inrush @ .1 and 0.01s the same on a 3phase Auto-XFMR?

Cheers,
Majesus

 

[dpc]

Yes, I believe it is basically a function of the transformer core size - so I base inrush assumptions on an equivalent 2-winding transformer rating.

BTW, the cold load pickup guidelines above are useful for setting overcurrent protection, but these are a function of the load on the transformer and not related to the transformer itself. You will have cold load pickup on a feeder, even without a transformer.

David Castor

http://www.cvoes.com

 

[majesus]

Thanks DPC,

I was after the magnetizing current info, but glad to have the insight

Cheers,
Majesus