What size breaker to avoid tripping on inrush

[Dumbo2929]

So my client wants to back-feed a 1000kVA transformer (13.8kV-480V) from the 480V side, to re-power a piece of equipment (connected off the 13.8kV side), and has asked what size circuit breaker would be needed to not trip on inrush. That transformer is about 1200A full load at 480V, and the ANSI inrush point is approx. 10-12X at 0.1 sec. I figure a 1000-1200A circuit breaker (with 10X instantaneous) would work, or a fuse of about 800A would clear the inrush point. Note that the load being fed is very small (<50kW) for a temporary installation. Anyone have any thoughts on these breaker sizes?

 

[dpc]

Maybe some help here: thread238-189353

When energizing from 480 V, the system impedance will tend to limit the inrush, but the duration of inrush can be longer and you'll definitely see the voltage dip.

 

[jraef]

Also read this thread; thread238-205768 for some other issues.

My suggestion would be to invest in a good breaker with an Electronic Trip Unit so that you have some better flexibility on the short time and instantaneous trip functions.

 

[Bronzeado]

Hi folks,

As your installation is temporary, my suggestion is to use a "series resistance" that can be bypassed by a switch after tranformer energizing.

This resistance will reduce the magnitude and duration of the inrush current. Its size could be calculated to reduce current to values so you can use the same circuit breaker you are using now. Beware the resistance heating.

Please, send us the results of this experience, if you go ahead.

Regards,

H. Bronzeado