Transformer Inrush and Frequency of Breaker Closures

[HenryOhm]

All,

Could anyone suggest any industry standards or otherwise as to the maximum energizations per time frame one should limit oneself to with transformer inrush? Will be doing some testing soon involving four identical 2800kVA three-winding transformers, 600:375:375VAC, with multiple breaker close and open cycles.

Obviously want to be cautious with the equipment. These transformers do have RTD's installed, though they were added many years after initial installation. Am concerned about hammering on the xfmrs without sufficient time for RTD to "see" ambient buildup. Or perhaps it is more mechanical stress rather than heat buildup that would be of concern?

Thanks for any and all help!

 

[JJHorak]

I have not heard of a rating/standard on frequent inrush limitations, but a couple tidbits that come to mind:
--Maybe you could reason through heating effects using some guesstimated average I^2R, keeping in mind that inrush I^2R is only in the primary. Then compare to rated continuous I^2R.
--I would think mechanical issues would be at more risk of creating problems than thermal issues. The inrush currents puts mechanical forces on the windings.
--If you are energizing that often, you may have breaker limitations.

 

[NorthSlope]

I agree, mechanical stress is much more of a concern than thermal overload. But transformers are designed and built to live through energisation inrush. If the down stream equipment can stand the energisation - de-energisation the transformer should be the least of your problems