ATS circuit breaker tripping

[living2learn]

The breaker downstream of the ATS trips. It trips when I go from Generator to Utility. It is a fast break before make, 400A, 480V ASCO ATS that transfers in 51-31msec with a current surge on the C phase of 4100A's. The downstream loads are only transformers from the 400A breaker feeding the ATS: (1) 225kVA [271 FLA] (2) 112.5kVA [136 FLA](1) 45kVA [54.2 FLA](1) 30kVA[36.1 FLA]
So, if you take 10x to 12x the FLA of all of them you get 6333A’s-7600A’s inrush for .1 seconds. and my breaker can only go up to 4000A's, but not sure if this surge current is from that or from the fast transition - help in understanding this is appreciated.

The transfer switch I believe allows a larger transfer time of 1 second - would that help? Would it help if the generator had more load on it? This is the last switch out of 12 to transfer. Is this current surge from the fast transition time or is it from the inrush to the transformers (asked already)? I might be able to install a time delay between the largest transformer via a large contactor with a timer.

The obvious solution would be to increase the breaker frame size to accomadate this surge (unfortunatly I inherited this mess and the engineer before me assured them this would work . A closed transition ATS would also work, but would have to evaluate fault currents for downstream equipment. Any other solutions.

 

[living2learn]

would improper grounding of the generator cause this? Meaning a 3 pole switch with both neutrals tied to ground. I don't think so only GFI problems.

 

[rcw retired EE]

If the ASCO switch does not have an in-phase monitor it can fast transfer from generator to normal power out of phase, causing high inrush currents. A neutral/off position delay might help, but I don't recall the ASCO switches having that feature.

 

[dpc]

What size and type is the breaker that is tripping? I'd crank up the instantaneous trips to maximum, although I suspect you've already done that.

 

[living2learn]

You are correct it is maxed out! with a 400AF/400AT set at 10x instantaneous...
It does have an in phase monitor and I am going to double check the settings and put it at 5 degrees at 1.5 seconds - thoughts? The in phase monitor should really eliminate the high surge currents correct?

 

[waross]

It may be that the phase monitor is incorrectly wired for that breaker. It may have reversed polarity or be connected to the wrong phase and be transferring with a 180 deg. or 120 deg error. Almost certain to trip the breaker.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[living2learn]

Thnanks, I am investigating that now and it only trips 1 out of 6 times.

 

[dpc]

Sounds like transformer inrush.

 

[living2learn]

Agreed. I guess 51 milli seconds is enough time to have to re-energize the transformer causeing high inrush currents...