Ups Switching Over To Bypass Nuisance

[pgb1925]

I am presently working on a problem with one of the UPS systems in the plant. There are two Single UPS systems (480-208/120) feeding two UPS buses (A and separately. The two separate bus circuits feeds a couple of 208VAC-24Vdc rectifiers which in turn individually feed as redundant power supplies to downstream PLCs, SIS, etc.

 

Of the 24Vdc rectifiers, the 24Vdc, 400A rectifier is the largest. The UPS systems always shifts to bypass everytime the UPS output MCCB is closed or this Rectifier is energized. The UPS alarm shows "Output Overload". We suspect that it was the downstream in-rush which is causing the UPS to nuisantly transfer. An idea by one of the contractor was to put a reactor on the input of the rectifier. A 5% impedance reactor was connected in series but still the UPS keep to transfering to bypass.

 

It is observed that the UPS output transformer secondary is wye and the downstream rectifier isolation transformer primary is delta...is it possible that the problem is also caused by phase mismatch between the UPS and downstream charger?

 

is there a possibility that the UPS settings may be too sensitive? is this caused by In-rush currents?

 

I am gathering all the data so I can post them here soon. But any help or guidance will be very much appreciated. Thank you. I am presently working on a problem with one of the UPS systems in the plant. There are two Single UPS systems (480-208/120) feeding two UPS buses (A and separately. The two separate bus circuits feeds a couple of 208VAC-24Vdc rectifiers which in turn individually feed as redundant power supplies to downstream PLCs, SIS, etc.

 

Of the 24Vdc rectifiers, the 24Vdc, 400A rectifier is the largest. The UPS systems always shifts to bypass everytime the UPS output MCCB is closed or this Rectifier is energized. The UPS alarm shows "Output Overload". We suspect that it was the downstream in-rush which is causing the UPS to nuisantly transfer. An idea by one of the contractor was to put a reactor on the input of the rectifier. A 5% impedance reactor was connected in series but still the UPS keep to transfering to bypass. Everytime the staticon charger is energized, the UPS transfers to bypass and indicates an "output overload" and after the staticon charger is started, it shifts back to the inverter. The concern also is that if there is a plant wide shutdown, then the UPS may not transfer to the batteries everytime the rectifier is energized since it sees a fault in its output.

 

It is observed that the UPS output transformer secondary is wye and the downstream rectifier isolation transformer primary is delta...is it possible that the problem is also caused by phase mismatch between the UPS and downstream charger?

 

is there a possibility that the UPS settings may be too sensitive?

 

The contractor's consultant also suspected that the downstream charger is in resonance with the UPS meaning, the UPS sees a zero impedance and erroneously see as a fault and was suggesting phase shifting isolation transformer between the UPS output and the downstream charger input to correct the phase shift mismatch. I do not see anything wrong of connecting a delta connected load from a wye source.

 

I am gathering all the data so I can post them here soon. But any help or guidance will be very much appreciated. Thank you.

 

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