UPS Nuisantly Transfers to Bypass 5

[nightfox1925]

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 B) 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?

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.

 

[Pikoplatt]

Nightfox, are you still having the UPS problem with transfer to Bypass when energizing the rectifier? Can you check the manual on the Static switch to determine what the threshold limts are for transfer to Bypass? These can be a low inverter output voltage threshold (Undervoltage), high Inverter output voltage (Overvoltage). There may also be Inverter output frequency limits and Inverter output current limits, but I am not sure of the last two parameters (Frequecy & Current). You can energize the rectifier, as suggested earlier in this thread, with the Bypass supply isolated, and determine Inverter output Current peaks (as already done), Voltage and Frequency excursion limits. You can then compare these with the transfer thresholds of the Static Switch to see which threshold (Voltage, Current or Frequency) is being violated.
You can also test the static switch to ensure that it transfers accurately at its set threshold limits. If the static switch is operating correctly at its preset threshold limits, then we are back to limiting inrush current on energising the rectifier.

 

[nightfox1925]

For update on this topic.

Staticon decided to internally modify the existing rectifiers by changing the transformers, line reactors and providing DC choke and some kind of an internal circuitry which will somehow soft-start the input transformer.

The resulting transformer in-rush was even less than the transformer FLA. It was good news but eventually we paid a lot in extra because of it.