You will need to provide some info to get a better answer. In general it sounds like you have some imbalance as pointed out in the other response, by listing each phase currnet and the neutral current you will liklely see the neutral current when the UPS is not connected as load imbalance.
When the UPS is connected you may be seeing harmonic current, you may be able to determine that if you have a good quality DMM and can get a frequency reading off the current, but likely you will need a power quality analyzer or recorder to properly display and document what you have on you system.
What does your voltage do when the UPS is brought on, does it rise or fall from the nominal value that it maintains when only the plant loads without the UPS are on? This could also be an indication of harmonics. Does the UPS have power correction capacitors installed? Have you investigated the UPS while it is on the utility source?
Again, better describe your system and the information you are getting, like measured values if you want better answers, otherwise you can only get speculation from what info you have provided so far.
Mike L.
United States / Canada
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