Oh Boy! Another {electrical} pissing contest!!
The problem has bee around for at least 20 years, but everyone hasn’t had to deal with it at the same time. The simplified problem is that the genset exciter wants to see a nice clean sinusoidal load, and the UPS wants to see an very stiff source, and so they end up fighting each other. It just continues as finger pointing until someone caves, and the end user {and/or electrical contractor, depending on contract verbiage} often get the very short end of the stick.
A delta-wye drive-isolation transformer or 3ø reactor between the two may help—like those used on AF drives, but don’t bet on it. The UPS and genset vendor susually insit that it’s not their problel, and it never happened before, but they LIE! Ususally the CSI-type specidfications don’t handle this very well, and reapozilbiblues need to be worked out before the problem occurs.
Good luck. Keep us posted.
