I must start with some background. We have a client who has a 480/277 Service into a 1000A Breaker. Neutral/Ground bond is located here via a busbar. The ground is a ground ring of unknown size and distance. There is also a supplemental ground wire (#6) attached to a ground rod. The service is then routed through an Cutler Hammer ATS. The service is then routed to a distribution panel. The panel has 2 600A 480/277 breakers. It also has various other "Utility" breakers that feed other distribution panels that go to HVAC units and lights/receptacles through another transformer. The 2 600A breakers go as follows. 1 goes to UPS as Mains 1 and the other goes as Mains 2. I am not sure how the Piller UPS manages these two Mains but there is also a MOTS bypass. This is simply a manual transfer switch to bypass the UPS. Now for the problem. The UPS just runs the power through it until it sees a problem. Now, as mentioned before the entire bldg is through an ATS. This ATS is connected to an 800kW Diesel generator with a Marathon DVR2000 Voltage Regulator. When this generator turns on either by power fail or exercise the UPS automatically triggers itself by some interconnect relay to close an internal switch that switches all power to the rectifiers. Now even though the ATS is connected to the utility company the UPS is rectifying power. (maybe in anticipation of transfer, the UPS tech cannot convey why this is) Now when the ATS transfers to ER Power (GenSet) the power is rectified from the Generator. This is raising havoc on our generator. We cannot stabilize the voltage. The current appears to stay the same...around 360 and the Freq is stable at 59.7. However the voltage is floating between 420 and 490. This causes the UPS to switch periodically to batteries. The "powers that be" recommended that we put filtering on the Voltage Regulator to filter and stabilize the voltage. We installed MOVs to clamp the 3 phase sensing wires on the VR and a 3% Line Reactor on the sensing wires. This, as I thought, had no effect. The VR seems to be doing what it is supposed to. We have about 175kW of coming out of the UPS and the GenSet is supplying about 260kW. So this means that about 85kW is being used for the Utility. This means that 67% of the load is PURE Reactive through the UPS rectifiers. We have paralleled a Resistive load bank with the UPS at 250kW and were able to stabilize the voltage. This tells me that we need to add some resistive load to this unit. HOW? Oh yeah...the UPS tech says that his machine must maintain a SEPERATE neutral/ground bond. That makes (2) Two complete neutral ground bonds on the 480/277 buss. Test Results from Previous power monitoring Voltage Harmonics THD=2.17% H1=276 H3=5.01 H5=0 H7=0 Current Harmonics THD=7.8% H1=86.95 H3=0 H5=3.86 H7=2.86 Also at the time when the Static Switch on the UPS triggers the rectifiers this is the power Before static switch Apparent Power 19500 VA Real Power 19130 W Reactive Power (Ind) 0 VAR Reactive Power (Cap) 3734 VAR 1 Min Later Gen Running withRectifiers Apparent Power 23960 VA Real Power 19700 W Reactive Power (Ind) 13640 VAR Reactive Power (Cap) 0 VAR How can we stabilize the voltage? IF more info is needed just ask.