Sounds like you have a system with the units operating in both frequency and voltage droop. VR3 AVR's and VR6 AVR's usually don't play well together in a cross current compensation circuit. Are the VR6's a retrofit or an original AVR?
Let's back up a bit, I need some more details. What engine models and what kind of governing? Newer engines will have their governing integral with the ECM (Engine Control Module), older engines will have either Woodward or CAT hydramechanical governors or Woodward electronic governors. Does the kW load share work as desired or do you have issues with it?
On the VAR sharing, do all of the units have droop CT's installed? On the VR6 equipped machines you have the option to use the original CAT nominal 1 amp CT (looks like a bar type CT instead of a donut or window) or a 5 amp nominal CT, which type do you have? On the VR3 equipped machine if it has a droop CT it will be the old style CAT CT, the VR3 does not work correctly with a "normal" type CT.
As noted above the VR3 is an obsolete AVR and frankly I change them out whenever I get the chance. The VR6 is still a production item and is actually a Basler AVC63-12 regulator, the CAT version is only available in 240 VAC sensing.
If you are satisfied with the operation of the kW load share and only wish to address the VAR share and voltage control, I think your best solution would be as follows,
Replace the VR3 AVR with a VR6 (or Basler AVC 63-12).
Install 5 amp nominal CT's on all the units, based on the info provided the rated current is about 720 amps, use 800:5 CT's installed on the B phase.
Set all AVR voltage droop pots the same, I would start at about 30% ot the potentiometer, It's likely you don't have a resistive/reactive load bank so this will be a good starting point. Also make sure you set the stability pots the same, I usually start at 50%.
Use the Basler instructions and wire the droop CT's in a cross current compensation circuit. (diagram on page 16 of this document available from Basler
please take the time to read the whole thing, I think you'll find it very helpful, also try this,
This is the Basler AVC63-12 manual, it also has a cross current description and schematic and I find it better than the CAT manual for the VR6).
I have done a large number of smaller systems, two to five generator sets paralleling using a Woodward kW load share line system like the 2301A and the LSM (Load Share Module)use and the AVR's connected in cross current.
A note on cross current, it works best when the units are the same size or well matched proportionally, and the same type AVR with similar droop and stability settings. When installed and setup correctly I find the usually VAR share to less than 5% and voltage droop is very low, usually no more than 2%. Systems trying to use CAT 1 amp CT's usually don't work worth a darn, some people get them to work for awhile, but I don't find them reliable. Also some folks feel you can do a cross current system with no voltage droop. You can make it work but I find them not very stable. Also if you go to a cross current system, keep people from messing with the voltage adjust pots. On the last system I did (and several before) the pots on the front of the panel are dummies. If you install and adjust the system correctly, the voltage will stay pretty close, and your VAR share will stay good. Biggest mistake is someone tries to maintian the system using panel meters that are not accurate. Set everything up with one good quality DMM, record the settings and let it do it's job, I have systems I have setup over 10 years ago that have stayed working well.
If you find that you desire a better form of control, and/or want to improve you kW share as well, there are several options. I lean towards controls made by Woodward, but there are others as well. More details about your system and what you would like to achieve can provide you with better recommendations.
Hope that helps,
Mike L.
