Unbalanced reactive power sharing of one Alternator on common bus of 3 Alternators

CRA
At an Oil and Gas supplier offshore platform, we are facing one problem with our GE alternator which was recently came back after rotor rewinding.

At our plant we have 3 alternators of same rating of 3500 KVA at 0.8 pf lag and same capacity, all are running by Gas Turbines. all are equipped with DECS-200 AVR at a droop setting of 4%. All 3 alternators are on the same bus which is having Power management system. Recently one of the Alternator went to repair Rotor winding and we reinstalled the same now. The values of IR and WR of all PMG winding, exciter main and field windings, main field winding and stator winding are normal and matching to original values. presently remaining 2 Alternators are sharing total load of 2800 KW load at 0.83 lag pf equally under the control of PMS. We started the 3rd alternator and found rated voltage at no load. after we synchronised this Alternator to common BUS with local control (not under PMS control) and increased the active load by 50 KW step wise. until 200 KW of load the Alternator worked fine. but for the above 200 KW of active load the power factor of this Alternator started to swinging slowly from lagging pf to leading pf up to 0.4 lead pf. and after some time it is settling to lead pf by drawing reactive power like from 90 KVAr to 340KVAr. the remaining Alternator are on the same bus but under PMS control are working fine.

We suspected the problem with AVR. so we interchanged the AVR from other Alternator and found the AVR is healthy. but the problem still persists.
what may be the reason for this behaviour?

Any solutions or explanations as to why this is the case would be appreciated.

Thanks
 
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