1) Check individual DG on load, make sure P (KW) value on all phases is positive. If found negative on any phase, reverse the CT polarities.

2) Crosscheck the Nominal Frequency settings in controller, must be similar in all Sync controllers.

3) Check the speed regulation of the DG which is tripping due to reverse power. Refer section 5.2.2 ‘Load picture after synchronising’ for slip frequency settings.

DESIGNER'S HANDBOOK (azureedge.net) 

4) In case reverse power occurs during initial KW sharing, adjust Load Ramp Up settings and ‘P Kp’ value