I have an AC microgrid model consisting of one grid-feeding and two grid-forming units. The control system includes droop control, secondary control, and synchronization loop for smooth connection to the grid. The microgrid works properly in islanded mode and can be synchronized successfully to the grid but when the tertiary control is enabled to manage the power flow, it causes a significant drop in PCC voltage amplitude that causes improper power sharing (the grid active and reactive power don't match the specified reference values). I tried different values of gains for the PI controllers but the problem still exists.

I applied the control loop as illustrated in the attached picture. Are there any adjustments or modifications I should consider?