FEM: How to calculate impedance in stator circuit for loaded generator
If you know the regulator equation with a transfer function of field current to output voltage and can model the effective series resistance , you can assume a starting point, knowing that no load is unstable by using 10% pre-load locally. Then switchover from stand-by to unknown load when system is in sync with grid. If no grid, then when up to speed switchover from standby load to external load. 

Perhaps you can model the behavior for various initial loads and choose better assumptions for crossover speed with a known internal pre-load on startup, then you understand how to avoid overshoot with dynamic loads and passive loads. You also know that matching source impedance to load will help optimize power transfer.