I'd take the opposite view to Power0020 - switching a heavy reactive load onto a lightly loaded generator during a black start is inviting a major voltage excursion.
You'll find that the AVR will boost the field as it tries to recover the severe transient depression. It may hit the over-excitation limiter which in some installations is an instantaneous trip and in others is a delayed trip. You will quite possibly see the AVR then swing onto the under-excitation limiter as it tries to suppress the resultant overshoot. Your AVR topology will make a big difference to how the machine behaves: a 2-quadrant (regenerative) rectifier connected via sliprings can actively suppress the field by feeding the excess energy back into the source, but a brushless machine can't do this and has to rely on the resistance of the rotor to dissipate the stored energy over a time determined by the rotor R-L time constant, during which time the UEL may well be operational. Normally this isn't a trip, just a warning that the machine is operating near the stability limit, which isn't the best place to be especially if you are going to be adding large blocks of load.
Can you arrange the system transformer to be softly energised at the same time as the GSU transformer? It may require modification to switching interlocks, and you probably don't have synchronising capability other than at the GSU transformer breakers but in a blackout this probably doesn't matter too much.
