If it is DC supply, relays emergency lighting inside the sub stations, breakers for tripping and closing etc. may be connected. If an emergency AC backup is provided then some critical equipments of the plant may also be supplied as per the requirment of the process.
It all depend upon the instant of synchronising the generator with the mains. Your set may be subjected to severe mechanical stresses if it is out of phase synchronised with mains. I personaly have seen a 1000 KVA generator breaking into two parts in one of the plant.
