In the opposite way on station dc system which one point of battery is grounded, the generator dc exciting system is isolated from ground. Therefore when a ground occurs within the field winding, the rotor body will be energized to a potential dependent on the location of the winding failure. Because the field circuit is normally ungrounded, this condition is not damaging. The concern is that a second ground will occur. The second ground will bypass a portion of the field winding and unbalance the air-gap flux. The unbalanced flux will produce vibration. The degree of unbalance and the resulting vibration depends on the location and extent of the winding bypassed. The vibration can be severe enough to cause massive damage.
Mason reports a bent rotor, broken bearing pedestals, and rotor–stator contact. On the other hand, the resulting vibration may be undetectable.
A double ground that does not significantly unbalance the air gap can still result in damaging vibration. The second failure can cause a substantial current in the rotor forging and unbalance winding currents. The local heating that results can cause rotor distortion that produces damaging vibration. This mode of failure would be characterized by a slowly increasing vibration, beginning 30 minutes to 2 hours after the second ground.
The first ground will provide a reference voltage on the rotor body. As operation continues, the field insulation will be stressed against this reference. System switching and faults induce transient voltages in the rotor can add to the reference voltage to produce additional voltage stress on the insulation. In short, the first ground increases the probability of the second ground.
Reference: Protective relaying for power generation system (DONALD REIMERT)