Loss of Field

To say shortly,Loss of field is disturbed terminal voltage of generator which in turn affects the stability of the system.for more details go through this discussion. It will make you clear. http://www.control.com/thread/1276103698

When a generator is synchronised to a bus and if during its operation, the generator's excitation system fails, then this generator would draw the reactive power (excitation) from the bus and would deliver active power to the bus (sort of an induction generator). This condition is called loss of field. It is quite dangerous, as the stator current could become 200% of rating, rotor heating, etc.