Loss of field or excitation can be caused in the generator due to excitation failure. In larger sized generator, energy for excitation is often taken from a separate auxiliary source or from a separately driven DC generator. The failure of auxiliary supply or failure of driving motor can also cause the loss of excitation in a generator. Failure of excitation that is failure of field system in the generator makes the generator run at a speed above the synchronous speed.
In that situation the generator or alternator becomes an induction generator which draws magnetizing current from the system. Although this situation does not create any problem in the system immediately but over loading of the stator and overheating of the rotor due to continuous operation of the machine in this mode may create problems in the system in long-run. Therefore special care should be taken for rectifying the field or excitation system of the generator immediately after failure of that system. The generator should be isolated from rest of the system till the field system is properly restored.
There are mainly two schemes available for protection against loss of field or excitation of a generator. In 1st scheme, we use an undercurrent relay connected in shunt with main field winding circuit. This relay will operate if the excitation current comes below its predetermined value. If the relay is to operate for complete loss of field along, it must have a setting lies well below the minimum excitation current value which can be 8 % of the rated full load current. Again when loss of field occurs due to failure of exciter but not due to problem in the field circuit (field circuit remains intact) there will be an induced current at slip frequency in the field circuit. This situation makes the relay to pick up and drop off as per slip frequency of the induced current in the field. This problem can be overcome in the following manner. 
In this case a setting of 5 % of normal of full load current is recommended. There is a normally closed contact attached with the undercurrent relay. This normally closed contact remains open as the relay coil is energized by shunted excitation current during normal operation of the excitation system. As soon as there is any failure of excitation system, the relay coil becomes de-energized and the normally closed contact closes the supply across the coil of timing relay T1.
As the relay coil is energized, the normally open contact of this relay T 1 is closed. This contact closes the supply across another timing relay T 2 with an adjustable pickup time delay of 2 to 10 seconds. Relay T 1 is time delayed on drop off to stabilize scheme again slip frequency effect. Relay T 2 closes its contacts after the prescribed time delay to either shut down the set or initiate an alarm. It is time delayed on pickup to prevent spurious operation of the scheme during an external fault.
For larger generator or alternator, we use a more sophisticated scheme for that purpose. For larger machines, it is recommended to trip the machine after a certain prescribed delay in presence of swing condition resulting from loss of field. In addition to that there must be subsequent load shedding to maintain stability of the system. In this scheme of protection, an automatic imposition of load shedding to the system is also inherently required if the field is not restored within the described time delay. The scheme comprises an offset mho relay, and an instantaneous under voltage relay. As we have said earlier that it is not always required to isolate the generator immediately in the event of loss of field, unless there is a significant disturb in system stability.
We know that system voltage is the main indication of system stability. Therefore, the offset mho relay is arranged to shut the machine down instantaneously when operation of generator is accompanied by a system voltage collapse. The drop in system voltage is detected by an under voltage relay which is set to approximately 70 % of normal rated system voltage. The offset mho relay is arranged to initiate load shedding to the system up to a safe value and then to initiate a master tripping relay after a predetermined time.