Alternator,when connected to the grid, it is preferred to operate it in lagging Power Factor mode and in lagging power factor mode it delivers Reactive power and Active power to the grid and the rotor speed is same as that of grid frequency, which is 3000 RPM as per 50 Hz in India.Loss of excitation will cause loss of synchronism between rotor field & stator field as in any AC rotating machine the condition for the rotor rotation is that number of field poles in both Rotor and Stator should be same. In case of Alternator also both stator and rotor fields rotate being magnetically locked with each other, but there are certain situations when the Alternator operates in Leading Power Factor mode and in that case it draws Reactive power from the grid. This results in the flow of currents in the rotor body and severe torque oscillations in the rotor shaft. So there is chance that in the Rotor Overheating may occur and the rotor may fall out of synchronism. Sometimes loss of excitation may be a problem with the excitation system also or it may be due to the unwanted tripping of the filed breaker, the field breaker may trip due to the loss of dc supply to the control circuit.
Protection Scheme for "Loss of Excitation" in Generator
There is also another reason for the loss of excitation that the machine may be absorbing Reactive Power from the grid for a longer time(operating in Leading power factor mode for a long time) so in that case the field current for the machine is very low(Below Minimum Field Current Limit) as the Alternator is Under Excited and there is always a limit for the Minimum Rotor Current in the Capability Curve of the Alternator. Because below a certain level of field current the machine will fall out of synchronism. For that purpose a Mho relay is used to trip the machine in the case of Under Excitation under Loss of Excitation Protection.
