Stator 100% E/F Protection

See:Earth resistance

see this:

http://cr4.globalspec.com/thread/19822/100-stattor-earth-fault-protection

Any generator generates not only the fundamental frequency voltage, but also all Harmonic voltages, due to slot harmonics. Amongst all the harmonics generated, the property of triplen (odd multiples of 3) harmonic currents is to accumulate in the neutral (as they are zero sequence, in nature). Now, any system live lines have got a leakage capacitance to earth. The tripen harmonic currents would easily flow through these leakage capacitive reactances (as capacitive reactance is less for higher frequency currents, as that of harmonic currents) back to the generator neutral, thro the neutral-earth impedance. So, there would always be a third harmonic current flowing thro the neutral-earth impedance and thus there would always be a third harmonic voltage drop across the neutral-earth impedance. This is under healthy conditions, as well as if the earth fault is away from the neutral, within the generator winding. But, if the earth fault is close to the neutral, say, within 5% of the neutral, then the impedance offered by the faulted winding path is lesser than the path offered by the earthing impeadnce. Thus, there would not be any triplen harmonic current flow thro the earthing impedance.Thus, there would be third-harmonic undervoltage across the earthing impedance. This undervoltage is monitored to indicate a close-to-neutral earth fault, particularly in impedance earthed generators.