Generator Stator Earth Fault Relay (64G) - 27TN
For high impedance grounded generators, a voltage relay across the secondary winding of the generator neutral grounding transformer is used to detect the ground fault on the generator stator winding.
The neutral over voltage function (59N) responds to the fundamental frequency voltage at the generator neutral that appears when a ground fault occurs.
Normally, it will protect up to 90≈ 95 % of the stator windings.
The neutral under voltage functions responds to 3rd harmonic voltage measured at the generator neutral. When used in conjunction with the neutral over voltage function, it provides 100% ground fault protection of the stator winding.
In healthy state, the machine 3rd harmonic voltage is distributed over the windings such that the maximum occur at the star point neutral and at the line terminal. For a ground fault close to the neutral, all the 3rd harmonic voltage is transferred to the line terminals. Consequently, the relay function will see a 3rd harmonic under voltage at the neutral end and will operate the protection element.