100% stator Earth Fault protection:
Stator is a stationary winding of generator. It is a heart of the generator. To protect the stator, we have many protections such as differential relay, percentage differential relay, restricted earth fault relay, standby earth fault relay, etc. But they are primary protection for the generator, if they failures to operate, we need a backup protection. Therefore 100% stator earth fault protection work as back up protection of the alternator.
Principle of 100% stator Earth Fault protection:
Most Commonly the generator neutral is earthen with the NGR Neutral grounding resistor. Here during normal condition, The current through the NGR will normally be 5 – 10 A (depending upon rating and resistance value) when subjected to the rated phase to earth voltage. Here the generators with step-up transformer, a neutral point voltage relay (27TN/59N) with typical setting 5 % of generator phase voltage will provide earth-fault protection for 95 % of the stator winding.
The above setting covers the generator/alternator’s bus duct, the low-voltage winding of the step-up transformer associated with the generator and other interconnection with the stator winding. If the earth-fault nearer to the neutral point of the generator will not give sufficient neutral point voltage to activate the neutral point voltage (95 %) relay. Therefore, additional protection methods are used to provide a 100% stator ground fault protection. Special protection systems based on the third harmonic analysis and on the subharmonic voltage injection can detect stator ground faults close to the neutral. These protection methods are strongly recommended for large generators since the entire stator winding must be protected.
Remaining 5% Stator Earth fault protection:
Normally, the Generator produces about 1 % or more third harmonic voltage under all service conditions, can have the entire stator winding, including the neutral point, protected using a scheme which combines the neutral point voltage function (95 % relay) and a third harmonic under voltage function. Under normal condition, typically 40 – 60 % of the generated third harmonic phase voltage will appear across the neutral point resistor and will activate the third harmonic voltage relay. If an earth-fault occurs close to the neutral point, the third harmonic voltage drops to a low value and the under voltage relay operates. Hence the remaining position of the generator windings are being covered.
Note: The relay must have a filter which prevents the fundamental frequency neutral point voltage from affecting the third harmonic voltage function. Therefore, an effective filter must make the third harmonic relay practically insensitive to fundamental frequency voltages.
Ref: ABB for Fair Use
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