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Generator Protection - Stator Earth Fault Protection

Stator Earth Fault Protection

In most countries, it is a common practice to ground the generator neutral through a Grounding Transformer having a loading resistor across its secondary. This method of earthing is called high impedance earthing where the earth fault current is limited to 5-10 Amps. Tuned reactors which limit the ground fault current to less than 1.0 A are also used.

The generator grounding resistor normally limits the neutral voltage transmitted from the high voltage side of the unit transformer in case of a ground fault on the H.V. side to maximum 2-3% of rated generator phase voltage.


Short circuits between the stator winding in the slots and the stator core are the most common electrical fault in generators. Interturn faults, which normally are difficult to detect, will quickly develop into a ground fault and will be tripped by the stator ground fault protection.

95% stator ground fault relay for generator

For generators with unit transformer and with high impedance grounding of the neutral, a neutral voltage relay with harmonic immunity and independent time delay is used. The relay is normally set to operate at 5% of maximum neutral voltage with a time delay of 0.3-0.5 second. With this voltage setting, it protects approximately 95% of the stator winding.

It also covers the generator bus, the low voltage winding of the unit transformer and the high voltage winding of the unit aux. transformer.

Relay details: 64 A/B- Neutral displacement relay having IDMT or definite time characteristic.



95% Stator Ground Fault Protection


100% stator ground fault relay for generator

Ground fault caused by mechanical damage may occur close to the generator neutral. Today there is a distinct trend towards providing ground fault protection for the entire stator winding (100% ground fault protection).

The principle diagram of the relay is shown in Fig. The 100% stator ground fault scheme includes a 95% unit-I, which covers the stator winding from 5% of the neutral and a 3rd harmonic voltage measuring unit -II which protects the rest of the stator winding.

For generators with 3rd harmonic voltage less than 1%, a filter is available with a damping factor of more than 100.

When the generator is running and there is no ground fault near the neutral, the 3rd harmonic voltage unit-II and the voltage check unit-IV are both activated and the relay contact used in alarm/trip circuit is open. If a ground fault occurs close to the generator neutral, the 3rd harmonic voltage unit will reset, operating relay contact will close and alarm or tripping is obtained.

The voltage check is included to prevent faulty operation of the relay at generator standstill or during the machine running up or running down period.

Generator which produces more than 1% 3rd harmonic voltage under all service conditions can have the entire stator winding up to and including the neutral point protected by the 100% stator ground fault relay.


   
100% Stator Ground Fault Protection
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