Best answer: Why only 95% of the stator winding is protected upon a phase to earth fault?

In case of an earth-fault on the stator winding and no additional fault resistance, the fundamental frequency neutral point voltage is increasing in proportion to the distance of the fault from the neutral point. Hence a voltage relay set to 5 % of the stator phase voltage will protect 95 % of the stator winding.

What percentage of generator winding can be protected against earth faults through restricted earth fault protection?

The setting should not be more than 33% of the maximum earth fault current of the generator, and a lower setting would be preferable, depending on grading considerations.

Which protection is used for stator fault?

Over current relays can not detect the winding temperature because electrical protection can not detect the failure of the cooling system. So to protect the stator against over heating, embed resistance temperature detector or thermocouples are used in the slots below the stator coils.

Which is the most important stator winding fault of an alternator?

01. Stator winding fault. It is the most severe fault and considered most important for alternator protection. This vital protection of alternator stator is Differential circulating current scheme or Mertz price differential current protection scheme of alternator is discussed in a separate post.

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How can we stop earth fault?

Earth fault protection of the LV winding is part of the excitation equipment earthing which consists of high impedance earthing through a resistor. To prevent voltages being transferred through interwinding capacitance coupling, an earthed screen is provided between windings.

Which relay is used for earth fault protection?

Relay R1 is used for providing protection against earth fault at the secondary side of 2.5MVA, 11/3.3kV transformer, whereas, relay R2 has to provide protection at bus B. Two CTs are used for protection. 200:5 CT is connected to instantaneous relay and 500:5 is connected to inverse current characteristic relay.

Do you need to earth a generator?

All generators need to be earthed unless floating. … But Earth can be, and is, used for fault protection with low-voltage generator based installations.

What is percentage differential protection?

Percentage restrained differential relays measure the individual branch currents and quantify the through current in the zone of protection. The percentage restraint characteristic requires that the differential current be greater than a percentage of the through (restraint) current.

When input to the prime mover of an alternator fails?

1. Failure of Prime-Mover: When input to the prime-mover fails, the alternator runs as a synchronous motor and draws some current from the supply system. This motoring conditions is known as “inverted running“. In case of turbo-alternator sets, failure of steam supply may cause inverted running.

Which abnormality is possible in alternator?

Alternators need to be protected not only from short circuits but also from abnormal operating conditions such as overexcitation, overvoltage, underexcitation, loss of field, unbalanced currents, loss of prime mover and abnormal frequency conditions.

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