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Relion 670 series generator protection

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1MRK 502 071-UEN -

Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062

Application manual

100% Stator earth fault protection, 3rd harmonic

based STEFPHIZ

Identification

Function description

100% Stator earth fault protection, 3rd

harmonic based

The stator earth-fault protection of medium and large generators connected to their

power transformers should preferably be able of detecting small earth current

leakages (with equivalent resistances of the order of several kΩ) occurring even in the

vicinity of the generator neutral. A high resistance earth fault close to the neutral is not

critical itself, but must anyway be detected in order to prevent a double earth fault

when another earthing fault occurs, for example near the generator terminals. Such a

double fault can be disastrous.

Short-circuit between the stator winding in the slots and stator core is the most

common type of electrical fault in generators. Medium and large generators normally

have high impedance earth, that is, earthing via a neutral point resistor. This resistor

is dimensioned to give an earth fault current in the range 3 – 15 A at a solid earth-fault

directly at the generator high voltage terminal. The relatively small earth fault currents

(of just one earth fault) give much less thermal and mechanical stress on the generator,

compared, for example to short circuit between phases. Anyhow, the earth faults in the

generator have to be detected and the generator has to be tripped, even if longer fault

time, compared to short circuits, can be allowed.

The relation between the amplitude of the generator earth fault current and the fault

time, with defined consequence, is shown in figure 212.

Voltage protection

identification

ANSI/IEEE C37.2

device number

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