10.6 100% Stator earth fault protection, 3rd harmonicbased STEFPHIZ10.6.1 IdentificationFunction description IEC 61850identificationIEC 60617identificationANSI/IEEE C37.2device number100% Stator earth fault protection, 3rdharmonic basedSTEFPHIZ - 59THD10.6.2 ApplicationThe stator earth-fault protection of medium and large generators connected to theirpower transformers should preferably be able of detecting small earth currentleakages (with equivalent resistances of the order of several kΩ) occurring even in thevicinity of the generator neutral. A high resistance earth fault close to the neutral is notcritical itself, but must anyway be detected in order to prevent a double earth faultwhen another earthing fault occurs, for example near the generator terminals. Such adouble fault can be disastrous.Short-circuit between the stator winding in the slots and stator core is the mostcommon type of electrical fault in generators. Medium and large generators normallyhave high impedance earth, that is, earthing via a neutral point resistor. This resistoris dimensioned to give an earth fault current in the range 3 – 15 A at a solid earth-faultdirectly 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 thegenerator have to be detected and the generator has to be tripped, even if longer faulttime, compared to short circuits, can be allowed.The relation between the amplitude of the generator earth fault current and the faulttime, with defined consequence, is shown in figure 212.1MRK 502 071-UEN - Section 10Voltage protectionGenerator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 429Application manual
