Function Range or value AccuracyCurve type IEEE2(0.18 ): ( 1)×= -TDIEEE t MEQUATION1645 V1 EN (Equation 93)where M = (E/f)/(Vn/fn)± 5% + 40 msMinimum time delay for inversefunction(0.000–60.000) s ± 0.5% ± 25 msAlarm time delay (0.00–9000.00) ± 0.5% ± 25 ms9.5 100% Stator ground fault protection, 3rd harmonicbased STEFPHIZ (59THD)9.5.1 IdentificationFunction description IEC 61850identificationIEC 60617identificationANSI/IEEE C37.2device number100% Stator ground fault protection, 3rdharmonic basedSTEFPHIZ - 59THD9.5.2 FunctionalityStator ground fault is a fault type having relatively high fault rate. The generator systemsnormally have high impedance grounding, that is, grounding via a neutral point resistor.This resistor is normally dimensioned to give an ground fault current in the range 3 – 15A at a solid ground-fault directly at the generator high voltage terminal. The relativelysmall ground fault currents give much less thermal and mechanical stress on the generator,compared to the short circuit case, which is between conductors of two phases. Anyhow,the ground faults in the generator have to be detected and the generator has to be tripped,even if longer fault time compared to internal short circuits, can be allowed.In normal non-faulted operation of the generating unit the neutral point voltage is close tozero, and there is no zero sequence current flow in the generator. When a phase-to-groundfault occurs the neutral point voltage will increase and there will be a current flow throughthe neutral point resistor.To detect a ground fault on the windings of a generating unit one may use a neutral pointovervoltage protection, a neutral point overcurrent protection, a zero sequenceovervoltage protection or a residual differential protection. These protections are simpleand have served well during many years. However, at best these simple schemes protect1MRK 502 048-UUS A Section 9Voltage protection309Technical manual
