9.2.7 Operation principleOverfrequency protection SAPTOF (81) is used to detect high power system frequency.SAPTOF (81) has a settable definite time delay. If the frequency remains above the setvalue for a time period greater than the set time delay the TRIP signal is issued. To avoidan unwanted TRIP due to uncertain frequency measurement at low voltage magnitude, avoltage controlled blocking of the function is available from the preprocessing function,that is, if the voltage is lower than the set blocking voltage in the preprocessing function,the function is blocked and no PICKUP or TRIP signal is issued.9.2.7.1 Measurement principleThe frequency measuring element continuously measures the frequency of the positivesequence voltage and compares it to the setting PUFrequency. The frequency signal isfiltered to avoid transients due to switchings and faults in the power system. If the voltagemagnitude decreases below the setting MinValFreqMeas in the SMAI preprocessingfunction, which is discussed in the Basic IED Functions chapter and is set as a percentageof a global base voltage parameter VBase, SAPTOF (81) is blocked and the outputBLKDMAGN is issued. All voltage settings are made in percent of the global parameterVBase. To avoid oscillations of the output PICKUP signal, a hysteresis has been included.PICKUP PICKUPTRIPPickup&TripOutputLogicDefinite Time DelayTimeDlyOperateBLOCKANSI09000033-1-en.vsdFrequency Comparatorf > PUFrequencyTRIPBLKDMAGNBLOCKORfreqNotValidANSI09000033 V1 ENFigure 150: Schematic design of overfrequency protection SAPTOF (81)1MRK 506 335-UUS A Section 9Frequency protection323Technical manual
