the set time delay the TRIP signal is issued. To avoid an unwanted trip due to uncertainfrequency measurement at low voltage magnitude, a voltage controlled blocking of thefunction is available from the preprocessing function, that is, if the voltage is lowerthan the set blocking voltage in the preprocessing function, the function is blocked andno PICKUP or TRIP signal is issued.10.1.7.1 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 thevoltage magnitude decreases below the setting MinValFreqMeas in the SMAIpreprocessing function, which is described in the Basic IED Functions chapter and isset as a percentage of a global base voltage parameter, SAPTUF (81) gets blocked, andthe output BLKDMAGN is issued. All voltage settings are made in percent of thesetting of the global parameter VBase.To avoid oscillations of the output PICKUP signal, a hysteresis has been included.10.1.7.2 Time delayThe time delay for SAPTUF (81) is a settable definite time delay, specified by thesetting tDelay.Trip signal issuing requires that the under frequency condition continues for at least theuser set time delay. If the PICKUP ceases during the delay time, and is not fulfilledagain within a defined reset time, the PICKUP output is reset.When the measured frequency returns to the level corresponding to the settingRestoreFreq, a 100ms pulse is given on the output RESTORE after a settable timedelay (tRestore).10.1.7.3 BlockingIt is possible to block underfrequency protection SAPTUF (81) completely, by binaryinput signal:BLOCK: blocks all outputsIf the measured voltage level decreases below the setting of MinValFreqMeas in thepreprocessing function, both the PICKUP and the TRIP outputs, are blocked.1MRK 502 043-UUS B Section 10Frequency protection293Technical Manual
