hysteresis area, the START condition must be fulfilled again and it is not sufficientfor the signal to only return back to the hysteresis area.The total time delay consists of the set value for time delay plus theminimum operate time of the start function (80-90 ms).On the RESTORE output of SAPTUF a 100ms pulse is issued, after a time delaycorresponding to the setting of tRestore, when the measured frequency returns tothe level corresponding to the setting RestoreFreq, after an issue of the TRIPoutput signal. If tRestore is se to 0.000 s the restore functionality is disabled, andno output will be given.10.1.6.3 Voltage dependent time delay M13354-19 v5Since the fundamental frequency in a power system is the same all over the system,except some deviations during power oscillations, another criterion is needed todecide, where to take actions, based on low frequency. In many applications thevoltage level is very suitable, and in most cases is load shedding preferable in areaswith low voltage. Therefore, a voltage dependent time delay has been introduced,to make sure that load shedding, or other actions, take place at the right location. Atconstant voltage, U, the voltage dependent time delay is calculated according toequation 53. At non-constant voltage, the actual time delay is integrated in asimilar way as for the inverse time characteristic for the undervoltage andovervoltage functions.( )ExponentU UMint tMax tMin tMinUNom UMin-= × - +-é ùê úë ûEQUATION1182 V1 EN-US (Equation 53)where:t is the voltage dependent time delay (at constant voltage),U is the measured voltageExponent is a setting,UMin, UNom are voltage settings corresponding totMax, tMin are time settings.The inverse time characteristics are shown in figure 127, for:UMin = 90%UNom = 100%tMax = 1.0 stMin = 0.0 sExponent = 0, 1, 2, 3 and 4Section 10 1MRK 505 394-UEN AFrequency protection270 Line differential protection RED650 2.2 IECTechnical manual
