8.1.3.4 Voltage instability mitigationThis setting is very much dependent on the power system characteristics, and thoroughstudies have to be made to find the suitable levels.8.1.3.5 Backup protection for power system faultsThe setting must be below the lowest occurring "normal" voltage and above the highestoccurring voltage during the fault conditions under consideration.8.1.3.6 Settings for Two step undervoltage protectionThe following settings can be done for two step undervoltage protection (UV2PTUV ,27).GlobalBaseSel: Selects the global base value group used by the function to define (IBase),(VBase) and (SBase).ConnType: Sets whether the measurement shall be phase-to-ground fundamental value,phase-to-phase fundamental value, phase-to-ground RMS value or phase-to-phase RMSvalue.Operation: Disabled/Enabled.UV2PTUV (27) measures selectively phase-to-ground voltages, or phase-to-phasevoltage chosen by the setting ConnType.This means operation for phase-to-ground voltage if:(%) ( )3< ×V VBase kVEQUATION1711 V1 EN (Equation 164)and operation for phase-to-phase voltage if:Vpickup (%) VBase(kV)< ×EQUATION1991-ANSI V1 EN (Equation 165)Characteristic1: This parameter gives the type of time delay to be used for step 1. Thesetting can be. Definite time/Inverse Curve A/Inverse Curve B. The choice is highlydependent of the protection application.OpModen: This parameter describes how many of the three measured voltages that shouldbe below the set level to give operation for step n (n=step 1 and 2). The setting can be 1out of 3, 2 out of 3 or 3 out of 3. It is sufficient that one phase voltage is low to give1MRK 506 334-UUS A Section 8Voltage protection257Application manual
