8.1.7.4 DesignThe voltage measuring elements continuously measure the three phase-to-neutralvoltages or the three phase-to-phase voltages. Recursive fourier filters or true RMS filtersof input voltage signals are used. The voltages are individually compared to the set value,and the lowest voltage is used for the inverse time characteristic integration. A speciallogic is included to achieve the 1 out of 3, 2 out of 3 and 3 out of 3 criteria to fulfill thePICKUP condition. The design of Two step undervoltage protection UV2PTUV (27) isschematically shown in Figure 138.PICKUPPU_ST1_APU_ST1_BPU_ST1_CTRST1PICKUPPU_ST2TRST2TRIPMinVoltSelectorPickup&TripOutputLogicStep 1Pickup&TripOutputLogicStep 2Phase CPhase BPhase APhase CPhase BPhase ATimert2Voltage PhaseSelectorOpMode2Time integratoror Timer t1Voltage PhaseSelectorOpMode11 out of 32 out of 33 out of 3TRIPTRIPORORORORORORPICKUPVA or VABVB or VBCVC or VCAComparatorV < Pickup11 out of 32 out of 33 out of 3PU_ST1ANSI08000016-3-en.vsdComparatorV < Pickup1ComparatorV < Pickup1ComparatorV < Pickup2ComparatorV < Pickup2ComparatorV < Pickup2ANSI08000016 V3 ENFigure 138: Schematic design of Two step undervoltage protection UV2PTUV (27)Section 8 1MRK 506 335-UUS AVoltage protection298Technical manual
