5-166 B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUALFLEXLOGIC CHAPTER 5: SETTINGS5Two bus differential zones can be configured. The zone is associated with its own bus differential protection element andCT trouble monitoring element.The bus differential zone is defined by providing the names of sources whose currents bound the differential zone (refer tosettings BUS ZONE 1A SOURCE to BUS ZONE 1F SOURCE).The connection status of a circuit with respect to the protected bus is dynamically provided by FlexLogic operandsassigned to the BUS ZONE 1A STATUS to BUS ZONE 1F STATUS settings. The selected operand must be ON when thecorresponding circuit is connected to the bus. The operands are selected from the contact inputs that reflect positions ofparticular circuit switches and/or breakers. If contact discrepancy filtering is needed, accomplish this using appropriateFlexLogic to form a Virtual Output, representing the current circuit status. If a given circuit (feeder) is fixed to the protectedbus section and cannot be attached to any other bus section, then use the FlexLogic "ON" operand to represent fixedcircuit status.The status signal is meant to exclude a given current from the bus zone if the circuit is connected to a different bus sectionand its non-zero values upsets the current balance causing a spurious differential signal. Therefore, it is not required norrecommended to use the position of the breaker to control the status signal of a given circuit. If the breaker is opened, thecircuit can remain included in the bus differential zone as the zero current values are measured and used when calculatingthe differential signal. Dynamically excluding or including a given current during the breaker operation can causeundesirable transients and race conditions for the relay algorithm.If a given circuit cannot be connected to any other bus section different than the protected one, the FlexLogic constant"On" is recommended for the status signal.5.6 FlexLogic5.6.1 FlexLogic operandsFor flexibility, the arrangement of internal digital logic combines fixed and user-programmed parameters. Logic uponwhich individual features are designed is fixed, and all other logic, from contact input signals through elements orcombinations of elements to contact outputs, is variable. The user has complete control of all variable logic throughFlexLogic. In general, the system receives analog and digital inputs that it uses to produce analog and digital outputs. Thefigure shows major subsystems of a generic UR-series relay involved in this process. BUS ZONE 1F STATUS:OffRange: FlexLogic operand
