5-386 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALCONTROL ELEMENTS CHAPTER 5: SETTINGS5In single-pole tripping applications, the scheme uses local fault type identification provided by the phase selector togetherwith information received from the remote terminal. The scheme generates output operands (HYBRID POTT TX1 throughHYBRID POTT TX4) that are used to transmit the signal to the remote terminal(s). Choices of communications channel includeremote inputs/outputs and telecommunications interfaces. When used with telecommunications facilities, assign theoutput operand to operate an output contact connected to key the transmitter at the interface. When used withtelecommunications facilities, assign the output operands to operate output contacts connected to assert the individualbits at the interface.To make the scheme fully operational as a stand-alone feature, the scheme output operands must be configured tointerface with other relay functions, output contacts in particular. Typically, you program the output operands to initiate atrip, breaker fail, and autoreclose, and drive a user-programmable LED as per user application.When used in conjunction with the trip output element, the scheme is pre-configured to initiate trip, breaker fail, andsingle-pole autoreclose actions.HYB POTT BLOCK — This setting allows the user to assign any FlexLogic operand to block the scheme. Contact inputs from apilot cut-out switch are typically used for this purpose.HYB POTT PERMISSIVE ECHO — If set to “Enabled”, the scheme sends a permissive echo signal to the remote end(s) using apre-programmed logic (see logic diagram below). If set to “Custom”, the echo signal is sent if a condition selected via theHYB POTT ECHO COND setting is satisfied. The echo is sent only once and then the logic locks out for the time specified bythe ECHO LOCKOUT setting. The duration of the echo pulse is settable as ECHO DURATION. Operation of the overreachingprotection elements (distance zone 2 or GND DIR O/C FWD setting) inhibits the echo.HYB POTT ECHO COND — This setting specifies a user-selected echo condition and applies if the HYB POTT PERMISSIVE ECHO isset to “Custom”.HYB POTT WEAK INFEED — If this setting is set to “Enabled”, the scheme activates both the keying and operating paths usinga pre-programmed weak infeed logic (see logic diagram below). If this setting is set to “Custom”, the weak infeed conditionis to be specified by the user via the HYB POTT W/I COND setting.HYB POTT W/I COND — This setting specifies user-selected weak infeed condition and applies if the HYB POTT WEAK INFEED isset to “Custom”.HYB POTT RX PICKUP DELAY — This setting enables the relay to cope with spurious received signals. Set the delay longer thanthe longest spurious TX signal that can be received simultaneously with the zone 2 pickup. The selected delay increases theresponse time of the scheme.TRANS BLOCK PICKUP DELAY — This setting defines a transient blocking mechanism embedded in the hybrid POTT schemefor coping with the exposure of both the over-reaching zone 2 and ground directional overcurrent function to currentreversal conditions.The transient blocking logic applies to both operate (trip) and send (transmit) paths. Identifying the fault as a reverse faultprevents the scheme from both operating and keying the channel. If the reverse fault condition prevails for TRANS BLOCKPICKUP DELAY, the blocking operation is extended by the transient blocking timer for TRANS BLOCK RESET DELAY. This allowsriding through current reversal conditions.However, if distance zone 1 picks up during the transient blocking condition, the blocking action is removed. This is to copewith evolving faults when an external fault is followed by an internal fault. Without the zone 1 feedback, the trip is delayedunnecessarily.Do not set the TRANS BLOCK PICKUP DELAY longer than the fastest possible trip time for faults on an adjacent line so thatextended blocking action can be established. Take into account the pickup time of the reverse-looking elements of thescheme.Do not set the delay too short, in order to avoid locking up a spurious reverse fault indication that can occur during internalfault conditions.TRANS BLOCK RESET DELAY — This setting defines a transient blocking mechanism embedded in the hybrid POTT scheme forcoping with the exposure of the overreaching protection functions to current reversal conditions (see also the TRANS BLOCKPICKUP DELAY).
