10-20 L90 Line Current Differential System GE Multilin10.5 PROTECTION SIGNALING SCHEMES 10 APPLICATION OF SETTINGS1010.5.7 DIRECTIONAL COMPARISON UNBLOCKINGa) OVERVIEWThe directional comparison unblocking scheme is used with a frequency shift keying (FSK) PLC that produces a loss-of-guard output during an actual loss-of-guard signal condition and during reception of the permissive keyed frequency whenthe received carrier signal changed from guard to permissive frequency. The loss-of-guard signal should preferably be anormally-closed contact, allowing the scheme to get this signal if the communications equipment failed. This loss-of-guardoutput should be connected to a contact input of the L90. The power line carrier should also provide an output contact whenthe permissive frequency is received. This output should be wired to any other contact input of the L90.The directional comparison unblocking scheme allows a window of opportunity for tripping the breaker fast if the permissivesignal gets attenuated by the fault to a level below the receiver's threshold. The guard signal is monitored, and the relay isallowed to trip for a loss of guard without receipt of the permissive signal, if the relay sees the fault in the forward, and notreverse direction by means of the forward looking distance zone 2 and ground directional forward, if configured, andreverse looking distance zone 4 and ground directional reverse, if configured.The scheme is active and ready for operation when the DCUB BLOCK setting is “Off” the DCUB SCHEME FUNCTION setting is“Enabled”. The user can disable or block the scheme by assigning any FlexLogic™ operand to the DCUB BLOCK setting.This operand can consist of any pre-defined logic, including a pilot cutout switch connected to any contact input of the L90.With this pilot cutout switch and the pre-defined logic state on and assigned to DCUB BLOCK, the scheme will be disabled.The directional comparison unblocking scheme supervises the operation and keying of all scheme functions. The lockoutfeature of the scheme also supervises all scheme functions. All timers are fully adjustable, with typical values indicated.Lockout of the directional comparison unblocking scheme is established as follows. Lockout of the scheme is started for atime duration set by DCUB LOG TRIP WINDOW setting if:• The scheme is enabled and not blocked (that is, the scheme is active), and• no permissive RX signal is received, and• any loss-of-guard signal is received.Lockout is sealed-in through an AND gate with the directional comparison unblocking scheme active and any loss-of-guardthrough GUARD HEALTHY RESET DELAY timer. The scheme will thus remain blocked during longer loss-of-guard or channelconditions for any of the configured channels. The GUARD HEALTHY RESET DELAY timer ensures that the scheme will remainlocked out slightly longer after the loss-of-guard signal(s) is reset. This allows for all channel(s) stabilizing and contactdenouncing. This timer determines how long the logic waits before enabling the directional comparison unblocking schemewhen the communications channel is restored.Operating of the directional comparison unblocking scheme consists of two parts: normal operation and loss-of-guard oper-ation.b) NORMAL OPERATIONNormal operation picks up under the following conditions:• The scheme is enabled (DCUB FUNCTION is “Enabled” and DCUB BLOCK is “Off”), and• any permissive signal is received with its associated loss-of-guard signal (e.g. DCUB RX1 and DCUB LOG1), and• a fault is seen in the forward zone 2 distance elements or ground directional forward function (if configured), and• the reverse zone 4 distance elements or ground directional reverse function (if configured) did not pickup to set thetransient blocking, and• the scheme is not locked out.If these conditions are met, the trip table is run, operating the scheme and asserting DCUB OP FlexLogic™ operand. Thetrip table with the aid of the local phase selector and received RX signals will determine what tripping operands will be oper-ated. During normal operation, the loss-of-guard signal will be seen momentarily before the permissive DCUB RX signal isreceived for each channel being operated, as the channel transitions from guard to permissive frequency, but the schemeoperates instantaneously after any DCUB RX signal is received.
