P12x/EN T/Eb6 - Schneider - error

P12x/EN AP/Eb6Page 26/78Application GuideMiCOM P120/P121/P122/P1238. BACK-UP DIAGRAM USING « TRANSFERRED SELECTIVE TRIPPING »In this application, the relay protecting the incomer can trip the circuit breaker of the faultyfeeder via the watchdog contact of the relay protecting the faulty feeder.Figure 8 illustrates this example:IncomertI>>P12150/51tI>>>CBCTP12150/51WDTrip&>= 1CB1CTP12150/51WDTrip&>= 1CB nCTFeeder 1Feeder nP0027ENaFIGURE 8: EXAMPLE OF A BACK-UP DIAGRAM USING " TRANSFERRED SELECTIVE TRIP"Thus, a fault occurring on a feeder can be cleared tripping the circuit breaker of the faultyfeeder, even if the relay protecting this feeder failed to operate. Without this function, thefault would normally be cleared by the opening of the circuit breaker of the incomer. Thiswould lead to a total loss of operation on the affected busbar.The relay protecting the incomer has two time delay output contacts available (amongothers):−−3rd threshold: tI>>> time delay at 60ms (active threshold for the high phase faults)2nd threshold: tI>> time delay selectively greater than for the third threshold, i.e. 360ms.The output contact associated with the 2nd threshold is wired in serie with the watchdogcontact of the downstream relays, so that it can activate the trip coil of the circuit breakers ofthe feeders. Regarding the output contact associated with the 2nd and 3rd threshold, thiscontact is directly wired to the trip coil of the incomer circuit breaker.Case n°1 → all relays operate normally:In this case, watchdog contacts of all the relays are open.Thus, for a phase fault on the busbar, threshold tI>> or tI>>> of the P121 located on theincomer will clear the fault.For a phase fault on one of the feeder, the thresholds tI>> and tI>>> of the relay located onthe incomer being selectively set to higher values than the ones set for the phase thresholdsof downstream relays, the fault shall be cleared selectively by the relay of the faulty feeder(selectivity between the relay of the incomer and relays of the feeders is ensured thanks tointervals of selectivity correctly chosen, or thanks to a suitable blocking diagram).