9-34 D60 Line Distance Protection System GE Multilin9.5 SINGLE-POLE TRIPPING 9 THEORY OF OPERATION99.5.5 PILOT SCHEME / PHASE SELECTOR COORDINATIONFor local fault type identification the pilot schemes use the Phase Selector. The latter may fail to respond to certain faultscenarios. Examples are: simultaneous forward and reverse fault, simultaneous SLG and LL fault involving different phases(e.g. AG and BC) or two simultaneous faults in the same direction but at very different locations. The Phase Selector is opti-mized to either indicate correctly the forward fault or to assert the Void flag. For example, a combination of AG and BC isnot a valid fault type - it is two different simultaneous faults and as such cannot be described by any single fault pattern,therefore, the Phase Selector would assert the Void flag.The VOID phase selection combined with a local trip request (such as high-set directional overcurrent) will result in three-pole trip as per the Trip Output logic.The Pilot Schemes, however, try to recover more information from the distance elements. Each scheme uses a forwardlooking, either underreaching or overreaching, distance zone. A given Pilot Scheme analyzes this zone for fault type identi-fication if the Phase Selector asserts its Void flag: the DUTT scheme uses Z1; all the other schemes use Z2. The schemesanalyze all six fault loops of the zone to determine the fault type.For example, simultaneous forward AG and reverse BG faults may result in the Void indication. The POTT scheme wouldanalyze the Z2 response. As only the AG element is picked up, the local phase selection is determined as AG. This is a cor-rect indication.Depending on the number of bits used for communications, the accuracy of the overall response will be further improved asillustrated in the next subsection.This enhanced operation of the pilot-aided schemes is the reason to use a short pilot scheme priority time when setting theTrip Output logic. The timer will force the scheme to wait for a decision from the pilot scheme for a short period of timebefore accepting any local trip request. The advantage, however, materializes only if more than one-bit communicationschannels are used, and is important only on parallel lines or when the application requires maximum accuracy of single-pole tripping. In other cases, it is not recommended to delay the local trip decision.
