GE MiCOM P40 Agile Technical Manual

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P44x/EN AP/Hb6 Application Notes(AP) 5-76 MiCOM P40 Agile P442, P444Relay A will underreach for faults beyond the tee-point with infeed from terminal C. Whenterminal C is a relatively strong source, the underreaching effect can be substantial. For azone 2 element set to 120% of the protected line, this effect may result in non-operation ofthe element for internal faults. This not only effects time delayed zone 2 tripping but alsochannel-aided schemes. Where infeed is present, it will be necessary for Zone 2 elements atall line terminals to overreach both remote terminals with allowance for the effect of tee-pointinfeed. Zone 1 elements must be set to underreach the true impedance to the nearestterminal without infeed. Both these requirements can be met through use of the alternativesetting groups in the P442 and P444 relays.ZbtAZatIa BIbCZctIcP3075ENVa = Ia Zat + Ib ZbtIb = Ia + IcVa = Ia Zat + Ia Zbt + Ic ZbtImpedance seen by relay A = VaIaZa = Zat + Zbt + Ic ZbtIaFigure 50: Teed feeder application - apparent impedances seen by relay3.4.2.2 Permissive Overreach SchemesTo ensure operation for internal faults in a POP scheme, the relays at the three terminalsshould be able to see a fault at any point within the protected feeder. This may demand verylarge zone 2 reach settings to deal with the apparent impedances seen by the relays.A POP scheme requires the use of two signalling channels. A permissive trip can only beissued upon operation of zone 2 and receipt of a signal from both remote line ends. Therequirement for an 'AND' function of received signals must be realised through use of contactlogic external to the relay, or the internal Programmable Scheme Logic. Although a POPscheme can be applied to a three terminal line, the signalling requirements make its useunattractive.3.4.2.3 Permissive Underreach SchemesFor a PUP scheme, the signalling channel is only keyed for internal faults. Permissivetripping is allowed for operation of zone 2 plus receipt of a signal from either remote line end.This makes the signalling channel requirements for a PUP scheme less demanding than fora POP scheme. A common power line carrier (PLC) signalling channel or a triangulatedsignalling arrangement can be used. This makes the use of a PUP scheme for a teed feedera more attractive alternative than use of a POP scheme.The channel is keyed from operation of zone 1 tripping elements. Provided at least one zone1 element can see an internal fault then aided tripping will occur at the other terminals if theoverreaching zone 2 setting requirement has been met. There are however two cases wherethis is not possible: