Permissive Overreach Schemes; Permissive Underreach Schemes; Teed Feeder Application - Apparent Impedances Seen By Relay - GE MiCOM P40 Agile Technical Manual

P44x/EN AP/Hb6
(AP) 5-76
Relay A will underreach for faults beyond the tee-point with infeed from terminal C. When
terminal C is a relatively strong source, the underreaching effect can be substantial. For a
zone 2 element set to 120% of the protected line, this effect may result in non-operation of
the element for internal faults. This not only effects time delayed zone 2 tripping but also
channel-aided schemes. Where infeed is present, it will be necessary for Zone 2 elements at
all line terminals to overreach both remote terminals with allowance for the effect of tee-point
infeed. Zone 1 elements must be set to underreach the true impedance to the nearest
terminal without infeed. Both these requirements can be met through use of the alternative
setting groups in the P442 and P444 relays.
A
Va = Ia Zat + Ib Zbt
Ib = Ia + Ic
Va = Ia Zat + Ia Zbt + Ic Zbt
Figure 50: Teed feeder application - apparent impedances seen by relay
3.4.2.2

Permissive Overreach Schemes

To ensure operation for internal faults in a POP scheme, the relays at the three terminals
should be able to see a fault at any point within the protected feeder. This may demand very
large 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 be
issued upon operation of zone 2 and receipt of a signal from both remote line ends. The
requirement for an 'AND' function of received signals must be realised through use of contact
logic external to the relay, or the internal Programmable Scheme Logic. Although a POP
scheme can be applied to a three terminal line, the signalling requirements make its use
unattractive.
3.4.2.3

Permissive Underreach Schemes

For a PUP scheme, the signalling channel is only keyed for internal faults. Permissive
tripping 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 for
a POP scheme. A common power line carrier (PLC) signalling channel or a triangulated
signalling arrangement can be used. This makes the use of a PUP scheme for a teed feeder
a more attractive alternative than use of a POP scheme.
The channel is keyed from operation of zone 1 tripping elements. Provided at least one zone
1 element can see an internal fault then aided tripping will occur at the other terminals if the
overreaching zone 2 setting requirement has been met. There are however two cases where
this is not possible:
Ia
Zat
C
MiCOM P40 Agile P442, P444
Ib
Zbt
Ic
Zct
Impedance seen by relay A = Va
Za = Zat + Zbt + Ic Zbt
Application Notes
B
Ia
Ia
P3075EN