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2.12.1 Permissive underreach protection (PUP)
(i) Application
When an underreach element operates at local terminal, and when the circuit breaker (CB) is
tripped by this element, a trip permission signal† is sent to other opposite terminals. When
the opposite terminals receive the signal, CBs at these terminals are tripped instantly if an
overreaching element at respective these terminals has been operated. In PUP, Z1S/Z1G
elements in ZS/ZG are used for the underreach element, and ZCSF/ZCGF elements are used
for the overreaching element.
The trip permission signal is not sent until it is observed that the fault exists in the
protection zone of Z1S/Z1G; hence, PUP can provide excellent security., but PUP cannot provide
sufficient dependability if faults exist on the line including open terminals or weak infeed
terminals. This is because Z1S/Z1G cannot operate for this case‡.
†Note: The trip permission signal yielded by the underreaching element is transmitted
only; hence, respective terminals can share a telecommunication channel; and a
simplex channel can be used.
‡Note: At remote terminals, Z1X/ Z2 have a delay timer. Thus, they can remove the
faults.
(ii) Scheme logic
Figure 2.12-1 shows PUP logic. When Z1S/Z1G operate at the local terminal, Z1S/Z1G issue a
single-phase trip signal or three-phase trip signal instantaneously; and a trip permission
signal (DISCAR-CS-*) is sent to the opposite terminals through the sending circuit. Figure
2.12-2 shows the sending circuit of PUP; the trip permission signals are (DISCAR-S: used for
the external carrier communication) and (INT.DISCAR-S-*: used for the integral carrier
communication). The selection of either the single-phase trip (1P) or the three-phase trip (3P)
is carried out using scheme switch [DisCAR-TPMD].
When the opposite terminal receives the trip permission signal, PUP at the opposite
terminal also issues a trip signal instantly when either ZCSF/ZCGF have been operated.
PUP can operate when PUP is set for scheme switch [Dis-CAR]. If PUP requires operating
ARC, set scheme switch [DisCAR-ARCBlk] Non. (For more information of ARC, see chapter
Relay application: Autoreclose
.)
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