Teleprotection
GARD 8000 SYS RFL Electronics
March 1, 2006 9-2 973.334.3100
9.2.1 PERMISSIVE TRANSFER TRIP
Permissive Transfer Trip schemes provide a high degree of security for transmission line protection.
This high degree of security in a permissive scheme is a result of the requirement that the
teleprotection receiver and the local relay must operate in order to trip. There are two basic types of
PTT schemes, Permissive Underreaching Transfer Trip (PUTT) and Permissive Overreaching
Transfer Trip (POTT). PTT schemes typically require trip times under 12ms. Security requirements for
PTT schemes can be less stringent since the local breaker must also operate. However, dependability is
stressed since fault clearing will not occur unless the teleprotection device operates. In an effort to
improve dependability, unblock timers can be used to allow operation even during channel losses
which are coincident with line faults.
Figure 9-1. Permissive Underreaching Transfer Trip
Figure 9-2. Permissive Overerreaching Transfer Trip
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