GE D90 Plus Instruction Manual page 270

CONTROL ELEMENTS
Directional comparison blocking
262
The hybrid permissive over-reaching transfer trip (hybrid POTT) scheme logic is shown
below.
Figure 207: Hybrid POTT scheme logic
The directional comparison blocking scheme compares the direction to a fault at all
terminals of the line. Unlike the permissive schemes, the absence of a blocking signal
permits operation of the scheme. Consequently, the scheme is biased toward
dependability and requires an on-off type of signaling.
By default, the directional comparison blocking scheme uses only a forward-looking over-
reaching zone 2 distance element to identify forward faults. Ground directional
overcurrent functions available in the relay can be used in conjunction with the zone 2
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION