I.L. 40-386.3
(10/94) 2-19
c. Carrier Receiving Logic (Figure 2-24)
Carrier signal from the receiver output will be directly applied to AND-47 to disable the
pilot tripping function.
d. Channel Indication (not shown in Figure 2-24)
Since the carrier channel turns “ON” for external faults only, the channel indicators
(Car Send and Rx Ch1) should not be sealed-in.
†
e. Channel Simulation
Same as for POTT scheme.
f. Programmable Reclosing Initiation (Figure 2-19)
Same as for POTT scheme.
2.5.2 Pilot Ground Overcurrent
Pilot Ground Overcurrent is more dependable on high resistance faults because it is
supplemented with FDOG and IOM (refer to Figure 2-25).
Pilot ground is more secure on POTT/unblocking schemes on some special power sys-
tem conditions, such as shown in Figure 2-26. A ØØG fault is on the paralleled line
section. Due to the system condition, fault current flows in the protected line would
be I1+I2 from A to B, and Io from B to A. The operation of pilot distance relays would
be a phase relay at A and a ground relay at B. The result would be erroneous direc-
tional comparison of an external fault as an “internal” one. The POTT/unblocking
scheme will incorrectly trip out of the protected line.
REL 302 POTT/Unblocking pilot ground unit (PLTG/FDOG) is supervised by the re-
verse-looking ground unit (RDOG). The “Reverse-Block” logic is as shown in Figure 2-
31. At terminal A, the RDOG disables the PLTG/FDOG trip/key functions via AND-35
and AND-30. At terminal B, it will receive no carrier signal for permissive trip. The re-
verse-block logic also provides the conventional TBM feature to prevent false operation
on power reversal. It should be noted that a “Block-the-Block” logic is also included in
the circuit, as shown in Figure 2-31. The Block-the-Block logic is to prevent the Re-
verse-Block logic from over-blocking (see the following system condition). If the breaker
is unequal-pole closing on a ØG fault, say pole-A, pole B and C close at a later time
(refer to Figure 2-27). If, due to breaker contact asymmetry, the first breaker contact to
close is the one of the faulted-phase, the zero-sequence (or Negative Sequ) polarizing
voltage will initially have a polarity opposite to its fault-derived polarity, the reverse-
looking ground unit could pick-up for a short period, issue a blocking order, and main-
tain it for 50 ms consequently, the correct tripping will be delayed. The Block-the-
Block logic would prevent this delaying. The Reverse-Block logic also includes the re-
verse looking Zone3Ø /Z3G units as shown in Figure 2-31.
2.5.3 High Resistance Ground Fault Supplement
Supplemental protection is provided on overreaching pilot systems to detect high re-
sistance ground faults. The instantaneous forward directional overcurrent ground
†
Refer to Table 4-1 for Setting Cross-reference.
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