ABB REL301 Instruction Manual page 40

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.
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.
(10/94)
Programmable Reclosing Initiation (Figure 2-19)
†
I.L. 40-386.3
2-19