High Resistance Ground Faults - Pilot Supplement; Power Reversal On Pott - ABB REL300 Instruction Manual

c. Carrier Receiving Logic
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 3-25)
Since the carrier channel turns "ON" for external faults only, the channel indicators (SEND
and RCVR) should not be sealed-in.
e. Channel Simulation
Same as for POTT scheme.
f. Programmable Reclosing Initiation (Figure 3-19, page 3-41)
Same as for POTT scheme.
3.5.4

High Resistance Ground Faults - Pilot Supplement

Pilot ground is more dependable on high resistance faults because it is supplemented with
FDOG and IOM (refer to Figure 3-26, page 3-46).
Supplemental protection is provided on overreaching pilot systems to detect high resistance
ground faults. The instantaneous forward directional overcurrent ground function (FDOG)
works in conjunction with the pilot ground distance unit. The FDOG directional unit is deter-
mined by the setting of DIRU (ZSEQ/NSEQ/DUAL). Refer to Section 3.4.11 (page 3-11) for the
setting of DIRU. FDOG is supervised by the IOM setting. A coordination timer FDGT (T/0) is
provided to allow preference for pilot ground distance (mho) unit operation. The delay time (T)
can be set from 0 to 15 cycles in 1 cycle steps. It is recommended to set the FDGT timer to 3
cycles or longer for security reasons.
3.5.5

Power Reversal on POTT

Pilot ground is more secure on POTT/unblocking schemes on some special power system con-
ditions, such as shown in Figure 3-27. A φφG 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 directional comparison of an external fault as an "in-
ternal" one. The POTT/unblocking scheme will incorrectly trip out of the protected line.
REL300 POTT/Unblocking pilot ground unit (PLTG/FDOG) is supervised by the reverse-look-
ing ground unit (RDOG). The "Reverse-Block" logic is as shown in Figure 3-32 (page 3-49). At
terminal A, the RDOG disables the PLTG/FDOG trip/key functions via OR-9A, AND-45A AND-
45 and AND-30. At terminal B, it will receive no carrier signal for permissive trip. The reverse-
block logic also provides the conventional TBM feature to prevent false operation on power re-
versal. It should be noted that a "Block-the-Block" logic is also included in the circuit. The Block-
the-Block logic is to prevent the Reverse-Block logic from over-blocking. If the breaker is un-
equal-pole closing on a φG fault, say pole-A, pole B and C close at a later time (refer to Figure
3-28). If, due to breaker contact asymmetry, the first breaker contact to close is the one of the
faulted-phase, the zero-sequence (or negative sequence) 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 maintain it for 50 ms consequently, the correct trip-
ping will be delayed. The Block-the-Block logic would prevent this delaying. The Reverse-Block
logic also includes the reverse looking Z3P/Z3G units as shown in Figure 3-32.
I.L. 40-385.7
3-21