Pilot Ground Overcurrent; High Resistance Ground Fault Supplement; Instantaneous Reverse Directional Overcurrent Ground; Supplement To Carrier Ground Start, Blocking Scheme - ABB REL 301 Instruction Leaflet

I.L. 40-386.4
2.5.2

Pilot Ground Overcurrent

Pilot Ground Overcurrent Supplement is added for high resistance faults and improves security
on POTT/unblocking schemes on some special power system conditions, such as shown in Fig-
ure 2-25. A ØØG fault is on the paralleled line section. Due to the system condition, fault current
flowing 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 "internal" one. The POTT/unblocking
scheme will incorrectly trip the protected line.
REL302 POTT/Unblocking pilot ground unit is supervised by the reverse-looking ground unit
RDOG as shown in Figure 2-31 (REVERSE BLOCK LOGIC). At terminal A, the RDOG disables
the PILOT KEY and PILOT TRIP functions via AND 35 and AND 30. Terminal B will not receive
a signal for permissive trip since none is sent. The reverse-block logic also provides the conven-
tional 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 REVERSE BLOCK LOGIC from over-blocking. If the
breaker is unequal-pole closing on a ØØG fault, say pole-A, pole B and C close at a later time
(see Figure 2-26). 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 ini-
tially have a polarity opposite to its fault-derived polarity. Reverse looking ground unit could
pick-up, start the reverse block logic and maintain it for 50 ms causing the correct tripping to be
delayed. The BLOCK-THE-BLOCK logic prevents this delay. The Reverse Block Logic also in-
cludes the reverse looking Zone-3 Ø /Zone-3 G (Z3P/Z3G) logic as shown in Figure 2-31.
2.5.3 High Resistance Ground Fault Supplement (Figure 2-27)
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 operation is deter-
mined by the setting of "Dir Type". Refer to Section 2.4.11 for the setting of "Dir Type". FDOG
is supervised by the "Iom" setting. A coordination timer "FDOGTime" (FDGT) is provided to
allow preference for pilot ground distance unit operation. The delay time can be set from 0 to 15
cycles in 1 cycle steps. It is recommended to set the "FDOGTime" to 3 cycles or longer due to
the sensitivity of FDOG.
2.5.4

Instantaneous Reverse Directional Overcurrent Ground

Similar to FDOG, the instantaneous reverse directional overcurrent ground function RDOG sup-
plements the pilot zone logic.
2.5.4.1

Supplement to Carrier Ground Start, Blocking Scheme

In the blocking system, RDOG, supervised by IOS, provides additional ground fault detection
(high resistance) beyond what is available by Z3G (reverse looking) for carrier start.
2.5.4.2

Pilot Ground Start, POTT

In the POTT/UNBLOCK systems, RDOG supervises PLTG and prevents keying or tripping on
reverse faults.
2-20