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voltage and/or zero sequence current) or Negative Sequ (NSEQ) (negative sequence
voltage and negative sequence current) for polarization (see 2.4.11, Selectable Ground
Directional Unit, Zero Sequence / Negative Sequ/Dual Polariz).
2.4.3
Loss of Potential Supervision
The ac voltage monitoring circuit is called loss-of-potential (LOP) circuit. In order to
prevent undesirable tripping due to the distance unit(s) pickup on loss-of-potential,
the following logic is used:
• (V
AN
This means that the LOP Blk (LOPB) will be set if any one of the voltages is below 7Vac
(without ∆I), or if the system detects 3Vo without 3Io (or 3I
2-10. The (loss-of-potential condition satisfies AND-1; output signal of AND-1 starts
the 8/500 ms timer. The timer output will satisfy AND-1C if there is no output from
AND-1B. Output signal of AND-1C will block all the distance unit (Z) tripping paths
via AND-2, AND-3, AND-4, AND-5, AND-6, AND-172 (also blocks AND-191 and AND-
187 for Pilot Systems), if LOP Blk (LOPB) is set at YES. Although all distance units are
blocked for tripping, the ground backup (GB) and high-set overcurrent units (Inst Ø
and Inst G) are operative and converted to non-directional automatically. The LOP Blk
(LOPB) blocking function can be disabled by setting the LOP Blk (LOPB) functional dis-
play at NO position. The output of the LOP timer will de-energize the Alarm 1 (AL1) re-
lay and cause the failure alarm.
When applying the LOP Blk (LOPB) to YES, it is the intent to block all distance units
from tripping, should LOP condition exist. However, under a special system condition
(refer to Figure 2-11), both circuits are energized without load current; with no source
at terminal B, fault at Ø where Ø is near terminal A, Zone2 relay at terminal B will be
blocked by LOP, and may fail to trip. This is because the relay at B sees no current,
and a low voltage condition exists before circuit breaker A opens. Another special sys-
tem condition involves two parallel lines with two symmetrical sources at both ends.
For an evolving flashover fault, at a point equidistant from both terminals, the conven-
tional LOP Blk (LOPB) logic will block trip, because the first external fault generates 3V0
and not 3I
timers circuit (in Figure 2-10) are for solving these problems. This logic unblocks the
LOP Blk (LOPB) circuit and provides a 3500 ms trip window for the distance units to
trip if the fault current is detected within 150 ms after LOP has been set up. This logic
has no effect on the conditions:
• if ∆I signal occurs ahead of LOP, or
• if LOP and ∆I signals occur simultaneously
NOTE:
2.4.4
Loss of Current Supervision (LOI)
The ac current monitoring circuit uses IOM (and not Vo) as criterion, as shown in Fig-
ure 2-12. Under ct short circuit or open circuit condition, IoM (and not Vo) satisfies
(10/94)
<7Vac) or (3Vo>7Vac) and not ∆I or not (3I
or V
or V
BN
CN
on the protected line. Logic AND-1A, 1B, -1C, and -1E 150/0, 3500/200 ms
0
The LOP Blk (LOPB) setting detects a blown fuse condition.
The distance units are designed to be blocked under the loss-of-poten-
tial condition, but the high set (Inst Ø and Inst G and ground backup
(GB) are functional and converted to non-directional automatically (see
also 2.4.6).
I.L. 40-386.3
>I
)
O
OS
> I
) as shown in Figure
O
OS
2-7
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