Loss Of Potential Supervision - ABB REL 301 Instruction Leaflet

Note:
The directional unit polarization is determined by the setting of "Dir Type" which can be set to
"Zero Sequence"; zero sequence voltage, "Dual Polariz"; zero sequence voltage and/or zero
sequence current, or "Negative Sequ" negative sequence voltage and negative sequence cur-
rent (see Section 2.4.11, Selectable Ground Directional Unit, Zero Sequence / Negative Se-
qu/Dual Polariz).
2.4.3 Loss of Potential Supervision (Figure 2-10)
The ac voltage monitoring circuit is referred to as the loss-of-potential (LOP) circuit. In order to
prevent undesirable tripping due to the distance unit(s) operation on loss-of-potential, the follow-
ing logic is used:
• (V or V
AN
This means that the LOP Block will be set if any one of the phase voltages is below 7 Vac (with-
out ∆I), or if the system detects 3Vo without 3Io (or 3I
loss-of-potential condition satisfies AND 1. The output signal of AND 1 starts the 8/0 millisecond
timer. The timer output pickups the 0/500 millisecond timer and satisfies AND 1C if there is no
output from AND 1B. Output signal of AND 1C will block all the distance unit 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" is set to "YES". All distance units are blocked from tripping but, the
ground backup, regardless of it directional setting, and high-set overcurrent units (Inst Ø and Inst
G) are operative and converted to non-directional operation automatically. If "LOP Blk" is set
to "All", all distance and overcurrent tripping functions will be blocked via AND 8 (Figure 2-5)
and the Protection In Service LED will go out. Loss of potential blocking function can be disabled
by setting the "LOP Blk" to "No" and the output of the LOP timer will operate the Alarm 1 relay
(Failure Alarm) only.
When applying the "LOP Blk" 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 near terminal
A, Zone-2 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 system condition involves two parallel lines with a symmetrical sources at both
terminals. For an evolving flashover fault, at a point equidistant from both terminals, the conven-
tional LOP logic will block trip, because the first external fault generates 3V0 and not 3I
protected line. Logic AND 1A, 1B, 1C, and 1E 150/0, 3500/200 millisecond timers circuit (in Figure
2-10) are for solving these problems. This logic unblocks the LOP 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. This logic has will be blocked (will have no effect) for the following conditions:
• if ∆I signal occurs ahead of LOP, or
• if LOP and ∆I signals occur simultaneously
The curves commonly used with electromechanical (e/m) overcurrent relays are
a composite, average result of considerable testing. Overcurrent characteristics
utilized in the REL301/302 are the result of calculations which do not exactly em-
ulate e/m overcurrent relay characteristics. Also it should be noted that the time
dial setting differs from e/m overcurrent relays. E/m relays have a continuously
adjustable time dial. REL301/302 attempts to emulate this feature by providing
time dial settings of 1 to 63 with the middle of the "time dial" range being 24 (e/m
approximate equivalent of time dial 5).
<7Vac) or (3Vo>7Vac) and not ∆I or not (3I
or V
BN CN
>I )
O OS
> I ) and without 52b as shown. The
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I.L. 40-386.4
on the
0
2-7