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ed 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: The LOPB setting detects a blown fuse
condition.
The distance units are designed to be
blocked under the loss-of-potential con-
dition, but the high set (ITP and ITG) and
ground backup (GB) are functional and
converted to non-directional automati-
cally (see also 3.4.6).
3.4.4 Loss of Current Monitoring (LOI)
The ac current monitoring circuit uses IoM (and not
Vo) as criterion, as shown in Figure 3-12. Under ct
short circuit or open circuit condition, IoM (and not
Vo) satisfies AND-23; the output signal of AND-23
starts the 500/500 ms timer. The timer output turns
"ON" the non-memory LOI indicator, which is shown
in the Metering mode, and drops out the AL1 relay
(Failure Alarm). If the LOI condition exists and LOIB
is set at YES, the trip will be blocked after the 500 ms
timer times out.
3.4.5 Overcurrent Supervision
For MDAR, as shown in Figure 3-13, the distance
units do not require overcurrent supervision; be-
cause the relay normally operates in a background
mode, they will not start the Zone 1 and pilot imped-
ance computation until a phase current or a phase
voltage disturbance is detected. This approach can
minimize the load problem when setting the phase
overcurrent units. However, the 3-phase units are
supervised by the load restriction logic, and the 21BI
unit is restricted by the setting of I
quently, the 3-phase units are supervised by the I
setting. For increasing the security. The application
of LOPB supervision function is also normally rec-
ommended.
For coordination purposes the ground trip units
(Z1G, Z2G, Z3G, PLTG, and FDOG) are supervised
by the medium set ground overcurrent unit (IoM).
The Ios setting and RDOG are used for carrier send
in a Pilot Blocking system.
(5/92)
3.4.6 Instantaneous Forward Directional Over-
The instantaneous overcurrent units (IAH, IBH, ICH
and IOH) are normally set forward directional and
high to detect those faults which occur in the Zone 1
area, therefore, their tripping will occur via OR-2 for
HST, as shown in Figure 3-14. These high set trip
functions can be disabled by setting the ITP (phase)
and/or ITG (ground) to the OUT position. The direc-
tional unit (ITP and ITG) will be automatically con-
verted to non-directional protection if the LOP
condition occurs and the setting of LOPB = YES.
3.4.7 Close-Into-Fault Trip (CIFT) and
There are three low voltage units (LVA, LVB and
LVC) in MDAR. Each unit senses the phase voltage
condition in the background mode. The unit can be
set from 40 to 60 volts, in 1.0 volt steps. For any
phase voltage below its preset value, the LV logic
will produce a logic "1" output signal. The low voltage
units are used in CIFT and weakfeed logic in MDAR.
In order to supplement distance unit operation, when
the circuit breaker is closed into a fault and line side
potential is used, the Close-Into-Fault Trip (CIFT)
circuit operates as shown in Figure 3-15. It includes
logic AND-22, OR-3 and 100/180 ms and 16/0 ms
timers. If any overcurrent unit (IAL, IBL, ICL or IOM)
operates OR-11, at the same time as one of the
phase voltages (VA, VB,VC) is below the preset lev-
el of the LV units. Then logic AND-22 is satisfied and
produces a trip signal (for 180 ms) after circuit break-
er closes (52b contact opens). Tripping will be via
OR-3, with RB and CIF targets.
The stub bus protection feature protects a line termi-
nal with the potential device on the line-side. With
units, conse-
the line disconnect switch open, the distance units
L
will lose their reference voltage, and may not func-
L
tion correctly when a fault occurs on the short part of
the bus between the ct location and the opened dis-
connect switch. The logic for stub bus protection is
independent of the operation of the circuit breaker(s)
and the line voltage condition. Also, it requires the
information from the disconnect switch (89b). The
stub bus protection logic, as shown in Figure 3-15,
includes the contact convertor for the 89b switch and
AND-22E and OR-22B logic.
current Trip/Highset Trip Logic
Stub-Bus Protection (SBP)
I.L. 40-385.1B
3-5
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