ABB MDAR Instruction Manual page 33

Section 9. OPERATION OF MDAR (REL 300) BASIC FUNCTIONS
9.1 LOSS-OF-POTENTIAL SUPERVISION (LOP)
The ac voltage monitoring circuit is called loss-of-
potential circuit.
In order to prevent undesirable tripping due to the
distance unit(s) pickup on loss-of-potential, logic (low
voltage and not VI) and (Vo and not Io) are included
in MDAR (REL-300) design, as shown in Figure 12A.
The low voltage and Vo units for the LOP logic are
set at a fixed value of 7 volts. Under loss-of-potential
condition, low voltage (or Vo) and not VI (or Io) satisfy
AND-1. The 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. The 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 sys-
tems) if LOPB is set to "YES". The LOPB blocking
function can be disabled by selecting the LOPB value
display to NO.
The output of the LOP timer will turn on the non-
memory LOP indicator on the metering display dur-
ing the LOP condition via AND-1F after 500 ms
delay. This output is also connected to the failure
alarm output relay GS (Alarm-3).
In selecting the LOPB to "YES", the intent is to block
all the distance units from tripping should a LOP con-
dition exist. However, for a condition as shown in Fig-
ure 12B, a single-end source energizes two parallel
circuits without pre-fault load current. The MDAR
(REL-300) at the remote end B of the protected line
may fail to trip for an internal fault near the source-
end A, because its LOP sets up before the terminal A
trips. This is because the relay at B sees no current,
but sees a low voltage condition before circuit
breaker A opens.
Also, for a condition as shown in Figure 12C, an
evolving fault at 50% location with symmetrical
source impedances, MDAR (REL-300)'s at both A
and B will not trip if LOPB is selected to "YES",
because neither MDAR (REL-300) sees a current
change (or no Io) on the protected line to set up the
LOPB logic before the second fault is applied.
Logic AND-1A,-1B,-1C,-1E and OR-1D, and 150/0, 0/
3500 ms timer circuits in Figure 12A are for solving
these problems. This logic unblocks the 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. The 150 ms timer can
be extended to 500 ms if a jumper (JMP3) on the
Microprocessor module is installed.
9.2 MONITORING OF AC CURRENT (LOI)
The ac current monitoring circuit is similar to the LOP
logic, except using I and not V
OM
shown in Figure 13. Under a ct short circuit or open
circuit condition, I and not Vo satisfies AND-23.
OM
The LOI output signal from AND-23 starts the 500/
500 ms timer. The output of the timer will turn on the
non-memory LOI indicator on the metering display
during the LOI condition. This output is also con-
nected to the failure alarm output relay GS (AL-3).
The LOI signal also can be used to block the MDAR
(REL-300) trip outputs under LOI condition if the
LOIB (loss-of-current block) is selected to YES.
9.3 OVERCURRENT SUPERVISION
(Figure 14)
The MDAR (REL-300) distance units do not require
overcurrent supervision, because the relay normally
operates in a background mode. They will not start
the zone-1 and pilot impedance computation until a
phase current or a phase voltage disturbance is
detected. This approach can minimize the load prob-
lem on setting the phase overcurrent units. However,
in order to meet the traditional practice, a medium set
phase overcurrent unit I
been added to MDAR (REL-300) versions 2.10/2.60
to supervise Z1P/Z2P/Z3P/PLTP trip functions, as
shown in Figure 14. This unit should not be set to
limit the zone-3 reach, and traditionally should be set
above the load current.
For increasing the security, the application of LOPB
supervision function is still normally recommended.
For coordination purposes, the ground trip units Z1G,
Z2G, Z3G, PLTG, and FDOG are supervised by the
medium set ground overcurrent unit I
Z3G/RDOG units (for carrier start on blocking
scheme, and for reverse blocking on permissive
scheme) are supervised by the low set IOS. The Iom
I.L. 40-385.5
as a criterion, as
o
(I or I or I ) has
M AM BM CM
, except the
OM
9-1