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6.122
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Applications Examples
Out-of-Step Logic Application Examples
positive-sequence impedance locus (Z
1
) in Zone 6 and Zone 7 when a power
swing or fault occurs. Two factors affect the Zone 6 and Zone 7 impedance set-
tings:
➤
The outermost overreaching zone of phase-distance protection that
you want to block.
➤
The load impedance that the relay measures during maximum load
(minimum load impedance locus).
NOTE:
This settings philosophy
provides the most time for the relay to
decide whether a fault or a power
swing has occurred.
Set inner Zone 6 (X1T6, R1R6, X1B6, and R1L6) to encompass the outermost
zone of phase-distance protection that you have selected for out-of-step blocking.
Set Zone 7 so that the closest minimum load impedance locus is outside the
Zone 7 characteristic for all loading conditions.
Resistance Blinders
Zone 2 is the outermost characteristic for this particular example. Include a safety
margin (20 percent for this example).
When you set Zone 6, SEL recommends that you assume that Z1ANG is at 90
degrees. This allows the user to set the resistive reach of the zone along the x-
axis. Internally, the relay will adjust the setting by the line angle Z1ANG when
the zone setting is applied. Adjusting the setting by Z1ANG allows the resistive
reach to be parallel with the line angle, as shown in
. Use
to set the reach of Zone 6 along the x-axis.
Equation 6.70
R1R6 :=
5.77
.
Zone 6 Resistance—Right (0.05–140
secondary)
Set Zone 7 outer resistance blinders according to maximum load. In other words,
set the Zone 7 outer right-hand resistance blinder just inside the corresponding
minimum export load impedance locus (maximum load locus). The maximum
load current is 2.41 A secondary, determined from load studies. The correspond-
ing line-to-neutral voltage during maximum load at Station S is 61.44 V second-
ary.
Determine the minimum load impedance that the relay measures:
Equation 6.71
where:
Z2MP = Zone 2 mho phase-distance element reach (see
).
I
L(max)
=2.41 A
VLN
=61.44 V
R1R6
1.2
Z2MP
2
----------------
•
=
1.2
9.60
2
-----------------
•
=
5.77
=
Z
L
min
V
LN
I
L MAX
---------------------
=
61.44 V
2.41 A
-------------------
=
25.49
=
Summary of Contents for SEL-421-4
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