5.50
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Out-of-Step Logic (Conventional)
Out-of-Step Logic (Conventional)
The SEL-421 offers both conventional and settingless (zero-setting) out-of-step
(OOS) functions. To use the conventional OOS function, set EOOS = Y. To use
the zero-setting OOS function, set EOOS = Y1.
The out-of-step (OOS) logic determines whether a power swing is stable. This
relay logic can be set to either block distance protection or allow tripping when
the measured positive-sequence impedance (Z
1
) remains between inner Zone 6
and outer Zone 7 longer than either the OOS blocking delay (setting OSBD) or
the OOS tripping delay (setting OSTD), respectively (refer to Figure 5.32).
NOTE: E50Q must be set to 1 or
greater for enabling 67Q1T override of
OOS blocking for Zone 1 (see
Figure 5.57
,
Figure 5.60
, and
Figure 5.63
).
The OOS logic detects all power swings that enter the OOS characteristics, even
if a single-pole open condition exists (Relay Word bit SPO equals logical 1). If
either negative-sequence directional element 67QUBF or 67QUBR (67Q1T for
Zone 1) picks up during a power swing and a single-pole open condition does not
exist (Relay Word bit SPO equals logical 0), the logic overrides OOS blocking
Figure 5.31 Load-Encroachment Characteristics
ZLIN
(Load In
Region)
(180˚)
(90˚)
X1
PLAR
PLAF
NLAR
NLAF
ZLF
ZLR
(270˚) (—90˚)
ZLOUT
(Load Out
Region)
(0˚)
R1
Table 5.44 Load-Encroachment Logic Relay Settings
Setting Prompt Range
Default
(5 A)
ELOAD Load Encroachment Y, N Y
ZLF Forward Load Impedance () (0.25–320)/I
NOM
9.22
ZLR Reverse Load Impedance () (0.25–320)/I
NOM
9.22
PLAF Forward Load Positive angle (°) –90.0 to +90 30.0
NLAF Forward Load Negative angle (°) –90.0 to +90 –30.0
PLAR Reverse Load Positive angle (°) 90.0–270 150.0
NLAR Reverse Load Negative angle (°) 90.0–270 210.0
Table 5.45 Load-Encroachment Logic Relay Word Bits
Name Description
ZLOAD ZLIN OR ZLOUT
ZLIN Import load impedance detected
ZLOUT Export load impedance detected
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