5.49
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Functions
Load-Encroachment Logic
Load-Encroachment Logic
The load-encroachment logic prevents load from causing phase protection to
operate. You can set the phase-distance and phase overcurrent elements indepen-
dent of load. Two independent positive-sequence impedance characteristics mon-
itor the positive-sequence load impedance (Z
1
) for both export and import load.
The positive-sequence voltage-polarized directional element (32P) is blocked
when the load-encroachment logic is enabled and load is detected. The phase-
distance elements cannot operate during balanced system conditions unless the
logic asserts the 32P element.
Figure 5.30 illustrates the load-encroachment logic. The logic operates only if the
positive-sequence current (I
1
) is greater than the positive-sequence threshold
(10 percent of the nominal relay current). Relay Word bit ZLOUT indicates that
load is flowing out with respect to the relay (an export condition). Relay Word bit
ZLIN indicates that load is flowing in with respect to the relay (an import condi-
tion). Figure 5.31 illustrates load-encroachment settings and corresponding char-
acteristics in the positive-sequence impedance plane. Either Relay Word bit
ZLOUT or ZLIN asserts if the relay measures a positive-sequence impedance
that lies within the corresponding hatched region. Relay Word bit ZLOAD is the
OR combination of ZLOUT and ZLIN.
Table 5.43 Series-Compensation Line Logic Relay Settings
Setting Prompt Range
Default
(5 A)
ESERCMP Series-Compensation Line Logic Y, N N
XC Series Capacitor Reactance () (OFF, 0.25–320 )/I
NOM
OFF
Figure 5.30 Load-Encroachment Logic Diagram
(Setting)
(Setting)
(Setting) (Setting)
Pos.-Seq.
I1
V1
V1
Z1 =
ZLOAD
Z1 (angle)
I1L
Z1 (magnitude)
Threshold
NLAR > Z1 > PLAR
PLAF > Z1 > NLAF
ZLR
ZLF
ZLOUT
Relay
Word
Bits
ZLIN
Pos.-Seq. Threshold = 0.1 • I
NOM
Load Out
Load In
(Setting)
(Setting)
+
–
+
–
+
–
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