Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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5.172
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Synchronism Check
Negative Slip Frequency
If the slip frequency is negative, V
S1
is slipping behind reference V
P
(the system
corresponding to V
S1
has a lower system frequency than the system correspond-
ing to V
P
; f
S1
< f
P
). For such a case, V
S1
rotates clockwise with respect to refer-
ence V
P
. Relay Word bit SLOW1 asserts to logical 1 (and Relay Word bit FAST1
deasserts to logical 0) to indicate this condition.
“No-Slip” Condition
If the absolute value of the slip is less than 0.005 Hz (|f
S1
–f
P
| < 0.005 Hz; a “no-
slip” condition), both Relay Word bits FAST1 and SLOW1 deassert to logical 0
and Relay Word bit SFZBK1 asserts to logical 1. A “no-slip” condition is con-
firmed when FAST1 and SLOW1 are deasserted, and SFZBK1 is asserted.
Synchronism-Check Element Output Effects
Compare the corresponding “slip—no compensation” cases in Figure 5.129 to
the previous “no-slip” cases in Figure 5.128 . Note that synchronism-check ele-
ment outputs 25W1BK1 and 25A1BK1 operate identically in all cases of the
“slip—no compensation” examples in
Figure 5.129 (both assert to logical 1 or
deassert to logical 0). The condition of “no slip” or “slip—no compensation”
does not affect the operation of element outputs 25W1BK1 and 25A1BK1 in the
scenarios depicted in
Figure 5.128 and Figure 5.129 .
The similarity of element outputs 25W1BK1 and 25A1BK1 for the “no slip”
condition (
Figure 5.128 ) and the “slip—no compensation” ( Figure 5.129 ) condi-
tion results from the maximum slip frequency setting 25SFBK1 := OFF. Setting
25SFBK1 has no effect in a “no slip” scenario (
Figure 5.128 ), but the setting
does affect the operation of synchronism-check element output 25A1BK1 (see
the “slip—no compensation” scenario,
Figure 5.129 ).
With setting 25SFBK1 := OFF, the relay does not compensate for the further
angular travel of V
S1
(with respect to reference V
P
) during the Circuit Breaker
BK1 close time setting TCLSBK1. The relay measures the phase angle directly
with no compensation between reference V
P
and V
S1
for synchronism-check ele-
ment output 25A1BK1.
The relay always measures the phase angle directly (without compensation)
between reference V
P
and V
S1
for element output 25W1BK1. Setting 25SFBK1,
time setting TCLSBK1, and whether system conditions are “no slip”
(
Figure 5.128 ) (see the “slip—no compensation” in Figure 5.129 ) have no effect
on element output 25W1BK1.
“Slip—With Compensation” Synchronism Check
Figure 5.130 is derived from Figure 5.129 , but with the maximum slip frequency
setting 25SFBK1 set to some value other than OFF; thus the SEL-421 compen-
sates for circuit breaker closing time with setting TCLSBK1. This results in a
compensated normalized synchronism-check voltage source V'
S1
.
Synchronism-check element output 25W1BK1 in Figure 5.130 operates the same
as in Figure 5.129 . Element output 25W1BK1 is unaffected by relay settings
25SFBK1 and TCLSBK1, and by whether system conditions are slipping. Ele-
ment 25W1BK1 follows normalized synchronism-check voltage source V
S1
.