Sel SEL-421 - Negative Slip Frequency; “No-Slip” Condition

R.2.63
Date Code 20111215 Reference Manual SEL-421 Relay
Auto-Reclosing and Synchronism Check
Synchronism Check
clockwise rotation of V
S1
with respect to reference V
P
, as shown in
Figure 2.30. Relay Word bit FAST1 asserts to logical 1 (and Relay Word bit
SLOW1 deasserts to logical 0) to indicate this condition.
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
corresponding to V
P
; f
S1
< f
P
). For such a case, V
S1
rotates clockwise with
respect to reference 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 confirmed when FAST1 and SLOW1 are deasserted, and
SFZBK1 is asserted.
Synchronism-Check Element Output Effects
Compare the corresponding “slip—no compensation” cases in Figure 2.30 to
the previous “no-slip” cases in Figure 2.29. Note that synchronism-check
element outputs 25W1BK1 and 25A1BK1 operate identically in all cases of
the “slip—no compensation” examples in Figure 2.30 (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 2.29 and Figure 2.30.
The similarity of element outputs 25W1BK1 and 25A1BK1 for the “no slip”
condition (Figure 2.29) and the “slip—no compensation” (Figure 2.30)
condition results from the maximum slip frequency setting 25SFBK1 := OFF.
Setting 25SFBK1 has no effect in a “no slip” scenario (Figure 2.29), but the
setting does affect the operation of synchronism-check element output
25A1BK1 (see the “slip—no compensation” scenario, Figure 2.30).
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 element 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 2.29) (see the “slip—no compensation” in Figure 2.30) have no
effect on element output 25W1BK1.
“Slip—With
Compensation”
Synchronism Check
Figure 2.31 is derived from Figure 2.30, but with the maximum slip frequency
setting 25SFBK1 set to some value other than OFF; thus the SEL-421
compensates for circuit breaker closing time with setting TCLSBK1. This
results in a compensated normalized synchronism-check voltage source V'
S1