4.137
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
A transient synchronism-check condition across open circuit breaker 52/2
could possibly occur if circuit breaker 52/1 recloses into a fault on one phase
of the transmission line. The other two unfaulted phases would be briefly
energized until circuit breaker 52/1 is tripped again. If channel VS of the
SEL-751(2) is connected to one of these briefly energized phases, synchro-
nism-check element 25A1 could momentarily assert to logical 1.
So that this possible momentary assertion of synchronism-check element
25A1 does not cause any inadvertent reclose of circuit breaker 52/2, make
sure the open interval timers in the SEL-751(2) are set with some appreciable
time greater than the momentary energization time of the faulted transmission
line. Or, run the synchronism-check element 25A1 through a programmable
timer before using it in the preceding 79CLS and 79STL settings for the
SEL-751(2) (see Figure 4.80). Note the built-in 2-cycle qualification of the
synchronism-check voltages shown in Figure 4.67.
EXAMPLE 4.18 Settings Example 2
Refer to subsection Synchronism-Check Elements on page 4.106. Also refer
to Figure 4.82 and Figure 4.83.
If the synchronizing voltages across open circuit breaker 52/2 are “slipping”
with respect to one another, the Reclose Supervision Limit Timer setting
79CLSD should be set greater than zero so there is time for the slipping
voltages to come into synchronism. For example:
79CLSD := 1.00 second
79CLS := 25A1
The status of synchronism-check element 25A1 is checked continuously
during the 60-cycle window. If the slipping voltages come into synchronism
while timer 79CLSD is timing, synchronism-check element 25A1 asserts to
logical 1 and reclosing proceeds.
In the previous referenced subsection, note item 3 under Synchronism-
Check Element Outputs on page 4.113, Voltages VP and VS are “Slipping.”
Item 3 describes a last attempt for a synchronism-check reclose before
timer 79CLSD times out (or setting 79CLSD := 0.00 and only one check is
made).
If E79 := 3 (which allows three automatic reclose attempts) and the slipping
voltages fail to come into synchronism while timer 79CLSD is timing
(resulting in a reclose supervision failure, causing RCSF to assert for one
processing interval), then the reclosing relay goes to the lockout state.
If E79 := C3 (which allows three automatic reclose attempts) and the
slipping voltages fail to come into synchronism while timer 79CLSD is timing
(resulting in a reclose supervision failure, causing RCSF to assert for one
processing interval), then the reclosing relay increments the shot counter
and starts timing on the next open interval. This operation emulates a
rotating drum timer style reclosing relay—going onto the next open interval
time and reclose opportunity if supervising conditions for the present
reclose opportunity are not true. If the reclosing relay increments to the last
shot value (no more open intervals left; see Figure 4.86 and Table 4.55), the
reclosing relay is then driven to the lockout state.
Reclose Logic
Note that input:
Reclosing Relay Open Interval Time-Out
in Figure 4.81 is the logic input that is qualified by SEL
OGIC control equation
setting 79CLS, and then propagated on to the close logic in Figure 4.80 to
automatically reclose a circuit breaker. The explanation that follows in this
reclosing relay subsection describes all the reclosing relay settings and logic
that eventually result in this open interval time-out logic input into
Figure 4.81. Other aspects of the reclosing relay are also explained. As many
as four (4) automatic reclosures (shots) are available.
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