4.113
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
The top of Figure 4.69 shows the Angle Difference decreasing—VS* is
approaching VP. Ideally, circuit breaker closing is initiated when VS* is in
phase with VP (Angle Difference = 0 degrees). Then when the circuit breaker
main contacts finally close, VS is in phase with VP, minimizing system shock.
The bottom of Figure 4.69 shows the Angle Difference increasing—VS* is
moving away from VP. Ideally, circuit breaker closing is initiated when VS* is
in phase with VP (Angle Difference = 0 degrees). Then when the circuit
breaker main contacts finally close, VS is in phase with VP. But in this case,
VS* has already moved past VP. To initiate circuit breaker closing when VS*
is in phase with VP (Angle Difference = 0 degrees), VS* has to slip around
another revolution, relative to VP.
Synchronism-Check Element Outputs
Synchronism-check element outputs (Relay Word bits 25A1 and 25A2 in
Figure 4.68) assert to logical 1 for the conditions explained in the following
text.
Voltages VP and VS Are “Static” or Setting TCLOSD := OFF. To imple-
ment a simple fixed-angle synchronism-check scheme, set TCLOSD := OFF
and 25SF = 0.50. With these settings, the synchronism check is performed as
described in the top of Figure 4.68.
If there is the possibility of a high slip frequency, exercise caution if you use
synchronism-check elements 25A1 or 25A2 to close a circuit breaker. A high
slip frequency and a slow breaker close could result in closing the breaker out-
side the synchronism-check window. Qualify the breaker close command with
a time delay, such as:
SV06 := 25A1
CL := CC and SV06T
Set SV06PU with enough pickup delay to ensure that the slip frequency is low
enough for the circuit breaker to close within the synchronism-check window.
Voltages VP and VS Are “Slipping” and Setting TCLOSD OFF.
Refer to bottom of Figure 4.68. If VP and VS are “slipping” with respect to
one another and breaker close time setting TCLOSD OFF, the Angle Differ-
ence (compensated by breaker close time TCLOSD) changes through time.
Synchronism-check element 25A1 or 25A2 asserts to logical 1 for any one of
the following three scenarios.
1. The top of Figure 4.69 shows the Angle Difference
decreasing—VS* is approaching VP. When VS* is in phase
with VP (Angle Difference = 0 degrees), synchronism-check
elements 25A1 and 25A2 assert to logical 1.
2. The bottom of Figure 4.69 shows the Angle Difference
increasing—VS* is moving away from VP. VS* was in phase
with VP (Angle Difference = 0 degrees), but has now moved
past VP. If the Angle Difference is increasing, but the Angle
Difference is still less than maximum angle settings 25ANG1
or 25ANG2, then corresponding synchronism-check elements
25A1 or 25A2 assert to logical 1.
In this scenario of the Angle Difference increasing, but still
being less than maximum angle settings 25ANG1 or 25ANG2,
the operation of corresponding synchronism-check elements
25A1 and 25A2 becomes less restrictive. Synchronism-check
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