4.41
Date Code 20100129 Instruction Manual SEL-751A Relay
Protection and Logic Functions
Voltage-Based Protection
Voltages VP and VS Are “Slipping” and Setting TCLOSD ≠ OFF. Refer to
bottom of Figure 4.21. If VP and VS are “slipping” with respect to one
another and breaker close time setting TCLOSD ≠ OFF, the Angle Difference
(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 4.22 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 4.22 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
breaker closing does not have to wait for voltage VS* to slip
around again in phase with VP (Angle Difference = 0 degrees).
There might not be enough time to wait for this to happen.
Thus, the “Angle Difference = 0 degrees” restriction is eased
for this scenario.
3. Refer to Reclose Supervision Logic on page 4.54.
Refer to the bottom of Figure 4.31. If timer 79CLSD is set
greater than zero (e.g., 79CLSD := 100 ms) and it times out
without SEL
OGIC control equation setting 79CLS (Reclose
Supervision) asserting to logical 1, the relay goes to the
Lockout State (see top of Figure 4.32).
Refer to the top of Figure 4.31. If timer 79CLSD is set to zero
(79CLSD := 0.00), SEL
OGIC control equation setting 79CLS
(Reclose Supervision) is checked only once to see if it is
asserted to logical 1. If it is not asserted to logical 1, the relay
goes to the Lockout State.
Refer to the top of Figure 4.22. 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. But
with time limitations imposed by timer 79CLSD, this may not
be possible. To try to avoid going to the Lockout State, the
following logic is employed.
If 79CLS has not asserted to logical 1 while timer 79CLSD is
timing (or timer 79CLSD is set to zero and only one check of
79CLS is made), the synchronism-check logic at the bottom of
Figure 4.21 becomes less restrictive at the “instant” timer
79CLSD is going to time out (or make the single check). It
drops the requirement of waiting until the decreasing Angle
Difference (VS* approaching VP) brings VS* in phase with VP
(Angle Difference = 0 degrees). Instead, it just checks to see
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