3.46
SEL-351A Relay Instruction Manual Date Code 20080213
Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Synchronism Check Elements (Not in SEL-351A-1)
V
S
* has already moved past V
P
. In order to initiate circuit breaker closing
when V
S
* is in phase with V
P
(Angle Difference = 0 degrees), V
S
* has to slip
around another revolution, relative to V
P
.
Synchronism Check Element Outputs
Synchronism check element outputs (Relay Word bits 25A1 and 25A2 in
Figure 3.28) assert to logical 1 for the conditions explained in the following
text.
Voltages V
P
and V
S
Are “Static” or Setting TCLOSD = 0.00
To implement a simple fixed-angle synchronism check scheme, set
TCLOSD = 0 and 25SF = 0.500. With these settings, the synchronism check is
performed as described in the top of Figure 3.28.
If there is the possibility of a high slip frequency, exercise caution if
synchronism check elements 25A1 or 25A2 are used to close a circuit breaker.
A high slip frequency and a slow breaker close could result in closing the
breaker outside the synchronism check window. Qualify the breaker close
command with a time delay, such as:
SV1 = 25A1
CL = CC * SV1T
Set SV1PU 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 V
P
and V
S
are “Slipping” and Setting TCLOSD ≠ 0.00
Refer to bottom of Figure 3.28. If V
P
and V
S
are “slipping” with respect to
one another and breaker close time setting ≠ 0.00, 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 3.29 shows the Angle Difference
decreasing—V
S
* is approaching V
P
. When V
S
* is in phase
with V
P
(Angle Difference = 0 degrees), synchronism check
elements 25A1 and 25A2 assert to logical 1.
2. The bottom of Figure 3.29 shows the Angle Difference
increasing—V
S
* is moving away from V
P
. V
S
* was in phase
with V
P
(Angle Difference = 0 degrees), but has now moved
past V
P
. 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 V
S
* to slip
around again in phase with V
P
(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.
Courtesy of NationalSwitchgear.com
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