3.41
Date Code 20080213 Instruction Manual SEL-351A Relay
Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Synchronism Check Elements (Not in SEL-351A-1)
System Rotation Can Affect Setting SYNCP
The solution in the preceding paragraph:
➤ Voltage input VA connected to Phase A
➤ Voltage input VS connected to Phase B
➤ Setting SYNCP = 120 degrees (V
S
constantly lags V
A
by 120°)
presumes ABC system rotation. If voltage input connections are the same, but
system rotation is ACB, then setting SYNCP = 240 degrees (V
S
constantly
lags V
A
by 240°). See the SEL Application Guide entitled Compensate for
Constant Phase Angle Difference in Synchronism Check with the SEL-351
Relay Family for more information on setting SYNCP with an angle setting.
Synchronism Check
Elements Operation
Refer to Figure 3.27 and Figure 3.28.
Voltage Window
Refer to Figure 3.27.
Single-phase voltage inputs V
P
and V
S
are compared to a voltage window, to
verify that the voltages are “healthy” and lie within settable voltage limits
25VLO and 25VHI. If both voltages are within the voltage window, the
following Relay Word bits assert:
59VP indicates that voltage V
P
is within voltage window setting limits
25VLO and 25VHI
59VS indicates that voltage V
S
is within voltage window setting limits
25VLO and 25VHI
As discussed previously, voltage V
A
determines the frequency on the voltage
V
P
side of the circuit breaker. Voltage V
A
is also run through voltage limits
25VLO and 25VHI to ensure “healthy voltage” for frequency determination,
with corresponding Relay Word bit output 59VA.
Other Uses for Voltage Window Elements
If voltage limits 25VLO and 25VHI are applicable to other control schemes,
Relay Word bits 59VP, 59VS, and 59VA can be used in other logic at the same
time they are used in the synchronism check logic.
If synchronism check is not being used, Relay Word bits 59VP, 59VS, and
59VA can still be used in other logic, with voltage limit settings 25VLO and
25VHI set as desired. Enable the synchronism check logic (setting E25 = Y)
and make settings 25VLO and 25VHI. Apply Relay Word bits 59VP, 59VS,
and 59VA in the desired logic scheme, using SEL
OGIC control equations.
Even though synchronism check logic is enabled, the synchronism check logic
outputs (Relay Word bits SF, 25A1, and 25A2) do not need to be used.
Block Synchronism Check Conditions
Refer to Figure 3.27.
The synchronism check element slip frequency calculator runs if both voltages
V
P
and V
S
are healthy (59VP and 59VS asserted to logical 1) and the
SEL
OGIC control equation setting BSYNCH (Block Synchronism Check) is
Courtesy of NationalSwitchgear.com
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