5.174
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Synchronism Check
For any case [(a), (b), (c), or (d)] in Figure 5.130, the location of V'
S1
is the loca-
tion of V
S1
a period later (this period is setting TCLSBK1, Circuit Breaker BK1
Close Time). Consider, for example, issuing a close command to Circuit Breaker
BK1. If case (b) in Figure 5.130 represents the time at which the close command
occurs, then V
S1
is the normalized synchronism-check voltage source position at
the instant the close is issued and V'
S1
is the position of V
S1
when Circuit
Breaker BK1 actually closes.
Slip Frequency
If the slip frequency exceeds setting 25SFBK1, synchronism check cannot pro-
ceed via element output 25A1BK1. Synchronism check stops because element
output 25A1BK1 deasserts to logical 0 for an out-of-range slip frequency condi-
tion, regardless of other synchronism-check conditions such as healthy voltage
magnitudes.
Synchronism check remains possible (although not necessarily advantageous) if
you use element output 25W1BK1 and the slip frequency exceeds setting
25SFBK1. Synchronism-check element 25W1BK1 does not measure slip. In this
instance, synchronism check occurs (25W1BK1 is logical 1) when the phase
angle difference between reference V
P
and V
S1
is less than angle setting
ANG1BK1.
Synchronism-Check Element Output Effects
A contradiction seems to result from analysis of case (a) in Figure 5.130; it
appears that element output 25A1BK1 should assert to logical 1 because V'
S1
is
within angle setting ANG1BK1. Note in this case, however, that V'
S1
is
approaching synchronism-check reference VP. This is where element output
25A1BK1 behaves differently than element output 25W1BK1, for setting
25SFBK1 set to some value other than OFF. As V'
S1
approaches V
P
, 25A1BK1
remains deasserted (equals logical 0) until the phase angle difference between
reference V
P
and V'
S1
equals zero degrees.
At this zero degrees difference between V
P
and V'
S1
point, element output
25A1BK1 asserts to logical 1. We know the systems will truly be in synchronism
(0 degrees between reference V
P
and V
S1
) a period later (this period is setting
TCLSBK1, Circuit Breaker BK1 Close Time). Thus, if a close command occurs
right at the instant that element output 25A1BK1 asserts to logical 1, then there
will be a zero degree phase angle difference across Circuit Breaker BK1 when
Circuit Breaker BK1 actually closes. Closing Circuit Breaker BK1 at a phase
angle difference of 0 degrees between reference V
P
and V'
S1
minimizes system
shock when you bring two asynchronous systems together.
Element output 25A1BK1 remains asserted to logical 1 as V'
S1
moves away from
reference V
P
. When the phase angle difference between reference V
P
and V'
S1
is
again greater than angle setting ANG1BK1, element output 25A1BK1 deasserts
to logical 0.
Alternative Synchronism-Check Source 2 Settings
You can program alternative input sources for the synchronism-check function in
the SEL-421. Alternative inputs give you additional flexibility to synchronize
other portions of your power system.
The SEL
OGIC control equation ALTS2 determines when the relay uses alternate
Synchronism-Check Voltage Source 2 in place of regular Synchronism-Check
Voltage Source 2. When ALTS2 is logical 1, the relay substitutes alternative Syn-
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