3.43
Date Code 20080213 Instruction Manual SEL-351A Relay
Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Synchronism Check Elements (Not in SEL-351A-1)
An application idea for SSLOW and SFAST is a small generator installation:
V
P
is from the generator side and V
S
is from the system side (other side of the
open circuit breaker). With some logic (perhaps to create pulsing signals),
SSLOW and SFAST are used as signals (via output contacts) to the generator
governor. “SSLOW” indicates that the generator (V
P
) is slower than (or equal
in frequency to) the system (V
S
), while “SFAST” indicates that the generator
(V
P
) is faster than the system (V
S
). If the enable into the slip frequency
calculator in Figure 3.27 is disabled (e.g., SEL
OGIC setting BSYNCH asserts
because the breaker closes; BSYNCH = 52A + …), then both SSLOW =
logical 0 and SFAST = logical 0, regardless of slip frequency.
Angle Difference Calculator
The synchronism check element Angle Difference Calculator in Figure 3.28
runs if the slip frequency is less than the maximum slip frequency setting
25SF (Relay Word bit SF is asserted).
Voltages V
P
and V
S
Are “Static”
Refer to top of Figure 3.28.
If the slip frequency is less than or equal to 0.005 Hz, the Angle Difference
Calculator does not take into account breaker close time—it presumes
voltages V
P
and V
S
are “static” (not “slipping” with respect to one another).
This would usually be the case for an open breaker with voltages V
P
and V
S
that are paralleled via some other electric path in the power system. The Angle
Difference Calculator calculates the angle difference between voltages V
P
and
V
S
:
Equation 3.1
For example, if SYNCP = 90 (indicating V
S
constantly lags V
P
= V
A
by
90 degrees), but V
S
actually lags V
A
by 100 angular degrees on the power
system at a given instant, the Angle Difference Calculator automatically
accounts for the 90 degrees and:
Equation 3.2
Also, if breaker close time setting TCLOSD = 0.00, the Angle Difference
Calculator does not take into account breaker close time, even if the voltages
V
P
and V
S
are “slipping” with respect to one another. Thus, synchronism
check elements 25A1 or 25A2 assert to logical 1 if the Angle Difference is
less than corresponding maximum angle setting 25ANG1 or 25ANG2.
Voltages V
P
and V
S
Are “Slipping”
Refer to bottom of Figure 3.28.
If the slip frequency is greater than 0.005 Hz and breaker close time setting
TCLOSD ≠ 0.00, the Angle Difference Calculator takes the breaker close time
into account with breaker close time setting TCLOSD (set in cycles; see
Figure 3.29). The Angle Difference Calculator calculates the Angle
Difference between voltages V
P
and V
S
, compensated with the breaker close
time:
Angle Difference V
P
V∠
S
–∠()=
Angle Difference V
P
V∠
S
–∠()10°==
Courtesy of NationalSwitchgear.com
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