Schweitzer Engineering Laboratories SEL-351 - Angle Difference Example; P and V S Are Slipping); S Are Slipping; Voltages V P and V S

3-46 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements Date Code 20001006
SEL-351 Instruction Manual
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
:
Angle Difference = (∠V
P
- ∠V
S
)
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:
Angle Difference = (∠ V
P
- ∠ V
S
) = 10°
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 3.27.
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 3.28). 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
) + [(f
P
- f
S
) x TCLOSD x (1 second/60 cycles) x
(360°/slip cycle)]
Angle Difference Example (Voltages V
p
and V
s
are “Slipping”)
Refer to bottom of Figure 3.27.
For example, if the breaker close time is 10 cycles, set TCLOSD = 10. Presume the slip frequency
is the example slip frequency calculated previously. 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
) + [(f
P
- f
S
) x TCLOSD x (1 second/60 cycles) x
(360°/slip cycle)]
Intermediate calculations:
(f
P
- f
S
) = (59.95 Hz - 60.05 Hz) = -0.10 Hz = -0.10 slip cycles/second
TCLOSD x (1 second/60 cycles) = 10 cycles x (1 second/60 cycles) = 0.167 second
Courtesy of NationalSwitchgear.com