Schweitzer Engineering Laboratories SEL-351 - P and V Are Slipping and Setting TCLOSD ≠ 0.00

Date Code 20001006 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements 3-49
SEL-351 Instruction Manual
The top of Figure 3.28 shows the Angle Difference decreasing – V
S
*
is approaching V
P
. Ideally,
circuit breaker closing is initiated when V
S
*
is in-phase with V
P
(Angle Difference = 0°). Then
when the circuit breaker main contacts finally close, V
S
is in-phase with V
P
, minimizing system
shock.
The bottom of Figure 3.28 shows the Angle Difference increasing – V
S
*
is moving away from V
P
.
Ideally, circuit breaker closing is initiated when V
S
*
is in-phase with V
P
(Angle Difference = 0°).
Then when the circuit breaker main contacts finally close, V
S
is in-phase with V
P
. But in this case,
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°), 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 3.27) 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
Refer to top of Figure 3.27.
If V
P
and V
S
are “static” (not “slipping” with respect to one another), the Angle Difference between
them remains constant – it is not possible to close the circuit breaker at an ideal zero degree phase
angle difference. 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.
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” and Setting TCLOSD ≠ 0.00
Refer to bottom of Figure 3.27. If V
P
and V
S
are “slipping” with respect to one another and
breaker close time setting TCLOSD ≠ 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 3.28 shows the Angle Difference decreasing – V
S
*
is approaching V
P
. When
V
S
*
is in-phase with V
P
(Angle Difference = 0°), synchronism check elements 25A1 and 25A2
assert to logical 1.
2. The bottom of Figure 3.28 shows the Angle Difference increasing – V
S
*
is moving away from
V
P
. V
S
*
was in-phase with V
P
(Angle Difference = 0°), 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.
Courtesy of NationalSwitchgear.com