Schweitzer Engineering SEL-351P, Quick Start Manual

Page: 211 / 674

Date Code 990430 Overcurrent, Voltage, Synchronism Check, and Frequency Elements 3-43
SEL-351P Manual Técnico
If the slip frequency is greater than 0.005 Hz, the Angle Difference Calculator takes the breaker
close time into account with breaker close time setting TCLOSD (set in cycles; see
Figure SECTION 3: .26). 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 SECTION 3: .25.
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
Resulting in:
Angle Difference =
⏐
(
∠
V
P
-
∠
V
S
) + [(f
P
- f
S
) x TCLOSD x (1 second/60 cycles) x
(360
°
/slip cycle)]
⏐
=
⏐
(
∠
V
P
-
∠
V
S
) + [-0.10 x 0.167 x 360
°
]
⏐
=
⏐
(
∠
V
P
-
∠
V
S
) - 6
°⏐
During the breaker close time (TCLOSD), the voltage angle difference between voltages V
P
and
V
S
changes by 6
°
. This 6
°
angle compensation is applied to voltage V
S
, resulting in derived
voltage V
S
*
, as shown in Figure SECTION 3: .26 ( Note : The angle compensation in
Figure SECTION 3: .26 appears much greater than 6
°
. Figure SECTION 3: .26 is for general
illustrative purposes only).
The top of Figure SECTION 3: .26 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 SECTION 3: .26 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
.