Schweitzer Engineering SEL-351P, Quick Start Manual

Page: 208 / 674

3-40 Overcurrent, Voltage, Synchronism Check, and Frequency Elements Date Code 990430
SEL-351P Manual Técnico
If synchronism check is not being used, Relay Word bits 59VP, 59VS, and 59VA can still be
used in other logic, with voltage limit settings 25VLO and 25VHI set as desired. Enable the
synchronism check logic (setting E25 = Y) and make settings 25VLO and 25VHI. Apply Relay
Word bits 59VP, 59VS, and 59VA in desired logic scheme, using SEL
OGIC
Control Equations.
Even though synchronism check logic is enabled, the synchronism check logic outputs (Relay
Word bits SF, 25A1, and 25A2) do not need to be used.
Block Synchronism Check Conditions
Refer to Figure SECTION 3: .24.
The synchronism check element slip frequency calculator runs if both voltages V
P
and V
S
are
healthy (59VP and 59VS asserted to logical 1) and the SEL
OGIC
Control Equation setting
BSYNCH (Block Synchronism Check) is deasserted (= logical 0). Setting BSYNCH is most
commonly set to block synchronism check operation when the circuit breaker is closed
(synchronism check is only needed when the circuit breaker is open):
BSYNCH = IN101 (input IN101 connected to a breaker auxiliary 52a contact)
BSYNCH = !IN101 (input IN101 connected to a breaker auxiliary 52b contact)
In addition, synchronism check operation can be blocked when the relay is tripping:
BSYNCH = ... + TRIP
Slip Frequency Calculator
Refer to Figure SECTION 3: .24.
The synchronism check element Slip Frequency Calculator in Figure SECTION 3: .24 runs if
voltages V
P
, V
S
, and V
A
are healthy (59VP, 59VS, and 59VA asserted to logical 1) and the
SEL
OGIC
Control Equation setting BSYNCH (Block Synchronism Check) is deasserted (=
logical 0). The Slip Frequency Calculator output is:
Slip Frequency = f
P
- f
S
(in units of Hz = slip cycles/second)
f
P
= frequency of voltage V
P
(in units of Hz = cycles/second)
(determined from V
A
)
f
S
= frequency of voltage V
S
(in units of Hz = cycles/second)
A complete slip cycle is one single 360
º
revolution of one voltage (e.g., V
S
) by another voltage
(e.g., V
P
). Both voltages are thought of as revolving phasor-wise, so the “slipping” of V
S
past V
P
is the relative revolving of V
S
past V
P
.
For example, in Figure SECTION 3: .24, if voltage V
P
has a frequency of 59.95 Hz and voltage
V
S
has a frequency of 60.05 Hz, the difference between them is the slip frequency:
Slip Frequency = 59.95 Hz - 60.05 Hz = -0.10 Hz = -0.10 slip cycles/second
The slip frequency in this example is negative, indicating that voltage V
S
is not “slipping” behind
voltage V
P
, but in fact “slipping” ahead of voltage V
P
. In a time period of one second, the
angular distance between voltage V
P
and voltage V
S
changes by 0.10 slip cycles, which translates
into: