Schweitzer Engineering Laboratories SEL-351A - Page 114

3.42
SEL-351A Relay Instruction Manual Date Code 20080213
Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Synchronism Check Elements (Not in SEL-351A-1)
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 = 52A (see Figure 6.1 and Figure 7.2)
In addition, synchronism check operation can be blocked when the relay is
tripping:
BSYNCH = ... + TRIP
Slip Frequency Calculator
Refer to Figure 3.27.
The synchronism check element Slip Frequency Calculator in Figure 3.27
runs if voltages V
P
, V
S
, and V
A
(or V
AB
for delta) 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
(or V
AB
for delta)]
f
S
= frequency of voltage V
S
(in units of Hz = cycles/second)
A complete slip cycle is one single 360-degree 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 3.27, 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:
0.10 slip cycles/second • (360°/slip cycle) • 1 second = 36°
Thus, in a time period of one second, the angular distance between voltage V
P
and voltage V
S
changes by 36 degrees.
The absolute value of the Slip Frequency output is run through a comparator
and if the slip frequency is less than the maximum slip frequency setting,
25SF, Relay Word bit SF asserts to logical 1.
Generator Application for SSLOW and SFAST
Relay Word bits SSLOW and SFAST in Figure 3.27 indicate the relative slip
of voltages V
P
and V
S
:
f
P
≤ f
S
: SSLOW = logical 1, SFAST = logical 0
f
P
> f
S
: SSLOW = logical 0, SFAST = logical 1
Courtesy of NationalSwitchgear.com