Date Code 20001006 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements 3-45
SEL-351 Instruction Manual
In addition, synchronism check operation can be blocked when the relay is tripping:
BSYNCH = ... + TRIP
Slip Frequency Calculator
Refer to Figure 3.26.
The synchronism check element Slip Frequency Calculator in Figure 3.26 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
SELOGIC 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§ 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 3.26, 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 x (360§/slip cycle) x 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§.
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.
Angle Difference Calculator
The synchronism check element Angle Difference Calculator in Figure 3.27 runs if the slip
frequency is less than the maximum slip frequency setting 25SF (Relay Word bit SF is asserted).
Voltages V
P
and V
S
are “Static”
Refer to top of Figure 3.27.
Courtesy of NationalSwitchgear.com
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