
3-40
Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Date Code 20020215
SEL-351R Instruction Manual
Slip Frequency Calculator
Refer to Figure 3.24.
The synchronism check element Slip Frequency Calculator in Figure 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 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 3.24, 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:
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 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.
Angle Difference Calculator
The synchronism check element Angle Difference Calculator in Figure 3.25 runs if the slip
frequency is less than the maximum slip frequency setting 25SF (Relay Word bit SF is asserted).
If SYNCP = 0, 30, …, 300, or 330 degrees (indicating how many degrees V
S
constantly lags V
P
=
V
A
), the Angle Difference Calculator automatically accounts for this angular difference.
Voltages V
P
and V
S
are “Static”
Refer to top of Figure 3.25.
If the slip frequency is less than or equal to 0.005 Hz, the Angle Difference Calculator does not
take into account breaker close time—it presumes voltages V
P
and V
S
are “static” (not “slipping”
with respect to one another). This would usually be the case for an open breaker with voltages
Содержание SEL-351R
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