Schweitzer Engineering Laboratories SEL-351-5, Instruction Manual

Page: 120 / 706

3.40
SEL-351-5, -6, -7 Relay Instruction Manual
Date Code 20070117
Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements
Synchronism Check Elements
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 3.26 , 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 • (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 3.26 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
An application idea for SSLOW and SFAST is a small generator installation:
V
P
is from the generator side and V
S
is from the system side (other side of the
open circuit breaker). With some logic (perhaps to create pulsing signals),
SSLOW and SFAST are used as signals (via output contacts) to the generator
governor. “SSLOW” indicates that the generator (V
P
) is slower than (or equal
in frequency to) the system (V
S
), while “SFAST” indicates that the generator
(V
P
) is faster than the system (V
S
). If the enable into the slip frequency
calculator in Figure 3.26 is disabled (e.g., SEL
OGIC
setting BSYNCH asserts
because the breaker closes; BSYNCH = 52A + …), then both SSLOW =
logical 0 and SFAST = logical 0, regardless of slip frequency.
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 .