Schweitzer Engineering Laboratories SEL-421-4 - Page 470

6.134
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
Out-of-Step Logic Application Examples
Equation 6.83
Angle Ang_R6 was specified at 120.0° as a design criterion for this application
example (see Zone 6 and Zone 7 Impedance Settings on page 6.121).
Equation 6.84
Apply a fast unstable swing frequency and calculate OSTD (for this application
example, 
slip
= 10 Hz for an unstable power swing).
Equation 6.85
The OSTD timer settings are in increments of 0.125 cycle; round up to the near-
est valid relay setting.
OSTD := 0.875. Out-of-Step Trip Delay (0.500–8000 cycles)
To find the effective slip rate for OOS tripping, solve Equation 6.85 for f
slip
:
Equation 6.86
where:
Z
T
= transfer impedance
Z1
S
= positive-sequence source impedance
Z1
L1
= positive-sequence impedance for Line 1
Z1
R
= Positive-sequence remote impedance
where:

NOM
= nominal power system frequency (Hz)

slip
= maximum slip frequency (Hz)
Z
T
Z
1S
Z
1L1
Z
1R
++=
Ang_R7 2
Z
T
2
---------
R1R7
---------------
atan• =
2
8.80  88 8.00  87.6 3.52  88++
2
-----------------------------------------------------------------------------------------------------------------
15.53 
-----------------------------------------------------------------------------------------------------------------
atan•




=
66.4=
OSTD
Ang_R6 Ang_R7–f
NOM
•
360
cycle
------------
f
slip
•
---------------------------------------------------------------------------=
120.0 66.4–60 Hz•
360
cycle
------------ 10 Hz•
--------------------------------------------------------------=
0.89 cycles=
f
slip
Ang_R6 Ang_R7–f
NOM
•
360
cycle
------------ OSTD•
---------------------------------------------------------------------------=
120 66.4–60 Hz•
360
cycle
------------ 0.875•
---------------------------------------------------------=
10.2 Hz=