Schweitzer Engineering Laboratories SEL-421-4 - Page 468

6.132
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
Out-of-Step Logic Application Examples
Phase-Distance Element Blocking
Enable the OSB function to prevent tripping when the positive-sequence imped-
ance locus enters the Zone 1 and Zone 2 distance protection characteristics
during an unstable power swing. Therefore, in this application example, the relay
trips after the Z
1
impedance locus exits Zone 6 (Zone 6 drops out).
Block Zone 1 and Zone 2 distance protection elements during power swings.
OOSB1 := Y. Block Zone 1 (Y, N)
OOSB2 := Y. Block Zone 2 (Y, N)
OOSB3 := N. Block Zone 3 (Y, N)
Zone 6 and Zone 7 Impedance Settings
The purpose of this OOS application example is to configure the relay to trip
when the power system reaches a critical angle limit to prevent system collapse.
Thus, the Zone 6 impedance setting differs from Out-of-Step Blocking on
page 6.120.
Resistance Blinders
If the angle of the power swing Z
1
trajectory passes 120 degrees with respect to
the transfer impedance, the power system cannot recover. The transfer impedance
is the total impedance of the power system (line AB in Figure 6.33). Set the
Zone 6 right-hand inner resistance blinder R1R6 so Ang_R6 equals 120 degrees.
Rearrange Equation 6.77 as shown in Equation 6.80:
Equation 6.80
R1R6 := 5.87. Zone 6 Resistance—Right (0.05 to 140  secondary)
Use the minimum load impedance Z
L(min)
(Equation 6.71) and the 90 percent
safety margin criterion (see Equation 6.73) applied earlier in Resistance Blinders
on page 6.122 to set the Zone 7 right-hand resistance blinder.
Equation 6.81
R1R7 := 15.53. Zone 7 Resistance—Right (0.05–140  secondary)
R1R6
Z
T
2
---------
Ang_R6
2
----------------------


tan
------------------------------------=
8.80  88 8.00  87.6 3.52  88++
2
-----------------------------------------------------------------------------------------------------------------
120
2
-----------


tan
-----------------------------------------------------------------------------------------------------------------=
10.16
120
2
-----------


tan
-------------------------=
5.87 =
R1R7 0.9 Z
L min
45 90 Z1ANG–+cos• • =
0.9 25.49 45 90 87.6–+cos• • =
0.9 25.49 47.4cos• • =
15.53 =