Schweitzer Engineering Laboratories SEL-421-4 - Figure 6.30 Calculating Setting R1 R7

6.123
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
Out-of-Step Logic Application Examples
Assume that the maximum load angle is ±45°. Use trigonometry to calculate
R1R7, which is the distance from the origin to the right-hand resistance blinder
along line OP, the c side of the right triangle (see Table 6.30). The resistance
blinders are parallel to the line characteristic impedance Z1L1, for which the
angle is setting Z1ANG.
Equation 6.72
Rearrange Equation 6.72 and multiply by a safety factor of 90 percent to calcu-
late R1R7.
Equation 6.73
R1R7 := 15.53. Zone 7 Resistance—Right (0.05–140  secondary)
where:
A = 45° + (90°–Z1ANG)
b = Z
L(min)
c = setting R1R7
Figure 6.30 Calculating Setting R1R7
Acos
c
b
---=
45˚
R
X
b
a
c
O
P
A
Z
IL1
B
C
Export
Load
Minimum Load Impedance
Safety Margin
R1R7 0.9% Z
L min
Acos• • =
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 =