Schweitzer Engineering Laboratories SEL-421-4 - Page 204

5.44
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Ground Directional Element
Reverse Threshold
If Z2R is greater than or equal to 0:
Equation 5.18
If Z2R is less than 0:
Equation 5.19
32V
Directional Calculation
Equation 5.20
Forward Threshold
If Z0F is less than or equal to 0:
Equation 5.21
If Z0F is greater than 0:
Equation 5.22
Reverse Threshold
If Z0R is greater than or equal to 0:
Equation 5.23
If Z0R is less than 0:
Equation 5.24
Z2RTH 0.75 Z2R 0.25
V
2
I
2
------
• +• =
Z2RTH 1.25 Z2R 0.25
V
2
I
2
------
• +• =
z0
Re 3V
0
I
G
1 Z0ANG• 
*
• 
I
G
2
---------------------------------------------------------------------------=
Z0FTH 0.75 Z0F 0.25
3V
0
I
G
---------
•


–• =
Z0FTH 1.25 Z0F 0.25
3V
0
I
G
---------
•


–• =
Z0RTH 0.75 Z0R 0.25
3V
0
I
G
---------
• +• =
Z0RTH 1.25 Z0R 0.25
3V
0
I
G
---------
• +• =