Siemens SIPROTEC Manual page 148

Functions
2.5 Distance Protection
Figure 2-53
Table 2-11
Loop
L1-E
L2-E
L3-E
1)
L1-E
1)
L2-E
1)
L3-E
L1-L2
L2-L3
L3-L1
1)
with consideration of earth impedance compensation
If there is neither a current measured voltage nor a memorized voltage available which is sufficient for measur-
ing the direction, the relay selects the Forward direction. In practice this can only occur when the circuit
breaker closes onto a de-energized line, and there is a fault on this line (e.g. closing onto an earthed line).
Figure 2-54 shows the theoretical steady-state characteristic. In practice, the limits of the directional character-
istic when using memorized voltages is dependent on both the source impedance and the load transferred
across the line prior to fault inception. Accordingly the directional characteristic includes a safety margin with
respect to the borders of the first quadrant in the R–X diagram (Figure 2-54).
148
Direction determination with unfaulted voltages (cross polarizing)
Voltage and current values for the determination of fault direction
Measuring
Current (Direc-
tion)
U
I
L1
I U
L2
I U
L3
1)
I - I U
L1 E
1)
I - I U
L2 E
1)
- I U
I
L3 E
- I U
I
L1 L2
I - I U
L2 L3
I - I U
L3 L1
Actual short-circuit voltage
L1-E
L2-E
L3-E
L1-E
L2-E
L3-E
- U
L1 L2
- U
L2 L3
- U
L3 L1
Unfaulted voltage
U - U
L2 L3
U - U
L3 L1
U - U
L1 L2
U - U
L2 L3
U - U
L3 L1
U - U
L1 L2
U - U
L2-L3 L3-L1
U - U
L3-L1 L1-L2
U - U
L1-L2 L2-L3
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011