Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
148
Figure 2-53
Direction determination with unfaulted voltages (cross polarizing)
Table 2-11
Voltage and current values for the determination of fault direction
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).
Loop
Measuring
Current (Direc-
tion)
Actual short-circuit voltage
Unfaulted voltage
L1-E
I
L1
U
L1-E
U
L2
- U
L3
L2-E
I
L2
U
L2-E
U
L3
- U
L1
L3-E
I
L3
U
L3-E
U
L1
- U
L2
I
L1
-
I
U
L1-E
U
L2
- U
L3
I
L2
-
I
U
L2-E
U
L3
- U
L1
I
L3
-
I
U
L3-E
U
L1
- U
L2
L1-L2
I
L1
-
I
L2
U
L1
- U
L2
U
L2-L3
- U
L3-L1
L2-L3
I
L2
-
I
L3
U
L2
- U
L3
U
L3-L1
- U
L1-L2
L3-L1
I
L3
-
I
L1
U
L3
- U
L1
U
L1-L2
- U
L2-L3
