Siemens siprotec 7SA6 User Manual page 238

Functions
6-56
U
L1
U
L3
U
L2–L3
a) Phase–earth loop (L1–E)
Figure 6-30 Direction determination with quadrature voltages
Table 6-9 Allocation of the measured values for the direction determination
Measured current
Loop
L1 – E
L2 – E
L3 – E
*
L1 – E )
*
L2 – E )
*
L3 – E )
L1 – L2
L2 – L3
L3 – L1
*) k = Z /Z ; if only one phase-earth loop is picked up, the earth current I
E E L
If there is neither a current measured voltage nor a memorized voltage available which
is sufficient for measuring 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 6-31 shows the theoretical steady-state characteristic. In practice, the position
of the directional characteristic when using memorized voltages is dependent on both
the source impedance as well as 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 6-31).
As each zone may be set )RUZDUG, 5HYHUVH or 1RQ'LUHFWLRQDO there is a
separate (mirrored) directional characteristic for the "forward" and "reverse" direction.
U
L2
Short-circuit loop
(direction)
I
L1
I
L2
I
L3
*
– k )
I · I
L1 E E
*
– k )
I · I
L2 E E
*
– k )
I · I
L3 E E
I – I
L1 L2
I – I
L2 L3
I – I
L3 L1
U
L3–L1
– U
L1–L2
U
L3–L1
U
L2–L3
b) Phase–phase loop (L2–L3)
Quadrature
voltage
U U
L1–E
U U
L2–E
U U
L3–E
U U
L1–E
U U
L2–E
U U
L3–E
U – U U
L1 L2 L2–L3
U – U U
L2 L3 L3–L1
U – U U
L3 L1 L1–L2
E
C53000-G1176-C133-1
U
L1–L2
voltage
– U
L2 L3
– U
L3 L1
– U
L1 L2
– U
L2 L3
– U
L3 L1
– U
L1 L2
– U
L3–L1
– U
L1–L2
– U
L2–L3
is considered
7SA6 Manual