Siemens siprotec 7SA6 User Manual page 291

Direction
Determination with
Negative Sequence
System
Blocking
Switching onto an
Earth Fault
7SA6 Manual
C53000-G1176-C133-1
Im
„reverse"
Figure 6-67 Directional characteristic using I
For the determination of direction a minimum current and a minimum polarization
quantity is required. The minimum polarizing voltage set as 8!. If the displacement
voltage is too small, the direction can only be determined if it is polarized with the
transformer star-point current and this exceeds a minimum value corresponding to the
setting ,<!. The direction determination with U
transformer mcb is reported via binary input.
It is advantageous to use negative sequence system values for the direction
measurement if the resulting zero sequence voltages during earth faults are too small
for an accurate measurement or when the zero sequence values are subject to
interference by for example mutual coupling from a parallel line.
Otherwise this function operates the same as the direction measurement with zero
sequence current and zero sequence voltage. The negative sequence signals 3I
3U are simply used instead of the signals 3I
2
a minimum magnitude of ,! or 8!.
The earth fault protection can be blocked by the distance protection. If in this case a
fault is detected by the distance protection, the earth fault protection will not operate.
This gives the selective fault clearance by the distance protection preference over
tripping by the earth fault protection.
The earth fault protection can also be blocked during the single-pole dead time of an
automatic reclose cycle. This prevents an incorrect measurement resulting from the
zero sequence current and voltage signals arising in this state. If the device is
combined with an external automatic reclose device or if single-pole tripping can result
from a separate (parallel tripping) protection device, the earth fault protection must be
blocked via binary input during the single-pole open condition.
To achieve fast tripping following manual closure of the circuit breaker on to an earth
fault, the manual close command from the control switch can be routed to the device
via a binary input. The earth fault protection can then trip three-pole without delay. The
stage(s) that should be activated for instantaneous tripping after manual closure can
be selected with setting parameters. (refer to logic diagrams Figure 6-61 to 6-63).
Re
„forward"
U = 3U
E 0
α
= – 3I
I
E 0
β
= I
I
P Y
as polarization quantity
Y
is inhibited when a trip of the voltage
E
und 3U . These signals must also have
0 0
Functions
und
2
6-109