Directional Unblocking Scheme - Siemens siprotec 7SA6 User Manual

Functions
6.6.1.2

Directional Unblocking Scheme

Principle
Sequence
6-124
The unblocking method is a permissive scheme. The difference to the Directional
Comparison Scheme (Sub-section 6.6.1.1) lies in that tripping is also possible when
no permissive signal from the opposite line end is received. Accordingly it is mainly
used on long lines where the signal is transmitted via the protected feeder by means
of power line carrier (PLC) and the attenuation in the signal transmission path at the
fault location can be so severe that reception of the signal from the opposite line end
cannot necessarily be guaranteed. A special unblock logic takes effect here.
Figure 6-73 shows the operation scheme.
Two signal frequencies which are keyed by the transmit output of the 7SA6 are
required for the transmission. If the transmission device has a channel monitoring,
then the monitoring frequency f
frequency) f . When the protection recognizes an earth fault in the forward direction,
U
it initiates the transmission of the unblock frequency f
during an earth fault in the reverse direction, the monitoring frequency f
transmitted.
If the unblock frequency f
signal is routed to the trip logic. A pre-condition for fast fault clearance is therefore that
the earth fault is recognized in the forward direction at both line ends.
The send signal can be prolonged by T
send signal only comes into effect if the protection has already issued a trip command.
This ensures that the permissive signal releases the opposite line end even if the earth
fault is very rapidly cleared by a different independent protection.
A
E/F.
frwd.
T
S
&
&
unblock-
logic
f –
quiescent frequency (monit. frequency)
0
f –
unblocking frequency (send frequency)
U
Figure 6-73 Operation scheme of the directional unblocking method
Figure 6-74 shows the logic diagram of the unblocking scheme for one line end.
The directional unblocking scheme only functions for faults in the "forward" direction.
Accordingly the overcurrent stage intended for operation in the direction comparison
mode must definitely be set to )RUZDUG (, ',5(&7,21); refer also to Sub-
section 6.5.2 under the margin heading "Teleprotection with Earth Fault Protection".
On three terminal lines, the send signal is routed to both opposite line ends. The
receive signals are then combined with a logical AND gate, as all three line ends must
transmit a send signal during an internal fault. Via the setting parameter /LQH
is keyed over to the working frequency (unblocking
0
is faultlessly received from the opposite end, a release
U
(parameter setting). The prolongation of the
S
f
U
≥1
transm.
f
0
trip
U
rec.
B
. During the quiescent state or
U
is
0
B
f
U
≥1
transm.
T
f
S
&
0
&
trip
U
unblock-
rec.
B
logic
U –
unblocking signal
B –
blocking signal
C53000-G1176-C133-1
E/F.
frwd.
7SA6 Manual