Siemens siprotec 7SD5 User Manual page 234

2 Functions
Figure 2-100
Sequence Figure 2-101 shows the logic diagram of the directional comparison scheme for one
line end.
The permissive overreach transfer trip only functions for faults in the „Forward" direc-
tion. Accordingly the overcurrent stage intended for operation in the direction compar-
ison mode must definitely be set to Forward (3I0... 3I0...), see also Section
2.82.8 at margin heading „Teleprotection with Earth Fault Protection".
On two terminal lines, the signal transmission may be phase segregated. Send and
receive circuits in this case are built up for each phase. On three terminal lines, the
transmit signal is sent to both opposite line ends. The receive signals are then com-
bined with a logical AND gate, as all three line ends must transmit a send signal during
an internal fault. With the setting parameter Line Config. (address 3202) the
device is informed as to whether it has one or two opposite line ends.
The occurrence of erroneous signals resulting from transients during clearance of ex-
ternal faults or from direction reversal resulting during the clearance of faults on par-
allel lines, is neutralized by the „Transient Blocking" (see margin heading „Transient
Blocking").
On lines where there is only a single sided infeed or where the starpoint is only earthed
behind one line end, the line end without zero sequence current cannot generate a
permissive signal, as fault detection does not take place there. To also ensure tripping
by the directional comparison in this case the device has special features. This „Weak
Infeed Function" (echo function) is referred to under the margin heading „Echo Func-
tion". It is activated when a signal is received from the opposite line end — in the case
of three terminal lines from at least one of the opposite line ends — without the device
having detected a fault.
234
Operation scheme of the directional comparison pickup
7SD5 Manual
C53000-G1176-C169-1