Siemens SIPROTEC 7SA522 Manual page 178

2 Functions
Sequence
178
Figure 2-76 shows the logic diagram of the directional comparison scheme for one line
end.
The directional comparison only functions for faults in the "forward" direction. Accord-
ingly the over current stage intended for operation in the direction comparison mode
must definitely be set to )RUZDUG (, ',5(&7,21); refer also to Subsection
2.7 at the 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 /LQH &RQILJ (address ) the
device is informed as to whether it has one or two opposite line ends. If the parameter
7HOHSURW () (address ) is set to 6,*1$/Y3URW,QW and the parameter
180%(5 2) 5(/$< (address ) is set to  UHOD\V, the device is informed about
two remote ends. The default setting is  UHOD\V, which corresponds to one remote
end.
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 star-point is only
earthed behind one line end, the line end without zero sequence current cannot gen-
erate 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 tripping" (echo function) is referred to at 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.
The circuit breaker can also be tripped at the line end with no or only weak infeed. This
"Weak-infeed tripping" is referred to in Section 2.9.1.
7SA522 Manual
C53000-G1176-C155-3