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Sequence
Figure 2-120 shows the logic diagram of the directional comparison scheme for one line end.
The directional comparison 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 Forward (3I0... DI-
RECTION); refer also to Section 2.8 under margin heading „Teleprotection with Earth Fault Protection".
On two terminal lines, the signal transmission may be phase segregated. In this case, send and receive circuits
operate seperately for each phase. On three terminal lines, the transmit signal is sent 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. With the 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 external faults or from direc-
tion reversal resulting during the clearance of faults on parallel 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 release signal as fault detection does not take
place there. To ensure tripping by the directional comparison also in this case, the device has special features.
This „Weak Infeed Function" (echo function) is referred to at the margin heading „Echo function". 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 2.11.2.
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
2.9 Teleprotection for Earth Fault Protection (optional)
Functions
251
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