Siemens SIPROTEC Manual page 254

Functions
2.9 Teleprotection for Earth Fault Protection (optional)
Sequence
Figure 2-122 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 overcur-
rent stage intended for operation in the directional unblocking scheme must definitely be set to Forward
(RICH.3I0...); refer also to Section 2.8 at the 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 separately for each phase. On three terminal lines, the transmit signals are 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.
An unblock logic is inserted before the receive logic, which in essence corresponds to that of the directional
comparison scheme, see Figure 2-123. If an interference free unblock signal is received, a receive signal, e.g.
„>EF UB ub 1", appears and the blocking signal, e.g. „>EF UB bl 1" disappears. The internal signal
„Unblock 1" is passed on to the receive logic, where it initiates the release of the tripping (when all remaining
conditions have been fulfilled).
If the transmitted signal does not reach the other line end because the short-circuit on the protected feeder
causes too much attenuation or reflection of the transmitted signal, the unblock logic takes effect: neither the
unblocking signal „>EF UB ub 1" nor the monitoring signal „>EF UB bl 1" are received. In this case, the
release „Unblock 1" is issued after a security delay time of 20 ms and passed onto the receive logic. This
release is however removed after a further 100 ms via the timer stage 100/100 ms. When the transmission is
functional again, one of the two receive signals must appear again, either „>EF UB ub 1"or „>EF UB bl
1"; after a further 100 ms (dropout delay of the timer stage 100/100 ms) the quiescent state is reached again,
i.e. the direct release path to the signal „Unblock 1" and thereby the usual release is possible. On three terminal
lines, the unblock logic can be controlled via both receive channels.
If none of the signals is received for a period of more than 10 s the alarm „EF TeleUB Fail1" is generated.
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".
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" is referred to in Section „Measures for Weak and Zero Infeed". The function 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.
254
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011