2.9 Teleprotection for Earth Fault Protection (optional)
237
7SD5 Manual
C53000-G1176-C169-1
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.
An unblock logic is inserted before the receive logic, which in essence corresponds to
that of the directional comparison scheme, see Figure 2-104. 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 „Un-
block 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 ex-
ternal faults or from direction reversal resulting during the clearance of faults on par-
allel 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 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 Function“ is referred to in Section „Measures for Weak and Zero Infeed“.
It is activated when a signal is received from the opposite line end — on three terminal
lines, from at least one of the opposite ends — without the device recognizing an earth
fault.
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