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To guarantee that the time period between pickup and tripping of the protection function is sufficient to open
and close the pilot wire loop, T1B must be delayed for a short period. If the pilot wire comparison is used with
two different types of devices at both line ends (e.g. 7SD5 at one line end and a standard protection relay at
the other end) care must be taken that the difference in pickup and trip delay of the two devices, which may be
considerable, does not lead to an unwanted release. This must also be taken into consideration for the delay
of T1B.
The quiescent state loop ensures a steady check of the pilot wire connections against interruptions. Since the
loop is interrupted during each fault, the signal for pilot wire failure is delayed by 10 s. The pilot wire comparison
supplement is then blocked. It does not need to be blocked from external as the pilot wire failure is recognized
internally. The other stages of the distance protection continue operating according to the normal grading co-
ordination chart.
Due to the low current consumption of the binary inputs it may be necessary to additionally burden the pilot wire
loop with an external shunt-connected resistor so that the binary inputs are not held by the charge of the pilot
wire after an interruption of the loop. As an alternative, it is possible to connect auxiliary relay combinations
(e.g. 7PA5210-3D).
Figure 2-97
Pilot wire comparison - principle
Please take note that both binary inputs are connected in series with each other and the resistance of the pilot
wires. Therefore the loop voltage must not be too low or the pickup voltage of the binary inputs must not be too
high.
Operation with three terminals is also possible if the device allows it. The following figure shows the logic for
two terminals.
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
2.7 Teleprotection for Distance Protection (optional)
Functions
207
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