2 Functions
196
7SD5 Manual
C53000-G1176-C169-1
For lines shorter than the shortest settable line please take into consideration that the
first distance zone is either set to disabled or that T1 is delayed for at least one grading
time interval.
If the line has single-end infeed an instantaneous trip for the whole line is possible.
Since no pickup occurs on the non-feeding line end, the loop is not interrupted at that
point, but only on the feeding line end. After the fault is detected within Z1B, the loop
will be closed again and the trip command is executed.
To guarantee that the time period between pickup and tripping of the protection func-
tion 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 inter-
nally. The other stages of the distance protection continue operating according to the
normal grading coordination chart.
Due to the low current consumption of the binary inputs it may be necessary to addi-
tionally burden the pilot wire loop with an external shunt connected resistor so that the
binary inputs are not hold 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 2-81
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.
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