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For more than two ends, a communication chain or a communication ring can be formed. A setup with a
maximum of six devices is possible.
Figure 2-9 shows a Communication Chain with four devices. The ends 1 and 2 are derived from the arrange-
ments of the current transformers shown on the left. Although this is actually only one line end, it should be
treated in terms of differential protection as two ends because the current is measured in two places. This is to
make sure that the transformation errors of both transformer sets are considered by the restraint, especially for
a high fault current flowing from end 1 to end 2 (external fault).
The communication chain starts at the protection data interface PI 1 of the device with index 1, reaches the
device with index 2 at PI 1, extends from the device with index 2 at PI 2 to the device with index 4, etc. until it
reaches the device with index 3 at PI 1. The example shows that the indexing of the devices does not neces-
sarily have to correspond to the sequence of the communication chain. Which protection data interface is con-
nected to which protection data interface is irrelevant. One device with one protection data interface at each
end of the chain is sufficient.
Figure 2-9
Figure 2-10 shows the same line arrangement as Figure 2-9. The communication links, however, have been
complemented to form a closed ring. A 7SD5 device with 2 protection data interfaces is necessary for each
terminal. This communication ring has the advantage, as compared to the chain shown in Figure 2-9, that the
entire communication system works even if one communications link fails. The devices detect the failure and
switch automatically over to the remaining paths of communication. In this example PI 1 is always connected
to PI 2 of the following device.
By the way, the two possibilities for two devices can be regarded as special cases of chains and rings. The
connection as shown in Figure 2-7 forms a communication chain with only one element. Figure 2-8 shows a
ring which has been compressed into one two-way connection.
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
Differential protection for four ends with chain topology
2.2 Protection Data Interfaces and Protection Data Topology
Functions
75
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