Siemens siprotec 7SD5 User Manual page 75

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 arrangements 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, es-
pecially for a high fault current flowing from end 1 to end 2 (external fault).
The communication chain begins at the protection data interface PI 1 of device with
the index 1, continues in the device with index 2 at PI 1, extents from 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 must not necessarily have to cor-
respond to the sequence of the communication chain. Which protection data interface
is connected to which protection data interface does not play a role. One device with
one protection data interface at each terminal 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 commu-
nication 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.
7SD5 Manual
C53000-G1176-C169-1
2.2 Protection Data Interfaces and Protection Data Topology
Differential protection for four ends with chain topology
75