Siemens siprotec 7SD5 User Manual page 91

The sequence of the devices in the communication chain need not correspond to the
indexation, as shown in Figure 2-17. The allocation is carried out during the parame-
terization of the topology, as explained in Section 2.2.1.
Figure 2-17
The communication chain can also be connected to a ring, as shown in dashed lines
in Figure 2-17. This provides for redundancy of transmission: even if one communica-
tion link fails, the entire differential protection system will be fully operational. The
devices detect communication failures and switch automatically to another commun-
ciation channel. It is also possible to switch off one line end, e.g. for a check or a revi-
sion, and put the local protection out of operation. With a communication ring, the rest
of the operation can proceed without disturbances.
You will find detailed information on the topology of device communication in Section
2.2.1.
Measured Value The devices measure the local currents in an asynchronous way. This means that
Synchronization each device measures, digitizes and pre-processes the associated currents of the
current transformers with its own, random processor pulse. If the currents of two or
more line ends are to be compared, it is necessary, however, to process all currents
with the same time base.
All devices which belong together exchange their time with each telegram. The device
with index 1 functions as a „timing master" thus determining the time base. The other
devices then calculate the time delay from the transmission and processing times
related on the „timing master". With this „rough synchronization" the equality of the
time bases with a precision of ± 0.5 ms is provided.
To reach a sufficiently precise synchronization all current values are marked with a
„time stamp" before they are transmitted from one device to the other as digital tele-
grams. This time stamp indicates at which point in time the transmitted current data
were valid. Therefore, the receiving devices can carry out an optimized synchroniza-
tion of the current comparisons based on the received time stamp and their own time
management, i.e. they can compare the currents which were actually measured at
exactly the same time (<5 µs tolerance).
The transmission periods are permanently monitored by the devices using the time
data stamps and considered at the respective receiving end.
7SD5 Manual
C53000-G1176-C169-1
Differential protection for a line with three ends
2.3 Differential Protection
91