In Figure 14, both local CTs on the left side are treated as separate
ends by the line differential protection function.
Current values from two CTs in double breakers, ring main or 1½ breaker system
ends with dual breaker arrangements need to be sent to the remote end. As a 64
kbit/s LDCM only has capacity for one three-phase current (duplex), this implies
that two communication channels are needed at both ends, and this is also the
normal solution.
To reduce the number of communication channels needed, it is also possible to sum
up the two local currents before sending by using a software in the IED. However,
this is not recommended as it reduces information about bias currents. Bias current
is considered the greatest phase current at any line end, and it is common for all
three phases. When sending full information from both local CTs to the remote
end, the principle works, but when the two local currents are summed up before
sending, information on real phase currents from the two local CTs will not be
available at the remote line end.
In 64 kbit mode, the decision on using one communication channel instead of two
(as show in Figure 14) must be made case by case. Correct information on bias
currents is always available locally while only distorted information is available at
the end that receives limited information using only one LDCM.
In 2 Mbit mode, it is possible to send up to 9 different analog signals from each
LDCM; therefore only one LDCM can be used.
4.2 Communication between channels via Line data
communication module
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Communication between two IEDs requires that each IED is equipped with a Line
Data Communication Module (LCDM). The LDCM then acts as an interface to 64
kbit/s and 2Mbit/s communication channels for duplex communication between the
IEDs.
Section 4 1MRK 505 382-UEN B
Analog and binary signal transfer for line differential protection
28 Communication set-up 670/650 series 2.2
Application Guide
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