P44x/EN AD/E44 Update Documentation
Page 74/82
MiCOM P441, P442 & P444
9.10.4.1
Setting Guidelines
The settings required for the InterMiCOM signalling are largely dependant upon whether a
direct or indirect (modem/multiplexed) connection between the scheme ends is used.
Direct connections will either be short metallic or dedicated fiber optic based and hence can
be set to have the highest signalling speed of 19200b/s. Due to this high signalling rate, the
difference in operating speed between the direct, permissive and blocking type signals is so
small that the most secure signalling (direct intertrip) can be selected without any significant
loss of speed. In turn, since the direct intertrip signalling requires the full checking of the
message frame structure and CRC checks, it would seem prudent that the “IM# Fallback
Mode” be set to “Default” with a minimal intentional delay by setting “IM# FrameSyncTim” to
10msecs. In other words, whenever two consecutive messages have an invalid structure,
the relay will immediately revert to the default value until a new valid message is received.
For indirect connections, the settings that should be applied will become more application
and communication media dependent. As for the direct connections, it may be appealing to
consider only the fastest baud rate but this will usually increase the cost of the necessary
modem/multiplexer.
In addition, devices operating at these high baud rates may suffer from “data jams” during
periods of interference and in the event of communication interruptions, may require longer
re-synchronization periods.
Both of these factors will reduce the effective communication speed thereby leading to a
recommended baud rate setting of 9600b/s. It should be noted that as the baud rate
decreases, the communications become more robust with fewer interruptions, but that
overall signalling times will increase.
Since it is likely that slower baud rates will be selected, the choice of signalling mode
becomes significant. However, once the signalling mode has been chosen it is necessary to
consider what should happen during periods of noise when message structure and content
can be lost.
If “Blocking” mode is selected, only a small amount of the total message is actually used to
provide the signal, which means that in a noisy environment there is still a good likelihood of
receiving a valid message. In this case, it is recommended that the “IM# Fallback Mode” is
set to “Default” with a reasonably long “IM# FrameSyncTim”.
If “Direct Intertrip” mode is selected, the whole message structure must be valid and checked
to provide the signal, which means that in a very noisy environment the chances of receiving
a valid message are quite small. In this case, it is recommended that the “IM# Fallback
Mode” is set to “Default” with a minimum “IM# FrameSyncTim” setting i.e. whenever a non-
valid message is received, InterMiCOM will use the set default value.
If “Permissive” mode is selected, the chances of receiving a valid message is between that
of the “Blocking” and “Direct Intertrip” modes. In this case, it is possible that the “IM#
Fallback Mode” is set to “Latched”. The table below highlights the recommended “IM#
FrameSyncTim” settings for the different signalling modes and baud rates:
Minimum Recommended “IM# FrameSyncTim”
Setting
Baud
Rate
Direct Intertrip Mode Blocking Mode
Minimum
Setting
Maximum
Setting
600 100 250 100 1500
1200 50 130 50 1500
2400 30 70 30 1500
4800 20 40 20 1500
9600 10 20 10 1500
19200 10 10 10 1500
TABLE 5 : RECOMMENDED FRAME SYNCHRONISM TIME SETTINGS
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