Na7
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MiCOM P741, P742
1.5.7.2 General features & implementation
InterMiCOM provides 8 commands over a single communications link, with the mode of
operation of each command being individually selectable within the “IM# Cmd Type” cell.
“Blocking” mode provides the fastest signaling speed (available on commands 1 - 4), “Direct
Intertrip” mode provides the most secure signaling (available on commands 1 - 8) and
“Permissive” mode provides the most dependable signaling (available on commands 5 - 8).
Each command can also be disabled so that it has no effect in the logic of the relay.
Since many applications will involve the commands being sent over a multiplexed
communications channel, it is necessary to ensure that only data from the correct relay is
used. Both relays in the scheme must be programmed with a unique pair of addresses that
correspond with each other in the “Source Address” and “Receive Address” cells. For
example, at the local end relay if we set the “Source Address” to 1, the “Receive Address” at
the remote end relay must also be set to 1. Similarly, if the remote end relay has a “Source
Address” set to 2, the “Receive Address” at the local end must also be set to 2. All four
addresses must not be set identical in any given relay scheme if the possibility of incorrect
signaling is to be avoided.
It must be ensured that the presence of noise in the communications channel isn’t
interpreted as valid messages by the relay. For this reason, InterMiCOM uses a combination
of unique pair addressing described above, basic signal format checking and for “Direct
Intertrip” commands an 8-bit Cyclic Redundancy Check (CRC) is also performed. This CRC
calculation is performed at both the sending and receiving end relay for each message and
then compared in order to maximize the security of the “Direct Intertrip” commands.
Most of the time the communications will perform adequately and the presence of the
various checking algorithms in the message structure will ensure that InterMiCOM signals
are processed correctly. However, careful consideration is also required for the periods of
extreme noise pollution or the unlikely situation of total communications failure and how the
relay should react. During periods of extreme noise, it is possible that the synchronization of
the message structure will be lost and it may become impossible to decode the full message
accurately. During this noisy period, the last good command can be maintained until a new
valid message is received by setting the “IM# FallBackMode” cell to “Latched”. Alternatively,
if the synchronization is lost for a period of time, a known fallback state can be assigned to
the command by setting the “IM# FallBackMode” cell to “Default”. In this latter case, the time
period will need to be set in the “IM# FrameSynTim” cell and the default value will need to be
set in “IM# DefaultValue” cell. As soon as a full valid message is seen by the relay all the
timer periods are reset and the new valid command states are used. An alarm is provided if
the noise on the channel becomes excessive.
When there is a total communications failure, the relay will use the fallback (failsafe) strategy
as described above. Total failure of the channel is considered when no message data is
received for four power system cycles or if there is a loss of the DCD line.
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