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MiCOM P116 Modbus register for Breaker (CB) Status - can the register format be configured by the user?

As per the MiCOM P116 user documentation, the Breaker Status information is provided via Modbus communication as a single word (16-bit) register using the vendor-specific "F30" format.

For example, as described in the manual, if the CB is closed (52A is true), then this register will contain a value of zero.  As a consequence, the 2 lowest-order bits will both have a value of zero, or [0 0].  If the CB is open (52B is true), then the register will contain a value of 1, and the 2 lowest-order bits will have the value [0 1] (with the "1" being at the least-significant bit position).

In some cases, a PLC or RTU connecting as a Modbus Master to the MiCOM relay may have default definitions for data such as the CB Status.  For example, one commonly used convention to report the CB status via communication uses 2 bits, with one bit representing the "closed" state, and the other bit representing the "open" state.  As described below, this convention allows us to define 2 valid states and 2 invalid states for the CB.  This method is also used in IEC61850 communication in Logical Nodes such as CSWI and XCBR.

[1 0] ==> CB closed and not open (valid state).

[0 1] ==> CB open and not closed (valid state).

[1 1] or [0 0] ==> CB showing closed and open, or not closed and not open (invalid states).  These are typically defined as "intermediate" states that should last only momentarily (e.g., a few milliseconds) and would likely trigger an alarm message after a user-defined timeout expires.  For some devices supporting IEC61850,  a user-settable parameter is provided that allows the user to decide whether or not these "invalid" states are reported by the device's communication system following a successful CB control operation.

If we consider the "F30" Modbus format used for the MiCOM P116 relay, we can see that the format does not follow the convention described above.  With the F30 format, if the CB is closed and no control faults are detected, the 2 lowest-order bits will have the value [0 0], which would appear to be an invalid state for an RTU or PLC using the convention described above.

At the present time (Q4 of 2014), it is not possible for a user to modify the format used for Modbus registers on the P116 relay.  Modifications to the format are technically possible, but a special request must be made to the Engineering Team and a customized firmware version would need to be created.

As a conclusion, if at all possible, for the situation described here, it would be preferable to modify the data definitions of the master RTU or PLC, rather than attempting to modify the format of the P116 Modbus registers.


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