R.5.18
SEL-421 Relay Reference Manual Date Code 20111215
SEL Communications Protocols
SEL MIRRORED BITS Communications
and receives MIRRORED BITS communications messages eight times per
power system cycle. Therefore, a security counter set to two counts will delay
a bit by approximately 1/4 of a power system cycle. Reference Table 5.17 for
the message rates based on the settings. You must consider the impact of the
security counter settings in the receiving device to determine the channel
timing performance.
Channel Synchronization
When an SEL-421 detects a communications error, it deasserts ROKA or
ROKB. The relay transmits an attention message until it receives an attention
message that includes a match to the TX_ID setting value. If the attention
message is successful, the relay has properly synchronized and data
transmission will resume. If the attention message is not successful, the relay
will repeat the attention message until it is successful.
Loopback Testing
Use the LOOP command to enable loopback testing. While in loopback
mode, ROKc is deasserted, and, LBOKc asserts and deasserts based on the
received data checks.
Channel Monitoring
Based on the results of data checks (described above), the relay collects
information regarding the 255 most recent communications errors. Each
record contains at least the following fields:
➤ Dropout Time/Date
➤ Pickup Time/Date
➤ Time elapsed during dropout
➤ Reason for dropout (see Message Decoding and Integrity
Checks on page R.5.17)
Use the COM command to generate a long or summary report of the
communications errors.
There is a single record for each outage, but an outage can evolve. For
example, the initial cause could be a data disagreement, but framing errors can
extend the outage. If the channel is presently down, the COMM record will
only show the initial cause, but the COMM summary will display the present
cause of failure.
When the duration of an outage on Channel A or B exceeds a user-definable
threshold, the relay will assert a user-accessible flag, RBADA or RBADB.
When channel unavailability exceeds a user-definable threshold for Channel A
or B, the relay asserts a user-accessible flag, CBADA or CBADB.
MIRRORED BITS Communications
Protocol for the Pulsar 9600-BPS Modem
To use a Pulsar MBT modem, set setting MBT:= Y. Setting MBT:= Y hides
setting SPEED and forces it to 9600, and hides setting RTSCTS and forces it
to a value of N. The relay also injects a delay (idle time) of 3 ms between
messages.
NOTE: Combine error conditions
including RBADA, RBADB, CBADA, and
CBADB with other alarm conditions
using SEL
OGIC control equations. You
can use these alarm conditions to
program the relay to take appropriate
action when it detects a
communications channel failure.
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