Sel SEL-421 - Size of an SEL Fast Message Synchrophasor Message

R.7.35
Date Code 20111215 Reference Manual SEL-421 Relay
Synchrophasors
SEL Fast Message Synchrophasor Protocol
Communications
Bandwidth
A phasor measurement unit (PMU) that is configured to transmit a single
synchrophasor (positive-sequence voltage, for example) at a message period
of one second places little burden on the communications channel. As more
synchrophasors are added, or if the message rate is increased, some
communications channel restrictions come into play.
In the SEL Fast Message synchrophasor protocol, the master device
determines the message period (the time between successive synchrophasor
message time-stamps) in the enable request. If the SEL-421 can support the
requested message period on that serial port, the relay acknowledges the
request (if an acknowledge was requested) and commences synchrophasor
data transmission. If the SEL-421 cannot support the requested message
period, the relay responds with a response code indicating bad data (if an
acknowledge was requested).
The SPEED setting on any serial port set with PROTO := PMU should be set
as high as possible, to allow for the largest number of possible message period
requests to be successful.
The SEL-421 Fast Message synchrophasor format always includes 32 bytes
for the message header and terminal ID, time information, frequency, and
status bits. The selection of synchrophasor data will add to the byte
requirements. Table 7.27 can be used to calculate the number of bytes in a
synchrophasor message.
Tab le 7.28 lists the baud settings available on any SEL-421 serial port (setting
SPEED), and the maximum message size that can fit within the port
bandwidth. Blank entries indicate bandwidths of less than 40 bytes.
Referring to Table 7.27 and Table 7.28, it is clear that the lower SPEED
settings are very restrictive.
Table 7.27 Size of an SEL Fast Message Synchrophasor Message
Item
Possible
Number of
Quantities
Bytes per
Quantity
Minimum
Number of
Bytes
Median
Number of
Bytes
Maximum
Number of
Bytes
Fixed 32 32 32
Synchrophasors 1, 4, or 8 8 8 32 64
Total (Minimum, Median, and Maximum) 40 64 96
Table 7.28 Serial Port Bandwidth for Synchrophasors (in Bytes)
Requested
Message
Period (ms)
Equivalent
Message
Rate
(messages
per second)
Port Setting SPEED
300 600 1200 2400 4800 9600 19200 38400 57600
1000 1 41 83 166 333 666 1332 2665 3998
500 2 41 83 166 333 666 1332 1999
250 (60 Hz only) 4 41 83 166 333 666 999
200 5 66 133 266 533 799
100 10 66 133 266 399
50 (60 Hz only) 20 66 133 199