Schweitzer Engineering Laboratories SEL-787, Instruction Manual, Page: 520 / 564

H.12
SEL-787 Relay Instruction Manual Date Code 20081022
Synchrophasors
C37.118 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 rate of
once per second places little burden on the communications channel. As more
synchrophasors, analog values, or digital status words are added, or if the
message rate is increased, some communications channel restrictions come
into play.
The C37.118 synchrophasor message format always includes 18 bytes for the
message header and terminal ID, time information, status bits, and CRC value.
The selection of synchrophasor data, numeric format, programmable analog,
and programmable digital data will add to the byte requirements.
Table H.9
can be used to calculate the number of bytes in a synchrophasor message.
Table H.10 lists the baud settings available on any SEL-787 serial port (setting
SPEED), and the maximum message size that can fit within the port
bandwidth. Blank entries indicate bandwidths of less than 20 bytes.
Referring to Table H.9 and Table H.10 , it is clear that the lower SPEED
settings are very restrictive.
The smallest practical synchrophasor message would be comprised of one
digital status word, and this message would consume 24 bytes. This type of
message could be sent at any message rate (MRATE) when SPEED := 4800 to
38400, up to MRATE := 5 when SPEED := 2400, and up to MRATE := 1
when SPEED := 600.
Another example application has messages comprised of eight
synchrophasors, one digital status word, and two analog values. This type of
message would consume 64 bytes. The 64-byte message could be sent at any
message rate (MRATE) when SPEED := 9600.
Table H.9 Size of a C37.118 Synchrophasor Message
Item
Possible Number
of Quantities
Bytes
per Quantity
Minimum
Number
of Bytes
Maximum
Number
of Bytes
Fixed 18 18
Synchrophasors 0, 1, 2, 3, 4, 5,
6, 8, 9, or 12
4 0 48
Frequency 2 (fixed) 2 4 4
Analog Values 0–4 4 0 16
Digital Status
Words 0–1 2 0 2
Total (Minimum and Maximum) 22 88
Table H.10 Serial Port Bandwidth for Synchrophasors (in Bytes)
Global
Setting
MRATE
Port Setting SPEED
300 600 1200 2400 4800 9600 19200 38400
1 21 42 85 170 340 680 1360 2720
2 21 42 85 170 340 680 1360
5 34 68 136 272 544
10 34 68 136 272