18.26
SEL-400 Series Relays Instruction Manual Date Code 20171006
Synchrophasors
C37.118 Synchrophasor Protocol
required to have the same SPEED setting, although the slowest SPEED setting on
a PROTO := PMU port will affect the maximum Global MRATE setting that can
be used.
Ethernet Operation
C37.118 Synchrophasors may be used over Ethernet if an Ethernet card is
installed in the relay. Four transport methods are supported: UDP_U, UDP_S,
UDP_T, and TCP.
UDP_U, UDP_S, UDP_T
UDP stands for User Datagram Protocol and is a network protocol used for the
Internet. UDP uses a simple transmission model without implicit handshaking
interchanges for guaranteeing reliability, ordering, or data integrity. As such,
UDP minimizes additional overhead needed to send messages. Time-sensitive
applications often use UDP because dropping packets is preferable to waiting for
delayed packets, which may not be an option in a real-time system. UDP_S is a
version of UDP that only sends data; no reverse messaging is used, thus provid-
ing streaming data in one direction only. UDP_T uses a TCP socket to command
and configure PMU measurements, and then uses a UDP socket for sending data
out. A user may choose to use UDP to minimize the additional overhead bits
added and thus minimize the communications bandwidth needed to send PMU
information out of a substation. UDP_S uses the least amount of overhead (and
provides some additional security as the PMU or PDC using this method is only
sending data and ignores any messages coming in).
TCP
TCP stands for Transmission Control Protocol and is a connection-oriented pro-
tocol, which means that it requires handshaking to set up end-to-end communica-
tions. Once a connection is set up, user data may be sent bi-directionally over the
connection. TCP manages message acknowledgment, retransmission, and time-
outs. With TCP, there are no lost data; the server will request the lost portion to
be resent. Additionally, TCP ensures that the messages are received in the order
sent. TCP provides the most robust connection, but it also adds additional over-
head bits to any message data.
PMU Setting Example
A power utility is upgrading the line protection on its 230 kV system to use the
SEL-421 relay as main protection. The grid operator also wants the utility to
install phasor measurement units (PMUs) in each 230 kV substation to collect
data for a new remedial action scheme, and to eventually replace their present
state-estimation system.
The PMU data collection requirements call for the following data, collected at 10
messages per second:
➤ Frequency
➤ Positive-sequence voltage from the bus in each substation
➤ Three-phase and positive-sequence current for each line terminal
➤ Indication when the line breaker is open
➤ Indication when the voltage or frequency information is unusable
➤ Ambient temperature (one reading per station)
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