U.4.72
SEL-421 Relay User’s Guide Date Code 20090715
Basic Relay Operations
Configuring High-Accuracy Timekeeping
Only one IRIG-B time source can be used by the SEL-421, and the signal
connected to the IRIG-B BNC connector (shown in Figure 4.64) takes priority
over the Serial Port 1 IRIG-B pins. If a signal is detected on the IRIG-B BNC
input, the IRIG-B pins of Serial Port 1 will be ignored.
The SEL-421 determines the suitability of the IRIG-B signal connected to the
BNC connector for high-accuracy timekeeping by applying two tests:
➤ Measuring whether the jitter between positive-transitions
(rising edges) of the clock signal is less than 500 ns.
➤ Decoding the time-error information contained in the IRIG-B
control field and determining that Analog Quantity TQUAL is
less than 10
-6
seconds (1 µs).
The SEL-421 will assert Relay Word bit TSOK only when these two tests are
met, indicating HIRIG mode. The TQUAL Analog Quantity can be viewed
with the MET PM command, and is shown beside the label
Time Quality Maximum time synchronization error:. See Figure 7.13
on page R.7.25 for a sample.
The IRIG-B control field is defined in the IEEE C37.118 standard. The
SEL-421 places the raw time quality information in Relay Word bits
TQUAL1, TQUAL2, TQUAL4, and TQUAL8; and the decoded maximum
clock error in Analog Quantity TQUAL, in seconds.
If the clock signal is determined to be of low quality, with more than 500 ns of
jitter, the SEL-421 will not assert the TIRIG Relay Word bit.
1k PPS Connection
Not Required
SEL-421 Relays Changed
Previous versions of the SEL-421 Relay required a 1k PPS clock signal in
addition to the IRIG-B signal to allow HIRIG mode (high-accuracy IRIG
timekeeping). The previous SEL-421 hardware included two BNC connectors
for timekeeping: 1k PPS and IRIG-B (see Figure 4.64). On newer SEL-421
relays, there is only one BNC connector, IRIG-B, as shown in Figure 4.63.
Starting with SEL-421 firmware version R112, HIRIG mode is available if a
sufficiently accurate IRIG-B time source is connected to the IRIG-B BNC
connector (see Table 4.9). A 1k PPS time source cannot be used.
IRIG-B Connection
Required
None BNC connector
(preferred), or Serial
Port 1
BNC connector
Relay Word bits TIRIG = logical 0
TSOK = logical 0
TIRIG = logical 1
TSOK = logical 0
TIRIG = logical 1
TSOK = logical 1
a
The SEL-421 internal clock can be synchronized via DNP3, Ethernet card, SEL-2030
Communications Processor, or M
IRRORED BITS communications.
b
The time source must include the IEEE C37.118 IRIG-B control bit assignments to provide the
Time Error estimate for the clock.
Table 4.9 SEL-421 Timekeeping Modes (Sheet 2 of 2)
Item Internal Clock IRIG
HIRIG (or High-
Accuracy IRIG)
NOTE: If the time-code signal
connected to the BNC connector
degrades in quality, the SEL-421 will
not
switch-over to the IRIG-B pins of
serial port 1. The SEL-421 will only
switch to Serial Port 1 if the signal on
the BNC connector completely fails
(e.g. the cable is un-plugged).
Courtesy of NationalSwitchgear.com
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