P.3.294
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Channel Monitoring and Alarming Logic
relay can constantly monitor if the channel is truly symmetrical during in-
service conditions, as a part of scheme commissioning, or during
troubleshooting. This is especially beneficial for multiplexed channels, or
when considering potential failure modes of any active communications
device between any two relays.
The relay, per the basic equations in 87L Theory of Operation, averages raw
channel delay measurements in the receiving and transmitting directions
during a period of 20 ms (5 packets). We define channel asymmetry as an
unsigned difference between the two delay measurements. This asymmetry
value is available as the analog quantity 87CHpAX.
Apply the 87CHpMA setting on a per-channel basis to monitor the value of
channel asymmetry, as Figure 3.202 shows. The 87CHpAM Relay Word bit
signals an alarm.
Figure 3.202 Channel Asymmetry Alarm Logic
Note that the asymmetry measurement depends on the time sources at both
ends of a given channel. If the 87TOK Relay Word bits are not asserted in both
relays working over a given channel, or the relay is not in time-based
synchronization mode (87CHpSN = T), this measurement is unavailable
(forced to zero). When the relay is set to use time-based synchronization, use
the channel asymmetry alarm logic to monitor the suitability of the channel
for use with Time Fallback Mode (87TFB) 3 or 4.
Lost Packet Count
This section applies to all active channels, both serial and Ethernet.
The relay counts and alarms on lost packets. Packet loss impacts 87L function
dependability and stresses protection security by creating an impairment for
relays.
The relay declares a lost 87L packet if any of the following occurs.
➤ The difference between the sequence number the relay receives
in the present packet and the sequence number it received in
the last packet is other than exactly 1, as expected.
➤ The time elapsed since the last packet the relay received
exceeds 120 percent of the normal time between the packets.
➤ The data, as detected by the integrity code (BCH, see 87L
Theory of Operation for more information), are corrupt.
The relay runs a lost packet counter (87CHpLX) to count packets that have
been lost among the last 10,000 scheduled packets (40 seconds of data
reception). The counter starts upon receipt of the first valid packet on a given
channel following relay startup.
In addition, the relay runs a 24-hour lost packet counter (87CHpLD), using the
following procedure. The relay records the number of lost or defective packets
during the last 24 hours in 15-minute intervals. At the end of a 15-minute
interval, the relay adds the newest 15-minute count and subtracts the oldest
15-minute count from the total 24-hour number of lost packets. Each
15-minute period begins every UTC quarter hour.
As Figure 3.203 shows, an alarming threshold is available for the lost packet
count.
Setting
Analog
Relay
Word Bit
87CHpMA
87CHpAX
87CHpAM
0.1 s
0.1 s
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