P.3.20
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Theory of Operation
In either mode of data alignment, the relay estimates the clock offset between
a pair of relays communicating over an 87L channel. As each relay appends
the data with time stamps in the local relay time, knowledge of the time offset
allows each receiving relay to correct the original time stamps taken in the
remote relay time to the local relay time and to use the data together with the
local currents and other remote currents.
As a result of this general approach, each relay uses local time to time-stamp
the transmitted local currents. A time offset between a pair of relay clocks
changes very slowly, only as the result of a finite accuracy of the relay
oscillators or finite accuracy of the connected external clocks. As a result, the
estimated time offset does not change rapidly in response to channel
interruption or switching. The relay therefore operates very reliably under a
variety of events related to 87L communications. Because the estimated clock
offset changes only very slowly, each relay averages this offset to further
increase the accuracy of data alignment and to improve resilience of alignment
to various communications events.
The channel-based mode uses an industry-recognized ping-pong algorithm to
determine the time offset between a pair of relays. The method works
accurately as long as the channel is symmetrical, meaning that channel
propagation delays in both directions are identical.
In the channel-based synchronization mode, Relay 1 sends its 87L packet and
timestamps the moment of transmission as t
0
. The packet is marked with a
sequence number to identify it at the time of use. Relay 1 uses its own local
time to capture time t
0
(refer to Figure 3.14).
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