Sel 411L - Figure 3.210 Request for Time Fallback from the P-Th 87 L Channel; Figure 3.211 Time Fallback Mode 1 Logic

P.3.302
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Time Fallback Logic
➤ The time reference is lost on either end of this channel, as
deassertion of the 87TOK Relay Word bit or the received ETL
bit indicate.
Note that when the channel is not working (87CHpOK deasserted) the
fallback call resets. When the channel recovers, the fallback call depends on
the other conditions at the time of recovery. In other words, the relay logic
does not latch the fallback call but rather evaluates whether to place the call
based on present channel conditions.
In applications over Ethernet, a loss of time source at either end of a required
channel triggers the 87CHpNS Relay Word bit (see 87L Channel
Synchronization Logic and Status), which in turn shuts down the master mode.
When no relays remain as masters, the 87L function responds according to
design and stops operating.
In the three-terminal outstation applications (E87CH = 3SS), the relay
remains in permanent outstation mode and does not calculate differential
quantities. If you set the channel to time-based synchronization and there is a
loss of time reference at the local or remote relay, no action occurs at the relay
set to 3SS. The other relay, however, is set to 3SM and executes a fallback
strategy according to its 87TFB setting.
Figure 3.210 Request for Time Fallback From the p-th 87L Channel
Time Fallback Mode 1
In this mode, relay logic blocks the 87L function both locally and remotely if
any channel calls for time fallback (as signaled by assertion of the 87CHpFB
Relay Word bit). Scheme logic distributes the blocking action to all remote
relays in the scheme, as 87L Enable and Blocking Logic shows. Assertion of
the 87LST Relay Word bit (see 87L Master, Outstation, and Loss of Protection
Logic) signals unavailability of the 87L function.
This simple fallback mode is heavily biased toward security. The 87TOK logic
provides some ride-through capability for temporary problems with the timing
sources. Upon a very short interruption of the IRIG-B signal or noise in the
IRIG-B connection, the 87TOK Relay Word bit will not deassert immediately
and the time fallback will not engage, so the 87L function remains in service
(see 87L Channel Synchronization Logic and Status for more details on the
87TOK logic).
Figure 3.211 Time Fallback Mode 1 Logic
Relay
Word Bit
Relay
Word Bits
Setting
87TOK
Received
on the p-th
channel
E87CH = 2SS, 2SD, 3SM
87CHpSN = T
87CHpOK
87CHpRQ
ETL
87CHpFB
Relay
Word Bit
Setting
87CHpFB
All active
channels
87TFB = 1
To the 87L
blocking logic