P.3.304
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Time Fallback Logic
➤ The logic checks whether the channel is symmetrical by
comparing the clock offset the relay measures (87CHpTOFF
analog measurement) through the channel-based method (see
87L Channel Synchronization Logic and Status for more
details) against a constant of 1.5 ms (calibration setting
87FBASM). If the measured clock offset is less than the
factory threshold of 1.5 ms, the channel has enough symmetry
(given the capability of the Alpha Plane to handle channel
asymmetry) to permit switchover to channel-based
synchronization. Supervision for this check comes from time
sources at both relays on the channel.
➤ Supervision for the operation comes from the high-quality time
offset measurement bit (87CHpCH). If this bit is deasserted at
the moment the logic assesses whether to switch to the
channel-based synchronization mode, the clock offset
measurement cannot be trusted, and the relay should not use
the clock offset for aligning the data.
➤ The channel has been working properly for at least 1 second
(87CHpOK asserted) to prevent engaging the fallback logic
after channel interruption. The logic engages only for loss of
the external time reference on a channel that has been working
without interruption or switching.
If all conditions occur, the logic forces the channel into channel-based
synchronization by asserting the force channel-based synchronization bit,
87CHpFC. The latch resets upon a channel call for fallback removal
(deassertion of the 87CHpFB Relay Word bit).
If at the moment the channel places the fallback call, the time offset
measurement is unreliable or greater than the 1.5 ms threshold (signifying
channel asymmetry), the channel has not been OK for as long as a second, or a
relay measured clock offset without enough precision, the logic forces out the
channel by asserting the 87CHpFO Relay Word bit. Consequences are similar
to those resulting from Mode 2 of the fallback logic.
If the 87CHpOK deasserts during the fallback situation, both latches reset.
This deasserts the forced use of the channel-based synchronization mode
(87CHpFC deasserts) and forces the channel out (87CHpFO deasserts). When
the channel recovers and 87CHpOK reasserts, but still calls for fallback, the
logic does not re-engage. Therefore, instead of forcing channel-based
synchronization, the relay forces the channel out. This is necessary because
there can be no guarantee of channel symmetry after the channel interruption.
If the channel-based synchronization was not forced, and the time sources had
not recovered, relay logic declares the channels to be unsynchronized (the
87CHpNS asserts as 87L Channel Synchronization Logic and Status explains)
and either the relay becomes an outstation or the 87L function ceases to
operate, depending on the status of the other channels and specific 87L
configuration.
Note that if any required channel is forced to use the channel-based
synchronization method (87CHpFC asserted), the 87L function switches to
more secure Alpha Plane settings via the 87SYNL bit, as 87L Differential
Elements explains.
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