P.3.287
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Channel Synchronization Logic and Status
less than about 15 ppm (15 microseconds / s), allowing for a
relatively long ride through given the 87TOK dropout
threshold of about 500 microseconds.
➤ The external time source lost connection with its time
reference (such as a GPS clock that loses its lock on satellites)
and reports possible error in its time output via the time quality
bits.
➤ A combination of previously listed factors.
In any case, the 87TOK signifies the worst-case possible timing error. When
this status bit is asserted, the relay can use local time safely for 87L protection
and 87L channel monitoring and alarming functions.
Use a high-quality time source for 87L applications. The source must report
its own quality via the embedded time quality bits. 87L Communication and
Timing provides application guidelines for engineering the time distribution
network, including for the 87L applications. Check the 87TOK status bit to
confirm that the relay can establish accurate time when connected to a time
source intended for the 87L application.
Meaningful applications of time for a given 87L channel require precise time
in both relays that communicate over the channel. Therefore, the 87L data
packet includes the 87TOK bit (as an external time locked (ETL) data item) to
inform peer relays about validity of the local time. Time-based calculations
receive subsequent supervision from the 87TOK bit in the local relay and the
ETL bit from the remote relay.
87L Data
Synchronization
Logic and Status
The relay uses either the channel-based or external time-based method to
synchronize 87L data over a given channel. This selection occurs on a per-
channel basis and is primarily controlled by your setting of 87CHpSN
according to the type of communications channel (symmetrical or
asymmetrical) and the acceptable degree of reliance on absolute time for
protection applications.
The data synchronization method setting is available only in applications over
serial channels. Applications over Ethernet use the external time-based
method.
Select 87CHpSN to C (channel-based synchronization) for channels that are
symmetrical or near-symmetrical given your intended setting for the Alpha
Plane blocking angle. The Alpha Plane can tolerate small channel asymmetry.
In this mode, the relay will not use external time for data alignment. As a
result, the protection scheme is independent of time sources. It is therefore not
exposed to possible time source failure modes and misbehavior. This is a
particular concern if you use the GPS system as a time source. It is of less
Table 3.151 87L Absolute Time Quality Relay Word Bit
Name Description
87TOK Local relay time valid for the 87L function and related 87L applications
Table 3.152 87L Synchronization Method Settings
Setting Description Range Default
87CH1SN 87L data synchronization method for the
87L Channel 1
C, T C
87CH2SN 87L data synchronization method for the
87L Channel 2
C, T C
NOTE: In order to use external
time-based mode for the data
synchronization, the GPS clock must
be C37.118 compliant, and the
SEL-411L global setting IRIGC must be
set to C37.118.
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