P.3.288
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Channel Synchronization Logic and Status
concern if you use a terrestrial system, such as the SEL ICON, as a time
source. In any case, using the channel-based mode, if the channel symmetry
allows, yields a better protection scheme by relying on less equipment and
associated engineering.
Select 87CHpSN to T (external time-based synchronization) for channels that
are or can become too asymmetrical because of communications network
behavior, given your intended setting for the Alpha Plane blocking angle.
When you use this mode, the 87L function depends on the time sources at the
relays working over the given 87L channel. These time sources must be of
adequate quality and must be engineered as a part of the protection system.
The relay monitors time source quality and provides for time fallback modes
upon stabilization of external time source quality (see 87L Time Fallback
Logic).
The data synchronization method in effect is largely the result of your choice
of settings, but the time fallback mode may dynamically force this method.
Therefore, as Figure 3.197 shows, a given 87L channel can be in a channel-
based synchronization mode (87CHpCS Relay Word bit asserted), external
time-based synchronization mode (87CHpTS asserted), or neither (87CHpNS
asserted). This refers to the method of estimating the clock offset between the
remote and local relay communicating over the p-th channel. If neither
method is in place at a given time, the relay assumes that the offset is unknown
and that it can neither align nor use the remote current data.
With reference to Figure 3.197, the channel-based synchronization mode is in
effect in the following cases.
➤ When the application uses serial channels (E87CH = 2SS,
2SD, 3SS, or 3SM).
➤ The channel is configured to work in the channel-based mode
(87CHpSN = C) or when the channel-based mode is forced by
the external-time synchronization fallback logic (87CHpFC
asserted).
➤ The channel-based measurement of the clock offset is actually
available either as a coarse (87CHpCL asserted) or precise
(87CHpCH asserted) value.
Forcing of the channel-based mode (via the 87CHpFC Relay Word bit) occurs
if the channel is configured to use external time for data synchronization, but
valid time is unavailable at the local or remote relay. This forcing is not
automatic but depends on other characteristics of the channel and user
preferences, according to the time fallback mode (see 87L Time Fallback
Logic).
The external time-based synchronization mode is in effect for the following
cases.
➤ In 87L applications over Ethernet (E87CH = 2E, 3E, or 4E) or
if the user configured the channel to work in the external time
mode (87CHpSN = T).
➤ The time is valid at both relays connected by the channel.
It may happen that a given channel is in neither mode (87CHpNS Relay Word
bit asserted), meaning that the relay is unaware of the clock offset between
itself and the remote relay connected over the 87L channel. For example, a
channel can be permanently broken in one direction from the local to the
remote relay while the local relay is configured for channel-based
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