P.3.300
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Time Fallback Logic
87L Time Fallback Logic
You can apply the relay with asymmetrical communications channels by using
the external time-based synchronization mode. In this mode, the external time-
based synchronization avoids synchronization errors that channel asymmetry
introduces (see more information in 87L Theory of Operation). In this case,
connect the time sources via the IRIG-B inputs and ensure the timing system
is appropriately engineered for this protection application (see 87L
Communication and Timing).
A fallback logic is necessary for 87L applications where at least one of the
required channels is configured for the external time-based synchronization
mode. The fallback logic responds to loss or degradation in external time
quality, as deassertion of the 87TOK Relay Word bit indicates. The fallback
logic controls the 87L function to provide proper security and, optionally,
maintain dependability of the 87L elements for a limited time or under
specific conditions.
In general, the relay provides for four different time fallback modes, each with
varying degrees of 87L protection function security and dependability.
Specifically, in the three-terminal outstation application (E87CH = 3SS) no
fallback strategy is necessary. Two relays act as a permanent outstation
serving data to the remote relay master. The outstation relay uses no current
data, so it is unaffected by loss of synchronization. Note that the master relay
in this configuration (E87CH = 3SM) would require a time fallback mode to
deal with loss of external time information at either end of the channel
connecting the outstation and master relays.
The only time fallback possible with applications over Ethernet is to inhibit
the 87L function; the 87L scheme is entirely external time-based and cannot
function if any relay loses the absolute time reference.
In summary, the time fallback mode setting, 87TFB (see Table 3.159), is
available only if E87CH = 2SS, 2SD, or 3SM. Table 3.160 summarizes the
modes, and Table 3.161 lists the merits of each. It is generally best to set
87TFB identically for all relays in a given application.
Table 3.159 Time Fallback Logic Setting
Setting Description Range Default
87TFB Time fallback mode 1–4 1
Table 3.160 Summary of Time Fallback Modes (Sheet 1 of 2)
87TFB Summary of the 87L Scheme Response
1 If any required time source is unavailable, the 87L function is effectively inhibited at all relays of the 87L differential sys-
tem. This mode is biased toward security of protection. There is no attempt to continue providing 87L protection upon
loss of a required timing source.
2 If a local and/or remote time source for a given channel is unavailable or degraded, the affected channel is forced out, i.e.
effectively marked as unusable. The relays respond by switching to a hot stand-by channel, switching to the outstation
mode, or disabling the 87L function entirely, depending on the application and the status of the other channels. This mode
provides no benefits in the 2SS applications, but it may maintain dependability in 2SD and 3SM applications if only one
channel operates in the external time-based synchronization mode.
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