P.3.301
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Time Fallback Logic
The previously discussed time fallback modes progress from simplest and
most secure (Mode 1) to the most elaborate solution, which attempts to
provide dependability based on extra channel monitoring functions (Mode 4).
Use caution with the 87TFB setting, and study the time fallback logic schemes
carefully before applying a fallback mode other than the most secure Mode 1.
Upon selecting a fallback mode, consider availability of the second, redundant
protection scheme, as well as regulatory constraints or internal utility practices
related to operating a line without redundant schemes capable of
instantaneous fault clearance.
This subsection explains details of each time fallback mode and highlights
channel monitoring functions that feed into the fallback logic.
Request for Time
Fallback
The relay allows application of the channel-based and time-based
synchronization modes on a per-channel basis; you can minimize use of
external time sources to only those asymmetrical channels that actually need
the external time reference for accurate data synchronization. As a result, the
relay also processes requests for fallback on a per-channel basis, and only for
those channels that use time for synchronization.
As Figure 3.210 shows, the relay places a call for fallback on the p-th channel
if, at a given time, all of the following occur.
➤ The 87CHpRQ Relay Word bit marks the channel as required.
➤ The 87CHpOK Relay Word bit signals proper operation of the
channel.
➤ The channel is set in the time-based synchronization mode
(87CHpSN = T).
3 If a local and/or remote time source for a given channel is unavailable, and the channel was symmetrical prior to loss of
the time reference, the logic forces the affected channel into the channel-based synchronization mode. The 87L Alpha
Plane settings switch into high security mode, and the relay continues to use the channel. If the switchover to channel-
based synchronization is impossible, the logic forces out the channel with consequences similar to those in fallback Mode 2.
4 If a local and/or remote time source for a given channel is unavailable, and the channel was symmetrical prior to loss of
the time reference, the logic forces the affected channel into the channel-based synchronization mode. The 87L Alpha
Plane settings switch into high security mode, and the relays continue to use the channel. This state continues until the
channel switches. The logic detects channel switching via a step change in the round-trip channel delay or temporary loss
of channel. If the logic detects channel switching during the channel-based synchronization mode, or if switchover to
channel-based synchronization is impossible, the logic forces the channel out, with consequences similar to those in fall-
back Mode 2.
Table 3.160 Summary of Time Fallback Modes (Sheet 2 of 2)
87TFB Summary of the 87L Scheme Response
Table 3.161 Merits of Time Fallback Modes Depending on the E87CH
Application Setting
E87CH Discussion of the 87TFB Setting
2SS or
3SM+3SS
Modes 3 and 4 can allow continued operation of the 87L scheme if the
channel is symmetrical at the moment of time reference loss. Use of
Mode 2 has no merit and will result in 87L function loss because no alter-
native channel is available in these applications.
2SD All modes are merited. In Mode 2, the scheme can continue operation
with the second channel. In Modes 3 or 4, the scheme can continue oper-
ation if the channel is symmetrical at the moment of time reference loss.
All relays are
3SM
All modes are merited. Mode 2 is merited if not all channels are synchro-
nized based on time. In Modes 3 or 4, the scheme can continue operation
if the channel is symmetrical at the moment of time reference loss.
NOTE: Both 87L channels use the
same external time reference.
Therefore, if both channels are set to
87CHpSN = T and the external time
source is lost, it will affect both
channels.
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