Sel 411L - Figure 3.28 Providing Extra Security for Critical 87 L Communications Bits Used for Unconditional Tripping (a Combination of Delay and Disturbance Detection Supervision); Figure 3.29 Providing Extra Security for Critical 87 L Communications Bits Used for Unconditional Tripping (Two Bits Used)

P.3.53
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
integrity code. Because the bits have little time separation and
may be affected by the same channel noise, use the inverted
logic for one of the bits, as Figure 3.29 shows. Use the
discrepancy check at the receiving relay to prevent assertion of
the critical bit and drive an alarm if necessary. Use this method
if both extra security and speed are essential, and spare bits are
available.
Figure 3.28 Providing Extra Security for Critical 87L Communications Bits
Used for Unconditional Tripping (a Combination of Delay and Disturbance
Detection Supervision)
Figure 3.29 Providing Extra Security for Critical 87L Communications Bits Used for Unconditional Tripping
(Two Bits Used)
87L Supervisory
Logic
This section describes 87L supervisory logic of the relay. These logic schemes
supervise operation of the 87L function to enhance its performance as follows.
➤ External fault detection logic asserts during noninternal faults
accompanied by elevated 87L zone currents and/or long-lasting
dc components in the currents that could cause CT saturation.
➤ Disturbance detection logic responds to disturbances in the
local currents or voltages and remote currents of the 87L zone
to guard against undetected errors in the communications
channel and other unusual conditions.
➤ Enhanced security setting switchover logic controls switching
between normal and more secure settings of the Alpha Plane in
response to a number of rare events that could jeopardize
security of the 87L elements set for typical operating
conditions.
➤ Open CT detection logic responds to problems with CTs and
wiring of the 87L zone currents.
87L Theory of Operation briefly describes these logic schemes.
This section provides comprehensive description of the four supervisory
elements to help you understand the benefits and limitations of their
performance. It is also intended to facilitate in-depth testing of the 87L
protection elements.
87DDL
87R01P1
RX Bit With
Extra Security
4ms
0
Relay
Word
Bit
Relay Transmitting on
Serial Port 1
Bit to be
transmitted
Relay Receiving on Serial Port 1
87R01P1
87T1P1
87T2P1
87R02P1
XOR
Alarm
RX Bit With Extra
Security
Relay
Word
Bits
87L Communications
Bits Setting