P.3.52
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
To ensure an acceptable level of data security, the relay secures the 87L
communications bits with a BCH data integrity code in applications over
serial channels (see 87L Theory of Operation for more detail). The BCH code
protects against corruption of any nine (or less than nine) bits in the 87L serial
packet. If we assume a binary symmetric channel, a uniform distribution of
the probability of corrupting any single bit in the packet, the probability of an
undetected error with the relay BCH code is less than 1.2 • 10
-10
.
In applications over Ethernet, the relay protects the entire Ethernet packet with
a 32-bit CRC. The relay uses an extra 8-bit checksum for user-programmable
bits and applies the 32-bit BCH the serial 87L applications use to 87L current
data and the 87L bits. If any of the three data integrity codes fails, the relay
assumes all data to be invalid and does not use the information.
A 32-bit data integrity check is sufficient if the channel is relatively free of
noise. However, protective relaying applications may need to assume a worst-
case scenario of standing noise in the communications channel such as might
result from a failing component in the communications equipment. Note that
the relay incorporates channel-monitoring functions that you can use to detect
channel problems. By attending to channel alarms and rectifying channel
issues, you can maintain high security and dependability of the user-
programmable 87L communications bits.
The worst-case scenario of elevated channel noise plays a role in such
applications as direct transfer tripping from the breaker failure protection, in
which a given user-programmable bit operates effectively for tripping without
any independent supervision with communications-independent signals. In
these applications, you can further increase security in one of the following
ways.
➤ Use a 4 ms or 8 ms pickup delay for the received bit. This
effectively calls for two or three consecutive 87L packets to be
consistent with respect to the received bit. The probability of
undetected bit errors in two or three consecutive packets is
beyond practical engineering concerns, considering the
probability of defeating BCH in a single packet being
1.2 • 10
–10
. In some cases, the extra 4 ms or 8 ms delay may
not be acceptable, so this solution is not allowed. Use this
method if you need extra security and can accept the additional
delay.
➤ Supervise the received bit with the local disturbance detector
(87DDL Relay Word bit) in a manner similar to that for the
87DTT logic (see 87DTT Direct Transfer Tripping Logic for
more details). Allow instantaneous assertion of the received bit
if the local disturbance detector picks up. Otherwise, apply a
short delay (4 ms in this example, see Figure 3.28). Note that
the disturbance detector is very sensitive and responds to both
currents and voltages (see 87L Theory of Operation for more
details). Disturbance detector supervision, therefore, does not
impact dependability, even when it supervises DTT-type
commands. In addition, when you use the solution of
Figure 3.28, even upon a failure of 87DDL to pick up, the
command will still execute after the short delay. Use this
method if extra security is essential, particularly when the
transmission of the received bit coincides with a disturbance.
➤ Use two bits to send the critical command. The probability of
corrupting both bits and defeating the BCH is substantially less
than corrupting a single bit and defeating the BCH data
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