Sel 411L - Page 168

P.3.60
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
➤ Disturbance detection in remote currents (Figure 3.36).
➤ Adaptive disturbance detection algorithm used with both the
local and remote disturbance detection schemes (Figure 3.37).
As Figure 3.34 shows, the relay declares a disturbance by asserting the 87DD
Relay Word bit when the logic detects a disturbance in both local (87DDL
Relay Word bit asserted) and remote signals (87DDR Relay Word bit
asserted). This approach of calling for both the local and remote data to
confirm a disturbance before allowing the 87L function to operate with no
delay guards against a number of conditions 87L Theory of Operation
explains, and which the following list includes.
➤ Undetected bit errors in the communications packets.
➤ Failure of the remote relay resulting in bad remote data, but
before the self-test procedures assert in the remote relay to
inhibit the entire 87L scheme.
➤ Failure of the local relay resulting in bad local data, but before
the self-test procedures assert in the local relay to inhibit the
entire 87L scheme.
➤ Misalignment of local and remote currents in response to
unusual channel conditions or transients.
Most of the previously stated benefits would be impossible without combining
the local and remote disturbance detection with an AND condition when
driving the supervisory 87DD Relay Word bit.
Because of the following, the relay disturbance detection logic causes no
impairment of 87L function dependability.
➤ The logic uses an adaptive, very sensitive algorithm to watch
each of the signals for disturbance. The algorithm measures the
standing variation in its input signal during load conditions and
adjusts its pickup threshold accordingly to provide the detector
with optimal sensitivity under given conditions, without
causing it to assert spuriously during normal load variations.
➤ The local disturbance detection logic responds to both currents
and voltages, making it dependable under weak feed
conditions.
➤ The remote disturbance detection logic maintains
dependability by asserting for a disturbance if the remote
current is very low, such as when the remote breaker is opened
or the remote terminal is very weak.
➤ The 87DD Relay Word bit supervises the instantaneous
operation of the 87LP, 87LQ, and 87LG elements, as well as
the 87DTT logic. In the absence of a disturbance, the logic still
allows these elements to operate after a short, intentional time
delay.
When the 87L function is in the test mode (87TEST Relay Word bit asserted)
no remote disturbance is necessary to drive the 87DD Relay Word bit. This
simplifies testing, especially in the single-terminal test mode of the relay (see
Section 11: Testing and Troubleshooting for more details).
When the 87L function is in the stub bus mode (ESTUB Relay Word bit
asserted), no remote disturbance is necessary to drive the 87DD Relay Word
bit. In stub bus mode, the 87L function uses only the local currents and
disturbance supervision from the remote line terminals does not apply. Stub