P.3.97
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
Additional considerations for charging current compensation with in-line
transformers are as follows.
➤ The compensation is more accurate and the settings are more
straightforward if the voltage sources used by the 87L function
(87LINEV) are installed on the line side of the in-line
transformer. This avoids errors in voltages as a result of the
voltage drop across the transformer and the impact of
magnetizing and inrush currents.
➤ When the line side voltages are used for compensation, the
relay correctly measures the zero-sequence line voltage and
compensates for the positive-, negative- and zero-sequence
components of the charging current. Subsequently, the zero-
sequence current is removed as part of the vector group
compensation (at least in typical applications from currents of
grounded wye and zigzag windings). Still it is recommended to
enter the actual value of the 87CCB0 susceptance setting (to
benefit the application for internal faults on the line side of
delta-connected windings).
➤ When the winding side voltages are used for compensation
(different than the line voltage), the relay may not correctly
measure the true zero-sequence voltage of the line. This is not
relevant as the zero-sequence current is subsequently removed
from the operating signal as a part of the vector group
compensation. Still it is recommended to enter the actual value
of the 87CCB0 susceptance setting (to benefit the application
for internal faults on the line side of delta-connected windings).
➤ Interaction between the line charging current and the shunt
nonlinear magnetizing branch of the in-line transformer may
alter the nature of the magnetizing inrush and overexcitation
currents compared with a typical case when the transformer
differential relay measures the magnetizing current only and
not the sum of the magnetizing and line charging currents. This
phenomenon should be investigated more carefully to ensure
the harmonic blocking and/or restraining provided per typical
transformer protection principles are adequate for security.
Note that the charging current compensation effectively
removes the charging current from the current the relay uses to
calculate the differential current. With the charging current
removed, the differential current now reflects the magnetizing
current only.
➤ The line capacitance may lead to transformer overvoltage
conditions for opened or lightly loaded lines terminated with
transformers. This needs to be factored in when providing
overexcitation protection for the transformer, typically
accomplished with overvoltage functions (assuming
insignificant variations in system frequency).
87L Watchdog
Monitor
The relay incorporates an 87L watchdog monitor that warns users when an
87L element operation was repeatedly avoided by the disturbance detector or
if the relay repeatedly receives an 87LDTT bit without an accompanying
pickup of the disturbance detector Relay Word bit 87DDL. The 87L watchdog
monitor is enabled by the Group SEL
OGIC setting EWDSEC. This setting is
set to 1 by default to enable the monitor; however, it can be set to 0 to be
disabled during testing. The watchdog outputs 87ERR1 and 87ERR2 will be
inhibited, but, the analog quantities 87WDCT1 and 87WDCT2 will still
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