P.3.38
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
tapped load. The relay provides an option to back up and
coordinate with the tapped load negative-sequence protection
(see Time-Overcurrent Differential Protection for more
details).
The pickup threshold you select should be low enough to meet the sensitivity
requirements you need for a given application. This threshold should be low
enough, with a margin, for the relay to detect phase-to-phase and phase-to-
phase-to-ground faults under minimum system short-circuit capacity and
maximum assumed fault resistance. In applications providing higher zero-
sequence current, you can use the zero-sequence differential element to
provide the necessary sensitivity for single-line-to-ground faults.
Setting the pickup threshold greater than the load current provides security
under open CT conditions. In some applications, however, you might want an
open CT to result in a trip. Alternatively, you can use relay open CT logic to
detect an open CT and respond accordingly (see Open CT Detection Logic for
more details).
87LQR and 87LQRS
These settings specify the Alpha Plane outer radius for the negative-sequence
differential element (the inner radius is a reciprocal of the outer radius). If the
calculated value for the Alpha Plane ratio based on the full-cycle cosine-
filtered quantities is less than the reciprocal of this setting, the element will
operate.
The extended security setting, 87LQRS, is available under advanced settings
(EADVS = Y). You should set 87LQRS less sensitive (greater than the 87LQR
setting value) to provide better security as necessary. The relay displays a
warning message if you attempt to enter a value for this setting less than that
for 87LQR. If advanced settings are disabled, setting 87LQR sets 87LQRS to
1.2 • 87LQR, but no greater than the setting maximum of 8.
External fault detection logic in the relay allows you to be liberal in your
choice of a radius setting.
87LQA and 87LQAS
These settings specify the Alpha Plane angle for the negative-sequence
differential element in degrees. If the calculated value for the Alpha Plane
angle based on the full-cycle cosine-filtered quantities is outside of the
blocking region this setting defines, the element will operate.
The extended security setting, 87LQAS, is available under advanced settings
(EADVS = Y). You should set 87LQAS less sensitive (higher than the value
for 87LQA) to provide better security as necessary. The relay displays a
warning message if you attempt to enter a value for this setting less than that
for 87LQA. If the advanced settings are disabled, setting 87LQA sets
87LQAS to 1.2 • 87LQA, but no greater than the setting maximum of
270 degrees.
Select the blocking angle setting to ensure security against measurement
errors in current angles that might result typically from CT saturation or
synchronization errors resulting from channel asymmetry.
External fault detection logic in the relay allows you to be liberal in your
choice of a blocking angle setting.
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