P.3.35
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
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 three-phase faults under
minimum system short-circuit capacity. We typically do not consider other
fault types for the 87LP element if we apply the negative-sequence or zero-
sequence differential elements.
Setting the pickup threshold greater than the load current provides security
against 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).
87LPR and 87LPRS
These settings specify the Alpha Plane outer radius for the phase 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 if
other conditions are met.
The extended security setting, 87LPRS, is available under advanced settings
(EADVS = Y). You should set 87LPRS less sensitive (higher than the 87LPR
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 87LPR. If advanced settings are disabled, setting 87LPR sets 87LPRS to
1.2 • 87LPR, but no greater than the setting maximum of 8.
Select a radius setting that ensures security against measurement errors in
current magnitudes. External fault detection logic in the relay addresses
potential CT saturation, so you can be liberal in your choice of a radius setting.
87LPA and 87LPAS
These settings specify the Alpha Plane blocking angle for the phase
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 if other
conditions are met.
The extended security setting, 87LPAS, is available under advanced settings
(EADVS = Y). You should set 87LPAS less sensitive (higher than the value
for 87LPA) 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 87LPA. If the advanced settings are disabled, setting 87LPA sets 87LPAS
to 1.2 • 87LPA, but no greater than the setting maximum of 270 degrees.
Select a 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.
When you use channels that are guaranteed to be symmetrical (for example,
direct fiber), channel asymmetry is not a concern and you can make this
setting more sensitive. Also, by applying external time-based synchronization
to asymmetrical channels, you reduce the danger of synchronization errors
and can use lower blocking angle values. Note that the time fallback logic that
responds to the loss or degradation of the time sources you use will force the
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