P.3.41
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
cases. The primary concern is line pickup with uneven breaker
pole closing or one pole stuck. By enabling line charging
current compensation, you can reduce the pickup threshold,
improving the element's sensitivity.
➤ Unmeasured tapped loads. Use the maximum zero-sequence
current from unmeasured taps with a safety factor that accounts
for faults in the tapped loads, cold load pickup, and inrush
upon line energization. Such inrush may include magnetizing
inrush from tapped transformers at the load point. The relay
provides an option to back up and coordinate with the tapped
load protection (see Time-Overcurrent Differential Protection
for more details). If the tapped load is connected via a step-
down transformer, the transformer winding connections can
impact dramatically the amount of zero-sequence current on
the line side for ground faults on the load side of the
transformer.
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 single-line-to-ground faults
under minimum system short-circuit capacity and maximum assumed fault
resistance. In applications providing greater negative-sequence current, you
can use the negative-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).
87LGR and 87LGRS
These settings specify the Alpha Plane outer radius for the zero-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 if other conditions are met.
The extended security setting, 87LGRS, is available under advanced settings
(EADVS = Y). You should set 87LGRS less sensitive (greater than the 87LGR
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 87LGR. If the advanced settings are disabled, setting 87LGR sets 87LGRS
to 1.2 • 87LGR, but no greater than the maximum setting limit of 8.
External fault detection logic in the relay allows you to be liberal in your
choice of a radius setting.
87LGA and 87LGAS
These settings specify the Alpha Plane blocking angle for the zero-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, 87LGAS, is available under advanced settings
(EADVS = Y). You should set 87LGAS less sensitive (greater than the value
for 87LGA) 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
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