P.3.7
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Theory of Operation
Figure 3.3 Traditional Alpha Plane Operating Characteristic for 87L Zone
With Two Currents
The Alpha Plane operating principle has proven secure, fast, and sensitive
because of a combination of factors including good immunity to data
synchronization errors and CT saturation on external faults, good sensitivity
and speed of the sequence differential elements (87LQ and 87LG), and good
sensitivity of the 87LQ and 87LG elements under the presence of line
charging currents.
The term “generalized Alpha Plane” refers to an algorithm with the following
characteristics:
➤ It measures any number of currents that bound the differential
zone.
➤ It allows arbitrary manipulation of the differential and
restraining auxiliary signals.
➤ It results in two equivalent currents that produce a complex
ratio on the Alpha Plane, with this ratio checked against the
traditional Alpha Plane operating characteristic.
The relay uses a generalized (expanded) Alpha Plane algorithm for the
following reasons.
➤ Ability to work in multiterminal lines with each terminal
having multiple currents bounding the 87L zone. For example,
in a four terminal line with each terminal connected in a dual-
breaker arrangement, eight currents bound the differential
zone. The traditional Alpha Plane expects two currents in the
differential zone.
➤ Ability to perform line charging current compensation while
not having to estimate the exact amount of charging current
from each terminal bounding the differential zone. The
I
L
I
L
I
R
I
R
Im(k)
Re(k)
RESTRAIN
OPERATE
Angle
Radius
-1
k =
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