P.3.90
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
The high-set operating path responds to the full-cycle filtered differential
current magnitude, 87IFM, and uses the 87LPU pickup threshold. Be sure to
set the 87LPU pickup sufficiently above the maximum magnetizing inrush
current of the in-line transformer. Operation of this path is signaled via the
87LU Relay Word bit. The 87LU Relay Word bit is ORed with the Alpha
Plane operating trip output. This ORed combination, after further supervision
as explained in 87L Elements, asserts Relay Word bit 87L that routes the
output of the logic to the trip logic. Also, the 87LUA, 87LUB and 87LUC
Relay Word bits are ORed to form Relay Word bit 87LU Relay for your
convenience.
The 87LP element incorporates the Alpha Plane comparator that responds to
the 87K and 87APA operating quantities. Because this Alpha Plane ratio
and angle are derived without harmonic restraint (in the same fashion as in
applications without in-line transformers), Relay Word bit 87HB supervises
the output from the Alpha Plane comparator and permits the Alpha Plane to
operate if the harmonic blocking operating mode is enabled and the level of
harmonic is low, signifying no magnetizing inrush or steady state
overexcitation of the transformer core (see Figure 3.45 and Figure 3.46).
Similarly, the 87LP element incorporates the harmonic restrained Alpha Plane
comparator responding to the 87KHR and 87HRA operating quantities (see
Table 3.36). Relay Word bit 87HR supervises this comparator and permits
the Alpha Plane to operate if the harmonic restraining operating mode is
enabled and the level of fifth harmonic is low, signifying no steady state
overexcitation of the transformer core (see Figure 3.45 and Figure 3.46).
Because the Alpha Plane ratio and angle are derived including harmonic
restraint, this comparator is not blocked with the second or fourth harmonics
to cope with the magnetizing inrush conditions.
An overcurrent condition, signaled by Relay Word bit 87L50 supervises the
two outputs of the two Alpha Plane comparators described above. Other
conditions, explained in 87L Elements, further supervise these two outputs
before asserting Relay Word bits 87L to route the Alpha Plane output to the
trip logic (see Figure 3.47).
The harmonic restrained Alpha Plane works with the restraining current
augmented with the level of harmonics in the differential current per
principles of harmonic restraining for transformer differential protection.
Figure 3.48 explains the harmonic restrained Alpha Plane in more detail. The
generalized Alpha Plane works with the restraining current. When second and
fourth harmonics are present, the Alpha Plane calculations increase the
restraining current with the second and fourth harmonics of the differential
current. More information on the generalized Alpha Plane and the impact of
increasing the restraining signal in the basic Alpha Plane calculations can be
found in 87L Theory of Operation.
Table 3.35 87LP Settings Specific to Applications With In-Line Transformers
Setting Description Range Default
87LPU Pickup threshold of the phase differential ele-
ment for the unrestrained operation (pu)
OFF, 3.0–15 OFF
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