P.3.93
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
87LPP and 87LPPS
The purpose of these settings is to restrain the Alpha Plane under unfavorable
system conditions. In particular, when setting 87LPP (normal mode value) and
87LPPS (extended security mode value) for in-line transformers, be sure to
account for CT errors and line charging current, as well as for steady state
overexcitation current (worst possible overvoltage and no load conditions).
87LPR, 87LPRS, 87LPA, and 87LPAS
Be sure that you meet the required transformer protection sensitivity when
setting the radius (87LPR and 87LPRS) and blocking angle (87LPA and
87LPAS) settings in the normal and extended security modes, for in-line
transformers. With this respect, it is worth emphasizing that the negative-
sequence differential element, 87LQ, when set correctly, provides much
higher sensitivity for transformer faults than the phase element, 87LP.
Note that when the harmonic restrained mode of operation is enabled by
selecting E87HR = Y, security of the 87LP element during magnetizing inrush
conditions depends on the relation among all four settings 87LPR (or 87LPRS
when extended security in place via the 87LPSEC Relay Word bit), 87LPA (or
87LPAS), 87PCT2 and 87PCT4. The amount of extra restraint generated from
harmonics in the differential current and controlled by the 87PCT2 and
87PCT4 settings required to stabilize the element is inversely proportional to
the security of the Alpha Plane settings (87LPR and 87LPA, or 87LPRS and
87LPAS).
87LQ Negative-Sequence Differential Element In-Line Transformers
The negative-sequence differential element provides sensitive protection for
both high-resistance line faults and internal low-current transformer faults.
Note that the external fault detection logic supervises the very sensitive 87LQ
element against CT errors during external faults, as explained in External
Fault Detection.
In reference to Figure 3.49, the negative-sequence differential element, 87LQ,
in applications with in-line transformers is based on the Alpha Plane with
harmonic blocking. The Alpha Plane comparator responds to the 87KQ and
87APAQ operating quantities and works as described in 87L Elements. Relay
Word bit 87QB supervises this comparator to provide security under
magnetizing inrush and steady state overexcitation conditions as Figure 3.45
shows. Relay Word bit 87QB functions in a similar manner as the harmonic
blocking bits related to the phase differential function. The blocking action of
the 87QB is instantaneous. After blocking for 1.5 power cycles, the blocking
action is extended for security for one extra power cycle. Should the block
reassert while the dropout timer is timing out, the dropout timer is stalled for
extra security until the 87QB Relay Word bit in Figure 3.45 deasserts again.
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