Sel 411L - Figure 3.49 87 LQ Logic in Applications with In-Line Transformers

P.3.94
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
Figure 3.49 87LQ Logic In Applications With In-Line Transformers
Refer to 87L Elements for more information regarding supervising the 87LQ
logic with the master mode (87MTR), test mode (87TEST), disturbance
detection (87DD), and the blocking function (87BLK). 87L Elements also
explains the application of normal and expanded security Alpha Plane settings
and their control via the 87LQSEC Relay Word bit.
Detailed Description of Settings
87LQP and 87LQPS
These settings (normal and extended security mode values), when set for in-
line transformers, need to account not only for CT errors and line charging
current, but also for steady state overexcitation current under worst possible
overvoltage condition and no load. These considerations apply to unbalanced
system conditions such as an open-pole condition (under balanced conditions
the negative-sequence differential current is zero even under charging current
or stationary overexcitation of the transformer core).
87LQR, 87LQRS, 87LQA, and 87LQAS
The radius (87LQR and 87LQRS) and blocking angle (87LQA and 87LQAS)
settings in the normal and extended security modes, when set for in-line
transformers, need to meet the required sensitivity of transformer protection.
The blocking angle setting may be set in a conservative way (less sensitive)
without impacting performance of the element because of the high
homogeneity of the negative-sequence network. Such conservative settings
allow greater overall communications security for conditions such as current
synchronization errors. The ratio setting may be set sensitive (lower) when
required because the external fault detection logic secures the function during
CT saturation.
Analogs
0
1
0
1
Settings
0
1
Relay Word Bit
87DD
Relay
Word Bit
87LQ
Relay Word Bit
Relay
Word Bit
Analog
(to trip logic)
2 cyc
Test point
87BLK
Relay Word Bits
(to test mode logic)
Relay
Word Bit
Stall the
dropout timer
ID
PKP
OC
Overcurrent Comparator
(Supervision)
RAD
BA
k
AP
Alpha Plane Comparator
(Harmonic
Blocking)
A
0
1.5 cyc
1 cyc
87IQFM
87LQP
87LQPS
87LQR
87LQRS
87LQA
87LQAS
87LQSEC
87QB
E87XFMR = Y
87L50Q
87KQ
87APAQ
87TEST
87MTR