Sel 411L - Figure 3.22 87 LQ Negative-Sequence Differential Element Logic

P.3.37
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
Figure 3.22 87LQ Negative-Sequence Differential Element Logic
Detailed Description of Settings
87LQP and 87LQPS
These settings specify the pickup threshold for the negative-sequence
differential element. The full-cycle cosine-filtered negative-sequence
differential current (3I2) must exceed the level this setting specifies before the
element can operate. These settings are in per unit of the 87L function. In
applications without in-line transformers, 1.0 pu is the highest primary of any
CT of the differential zone (see 87L Theory of Operation for more details).
The extended security setting, 87LQPS, is available under advanced settings
(EADVS = Y). Set this higher to provide better security, as necessary. You
should see a warning message if you attempt to make this setting less than
87LQP. If the advanced settings are disabled, setting 87LQP to a value other
than OFF sets 87LQPS to 1.2 • 87LQP, but no greater than the maximum
setting limit of 2 pu.
Select a pickup setting that ensures security for differential currents not
resulting from internal faults. Considerations may include the following.
➤ Line charging current. The maximum line charging current
should be considered with a safety factor of 2 to 3 to account
for inrush charging current during line energization. Under
normal conditions, the negative-sequence charging current is
typically a relatively small percentage of the phase charging
current. During external faults, the voltages change, and the
line may draw more negative-sequence charging current.
However, the elevated fault currents provide sufficient
restraining in such 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 negative-
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 a
magnetizing inrush from transformers used to connect the
Analog
87LQFM
Analogs
87KQ
87APAQ
Relay Word Bit
87L50Q
Relay Word Bit
87LQ
(to trip logic)
2
cyc
0
Relay Word Bit
87DD
Relay Word Bit
87MTR
87BLK
87TEST
Test Point
(to test mode logic)
Settings
87LQP
87LQPS
Relay Word Bit
87LQSEC
87LQR
87LQRS
87LQA
87LQAS
RAD
BA
AP
Alpha Plane
Comparator
k
A
ID
PKP
OC
Overcurrent
Comparator
0
1
0
1
0
1