Sel 411L - Figure 3.23 87 LG Zero-Sequence Differential Element Logic; Table 3.10 87 LG Zero-Sequence Differential Element Analog Quantities

P.3.40
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
Figure 3.23 87LG Zero-Sequence Differential Element Logic
Detailed Description of Settings
87LGP and 87LGPS
These settings specify the pickup threshold for the zero-sequence differential
element. The full-cycle cosine-filtered zero-sequence differential current (3I0)
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, a 1.0 per unit is the highest primary of any CT of the
differential zone (see 87L Theory of Operation for more details).
The extended security setting, 87LGPS, 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
87LGP. If the advanced settings are disabled, setting the 87LGP to a value
other than OFF sets the 87LGPS to 1.2 • 87LGP, 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 zero-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 zero-sequence charging current. However,
the elevated fault currents provide sufficient restraining in such
Table 3.10 87LG Zero-Sequence Differential Element Analog Quantities
Name Description Units
87IGFM Zero-sequence differential current (3I0) magnitude, full-
cycle cosine-filtered
pu
87KG Alpha Plane ratio, zero-sequence –
87APG Alpha Plane angle, zero-sequence Deg (±180°)
Analog
87LGFM
Analogs
87KG
87APAG
Relay Word Bit
87L50G
Relay Word Bit
87LG
(to trip logic)
2
cyc
0
Relay Word Bit
87DD
Relay Word Bit
87MTR
87BLK
87TEST
Test Point
(to test mode logic)
Settings
87LGP
87LGPS
Relay Word Bit
87LGSEC
87LGR
87LGRS
87LGA
87LGAS
RAD
BA
AP
Alpha Plane
Comparator
k
A
ID
PKP
OC
Overcurrent
Comparator
0
1
0
1
0
1