Date Code 20001006 Loss-of-Potential, Load Encroachment, and Directional Element Logic 4-33
SEL-351 Instruction Manual
The 50GRP setting (3I
0
current value) is the pickup for the reverse fault detector 50GR of the
zero-sequence voltage-polarized and channel IN current-polarized directional elements (see
Figure 4.6). Ideally, the setting is above normal load unbalance and below the lowest expected
zero-sequence current magnitude for unbalanced reverse faults.
50GFP and 50GRP Set Automatically
If enable setting E32 = AUTO, settings 50GFP and 50GRP are set automatically at:
50GFP = 0.50 A secondary (5 A nominal phase current inputs, IA, IB, IC)
50GRP = 0.25 A secondary (5 A nominal phase current inputs, IA, IB, IC)
50GFP = 0.10 A secondary (1 A nominal phase current inputs, IA, IB, IC)
50GRP = 0.05 A secondary (1 A nominal phase current inputs, IA, IB, IC)
a0 – Positive-Sequence Current Restraint Factor, I
0
/I
1
Setting Range:
0.02 - 0.50 (unitless)
If preceding setting ORDER does not contain V or I (no zero-sequence voltage-polarized or
channel IN current-polarized directional elements are enabled), then setting a0 is not made or
displayed.
Refer to Figure 4.6.
The a0 factor increases the security of the zero-sequence voltage-polarized and channel IN current-
polarized directional elements. It keeps the elements from operating for zero-sequence current
(system unbalance), which circulates due to line asymmetries, CT saturation during three-phase
faults, etc.
a0 Set Automatically
If enable setting E32 = AUTO, setting a0 is set automatically at:
a0 = 0.1
For setting a0 = 0.1, the zero-sequence current (I
0
) magnitude has to be greater than 1/10 of the
positive-sequence current (I
1
) magnitude in order for the zero-sequence voltage-polarized and
channel IN current-polarized directional elements to be enabled ( I I
0 1
01> ×. ).
Courtesy of NationalSwitchgear.com
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