Date Code 990430 Loss-of-Potential, Load Encroachment, and Directional Element Logic 4-31
SEL-351P Manual Técnico
50QFP – Forward Directional Negative-Sequence Current Pickup
50QRP – Reverse Directional Negative-Sequence Current Pickup
Setting Range:
0.05 - 1.00 A secondary (1 A nominal phase current inputs, IA, IB, IC)
The 50QFP setting (3I
2
current value) is the pickup for the forward fault detector 50QF of the
negative-sequence voltage-polarized directional elements (see
Figure SECTION 4: .5). Ideally,
the setting is above normal load unbalance and below the lowest expected negative-sequence
current magnitude for unbalanced forward faults.
The 50QRP setting (3I
2
current value) is the pickup for the reverse fault detector 50QR of the
negative-sequence voltage-polarized directional elements (see
Figure SECTION 4: .5). Ideally,
the setting is above normal load unbalance and below the lowest expected negative-sequence
current magnitude for unbalanced reverse faults.
50QFP and 50QRP Set Automatically
If enable setting E32 = AUTO, settings 50QFP and 50QRP are set automatically at:
50QFP = 0.10 A secondary (1 A nominal phase current inputs, IA, IB, IC)
50QRP = 0.05 A secondary (1 A nominal phase current inputs, IA, IB, IC)
a2 – Positive-Sequence Current Restraint Factor, I
2
/I
1
Setting Range:
0.02 - 0.50 (unitless)
Refer to
Figure SECTION 4: .5.
The a2 factor increases the security of the negative-sequence voltage-polarized directional
elements. It keeps the elements from operating for negative-sequence current (system
unbalance), which circulates due to line asymmetries, CT saturation during three-phase faults,
etc.
a2 Set Automatically
If enable setting E32 = AUTO, setting a2 is set automatically at:
a2 = 0.1
For setting a2 = 0.1, the negative-sequence current (I
2
) magnitude has to be greater than 1/10 of
the positive-sequence current (I
1
) magnitude in order for the negative-sequence voltage-polarized
directional elements to be enabled (
II
21
01>⋅.).
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