4.43
Date Code 20080213 Instruction Manual SEL-351A Relay
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control Settings (Not in SEL-351A-1)
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 4.6). 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 = 0.50 A secondary (5 A nominal phase current inputs, IA, IB, IC)
50QRP = 0.25 A secondary (5 A nominal phase current inputs, IA, IB, IC)
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 4.6.
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
(|I
2
| > 0.1 • |I
1
|).
k2—Zero-Sequence Current Restraint Factor, I
2
/I
0
Setting Range:
0.10–1.20 (unitless)
Note the internal enable logic outputs in Figure 4.6:
➤ 32QE internal enable for the negative-sequence voltage-
polarized directional element that controls the negative-
sequence and phase overcurrent elements
➤ 32QGE internal enable for the negative-sequence voltage-
polarized directional element that controls the neutral-ground
and residual-ground overcurrent elements
The k2 factor is applied to internal enable 32QGE. The negative-sequence
current (I
2
) magnitude has to be greater than the zero-sequence current (I
0
)
magnitude multiplied by k2 in order for the 32QGE internal enable (and
following negative-sequence voltage-polarized directional element in
Figure 4.9) to be enabled:
Courtesy of NationalSwitchgear.com
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