Schweitzer Engineering Laboratories SEL-311C - Page 152

4.24
SEL-311C Relay Instruction Manual Date Code 20060320
Loss-of-Potential, CCVT Transient Detection, Load-Encroachment, and Directional Element Logic
Directional Control Settings
50QFP and 50QRP Set Automatically
If configuration 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 configuration 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 directional enable logic outputs in Figure 4.6:
➤ 32QE (enable for the negative-sequence voltage-polarized
directional element that controls the phase distance and
negative-sequence overcurrent elements)
➤ 32QGE (enable for the negative-sequence voltage-polarized
directional element that controls the ground distance and
residual ground overcurrent elements)
The k2 factor is applied to 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 enable (and following negative-
sequence voltage-polarized directional element in Figure 4.9) to be enabled:
Equation 4.1
This check assures that the relay uses the most robust analog quantities in
making directional decisions for the ground distance and residual ground
overcurrent elements.
I
2
k2 I
0
• >