Schweitzer Engineering Laboratories SEL-351A, Manual, Page: 170 / 666

4.44
SEL-351A Relay Instruction Manual
Date Code 20080213
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control Settings (Not in SEL-351A-1)
Equation 4.13
This check assures that the relay uses the most robust analog quantities in
making directional decisions for the neutral-ground and residual-ground
overcurrent elements.
The zero-sequence current (I
0
), referred to in the above application of the k2
factor, is from the residual current (I
G
), which is derived from phase currents
I
A
, I
B
, and I
C
:
Equation 4.14
If both of the internal enables:
32VE internal enable for the zero-sequence voltage-polarized directional
element that controls the neutral-ground and residual-ground overcurrent
elements
32IE internal enable for the channel
IN
current-polarized directional
element that controls the neutral-ground and residual-ground overcurrent
elements
are deasserted, then factor k2 is ignored as a logic enable for the 32QGE
internal enable. This effectively puts less restrictions on the operation of the
negative-sequence voltage-polarized directional element.
k2 Set Automatically
If enable setting E32 = AUTO, setting k2 is set automatically at:
k2 = 0.2
For setting k2 = 0.2, the negative-sequence current (I
2
) magnitude has to be
greater than 1/5 of the zero-sequence current (I
0
) magnitude in order for the
negative-sequence voltage-polarized directional elements to be enabled (|I
2
| >
0.2 •|I
0
|). Again, this presumes at least one of the internal enables 32VE or
32IE is asserted.
50GFP—Forward Directional Residual-Ground Current Pickup
50GRP—Reverse Directional Residual-Ground Current Pickup
Setting Range:
0.05–5.00 A secondary (5 A nominal phase current inputs, IA , IB , IC )
0.01–1.00 A secondary (1 A nominal phase current inputs, IA , IB , IC )
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 settings 50GFP and 50GRP are not made or displayed.
The 50GFP setting (3I
0
current value) is the pickup for the forward fault
detector 50GF of the zero-sequence voltage-polarized and channel
IN
current-
polarized directional elements (see Figure 4.7 ). Ideally, this setting is above
normal load unbalance and below the lowest expected zero-sequence current
magnitude for unbalanced forward faults.
I
2
k2 I
0
•
>
I
0
I
G
3
----- =
3I
0
I
G
I
A
I
B
I
C
+ + = =
Courtesy of NationalSwitchgear.com