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I.L. 40-386.3
3.3.23 Polarizing Settings
Settings for the directional ground overcurrent polarization is controlled by the setting
of Dir Type. It has 3 selections:
Zero Sequence —Zero sequence voltage polarization.
Negative Sequ—Negative sequence voltage polarization.
Dual Polariz—Both zero sequence voltage and current polarization.
3.3.24 Overcurrent Ground Backup
The overcurrent ground backup function provides seven sets of curves, which are
similar to the CO curves, for backing up the ground distance protection. Four set-
tings (GB Type, GB Pickup, GBT Curve and GB Dir) must be determined for applying
this function.
a.
GB Type is the ground backup curve selection. Seven sets of familiar CO curves are pro-
vided (C02,5,6,7,8,9 and 11), and are shown in Figures 2-32 thru 2 -38 . The selection is
based on the application and coordination time. A selection of Disabled prevents the
ground backup function from operating.
b.
GB Pickup is the current level setting. The setting range is 0.5 to 4.0 amperes in 0.5 steps.
In general, the current level setting criteria is:
(I
Fmin
where I
For the best sensitivity, GB Pickup should be set at 0.5 amperes, this is normally
adequate for most applications.
c.
GBT Curve is the time delay setting of the ground backup function. As shown in Figures
2-32 thru 2 -38 , the GBT Curve is settable in steps of one from 1 to 63. As with any time
delay overcurrent function, the time delay setting must be coordinated with other overcur-
rent devices.
d.
GB Dir is the setting for directional control selection. The ground backup function becomes
a forward-directional torque control overcurrent ground function if GB Dir is set to YES. If
GB Dir is set to No, the overcurrent ground backup function is non-directional and will pro-
duce a trip output for faults in the forward and reverse direction.
The following equation can be used to calculate the trip time for all CO curves from CO-2 thru
CO-11 :
T (sec) =
T (sec) =
Where 3I
GB Pickup = Pickup setting
3-12
/2) > GB Pickup > 2 x Max. Residual 3I
= Minimum ground fault current for a fault two buses away
Fmin
K
GBT Curve
×
T
+
------------------------- -
------------------------------- -
0
(
)
P
24 000
3I
–
C
0
R
GBT Curve
×
-------------------- -
------------------------------- -
(
)
,
3I
–
1
24 000
0
I
F
=
------------------------------ -
0
GB Pickup
0
(for 3I 0 ≥ 1.5)
,
(for 1<3I
< 1.5)
0
(10/94)
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