I.L. 40-385.5
13-4
This is the minimum setting of the outer blinder
for a 20
°
per cycle swing rate.
For this example, if Z
inner
= 0.104 Z
T
,
Z
outer
= 0.5 Z
T
, RT = 1.5
Set MDAR (REL-300) RU = 7.20
(k)
Overcurrent ground backup unit GB
The overcurrent ground backup unit GB pro-
vides the following time curves, which are simi-
lar to the CO and MCO relays, for backing up
the distance ground on high resistance ground
fault.
CO-2
Short time curve
CO-5
Long time curve
CO-6
Definite time curve
CO-7
Moderately inverse time curve
CO-8
Inverse time curve
CO-9
Very inverse time curve
CO-11
Extremely inverse time curve
Four settings, GBCV, GBPU, GCT and GDIR
need to be determined for applying this unit.
(1) GBCV is the setting for the selection of the
time-current curve. In general, the selec-
tion is based on the application and the
coordination. A selection of “OUT” dis-
ables the ground back up function. In gen-
eral, CO-8, CO-9 and CO-11 are normally
be used for line protection, except CO-6
may be used for short line application.
(2) GBPU is the current level setting. Its range
is 0.5 to 4.0 amperes in 0.5 steps. In gen-
eral, the current level setting criteria is
(I
Fmin
/2) > Setting Level > 2 x (Max. resid-
ual load, 3I
o
)
where I
Fmin
= Minimum ground fault cur-
rent for a fault two buses away.
For better sensitivity, GBPU should be set
at 0.5 amp. This would be adequate for
most of the applications.
(3) GCT is the time delay setting of the GB
unit. As shown in the relay time-current
performance curves (not included in this
write-up, please refer to I.L.), There are 63
setting selection, from 1 to 63 in 1.0 steps.
In general, the time delay setting should
be coordinated with any protective device
downstream of the line section.
(4) GDIR is the setting for directional control
selection. The GB unit will become a
directional torque control overcurrent
ground unit if GDIR is set at YES.
(5) The polarizing approach for the directional
ground overcurrent unit is controlled by the
setting of DIRU. It has 3 selections:
ZSEQ — Zero sequence voltage polariza-
tion only.
DUAL — Both zero sequence voltage and
current polarizations
NSEQ — Negative sequence voltage and
current operated.
For example, Assuming that the applica-
tion needs DUAL directional control, and a
CO-8 curve is required, the maximum 3Io
of unbalanced load is 0.2 amperes, the
minimum ground fault current for a fault
two buses away is 10 amperes and 0.7
seconds is required for coordination with
current of 20 times GBPU setting, then the
settings of the GB function should be as
shown below:
10 / 2 > GBPU > 2 x 0.2
Set GBPU = 0.5
From time-current performance curves, for
0.7 seconds at 20 times of GBPU setting,
GTC should be set to 24. Select CO-8
from GBCV. Set GDIR = YES if directional
control is required.
Set MDAR (REL-300) DIRU = DUAL
GBCV = CO-8
GBPU = 0.5
GCT = 24
GDIR = YES
(l)
Timer settings:
(1) Zone-1 trip delay timer T1 is normally set
Z
outer
Z
inner
---------------
0.500
0.104
-------------
4.8
=
=
Z
outer
Z
inner
---------------
R
U
R
T
-------
4.8
=
=
R
U
4.8
1.5
×
7.2
=
=
