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b. Setting the Outer Blinder (21BO)
For slow out-of-step swings, a reasonably close placement of outer to inner blinder char-
acteristic is possible. The separation must, however, be based on the fastest out-of-step
swing expected. A 50 ms interval is inherent in the out-of-step sensing logic, and the outer
blinder must operate 50 ms or more ahead of the inner blinder.
Since the rate of change of the ohmic value manifested to the blinder elements is depen-
dent upon accelerating power and system WR 2 , it is impossible to generalize. However,
based on an inertia constant (H) equal to 3, and the severe assumption of full load rejection,
a machine will experience (assuming a uniform acceleration) an angular change in position
of no more than 20° per cycle on the first half slip cycle.
If the inner blinder were set for (0.104Z T) , and the very severe 20° per cycle swing rate
were used, the outer blinder should be set for approximately:
This is the minimum setting of the outer blinder for a 20° per cycle swing rate.
For example,
5.1.7
Overcurrent Ground Backup Unit (GB)
The overcurrent ground backup unit GB provides seven sets of curves, which are similar to the
CO and MCO curves, for backing up the distance ground on high resistance ground faults. Four
settings (GBCV, GBPU, GTC and GDIR) should be determined for applying this unit.
a. GBCV is the ground backup curve selection. Seven sets of familiar CO curves are provid-
ed (C02,5,6,7,8,9 and 11), and are shown in Figures 2-1 thru 2-7. The selection is based
on the application and coordination time. A selection of "OUT" disables the ground backup
function.
b. GBPU is the current level setting. Its range is 0.5 to 4.0 amperes in 0.5 steps. In general,
the current level setting criterion is:
Z
= 0.5 Z
outer
T
where
Z
= Z
+ Z
T
A
if Z
= 0.104 Z
inner
(I
/2) > Setting level > 2 x (Max.
Fmin
residual load, 3I
)
0
where I
= Minimum ground fault current
Fmin
for a fault two buses away
primary ohms
+ Z
B
L
, Z
= 0.5 Z
T
outer
T
Z outer
0.500
----------------- -
=
------------ -
=
4.8
Z inner
0.104
Z outter
R U
------------------- -
=
------- -
=
4.8
Z inner
R T
×
R U
=
4.8 1.5
=
7.2
I.L. 40-385.7
, RT = 1.5
(3)
5-5
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