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I.L. 40-385.1B
c. The medium set ground overcurrent unit is used
for supervising the Zone 1, Zone 2 and Zone 3
ground distance units (Z1G, Z2G and Z3G), the
forward directional overcurrent ground unit (FDOG).
Generally, it is recommended to be set 2 times the
IOS setting.
IOM = 2 x IOS = 1.0
d. The directional high set overcurrent phase and
ground units (ITP and ITG) are used for direct trip
function. The general setting criterion for the instan-
taneous direct trip unit is:
The unit should be set higher than 1.15 times the
maximum fault on the remote bus, where the factor
of 1.15 is to allow for the transient overreach. For this
example, assume that the maximum load is not high-
er than the maximum forward end zone fault current,
and the maximum phase and ground fault currents
on the remote bus are 20 and 24 amperes, respec-
tively, then the settings of the high-set phase (ITP)
and the high-set ground (ITG) should be:
ITP = 20 x 1.15 = 23
ITG = 24 x 1.15 = 27.6
5.1.6 OSB Blinder Settings (RT and RU)
The requirements for setting the blinder units are:
• Inner blinder must be set to accommodate maxi-
mum fault resistance for internal 3-phase fault
• Inner blinder should not operate on severe stable
swings
• Outer blinder must have adequate separation
from inner blinder for fastest out-of-step swing to
be acknowledged as an out-of-step condition
•
Outer blinder must not operate on load
a. Setting the Inner Blinder
If the OSB is used to supervise tripping of the 3φ unit
on heavy load current, the inner blinder 21BI must be
set sufficiently far apart to accommodate the maxi-
mum fault arc resistance. A reasonable approxima-
tion of arc resistance at fault inception is 400 volts
per foot. If a maximum ratio of "line voltage per spac-
ing" is 10,000 volts/ft. for a high voltage transmission
line, and if a minimum internal 3-phase fault current
is calculated as:
I
= [E / 1.73(Z
min.
where Z
is maximum equivalent source impedance,
A
Z
is line impedance and E is line-to-line voltage.
L
5-2
+Z
)]
A
L
then R
= 400 x FT / I
max.
= 400x1.73(Z
+Z
A
L
= 0.0693 (Z
+Z
)
A
L
Adding a 50% margin to cover the inaccuracies of
this expression:
R
= 0.104(Z
+Z
max.
A
R
= 0.104(Z
+Z
S
A
L
Set inner blinder to:
R
= R
x COS (90° - PANG)
T
S
This is the minimum permissible inner blinder setting
when it is used to provide a restricted trip area for a
distance relay.
Another criterion that may be considered is based
upon the rule of thumb that stable swings will not in-
volve an angular separation between generator volt-
ages in excess of 120°. This would give an approxi-
mate maximum of:
Z
= (Z
+Z
+Z
)/ (2x1.73)
inner
A
L
B
= 0.288(Z
+Z
+Z
A
L
Z
= 0.288(Z
+Z
+Z
inner
A
L
where Z
is the equivalent maximum source imped-
B
ance at the end of the line away from Z
An inner blinder setting between the extremes of
equations (1) and (2) may be used. This provides op-
eration for any 3-phase fault with arc resistance, and
restraint for any stable swing. Except in those cases
where very fast out-of-step swings are expected, the
larger setting can be used.
It will usually be possible to use the minimum inner
blinder setting of 1.5 ohms.
b. Setting the Outer Blinder
For slow out-of-step swings, a reasonably close
placement of outer to inner blinder characteristic 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 mani-
fested to the blinder elements is dependent upon ac-
min.
)/10000
) primary ohms
L
)R
/R
secondary ohms
C
V
) primary ohms
B
)R
/R
secondary ohm
B
C
V
.
A
(5/92)
(1)
(2)
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