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I.L. 40-386.3
c.
The medium set ground overcurrent unit is used for supervising the Zone1, Zone2 and
Zone3 ground distance units, the forward directional overcurrent ground unit (FDOG). Gen-
erally, it is recommended to be set 2 times the 3I0s setting.
3I0M = 2 x 3I0s = 1.0
d.
The directional high set overcurrent phase and ground units (Inst. Ø and Inst. G) are used
for direct tripping functions. The general setting criterion for the instantaneous 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 higher 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, re-
spectively, then the settings of the high-set phase (ITP) and the high-set ground (ITG)
should be:
Inst. Ø = 20 x 1.15 = 23
Inst. G = 24 x 1.15 = 27.6
3.2.7
Out-of-Step Block (OS Block) Blinder Settings (OS Inner and OS Outer)
The requirements for setting the blinder units are:
• Inner blinder must be set to accommodate maximum 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 3Ø 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 approximation of arc resistance at fault
inception is 400 volts per foot. If a maximum ratio of "line voltage per spacing" is
10,000 volts/ft. for a high voltage transmission line, and if a minimum internal 3-
phase fault current is calculated as:
I
min.
where Z
is line-to-line voltage.
then R
= 400 x 1.73(Z
= 0.0693 (Z
Adding a 50% margin to cover the inaccuracies of this expression:
R
max.
R
= 0.104(Z
S
3-6
= [E / 1.73(Z
+Z
)]
A
L
is maximum equivalent source impedance, Z
A
= 400 x FT / I
max
min.
+Z
)/10000
A
L
+Z
)
A
L
= 0.104(Z
+Z
) primary ohms
A
L
+Z
)R
/R
secondary ohms
A
L
C
V
is line impedance and E
L
(10/94)
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