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I.L. 40-385.7
a. Setting the Inner Blinder (21BI)
If the OSB is used to supervise tripping of the 3φ unit on heavy load current, the inner blind-
er 21BI must be set sufficiently far apart to accommodate the maximum 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 transmis-
sion line, and if a minimum internal 3-phase fault current is calculated as:
where Z
voltage.
Adding a 50% margin to cover the inaccuracies of this expression:
Set inner blinder to:
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 involve an angular separation between generator voltages in excess of 120°. This would
give an approximate maximum of:
where Z
An inner blinder setting between the extremes of equations (1) and (2) may be used. This pro-
vides operation for any 3-phase fault with arc resistance, and restraint for any stable swing. Ex-
cept 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.
5-4
I
= [E / 1.73(Z
min.
is maximum equivalent source impedance, Z
A
then R
= 400 x FT / I
max
.
= 400x1.73(Z
= 0.0693 (Z
R
= 0.104(Z
max.
R
= 0.104(Z
S
R
= R
S
T
Z
= (Z
inner
= 0.288(Z
Z
= 0.288(Z
inner
is the equivalent maximum source impedance at the end of the line away from Z
B
+Z
)]
A
L
min.
+Z
)/10000
A
L
+Z
)
A
L
+Z
) primary ohms
A
L
+Z
)R
/R
secondary ohms
A
L
C
V
x COS (90° - PANG)
+Z
+Z
)/ (2x1.73)
A
L
B
+Z
+Z
) primary ohms
A
L
B
+Z
+Z
)R
/R
secondary ohm
A
L
B
C
V
is line impedance and E is line-to-line
L
(1)
(2)
.
A
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