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Set inner blinder to:
R
= R
T
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 sta-
ble swings will not involve an angular separation between generator voltages in ex-
cess of 120°. This would give an approximate maximum of:
OS Inner = (Z
= 0.288(Z
OS Inner = 0.288(Z
where Z
from Z
An inner blinder setting between the extremes of equations (1) and (2) may be
used. This provides operation for any 3-phase fault with arc resistance, and re-
straint 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 manifested to the blinder elements is
dependent upon accelerating power and system WR
ize. However, based on an inertia constant (H) equal to 3, and the severe assump-
tion 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:
OS Outer = 0.5 Z
where
Z
= Z
T
This is the minimum setting of the outer blinder for a 20° per cycle swing rate.
3.2.8
Timer Settings (Definite Time Setting)
a.
Zone2 timers (T2Ø and T2G Time) setting should be coordinated with the Zone1 and other
high-speed trip units on the adjacent line terminal. Coordination Time Interval of 0.3 to 0.5
seconds is recommended. For example, if T2Ø and T2G Time of 0.4 seconds is used, then
the phase and ground Zone2 timers should be set as follows:
T2Ø Time = 0.4 seconds and T2G Time = 0.4 seconds
(10/94)
x COS (90° - PANG)(1)
S
+Z
+Z
)/ (2x1.73)(2)
A
L
B
+Z
+Z
) primary ohms
A
L
B
+Z
+Z
A
L
B
is the equivalent maximum source impedance at the end of the line away
B
.
A
primary ohms
T
+ Z
+ Z
A
B
L
)R
/R
secondary ohm
C
V
I.L. 40-386.3
2
, it is impossible to general-
(3)
3-7
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