Table of Contents
Advertisement
(h)
OSB blinder settings, RT and RU:
The requirements for setting the blinder units
are:
(1) Inner blinder must be set to accommo-
datemaximum fault resistance for internal
3-phase faults.
(2) Inner blinder should not operate on severe
stable swings.
(3) Outer blinder must have adequate separa-
tion from inner blinder for the fastest out-
of-step swing to be acknowledged as an
out-of-step condition.
(4) Outer blinder must not operate on load.
(i)
Setting the Inner Blinder (for heavy load restric-
tion):
If the OSB is used to supervise tripping of the
3Ø-unit on heavy load current, the inner blind-
ers 21BI must be set sufficiently far apart to
accommodate the maximum fault arc resis-
tance. A reasonable approximation of arc resis-
tance 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, minimum internal 3-phase fault current is
calculated as:
Imin. = [E / 1.73(Z
where Z
is max. source impedance, Z
A
impedance and
E is line-to-line voltage.
then, R
.= 400 x FT. / I
max
= 400x1.73(Z
= 0.0693(ZA+ZL)
Adding a 50% margin to cover the inaccuracies
of this expression.
R
. = 0.104(Z
max
R
= 0.104(Z
S
ohms.
Set inner blinder to
R
= R
x
T
S
This is the minimum permissible inner blinder
setting, where it is used to provide a restricted
trip area for a distance relay.
Another criteria that may be considered is
+Z
)]
A
L
is line
L
.
min
+Z
) / 10000
A
L
+Z
) primary ohms.
A
L
+Z
)R
/R
secondary
A
L
C
V
COS(90°
- PANG)
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
Z
inner
Z
= 0.288 (Z
inner
Where Z
is the max. equivalent source imped-
B
ance at the end of the line away from Z
An inner blinder setting between the extreme 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.
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.
Set MDAR (REL-300) RT = 1.50
(j)
Setting the Outer Blinder (for OSB function):
For slow out-of-step swings, a reasonably
close placement of the outer to the inner
blinder characteristic is possible. The separa-
tion 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 50ms 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
2
WR
, 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 20o per cycle on the first half slip
cycle.
If the inner blinder were set for (0.104Z
the very severe
(1)
used, the outer blinder should be set for
approximately:
Z
outer
where Z
I.L. 40-385.5
= (Z
+Z
+Z
)/(2x1.73)
A
L
B
= 0.288(Z
+Z
+Z
) pri. ohms.
A
L
B
+Z
+Z
)R
/R
sec. ohms. (2)
A
L
B
C
V
20°
per cycle swing rate were
= 0.5 Z
primary ohms
T
= Z
+ Z
+ Z
.
T
A
L
B
°
.
.
A
), and
T
13-3
Advertisement
Table of Contents
