TYPE KAB HIGH IMPEDANC E
B U S DIFF E R E NTI A L R ELAY
way lead for phase fault, two way
lead for phase to ground fault).
IF =Maximum external fault current ( RMS
amp . ) contributed by the bus.
N
= ct turn's ratio.
K
=
Margin factor
The margin factor is a modification number. It
varies with the reciprocal of the ct saturation
.
+
RL ) IF
factor,
1.e.
[
N VK
Figure 3 shows the margin factor curve, which
is based on the High Power Lab tests on the
KAB relay and considers a safety factor of 2 .
The use of this curve i s explained i n the sample
calculation.
2.
SETTING C URRENT UNIT
This unit is not intended for speed improvement.
The High Power Lab . test data shows that its
operation is always slower than the V -unit.
Therefore, the setting c alculation for this unit
is rather simple. First, calculate the value of
(R s
+
RL )
then from curves i n Fig. 4
N
find the value for the setting. These curves
have considered a safety factor of 2.
3. Example:
Assume a 6 circuit bus, for which the maximum
extPrnal 3-phase fault c urrent is 60, 000 amperes
rms, symmetrical, maximum external phase to
gro und faul t current is 45,000 amperes, and the
minimum internal fault cur rent is 1 0,000 amperes.
Assume
the c
t
'
s ratios are 2000:5, cl ass C400,
VK
=
375 \'Olts, secondary winding resistance
Rs
-
0.93
n,
and one way l ead resistance to
junction
point RL = 1. 07D .
A. SETTING OF VOL TAGE UNIT
4
a) for 3-phase fault condition
( ( Rs
t
RL
)_l_r
] I
VK
�
[ ( 0. 93
+
1.07)
N
from Fig. 3 K ::: 0. 76
60000
l
I 375
400
0.9
.·.
VR
=
K ( RS
t
�
)
X
>
60000
- 2 3
VR
=
0.
76 ( 0.93
+
1.07)
X
450 -
4
volts,
b) for phase to ground fault condition
[ ( Rs
+
RL )IF] I VK
�
[ ( 0. 93
+
2
X
1.07 ) x
N
45000
l
400
0 . 89
from Fig. 3 K
?
0. 77
IF
. . VR
_>
K ( Rs
+
RL )
-
N
/375
VR
>
0. 77 ( 0. 93
+
2
X
1. 07) x 45000
=
265
=
400
volts.
choose the maximum of (a) and ( b ) , to prevent
the relay from fal se pickup on external
faults, its minimum setting should be at
least this maximum 265 volts. This adjust
ment is made by varying the spring tension.
See ''Routine Maintenance''.
c ) The minimum fault c urrent required to oper
ate the relay at the setting of 265 volts.
Assume that from the ct saturation curve
I e
0. 045 amp . at 265 volts. And from
figure 2b Iv = 0. 16 amp. at 265 volts. (RMS) .
=
( 6
X
0. 045
+
265
+
0. 16) 400
2600
= 0.53 2 x 400
=
213 amp.
B. SETTING OF C URRENT UNIT
Since (RS
+
RL)
I F= (0.93
+
1 . 07) x 60000 =3 00
N
400
for 3-phase fault.
IF
45000
_
( RS
+
RL)N
=
(0.93
+
2 x 1. 07) x
� -
3 3 0
for phase to ground fault.
From Fig. 4, using the higher number of 3 3 0
the current unit setting is determined to be 4 2
amperes. ( fo r R S
+
RL
=
3 D). Set the o ver
current unit at 45 amp.
I N S TALLAT I O N
The relays should be mounted on switchbo ard
panels or their equivalent in a location free from
dirt, moisture, excessive vibration and heat. Mount
www
. ElectricalPartManuals
. com
