INSTALLATION
Westinghouse
I . L. 41-337.4A
•
OPERATION
•
MAINTENANCE
INSTRUCTIONS
TYPE KAB HIGH IMPEDANCE
BUS DIFFERENTIAL RELAY
CAU T I ON:
Before putting relays into servic e , remove
all blocking which may have been inserted for the
purpose of securing the parts during shipment, make
sure that all moving parts operate freely , inspect
the contacts to see that they are clean and close
properly, and operate the relay to check the settings
and electrical connections .
AP P L I CAT I O N
The type KAB relay i s a n instantaneous relay of
the high impedance type used for bus differential
protection.
*
AP P L I CAT I O N C ON S I D E RAT I ON S
The type KAB relay can b e applied for bus
protection in most cases where bushing type ct's
are in use, and in metal-clad equipment where ct's
with toroidally wound cores have their windings
completely distributed are employed. Dwg.
265C 196
shows the external connection .
The following points should be considered or
should be known on any proposed type KAB relay
application :
1 .
All ct's in the bus differential circuit should
have the same ratio, and should be operated on
their full tap. If tap connection cannot be avoid
ed, the winding section between the tap s being
used must be fully distributed and the high
voltage which may appeaT at the full tap terminal
due to the auto-trans former action should b e
checked.
2.
The leakage impedance of the ct 's which are to
be used should be low.
3.
The use of the auxiliary ct's is not recommend
ed. If this cannot be avoided the additional
impedance from the auxiliary ct's and the high
voltage which transformed by the auxiliary ct
should be checked.
SUPE RSEDES I .L. 41-337.4
*Denotes change f rom superseded 1ssue.
4.
The best location for the junction points is
equidistant from all ct's.
5 .
The lead resistance from the junction points to
the relay terminals is not critical.
6.
A lockout relay contact is recommended to short
circuit the varistor following the relay operation
in order to prevent the varistor from ove rheating.
7 .
To insure a substantial margin of operation the
voltage-unit should not be set higher than VK•
where VK =the k nee voltage value of the poorest
ct which be connected to the r elay. The k nee
voltage is defined as the inters ection of the
extension of the two straight line portions of the
saturation cur v e , ordinate and abscissa must
be same scale for each decade.
8. The maximum number of circuit s which can be
connected to the relay or the minimum internal
fault current required to ope rate the r elay c an
be estimated from the following equation
lmin. = ( XJe
+
lR
+
ly)
N
where Imin. =minimum internal fault current , RMS.
Ie
= ct secondary excitation current at
a voltage equal to the setting
value of V-unit .
I
R
=
Current in V-unit at setting voltage
VR (i . e. lR =
VR,/2600)
Iy
= Current in varistor circuit at a
voltage equal to the setting value
of V-unit.
N
=
ct turn's ratio.
X
= Number of circuits connected to
the bus .
CON S T R U C T I O N
The relay consists of a high speed overvoltage
cylinder unit (V) a high speed overcurrent unit (IT) ,
EFFE C T I V E DECEMBER 1 973
www
. ElectricalPartManuals
. com
