I N ST A L L ATI O N
Westinghouse
I . L. 41-337. 1 D
•
OPERATION
•
MAINTENANCE
INSTRUCTION S
TYPE CA-6 PERCENTAGE DIFFERENTIAL RELAY
FOR BUS AND TRANSFORMER PROTECTION
C A UT I O N
Before putting relays into service , re
move all blocking which may have been inserted for
the purpose of securing the parts during shipment,
make sure that all moving parts operate freely, in
spect the contacts to see that they are clean and
close properly, and operate the relay to check the
settings and electrical connections .
APP LICATION
The type CA-6 percentage differential relay is
usually applied for the differential protection of
multi-circuit buses.
The application of the CA-6
relays for bu s differential protection may be rti vided
into three categories. (In the following discussion
an equivalent circuit is defined as a major source or
a group of feeder circuit s . The fault current contri
bution of all the paralleled feeders should not ex
ceed 5% of the total phase or ground fault current ,
with normal bus connections. )
GENERATING STATION BUSES
•
FOU R
EOUIV ALENT CIRCUITS OR LESS
•
DC
TIME CONSTANT
01<"
0 . 6 CYCLES
O R MORE.
If the following requirements are met, the
CA-6 may be satisfactorily applied:
1) The
maximum external fault current
through the bu s should not exceed 100
symmetrical RMS secondary amperes.
2)
The magnetizing current of the current
transforme rs carrying a total external
fault current of 100 symmetrical second
ary amperes should not exceed one sec-
ondary ampere.
B) GENERATING STATION BUSES
•
MORE
THAN FOUR EQUIVALENT CIRCUITS
•
DC
TIME CONSTANT OF 0 . 6 CYCLES
OR MORE.
The same requirements as in (A) apply. How-
S UPERSEDES I . L. 41-337. 1C
*Denotes ch an ge from superseded i ssue.
ever, consideration must be given to the re
straint coil connections.
Where more than 4 equivalent circuits are
present , it is rarely ne cessary to provide two
relays per phase. With little or no compromise
in the protection scheme, one relay can handle
at least
8
equivalent bu s circuits. (See "Con
nections" for further discussion. )
C ) SUBSTATIOr BUSES - DC TIME CONSTANT
OF 0 . 6 CYCLES
O R LESS.
If the following requirements are met , the
CA-6 may be satisfactorily applied:
1) The
maximum external fault current
through the bus should not exceed 100
symmetrical RMS secondary amperes.
2)
The magnetizing current of the current
transformers carrying a total external
fault current of 100 symmetrical s econd
ary amperes should not exceed
ten sec
ondary amperes.
Where more than 4 equivalent circuits are
present, it is rarely necessary to provide two
relays per phase. With little or no compromise
in the protection scheme, one relay can handle
at least eight equivalent bus circuits.
(See "Connections " for further discussion. )
A type CA-6 transformer relay i s also available
for 3 winding transformer protection. The standard
bus relay should not be utilized for transformer dif
ferential applications.
CONSTRUCTION
The type CA-6 relay consists of three restrain
ing eleme nts (two restraining windings per element),
one operating element , and an indicating contactor
switch. The type CA-6 transformer relay also con
tains an indicating instantaneous trip unit.
EFFECTI V E SEPTEM B E R 1 960
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