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TYPE CA PERCENTAGE DIFFERENTIAL RELAY
Before putting relays into service, remove
CAUTION
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 prop
erly, and operate the relay to check the settings and
electrical connections.
A PP LICA TION
The type CA percentage differential relay is de
signed for the differential protection of rotating a-c
machinery such as generators, motors and frequency
changers.
CONSTRUCTION
The type CA relay consists of a perc entage dif
ferential unit and an indicating contactor switch. The
principal component parts of the relay and their loca
tions are shown in Fig.
1-3.
Percentage Differential Unit
:This unit is an induction disc unit with an electro
magnet that has poles above and below the disc a s
shown i n Fig.
2.
Two restraint coils are placed on
the
lower left-hand pole (front view) and are con
nected
in series. Their junction point is connected
to the operating coil wound on the lower right-hand
pole. A transformer winding is supplied on both the
left and right hand poles and these are connected in
parallel
supply current to the upper pole wind
to
The upper pole current generates a flux which
ings.
is
in quadrature with the lower pole resultant flux,
an d the two fluxes react to produce a torque on the
disc.
If the
operating winding is energized, this
torque is in the contact closing direction; if current
flows
through
the two restraining winding in the
same direction a contact opening torque is produced.
Indicating Contactor Switch Unit (ICS)
The
d-e
indicating contactor s witch is a small
SUPERSEDES I.L. 41-331.2A
*Denotes change from superseded issue.
INSTALLATION
INSTRUCTIONS
FOR GENERATOR PROTECTION
Westinghouse
•
OPERATION
clapper type devi ce. A magnetic armature, to which
leaf-spring
mounted contacts are attached,
tracted to the magnetic core upon energization of the
switch. When the switch closes, the moving contacts
bridge two stationary contacts, comple ting the trip
circuit. Also during the operation two fingers on the
armature deflect a spring located on the front of the
switch which allows the operation indicator target
to drop. The target is reset from the outside of the
case by a push rod located at the bottom of the cover.
The front spring, in addition to holding the target ,
provides restraint for the armature and thus controls
the pick-up value of the switch.
OPERA TION
With the relay connected as in the schematic dia
gram Fig. 4A, a through fault causes currents to flow
through two restraint windings in the same direction.
If the current transformers operate properly, these
restraining currents are equal and no current flows in
the operating coil winding, and hence only
opening torque is produced. If the currents in the two
restraining windings are unequal, the difference must
flow in the operating coil. The operating coil current
required to overcome the restraining torque and close
the relay contacts is a function of restraining current.
In
the case of heavy internal fault, when an ex
ternal
source
feeds current into the fault, the re
straining currents are in opposite directions and re
straining torque tends to cancel out as illustrated in
Fig.
4B. When the currents fed from the two sides
are
equal, the restraint is totally cancelled. When
unequal currents flow in from the two sides, the re
straint is equivalent to the difference in the two cur
rents, divided by two, but since the more sensitive
operating coil is e nergized by the sum of the two
currents, the restraint in this case is inconsequential,
and a large amount of closing torque is produced.
I.L. 41-331. 2B
•
MAINTENANCE
is at
contact
EFFECTIVE A PRIL 1965
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