Westinghouse KRD-4, Installation, Operation, & Maintenance Instruction

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TYP E K RD-4 DI RECTIONAL O V E RCURR ENT
GROUND
transformer, c apacitor, reactor, and resi stor. Elec­
trically thi s n etwork i s e quivalent to the polarizing
circuit of the indu ction cylinder unit and i s utilized
to minimize the coupling between the current and
pot ential polarized sources.
B. I NSTAN TAN EOUS OVERCURRENT UN I T (I)
The instantaneous overcurrent unit consi sts of an
induction cylinder unit , c apacitor, varistor, and a
tran sformer. The components are connected such that
a contact closing torque is produced when the current
exceeds a specifi ed value.
1.
Cyl inder Unit
The cylinder unit i s similar in construction to the
cylinder unit o f the directional unit e x c ept that all
coil s are simil ar . The phase relatio nship of the two
air gap flux e s necess ary for the development of tor­
que i s achieved by m ean s of a capacitor connected
i n seri e s with one pair of pole windings.
2.
Tran sformer
The transformer is a s aturating type consi sting
of a tapped primary winding and a secondary wind­
ing. A varistor is connect ed across the secondary
winding to reduce the voltage p e ak s applied to the
cylinder unit and phase shifting capacitor.
C. I NDICATI NG CONTACTOR SWI TCH (I CS)
The indicating contactor s witch is a small d-e
operated cl apper type device. A magnetic armature
to which l e af- spring m ounted contacts are attached,
is attracted to the m agnetic core upon energization
of the switch. When the switch clo s e s , the moving
contacts bridge two stationary contacts, completing
the trip circuit. Also during this operation two fingers
on the armature deflect a spring locat ed on the front
of the switch, which allo w s the operatio n indicator
target to drop. The target is reset from outside of the
case by a push rod located at the bottom of the cover.
The front spring, in addition to holding the tar­
get, provides re straint for the armature and thus con­
trol s the pickup value of the s witch.
O P E R A T I O N
The type KRD- 4 relay i s connected to the pro­
tected transmi s sion line as s hown in Fig. 4. I n such
a connection , the relay o perat e s to di sconnect the
line for ground faults of a definite magnitude that are
flowing in a specified direction.
4
The directio nal unit of the relay compar e s the
phas e an gl e between the fault current and the polari­
zing quantitie s of the system and either produc e s a
contact closing torque for faults in the trip direction
or produces a contact opening torque for faults in the
non-trip directio n . Relay op eration occurs when both
the directional unit and the instantaneous overcurrent
unit close their contacts. H enc e , the fault current
must be greater than the tap s etting of the ov ercurrent
unit.
For faults in the non-trip directio n, a co ntact
opening torque is produced by the dire ctional unit
such that the norm ally clo sed contact o f this unit
s horts out a pair o f windings on the overcurrent unit.
This prevents th e overcurrent unit from developing
torque to clos e its contacts . For faults in the trip
direction, the directional unit will pickup and re­
move this short circuit , allowing the overcurrent con­
tact to commence closing alm o st simultaneou sly
with the directional contact for high speed operation.
C H A R A C T E R I S T I C S
The r el ays are avail abl e in the following current
ranges:
Ran g e
0 . 5- 2 Amps. 0.5 0 . 7 5
1-4
4- 1 6
10-40
1 . 0
1 . 5
4 . 0
6 . 0
1 0
1 5
Tap s
1 . 0 1 . 25 1 . 5
2
2 . 0
2 . 5
8 . 0 9 . 0
20 2 5
3 . 0
4 . 0
1 2
16
30 40
The tap value i s the minimum current r equired to
just close the overcurrent relay co ntacts. For pickup
s ettings i n between taps refer to the s e ction under
SETTINGS.
The KRD-4 relay is designed for dual polarizing
and c an be polarized from a potential sour c e , a l o c al
ground source or from both simultaneou sly. When the re­
l ay is potential polarized, the m aximum torque of the
relay occurs when t h e op erating current lags the
pol arizing voltag e by approximately 6 5 d egree s . When
the r elay is current polarized, the m aximum torque of
the relay o ccurs when the operating current i s in
phase with the pol arizing current.
TIME CURVES
The time curv e s for the KRD-4 relay are shown
i n Fig. 5 and 6. Fig. 5 includes three curve s which
are :
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