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TY PE
GENERATING STATION
(3---l
I
LINE PROTECTION
ZONE
BUS PROTECTIOII
ZONE
Fig. 2. 5ampl e System to Show Advantages of Breaker- Fa i lure Protection.
for bus-fault B in Fig. 2. Break er-failure protection
will promptly trip breaker 2, but the fault is still fed
by breaker 8. Like wi se , if breaker 2 fails with a line
fault at M, a remote breaker must trip to clear the
fault. Breaker-failure p rotection trips breaker 3 , but
bre ak er 8 still feeds the fault. Although breaker
failure protection does not complete the job in these
examples , it does e xpeditiously trip the lo cal bre aker,
making it easier for the remote relays to detect the
fault.
C O N S T R U C T I O N
P h a s e O v e rc urrent U n i t
(11
& 13 )
The phase overcurrent unit is an induction
cylinder unit. The time-phase relationship of the two
air gap fluxes necessary for the development of torque
is achieved by m eans o f a cap acitor connected in
series with one pair of po le windings.
Mechanically , the overcurrent unit is composed
of four basic components: a die-cast aluminum frame ,
an electromagnet , a moving element assembly , and a
molded bridge .
The frame serves as the mounting ·structure for
the magnetic c ore. The magnetic core which houses
the lower pin bearing is secured to the fram e by a
locking nut. The bearing can be replaced, if neces
sary, without h aving to remove the magnetic core from
the frame.
The electromagnet has two p airs of coils. The
coils of each p air. are mounted diametrically opposite
one another. In addition, there are two locating pins.
The lo cating pins are used to accurately position
the lower pin be aring, which is threaded into the
bridge . The electromagnet is secured to the frame
by four mo unting screw s.
The moving element as sembly consist s of a spiral
spring, contact c arrying memb er , and an aluminum
cylinder as sembled to a molded hub which holds the
shaft. The s haft has removable top and bottom j ewel
bearings . The shaft rides between the bottom pin
bearing and t he upper pin be aring with the cylinder
rotating in an air gap formed by the electromagnet
and the m agnetic core.
The bridge is secured to the electromagnet and
frame by two mounting screws. In addition to holding
the upper pin bearing, the bridge is used for mounting
the adjustable stationary contact housing . The sta
tionary contact housing is held in position by a spring
type clamp . The spring adjuster is located on the
underside of the bridge and is attached to the moving
contact arm by a spiral spring. The spring adjuster
is also held in place by a spring type clamp.
With the contact clo sed , the electrical connec
tion is made through the stationary contact housing
clamp , to the moving contact , through the spiral
spring out to the spring adj uster clamp.
3
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Summary of Contents for KC-4
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