The
equipments intended to connect and disconnect power circuits are known
collectively as switchgear (please—not switchgears and not switch-gear).
Switchgear units range from the small, molded-case circuit breakers in a household
panel board to the huge, air break switches on 750-kV transmission lines. They
are generally divided into the four groups of disconnect or isolator switches,
load break switches, circuit breakers, and contactors .
Disconnect or isolator switches are
used to connect or disconnect circuits at no load or very light loads. They
have minimum arc-quenching capability and are intended to interrupt only
transmission line charging currents or transformer exciting currents at most.
They are
usually the least expensive type of switch. Mechanically, they are designed to
provide suffi cient contact pressure to remain closed through fault currents despite
the high mechanical forces these currents may cause. Simple knife switches rely
on multiple leaves for contact and frictional forces to maintain contact.
Others types
have over-center latches, while still others have clamping locks that toggle
toward the end of the closing cycle. All operate in air and have visible contacts
as a safety provision, although low-voltage safety switches rely on handle position.
All have provisions for lockout.
Medium- and high-voltage disconnect
switches are available as indoor designs that are typically mounted in
metal switchgear enclosures or as outdoor switches incorporated into elevated
structures. Both horizontally and vertically operating switches are available in
outdoor designs, and most are available with motor operators. Some have
optional
Load break switches generally follow
the basic design arrangements of disconnect switches
except that they are equipped with arc chutes that enable them to interrupt the current they
are designed to carry. They are not designed to interrupt fault currents; they must remain
closed through faults.
Again, motor
operators are available in most designs. Motor-operated
load break switches can be a lower cost alternative to circuit breakers in some
applications where remote control is required.
Circuit breakers are the heavy-duty
members of the switchgear family. They are rated thermally
for a given continuous load current, as well as a maximum fault current that they can interrupt.
The arcing contacts are in air with small breakers, but the larger types have
contacts in a vacuum or in oil.
High-voltage
utility breakers may utilize sulfur hexafl uoride (SF 6 ) gas. Most breakers
have a stored energy operating mechanism in which a heavy spring is wound up by
a motor and maintained in a charged state. The spring energy then swiftly parts
the contacts on a trip operation.
Typically,
the circuit iscleared in 3 to 5 cycles, since rapid interruption is essential
to minimize arc heating and contact erosion. Indoor breakers are usually in
metal cabinets as part of a switchgear lineup, whereas outdoor breakers may be
stand-alone units.
Some caution
should be used when specifying vacuum circuit breakers. When these breakers
interrupt an arc, the voltage across the contacts is initially quite low. As
the current drops to a low value, however, it is suddenly extinguished with a
very high di/dt .
This current
is termed the chop current, and it can be as high as 3 to 5 A. If the breaker is
ahead of a transformer, the high di/dt level can generate a high voltage
through the exciting inductance of the transformer, and this can be passed on
to secondary circuits.
The required
voltage control can be obtained with arresters on the primary or metal oxide varistors
(MOVs) on the secondary of the transformer. The MOV should be rated to dissipate
the transformed chop current at the clamping voltage rating of the MOV. It also
must be rated for repeated operations while dissipating the 1/2 LI 2 energy of
the primary inductance where I is the chop current.
Molded case
breakers are equipped with thermal and magnetic overload elements that are self-contained.
They are rated by maximum load current and interrupt capacity. Thermal types
employ selectable heaters to match the load current for overload protection.
Larger breakers are operated from external protective relays that can provide
both overload and short circuit protection through time overcurrent elements
and instantaneous elements.
Nearly all
relays are operated from current transformers and most are now solid-state. Because
of their heavy operating mechanisms, circuit breakers are not rated for
frequent operation. Most carry a maximum number of recommended operations
before being inspected and repaired if necessary. Also, after clearing a fault,
breakers should be inspected for arc damage or any mechanical problems.
The real
workhorses of switchgear are the contactors. These are electromagnetically operated
switches that can be used for motor starting and general-purpose control. They
are rated for many thousands of operations.
Contactors can employ air breaks at low voltages
or vacuum contacts at medium voltages. Most have continuously energized operating
coils and open when control power is removed. Motor starters can handle
overloads of five times rated or more, and lighting contactors also have
overload ratings for incandescent lamps.
The
operating coils often have a magnetic circuit with a large air gap when open
and a very small gap when closed. The operating coils may have a high inrush
current when energized, and the control power source must be able to supply
this current without excessive voltage drop. Some types have optional DC coils that use a contact
to insert a current reducing resistor into the control circuit as the contactor
closes.
Any piece of
electrically operated switchgear, whether breaker or contactor, has inductive
control circuits that can develop high voltages in control circuits when
interrupted. Good design practice calls for R/C transient suppressors on operating
coils or motors.
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