What Are Circuit Switchers?
Circuit switchers are mechanical
switching devices suitable for frequent operation; not necessarily
capable of high-speed reclosing; capable of making, carrying, and
breaking currents under normal circuit conditions; capable of making,
and carrying for a specified time, currents under specified abnormal
conditions; and capable of breaking currents under certain other
specified abnormal circuit conditions.
They may include an integral isolating
device. Circuit switchers available today use SF6 as an interrupting
medium and may be equipped with a trip device connected to a relay to
open the circuit switcher automatically under specified abnormal
conditions, such as overcurrent or faults.
A circuit switcher, like a circuit
breaker, must carry normal load currents within a specified
temperature range to prevent damage to key components such as
contacts, linkage, terminals, and isolating device parts.
Principal designating parameters of a
circuit switcher are maximum operating voltage, BIL, rated load
current, interrupting current, whether an isolator is required,
whether a trip device is required, and whether manual or motorized
operation is required.
A circuit switcher essentially combines
the functions of a circuit breaker (without reclosing capability) and
a disconnecting switch (by providing visible isolation, but not
necessarily meeting the safety requirements of all users).
A circuit switcher provides a
cost-effective alternative means of transformer protection and
switching, line and loop switching, capacitor or reactor switching,
and load management, with protection in most instances.
Evolution of the circuit switcher
concept provides a more in-depth understanding of its application
versatility and its limitations.
History of Circuit-Switcher
Development
After World War II, the drive to
electrify the remaining rural and sparsely populated areas of the
United States was renewed. Providing fully rated circuit breakers for
switching loaded circuits was frequently beyond budget limitations.
This created a need for new transmission and subtransmission voltage
circuit-switching devices.
One such device could be described as a
load interrupter. It appeared in a wide variety of forms. Most were
attachments to disconnect switches.
Initially, most of these devices used
low-volume oil as an interrupting medium. Ablative gas generating
devices and later vacuum displaced oil. With rare exceptions, these
devices had deficiencies. In the mid-1950s, SF6 was first employed as
an interrupting medium. The application was an interrupter attachment
for disconnect switches.
Whereas ablative devices and vacuum
bottles are limited to approximately 30-kV recovery voltage per gap,
this single-gap SF6 device was readily applied on 138-kV systems for
up to 600 A load switching.
Most of these vacuum, ablative, and SF6
devices were shunted into the circuit during the disconnect switch
opening process. As the 1960s approached, the circuit switcher was
born. It appeared as an in line device. While the first version
employed a number of ablative devices in series, it soon evolved into
the use of SF6 as a medium.
Because of the unfavorable experience
with the earlier devices, the general acceptance of the circuit
switcher took much effort and considerable time. A typical
installation is shown below.
Applications for circuit switchers have
been primarily for transformer protection. The circuit switcher
provides load-switching capability and mainly protection for faults
that originate on the secondary side of the substation transformer.
The zone of protection for circuit
switchers in this location is typically from the current transformers
inside the transformer on the high-voltage bushings to the secondary
feeder breakers. There is generally shorter strike distance on the
secondary bus and more exposure to flashover from wildlife and other
causes.
Therefore, circuit switchers are
specifically tested to interrupt the higher transient recovery
voltages (TRVs) associated with faults initiated on the secondary of
the transformer and cleared by the high-side protective device. For
application where the available high-side short-circuit current
exceeds the device’s capability, blocking relays can be used.
However, in most applications this is not necessary.
No comments:
Post a Comment