In the electric power industry,
switchgears refer to a
collection of switching devices
(for example, electrical disconnects
and/or circuit breakers). These play
a vital role in electrical protection
(for example, protection against
overload currents, short circuit
currents and insulation failures)
and safe isolation from live parts so
that the personnel can carry out
the maintenance/installation work
with safety. Located on both the
high voltage and low voltage
side of the transformer,
switchgears also come handy for
local or remote switching purposes
such as functional switching,
emergency switching, emergency
stopping and switching off for
mechanical maintenance.
Classification of switchgears
Switchgears can be classified on
the basis of technology and
voltage levels and end-users.
There are two classifications based
on technology, namely sulfur
hexafluoride (SF6) based
switchgears and vacuum
based switchgears.
Based on the voltage,
switchgears can be classified into
three types, namely, high voltage
(HV; more than 38 kV), medium
voltage (MV; 1 to 38 kV) and low
voltage (LV; less than 1 kV).
However, the voltage level
classification of switchgears may
vary depending on the market.
The HV level classification can
further be broken down to include
equipment such as SF6 circuit
breakers live tank, dead tank, and
circuit switchers and disconnectors
(centre break or double break).
The MV product category
includes indoor switchgear,
outdoor switchgears, and outdoor
circuit breakers and disconnect
switches. The LV switchgear can be
further broken down to include air
circuit breaker (ACB), molded case
circuit breakers (MCCB), contactors,
distribution switchboards and
panel boards (excludes ACBs,
MCCBs and contactors).
Technology scenario
The use of oil as the insulating
medium in switchgears has been
replaced with vacuum and sulfur
hexafluoride (SF6) insulated units.
The favourable electro-technical,
chemical and physical features of
the gas have considerably
influenced the development of
high- and medium-voltage
switchgear systems.
In high voltage (HV) and extra
high voltage (EHV) circuit breakers
use SF6 exclusively as an arc
quenching medium. In gas
insulated switchgears (GIS), SF6 is
also used as insulation in addition
to being used as an arc quenching
medium. In medium voltage
circuit breakers and switches, the
other quenching media including
SF6 is installed inside a metal
enclosure. In other designs such as
SF6 insulated metal-enclosed
switchgear, SF6 gas provides the
main insulation of the equipment.
The use of SF6 as arc quenching
and insulating medium in
switchgears helps not only in
achieving the compact design but
also has the potential to increase
system reliability and less
maintenance and a longer service
life. Other technology advantages
of using SF6 as against air include
the reduction of insulation
volumes, better heat dissipation,
high braking capacity and low
operating energy. SF6 quenching
properties are 100 times higher
than air.
Other conditional technology
advantages include non-toxic and
non-inflammable nature of SF6 in
its pure state and protection
against ambient conditions
because of the use of sealed
....CONTD
