In an electric power system, switchgear is the combination of electrical disconnect switches,
fuses or circuit breakers used to control, protect and isolate electrical equipment.
Switchgear is used both to de-energize equipment to allow work to be done and to clear faults
downstream. This type of equipment is directly linked to the reliability of the electricity
supply. The very earliest central power stations used
simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels
and voltages rapidly escalated, making opening manually operated switches too dangerous for
anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc
energy to be contained and safely controlled. By the early 20th century, a switchgear line-up
would be a metal-enclosed structure with electrically operated switching elements, using oil circuit
breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum,
or SF6 equipment, allowing large currents and power levels to be safely controlled by
automatic equipment incorporating digital controls, protection, metering and communications.
The high-voltage switchgear was invented at the end of the 19th century for operating
motors and other electric machines. The technology has been improved over time and can now be
used with voltages up to 1,100 kV. Typically, switchgears in substations are
located on both the high-voltage and low-voltage side of large power transformers. The switchgear
on the low-voltage side of the transformers may be located in a building, with medium-voltage
circuit breakers for distribution circuits, along with metering, control, and protection
equipment. For industrial applications, a transformer and switchgear line-up may be
combined in one housing, called a unitized substation or USS.
History
Switchgear is as old as electricity generation. The first models were very primitive: all
components were simply fixed to a wall. Later they were mounted on wooden panels. For reasons
of fire protection, the wood was replaced by slate or marble. This led to a further
improvement, because the switching and measuring devices could be attached to the front, while
the wiring was on the back. Housing
Switchgears for low voltages may be entirely enclosed within a building. For transmission
levels of voltage, switchgears are often mounted outdoors and insulated by air, although this
requires a large amount of space. Gas insulated switchgears used for transmission-level voltages
save space compared with air-insulated equipment, although the equipment cost is higher. Oil
insulated switchgears present an oil spill hazard.
At small substations, switches may be manually operated, but at important switching stations
on the transmission network all devices have motor operators to allow for remote control.
Types A switchgear may be a simple open-air isolator
switch or it may be insulated by some other substance. An effective although more costly
form of switchgear is the gas insulated switchgear, where the conductors and contacts are insulated
by pressurized sulfur hexafluoride gas. Other common types are oil or vacuum insulated switchgear.
The combination of equipment within the switchgear enclosure allows them to interrupt fault currents
of thousands of amps. A circuit breaker is the primary component that interrupts fault
currents. The quenching of the arc when the circuit breaker pulls apart the contacts open
requires careful design. Circuit breakers fall into these five types:
Oil
Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil along
the path of the arc. The vapor released by the arcing consists of hydrogen gas.
Air Air circuit breakers may use compressed air
or the magnetic force of the arc itself to elongate the arc. As the length of the sustainable
arc is dependent on the available voltage, the elongated arc will eventually exhaust
itself. Alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping
of the displaced air thus blowing out the arc.
Circuit breakers are usually able to terminate all current flow very quickly: typically between
30 ms and 150 ms depending upon the age and construction of the device.
Gas
Gas circuit breakers sometimes stretch the arc using a magnetic field, and then rely
upon the dielectric strength of the SF6 gas to quench the stretched arc.
Hybrid
A hybrid switchgear is one that combines the components of traditional air-insulated switchgear
and SF6 gas-insulated switchgear technologies. It is characterized by a compact and modular
design, which encompasses several different functions in one module.
Vacuum Circuit breakers with vacuum interrupters
have minimal arcing characteristics(as there is nothing to ionize other than the contact
material), so the arc quenches when it is stretched by a small amount. Near zero current
the arc is not hot enough to maintain a plasma, and current ceases; the gap can then withstand
the rise of voltage. Vacuum circuit breakers are frequently used in modern medium-voltage
switchgear to 40,500 volts. Unlike the other types, they are inherently unsuitable for
interrupting DC faults. Carbon dioxide
Breakers that use carbon dioxide as the insulating and arc extinguishing medium work on the same
principles as an SF6 breaker. By switching from SF6 to CO2 it is possible to reduce the
CO2 emissions by 10 tons during the product life cycle.
Classification Several different classifications of switchgear
can be made: By the current rating.
By interrupting rating Circuit breakers can open and close on fault
currents Load-break/Load-make switches can switch normal
system load currents Isolators may only be operated while the circuit
is dead, or the load current is very small
By voltage class: Low voltage
Medium voltage High voltage
Extra high voltage, ultra high voltage
By insulating medium: Air
Gas Oil
Vacuum Carbon dioxide
By construction type: Indoor class or NEMA enclosure type)
Outdoor Industrial
Utility Marine
Draw-out elements Fixed elements
Live-front Dead-front
Open Metal-enclosed
Metal-clad Arc-resistant
By IEC degree of internal separationNo Separation Busbars separated from functional units
Terminals for external conductors separated from busbars
Terminals for external conductors separated from functional units but not from each other
Functional units separated from each other Terminals for external conductors separated
from each other Terminals for external conductors separate
from their associated functional unit
By interrupting device: Fuses
Air Circuit Breaker Minimum Oil Circuit Breaker
Oil Circuit Breaker Vacuum Circuit Breaker
Gas Circuit breaker CO2 Circuit Breaker
By operating method: Manually operated
Motor/stored energy operated Solenoid operated
By type of current: Alternating current
Direct current
By application: Transmission system
Distribution
By purpose Isolating switches
Load-break switches. Grounding switches
A single line-up may incorporate several different types of devices, for example, air-insulated
bus, vacuum circuit breakers, and manually operated switches may all exist in the same
row of cubicles. Ratings, design, specifications and details
of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards
are used, much of the rest of the world uses IEC standards, sometimes with local national
derivatives or variations. Functions
One of the basic functions of switchgear is protection, which is interruption of short-circuit
and overload fault currents while maintaining service to unaffected circuits. Switchgear
also provides isolation of circuits from power supplies. Switchgear is also used to enhance
system availability by allowing more than one source to feed a load.
Safety To help ensure safe operation sequences of
switchgear, trapped key interlocking provides predefined scenarios of operation. For example,
if only one of two sources of supply are permitted to be connected at a given time, the interlock
scheme may require that the first switch must be opened to release a key that will allow
closing the second switch. Complex schemes are possible.
Indoor switchgear can also be type tested for internal arc containment. This test is
important for user safety as modern switchgear is capable of switching large currents.
Switchgear is often inspected using thermal imaging to assess the state of the system
and predict failures before they occur. Other methods include partial discharge testing,
using either fixed or portable testers, and acoustic emission testing using surface-mounted
transducers or ultrasonic detectors used in outdoor switchyards. Temperature sensors fitted
to cables to the switchgear can permanently monitor temperature build-up. SF6 equipment
is invariably fitted with alarms and interlocks to warn of loss of pressure, and to prevent
operation if the pressure falls too low. The increasing awareness of dangers associated
with high fault levels has resulted in network operators specifying closed door operation
for operating earth switches and racking breakers. Many European power companies have banned
operators from switch rooms while operating. Remote racking systems are available which
allow an operator to rack switchgear from a remote location without the need to wear
a protective arc flash hazard suit. ...
See also Arc flash
Circuit breaker Disconnector
Electrical safety Electric arc
High voltage Remote racking system
Short circuit References
External links The basic functions of LV switchgear
(English) IEC TC17: Switchgear & ControlgearTechnical Committee, on tc17.iec.ch
(English) IEEE Switchgear Committee, on ewh.ieee.org (German) Course ETHZ, handouts_2.pdf, on eeh.ee.ethz.ch