Switchgear

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.

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 open manually-operated switches too dangerous to use 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 Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls,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.

Locations

Switchgears are located anywhere that isolation and protection may be required. These locations include generators, motors, transformers and substations.

Substations

Typically switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. The switchgear located on the low voltage side of the transformers in distribution type substations, now are typically located in what is called a Power Distribution Center (PDC). Inside this building are typically smaller, medium-voltage (~15kV) circuit breakers feeding the distribution system. Also contained inside these Power Control Centers are various relays, meters, and other communication equipment allowing for intelligent control of the substation.

For industrial applications, a transformer and switchgear line-up may be combined in one housing, called a unit substation.

Housing

Switchgear for low voltages may be entirely enclosed within a building. For transmission levels of voltage (high voltages over 66 kV), often switchgear will be mounted outdoors and insulated by air, though this requires a large amount of space. Gas- [or oil- or vacuum-] insulated switchgear used for transmission-level voltages saves space, although it has a higher equipment cost.

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 piece of 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 "gas insulated switchgear" (GIS), where the conductors and contacts are insulated by pressurized (SF6) sulfur hexafluoride gas. Other common types are oil [or vacuum] insulated switchgear.

Circuit breakers are a special type of switchgear that are able to interrupt fault currents. Their construction allows them to interrupt fault currents of many hundreds or thousands of amps. The quenching of the arc when the contacts open requires careful design, and falls into four types:

* Oil circuit breakers rely upon vaporisation of some of the oil to blast a jet of oil through the arc.
* Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 to quench the stretched arc.
* Vacuum circuit breakers have minimal arcing (as there is nothing to ionise other than the contact material), so the arc quenches when it is stretched a very small amount (<2-3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts.
* Air circuit breakers may use compressed air to blow out the arc, or 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.

Several different classifications of switchgear can be made [ Robert W. Smeaton (ed) "Switchgear and Control Handbook 3rd Ed.", Mc Graw Hill, new York 1997 ISBN 0-07-058451-6] :

* By the current rating.
* By interrupting rating (maximum short circuit current that the device can safely interrupt)
** 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 (less than 1000 volts AC)
** Medium voltage (1000-35,000 volts AC)
** High voltage (more than 35,000 volts AC)
* By insulating medium:
** Air
** Gas (SF6 or mixtures)
** Oil
** Vacuum
* By construction type:
** Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure type)
** Outdoor
** Industrial
** Utility
** Marine
** Draw-out elements (removable without many tools)
** Fixed elements (bolted fasteners)
** Live-front
** Dead-front
** Open
** Metal-enclosed
** Metal-clad
** Metal enclose & Metal clad
** Arc-resistant
** By IEC degree of internal separation [ IEC Standard EN 60439 part 1 Table 6A]
*** No Separation (Form 1)
*** Busbars separated from functional units (Form 2a, 2b, 3a, 3b, 4a, 4b)
*** Terminals for external conductors separated from busbars (Form 2b, 3b, 4a, 4b)
*** Terminals for external conductors separated from functional units but not from each other (Form 3a, 3b)
*** Functional units separated from each other (Form 3a, 3b, 4a, 4b)
*** Terminals for external conductors separated from each other (Form 4a, 4b)
*** Terminals for external conductors separate from their associated functional unit (Form 4b)
*By interrupting device:
** Fuses
** Air Blast Circuit Breaker
** Minimum Oil Circuit Breaker
** Oil Circuit Breaker
** Vacuum Circuit Breaker
** Gas (SF6) Circuit breaker
* By operating method:
** Manually-operated
** Motor-operated
** Solenoid/stored energy operated
* By type of current:
** Alternating current
** Direct current
* By application:
** Transmission system
** Distribution.

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 also is 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. James Harry Castell ( [http://www.castell.com] )invented this technique in 1922. 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. ( [https://www.energy.siemens.com/cms/00000013/aune/Documents/Medium%20Voltage%20Arc%20Fault%20Containment.pdf] )

References

External links

* [http://www.electrical-installation.merlingerin.com/guide/h-electrical-switchgear.htm the basic functions of LV switchgear]