What Is Switchgear And Its Types?
In an electric power system, switchgear is composed 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.
Typically, switchgear in substations are located on both the high- and low-voltage sides 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 (USS).
A switchgear assembly has two types of components:
- Power conducting components, such as switches, circuit breakers, fuses, and lightning arrestors, that conduct or interrupt the flow of electrical power.
- Control systems such as control panels, current transformers, potential transformers, protective relays, and associated circuitry, that monitor, control, and protect the power conducting components.
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.
Switchgears for lower voltages may be entirely enclosed within a building. For higher voltages (over about 66 kV), switchgear is typically mounted outdoors and insulated by air, although this requires a large amount of space. Gas-insulated switchgear saves space compared with air-insulated equipment, although the equipment cost is higher. Oil insulated switchgear presents an oil spill hazard.
Switches may be manually operated or have motor drives to allow for remote control.
Several different classifications of switchgears can be made:
- By the current rating.
- By interrupting rating (maximum short circuit current kA 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 are off load disconnectors which are to be operated after Circuit Breakers, or else if the load current is very small
- By voltage class:
- Low voltage (less than 1 kV AC)
- Medium voltage (1 kV AC through to approximately 75 kV AC)
- High voltage (75 kV to about 230 kV AC)
- Extra high voltage, ultra high voltage (more than 230 kV)
- By insulating medium:
- Gas (SF6 or mixtures)
- Carbon dioxide (CO2)
- By construction type:
- Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure type)
- Draw-out elements (removable without many tools)
- Fixed elements (bolted fasteners)
- Metal-enclosed (ME) — A switchgear assembly completely enclosed on all sides and the top with sheet metal.
- Metal-clad (MC) — A more expensive variety of metal-enclosed switchgear that has the following characteristics: the main switching and interrupting device of removable type; grounded metal barriers to separate compartments and enclose all major circuits and parts; mechanical interlocks; insulated bus conductors and other features.