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 important because it 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 Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls,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.
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 be used with voltages up to 1,100 kV.
Typically, the switchgear in substations is located on both the high voltage and the 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.
Switchgears are as old as the 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.
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, although this requires a large amount of space. Gas insulated switchgear used for transmission-level voltages saves space compared with air-insulated equipment, although it has a higher equipment cost. Oil insulated switchgear presents 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.
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 sulfur hexafluoride gas (SF6). 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 (within a switchgear enclosure) is the primary component that interrupts fault currents. The quenching of the arc when the circuit breaker pulls apart the contacts open (disconnects the circuit) requires careful design. Circuit breakers fall into these four types:
Oil circuit breakers rely upon vaporization 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 ionize other than the contact material), so the arc quenches when it is stretched by a small amount (<2–3 mm). At or 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 35,000 volts. Unlike the other types, they are inherently unsuitable for interrupting DC faults.
Air circuit breakers may use compressed air (puff) 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.
Several different classifications of switchgear can be made:
- 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 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)
- By construction type:
- By IEC degree of internal separation
- 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:
- Air Circuit Breaker
- Minimum Oil Circuit Breaker
- Oil Circuit Breaker
- Vacuum Circuit Breaker
- Gas (SF6) 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
- By purpose
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.