Substations generally have switching, protection and control equipment, and transformers. In a large substation,
circuit breakers are used to interrupt any
short circuits or overload currents that may occur on the network. Smaller distribution stations may use
recloser circuit breakers or
fuses for protection of distribution circuits. Substations themselves do not usually have generators, although a
power plant may have a substation nearby. Other devices such as
capacitors,
voltage regulators, and
reactors may also be located at a substation. Substations may be on the surface in fenced enclosures, underground, or special-purpose buildings. High-rise buildings may have several indoor substations. Indoor substations are usually found in urban areas to reduce the noise from transformers, improve appearance, or protect switchgear from extreme climate or pollution. Substations often use
busbars as conductors between electrical equipment. Busbars may be aluminum tubing thick, or else wires (strain bus). Outdoor, above-ground substation structures include wood pole, lattice metal tower, and tubular metal structures, although other variants are available. Where space is plentiful and appearance of the station is not a factor, steel lattice towers provide low-cost supports for transmission lines and apparatus. Low-profile substations may be specified in suburban areas where appearance is more critical. Indoor substations may be
gas insulated substations (GIS) (at high voltages, with gas insulated switchgear), or use metal-enclosed or metal-clad switchgear at lower voltages. Urban and suburban indoor substations may be finished on the outside so as to blend in with other buildings in the area. A
compact substation is generally an outdoor substation built in a metal enclosure, in which each item of the electrical equipment is located very near to each other to create a relatively smaller footprint size of the substation.
Switchgear High-voltage
circuit breakers are commonly used to interrupt the flow of current in substation equipment. At the time of interruption, current could be normal, too high due to excessive load, unusual due to a fault, or tripped by protective relays prior to anticipated trouble. The most common technologies to extinguish the
power arc from separating the conductors in the breaker include: • Air at atmospheric pressure (air-insulated switchgear (AIS)), which is the most common worldwide. Air is the cheapest insulator and is easy to modify, but AIS takes up more space, and leaves equipment exposed to the outside environment. One drawback of AIS is the visual impact of a larger substation with overhead power lines entering and exiting, which may be unacceptable in scenic or urban areas. AIS requires additional bracing in a seismically active area, and emits more
electromagnetic fields and noise than alternative technologies. • Gas (gas circuit breaker (GCB) or gas-insulated switchgear (GIS)), most commonly
sulfur hexafluoride (SF6) or a mixture of gases including SF6. Although it is the most expensive, these gases are a much more effective insulator than air. GIS require only 10 to 20 percent of the land area as AIS, which can save on land acquisition cost in urban areas, and allow the substation to be built at the exact location where its power is being used in an industrial or urban area—which can be a significant cost saving. On the generation side, GIS can be installed closer to the generator which allows cost savings in cabling, bus duct connections, and civil construction and can increase reliability. GIS can replace AIS if power requirements increase without requiring additional land area. Additionally, GIS is commonly installed in an enclosed building that keeps the equipment protected from pollution and salt. Unless the substation is often used for switching, maintenance cost can be very low or even zero for many years. Because SF6 turns to solid around , in some climates these circuit breakers require heaters to function in extremely cold weather. SF6 has been used in switchgear since the 1960s. •
Mineral oil (called OCB for oil circuit breaker) provides a high resistance between the opened contacts, effectively stopping the flow of current. Although oil circuit breakers are suitable for a wide range of voltages, the oil becomes contaminated during the suppression of arcs and must be filtered or replaced periodically. •
Vacuum is a better insulator than air but less than gas or oil. Vacuum circuit breakers (VCB) are smaller than air circuit breakers and are commonly used in distribution and other switchgear under 35 kV. • Mixed, including both gas and air insulation. Although it’s the least common option it can be useful when an air-insulated substation needs to be expanded but there is very limited location for additional construction.
Reclosers are similar to breakers, and can be cheaper because they do not require separate protective relays. Often used in distribution, they often are programmed to trip when the amps exceed a certain amount over a period of time. Reclosers will attempt to re-energize the circuit after a delay. If unsuccessful for a few times, the recloser will have to be manually reset by an electrical worker.
Capacitors Capacitor banks are used in substations to balance the lagging current draw from
inductive loads (such as motors, transformers, and some industrial equipment) with their
reactive load. Additional capacitor capacity may be needed if
dispersed generation (such as small diesel generators, rooftop
photovoltaic solar panels, or
wind turbines) are added to the system. Capacitors can reduce the current in wires, helping stem system losses from
voltage drop or enabling extra power to be sent through the conductors. Capacitors may be left on in response to constant inductive load or turned on when inductive load is increased, such as in the summer for
air conditioners. The switching may be remote and can be done manually or automatically.
Control rooms s. Larger substations have control rooms for the equipment used to monitor, control, and protect the rest of the substation equipment. It often contains protective relays, meters, breaker controls, communications, batteries, and recorders that save detailed data about substation operations, particularly when there is any unusual activity, to help reconstruct what happened after the fact. These control rooms typically are heated and air conditioned to ensure the reliable operation of this equipment. Additional equipment is necessary to handle power surges associated with intermittent renewable energy such as dispersed generation from wind or solar.
Transformers Most transformers lose between 5 and 1.5 percent of their input as heat and noise. Iron losses are no-load and constant whenever the transformer is energized, while copper and auxiliary losses are proportionate to the square of the current. Auxiliary losses are due to running fans and pumps which is noisy when the transformer is operating at maximum capacity. To reduce noise, enclosures are often built around the transformer and can also be added after the substation is built. Oil-based transformers are often built with bunded areas to prevent the escape of flaming or leaking oil. Fire separation areas or firewalls are built around the transformer to stop the spread of fire. Firefighting vehicles are allowed a path to access the area. ==Maintenance==