A substation is a high-voltage electric facility containing equipment to regulate and distribute electrical energy. While some substations are small with little more than a transformer and associated switches, other substations are large and complex.

Functions of a substation include receiving power from a generating facility, regulating distribution, stepping voltage up and down, limiting power surges, and converting power from direct current to alternating current or vice versa.

Components in an electric substation

Many electrical components work together in a substation to carry out its functions, these include

Lightning Arresters – protect a substation from voltage surges and are installed on power poles, towers, transformers and circuit breakers to protect them from damage during electrical storms. Lightning Arresters look similar to standoff insulators and bushings, but their unique characteristics is that they have earthing terminals at the bottom where a large ground cable is connected and runs down the structure that connects to the station ground.

Switches - measure, regulate, and switch electrical transmissions within the substation as necessary. Switchers turn circuits on and off the grid.

Distribution Bus - is an array of switches that direct power out of the substation. Distribution buses are usually pyramid-shaped or rectangular.

Circuit Breakers - there are two forms of open circuit breakers, namely, dead tank and live tank. The form of circuit breaker influences the way in which the circuit breaker is accommodated, this may be as a ground mounting and plinth mounting, retractable circuit breaker and suspended circuit breakers.

Current Transformers - may be accommodated in one of six manners, namely

• Installed over circuit breaker bushings or on pedestals
• In separate post type housings
• Installed over moving bushings of some types of insulators
• Installed over power transformer or reactor bushings
• Installed over wall or roof bushings
• Installed over cables

In all except in separate post type housing, the current transformer occupies incidental space and does not affect the size of the layout. Installation of current transformers over isolator bushings, or bushings through walls or roofs, is usually confined to indoor substations.

Isolators - are essentially off load devices although they are capable of dealing with small charging currents of busbars and connections. The design of isolators is closely related to the design of substations. Isolator design is considered in the following aspects, namely space factor, insulation security, standardisation, ease of maintenance and cost.

Some types of isolators include horizontal isolation, vertical isolation and moving bushing.

Conductor Systems - The most suitable material for a conductor system is copper or aluminium. Steel may be used but has limitations due to poor conductivity and high susceptibility to corrosion. An ideal conductor may be flat surfaced, stranded or tubular.

An ideal conductor should fulfil the following requirements:

• Capable of carrying specified load currents and short time currents
• Able to withstand forces on it due to its situation. These forces comprise self weight, and weight of other conductors and equipment, short circuit forces and atmospheric forces such as wind and ice loading
• Corona free at rated voltage
• Have minimum number of joints
• Need minimum number of supporting insulators
• Be cost effective

Insulation - insulation security is of high importance in a well designed substation. Extensive research is done on improving flashover characteristics. Increased creepage length, resistance glazing, insulation greasing and line washing have been used with varying degrees of success.

A standoff insulator is required to keep high voltage overhead conductors in position and at a certain distance from conductors in other phases or neighbouring equipment. It also insulates between a conductor and ground. An insulator is always mounted on an earthed support and contains a porcelain section that is mounted between a base plate and a top mounting plate. The base plate is fixed on the grounded support and the top mounting plate is equipped with bus support hardware, on which the overhead conductor is clamped.

Power Transformers – Transformers are large, box-shaped structures connected to multiple wires and are usually the largest single item in a substation. Transformers are usually located on one side of a substation, and the connection to switchgear is by bare conductors. Because of the large quantity of oil, it is essential to take precaution against fire hazards. Hence, a transformer is usually located around a sump used to collect excess oil.

Power from a generating station is sent at a much higher voltage than required for home appliances. Step-down transformers decrease voltage of transmission lines en route to neighbourhoods. Auto transformers can offer advantage of smaller physical size and reduced losses.

Overhead Line Terminations – there are two methods used to terminate overhead lines at a substation, namely, tensioning conductors to substation structures or buildings and tensioning conductors to ground winches. The choice is influenced by the height of towers and the proximity to the substation.

Bushings - This is the component that allows electricity to enter electrical equipments safely, preventing it from shorting to another phase. There are different types of bushings, namely oil filled, gas filled and dry solid porcelain. Unique characteristics of bushings are that the porcelain section is between an oil expansion chamber and a mounting flange.