What Are Air Insulated Substations?

Air insulated substations (AIS) are electrical substations in which the high-voltage conductors, busbars, switching equipment, and protective devices are insulated from one another and from grounded structures by open air rather than by a compressed gas or a solid dielectric medium. They are the most widely deployed substation type worldwide, used at voltage levels ranging from distribution (1 kV) through ultra-high-voltage transmission (800 kV and above), and their outdoor, open-structure layout makes them accessible for maintenance and visually identifiable at a distance.

In contrast to gas-insulated substations (GIS), which encapsulate live components in sulfur hexafluoride (SF6) at elevated pressure, an AIS relies on carefully designed spacing between conductors and between conductors and grounded metal to maintain electrical isolation under normal operating conditions and during transient overvoltages caused by lightning or switching events. The tradeoff is a larger footprint: the clearances required for safe operation at transmission voltages can result in installations covering several hectares.

Electrical Clearances and Insulation Coordination

The fundamental engineering parameter of an air insulated substation is the air clearance: the minimum unobstructed distance between a live conductor and any adjacent grounded structure, another live conductor at a different potential, or personnel working in the vicinity. Clearances are derived from the voltage levels the substation must withstand, including both the continuous operating voltage and the peak transient voltages that surge arresters are designed to limit. The IEEE 1427-2020 Guide for Recommended Electrical Clearances and Insulation Levels in Air Insulated Electrical Power Substations establishes recommended operating clearances, safety clearances, and insulation coordination procedures for three-phase AC systems from 1 kV to 800 kV, providing the framework that designers use to establish equipment spacing and to verify that the weakest insulating path in the system is protected by the appropriate surge arrester or gap device.

Bus Design and Conductor Configuration

The busbars that carry current through an AIS are typically rigid tubular conductors, flexible stranded cables, or combinations of both. IEEE Std 605, the Guide for Bus Design in Air Insulated Substations, addresses conductor selection, mechanical loading from wind and ice, thermal ratings, and the structural considerations for the gantry structures and insulators that support the bus. Designers must balance electrical clearance requirements against the mechanical forces acting on conductors, and must select conductor cross-sections that can carry continuous load current without overheating while meeting the fault-current thermal withstand requirements defined in the substation's protection study. Strain insulators, post insulators, and suspension strings made from porcelain, glass, or polymer materials provide both mechanical support and the required dielectric isolation.

Switching Equipment and Protection

The primary switching devices in an AIS are circuit breakers, disconnect switches, and earthing switches, all of which operate in open air and are rated for the maximum short-circuit current the network can deliver. Circuit breakers use various arc-extinction methods, including oil, vacuum, and SF6 interrupters housed in individual chambers, but their external terminals and buswork remain air-insulated. Disconnect switches (isolators) provide visible, confirmed open gaps when equipment must be de-energized for maintenance, a function that relies on the inspector being able to see that an air gap exists between the open contacts. Protection systems include current transformers, voltage transformers, and surge arresters, which together provide the measurements and overvoltage clamping needed for relay protection and insulation coordination. The IEEE Power, Switchgear, Substations and Relays Standards Collection catalogs the full set of applicable standards, from circuit breaker ratings to grounding requirements for outdoor installations.

Applications

Air insulated substations have applications across the entire electric power infrastructure, including:

• High-voltage transmission substations at 115 kV to 800 kV interconnecting generation sources and load centers
• Distribution substations at 1 kV to 35 kV serving industrial and residential customers
• Renewable energy plant step-up substations for wind and solar farms
• Industrial facilities requiring dedicated high-power supply points
• Grid interconnection points and HVDC converter stations where AC overhead lines terminate