What Are Air Insulated Substations

A substation is an important part of the electrical infrastructure. Its function is to step-down or “transform” high voltage from the power transmission lines to a lower voltage that can be sent out through the distribution lines for power supply into homes and businesses.

An Air Insulated Substation (AIS), commonly known as an Outdoor Substation, has all bus-bars, switchgear components, and all other switchyard equipment installed outside, unlike their indoor counterparts that have the apparatus installed within the building of the substation. The latter is only deployed in saline environments or places with high pollution rates.

However, indoor substations  are not as common as AIS systems, which account for more than 70 percent of the substations in the world. Outdoor substations can be classified into pole-mounted and foundation-mounted substations. Here's an overview of how each of them works.

Pole-Mounted AIS

These are designed to support distribution transformers with a capacity of 250 KVA, which are small, simple, and cheap compared to other distribution systems. In this setup, the substation equipment is mounted on the support structures of high-tension distribution lines, which are switched on/off using a mechanically-operated triple-pole switch. Pole-mounted AIS are earthed at multiple points.

Foundation-Mounted AIS

These are assembled on the ground, out in the open, and the area cordoned off by a fence for safety. Foundation-mounted substations are built for both primary and secondary transmission and distribution above 250 KVA. Since the equipment required for these AIS systems is heavy, the sites selected for such outdoor setups need to have adequate pathways for heavy transport.

Who Runs Air Insulated Substations?

Substations are usually owned and run by electrical utility companies. However, it is not unusual for large-scale industrial and commercial entities to have privately-owned AIS powering their manufacturing or processing plants.

One of the main advantages that Air Insulated Substations offer is there’s no risk of a fault appearing at one point in the system being propagated to another section. The distance between adjoining connections is so spaced out that it eliminates any possibility of this happening, making them virtually foolproof.