Air-insulated substations are the classic image that comes to mind when you hear the term "power substation": a vast yard, steel structures, porcelain insulators, and equipment exposed under the open sky. Air itself is the insulating medium — and that is the secret behind both its advantages and its drawbacks.
What Is an AIS Substation?
An Air Insulated Substation (AIS) uses atmospheric air as the insulating medium between its live components, and between those components and ground. Because air is a relatively modest insulator, the required distances between conductors (clearances) must be large, so the substation extends over a wide area and all of its equipment is exposed and visible.
Practical Characteristics
- The oldest and most widespread: a technology proven over decades, typically covering voltages from 132 to 400 kV in transmission substations.
- Visible equipment: circuit breakers, disconnectors, instrument transformers, and busbars are all visible — an advantage for ease of visual inspection.
- Lower initial construction cost: cheaper from the outset than its gas-insulated GIS counterpart.
- Exposed to external factors: dust, rain, humidity, salinity, and bird droppings and nests, as well as rodents — all of which increase maintenance work and cost.
- Large footprint: air clearance requirements impose vast areas, which raises land cost, especially within cities.
Impact on Maintenance
Because the components are exposed with no barriers preventing access, AIS substations require higher maintenance: periodic cleaning of insulators contaminated by pollution, inspection of weather-exposed connections, control of bird nests and rodents, and continuous inspection after storms. For this reason, many networks are gradually shifting toward gas-insulated substations despite their higher initial cost.
Sample answer: An AIS substation uses air as the insulating medium between its components, so its equipment is exposed and visible and requires large clearance distances and extensive areas. Its capital cost is lower, but it requires higher maintenance because its components are directly exposed to weather and degradation factors: dust, rain, and bird and rodent activity, which necessitates intensive periodic cleaning and inspection that raise long-term operating costs.
Judging AIS as "outdated technology." The choice between AIS and GIS is an economic and site-specific decision: in areas with cheap land and a moderate climate, AIS may remain the most sensible choice, while GIS excels within cities and in harsh environments.
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