Technical Comparison of Air-Insulated Switchgear (AIS) and Gas-Insulated Switchgear (GIS)

1. Introduction

The selection of appropriate switchgear is crucial for power system reliability, safety, and efficiency. Air-Insulated Switchgear (AIS) and Gas-Insulated Switchgear (GIS) are two predominant technologies used in electrical power distribution and transmission. While both serve the same fundamental purpose of controlling, protecting, and isolating electrical equipment, they differ significantly in design, insulation medium, footprint, cost, and maintenance requirements. This article provides a comparative analysis of AIS and GIS, highlighting their applications, advantages, disadvantages, and applicable international standards.

2. Comparison of AIS and GIS

2.1 Insulation Medium

• AIS: Uses ambient air as the insulating medium.
• GIS: Uses sulfur hexafluoride (SF₆) gas, a highly effective dielectric medium with superior insulating properties.

2.2 Physical Footprint and Space Requirements

• AIS: Requires a large footprint due to the air gaps necessary for insulation and clearance.
• GIS: Has a compact design, making it ideal for space-constrained environments such as urban substations or underground installations.

2.3 Installation Considerations

• AIS: Typically installed outdoors due to large spatial requirements; indoor installations require extensive ventilation.
• GIS: Designed for indoor or underground installations, often used in densely populated areas where land is limited.

2.4 Cost Implications

• AIS: Lower initial capital expenditure due to simpler construction and lower material costs.
• GIS: Higher upfront cost due to specialized components, SF₆ gas handling, and compact modular design.

2.5 Maintenance and Reliability

• AIS: Requires regular maintenance due to exposure to environmental factors such as dust, moisture, and pollution, which can lead to insulation degradation.
• GIS: Minimal maintenance due to its sealed construction, reducing exposure to external contaminants, resulting in higher reliability and longevity.

2.6 Safety Considerations

• AIS: Higher risk of arc faults in open-air configurations, necessitating strict operational safety measures.
• GIS: Enclosed design reduces the risk of arc faults, enhancing operational safety.

2.7 Environmental Impact

• AIS: Environmentally friendly as it does not use greenhouse gases.
• GIS: Uses SF₆, a potent greenhouse gas; proper gas handling and leakage prevention are crucial to minimize environmental impact.

3. Applications

3.1 AIS Applications:

• Outdoor substations in rural and suburban areas.
• Large industrial plants where space is not a constraint.
• Transmission networks in less polluted environments.
• Cost-sensitive projects requiring lower initial investment.

3.2 GIS Applications:

• Urban substations where space is limited.
• Underground substations and offshore platforms.
• High-voltage applications (220 kV and above).
• Renewable energy substations, including offshore wind and solar farms.

4. International Standards

Both AIS and GIS must comply with various international standards to ensure operational safety, performance, and environmental compliance. Some key standards include:

4.1 AIS Standards:

• IEC 62271-200 – Metal-enclosed switchgear.
• IEC 62271-100 – High-voltage AC circuit breakers.
• IEEE C37.20.2 – Metal-clad switchgear.
• IEEE C37.04 – High-voltage circuit breakers.
• ANSI C37.20.1 – Switchgear specifications.

4.2 GIS Standards:

• IEC 62271-203 – GIS for rated voltages above 52 kV.
• IEC 62271-200 – Metal-enclosed switchgear.
• IEEE C37.122 – GIS for voltages up to 800 kV.
• ANSI C37.122.1 – GIS for voltages up to 72.5 kV.
• IEC 60376 – Handling of SF₆ gas.

5. Conclusion

The choice between AIS and GIS depends on several factors, including space availability, cost considerations, maintenance requirements, and environmental impact. AIS is preferred for open areas with lower initial costs and easier maintenance, while GIS is suited for urban, high-voltage, and space-restricted applications due to its compactness and reliability. Understanding these differences enables engineers to make informed decisions when designing power distribution and transmission systems.

Comparison of Air-Insulated Switchgear (AIS) and Gas-Insulated Switchgear (GIS)