When designing substations and high-voltage transmission systems, engineers often encounter four terms: AIS, GIS, HGIS, and GIL.
They are all related to high-voltage equipment and insulation technologies, but their functions are fundamentally different.
AIS, GIS, and HGIS are mainly used for high-voltage switchgear arrangements inside substations, while GIL is designed as a high-voltage power transmission channel.
The first three systems perform functions such as:
- Switching
- Isolation
- Earthing
- Measurement
- Protection connection
GIL, however, has a different purpose: transmitting high-voltage electrical power between two locations through an enclosed transmission path.
Therefore, selecting between AIS, GIS, HGIS, and GIL should not only depend on the equipment name. Engineers need to consider:
- Available land area
- Environmental conditions
- Transmission route limitations
- Construction requirements
- Investment budget
- Operation and maintenance capability
- Long-term reliability requirements
1. AIS: Air Insulated Switchgear
What is AIS?
AIS stands for Air Insulated Switchgear.
It is the traditional open-air high-voltage switchgear solution. The main components, including:
- Busbars
- Circuit breakers
- Disconnectors
- Current transformers
- Voltage transformers
- Surge arresters
are installed separately in an outdoor environment.
The insulation between energized components mainly relies on natural air insulation, which requires sufficient safety clearance.
Many conventional outdoor substations with visible steel structures, insulators, and disconnect switches are typical AIS installations.
Advantages of AIS
The biggest advantage of AIS is its simple structure and easy maintenance.
Because all components are visible:
- Operators can easily inspect equipment conditions;
- Fault locations are easier to identify;
- Maintenance work is more straightforward;
- Individual components can be replaced without affecting the whole system.
Inspection methods such as:
- Infrared temperature measurement
- Electrical testing
- Visual inspection
can be applied directly.
AIS also has a relatively lower initial investment cost. For substations with sufficient space and good environmental conditions, AIS remains a reliable and economical solution.
Limitations of AIS
The main limitation of AIS is the requirement for large installation space.
As voltage levels increase, insulation distances become larger, resulting in bigger substations.
Because equipment is exposed to outdoor conditions, AIS can be affected by:
- Salt fog
- Dust pollution
- Rain and snow
- Bird contamination
- Strong wind
- Pollution flashover risks
For coastal areas, chemical plants, and urban locations, environmental impact and maintenance requirements must be carefully evaluated.
2. GIS: Gas Insulated Switchgear
What is GIS?
GIS stands for Gas Insulated Switchgear.
Unlike AIS, GIS places high-voltage components inside a sealed metal enclosure.
Typical GIS components include:
- Circuit breakers
- Disconnect switches
- Earthing switches
- Busbars
- Current transformers
- Voltage transformers
The enclosure is filled with insulating gas to provide electrical insulation.
Traditional GIS systems mainly use SF₆ gas, while environmentally friendly gas mixtures are increasingly being developed.
The basic principle remains the same:
High-voltage conductors are completely enclosed inside a grounded metal housing.
Advantages of GIS
The biggest advantage of GIS is its compact design.
Compared with AIS:
- Installation area is significantly reduced;
- External energized parts are minimized;
- Environmental influence is greatly reduced.
GIS is especially suitable for:
- Urban substations
- Underground substations
- High-load-density areas
- Locations with limited land availability
Maintenance Requirements of GIS
Although GIS is a closed system, it does not mean maintenance-free.
Important monitoring items include:
- Gas pressure
- Moisture level
- Sealing condition
- Partial discharge
- Mechanical operating performance
GIS requires strict control during:
- Manufacturing
- Transportation
- Installation
- Testing
Contamination inside the gas chamber, poor sealing, or improper field assembly may create long-term reliability problems.
Compared with AIS, GIS fault diagnosis and repair are usually more complicated because internal components are not directly accessible.
3. HGIS: Hybrid Gas Insulated Switchgear
What is HGIS?
HGIS stands for Hybrid Gas Insulated Switchgear.
It is a solution between AIS and GIS.
In an HGIS system, key switching components such as:
- Circuit breakers
- Disconnectors
- Earthing switches
- Current transformers
are integrated into compact gas-insulated modules.
These modules are then connected with external equipment through:
- Bushings
- Conductors
- Open busbars
HGIS combines the advantages of GIS compactness with some of the accessibility of AIS.
Typical Applications of HGIS
HGIS is commonly used for:
- Substation expansion projects
- Existing AIS station upgrades
- Sites with limited space
- Projects where full GIS conversion is unnecessary
For example:
An existing AIS substation needs additional switchgear bays because of increasing power demand.
However:
- Available space is limited;
- Complete GIS replacement is expensive;
- Large-scale shutdown is undesirable.
In this case, HGIS can provide a practical solution.
Advantages of HGIS
The main advantage of HGIS is flexibility and balance.
It can:
- Reduce installation space;
- Lower modification difficulty;
- Control project cost;
- Maintain familiar maintenance methods.
4. GIL: Gas Insulated Line
What is GIL?
GIL stands for Gas Insulated Line.
Although GIL uses a similar concept as GIS, including:
- Metal enclosure
- High-voltage conductor
- Insulating gas
its purpose is completely different.
GIS is a switching system.
GIL is a high-voltage transmission system.
GIS performs:
- Switching
- Isolation
- Earthing
- Measurement
GIL performs:
- High-capacity power transmission between two locations
Structure of GIL
A typical GIL system includes:
- Aluminum or metal enclosure
- Central conductor
- Insulating supports
- Insulating gas
- Expansion joints
- Grounding system
The high-voltage conductor operates inside a grounded enclosure, while the insulating gas provides electrical insulation.
Applications of GIL
GIL is mainly used in special transmission conditions, such as:
- Underground utility tunnels
- Mountain tunnels
- River-crossing projects
- Offshore transmission routes
- Nuclear power plants
- Large energy bases
- Urban transmission corridors
When overhead transmission lines cannot be constructed and cables cannot meet requirements for:
- Capacity
- Heat dissipation
- Loss performance
- Reliability
GIL becomes a possible solution.
Advantages and Challenges of GIL
Advantages:
- High current carrying capacity
- Low transmission losses
- Low electromagnetic interference
- High reliability
- Excellent environmental protection
However, GIL also requires:
- High installation accuracy
- Strict sealing control
- Professional testing
- Complex transportation and assembly planning
GIL is not simply a replacement for conventional cables.
It is a complete high-voltage enclosed transmission system.
AIS vs GIS vs HGIS vs GIL Comparison Table
| Item | AIS | GIS | HGIS | GIL |
|---|---|---|---|---|
| Full Name | Air Insulated Switchgear | Gas Insulated Switchgear | Hybrid Gas Insulated Switchgear | Gas Insulated Line |
| Main Function | Substation switching | Substation switching | Substation switching | Power transmission |
| Insulation Method | Air | Gas | Gas + Air | Gas |
| Installation Space | Large | Small | Medium | Depends on route |
| Initial Cost | Low | High | Medium | Very High |
| Maintenance | Easy | Professional | Medium | Highly specialized |
| Typical Application | Outdoor substations | Urban/underground substations | Expansion projects | Special transmission corridors |
How to Select Between AIS, GIS, HGIS, and GIL?
Choose AIS when:
- Land is available;
- Environmental conditions are acceptable;
- Simple maintenance is preferred;
- Budget control is important.
Choose GIS when:
- Space is limited;
- Environmental pollution is severe;
- High reliability is required;
- Underground installation is needed.
Choose HGIS when:
- Existing substations require expansion;
- Space is restricted;
- Full GIS replacement is unnecessary;
- Construction interruption must be minimized.
Choose GIL when:
- Transmission corridors are restricted;
- Large power capacity is required;
- Underground or special routes are involved.
Conclusion
AIS, GIS, HGIS, and GIL are not simply different levels of technology upgrades.
Each system is designed for different engineering conditions:
- AIS — Simple, mature, economical, and easy to maintain.
- GIS — Compact, enclosed, reliable, and suitable for complex environments.
- HGIS — Flexible and practical for expansion and retrofit projects.
- GIL — A high-capacity enclosed transmission solution for special routes.
The best engineering solution is not always the newest technology.
A successful high-voltage system must match:
- Site conditions
- Operating requirements
- Investment plans
- Maintenance capability
A well-designed system should remain reliable not only during commissioning, but throughout decades of operation.