Air-Insulated Switchgear (AIS): The Definitive Guide for Modern Substations
- Derrel Gerary
- Jul 28
- 6 min read
In the vast and complex world of electrical power systems, switchgear serves as the critical nerve center. It is the essential equipment responsible for protecting, controlling, and isolating electrical circuits, ensuring the safe and reliable flow of energy from generation plants to end-users. Among the various technologies available, one stands as the most widely deployed, time-tested, and foundational solution for medium-voltage (MV) and high-voltage (HV) substations worldwide: Air-Insulated Switchgear (AIS).
While newer, more compact technologies like Gas-Insulated Switchgear (GIS) have found their niche, AIS remains the backbone of the global power grid. Its enduring prevalence is a testament to its inherent reliability, operational flexibility, and compelling economic advantages.
This comprehensive guide from leistungenergie.com.au will provide a deep, technical dive into the world of AIS. We will explore its fundamental principles, conduct a detailed anatomical breakdown of its core components, analyze key design considerations, and provide a clear comparison with other technologies to help engineers, planners, and asset managers make informed decisions.
What is Air-Insulated Switchgear (AIS)?
Air-Insulated Switchgear is a type of electrical switchgear where atmospheric air is used as the primary dielectric medium to insulate the energized conductors from each other and from the ground.
The core principle of AIS is elegantly simple: physical distance. By maintaining sufficient clearance between high-voltage components, the natural dielectric strength of the surrounding air is utilized to prevent electrical flashovers. This design philosophy directly influences the character of AIS substations—they are typically installed outdoors and require a significant physical footprint to accommodate these necessary safety clearances.
This stands in stark contrast to Gas-Insulated Switchgear (GIS), where conductors are placed much closer together within a sealed metal enclosure filled with a gas that has a much higher dielectric strength than air (traditionally Sulphur Hexafluoride, SF₆).
This difference in insulation philosophy is the primary reason for the vast disparity in size, cost, and application between the two technologies.
The Anatomy of an AIS Substation
An AIS substation is an assembly of several distinct, standalone pieces of equipment, interconnected by busbars or conductors. Understanding the function of each component is essential to grasping the system as a whole.
Circuit Breakers: This is the most critical component, designed to interrupt both normal load currents and, more importantly, high-magnitude short-circuit fault currents. Modern HV AIS circuit breakers are sophisticated devices, often using SF₆ gas or vacuum interrupters. They can be categorized as "live tank" (where the interrupter is at high potential) or "dead tank" (where the interrupter is housed within a grounded enclosure).
Disconnectors (Isolators): These are mechanical switches used to provide a clear, visible point of isolation in a circuit. They are crucial for ensuring the safety of maintenance personnel. Disconnectors are designed to be operated only under no-load conditions (i.e., after the circuit breaker has first interrupted the current). Common types include center-break, double-break, and pantograph disconnectors.
Earthing Switches: A key safety device, the earthing switch is used to connect a de-energized and isolated circuit to the substation's ground grid. This ensures that no residual charge or accidental energization can harm personnel working on the equipment. Some earthing switches are designed with fault-making capabilities.
Current Transformers (CTs): These are the primary sensors for current. A CT is an instrument transformer that produces a reduced, secondary current that is directly proportional to the high primary current. This low-level signal is safe to use for protection relays and metering instruments.
Voltage Transformers (VTs): Also known as Potential Transformers (PTs), these devices are the sensors for voltage. They step down the high system voltage to a standardized low voltage (e.g., 110V) for use by protection relays, meters, and synchronizing equipment. At higher voltages, Capacitor Voltage Transformers (CVTs) are often used.
Surge Arresters: These are the substation's primary protection against overvoltage events caused by lightning strikes or switching operations. A surge arrester acts as a high-speed pressure relief valve for electricity, diverting the excess surge energy safely to the ground before it can damage more expensive equipment like power transformers.
Busbars: These are the main electrical conductors that connect the various bays and equipment within a substation, acting as the primary electrical highways. In AIS, busbars are typically made of rigid aluminum alloy tubes or flexible stranded conductors (like ACSR) strung between support structures.
Support Structures and Insulators: A network of galvanized steel structures provides the physical support for all the equipment. High-voltage insulators (typically made of porcelain or modern composite polymer materials) are used to mechanically support the energized conductors while electrically isolating them from the grounded structures.
AIS vs. GIS
The decision between AIS and GIS is one of the first and most significant choices in substation design. It involves a trade-off between space, cost, flexibility, and maintenance philosophy.
Parameter | Air-Insulated Switchgear (AIS) | Gas-Insulated Switchgear (GIS) |
Footprint / Land Area | Very Large | Very Compact (can be up to 90% smaller than an equivalent AIS) |
Primary Insulation | Atmospheric Air | SF₆ Gas or eco-friendly gas alternatives |
Initial Capital Cost | Lower | Significantly Higher (2-3 times or more) |
Installation & Commissioning | Long and complex, requiring on-site assembly of numerous individual components. | Shorter and faster, as it arrives in factory-assembled, modular bays. |
Maintenance Requirements | Higher Frequency. Requires periodic cleaning of insulators, especially in polluted areas. | Lower Frequency. Components are sealed for life, requiring minimal maintenance. |
Flexibility & Scalability | Very High. It is simple to add a new bay or modify the busbar configuration in the future. | Low. The rigid, gas-filled bus duct system makes extensions and modifications complex and expensive. |
Fault Location & Repair | Easier. Faults are often visually apparent, and individual components can be easily accessed and replaced. | More Complex. Requires specialized equipment for gas handling and locating internal faults. Downtime is typically longer. |
Environmental Impact | Minimal, as it uses air as an insulator. | Higher, due to the high Global Warming Potential (GWP) of SF₆ gas if it leaks. |
Ekspor ke Spreadsheet
When to Choose AIS: AIS is the optimal choice when land space is readily available and the initial capital budget is a primary constraint. Its unparalleled flexibility makes it ideal for projects where future expansion is anticipated.
Key Design and Layout Considerations for AIS Substations
The design of an AIS substation is a complex process governed by international standards (like IEC and IEEE) and national regulations.
Busbar Configuration: The layout of the busbars determines the substation's operational flexibility and reliability. Common schemes include:
Single Bus: Simple and low-cost, but less reliable as a bus fault will de-energize the entire substation.
Double Bus, Single Breaker: More flexible, allowing load transfer between buses for maintenance.
Breaker-and-a-Half: Highly reliable and flexible, commonly used in critical transmission substations, but at a higher cost.
Electrical Clearances: This is the paramount design factor that dictates the physical size of the substation. Minimum required distances (phase-to-phase and phase-to-ground) must be maintained at all times to prevent flashovers. These distances are a function of the system voltage (BIL - Basic Insulation Level).
Creepage Distance: For insulators, the creepage distance is the total distance along the surface of the insulator from the energized end to the grounded end. In areas with high atmospheric pollution (e.g., coastal regions with salt spray or industrial zones), a longer creepage distance is required to prevent contaminant build-up from creating a conductive path and causing a flashover.
Substation Grounding Grid: A comprehensive earthing mat, typically made of buried copper conductors, is installed across the entire substation. This is a critical safety system designed to control step and touch potentials during a ground fault, ensuring the safety of personnel within the substation.
Civil and Structural Design: Foundations for all equipment and support structures must be designed to handle static loads (equipment weight) and dynamic loads (wind, ice, seismic activity, and short-circuit forces).
Advantages and Disadvantages of Air-Insulated Switchgear
Let's consolidate the pros and cons into a clear summary.
Advantages:
Lower Capital Cost: The initial investment for equipment and construction is significantly lower than for GIS.
Flexibility and Scalability: The open and modular nature of AIS makes it exceptionally easy to extend, modify, or upgrade the substation in the future.
Simplicity and Ease of Maintenance: Faults are often easy to locate visually, and individual components from various manufacturers can be easily replaced without requiring specialized gas-handling equipment.
Proven Technology: AIS is a time-tested technology with a long history of reliable operation and a well-understood performance lifecycle.
Disadvantages:
Large Footprint: AIS requires a substantial amount of land, making it impractical or prohibitively expensive for dense urban or industrial locations.
Susceptibility to Environmental Factors: Being exposed to the elements, AIS is vulnerable to performance degradation from industrial pollution, salt spray, lightning, and wildlife, which can lead to faults.
Higher Maintenance Frequency: While the tasks may be simpler, AIS requires more frequent maintenance, particularly the regular cleaning of insulators to prevent flashovers.
Longer Installation Time: The on-site assembly of numerous individual components and the extensive civil works required result in longer project timelines.
Conclusion
While newer technologies like GIS and Hybrid Switchgear offer compelling advantages in terms of compactness, Air-Insulated Switchgear remains the undisputed foundation of reliable and cost-effective power grids across the globe. Its inherent simplicity, operational flexibility, and economic value ensure that it will continue to be the right choice for a vast range of greenfield substation projects for the foreseeable future.
The decision to specify AIS is a strategic one, based on a clear understanding of its strengths in applications where space is available and future scalability is a priority. At Leistung Energie, we possess deep expertise in the engineering, design, and supply of high-quality AIS components and complete, turnkey substation solutions. We are committed to helping our clients build the robust, reliable, and scalable electrical infrastructure that their projects demand.
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