What is a Gas-Insulated System (GIS)? Comprehensive Guide

Een Gas-Insulated System (GIS) is a compact, high-voltage electrical system where components like switchgear, circuit breakers, disconnectors, and transformers are housed within a sealed enclosure filled with an insulating gas, typically sulfur hexafluoride (SF6), instead of air at atmospheric pressure. This design significantly reduces the space required compared to air-insulated substations (AIS). The use of SF6 gas, with its superior dielectric strength and arc-quenching capabilities, allows for a much smaller footprint, making GIS ideal for urban areas, underground installations, and other space-constrained environments. The enclosed design also protects the components from environmental factors such as pollution, salt spray, and extreme weather, enhancing reliability and reducing maintenance.

1. Introduction

Gas-Insulated Systems (GIS) represent a significant advancement in high-voltage electrical technology. Traditional air-insulated substations (AIS) require large clearances between energized components due to the relatively low dielectric strength of air. This results in large substation footprints, making them unsuitable for many urban and space-constrained locations. GIS technology overcomes this limitation by enclosing all live parts within a grounded metallic enclosure filled with a pressurized insulating gas, most commonly sulfur hexafluoride (SF6).

The concept of using a gas as an insulating medium is not new, but the development of SF6 gas with its exceptional properties revolutionized the design of high-voltage equipment. SF6 has a dielectric strength approximately 2.5 to 3 times that of air, allowing for a dramatic reduction in the size of the equipment. It also has excellent arc-quenching capabilities, making it ideal for use in circuit breakers.

2. Why Use a Gas-Insulated System?

The adoption of GIS technology is driven by several key advantages:

• Space Saving: The most significant advantage of GIS is its compact size. Een GIS substation can occupy as little as 10% of the space required by a comparable AIS substation. This is particularly crucial in urban areas where land is expensive and scarce.
• High Reliability: The enclosed design of GIS protects the internal components from environmental factors such as pollution, salt spray, humidity, and animal intrusion. This significantly reduces the risk of faults and enhances the overall reliability of the system.
• Enhanced Safety: All live parts are enclosed within a grounded metallic enclosure, minimizing the risk of accidental contact and improving safety for personnel.
• Reduced Maintenance: The sealed environment and the use of SF6 gas, which is chemically inert and non-degrading, significantly reduce the need for maintenance compared to AIS.
• Indoor and Underground Installation: GIS is suitable for both indoor and outdoor installations. Its compact size and enclosed design make it ideal for underground substations, further minimizing its visual impact.
• Aesthetic Appearance: GIS substations have a much smaller and more aesthetically pleasing appearance compared to AIS substations.

3. Components of a GIS

A typical GIS consists of the following main components:

• Circuit Breakers: Used to interrupt fault currents and isolate sections of the system. GIS circuit breakers typically use SF6 gas for both insulation and arc quenching.
• Disconnectors (Isolators): Used to provide visible isolation of equipment for maintenance purposes. They are not designed to interrupt load current.
• Earthing Switches: Used to ground de-energized equipment, ensuring safety during maintenance.
• Current Transformers (CTs): Used to measure the current flowing in the high-voltage conductors for protection and metering purposes.
• Voltage Transformers (VTs): Used to measure the voltage of the high-voltage system for protection and metering purposes.
• Busbars: The main conductors that carry the current between different components of the GIS.
• Surge Arresters: Used to protect the GIS from overvoltages caused by lightning strikes or switching operations.
• Gas-Tight Enclosure: The grounded metallic enclosure that houses all the components and contains the SF6 gas. The enclosure is typically made of aluminum or steel.
• Gas Monitoring System: Monitors the pressure, density, and purity of the SF6 gas.

4. The Role of SF6 Gas

Sulfur hexafluoride (SF6) is a colorless, odorless, non-toxic, and non-flammable gas that has exceptional dielectric and arc-quenching properties. These properties make it the ideal insulating medium for GIS.

Key Properties of SF6 Gas:

• High Dielectric Strength: SF6 has a dielectric strength approximately 2.5 to 3 times that of air at the same pressure. This allows for much smaller clearances between energized components.
• Excellent Arc-Quenching Capability: SF6 is very effective at extinguishing arcs that occur when a circuit breaker interrupts current. It absorbs the energy of the arc and rapidly cools it down, preventing re-ignition.
• Chemical Stability: SF6 is chemically inert and does not decompose under normal operating conditions. This ensures long-term stability and performance.
• Thermal Stability: SF6 has good thermal stability, allowing it to operate over a wide range of temperatures.
• Non-Toxicity: SF6 itself is non-toxic. Echter, in the presence of an arc, it can decompose into byproducts that may be toxic. Proper handling and ventilation procedures are necessary.

While SF6 is a very useful gas for GIS, it is also a potent greenhouse gas, with a global warming potential (GWP) approximately 23,500 times that of carbon dioxide (CO2). Because of this, there is increasing focus on minimizing SF6 emissions and finding alternative gases.

5. Alternatives to SF6 Gas

Due to the high GWP of SF6, the industry is actively researching and developing alternative insulating gases for GIS. These alternatives aim to reduce the environmental impact while maintaining the performance and reliability of GIS. Some potential alternatives include:

• **Fluoroketones:** These have a significantly lower GWP than SF6 and good dielectric properties.
• **Fluoronitriles:** These also offer a lower GWP and good dielectric strength.
• **Mixtures of Gases:** Combining gases like CO2, N2, and O2 with small amounts of fluorinated compounds can reduce the overall GWP while maintaining acceptable performance.
• **Clean Air (Dry Air):** While air has a lower dielectric strength than SF6, advances in design and higher operating pressures are making "clean air" GIS a viable option for some voltage levels.

The transition to alternative gases is ongoing, with ongoing research and development efforts focused on optimizing performance, kosten, and environmental impact.

6. Monitoring and Maintenance of GIS

Although GIS requires less maintenance than AIS, regular monitoring and maintenance are still essential to ensure its long-term reliability and safety. Key aspects of GIS monitoring include:

• SF6 Gas Monitoring: Regularly monitoring the pressure, density, and purity of the SF6 gas is crucial. Leaks can reduce the insulating properties of the gas and increase the risk of failure. Gas density monitors and leak detection systems are used for this purpose.
• Partial Discharge (PD) Monitoring: PD is a localized electrical discharge that can occur within insulation defects. PD monitoring can detect early signs of insulation degradation, allowing for timely intervention before a major fault occurs. Techniques used for PD monitoring in GIS include UHF (Ultra-High Frequency) sensors, acoustic emission (AE) sensors, and HFCT (High-Frequency Current Transformer) sensors.
• Temperature Monitoring: Overheating can indicate various problems, such as poor connections, excessive current, or insulation degradation. Temperature monitoring can be performed using traditional sensors (thermocouples, RTDs), infrared (IR) thermography, of fiber optic sensors. Fluorescence-based fiber optic sensors are particularly well-suited for GIS due to their immunity to EMI, small size, and high accuracy.
• Circuit Breaker Monitoring: Monitoring the operation of circuit breakers, including operating times, contact wear, and mechanism performance, is essential for ensuring reliable fault interruption.
• Visual Inspections: Periodic visual inspections can help identify external signs of problems, such as gas leaks, corrosie, or damage to the enclosure.

7. Applications of GIS

GIS technology is widely used in various applications, including:

• High-Voltage Substations: GIS is the preferred Technologie for high-voltage substations in urban areas where space is limited.
• Underground Substations: The compact size and enclosed design of GIS make it ideal for underground substations.
• Offshore Platforms: GIS is used in offshore wind farms and oil and gas platforms due to its compact size, reliability, and resistance to harsh environmental conditions.
• Industrial Plants: GIS is used in industrial plants with high power requirements, such as steel mills, aluminum smelters, and chemical plants.
• Hydroelectric Power Plants: GIS is often used in hydroelectric power plants, where space may be limited and environmental considerations are important.
• Mobile Substations: GIS can be used in mobile substations for temporary power supply during emergencies or maintenance.

8. Advantages and Disadvantages of GIS

Advantages:

• Compact size (significantly smaller footprint than AIS).
• High reliability.
• Enhanced safety.
• Reduced maintenance.
• Protection from environmental factors.
• Suitable for indoor, outdoor, and underground installations.
• Aesthetically pleasing.

Disadvantages:

• Higher initial cost compared to AIS.
• Complexity of design and installation.
• Use of SF6 gas, a potent greenhouse gas (although alternatives are being developed).
• Specialized equipment and expertise required for maintenance and repair.
• Longer outage times for repairs compared to AIS (in some cases).

9. Frequently Asked Questions (FAQ)

Wat is een Gas-Insulated System (GIS)?

A Gas-Insulated System (GIS) is a compact, high-voltage electrical system where components like switchgear, circuit breakers, disconnectors, and transformers are housed within a sealed enclosure filled with an insulating gas, typically sulfur hexafluoride (SF6), instead of air at atmospheric pressure. This design significantly reduces the space required compared to air-insulated substations (AIS).

What are the advantages of using a GIS?

GIS offers several advantages, including: significantly reduced space requirements, higher reliability, enhanced safety, reduced maintenance, protection from environmental factors (pollution, salt spray, enz.), and suitability for indoor and underground installations.

What is the typical insulating gas used in GIS?

Sulfur hexafluoride (SF6) is the most commonly used insulating gas in GIS due to its excellent dielectric strength, arc-quenching properties, and chemical stability. Echter, due to its high global warming potential, there is ongoing research into alternative gases.

What are the main components of a GIS?

The main components of a GIS include: circuit breakers, disconnectors (isolators), earthing switches, current transformers (CTs), voltage transformers (VTs), busbars, surge arresters, and the gas-tight enclosure.

How is temperature monitored in a Gas-Insulated System?

Temperature monitoring in a GIS is crucial for detecting overheating and preventing failures. Several methods are used, including: traditional sensors (thermocouples, RTDs), infrared (IR) thermography, en fiber optic sensors. Fluorescence-based fiber optic sensors are considered the best option due to their high accuracy, immunity to electromagnetic interference (EMI), small size, and ability to be directly integrated into GIS components like joints and terminations without compromising the insulation integrity.

What are some applications of GIS?

GIS is commonly used in: high-voltage substations in urban areas where space is limited, underground substations, offshore platforms, industrial plants, and hydroelectric power plants.

10. Conclusion

Gas-Insulated Systems (GIS) have become a cornerstone of modern high-voltage electrical infrastructure. Their compact size, high reliability, and enhanced safety make them an ideal solution for a wide range of applications, particularly in space-constrained environments. While the use of SF6 gas presents environmental concerns, ongoing research and development efforts are focused on finding suitable alternatives. With continuous advancements in monitoring technologies and a growing emphasis on sustainability, GIS is expected to remain a vital technology for the power industry for years to come.