We also refer to a Gas Insulated Switchgear (GIS) as a Gas Insulated Substation.
- Gas Insulated Substation (GIS) refers to the entire substation setup that uses SF6 gas for insulation.
- Gas Insulated Switchgear (GIS) typically refers to the assembly of the gas insulated components within the substation. A GIS includes circuit breakers, isolators, and busbars, potential transformers, etc in SF6 gas-insulated encapsulation.
- Gas Insulated System is a more general term that we use to describe any high voltage system using SF6 gas as insulation.
Why is GIS the Future of High-Voltage Power Systems?
There are several reasons for preferring a Gas-Insulated System (GIS) over an Air Insulated System (AIS).
- The dielectric strength of air is approximately 3 kV/mm (kilovolts per millimeter) or 30 kV/cm under standard atmospheric conditions (1 atm pressure and 25°C temperature). On the other hand, the dielectric strength of SF6 gas is approximately 9 kV/mm under the same pressure and temperature conditions.
- SF6 gas has significantly superior arc-quenching properties compared to air due to its high electronegativity. It is 3 to 4 times better than air at equal pressure.
- A Gas Insulated Substation typically occupies 10% to 20% of the space required by a same-rated Air-Insulated Substation. This means the SF6-insulated system requires 80% to 90% less space than AIS. Therefore, this makes a GIS ideal for urban areas and constrained environments.
- The enclosed or encapsulated design of gas insulated system protects electrical components from dust, humidity, salt, and pollution. These are the factors that degrade insulation in AIS. In contrast, the encapsulated design of GIS eliminates flashovers and insulation failures caused by contaminants. With a controlled gas-insulated environment, GIS minimizes the weather-related outages. As a result, a gas insulated substation ensures greater reliability and stable operation, even in harsh conditions.
- Because of its full isolation from the surrounding environment, GIS offers a much longer lifespan than AIS. The expected operational lifespan of a Gas-Insulated Substation is typically 40 to 50 years under normal operating conditions. Air-Insulated Switchgears (AIS), usually last around 25 to 30 years.
Constructional Details of Gas Insulated Substation (GIS)
A GIS consists of several key modules, each performing specific functions. We can also consider modules as transportable units. Engineers assemble multiple modules to form a complete GIS bay. Each module may include more than one compartment. The module is a metal enclosure that provides a shielded, gas-tight housing for GIS components. Manufacturers use aluminum or stainless steel to make modules for durability and corrosion resistance. Nowadays, they mostly prefer aluminum-cast modules for this purpose.
The above image provides a schematic representation of the key components in a SF6 Insulated System and their interconnections.
Gas Insulated Busbars
The system uses two busbars (Bus 1 and Bus 2) for redundancy, ensuring reliability in case of faults and maintenance. A gas-tight, shielded enclosure houses each of Bus 1 and Bus 2. Bus 1 includes a disconnector with an earthing switch (DS/ES), while Bus 2 includes a disconnector (DS). These disconnectors with earthing switch (DS/ES) and disconnectors (DS) help to isolate and ground the lower section of the system during maintenance.
Gas Insulated Circuit Breaker
The Gas-Insulated Circuit Breaker (GCB) is one of the most critical components in GIS. It operates using a motor-charged spring mechanism, ensuring a fast and reliable response.
Current Transformer
A Current Transformer (CT) sits below the breaker. The cores of the CT surround the GIS enclosure, so they are not gas-insulated, as shown in the figure above.
Further down the system, an additional disconnector with an earthing switch provides further isolation capabilities. Earthing switches before and after the circuit breaker ground both sides of the breaker. The High-Speed Earthing Switch (H-ES) rapidly grounds the cable portion of the line. The Lightning Arrester (LA) sits at the end of the circuit. The Cable Head (CH) serves as the interface for underground cable connections. Engineers always recommend placing the cable termination arrangement in a separate gas-tight compartment.
GIS Compartment
Each GIS compartment includes four gas management components. Each compartment has one Gas Density Monitor, one Gas Handling Valve, one Rupture Disc, and one Adsorbent. The Gas Density Monitor continuously checks SF6 pressure of the compartment to detect leaks. The Gas Handling Valve of the compartment allows filling and evacuation of SF6 gas. To prevent overpressure, the Rupture Disc releases excess gas from the compartment if necessary. The Adsorbent is used to absorb moisture from the gas inside the compartment.
One key feature of SF6 insulated system is the use of gas-tight spacers. Engineers place these spacers at critical junctions to maintain gas isolation between compartments while allowing electrical continuity. Aluminum or stainless steel housings enclose the system, providing durability and corrosion resistance. Today, manufacturers prefer aluminum-cast enclosures for their lightweight and robust properties.
The modular design of GIS ensures compactness, reliability, and minimal maintenance. Each module consists of several gas-insulated compartments, with dedicated functions like circuit breaking, isolation, measurement, and grounding.