A Gas Insulated Substation (GIS) is sometimes also referred to as Gas Insulated Switchgear or Gas Insulated System.
- Gas Insulated Substation (GIS) refers to the entire substation setup that uses SF₆ gas for insulation.
- Gas Insulated Switchgear (GIS) typically refers to the switchgear components within the substation, including circuit breakers, isolators, and busbars, all housed in a SF₆ gas insulated encapsulation.
- Gas Insulated System is a more general term used to describe any high voltage system using SF₆ gas insulation.
Why GIS is 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 GIS requires 80% to 90% less space than AIS. This makes a GIS ideal for urban areas and constrained environments.
- The enclosed or encapsulated design of GIS protects electrical components from dust, humidity, salt, and pollution. These are the factors, which degrade insulation in AIS. The encapsulated design of GIS eliminates flashovers and insulation failures caused by contaminants. With a controlled, gas-insulated environment, weather-related outages are minimal in GIS. As a result, GIS 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 (GIS) is typically 40 to 50 years under normal operating conditions. Air-Insulated Switchgears (AIS), usually last around 25 to 30 years.
Constructional Details of GIS
A GIS consists of several key modules, each performing specific functions. Modules may also be considered transportable units. More than one module is assembled together 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. Modules are made of aluminum or stainless steel for durability and corrosion resistance. Nowadays, aluminum-cast modules are mostly preferred for this purpose.
The above image provides a schematic representation of the key components in a GIS and their interconnections.
The system uses two busbars (Bus 1 and Bus 2) for redundancy, ensuring reliability in case of faults and maintenances. Each of Bus 1 and Bus 2 is enclosed in a gas-tight, shielded enclosure along with disconnectors with earthing switch (DS/ES) and disconnectors (DS) respectively. These , which disconnectors with earthing switch (DS/ES) and disconnectors (DS) help to isolate and ground the lower section of the system during maintenance.
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. Below the breaker, a Current Transformer (CT) is placed. It is important to note that the cores of the current transformer surround the GIS enclosure and, hence, the CT cores 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. The earthing switches before and after the circuit breaker are used to earth both sides of the breaker. The High-Speed Earthing Switch (H-ES) is used to rapidly ground the cable portion of the line. The Lightning Arrester (LA) is installed at the end of the circuit. The Cable Head (CH) serves as the interface for underground cable connections. It is always recommended to place the cable termination arrangement in a separate gas-tight 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 of the key features of GIS is the use of gas-tight spacers, which are placed at critical junctions to maintain isolation between compartments while allowing electrical continuity. The system is enclosed in aluminum or stainless steel housings, providing durability and resistance to corrosion. Aluminum-cast enclosures are now preferred due to 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.