Introduction to GIS sections / bays
Gas-insulated switchgear (GIS) is a piece of high voltage equipment that is being constantly developed day by day. The basics of GIS technology is more or less the same, but everything else under the hood is improved a lot comparing to just a few years ago. This article explains major GIS components and their characteristics.

GIS are available internationally, covering the complete voltage range from 11 kV to 800 kV. The thermal current-carrying capacities and the fault-withstanding capabilities are tailored to meet all the substation requirements. More than 200,000 GIS bays have been in service all over the world since the introduction of such substation systems in the transmission and distribution field.
Figure 1 shows a single line diagram of a section at a substation identifying different components. Single busbar, double busbar and 3/2 circuit breaker are popular configurations at substations.


In GIS, the modular components are assembled together to form a desired arrangement for a section or a bay. Figure 2 shows a cross-section of a double bus GIS section. Here, the constituent components are assembled side by side. The porcelains and connections (ACSR conductors), as required in a yard substation, are totally eliminated in this new configuration.
The high voltage conductors (bus bars) are supported on simple disc insulators.


Where typical double busbar feeder components are:
- Circuit-breaker interrupter unit
- Stored-energy spring mechanism
- Circuit-breaker control unit
- Busbar I
- Busbar disconnector I
- Busbar II
- Busbar disconnector II
- Work-in-progress earthing switch
- Work-in-progress earthing switch
- Outgoing-feeder disconnector
- Make-proof earthing switch (high-speed)
- Current transformer
- Voltage transformer
- Cable sealing end
GIS components
The following are the principle gas insulated modules for a substation:
The auxiliary gas insulated module or accessories, excluding control panel, that are required to complete a substation are terminations, instrument voltage transformer and surge and lightning arrester.
1. Busbar
The busbar is one of the most elementary components of the GIS system. Co-axial busbars are common in isolated-phase GIS as this configuration results in an optimal stress distribution. Busbars of different lengths are used in GIS to cater to the requirement of circuit or the bay formation.
Various sizes of the bus enclosures exist nowadays.
1.1 Connectors
The high voltage and high current electrical connections from one module to another in a gas insulated substation system are carried out with the help of the spring loaded plug-in contacts. Plug-in contact systems impart the maximum flexibility during assembly and dismantling. These contacts offer plug-in features and are suitable for tubular conductors.
The connections made are reliable without the need for any additional hardware to secure their location.


1.2 Insulating Materials and Insulators
The following insulating materials are commonly used in low tension (LT) and air insulated substation applications:
- Sheet moulding compound (SMC),
- Dow moulding compound (DMC),
- Glass fibre reinforced plastics,
- Compression and thermo-setting plastics, and
- Refractory-based in-sulating materials (like cordrite and alumina)
Of these insulations, glass/silica-based systems are generally found unsuitable for SF6 applications due to their weak resistance to hydrofluoric acid (a by-product of moisture and decomposed SF6). Large shrinkage and instability at higher working temperatures prohibit the use of plastics in GIS.
Stable polymers like PTFE (poly tetra fluoroethane) are selectively used in GIS and associated accessories.
Insulating materials like PTFE (teflon) with very high volume resistivity retain electrical charges for long durations. This material property is sometimes undesirable and causes a deterioration in the performance of GIS (critically for direct current applications).
Alumina-filled epoxy matrix is a common insulating material for GIS-related applications. The filler alumina offers good resistance to decomposed SF6 products like hydrofluoric acid (HF) as compared to silica or felspar (common fillers used with epoxy).
2. Disconnectors
Disconnectors (or disconnect switches) are placed in series with the circuit breaker to provide additional protection and physical isolation. In a circuit, two disconnectors are generally used, one on the line side and the other on the feeder side. Disconnect switches are designed for the interruption of small currents, induced or capacitively coupled.
During the closing operation, this gap is bridged by the moving contact. The moving contact is attached to a suitable drive, which imparts the desired linear displacement to the moving contact at a pre-determined design speed.
A firm contact is established between the two contacts with the help of spring-loaded fingers or the multi-lam contacts. The isolation gap is designed for the voltage class of the isolator and the safe dielectric strength of the gas.
Figure 4 shows a cross-section of an isolated-phase GIS diconnector.