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Home / Technical Articles / Major components you can spot while looking at HV/EHV GIS (Gas-insulated switchgear)

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.

Major components you can spot while looking at HV/EHV GIS (Gas-insulated switchgear)
Major components you can spot while looking at HV/EHV GIS (Gas-insulated switchgear)

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.

High voltage substation generally consists of many sections/bays. The main equipment in a section consists of circuit breakers, isolators or disconnect switches, earth switches, current transformers, surge arresters, etc.

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.

Single line diagram for a double bus section
Figure 1 – Single line diagram for a double bus section

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.

Cross-section of a double bus GIS section
Figure 2 – Cross-section of a double bus GIS section

Where typical double busbar feeder components are:

  1. Circuit-breaker interrupter unit
  2. Stored-energy spring mechanism
  3. Circuit-breaker control unit
  4. Busbar I
  5. Busbar disconnector I
  6. Busbar II
  7. Busbar disconnector II
  8. Work-in-progress earthing switch
  9. Work-in-progress earthing switch
  10. Outgoing-feeder disconnector
  11. Make-proof earthing switch (high-speed)
  12. Current transformer
  13. Voltage transformer
  14. Cable sealing end

GIS components

The following are the principle gas insulated modules for a substation:

  1. Busbar
  2. Disconnecting switch
  3. Circuit breaker
  4. Current transformer and
  5. Earth switch
  6. Accessories

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.

The high voltage conductor (copper/aluminium) is centrally placed in a tubular metal enclosure. The conductor is supported, at a uniform distance, by the disc or post insulator to maintain concentricity. Two sections of bus are joined by using plug-in connecting elements.

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.

An example of busbar module for switchgear type 8DN9
Figure 3 – An example of busbar module for switchgear type 8DN9 up to 245 kV (three-phase encapsulated passive busbar)

1.2 Insulating Materials and Insulators

The following insulating materials are commonly used in low tension (LT) and air insulated substation applications:

  1. Sheet moulding compound (SMC),
  2. Dow moulding compound (DMC),
  3. Glass fibre reinforced plastics,
  4. Compression and thermo-setting plastics, and
  5. 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).

The stagnation of charge locally modifies local potential and the electrical field. The electrical stresses in the system thus get modified unpredictably from the designed values. In an AC system, this trapped charge concentration also varies with time and adversely affects the electric field intensities. The use of materials promoting charge concentration is thus avoided in gas insulated systems.

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).

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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.

Disconnect switches can be motorized or driven manually. In GIS systems, motorized isolators are preferred. A pair of fixed contacts and a moving contact form the active parts of disconnect switch. The fixed contacts are separated by an isolating gas gap.

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.

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Edvard Csanyi - Author at EEP-Electrical Engineering Portal

Edvard Csanyi

Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.

I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.

Profile: Edvard Csanyi

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