Loss less (less losses) transmission through Gas Insulated Transmission system.

Introduction:

     Gas Insulated Transmission line (GIL) is best choice where High rating power transmission is needed. It is nothing but 2 Conductors, Gas mixture and supporters (Resistive support). Gas Insulated Lines are environmental friendly and technically easy to implement. Its operational features are also excellent.

Construction:

     GIL consists of 2 concentric Aluminum tubes. The tubes are made of corrosion resistive aluminum alloy. Outer conductor is to cover inner conductor and gas filled in it.  Tubes are welded by computerized process and tested by ultrasonic device to inspect and remove cracks and to ensure that system is gas-tight. Outer tube is taken as ground potential. A particle trap is inserted in the tubes to increase their life.

Cast resin insulators are used to support inner conductor. These insulators also helps to put inner Aluminum tube at center.

     Mixture of gas is filled between inner and outer tube. This increases insulating capacity of Transmission line. Mixture of gas consists of 80% Nitrogen and 20% SF6 (sulphur hexafluoride) which provides electrical insulation.

     Length of GIL lies between 60km to 100km. Because of the large cross section of the conductor, electrical losses in GIL are less when compared with other transmission systems. So, transmission and operating cost reduces.

Advantages:

Cooling system:

     Due to large outer tube surface GIL don’t require any sophisticated cooling system.

Reactive compensation:

     Gil have low capacitance. Dielectric losses are also less, so, reactive compensation is not needed.

High safety:

     Due to presence of SF6, GIL are capable of extinguishing any arc produced in it at the time of failure. GIL are fire resistant. So safety to person and environment is ensured.

Highest reliability:

     GIL is being used all over world from last 35+ years without any case of failure.

Maintenance is less:

     Electrical and thermal aging in case of GIL is less hence very less maintenance is required. (Once in 50 years)

Low emission of magnetic fields:

     The conductor current in the inner aluminum tube induces a reverse current of the same size. Consequently the electromagnetic field outside the GIL is negligible. No special shielding is required even in areas which are critical with respect to EMC (e.g. airports or computer centres or in populated areas).

Landscape view:

     GIL can be used underground without any special shielding. It can be replaced by high towers and landscape view of surrounding can be maintained. It also reduces unwanted usage of land.

Arrangement:

     GIL can be used horizontally, vertically or in any inclined arrangement.

Applications:

     GIL are commonly used for transmission systems with 345-550 kV operative voltages and 2500-5000 Amp operation current. Following are general sites of applications;

1. Feeder connections in power plants

2. Transmission systems in tunnels beneath cities

3. Transmission of EHV (Extra High Voltage) is required within restricted places.

Some technical information:

Typical Technical voltage : 220-500 kV

Maximum operation voltage: 245-550 kV

Nominal frequency: 50 / 60 Hz (as per system design)

Housing and conductor material: Aluminium alloy

Insulation gas: Gas-mixture (80% N2 and 20% SF6)

Nominal pressure (absolute): 0.7 MPa

Environmental temperatures: -30°C up to +50°C

Weight per phase: 50-70 kg/m (Approx.)

Capacitance: 55 nF/km

Inductance: 0.205 mH/km

Surge impedance: approx. 60 Ohm

Electromagnetic field: negligible µT

Source: Siemens

(This post is demanded by Pratik Kerimane.)