GIS- Gas Insulated substation

GIS- Gas Insulated substation

Because of their high reliability, easy maintenance, and small ground space requirement, the gas insulated substations (GIS) have been used in power systems over the last three decades.

The GIS is totally enclosed and therefore is free from any atmospheric contamination. Conductive particles, particle discharges, and contamination are factors which affect the life of GIS.

To reduce the breakdown level of Gas insulated systems, conductive particles inside the enclosure are known. From conductive particles, partial discharges can develop, contamination, and defects during the manufacturing process, etc.

Because of its compactness, the GIS requires less number of lightning arresters compared to a conventional substation.

Air can withstand very high voltages for very short time. However, the withstand voltage falls off considerably, as the duration of voltage increases.

SF6 exhibits a flat characteristic. For GIS, the ratio of basic lightning impulse level is close to unity, whereas this ratio varies between 0.6 and 0.86 for the conventional substations.

GIS are in service up to the highest voltage level of 800 kV. GIS is meeting almost all the requirements in urban, industrial and rural areas.

The SF6 gas insulated substations were almost exclusively used in the initial stages. This type of substations is more popular where space limitations, site restrictions or exponential ambient conditions. In this condition, it is difficult to use conventional air-insulated substations.

Characteristics of SF6:

1. SF6 is colorlessly, odorless and a chemical neutral gas Characteristics of SF6 chemical neutral gas.

2. SF6 is 5 times heavier than air and it is not toxic and has no dangerous components inside.

3. SF6 is no hazardous material.

4. SF6 has no eco-toxic potential.

5. SF6 has no impact for the ozonosphere.

6. SF6 is a potent greenhouse gas.

7. SF6 has excellent electrical characteristics.

Features of GIS:

1. For standardized component models, widespread application of aluminum enclosure materials

2. The enclosures have low eddy-current losses, good conductivity, and a high resistance to corrosion and the enclosures is light in weight.

3. Easy handling, reduced stresses on foundation and support structure are additional features for the construction of GIS.

4. Good co-ordination between the manufacturer and the user, standard arrangements can be easily modified and extended.

5. For the separation of gas compartments, a gas-tight barrier insulator in switchgear serve and during maintenance prevents neighboring switchgear parts from being affected.

Components of GIS:

1. Circuit breaker

2. Disconnector switch

3. Earthing switch

4. Current transformer

5. Voltage transformer

6. Bus bar & connectors

7. Power transformer

8. Surge Arrester

9. Cable termination

10. SF6 / air or SF6 / oil bushing

Advantages of GIS:

During the last fifteen years, the application of GIS has been very rapid. The growth in GIS application is because of the following special advantages of GIS;

1. They offer to save on land area and construction costs.

2. Because of lesser maintenance, downtime and repair costs, more optimal life cycle costs.

3. Due to the earthed metal housing of all high voltage parts, greatly improved safety and reliability. SF6 gas as insulation much higher intrinsic strength.

4. Based on large factory assembled and tested shipping units, short on-site erection times.

5. Due to non-exposure of the use of high voltage parts to atmosphere influences high service reliability.

6. Reduction in radio interference with the use of earthed metal enclosures

7. Use as mobile substations for transportation to load centers on standard tracks. These substations can be located closer to load centers thereby reducing transmission losses and expenditure in the distribution network.

8. Insensitivity to external influences because of grounded metal enclosures.

9. It is not necessary that high voltage or extra high voltage switchgear has to be installed outdoors.

Disadvantages of GIS:

A lot of problems encountered in practice need fuller understanding, although GIS has been in operation for several years. Some of the problems are:

1. Switching operation generate Very Fast Transients Over Voltages.

2. Very Fast Transients Over Voltages may cause secondary breakdown inside a GIS and Transient Enclosure Voltages outside the GIS.

3. Field non-uniformity reduce withstanding levels of a GIS.

4. Prolonged arcing may produce corrosive/toxic by-products.

5. Support spacers can be weak points when arc by-products and metallic particles are present.