The gas-insulated substation (GIS) have been used in power systems over the last three decades. It has many advantages over conventional substations like high reliability, easy maintenance, and small ground space requirement.
The gas-insulated substation totally enclosed and therefore is free from any atmospheric contamination. The life of this substation depends on the conductive particles, particle discharges, and contamination.
Conductive particles are used inside the enclosure to reduce the breakdown level of Gas-insulated systems.
Because of its compactness, the GIS requires less number of lightning arresters compared to a conventional substation.
Air can withstand very high voltages for a very short time. However, as the duration of voltage increases, withstand voltage falls off considerably.
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 is in service up to the highest voltage level of 800 kV. The gas-insulated substation 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 substation is more popular where space is limited, site restrictions, or exponential ambient conditions. In this condition, it is difficult to use conventional air-insulated substations.
Characteristics of SF6 Gas
- It is colorless and odorless.
- It is 5 times heavier than air and it is not toxic and has no dangerous components inside.
- no hazardous material.
- no eco-toxic potential.
- It has no impact on the ozonosphere.
- greenhouse gas.
- SF6 has excellent electrical characteristics.
Features of Gas-Insulated Substation
- For standardized component models, widespread application of aluminum enclosure materials
- The enclosures have low eddy-current losses, good conductivity, and high resistance to corrosion and the enclosures are light in weight.
- Easy handling, reduced stresses on foundation, and support structure is additional features for the construction of GIS.
- Good coordination between the manufacturer and the user, standard arrangements can be easily modified and extended.
- 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 Gas-Insulated Substation
- Circuit breaker
- Disconnector switch
- Earthing switch
- Current transformer
- Voltage transformer
- Bus bar & connectors
- Power transformer
- Surge Arrester
- Cable termination
- SF6 / air or SF6 / oil bushing
During the last fifteen years, the application of gas-insulated substation increases rapidly. The growth in GIS application is because of the following special advantages of GIS:
- They offer to save on land area and construction costs.
- Because of lesser maintenance, downtime and repair costs, more optimal life cycle costs.
- Due to the earthed metal housing of all high voltage parts, greatly improved safety and reliability. SF6 gas as insulation much higher intrinsic strength.
- Based on large factory assembled and tested shipping units, short on-site erection times.
- Due to the non-exposure of the use of high voltage parts to atmosphere influences high service reliability.
- Reduction in radio interference with the use of earthed metal enclosures.
- Use as mobile substations for transportation to load centers on standard tracks. These substations can be located closer to load centers thereby reducing transmission line losses and expenditure in the distribution network.
- Insensitivity to external influences because of grounded metal enclosures.
- It is not necessary that extra-high voltage switchgear has to be installed outdoors.
A lot of problems encountered in practice need fuller understanding, although GIS has been in operation for several years. Some of the problems are:
- Switching operation generates Very Fast Transients Over Voltages.
- Very Fast Transients Overvoltages may cause secondary breakdown inside a GIS and Transient Enclosure Voltages outside the GIS.
- Field non-uniformities reduce withstanding levels of a GIS.
- Prolonged arcing may produce corrosive/toxic by-products.
- Support spacers can be weak points when arc by-products and metallic particles are present.
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