• Electrical India
  • Nov 5, 2015

Recent Trend And Technological Development  In Transformer Insulation

This article gives the idea of power transformers and their insulations at various levels of voltages. It also exclusively presents the recent trends in the transformer insulation and technological development, especially like Gas insulated Transformers...

- Dr N Kumarappan,

P Balaji, N S Srinath


 Power transformers have been used since the beginning of commercial power distribution in the late 1800s. They step-up and step-down voltages (and inversely, the magnitude of currents when power in = power out), to allow the efficient transmission of power, by reducing losses caused by high currents in long conductors. The first transformers were of wood construction, air insulated and had very low current ratings. As power requirements increased, transformers were constructed with their windings mounted inside sealed tanks, full of insulating oil. This general design concept allowed for better cooling and higher voltage ratings, and has remained in place for over 100 years. In the past 30 years, the use of gas as a dielectric has emerged as a safer, more environmentally friendly and as a replacement for oil. This article looks at Sulfur Hexafluoride (SF6) as a dielectric, current designs, reasons for implementation and the future trends of Gas Insulated Transformers (GIT).

Construction

  The construction of a GIT is basically the same as an Oil Immersed Transformer (OIT), with the exception of insulating material and cooling medium.

  Therefore, broad experience of OIT technology can be applied to GIT design, manufacturing and maintenance 

Alstom Gas Insulated Transformers at 145kV Gas-insulated switchgear (F35) at the Shuqaiq HV substation in Saudi Arabia...

Toshiba 15/150 MVA 110 KV (GIT in underground substation)...[Low pressure Design 0.14 MPa-g]

Toshiba 400 (MVA GIT in underground substation)...[High Pressure Design 0.43 MPa-g]

Cooling System

  The concept of the external cooling design for a GIT is nearly the same as that for an OIT; for this reason various types of cooling methods can be applied.

Flexibility in Substation Design

  Since the head pressure of SF6 gas is negligibly low, cooling equipment can be located above the GIT level as indicated in the figure. The GIT offers excellent flexibility when locating components such as radiators or coolers.

Toshiba Gas Insulated Transformer (GIT)...

Water Cooling System for Urban Substation

  A water cooling system is widely used in large-capacity underground and/or indoor substations. The heat generated from a GIT is efficiently transferred to water-air heat exchangers by water flow. The cooling system is usually located on the top floor of the substation building.

Toshiba Gas Insulated Transformer (GIT)...

GIT - Diagnostics and Maintenance

  The integrity of GITs can be checked and a failure cause analysis can be made using gas chromatography. Using a simple gas detective tube device, GIT integrity can be confirmed at site.

GIT Interface with Other Substation Equipments

  • Cable connection
  • Plug-in connector interface
  • Gas insulated bus bar (Direct Connection).

Main Applications of Gas Insulated Transformers

  • GITs excel in explosion-proof safety and are widely used in underground and indoor substations in urban areas. Significant tank pressure increase does not occur during internal failures due to the gas characteristics. The risk of a tank explosion or fire incident can be completely eliminated.
  • GITs are also installed in environmentally critical areas where oil leakage is prohibited.

Temperature Analysis for GIT design and GIT Room Considerations

Computer Aided Engineering (CAE)

  In the cooling design for the GIT winding or substation buildings, Computer Aided Engineering (CAE) methods can be applied. The calculation using CAE for the airflow inside the building and/or outside is very useful during the planning stage of the substation building.

Winding Cooling...

Transformer Room Cooling...

Least considered Disadvantage of GITs

  • Gas Insulated Transformers are more expensive; however, this cost difference may balance out over the life of the transformer, as maintenance is cheaper and minimum routine.
  • Environmental regulations on greenhouse gasses require SF6 to be handled carefully, and recycled where possible. This can create additional installation & repair costs, depending on local laws.
  • A GIT requires complex cooling due to SF6's low thermal conductivity, and generally the cooling system (blowers, heat exchangers etc.) need redundancy to ensure that a small equipment failure or maintenance does not dramatically reduce the transformer rating.

Transformers in the Future

  Dry type transformers are still the choice for most electricity distributors for low voltage distribution transformers, where oil cannot be used. Most of the European Countries opted for GITs for Higher level of Voltages. Sydney’s requirements are that any transformer that is not on or below ground level (i.e., any transformer in upper level substations) is required to be oil free. Currently, Ausgrid has around 100 upper level substations with 2 or 3 dry type transformers in each. It is most likely that within the next 5 years, gas insulated distribution transformers will be used instead. Although Dry Type Transformer with SF6 have numerous advantages, transportation, handling and recycling of SF6 may become financially prohibitive.

  “Octafluorocyclobutane is a possible replacement for SF6”. Its molecule is made up of fluorine and carbon atoms, removing the risk of creating poisonous sulphur based gases during arcing. It is denser than SF6 and has a higher dielectric strength of 3.6 times the breakdown voltage of air (as opposed to 3.0). It is also more environmental friendly than SF6.

Conclusion

  It is clear from the above mentioned points that Gas Insulated Transformers (GITs) have many benefits and will slowly replace most types of transformers in the future. It is possible that SF6 may not be used; however, an equivalent gas will perform in much the same way as sulphur hexafluoride. Hence, GITs out stands with the following advantages:

  • Environmental Friendly
  • Explosion Free.

Dr N Kumarappan is Professor in the Annamalai University, India. P Balaji is currently pursuing M.E. degree in Power Systems at Annamalai University, India. N S Srinath is currently working towards the M.E. degree in Power Systems atAnnamalai University, India.

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