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What are the various types of OHE Railway Insulators used in Railway Industry?

A high-voltage apparatus chamber uses the bushing as an insulated entry for a charged conductor. The transformer bushing can also be used to support other electrical components of the equipment.

The unexpected failure of bushing can be minimised when moisture is prevented from getting into the system. Insulation in the bushing can degrade to the point of failure due to moisture exposure. This can lead to serious equipment and safety hazards for both workers and the general public.

Maintenance and testing of High-Voltage Transformer Bushings should be performed on a regular basis to help avoid electrical disasters and maximise bushing service life.

Maintenance of High-Voltage Transformer Bushings:

  1. Visual Inspection:

    Cracks and contamination can occur in porcelain bushings. Carbon tracking, or "treeing," may form on the surface of the bushing if the bushing is broken or excessively polluted. It is possible for the bushings to fail if they are not cleaned on a regular basis.

    Make sure there are no oil leaks by inspecting the bushing. Oil levels can be checked by looking through an oil sight glass or by examining a gauge. If you have a level gauge with a pointer, take note of where it is at all times, as the level should fluctuate slightly with temperature.

    If the level of the liquid never changes, even though the temperature of the environment around it varies widely, it is quite likely that the gauge is broken and should be checked during the next outage that is available. A bushing failure can be catastrophically caused by a defective gauge pointer, which, when combined with even a minor oil leak, can damage the apparatus and other switchyard equipment in the immediate area.

    Checking the mounting bolts to ensure that they have appropriate torque and evaluating the gasket to ensure that it has the proper compression should be done in order to fix the low level of oil inside the busing. If it is determined that the compression and torque are accurate, the bushing will need to be replaced because of the leak. It is of the utmost importance to use the appropriate level of compression and install the appropriate gasket for the application.

  2. Infrared Inspection:
  3. An infrared inspection can be carried out to inspect High-Voltage transformer bushings. This indicates that there may be a loose connection if any of the bushings get hotter than normal under the same load.

    There are many cases where the top area connection of the bushing has issues; however, the higher temperature found at the top indicates a poor connection inside the transformer tank. In DGA samples, hot metal gases like ethylene and ethane are typically found in transformers with bad connections.

    It is possible to see the ionisation of the air at the tops of bushings during the night or at dusk, especially when there is heavy fog, mist, rain, as well as high humidity. At first, the corona appears normal towards the bushing top; however, as contamination increases, the corona will descend further.

    Indoors and outdoors at night, a corona scope can be used to see and photograph low amounts of the corona. If a dark background is present, such as buildings, canyon walls, trees, etc., high levels of corona can be observed outdoors in the daytime. This check will be necessary more frequently in environments where bushings accumulate salt and dust.

    Electrically test, visually inspect, and clean the transformer bushings as rapidly as possible if the corona appears lower than the top of the bushing. For night and indoor use, the corona scope is best suited. It cannot be utilised with a clouded sky.

  4. Cleaning Methods:
  5. If bushings are not cleaned regularly, a phase-to-ground fault may occur when corona is close to the grounded transformer tank. This fault may result in the destruction of the bushing and an extended outage.

    The manufacturer's documentation should be consulted to obtain particular instructions on cleaning and repairing the various bushing surfaces. There is a possibility that different solvents, wiping agents, and cleaning procedures will be necessary for the various bushings.

    The porcelain surface needs to be sealed if there are even the tiniest hairline cracks since collected dirt and moisture have the ability to find their way into the crack, which could then result in flashover. You can also use Epoxy to fix larger chips, but for smaller cracks and chips, you might need to utilise a different kind of repair process.

    Applying silicone wax to bushings may be sufficient, and then wiping them down with a soft cloth to clean them. To remove more tenacious pollutants, you might need steel wool, solvents, and brushes. It is possible that salt and other water-soluble deposits cannot be removed without the use of high-pressure water. Removing salt cake, chemicals, metallic oxides, as well as any other kind of hard pollutant can be done risk-free by using limestone powder in combination with dry air.

    Potter's clay, the shells of walnuts or pecans, or crushed coconut shells are some of the other materials that have been proven to safely remove hard impurities from high-voltage transformer bushings. The use of carbon dioxide (CO2) pellets for blasting is an alternative that is more expensive but completely removes the need for cleanup by evaporation.

  6. Power Factor Testing:
  7. High voltage insulation can get wet, dry, or deteriorate due to a variety of factors, including moisture in the insulation or a decrease in insulation power factor. There are two types of leakage current: resistive and capacitive. Since no insulation is perfect, there will always be some loss and a phase angle less than 90 degrees between total current and total voltage.

    A tap is generally included with high voltage transformer bushings for testing purposes. An important part of a bushing's nameplate is the measurement of the power factor for C2 insulation as well as tap capacitance.

  8. Bushing Maintenance Tips:
  9. Wood and porcelain don't insulate and can generate erroneous power factor readings. Future testing should be compared to past results.

    Some of the transformer bushings contain a nitrogen layer above the oil, which expands and pressurizes. If the fill plug is removed while hot, pressurised oil may gush out, inflicting burns.

    Never open oil fill plugs with a hot bushing. Consult the manufacturer's handbook for the safe opening temperature range.

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