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Testing and maintenance of high-voltage bushings for transformers

Yash Highvoltage

Transformer bushings

Bushings should undergo a Doble test as an acceptability test when they are brand new. For information on what constitutes acceptable performance, consult the instructions for the M4000 Doble test set, the Doble Bushing Field Test Guide [9], and the manufacturer's data.
CAUTION:
A bushing should never be tested while it is resting on wood or inside of its wooden shipping carton. Because wood does not insulate as well as porcelain, the readings will be off. Save the test results as a reference point for comparison with subsequent exams.

Check for cracks and/or contaminants in the external porcelain after a month of service and once a year (requires binoculars). There is never a "perfect insulator," yet there is always some leakage current. The high-voltage conductor's current "leaks" through and along the bushing's surface to the ground. Carbon tracking (treeing) on the surface of the bushing may be seen as obvious evidence if the bushing is severely damaged or contaminated. If the bushings are not routinely cleaned, flashovers could happen.

Keep a close eye out for oil leaks. View the oil-sight glass or the oil level gauge to check the bushing oil level. Look closely whether the bushing has a gauge with a pointer because the oil level ought to move somewhat as the temperature does. The gauge needs to be checked at the following outage if the pointer remains stationary despite significant changes in load and ambient temperature. An explosive bushing failure brought on by a blocked gauge pointer and a tiny oil leak can harm the transformer and other switchyard machinery. The result is an expensive extended outage.

Check the bolts for adequate torque and the compression of the gasket if the oil level is low and there is an exterior oil leak. If compression and torque are accurate, a spare bushing must be used in lieu of the damaged one. Pay close attention to the directions in the transformer's manual. It is crucial to instal the right kind of gasket and use the right amount of compression. Check the most recent transformer because it likely has a faulty gasket that is also allowing air and water into the transformer.

DGA for high oxygen and moisture levels. There might be an internal leak around the lower seal into the transformer tank if the oil level is low and there is no apparent external leak. Refill the bushing with the same oil, if feasible, and pay close attention to the level and volume needed to fill it to the right level. Plan to replace the bushing if it uses more than a quart of oil. The bushing cannot be repaired in the field; it must be sent to the factory for repair or it must be discarded.

CAUTION:

If a bushing has a fill plug and the temperature is high, never open the fill plug. A nitrogen blanket may be present on top of the oil in some bushings, which pressurises as the oil expands. Always refer to the manufacturer's instructions handbook for information on the appropriate range of temperatures for opening a bushing. This will typically range from 15 °C (59 °F) to 35 °C (95 °F). If the fill plug is removed when the temperature is high, pressurised hot oil could unexpectedly spill out of it, creating burn hazards. If the temperature gauge has been tested as described in section 4.3, it may be used as a reference since the bushing will typically be a little colder than the top oil temperature.
Moisture entering through faulty gaskets, seals, or cracks accounts for about 90% of all avoidable bushing failures. Doble testing can be used to find internal moisture. For issues and solutions, see FIST 3-2 [10] and Doble Bushing Field Test Guide [9]. Internal moisture damages the insulation of the bushing and can lead to an explosive failure, seriously damaging the transformer and other equipment and posing risks to the workers. Examine the bushings with an IR camera after one month of operation and once a year [4, 8]; if one phase displays a much higher temperature, there is likely a faulty connection.

Perform a thorough physical examination and cleaning of the bushings every three to five years [10]. Pay close attention for cracks, leaks, and carbon trailing. The frequency of this inspection will increase in environments where the bushings develop salt and dust deposits. A modest coating of GE Insulgel or Dow Corning grease DC-5 will assist lower the possibility of external flashover under conditions that lead to deposits. Grease accumulation could happen as a result of this procedure, which is a drawback. A higher-quality silicone paste wax applied to the porcelain in humid and wet places can be a preferable option because it will lessen the chance of flashover.

If the deposits aren't too tough, a spray-on wax with silicone, like the Turtle Wax brand, has been found to be particularly helpful for cleaning and waxing in one step. Wax minimises the chance of flashover by causing water to form beads rather than a continuous sheet. Simply applying Turtle Wax and wiping the surface with a soft cloth may be sufficient for cleaning. It might also be helpful to use a lime-removal solution like "Lime Away." Steel wool, brushes, and solvents may be needed to remove more tenacious pollutants. To get rid of salt and other water soluble deposits, a high pressure water stream could be necessary.

Dry air limestone powder blasting can safely remove salt cake, chemicals, metallic oxides, and other hard contaminants. For hard pollutants, other materials are often employed, such as pottery clay, crushed walnut, pecan, or coconut shells. Although carbon dioxide (CO2) pellet blasting is more expensive, because it evaporates, cleanup is essentially eliminated. Blasting with ground-up corn cobs helps get rid of soft contaminants like old grease buildups. When using any of these treatments, a qualified, experienced contractor should be engaged, and a comprehensive, written job hazard analysis (JHA) should be carried out.

At dusk or at night, particularly in times of rain, mist, fog, or excessive humidity, corona (air ionisation) may be seen at the tops of towers. Corona is regarded normal at the top, but it will gradually descend as a bushing becomes more and more contaminated. When the corona gets close to the grounded transformer top, if the bushing is not cleaned, flashover will happen. Inspection, Doble testing, and cleaning of the bushing should be done as soon as possible if corona appears to be lower than the top of the bushing. Flashover (phase to ground fault) could damage the bushing and result in a protracted power outage. If all of the bushings are polluted and a flashover occurs, line-to-line faults may also happen. Low-level corona can be seen and captured with a corona scope both inside under normal lighting and outside at dusk or night. If a black background, such as a canyon wall, building, or other object is present, high levels of corona can be seen outside during the day. The corona scope design is primarily intended for usage at night and inside; it cannot be utilised against a background of a cloudy or blue sky. The Technical Service Center (TSC), D-8450, has this technology on hand.

The bushings should undergo double testing every three to five years, depending on the environment and service conditions. For information on suitable values and test procedures, consult the Doble M-4000 test set instructions, Doble Bushing Field Test Guide [9], FIST 3-2 [10], and the manufacturer's instructions. Cleaning of the bushings is recommended before Doble testing. The results will be incorrect if the insulating surface is contaminated. Testing can be carried out both before and after cleaning to verify cleaning techniques. Reduce the testing interval to a year as the bushings get older and start to deteriorate. Keep thorough notes of the results so that replacements may be purchased beforehand and bushings don't need to be taken out of service.

CAUTION:

For thorough directions on cleaning and mending your particular bushing surfaces, consult the transformer manual. For various bushings, various cleaning techniques, materials, and solvents might be needed. Small cracks and chips may also require different restoration methods. Typically, hairline cracks, chips, and minor scratches can be fixed using glyptal or insulating varnish. Sharp chip edges should be filed down, and an insulating varnish should be applied to the damaged area to give it a glossy sheen. Because accumulated dirt and moisture in a crack may induce flashover, hairline cracks in the porcelain's surface need to be sealed. For larger chips, epoxy should be utilised for repair. A bushing insulator must be replaced if it has a significant chip that shortens the flashover distance or a significant crack that completely penetrates the insulator. Some manufacturers provide repairs for broken bushings that can't be fixed on the spot. If you have inquiries about repairs, get in touch with the manufacturer of your specific bushings. https://yashhv.com