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Transformer in-service interruptions and failures usually result from dielectric breakdown, winding distortion caused by short circuits, hots spots caused by localized deviations in winding and electromagnetic fields, deterioration of insulation, effects of lightning and other electrical disturbances, inadequate maintenance, loose connections ...
Arcing horns protecting bushings on a distribution transformer. Arcing horns form a spark gap across the insulator with a lower breakdown voltage than the air path along the insulator surface, so an overvoltage will cause the air to break down and the arc to form between the arcing horns, diverting it away from the surface of the insulator. [3]
In electric power, a bushing is a hollow electrical insulator that allows an electrical conductor to pass safely through a conducting barrier such as the case of a transformer or circuit breaker without making electrical contact with it. Bushings are typically made from porcelain, though other insulating materials are also used.
This is because the line might become inadvertently charged (e.g. through a back-charged transformer, possibly as a result of an improperly connected, inadequately isolated emergency generator at a customer facility), or inductively coupled from an adjacent in-service line. To prevent this, the line is first grounded via a clamp known as a bond ...
Various forms of flexible terminals, expansion joints, and weatherproof or fire-proof bushings and terminals are used with isolated-phase bus. Some types of apparatus such as disconnecting switches, circuit breakers, and instrument transformers are made in enclosures that can be welded to become an integral part of the isolated-phase bus system.
Single- and three-phase pad-mounted transformers are used in underground industrial and residential power distribution systems, where there is a need for safe, reliable, and aesthetically appealing transformer design. Their enclosed construction allows the installation of pad-mount transformers in public areas without protective fencing.
Transformers designed for below-grade installation can be designed for periodic submersion in water. [14] Distribution transformers may include an off-load tap changer, which slightly adjusts the ratio between primary and secondary voltage to bring the customer's voltage within the desired range on long or heavily loaded lines. [citation needed]
Switching events may be planned or unplanned. A transmission line or other component may need to be de-energized for maintenance or for new construction, for example, adding or removing a transmission line or a transformer. To maintain reliability of supply, companies aim at keeping the system up and running while performing maintenance.