Electrical bushings are hollow electrical insulators, providing a passage and barrier for a conductor. Their primary function is to transmit electrical power in or out of electrical apparatuses such as transformers, circuit breakers, shunt reactors, and power capacitors. The bushing conductor can be built into the apparatus, or as a separate component that may be drawn through the center of the bushing.
As electrical power is drawn out of voltage and current, the insulation within the bushing must have the capacity to resist the voltage that is applied, and the current-carrying conductor must be able to carry rated current without overheating the adjacent insulation. As such, an electrical bushing is defined by the maximum voltage and current needed for a particular application. With this in mind, there are a myriad of electrical bushings that serve a number of assemblies. Electrical bushings are designed as either solid type or capacitance-graded type bushings.
Solid-type bushings are constructed with a central conductor and porcelain or epoxy insulators, and they are used with voltages lower than 25 kV. They are often utilized for small distribution transformers, circuit switchers, large generator step-up transformers, and hydrogen-cooled power generators.
While more complex in construction and less cost-effective, capacitance-graded bushings are used for all voltage ratings exceeding 25 kV. They consist of conducting layers at specific radial intervals within an oil-impregnated sheet, or similar insulating material, sandwiched between the central conductor and the insulator.
Early designs of capacitance-graded bushings included the use of concentric porcelain cylinders with metal surfaces that were embedded in conductive layers. Today, conductive foils in oil-impregnated paper are used. These bushings are also equipped with a central circular conductor with a wound capacitance-graded core, top and lower insulators, a mounting flange, an oil and oil-expansion cap, and top and bottom terminals.
Electrical Bushings Based on Insulation
Electrical bushings may also be defined by the insulating media utilized at their ends. Air-to-oil bushings are the first type, which consist of air insulation at one end and oil insulation at the other. This bushing is typically used between atmospheric air and any oil-filled apparatus. Next, air-to-air bushings are designed with air insulation on both ends, and they are usually implemented in applications where one end of the bushing is exposed to outdoor atmospheric conditions while the other end is exposed to indoor conditions.
Another way of classifying electrical bushings is according to the insulation material used inside of the bushing.
Air-insulated bushings employ air at atmospheric pressure between the conductor and insulators.
Oil-Insulated or Oil-Filled Bushings
Oil-insulated or oil-filled bushings contain electrical-grade mineral oil between the conductor and insulators in solid-type bushings. Similarly, capacitance-graded bushings also use mineral oil for the purpose of impregnating the paper and transferring heat from the conducting lead.
Oil-Impregnated Paper-Insulated Bushings
These types of bushings use mineral oil’s dielectric synergy and electric-grade craft paper to generate a composite material with premium dielectric-resistant characteristics. This material is commonly found in capacitance-graded cores.
Resin-Bonded or Impregnated Paper-Insulated Bushings
Resin-bonded paper-insulated bushings use a resin-coated craft paper to create the capacitance-graded core while resin-impregnated paper-insulated bushings utilize papers impregnated with resin for the same core.
This type of bushing is constructed with a solid-cast material that may or may not feature an inorganic filler.
The last type is a gas-insulated bushing, which uses pressurized gas to provide insulation between the central conductor and the flange, and it is normally used in SF6 circuit breakers.
If you find yourself in need of electrical bushings, current transformers, power generators, or other related components, rely on ASAP 360 Unlimited. ASAP 360 Unlimited is a leading distributor of aircraft, aerospace, and aviation parts, all of which have been subjected to a myriad of quality assurance measures, tests, verifications, and assessments. As a premier purchasing platform, we are committed to providing top-quality components with competitive pricing and unmatched lead-times. Kickoff the procurement process by filling out an RFQ form, and a dedicated representative will reach out to you in no time.
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