Otis Singleton
Electrical bushings are essential components for a wide range of electrical equipment, such as power transformers, shunt reactors, circuit breakers, and capacitors. They are hollow electrical insulators that allow 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. The purpose of an electrical bushing is to transmit electrical power in or out of enclosures, i.e., barriers, of an electrical apparatus. Bushings are typically made from porcelain, though other insulating materials such as paper, resin, and silicon rubber are also used.
| Characteristics | Values |
|---|---|
| Definition | An insulating structure that allows a conductor to pass through a grounded conducting barrier |
| Function | Transmit electrical power in or out of enclosures |
| Types | Solid-type, air-insulated, oil-insulated, gas-insulated, condenser bushings |
| Materials | Porcelain, epoxy, electrical-grade mineral oil, SF6 gas, resin, paper, silicon rubber, aluminium, copper |
| Installation | Indoor or outdoor |
| Design considerations | Ability to withstand voltage, electrical field strength, electrical energy, rigours of transportation, handling and installation |
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What You'll Learn
Bushings are hollow electrical insulators
Bushings are typically made from porcelain, although other insulating materials such as paper, resin, rubber, and oil are also used. The design of a bushing must ensure that the electrical strength of the insulating material is able to withstand the penetrating electrical energy passing through the conductor. The bushing controls the shape and strength of the field and reduces electrical stresses in the insulating material.
The height of bushings is dependent on the insulation level and pollution class. Bushings can be installed either indoors or outdoors, and the selection of insulation is determined by the location of installation and the electrical service duty on the bushing. For a bushing to work successfully over many years, the insulation must remain effective in both composition and design shape.
Electrical bushings are essential components for a wide range of electrical equipment, such as power transformers, shunt reactors, circuit breakers, and capacitors. They perform the critical function of carrying current at high voltages through equipment enclosures.
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They allow an electrical conductor to pass through a barrier
Electrical bushings are essential components for a wide range of electrical equipment, such as power transformers, shunt reactors, circuit breakers, and capacitors. They are hollow electrical insulators that allow 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. This is achieved by providing an insulating barrier between the live conductor and the metallic (conducting) body of the electrical apparatus, which is at ground potential.
The design of a bushing must take into account various factors, such as the ability of the central conductor to carry the anticipated load without overheating the surrounding insulation. The internal insulation of a bushing must also be able to withstand the nominal operating and occasional transient electric field stresses placed upon it due to potential differences between the live conductor and grounded external surroundings.
Bushings are typically made from porcelain, although other insulating materials such as epoxy resin, silicon rubber, paper, mineral oil, and gas are also used. The material used depends on the location of installation and the electrical service duty on the bushing. For example, air-insulated bushings are used with air-insulated apparatus, while oil-insulated bushings use electrical-grade mineral oil between the conductor and the insulators.
The height of bushings is also an important consideration and is dependent on the insulation level, pollution class, internal structure, electric field distribution, and value of the operating field strength. Bushings must be designed to withstand the electrical field strength produced in the insulation when any earthed material is present.
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Bushings are typically made from porcelain
In electric power, a bushing is a hollow electrical insulator that allows an electrical conductor to pass safely through a conducting barrier, such as a transformer or circuit breaker, without making electrical contact with it. Bushings are typically made from porcelain, although other insulating materials are also used.
Porcelain was originally used for bushings due to its imperviousness to moisture once sealed by a fired glaze, and its low manufacturing cost. However, porcelain has a small value of linear expansion, which means that flexible seals and substantial metal fittings must be used, presenting manufacturing and operational problems. The design of a bushing must ensure that the electrical strength of the insulated material is able to withstand the penetrating electrical energy passing through the conductor.
Porcelain bushings are used for OIP (Oil-Impregnated Paper) condensers, while silicon rubber is used for RIP (Resin-Impregnated Paper) condensers. Both serve the purpose of limiting the flow of leakage current and preventing external flashovers. OIP condenser bushings are also fitted with a spring-loaded expansion chamber to allow for oil volume fluctuations due to varying temperatures.
Solid-type electrical bushings are typically made with a central conductor and porcelain or epoxy insulators at either end and are used at lower voltages up to 25 kV. Solid bushings are commonly used in applications ranging from small distribution transformers and circuit switchers to large generator step-up transformers and hydrogen-cooled power generators.
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They control the shape and strength of the electric field
Electrical bushings are hollow insulators that allow a conductor to pass through a conducting barrier, such as a transformer or circuit breaker, without making electrical contact. All materials carrying an electric charge generate an electric field. When an energised conductor is near an earthed material, it can form very high field strengths, especially where the field lines are forced to curve sharply around the earthed material.
The bushing controls the shape and strength of the electric field, reducing electrical stress in the insulating material. The design of the bushing must ensure that the electrical strength of the insulating material is able to withstand the penetrating electrical energy passing through the conductor. As the strength of the electric field increases, leakage paths may develop within the insulation. If the energy of the leakage path overcomes the dielectric strength of the insulation, it may puncture and conduct electrical energy to the nearest earthed material, causing burning and arcing.
The design of a bushing must take into account the anticipated load or fault currents, without overheating the surrounding insulation. The internal insulation must be able to withstand the nominal operating and occasional transient electric field stresses. These stresses arise due to the potential differences between the live conductor and the grounded external surroundings.
The height of a bushing is determined by the internal structure, electric field distribution, value of the operating field strength, and external insulation test data. The bushing's design and construction should be robust enough to withstand transportation, handling, and installation.
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Bushings can be installed indoors or outdoors
Bushings are hollow electrical insulators that allow an electrical conductor to pass safely through a conducting barrier, such as a transformer or circuit breaker, without making electrical contact. They are typically made from porcelain, although other insulating materials are also used.
Bushings can be installed both indoors and outdoors, and the selection of insulation is determined by the location of installation and the electrical service duty on the bushing. The insulation must be effective in both composition and design shape for the bushing to work successfully over many years.
For example, in DC yards, the bushings that connect the valves to the DC line can either be exposed to external ambient conditions or kept entirely indoors. Similarly, DC reactors can be placed outdoors with bushings exposed to the elements, or the bushings can be installed indoors and protected from ambient pollution.
The type of bushing used will depend on the specific application and requirements. For instance, in high pollution sites, the preferred solution for HVDC transformer bushings above a certain voltage is indoor installation. In contrast, outdoor wall bushings in such environments will require composite envelopes to limit total bushing length and related mechanical issues.
The installation process for bushings can vary, but the most common method is to push them inside the housing. This involves chamfering the overture to the housing by 20 degrees +/- 5 degrees to a measurement of 1-2mm, deburring and cleaning the mating areas, and then greasing the outside material of the bushing before fitting it.
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Frequently asked questions
An electrical bushing is a hollow electrical insulator that allows an electrical conductor to pass safely through a conducting barrier without making electrical contact with it.
Electrical bushings carry current at high voltage through equipment enclosures. They provide an insulating barrier between the live conductor and the metallic (conducting) body of the electrical apparatus.
Electrical bushings are typically made from porcelain, though other insulating materials such as epoxy resin, paper, mineral oil, silicon rubber, and gas are also used.
Electrical bushings are essential components for power transformers, shunt reactors, circuit breakers, and capacitors.
