Electrical bushings: types, purpose and construction with diagrams (2023)

Definition

According to IEEE/ANSI Std. oneelectrical socketis defined as "an insulating structure comprising a continuous conductor or providing a central passage for such a conductor, with the possibility of applying a barrier, conductive or not, to isolate the conductor from the barrier and to conduct the current on a to guide side of the barrier to the other".

Purpose of electrical feedthroughs

We can simply say that the purpose of an electrical feedthrough is to carry electrical energy into or out of enclosures, i.e. barriers, of an electrical device such as an electrical applianceTransformer,circuit breaker, shunt reactors and power capacitors.

The feed-through conductor may take the form of a conductor constructed directly as part of the feed-through, or alternatively as a separate conductor normally threaded through the center of the feed-through.

Because electrical energy is the product of voltage and current, a bushing's insulation must withstand the applied voltage and its live conductor must be able to carry the rated current without overheating the adjacent insulation.

For practical reasons, jacks are not classified according to the power they carry; Instead, they are rated by the maximum voltage and current they are rated for.

Lerinsulating materials in cables.

Types of Electrical Feedthroughs

There are many methods of classifying jack types. These classifications are based on practical reasons that will become apparent in the following discussion of three main areas. Bushings can be classified:

1. Depending on the construction:Depending on the construction, there are two types of bushings -
   1. Solid or mass bushings
   2. Capacitor type or capacitive feedthrough
2. Depending on the insulating medium at the ends:This classification mainly depends on the final application of the bushing. It can be classified as
   1. Air-Oil Bushing
   2. Bucha ar-ar
   3. Air ducts for SF6
   4. SF6 to Oil Bushings
   5. Oil to Oil Bushings
   6. Air-Oil Bushing
3. According to the insulation in the bushing:According to the internal insulation of the bushings, they are classified as -
   1. air-insulated bushings
   2. Oil insulated or oil filled bushings
   3. Insulated bushings made of oil-soaked paper
   4. cast insulating bushes
   5. gas insulated bushings

Types of electrical feedthroughs depending on the design

As discussed above, bushings are classified according to their construction. They are

1. Solid Type Bushings (Mass Type)
2. Capacitance rated bushings (capacitor type)

1. Solid bushings

Solid type electrical feedthroughs are typically made with a center conductor and porcelain or epoxy insulators at each end and are mainly used at lower voltages up to 25kV.

Solid Bushing Applications and Limitations

Solid bushings are commonly used in applications ranging from small distribution transformers and line breakers to large generator step-up transformers and hydrogen-cooled power generators.

The main limitation of the solid bushing is its ability to withstand voltages of 60 Hz in excess of 90 kV. Therefore, its applications are limited to 25 kV devices that have test voltages of 70 kV.

Recent applications require low partial discharge limits at the 25kV terminals during transformer testing and have led to further restrictions on the use of this type of bushing.

In these cases, a specially designed solid bushing with uniquely rated shielding that allows for low inherent partial discharges, or a more expensive capacitance rated bushing should be used.

2. Jacks classified by capacity

This design is currently used for virtually all nominal voltages above 25 kV system voltage and has been used for bushings up to 1500 kV system voltage.

Construction of capacity classified bushings

This design uses conductive layers at predetermined radial spacings within oil-impregnated paper or other insulating material located in the space between the center conductor and the insulator.

Various manufacturers have used a variety of materials and processes to create capacitive feedthroughs.

The first methods were to use concentric porcelain cylinders with metalized surfaces or laminated cardboard tubes with embedded conductive layers.

Later designs used conductive foils, usually aluminum or copper, on oil-impregnated kraft paper.

An alternative method is to print semiconductor ink (different manufacturers used different conductivities) on all or some of the oil-impregnated kraft paper sleeves.

The main elements are the central circular conductor on which the graded capacitance core is wound; the upper and lower insulators; the mounting flange; the oil and an oil equalizing cap; and the top and bottom connectors.

Capacitance rated jacks involve much more engineering and manufacturing detail than solid jacks and are therefore more expensive. This includes the insulation/conductive layer system, the equipment to wind the capacitor core and the oil to saturate the capacitor.paper insulation.

However, it should be noted that the required radial dimension for the graded capacity bushing is much smaller than for a solid construction and this saves material within the bushing as well as in the apparatus in which the bushing is used.

In addition, high-voltage bushings could not be made in bulk from a practical point of view.

Types of electrical feedthroughs based on end insulation

As shown in the section above, bushings are classified into six types based on the insulating medium at the ends. Some of them are explained in this section.

1. Air-Oil Bushing

An air-oil bushing has air insulation on one end of the bushing and oil insulation on the other. Because oil is more than twice as dielectrically strong as air at atmospheric pressure, the oil side is about half the length (or shorter) of the air side.

This type of bushing is commonly used between atmospheric air and oil filled equipment.

2. Bucha ar-ar

An air-to-air transit has air insulation at both ends and is typically used in construction applications where one end is exposed to outdoor weather conditions and the other end is exposed to indoor conditions.

Special application bushings have limited use and include:

• Air ducts for SF6, commonly used in SF6 insulated circuit breakers;
• SF6 to Oil Bushingsas transitions between SF6 busbar channels and oil-filled devices;
• Oil to Oil Bushings, used between oil bus pipelines and oil filled equipment.

Types according to the insulation inside the electrical bushing

Another classification concerns the insulating material used in the bushing.

In general, these materials can be used in solid or capacitive graded construction, and in different types more than one of these insulating materials can be used together.

The following text provides a brief description of these types:

1. Air insulated bushings

Air insulated bushings are generally used only with air insulated equipment and are solidly constructed using air at atmospheric pressure between the conductor and the insulators.

2. Oil insulated or oil filled bushings

Oil insulated or oil filled bushings have electrical grade mineral oil between the conductor and the insulators in solid bushings.

This oil can be contained within the bushing or it can be shared with the device in which the bushing is used.

Capacitive bushings also use mineral oil, normally contained in the bushing, between the insulating material and the insulators to impregnate the kraft paper and transfer heat from the conductive line.

3. Oil impregnated paper insulated bushings

Oil-impregnated paper insulated bushings utilize the dielectric synergy of mineral oil and electrical kraft paper to produce a composite material with superior dielectric strength properties.

This material has been used extensively as an insulating material in capacitive cores for about 50 years.

4. Insulated paper bushings, resinated or impregnated

Resin impregnated paper insulated jacks use a resin coated kraft paper to make the capacitive core while resin impregnated paper insulated jacks use resin impregnated papers which are then used to make the rated capacitance core.

The last type of bushing has superior dielectric properties comparable to bushings insulated with oil-impregnated paper.

5. Cast insulation bushes

Cast insulation bushings are manufactured from a solid cast material with or without an inorganic filler.

These feedthroughs can be solid or capacitive, although the former type is more representative of current technology.

6. Gas insulated bushings

Gas insulated bushings use pressurized gas such as SF6 gas to insulate between the center conductor and the flange. This type of socket is used inSF6 circuit breaker.

The bushing is one of the simplest designs and is typically used with circuit breakers.

It uses the same compressed gas as the circuit breaker, has no capacitance rating, and uses the dimensions and placement of the ground shield to control electric fields.