With electrical energy, Bushing is an insulated device that allows an electrical conductor to safely pass through a grounded conductive barrier, such as in the case of a transformer or circuit breaker. The bushings are usually made of porcelain. Although other insulating materials are possible, porcelain is generally used.
Types:
Porcelain Insulation Bushing
The earliest socket designs use porcelain for indoor and outdoor applications. Porcelain was originally used because, after being sealed by fired glaze, it is impervious to moisture and has low manufacturing costs. The main disadvantage of porcelain is that its low linear expansion value must be offset by the use of flexible seals and essential metal fittings, which pose both manufacturing and operational problems.
A basic porcelain socket is a hollow porcelain shape that fits through a hole in a wall or metal case, allowing a conductor to pass through its center and connect to other devices at both ends. Bushings of this type are often made of wet-process porcelain, which is then glazed. A semiconducting glaze can be used to help balance the electrical potential gradient along the length of the socket.
The interior of the porcelain socket is often filled with oil to provide additional insulation, and sockets of this design are often used up to 36 kV when higher partial discharges are allowed.
When partial discharge in accordance with IEC60137 is required, paper and resin insulated conductors in conjunction with porcelain are used for unheated indoor and outdoor applications.
The use of resin (polymer, polymer, composite) insulated bushings for high voltage applications is common, although most high voltage bushings usually consist of resin impregnated paper insulation around the conductor with porcelain or polymer weather scales for the outer end and occasionally for the inner end.
Paper insulation
Another early form of insulation was paper. However, paper is hygroscopic and absorbs moisture, which is harmful and disadvantageous due to the inflexible linear construction. Cast resin technology has dominated insulated products since the 1960s due to their flexibility in shape and their higher dielectric strength.
Typically, paper insulation is later impregnated with either oil (historically) or today, usually with resin. In the case of resin, the paper is film coated with a phenolic resin to become synthetic resin-bound paper (SRBP) or, after dry-wrapping, impregnated with epoxy resins to become resin-impregnated paper or epoxy resin-impregnated paper (RIP, ERIP).
SRBP insulated bushings are typically used up to voltages around 72.5 kV. Above 12 kV, however, there is a need to control the external electrical field and balance the internal energy storage, which marginalizes the dielectric strength of the paper insulation.
To improve the performance of paper insulated bushings, metal foils can be used during the winding process. These serve to stabilize the generated electric fields and to homogenize the internal energy using the capacitance effect. This function led to the capacitor / capacitor socket.
The capacitor socket is produced by inserting very fine layers of metal foil into the paper during the winding process. The inserted conductive foils create a capacitive effect that distributes the electrical energy more evenly over the insulated paper and reduces the electrical field voltage between the live conductor and any grounded material.
Capacitor bushings create electrical voltage fields that are much less strong around the mounting flange than designs without foils, and in conjunction with resin impregnation, they produce bushings that can be used with great success at operating voltages over a million.
Resin insulation
Resin materials have been used in all types of bushings up to the highest voltages since the 1965s. The flexibility of using a pourable form of insulation has replaced paper insulation in many product areas and dominated the existing insulated bushing market.
As with paper insulation, the control of the electrical voltage fields remains important. The resin insulation has a higher dielectric strength than paper and requires less voltage control at voltages below 25 kV. However, some compact, higher rated switchgear have grounded materials that are closer to the sockets than in the past, and these designs may require voltage control screens in resin sockets that operate only 12 kV. Attachment points are often an integral part of the main resin mold and there are fewer problems with grounded materials than with metal flanges for paper bushings. However, caution should be exercised in resin insulated bushing designs that use internally molded sieves so that the benefit of controlling the electric field is not compromised by an increase in partial discharge caused by the difficulty in removing micro voids in the resin around the sieves process caused during casting. The need to remove voids in the resin becomes more sensitive as the voltage increases, and it is normal to use resin-impregnated, film-coated paper insulation for sockets with a nominal voltage above 72.5 kV.