Bushings
Where transformers are enclosed within tanks to contain the insulating
and cooling fluid, it is necessary to link the winding connections to
the network using through bushings which penetrate the tank.
Low-voltage bushings are generally solid. They are made of porcelain, ceramic or epoxy insulation with sufficient electrical strength to withstand abnormal voltages due to lightning activity or switching operations, and to withstand the service voltage over the lifetime of the transformer. Some low-voltage bushings carry high current and cooling is necessary. In such cases, it is usual to employ a hollow porcelain construction in which the internal connections are cooled by the oil in the tank.
High-voltage bushings must withstand much higher voltage transients. They are usually of composite construction with a core of oil-impregnated or resin-bonded paper in an outer porcelain or epoxy cylinder. This outer cylinder is ‘shedded’ on the outside to increase its electrical strength under wet conditions. A typical high-voltage bushing with an oil-paper core is shown in Fig (a) its internal construction detail is shown in Fig(b).
Low-voltage bushings are generally solid. They are made of porcelain, ceramic or epoxy insulation with sufficient electrical strength to withstand abnormal voltages due to lightning activity or switching operations, and to withstand the service voltage over the lifetime of the transformer. Some low-voltage bushings carry high current and cooling is necessary. In such cases, it is usual to employ a hollow porcelain construction in which the internal connections are cooled by the oil in the tank.
High-voltage bushings must withstand much higher voltage transients. They are usually of composite construction with a core of oil-impregnated or resin-bonded paper in an outer porcelain or epoxy cylinder. This outer cylinder is ‘shedded’ on the outside to increase its electrical strength under wet conditions. A typical high-voltage bushing with an oil-paper core is shown in Fig (a) its internal construction detail is shown in Fig(b).
Fig (a)High-voltage oil–air bushing
Fig (b)Internal construction of high-voltage oil–air bushing
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Siemens(Trench) Condenser High Voltage Oil Air Bushing
Oil Impregnated Paper
Condenser-type transformer-to Air bushings covers rated voltages from
40.5kV to 1100kV. Technology resources are Trench UK and Trench France.
The
bushings consist of a paper insulator wound onto a central tube or
conductor, having aluminum foils inserted to give electrical street
grading through the insulation thickness and along its surface,
producing an efficient, compact design.
The paper winding is
enclosed within porcelain insulators at the outdoor and oil immersed
ends and a central metallic housing which also provides the mounting
flange. The wound core is impregnated with high-grade degassed mineral
oil, following a carefully controlled heat and vacuum drying process.
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