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
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.