18-03-2014, 02:24 PM
BUSHINGS
BUSHINGS.pdf (Size: 606.4 KB / Downloads: 34)
Introduction
The bushings are insulators which are used to take high voltage conductors through earthed
barriers such as walls, floors, metal tanks etc. Bushings must provide
1. Electrical insulation for the rated voltage and for service overvoltages.
2. Mechanical support for the conductor and external connections.
3. Stress control and avoid damages.
If bushings are not used, the current that flowing through the conductor will spark between high
voltage conductor and earthed body.
Difference between bushings and insulators: The insulators are used to insulate two different
voltage points whereas bushings are used to conduct one side voltage to the other side through
zero potential body.
Condenser bushing or Capacitance bushing:
The bushing in which cylindrical conducting layers are arranged coaxially within the
insulating material. The length and diameter of the cylinders are designed to control the
distribution of the electric field in and over the outer surface of the bushing. Condenser bushings
may be one of several types:
i. Synthetic resin-bonded paper (s.r.b.p) insulation
ii. Oil-impregnated paper insulation (o.i.p)
iii. Epoxy resin impregnated paper
In synthetic resin-bonded paper bushings, the paper is first coated with synthetic resin then
wound into a cylindrical form under heat and pressure and inserting conducting layers at
appropriate intervals. The conducting layers of metallic foil form a complete cylinder.
Gas insulated bushing:
An improved gas-insulated bushing having a high withstand voltage and an increased
capacity to cool a conductor, wherein the upper end portion of a potential-controlling capacitor
in the form of a stopped tube is directly air tightly fitted to the outer surface of the central
conductor, thereby enlarging a distance between the capacitor and the inner wall of the porcelain
insulator and elevating the withstand voltage of the bushing; an internal pipe open at the bottom
is inserted into the central conductor; the upper portion of the internal pipe communicates with a
cooling case provided at the top of the porcelain insulator; a cooling gas is circulated through a
route consisting of the cooling case