Bushings are insulating equipment which are used to bring out a conductor from a metallic enclosure such as transformer tank or a the enclosure of a breaker.
Bushings, generally use oil or gas as the insulating material. They consist of a conductor surrounded by an insulating medium and then an non conducting enclosure.
The electrostatic stress is highest on the insulating material closer to the live conductor and gradually decreases towards the periphery. Because of this, a large quantity of material is needed for bushings used at higher voltages. This makes the bushing bigger in size and costlier.
The condenser bushing consists of the metallic conductor surrounded by a cylindrical sheath of paper and then by a metallic sheet. This is again followed by a paper sheet and then by a metallic sheet. This order continues till the periphery. This can be visualized as a series of concentric capacitors.
This ensures that the electrostatic stress is evenly distributed across all the insulating material. Condenser bushings are smaller, use less material and are therefore cheaper.
Condenser bushings are usually used above 52 kV