The Bushing of power transformer is a type of porcelain or ebonite post insulator put on the top or side of the transformer tank through which connections are made to the external circuit.
The types of bushing are as follows:-
1. Porcelain insulator bushing used upto 11 KV.
2. Oil filled bushings consisting of a hollow porcelain cylinder of special shape with a hollow tube shaped conductor through its centre used for the voltage above 33 KV.
3. Capacitor type bushing made of thick layers of bakelized paper alternating with thin graded layers of tin foil being covered by a porcelain rain shed and filled up with bitumen in the annular space between the rain shed and the bushing used in outdoor sub-station for the voltage above 33 KV.
The general constructional details , are shown in Fig. 1 . The active part of the bushing consists of an oil Impregnated Paper (OIP) core manufactured from superior grade Kraft insulating paper with condenser graded layers for field control wound under tension on central tube/ conductor . The innermost condenser layer is electrically connected to the fixing flange through a test tap . The core is dried under heat and vacuum and then impregnated with superior grade of insulating oil. Porcelain insulators on the upper and lower sides of the bushing , oil resistance high quality rubber gaskets & ‘O’ rings are held together with the central tube by means of a set of powerful springs . The fixing flange is provided with a test tap for the measurement of Capacitance and Tan delta, air releasing screw to release the air trapped in the transformer during topping up of oil or can be connected to Buchholz relay, name plate giving the product details, handle / eyebolt for lifting and space for ring type CT’s on the oil end of the bushing. At the oil end, an epoxy coated stress shield or base plate is provided to control the high stresses in oil inside the transformer. A detachable bottom arc shield is provided for bushing rated 245 KV and above. The intervening space between the core of the transformer and the porcelain insulators is filled with specially treated mineral oil under vacuum. A prismatic ( upto 300KV )or magnetic ( upto 400 KV ) oil sight glass is provided on the expansion bowl . A top terminal is provided to connect to the over head line. Arcing horns are provided on request in between top shield and base plate.
The test tap is provided for the measurement of Capacitance, Tan delta and Insulation Resistance ( IR ) value of the bushing. It is connected with a copper lead to the last condenser foil of the core directly. During normal service this test tap is electrically connected to the mounting flange trough test tap cover. The threaded test tap cover must be fixed properly before putting into service.