Bushing of Transformer

A porcelain or ebonite post insulator known as a power transformer bushing is used to make connections to the external circuit and is installed on the top or side of the transformer tank.

The several bushing varieties are as follows:-

• Up to 11 KV, porcelain insulator bushing is used.

• For voltages above 33 KV, oil-filled bushings made of a hollow porcelain cylinder with a specific shape conductor running through the centre are utilised.

• Capacitor type bushing is used in outdoor sub-station for voltage above 33 KV. It is formed of thick layers of bakelized paper alternating with thin graded layers of tin foil. The annular space between the rain shed and the bushing is filled with bitumen.

Condenser bushing design and Construction

Depicts the general constructional elements. Oil Impregnated Paper (OIP) core made from top grade Kraft insulating paper with condenser graded layers for field control wound under tension on central tube/conductor makes up the active portion of the bushing. Through a test tap, the innermost condenser layer is electrically connected to the mounting flange. The core is heated and vacuum-dried before being impregnated with high-grade insulating oil. A series of strong springs hold the central tube and the porcelain insulators on the upper and lower sides of the bushing, as well as the oil-resistant, high-quality rubber gaskets and "O" rings, together. The fixing flange is equipped with a test tap for capacitance and Tan delta measurements, an air release screw to let out trapped air in the transformer during oil topping up, a name plate with the product's specifications, a handle or eyebolt for lifting, and space for ring type CTs on the oil end of the bushing.

An epoxy-coated stress shield or base plate is provided at the oil end to reduce the high stresses in the transformer's oil. For bushings rated 245 KV and higher, a removable bottom arc shield is offered. Under vacuum, a specific type of mineral oil is poured into the gap between the transformer's core and the porcelain insulators. On the expansion bowl is a prismatic (up to 300 KV) or magnetic (up to 400 KV) oil sight glass. For connecting to the overhead line, a top termination is offered. On request, arcing horns are offered between the top shield and base plate.

Tap Test:

The test tap is available for measuring the bushing's capacitance, tan delta, and insulation resistance (IR) value. It is immediately connected to the final condenser foil of the core by a copper lead. This test tap is electrically connected to the mounting flange trough test tap cover during normal operation. Before being used, the threaded test tap cover needs to be firmly fixed.

Regular inspection and upkeep of the transformer bushing:

There is no particular maintenance to be done because bushing is a self-contained device. However, routinely checking the oil level and cleaning the porcelain should be plenty.

During yearly maintenance, measurements of capacitance and tan delta may be made to assess the health of the bushing. These numbers should be compared to the outcomes of the pre-commissioning test. Tan delta value greater than 0.007 and capacitance increase of 10% or more