Electrical Bushing in Transformer
Electrical Bushing is a hollow electrical insulator mounted over the main tank that is used to form barrier between terminal and main tank of the transformer. It is an important part of transformer. It allows an electrical conductor (i.e terminal) to pass safely through a conducting barrier (i.e main tank) without making electrical contact with it.
Electrical Bushing is generally made up of porcelain, paper and epoxy resins. They provide safe passage for the conductors connecting terminals to the winding for conducting current from one side to another.
Function of Electrical Bushing in Transformer
The main function of the electrical bushing in transformer is to isolate the terminals of the transformer from the main tank to transfer electrical energy in or out safely. It helps to connect the incoming and outgoing conductor to the winding without making contact with the main tank of the transformer.
Properties of Electrical Bushing
As electrical bushing is an insulator, it must have the properties of an insulator. Some of them are listed below.
- High resistance
- High Mechanical strength
- Non-hygroscopic
- Non-flammable
- High Dielectric strength
Construction of Electrical Bushing
The construction of Electrical Bushing is very simple as aspect of design and easy to install. It is either provided with a conductor of copper or aluminium as its part at its center for terminal from the time of manufacture or is provided with a hole at its center for the terminal.
Working of the Electrical Bushing
When the transformer is connected to the supply and load, then it resists the current passing through its conductor to flow to the main tank.
Types of Electrical Bushing
It is classified in the basis of construction, voltage, types of insulation used, inside insulation, end insulation and so on. Its classification on the basis of construction has listed below.
- Solid or Bulk type
- Capacitance-Graded or Condenser type
Its classification on the basis of Voltage has listed below.
- Low-voltage
- High-voltage
Its classification on the basis of types of insulation used has listed below.
- Porcelain insulation type
- Paper insulation type
- Resin insulation type
Its classification on the basis of inside insulation has listed below.
- Air-insulated
- Oil-insulated or oil-filled
- Oil-impregnated paper-insulated
- Cast-insulation
- Gas-insulation
- Resin-bonded or impregnated paper insulated
Its classification on the basis of End insulation has listed below.
- Air to air
- Air to oil
- Air to SF6
- Sf6 to oil
- Oil to oil
- Sf6 to air
- Oil to air
- Oil to Sf6
- Sf6 to Sf6
Explanation of Types of Electrical Bushing in short
On the basis of construction
1) Solid or Bulk Bushing
A Solid or Bulk type Electrical Bushing is consists of a central conducting rod usually of copper or aluminium encased by an insulator. The encased insulator may be of porcelain or composite resin silicon rubber.
Traditional porcelain insulator provides mechanical sturdiness and longer lifespan. The composite resin silicon rubber insulator is becoming popular due to its low cost and its surface hydrophobicity reduces the risk of pollution related flash-overs.
Due to dielectric strength limitations, its use is limited to machine rated up-to 25 kV and system voltage up-to 72kv. It is used for small distribution transformer, step-up transformer, circuit switchers, large generators, hydrogen-cooled power generators.
2) Capacitance-Graded or Condenser Bushing
A capacitance Graded or condenser bushing is formed with an inner capacitance-graded insulated core which is sandwiched between the central current carrying tube and external insulator to over-come the high electric field stresses generated at high voltage level.
The condenser core consists of co-axial layers of electric grade Kraft paper with inserted conducting foil of different lengths. Foils are inserted at the fixed radial intervals to stabilize the electric field across the insulation. These conducting foils initiate the capacitive elements that is connected in series to link the high voltage conductor to ground. That’s why it is also known as capacitance-graded bushing.
Conclusion
Hope, you have clearly understand about the Electrical Bushing of the transformer, its function, properties, types on the basis of construction, voltage, insulation type, insulation end and inside insulation. Make sure to leave your opinion and suggestion about this post in the comment section below.