The basic construction of transformer is shown in figure. 1 phase or 3 phase transformer mainly consists of the following:
- Magnetic circuit
- Electric circuit
- Dielectric circuit
- Transformer Tank and Accessories
The magnetic circuit of a transformer is made of cores which provides the path to the flow of magnetic flux. The alternating flux flow through magnetic core and it causes two type of losses, one is Hysteresis and another one is Eddy current loss. For magnetic circuit basically two type of construction is prefer as core type or Shell type.
The electric circuit of a transformer is made of primary and secondary windings, primary connected to the supply and secondary connected to the load.
The winding of transformer are usually made of copper but recently aluminium conductors also are being used. Alluminium is cheaper in cost as compare to copper, so transformer of alluminium winding is also cheaper in cost.
The dielectric circuit is made of insulation, which is used at different places in the transformer for insulating the conducting and the magnetic parts from each other.
Core and transformer winding is insulated from each other with the help of Bakelite or paper. Same material is used to separate high voltage and low voltage windings.
Transformer Tank and Accessories:
In construction of Transformer main part is tis accessories. These are explain below one by one.
Steel tank of Transformer
Whole assembly of transformer is placed in a fabricated mild steel tank housing. Inside this housing core and winding assembly of transformer is immersed in oil which is used as an insulator as well as coolant.
Remaining essential accessories are connected with tank is shown in figure and explain in detail bellow.
Breather of transformer
Transformer take breath with the help of breather. In large transformer oil is present inside the tank and temperature of this oil change with the loading operation. With change in temperature, the volume of transformer oil is also change. This cause change in volume of air in conservator tank and in this operation air is in and out.
If air contain moisture then this moisture is come in contact with transformer oil and it cause deterioration of oil takes place. The moisture in the oil is reduce the dielectric strength of transformer oil.
The main function of breather is to arrest the moisture in air before entering inside the transformer tank. For this operation silica gel is mainly prefer in breather. The colour of fresh dry silica gel is Blue and colour of moisture absorb gel is in pure white or light pink colour.
Silica gel Breather is a protective device which is fitted with the conservator through the hollow pipe.
Explosion vent is fitted on top side of transformer as a pressure release device. The end side of explosion pipe is tightly closed using thin glass or laminated sheet.
When transformer is over heated due to overloading or short circuit, inside the transformer tank high pressure is created which may damage the tank.
Pressure created inside the transformer tank may break the thin glass or laminated sheet of the explosion vent. Thereby tank can saved from severe damage.
Cooling Tubes / Radiator
When the construction of transformer is connected with supply and load, iron loss and copper loss is takes place in it. This loss increase the temperature of transformer oil.
Heat generated inside the transformer should be radiated to the atmosphere to keep the oil temperature down.
This high temperature oil come in cooling tube and dissipate heat to atmosphere by surface contact with natural air.
It is a drum like, mounted on the top of the transformer. An oil level indicator is fitted with the conservator tank. Conservator is connected to the transformer tank through a pipe and in between buchholz relay is present.
The conservator tanks carries the transformer oil up to a specified level. When the transformer temperature is increase due to loading operation, the oil expands and the level of oil in conservator tank is increased and vice versa.
A pipe connected with the conservator tank above the transformer oil level, it allow the internal air to out or in through the breather.
Buchholz relay is a protective device which is connected between the transformer oil tank and the conservator tank. This relay is consist of cast iron chamber with two floating mercury contact switches.
One switch is used for gas release alarm when minor fault occurs, which operate on simple floating mechanism. And another one is used for short circuit fault alarm and which is operated on oil pressure release.
Bushings of Transformer
Transformer Bushings are provided with each input and output terminals of transformer to provide an electrical isolation for the winding terminals and material that present near to the conductors.
A bushing is a hollow cylindrical shaped arrangement made by porcelain material discs which is fitted to the top portion of the transformer. And the energized conductors are passed through the center of the bushings.
Porcelain offers very good and reliable electrical insulation for high voltages and also they have high dielectric strength.
It is just a temperature indicating instrument, which fitted to the transformer to indicate the temperature of the transformer oil for monitoring.
This is the basic construction of transformer which is explain above in detail. And Some important key points on transformer are listed below.
Important Key Points for Transformer
- Core lamination are typically 0.35 mm to 0.5 mm.
- Synthetic enamel insulation is used for conductor.
- Paper or Bakelite Insulation is used for two winding and winding – core separation.
- In transformer Mineral oil (naphthenic, silicone and bio-based) is used.
- Bushings are tightly sealed with glaze.
- Buchholz relay is a gas operated Relay.