Parts of a Power Transformer
What Is a Transformer?
A transformer is an electrical device that transfers electrical energy from one circuit to another by electromagnetic induction (also called transformer action). It is most often used to step up or step down voltage levels.
Components of a Transformer
Basic Parts of a Transformer
These are the basic components of a transformer.
- Laminated core
- Insulating materials
- Transformer oil
- Tap changer
- Cooling tubes
- Buchholz Relay
- Explosion vent
Of the above, the first four are found in almost all the transformers, whereas the rest are found only in transformers that are more than 50 KVA.
The core is used to support the windings in the transformer. It also provides a low reluctance path to the flow of magnetic flux. It is made of laminated soft iron core in order to reduce eddy current loss and Hysteresis loss. The composition of a transformer core depends on such factors as voltage, current, and frequency. The diameter of the transformer core is directly proportional to copper loss and is inversely proportional to iron loss. If the diameter of the core is decreased, the weight of the steel in the core is reduced, which leads to less core loss of the transformer and the copper loss increase. When the diameter of the core is increased, the opposite occurs.
Why Are Windings Made of Copper?
- Copper has high conductivity. This minimizes losses as well as the amount of copper needed for the winding (volume & weight of winding).
- Copper has high ductility. This means it is easy to bend conductors into tight windings around the transformer's core, thus minimizing the amount of copper needed as well as the overall volume of the winding.
There are two windings wound over the transformer core that are insulated from each other. Windings consists of several turns of copper coils bundled together, and each bundle is connected in series to form a winding.
Windings can be classified in two different ways:
- Based on the input and output supply
- Based on the voltage range
Within the input/output supply classification, windings are further categorized:
- Primary windings - These are the windings to which the input voltage is applied.
- Secondary windings - These are the windings to which the output voltage is applied.
Within the voltage range classification, windings are further categorized:
- High voltage winding - These are made of copper coil. The number of turns is the multiple of the number of turns in the low voltage windings. The copper coils are thinner than those of the low voltage windings.
- Low voltage windings - These have fewer turns than the high voltage windings. It is made of thick copper conductors. This is because the current in the low voltage windings is higher than that of high voltage windings.
Transformers can be supplied from either low voltage (LV) or high voltage (HV) windings based on the requirement.
Insulating paper and cardboard are used in transformers to isolate primary and secondary windings from each other and from the transformer core.
Transformer oil is another insulating material. Transformer oil can actually have two functions: in addition to insulating it can also work to cool the core and coil assembly. The transformer's core and windings must be completely immersed in the oil. Normally, hydrocarbon mineral oils are used as transformer oil. Oil contamination is a serious problem because contamination robs the oil of its dielectric properties and renders it useless as an insulating medium.
Parts of the Transformer
The conservator conserves the transformer oil. It is an airtight, metallic, cylindrical drum that is fitted above the transformer. The conservator tank is vented to the atmosphere at the top, and the normal oil level is approximately in the middle of the conservator to allow the oil to expand and contract as the temperature varies. The conservator is connected to the main tank inside the transformer, which is completely filled with transformer oil through a pipeline.
The breather controls the moisture level in the transformer. Moisture can arise when temperature variations cause expansion and contraction of the insulating oil, which then causes the pressure to change inside the conservator. Pressure changes are balanced by a flow of atmospheric air in and out of the conservator, which is how moisture can enter the system.
If the insulating oil encounters moisture, it can affect the paper insulation or may even lead to internal faults. Therefore, it is necessary that the air entering the tank is moisture-free.
The transformer's breather is a cylindrical container that is filled with silica gel. When the atmospheric air passes through the silica gel of the breather, the air's moisture is absorbed by the silica crystals. The breather acts like an air filter for the transformer and controls the moisture level inside a transformer. It is connected to the end of breather pipe.
The output voltage may vary according to the input voltage and the load. During loaded conditions, the voltage on the output terminal decreases, whereas during off-load conditions the output voltage increases. In order to balance the voltage variations, tap changers are used. Tap changers can be either on-load tap changers or off-load tap changers. In an on-load tap changer, the tapping can be changed without isolating the transformer from the supply. In an off-load tap changer, it is done after disconnecting the transformer. Automatic tap changers are also available.
Cooling tubes are used to cool the transformer oil. The transformer oil is circulated through the cooling tubes. The circulation of the oil may either be natural or forced. In natural circulation, when the temperature of the oil rises the hot oil naturally rises to the top and the cold oil sinks downward. Thus the oil naturally circulates through the tubes. In forced circulation, an external pump is used to circulate the oil.
The Bochholz Relay is a protective device container housed over the connecting pipe from the main tank to the conservator tank. It is used to sense the faults occurring inside the transformer. It is a simple relay that operates by the gases emitted due to the decomposition of transformer oil during internal faults. It helps in sensing and protecting the transformer from internal faults.
The explosion vent is used to expel boiling oil in the transformer during heavy internal faults in order to avoid the explosion of the transformer. During heavy faults, the oil rushes out of the vent. The level of the explosion vent is normally maintained above the level of the conservatory tank.
More About Transformers
I have written a series of articles to help the reader understand power transformers. I've listed two here, and if you are interested in finding more, you can find them by clicking on my author profile at the top of this article.
How Does a Transformer Work - Basic working principles of the transformer.
Classification of Transformers - The various classifications of transformers and how they differ from each other.
More by this Author
Buchholz relay is a type of protection relay universally used on all oil immersed transformers having rating more than 500 kVA. From this article its principle and working can be understood.