The main function of Transformer is to increase (step up) or decrease (step down) the voltage level of AC electrical power without changing its frequency.
Since it is a static electrical device due to the absence of rotating parts, it has very high efficiency ( more than 95% ).
The basic parts of a transformer are its core, primary, and secondary windings. Apart from these, there are many different types of equipment installed in transformers that are also considered as parts of the transformer, such as its cooling arrangements, protection relay ( Buchholz relay ), HT and LT terminals and bushings, breather, conservator, oil tank, explosion vent, tap changer, etc. So let’s discuss all these different parts of the transformer and their functions in detail.
Parts of Transformer and their Functions
Following are the various parts of the transformer:
- Laminated core
- Insulating material
- Terminals and bushings
- Transformer oil
- Tap changer
- Buchholz relay
- Oil conservator
- Explosion vent
- Radiator and fans
From all the above-listed parts of the transformer, laminated soft iron core, windings, and insulating material are the basic parts of the transformer. These three are available in all types of transformers. Whereas rest of all these parts of the transformer can be seen generally in power transformers of rating more than 100 kVA. So let’s discuss each part of the transformer one by one in detail and their functions.
laminated core is the most important part of the transformer, used to support the windings of the transformer. It is made up of laminated soft iron material to reduce eddy current loss and hysteresis loss. Nowadays in the core of the transformer, laminated sheets are used to minimize eddy current losses, and CRGO steel material is used to minimize hysteresis losses. The composition of the core material depends on the voltage, current, and frequency of supply to the transformer.
The diameter of the transformer core becomes directly proportional to copper losses and inversely proportional to iron losses or core losses. A laminated core also provides a low reluctance path for the magnetic flux that minimizes leakage flux and maximizes the strength of the main working flux for the transformer.
In a transformer always two sets of windings are placed on a laminated core and these are insulated from each other. Winding consists of several no of turns of copper conductors that are bundled together and connected in series.
The main function of windings is to carry current produce working magnetic flux and induce mutual EMF for transformer action. Windings are classified in two ways:
- Based on the input and output of the supply
- Based on the voltage level of the supply
Based on the input and output of the supply, windings are further classified as:
- Primary Winding:- The winding at which the input supply is connected is known as the primary winding.
- Secondary Winding:- The winding from which output is taken to the load is known as the secondary winding.
Whereas based on the voltage level of supply, windings are further classified as:
- High Voltage (HV) Winding:- The winding that is connected with higher voltage is known as high voltage winding. It is made up of a thin copper conductor with a large no of turns. It can be either the primary or secondary winding of the transformer.
- Low Voltage (HV) Winding:- The winding that is connected with lower voltage is known as low voltage winding. It is made up of a thick copper conductor with few no. of turns. It can also be either the primary or secondary winding of the transformer. Hence input and output to the transformer can be connected either on LV-winding or HV-winding as per requirement.
Why Transformer Windings are made up of Cooper?
Transformer and all other electrical machine windings are made of good quality copper material due to these properties of copper.
- Copper is a good conductor of electricity due to its higher conductivity as compared to other materials. So this minimizes the power losses in the windings.
- Another interesting property of copper is it has higher ductility. This means it is very easy to bend the conductor into tight winding around the transformer core, which helps minimize the amount of copper needed as well as the volume and weight of copper.
Insulation failure can cause the most severe damage to the transformer. So insulation and insulating material should be high grade and it is the most important part of transformer. Insulation is required between each turn of windings, between windings, winding, and core, and all current-carrying parts and the tank of the transformer.
The main function of insulating material is to protect the transformer against short circuits by providing insulation to windings so that it does not come in contact with the core and any other conducting material. The insulating material of the transformer should have high dielectric Properties and also good mechanical strength and temperature withstand capability. Synthetic material, papers, cotton cloth, etc. are used as insulating material in transformers.
The main tank is the robust part of the transformer that serves mainly two purposes:
- It protects the core and windings from the external environment and provides housing for them.
- It is used as a container for transformer oil and provides support for all other external accessories of the transformer.
Tanks are made up of fabricated rolled steel plates. They are provided with lifting hooks and inbuilt cooling tubes. In order to minimize the weight and stray losses, aluminum sheets are also being used instead of Steel plates. However, due to its lightweight property, nowadays aluminum tank is more familiar and costly than a steel tank.
Terminals and Bushings
Terminals and bushings are also important parts of the transformer that are used to connect incoming and outgoing cables of supply and load. These are connected with the ends of the windings conductor.
Bushings are mainly insulators made up of porcelain or epoxy resins. They are mounted over the tank and form a barrier between the terminals and the tank. They provide safe passage for the conductor connecting terminals to the windings.
As windings are of two types and so bushings are also of two types as named below:
- High-Voltage Bushings
- Low-Voltage Bushings
The function of transformer oil is to provide insulation between windings as well as cooling due to its chemical properties and very good dielectric strength.
It dissipates the heat generated by the core and windings of a transformer to the external environment. When the windings of the transformer get heated due to the flow of current and losses, the oil cools down the windings by circulating inside the transformer and transferring heat to the external environment through its cooling tubes. Hydro-carbon mineral oil is used as transformer oil and acts as a coolant. It is composed of aromatics, paraffin, naphthenes, and olefins.
- On-Load Tap Changer:- In an on-load tap changer, tapping can be changed without isolating the transformer from the supply. Hence it is capable of operating without interrupting the power supply.
- Off-Load Tap Changer:- In the off-load tap changer, the transformer needs to isolate from the supply to change its tapping (turns ratio).
Buchholz relay is the most important part of a power transformer rated more than 500kVA. It is a gas-actuated relay mounted on the pipe connecting the main tank and conservator tank. The function of the Buchholz relay is to protect the transformer from all internal faults such as short circuit faults, inter-turn faults, etc.
When a short circuit occurs in winding it generates enough heat to decompose transformer oil into gases ( hydrogen, carbon monoxide, methane, etc.). These gases move toward the conservator tank through a connecting pipe, and then due to these gases, the Buchholz relay gets activated. It sends a signal to trip and alarm circuits and activates it. Then circuit breaker disconnects the transformer from the supply.
The function of the oil conservator tank is to provide adequate space for the expansion and contraction of transformer oil according to the variation in the ambient temperature of transformer oil inside the main tank.
It is a cylindrical drum-type structure installed on the top of the main tank of the transformer. It is connected to the main tank through a pipe and a Buchholz relay mounted on the pipe. A level indicator is also installed on the oil conservator to indicate the quantity of oil inside the conservator tank. It is normally half-filled with transformer oil.
The breather is a cylindrical container filled with silica gel and directly connected to the conservator tank of the transformer. The main function of the breather is to supply moisture-free fresh air to the conservator tank during the expansion and contraction of transformer oil. This is because the transformer oil when reacting with moisture can affect the insulation and cause an internal fault in a transformer. That’s why the air entering in conservator tank should be moisture-free for a better life of transformer oil.
In a breather, when air passes through silica gel the moisture present in the air is absorbed by the silica gel crystal and hence moisture-free dry air is supplied to the conservator tank. Thus we can also say that the breather is acting as an air filter for the transformer.
An explosion vent is a metallic pipe with a diaphragm at one end and installed on the main tank slightly above than conservator tank. It is available only in high-rated power transformers. The main function of the explosion vent is to protect the power transformer against explosion during excessive pressure build-up in the main tank due to severe internal faults. It acts as an emergency exit for oil and hot air gases inside the main tank of the transformer.
The explosion vent works on the same principle as the safety valve works in the pressure cooker. Hence In other words we can also call the explosion vent as safety valve of the transformer.
Radiator and Fans
Since Power losses in the transformer are dissipated in the form of heat. So a cooling arrangement is required for the power transformer. Dry-type transformers are generally naturally air-cooled. But when we talk about oil-immersed transformers several cooling methods are used depending upon kVA rating, power losses, and level of cooling required. Hence to provide proper cooling, radiators and fans are installed on the main tank of the power transformer. Radiators are also called cooling tubes.
The main function of cooling tubes or radiators is to transfer heat generated by the core and windings to the environment by circulating heated oil throughout the cooling tubes. In a large power transformer, forced cooling is achieved with the help of cooling fans fitted on the radiator.
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