Different Types of Transformer

There are different types of transformers based on the application, nature of use and number of phases. Based on the application, transformers are widely used in the electric power system to transmit the electric power between power plants, substations, industries and domestic customers.

Before going into the types of transformers, Do you know how it is constructed? Do you know how a transformer works? Learn about the Construction And Working Principle Of Transformer.

What are the different types of transformers?

The different types of transformers based on different classifications are shown in the below figure.

Based on Turns ratio

Based on the turns ratio or voltage levels, transformers are classified into step-up transformer, step down transformer and isolation transformer

Step up transformer

The Step-up transformer increases the voltage level at the secondary side. The low primary voltage is boosted up by adding more number of turns in the secondary winding. The ratio of the primary winding to secondary winding is less than 1.

Turns ratio in a transformer: The ratio of the number of turns in the primary winding in a transformer to the number of turns in the secondary winding is called as turns ratio.

This type of transformer is used in an electric power system for transmitting the power over the transmission line. It is also used in applications where high voltage is required than the rated voltage.

Step down transformer

The step-down transformer decreases the voltage level at the secondary side. The high primary voltage is reduced to a low secondary voltage by adding less number of turns in the secondary winding. The ratio of the primary winding to secondary winding is greater than unity.

Step-down transformers are used in electric power systems for the distribution of power to consumers. It is employed in electrical and electronic appliances where the voltage required is 5V, 10V, 12V and so on.

Isolation Transformer

It is similar to the normal transformer, but it isolates the electrical devices from the power supply. It is used as a protection device for the electrical circuits on the secondary side. The primary and secondary windings will have an equal number of turns. For isolation transformer, the turns ratio is 1.

They are used in many measurement devices, pulse circuits, testing equipment, power quality applications, etc.

Based on construction

Core type, shell type, and berry type transformers are the different classification of transformer based on the construction of the transformer.

Core type Transformer

In this type, the windings or coils are wounded around the two limbs of the rectangular core in a single-phase transformer. In a three-phase transformer, the windings are wounded around the three limbs in the three-phase transformer.

Each limb holds one-half of the primary winding and one-half of the secondary winding so as to reduce the leakage reactance.

The LV winding is wound on the inside nearer to the core and the HV winding is wound over the LV winding, which is slightly away from the core.

Since the windings surround a considerable part of the core, this transformer is called a core type transformer.

Shell Type Transformer

In this type, the coils are wound on the central limb of a three-limb core. The entire flux passes through the central limb and divides into two parts, which go to the side limbs.

The advantage of a shell-type transformer is the core is being used to protect the winding from mechanical damage. It provides better support against electromechanical forces.

Core type of transformer holds more number of turns and provides more space for insulation. Hence they are best suited for extra-high voltage applications. But shell type of transformer is commonly used for construction of small transformers. Also, core type transformer permits visual inspection of coils in the case of fault and ease of repair at substation site.

However, the selection of core type transformer and shell type transformer is decided by more factors like weight, voltage rating, KVA rating, etc

Berry type Transformer

The core of this transformer looks like the spokes of the wheel. They are fitted in tight metal sheet tanks, which are then housed in an oil tank. The operation of this transformer is similar to that of the normal transformer.

Based on the number of phases

Based on the number of phases, transformers are classified into single-phase transformers and three-phase transformers.

Single phase transformer

The transformer that is operated with the single-phase supply is called a single-phase transformer. This type of transformer has two windings, one at the primary side and the other on the secondary side. Such transformers are used in applications where a single-phase supply is used.

Single phase transformers are used in electronic devices, communication systems, voltage regulators, mobile chargers, etc.,

Three phase transformer

The transformer that is operated with the three-phase supply is called a three-phase transformer. They have three windings on the primary side and three windings at the secondary side. It is similar to three single-phase transformers connected together to form a three phase transformer.

The three phase transformer is used in an electric power system for transmitting the power over long distances.

Auto transformer

In an auto transformer, the primary and secondary windings are electrically connected as a single winding. This winding is wound over a common core and a part of it is common to both sides. But in a normal transformer, both sides have separate winding. The circuit diagram of auto transformer is shown below.

Auto transformers are used in induction motor starters, variable voltage applications, laboratory purposes, interconnection of HV systems, etc.

Based on the type of services offered

Based on the type of services offered or application, the transformers are classified into power transformer, distribution transformer and instrument transformer.

Power Transformer

The power transformers are used for power rating more than 250 KVA. They are employed in generating stations for transforming the voltage at every end of the transmission line. They may be either three-phase three-wire delta-delta or delta-star connected.

Power transformers are operated in parallel and put in operation during peak hours. During the light load hours, the transformers are disconnected depending upon the load.

Power transformer is designed to have maximum efficiency at full load condition. These transformers may be self-oil cooled, forced cooled or forced water-cooled type.

Distribution Transformer

Distribution transformers are used for rating up to 200 KVA. It is used to step down the voltage suitable for domestic applications. They are usually three phases, four wire delta-star connected.

They are operated for 24 hours a day whether they are carrying any load or not. Because of such operation, iron loss occurs always but the copper loss will be there, whenever the transformer is loaded.

So, distribution transformer is designed with lower iron loss compared to the full load copper loss. The distribution transformer will have good All day efficiency but they don’t have more efficiency at full load. These transformers may be self-cooling type and are almost oil-immersed type.

Instrument Transformer

Instrument transformers are used to measure electrical quantities like current, voltage, power, frequency, power factor, etc. The great advantage of this transformer is that the transmission line parameters can be measured with small rating meters.

Instrument transformer are classified into two types:

• Current transformer : It is used to step down the current of a transmission line to a lower level to make it feasible to be measured by small rating Ammeter (say 5A).
• Potential transformer : It is used to step down the voltage of a transmission line to a lower level to make it feasible to be measured by small rating voltmeter(say 120V).

Based on the type of cooling

The huge-sized power transformer requires cooling mechanism to radiate the heat produced in the core and windings. Based on the cooling methods provided to the transformer, they are classified into several types.

Oil filled air cooled transformer

The transformers on the distribution side are kept in a transformer tank filled with oil. No special cooling system is provided in this medium-sized transformer. The air around the transformer act as a cooling medium.

The heat produced by the core and windings are radiated out of the transformer tank through the insulating oil. A distribution transformer is a good example of oil filled air-cooled transformer.

Oil filled water cooled transformer

For a transformer of high rating, an air cooling system is not economical. Temperature will increase in the insulating oil due to overheating of the core and windings.

This rise in temperature should be cooled down to a safe value in order to operate the transformer in a long run. So, water cooling system is provided to such transformer to remove the heat produced. Cooling tubes are installed near the surface of the oil and cool water is circulated to remove the heat in the insulating coil.

The transformers used for transmitting voltage over the transmission line uses oil-filled water-cooled transformers.

Air blast type transformer

As the name says, this type of transformer is cooled by circulating a blast of cool air with an external fan. This forced air removes the heat from the core and windings. While using fans, filters are used to prevent the accumulation of dust particles in the ducts. High-rated transformers of more than 3 KVA uses air blast type of cooling.

FAQS