A Three Phase Transformer is an important electrical device used to step up or step down voltage in three-phase power systems. These systems are commonly used in industrial and commercial settings due to their efficiency in power distribution. A three-phase transformer either comprises three single-phase transformers connected together or is constructed as a single unit with three windings on a common magnetic core.

Electric power generation, transmission, distribution, and consumption have evolved significantly over the years. Today, generation and transmission of power primarily rely on three phase systems as they are more efficient and economical compared to single phase systems. At the heart of any three phase power system lies the three phase transformer. 

In this article, we will discuss three phase transformers in detail including their working principle, construction, connections, applications, and advantages over single phase transformers. The information in this article helps you extensively in your SSC JE Electrical and GATE Electrical preparation journey.

What is a Three Phase Transformer?

A three phase transformer is a special transformer that is used in three phase electric power systems. It consists of three single phase transformer units built into a single core with each unit responsible for one phase. Each single unit has a primary and secondary winding associated with it. All three units share a common magnetic core.

• Three phase transformers can be constructed in two ways - using three individual single phase transformer cores placed together or using a single core with all three windings placed together.

Faraday‘s Law of Induction

The working of a three-phase transformer is based on Faraday's law of induction which states that a change in the magnetic field around a conductor will induce voltages in it. In a transformer, the primary winding is connected to an alternating three phase supply. When current flows through the primary, it produces an alternating magnetic field around the core. 

Fig- Electromagnetic Induction Principle

This changing magnetic field induces voltages in the secondary winding wound around the same core (working principle of three phase transformer). The transformer safely transfers power between the primary and secondary circuits through electromagnetic induction 

Three phase transformers are commonly built using one of the following two types of constructions:

Types of three phase transformer:

Different types of three phase transformer diagram are given below:

Core-Type Transformer

 All three limb cores are assembled together in an interesting magnetic circuit with primary and secondary windings wound on them. This type is preferred for higher power ratings.

Fig- Core type Transformer

Shell-Type Transformer

 In this, the primary and secondary windings of the three single phase transformer units are placed in separate limb cores inside a steel shell structure. This type is used in lower power ratings.

Fig- Shell-type Transformer

Parts of Three Phase Transformer

Other main parts of a three phase transformer include:

• Insulating oil/gas: Used as an insulating and cooling medium.
• Windings: made of copper or aluminum and insulated properly.
• Core: Made of high-grade silicon-coated steel laminations to reduce eddy current losses.
• Conservator tank: Houses the insulating oil and maintains the required oil level.
• Breather: Allows air in/out and protects from pressure variations.
• Cooling system: Can be oil, air, or forced water-cooled based on power rating.
• Bushings: Insulated entries for high voltage bushing leads.
• On load tap changer: Helps regulate the output voltage.
• Temperature sensors: Monitors hot-spot temperature of winding.

The choice of construction depends on factors like power rating, reliability, efficiency, overall size, and cost (three phase transformer construction). 

Core-type offers better cooling, but shell-type is more versatile in the use of core materials.

Working of Three Phase Transformers

When a three phase supply is connected to the primary winding of a three phase transformer, it produces an alternating flux in the core. According to Faraday's law of induction, this alternating flux induces voltages in the secondary windings (working principle of three phase transformer).

The primary and secondary voltages are directly proportional to their respective number of turns as given, 

Due to properties of three phase supply, the induced voltages in the three secondary windings are balanced in magnitude and equally displaced in space by 120°. They can supply power to three-phase equipment suitably.

Transformer Power and Efficiency

The power rating of a three phase transformer depends on factors like - 

the volt-ampere (VA) rating, cooling method used, frequency, temperature rise limits, and insulation class.

The efficiency of a transformer is given as -

It is typically above 95% for distribution transformers due to reduced losses with quality core material, optimized winding design, and proper cooling. Three phase transformers have better efficiency compared to single phase ones

Three Phase Transformer Connections

Three phase transformers can have various winding arrangements depending on the application. The common connection types are (three phase transformer connections):

• Star-Star (Y-Y): Used when balanced three-phase loads are connected at primary and secondary sides.
• Star-Delta (Y-Δ): Used for stepping down voltages generally.
• Delta-Star (Δ-Y): Preferred for stepping up voltages mainly.
• Delta-Delta (Δ-Δ): Used when the neutral is not required and all three phases are to be insulated.

Proper configuration is chosen to get the desired voltage level, current rating, and phase shift between primary and secondary windings. The connections impact parameters like voltage, current, and power rating of each phase.

Three Phase Transformer Windings

A three phase transformer made of six windings, three for the primary and three for the secondary. The windings on each side (primary and secondary side) can be connected in either star or delta configurations. These windings can be viewed as separate single-phase windings. 

Fig- Delta-Star connection of three phase transformer

In theory, To create a three-phase transformer, we use three single-phase transformers.

Three Phase Voltages and Currents

VL is the line-to-line voltage, and VP is the phase-to-neutral voltage on either the primary or the secondary side. Other possible connections for three phase transformers are star-delta Yd, where the primary winding is star-connected and the secondary is delta-connected or delta-star Dy with a delta-connected primary and a star-connected secondary.

Delta-star connected transformers are widely used in low power distribution with the primary windings providing a three-wire balanced load to the utility company while the secondary windings provide the required 4th-wire neutral or earth connection.

Where,

n equals the transformers “turns ratio” (T.R.) of the number of secondary windings NS, divided by the number of primary windings NP. ( NS/NP ) and VL is the line-to-line voltage with VP being the phase-to-neutral voltage.

Advantages of a Three Phase Transformer

Advantages of Three Phase Transformers include:

• They are smaller, compact, and more economical compared to equivalent single-phase transformers.
• Losses are lower in three-phase transformers due to the balanced loading of the core.
• Reliability is better in three-phase transformers since failure of one phase does not pull out the whole transformer.
• Their construction using a single core makes them more robust and durable compared to individual single-phase units.
• Can support sensitive loads requiring power quality like variable speed drives.
• Stepping up/down of voltages can be achieved efficiently using delta/star connections respectively.

Some disadvantages include slight complexity in protection and control compared to single-phase ones. But overall, three phase transformers are a key building block of modern power systems.

Application of Three Phase Transformer

Some common applications of three-phase transformers include -

• Power generation: Step-up transmission voltages from generators to transmission lines
• Transmission and distribution networks: Matches voltages between substations, distribution, and consumer levels.
• Industrial applications: Supports large three-phase motors, variable speed drives, welding systems, etc.
• Commercial facilities: Providing three-phase supplies to malls, apartments, offices, etc.
• Transportation systems: Power railways, metros, electric vehicles charging infrastructure.

They form a crucial link between different voltage levels in power systems and enable reliable power transfer matching supply to demand. With advances in materials, their performance is continuously improving.

Conclusion

In this detailed article, we discussed the working, construction, connections, types, advantages and applications of three phase transformers which are the backbone of modern power infrastructure. Their balanced and efficient operation has revolutionized power systems. Aspiring electrical engineers must have a solid understanding of three phase transformer fundamentals and configurations to pursue careers in power utilities, equipment manufacturing or other relevant fields. The three-phase principle brings unmatched benefits in generation, transmission and use of electric power.

This article summarises all the information related to Three Phase Transformer, which helps in propelling your preparation for various AE/JE and ESE examinations. You can visit the Testbook app to keep yourself updated with all the exam-oriented information related to the upcoming examination, like SSC JE, GATE, ESE, RRB JE, and state AE/JE Exams.