Transformers - 4 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE) PDF Download

THREE PHASE TRANSFORMER
• When three identical units of single-phase transformers are used, the arrangement is commonly called a bank of three transformers or a three- phase transformer bank.
• Three phase current in three primaries produce three-phase fluxes mutually time-displaced by 120°. These fluxes flow through their respective
yokes and then through the central limbs placed together. The resultant flux in the three central limbs must be zero.

Core Type Transformer

Transformers - 4 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

Three-phase core-type transformer from three single-phase units
• The reluctance offered to the flux of central limb II is less than the reluctance offered to the outer two fluxes produced in legs I and III. Consequently, the exciting current for the phase winding on the central limb is less than it is for the outer two phase windings. Shell Type Transformer
• A three- phase shell type transformer is obtained if three single-phase shell type cores are placed side by side.

Transformers - 4 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

Three-phase shell type transformer with three windings wound in the same direction
• In the magnetic circuits marked 2 and 3, the resultant flux is seen to be equal to the phasor difference of the component fluxes.

Transformers - 4 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

• For the same flux density, the cross-sectional areas at 2 and 3 are 86% of the central core area.

parallel Operation of 3-ø Transformers
• The various conditions that must be fulfilled, for the successful parallel operation of 3-phae transformers, are as follows:
• The line voltage rations of the transformers must be the same.
• The transformers should have equal per unit leakage impedances.
• The ratio of equivalent leakage reactance to equivalent resistance should be same for all the transformers.

• The transformers should have the same polarity.
• In addition to these four conditions, two more essential conditions that must be fulfilled for the parallel operation of three-phase transformers are as follows:

 Relative phase displacement: The relative phase displacement between the secondary line voltage of all the transformers must be zero i.e. the transformers to be connected in parallel, must belong to the same group number.

Transformers - 4 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

Transformers - 4 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

Note:
• Transformers for group numbers 3 and 4 can be successfully operated in parallel.
• Phase Sequence: An improper phase sequence as shown in figure (b) below, would give zero voltage across switch S1 and line voltages across switch S2 and S3. Consequently the parallel operations is not possible.

Transformers - 4 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

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FAQs on Transformers - 4 - Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

1. What is an electrical transformer?
Ans. An electrical transformer is a device that transfers electrical energy between two or more circuits through electromagnetic induction. It is used to increase or decrease the voltage of an alternating current (AC) power supply.
2. How does a transformer work?
Ans. A transformer works on the principle of electromagnetic induction. It consists of two coils, known as the primary and secondary winding, which are wrapped around a core. When an alternating current flows through the primary winding, it creates a changing magnetic field in the core. This changing magnetic field induces a voltage in the secondary winding, resulting in the transfer of electrical energy from the primary to the secondary circuit.
3. What are the different types of transformers?
Ans. There are several types of transformers used in electrical engineering. Some common types include power transformers, distribution transformers, autotransformers, instrument transformers, and isolation transformers. Each type is designed for specific applications and has different voltage and power ratings.
4. What are the main components of a transformer?
Ans. The main components of a transformer include the core, primary winding, secondary winding, and insulation. The core is typically made of laminated iron or steel to minimize energy losses. The primary and secondary windings are coils of insulated wire, which are wound around the core. Insulation is used to prevent short circuits and maintain electrical isolation between the windings.
5. What are the advantages of using transformers in electrical systems?
Ans. Transformers offer several advantages in electrical systems. They allow for efficient transmission of electrical power over long distances by stepping up the voltage at the generation side and stepping it down at the distribution side. Transformers also provide electrical isolation between different circuits, ensuring safety and protection against electric shocks. Additionally, they help regulate voltage levels, improve power quality, and reduce energy losses in the transmission and distribution networks.
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