Transformer Basics

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Questions and Answers

What is the primary function of a transformer in an electrical power system?

To change the voltage and current levels of the electrical power (correct)
To increase the power transmission losses
To convert AC to DC power
To change the frequency of the electrical power

What type of transformer is used to step-up the generated voltage to an appropriate level for power transmission?

Measuring transformer
Distribution transformer
Power transformer (correct)
Autotransformer

What is the primary method of cooling used for transformers with voltage ratings below 25 KV?

Oil Natural Air Natural Type (O.N.A.N.)
Air Natural Type (A.N.) (correct)
Oil Forced Air Forced Type (O.F.A.F.)
Oil Forced Water Forced (O.F.W.F.)

What is the main advantage of using transformers in electrical power systems?

To reduce the size and cost of transmission lines (B) Signup and view all the answers

Which type of cooling is used for oil filled transformers up to 30 MVA?

Oil Natural Air Natural Type (O.N.A.N.) (B) Signup and view all the answers

What is the purpose of the iron core in a transformer?

To carry the magnetic flux (A) Signup and view all the answers

What is the purpose of fans in Air Forced Type (A.F.) cooling?

To increase the rate of heat dissipation (A) Signup and view all the answers

What type of transformer is used to match the source and load impedances for maximum power transfer?

Any type of transformer can be used (C) Signup and view all the answers

Why do we need transformers in electrical power systems?

To perform three main tasks: changing voltage and current levels, matching source and load impedances, and providing electrical isolation (A) Signup and view all the answers

What is the primary purpose of providing cooling tubes in Oil Natural Air Natural Type (O.N.A.N.) cooling?

To increase the cooling area (C) Signup and view all the answers

What type of cooling is used for very large transformers with ratings of hundreds of MVA?

Oil Forced Water Forced (O.F.W.F.) (A) Signup and view all the answers

What is the difference between a single-phase transformer and a poly-phase transformer?

Single-phase transformer has one phase, while poly-phase transformer has multiple phases (C) Signup and view all the answers

What type of material is used to construct the magnetic core of a transformer?

Laminated iron (D) Signup and view all the answers

What are the two coils wound on a laminated steel core in a transformer?

Primary and secondary windings (B) Signup and view all the answers

What is the purpose of using fans in Oil Natural Air Forced Type (O.N.A.F.) cooling?

To facilitate the removal of heat from the core and winding of transformers (A) Signup and view all the answers

What is the main difference between Oil Natural Air Natural Type (O.N.A.N.) and Oil Natural Air Forced Type (O.N.A.F.) cooling?

The use of fans in Oil Natural Air Forced Type (O.N.A.F.) cooling (A) Signup and view all the answers

What is the typical voltage range for generating electric power on a large scale?

13.2 kV or somewhat higher (D) Signup and view all the answers

What is the purpose of 3-phase transformers in power transmission?

To step up the generated voltage to transmission level (B) Signup and view all the answers

What is a bank of three transformers?

Three identical units of single-phase transformers (C) Signup and view all the answers

What is the advantage of a single 3-phase transformer unit over a bank of three transformers?

It is lighter, cheaper and more efficient (C) Signup and view all the answers

What is the structure of a single-unit 3-phase core-type transformer?

A three-limbed core with one limb for each phase winding (C) Signup and view all the answers

How are the windings arranged in a single-unit 3-phase core-type transformer?

L.V. winding is placed adjacent to the laminated steel core and then H.V. winding is placed over the L.V. winding (D) Signup and view all the answers

What is the purpose of insulation in a single-unit 3-phase core-type transformer?

To provide electrical insulation between the core and L.V. winding and also between the two windings (D) Signup and view all the answers

What are the different methods available for transforming 3-phase voltages?

Various methods, including 3-phase transformer connections (D) Signup and view all the answers

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Study Notes

Transformer

A static device that transfers electrical power from one circuit to another through a magnetic field, changing voltage and current levels without changing frequency.
Works only with AC.
Main active materials: magnetic materials and conductors.
Magnetic material: laminated iron core carrying flux linked to windings.
Iron core provides a low reluctance path to magnetic flux, reducing magnetizing current.

Importance of Transformers

Change voltage and current levels in electrical power systems.
Step up generated voltage for power transmission.
Step down transmission voltage for distribution and power utilization.
Match source and load impedances for maximum power transfer.
Provide electrical isolation between circuits.

Transformer Classification

By number of phases:
Single-phase transformer
Poly-phase transformer
By function:
Power transformer
Distribution transformer
Measuring transformers
- Voltage transformer
- Current transformer
Autotransformer - Tapped autotransformer

Power Transformer

Connected to a generator's output to step up voltage to transmission level.
Higher voltage and lower current reduce transmission line size and cost, and transmission losses.

Cooling Methods for Transformers

For dry type transformers (below 25KV):
Air Natural Type (A.N.)
Air Forced Type (A.F.)
For oil immersed transformers:
Oil Natural Air Natural Type (O.N.A.N.)
Oil Natural Air Forced Type (O.N.A.N.)
Oil Forced Air Forced Type (O.F.A.F.)
Oil Forced Water Forced Type (O.F.W.F.)

Essential Elements of a Transformer

Two coils with mutual inductance wound on a laminated steel core: primary and secondary windings.

Three-Phase Transformers

Step up or step down transformation requires:
Three identical single-phase transformers (a bank of three transformers)
A single 3-phase transformer unit wound on a single 3-legged core (lighter, cheaper, and more efficient)
Single-unit 3-phase core-type transformer uses a three-limbed core, one limb for each phase winding.

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