EL25 Topic Assessment 2: Auto-transformers

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Questions and Answers

With respect to the double-wound transformer, what is a major disadvantage of an auto-transformer?

Auto-transformer efficiency is the lowest of all the transformer types

Auto-transformer voltage regulation is poor

Auto-transformers present a high risk to both people and equipment when stepping down a high voltage to a low voltage (correct)

Auto-transformers have much higher copper loss

What are FOUR major advantages of auto-transformers? (Select one or more)

They have a smaller size than an ordinary transformer of the same rating (correct)

They have a larger short circuit current

They have better voltage regulation and operate at a higher efficiency than a 2-winding transformer of the same rating (correct)

They are safer than most other transformers

They work on both AC and DC

They require a smaller magnetising current than a 2-winding transformer of the same rating (correct)

They require less copper than an ordinary transformer of a similar rating (correct)

They are useful for high voltage changes

The primary of a potential/instrument transformer is connected:

In order of size

On top of each other

In parallel with the supply (correct)

In series with the supply

To ensure safe removal of a current transformer you must:

Short the secondary winding Signup and view all the answers

Determine the actual values of voltage and current that would be applied to the meters if the line voltage was 10,950V and the line current was 140A.

109.5V, 3.5A Signup and view all the answers

What is a current transformer most commonly used for?

Metering purposes Signup and view all the answers

Instrument transformers are used in test laboratories.

True Signup and view all the answers

The core of a current transformer is typically:

A continuously wound lamination that is toroidal in shape Signup and view all the answers

In installing a current transformer, what TWO circuit components are connected in series with the CT primary conductor? (Select one or more)

The load Signup and view all the answers

What does a rating of 300:5 mean in relation to a current transformer?

300 primary amps at full load will produce 5 amps of secondary current when 300 amps flow through the primary winding Signup and view all the answers

Select a suitable current and potential transformer for a load connected to a three-phase, three-wire 11000/415V line-to-line potential requiring measurement of 3000 kVA.

Current transformer with 200:5 ratio and potential transformer with 100:1 ratio Signup and view all the answers

Select a suitable current and potential instrument transformer for a 1000kVA load supplied from a three-phase four-wire 11000V line.

Current transformer with 75:5 ratio and potential transformer with 100:1 ratio Signup and view all the answers

Study Notes

Auto-Transformers

Major disadvantage of auto-transformers: high risk to safety and equipment when reducing high voltage to low voltage.
Advantages of auto-transformers include:
- Less copper required than conventional transformers of similar ratings.
- Suitable for high voltage changes.
- Lower magnetizing current needed compared to 2-winding transformers.
- Smaller size compared to ordinary transformers of the same rating.

Instrument Transformers

Potential/instrument transformers are connected in parallel with the supply.
Safe removal procedure for current transformers involves shorting the secondary winding.
Instrument transformers are utilized in test laboratories for accurate measurements.

Current Transformers

Most common use of current transformers: metering applications.
Core construction often involves a continuously wound toroidal lamination.
Installing a current transformer requires connecting the load supply cable and the load in series with the CT primary conductor.

Current Transformer Ratings

A rating of 300:5 indicates that 300 primary amps yield 5 amps of secondary current.
Suitable CT and PT selections for different applications include:
- For a 3000 kVA load, a CT with a 200:5 ratio and a PT with a 100:1 ratio is appropriate.
- For a 1000 kVA load, a CT with a 75:5 ratio and a PT with a 100:1 ratio is ideal.

Exam Performance

Achieved full marks (100%) in assessment, demonstrating proficiency in the subject matter related to auto-transformers and instrument transformers.

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Description

Test your knowledge on auto-transformers and instrument transformers with this assessment. The quiz is designed to evaluate your understanding of key concepts and applications in transformer technology. Achieve a perfect score and reinforce your learning in this important topic.