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

Auto-transformer voltage regulation is poor

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

Auto-transformers have much higher copper loss

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

They are useful for high voltage changes (correct)

They work on both AC and DC

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

They have a larger short circuit current

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

They are safer than most other transformers

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

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

The primary of a potential/instrument transformer is connected:

In parallel with the supply (correct)

On top of each other

In series with the supply

In order of size

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

A current transformer is a type of instrument transformer. 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

A load connected to a three-phase, three-wire 11,000/415 V line-to-line potential requires measurement. It is estimated to be 3000 kVA. What type of CT and PT are suitable?

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

A 1000kVA load is supplied from a three-phase four-wire 11,000 V line. What type of CT and PT are suitable?

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

Study Notes

Auto-transformers

Higher copper loss is a major disadvantage compared to double-wound transformers.
Auto-transformers require less copper than ordinary transformers, making them more efficient.
Suitable for high voltage applications, they enhance safety, consume less magnetising current, and operate with better voltage regulation.

Instrument Transformers

Potential transformers are connected in parallel with the supply to measure voltage accurately.
Current transformers (CTs) are primarily used for metering purposes in electrical systems, enabling safe measurement of high currents.
Safety measures include shorting the secondary winding for safe removal of a current transformer during maintenance.

Technical Specifications

A 300:5 rating in current transformers indicates that when 300 amps flow through the primary winding, 5 amps will be produced in the secondary circuit.
Common configurations involve series connections of load supply cable and the load itself with the primary conductor of a CT.

Questions & Answers Overview

Correct identification of voltage and current values in transformer setups shows effective understanding; for example, a line voltage of 10,950V translates to monitoring values of 109.5V and 3.5A.
For a three-phase 11,000V line supplying a 3000 kVA load, appropriate transformers must be selected; a suitable combination is a current transformer with a 200:5 ratio and a potential transformer with a 100:1 ratio.

Practical Applications

Instrument transformers are vital in testing laboratories to create safe test environments while dealing with high voltage systems.
Select transformers based on specific load requirements and ensure precise metering by matching ratios according to intended usage (e.g., 75:5 ratio for a 1000 kVA load).

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Description

This quiz assesses your understanding of auto-transformers and instrument transformers. It covers key concepts and applications associated with these electrical devices, designed for students of the EL25 course. Prepare to demonstrate your knowledge and skills in this fundamental topic of transformer technology.