Questions and Answers
With respect to the double-wound transformer, what is a major disadvantage of an auto-transformer?
What are FOUR major advantages of auto-transformers? (Select one or more)
The primary of a potential/instrument transformer is connected:
To ensure safe removal of a current transformer you must:
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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.
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A current transformer is a type of instrument transformer. What is a current transformer most commonly used for?
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Instrument transformers are used in test laboratories.
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The core of a current transformer is typically:
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In installing a current transformer, what TWO circuit components are connected in series with the CT primary conductor? (Select one or more)
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What does a rating of 300:5 mean in relation to a current transformer?
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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?
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A 1000kVA load is supplied from a three-phase four-wire 11,000 V line. What type of CT and PT are suitable?
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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.
