Daily review: Difference Between Current Transformer & Potential Transformer | Scott-T Connection of Transformer | Operation of an 11kV to 400/230V Distribution Transformer Quiz

Q: In a Current Transformer (CT), the secondary winding is always kept closed. Why should it never be left open-circuited while the primary is energized?

The high magnetizing MMF will induce a dangerously high voltage across the secondary terminals — An open-circuited CT secondary produces high flux, leading to saturation and dangerously high induced voltages that can damage insulation.

Q: What is the primary function of the Scott-T connection of transformers?

To convert 3-phase supply to 2-phase supply — Scott-T connection utilizes two single-phase transformers (Main and Teaser) to transform a 3-phase supply into a 2-phase supply.

Q: In a 11kV/400V distribution transformer, what is the connection type commonly used for the secondary side to provide both 3-phase and single-phase supply?

Star (Wye) with a neutral point — A star-connected secondary provides a neutral point, allowing for both 3-phase (400V) and single-phase (230V) consumer loads.

Q: Which of the following statements correctly describes a Potential Transformer (PT)?

It is a step-down transformer used to measure high voltages — A PT is a step-down instrument transformer used to step down high line voltage to a safe, measurable level for meters and relays.

Q: In the Scott-T connection, what is the purpose of the 'Teaser' transformer?

To provide a voltage displaced by 90 degrees from the Main transformer — The Teaser transformer is connected to the midpoint of the Main transformer, producing a phase shift necessary for 2-phase output.

Q: Why is the core of an 11kV distribution transformer typically made of silicon steel laminations?

To minimize hysteresis and eddy current losses — Silicon steel is used to reduce magnetic losses (hysteresis) and thin laminations minimize eddy current losses, increasing efficiency.

Q: How are Current Transformers usually connected to the system?

Series with the line — CTs are connected in series with the conductor to sense the full load current passing through the line.

Q: For a Scott-T connection, what is the turns ratio requirement for the Teaser transformer compared to the Main transformer?

It must have 86.6% (?3/2) of the primary turns of the Main transformer — To maintain balanced voltages, the Teaser transformer's primary winding requires ?3/2 (approx 86.6%) of the turns of the Main transformer.

Q: What is the typical function of the 'tap changer' in an 11kV/400V distribution transformer?

To regulate the output secondary voltage — Tap changers adjust the turns ratio of the transformer to compensate for variations in input supply voltage.

Q: Which of the following is true regarding the design difference between CT and PT?

CTs have fewer turns on the primary and many on the secondary — CTs have few primary turns (series) and many secondary turns to step down current, whereas PTs have many primary turns to handle high voltage.

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Question 1 of 10

In a Current Transformer (CT), the secondary winding is always kept closed. Why should it never be left open-circuited while the primary is energized?

An open-circuited CT secondary produces high flux, leading to saturation and dangerously high induced voltages that can damage insulation.

Question 2 of 10

What is the primary function of the Scott-T connection of transformers?

Scott-T connection utilizes two single-phase transformers (Main and Teaser) to transform a 3-phase supply into a 2-phase supply.

Question 3 of 10

In a 11kV/400V distribution transformer, what is the connection type commonly used for the secondary side to provide both 3-phase and single-phase supply?

A star-connected secondary provides a neutral point, allowing for both 3-phase (400V) and single-phase (230V) consumer loads.

Question 4 of 10

Which of the following statements correctly describes a Potential Transformer (PT)?

A PT is a step-down instrument transformer used to step down high line voltage to a safe, measurable level for meters and relays.

Question 5 of 10

In the Scott-T connection, what is the purpose of the 'Teaser' transformer?

The Teaser transformer is connected to the midpoint of the Main transformer, producing a phase shift necessary for 2-phase output.

Question 6 of 10

Why is the core of an 11kV distribution transformer typically made of silicon steel laminations?

Silicon steel is used to reduce magnetic losses (hysteresis) and thin laminations minimize eddy current losses, increasing efficiency.

Question 7 of 10

How are Current Transformers usually connected to the system?

CTs are connected in series with the conductor to sense the full load current passing through the line.

Question 8 of 10

For a Scott-T connection, what is the turns ratio requirement for the Teaser transformer compared to the Main transformer?

To maintain balanced voltages, the Teaser transformer's primary winding requires ?3/2 (approx 86.6%) of the turns of the Main transformer.

Question 9 of 10

What is the typical function of the 'tap changer' in an 11kV/400V distribution transformer?

Tap changers adjust the turns ratio of the transformer to compensate for variations in input supply voltage.

Question 10 of 10

Which of the following is true regarding the design difference between CT and PT?

CTs have few primary turns (series) and many secondary turns to step down current, whereas PTs have many primary turns to handle high voltage.

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