Q1. In a three-phase transformer, the main advantage of using a single three-phase unit over three single-phase units is:
A. Higher efficiency
B. Better voltage regulation
C. Reduced cost, weight and space
D. Easier transportation
Correct Option: C
Explanation:
A single three-phase transformer uses a common magnetic core for all phases, reducing copper, iron, oil, and tank requirements. This makes it more economical, compact, and lighter compared to three separate single-phase units, while efficiency remains nearly the same.
Q2. Which connection is preferred for a three-phase transformer when neutral grounding is required on both sides?
A. Δ–Δ
B. Y–Y
C. Δ–Y
D. Y–Δ
Correct Option: B
Explanation:
In Y–Y connection, both primary and secondary are star-connected, providing a neutral point on both sides. This allows grounding of neutrals for system protection and stability.
Q3. The major drawback of Star–Star (Y–Y) connection is:
A. Low efficiency
B. Circulating current
C. Third harmonic voltages
D. Poor cooling
Correct Option: C
Explanation:
In Y–Y connection, there is no closed path for third harmonic currents. As a result, third harmonic voltages appear in phase voltages, causing waveform distortion and neutral instability.
Q4. In a Y–Y connected transformer, third harmonic currents:
A. Flow in line conductors
B. Flow in neutral only
C. Cannot flow and cause waveform distortion
D. Improve voltage regulation
Correct Option: C
Explanation:
Third harmonic currents require a closed loop. In Y–Y connection without delta or neutral path, such a loop is absent, so harmonics cannot flow and instead distort the voltage waveform.
Q5. Which transformer connection is most suitable for heavy industrial loads?
A. Y–Y
B. Δ–Δ
C. Y–Δ
D. Open-Δ
Correct Option: B
Explanation:
Δ–Δ connection can handle large unbalanced loads and provides a closed path for third harmonic currents. It ensures stable operation under fluctuating industrial load conditions.
Q6. A delta–delta connected transformer has the advantage of:
A. Neutral availability
B. Suppression of third harmonic currents
C. Lower insulation requirement
D. High voltage capability
Correct Option: B
Explanation:
Delta connection provides a closed path for third harmonic currents, preventing their appearance in line voltages and maintaining sinusoidal waveform.
Q7. If one transformer in a Δ–Δ bank fails, the remaining transformers can operate as:
A. Y–Y
B. Δ–Y
C. V–V
D. Y–Δ
Correct Option: C
Explanation:
When one unit fails in a Δ–Δ bank, the remaining two can continue supplying power in open-delta (V–V) connection, ensuring continuity of supply at reduced capacity.
Q8. In an open-delta (V–V) connection, the maximum power supplied compared to closed-delta is:
A. 50%
B. 57.7%
C. 66.7%
D. 86.6%
Correct Option: B
Explanation:
An open-delta bank can supply only 57.7% of the power of a closed-delta bank using the same transformers.
Q9. Two transformers connected in V–V supply a load of 60 kVA. The rating of each transformer is:
A. 30 kVA
B. 34.6 kVA
C. 50 kVA
D. 60 kVA
Correct Option: B
Explanation:
In V–V connection, transformer rating = Load / √3. Hence, each transformer rating = 60 / √3 ≈ 34.6 kVA.
Q10. The phase difference between primary and secondary line voltages in Y–Δ connection is:
A. 0°
B. 30°
C. 60°
D. 90°
Correct Option: B
Explanation:
Y–Δ connection introduces a 30° phase shift between primary and secondary line voltages due to vector group displacement.
Q11. Which connection is commonly used in step-down transformers for distribution?
A. Δ–Δ
B. Y–Y
C. Y–Δ
D. Δ–Y
Correct Option: D
Explanation:
Δ–Y connection is used in distribution transformers because delta primary traps harmonics and star secondary provides neutral for domestic loads.
Q12. In Δ–Y connection, the secondary phase voltage is:
A. Equal to line voltage
B. √3 times line voltage
C. 1/√3 times line voltage
D. Zero
Correct Option: C
Explanation:
In star-connected secondary, phase voltage equals line voltage divided by √3.
Q13. The main purpose of Scott (T–T) connection is to:
A. Reduce losses
B. Improve efficiency
C. Convert three-phase to two-phase supply
D. Reduce harmonics
Correct Option: C
Explanation:
Scott connection is specifically used to convert three-phase supply into two-phase supply while maintaining balanced voltages.
Q14. In Scott connection, the teaser transformer has a turns ratio of:
A. 1 : 1
B. 0.866 : 1
C. 1 : 0.866
D. √3 : 1
Correct Option: B
Explanation:
The teaser transformer has a turns ratio of 0.866:1 to ensure equal magnitude and balanced two-phase output voltages.
Q15. The angle between the two secondary voltages in Scott connection is:
A. 60°
B. 90°
C. 120°
D. 180°
Correct Option: B
Explanation:
Scott connection produces two secondary voltages displaced by 90°, forming a two-phase system.
Q16. Which transformer connection provides phase shift of 30° between primary and secondary?
A. Y–Y
B. Δ–Δ
C. Y–Δ and Δ–Y
D. Open-Δ
Correct Option: C
Explanation:
Only Y–Δ and Δ–Y connections introduce a 30° phase shift. Y–Y and Δ–Δ have zero displacement.
Q17. A three-phase transformer supplies balanced load. Which harmonic is eliminated automatically in delta winding?
A. Fundamental
B. Second
C. Third
D. Fifth
Correct Option: C
Explanation:
Delta winding provides a closed path for third harmonic currents, preventing them from appearing in line voltages.
Q18. Why is Y–Y connection rarely used without tertiary winding?
A. Low efficiency
B. Poor cooling
C. Third harmonic instability
D. High copper loss
Correct Option: C
Explanation:
Without a delta tertiary winding, Y–Y connection suffers from third harmonic voltage distortion and neutral instability.
Q19. If a closed-delta transformer supplies 300 kVA, the maximum load in V–V connection is:
A. 173 kVA
B. 200 kVA
C. 260 kVA
D. 300 kVA
Correct Option: A
Explanation:
Open-delta can supply only 57.7% of closed-delta rating. Hence, 0.577 × 300 ≈ 173 kVA.
Q20. Scott connection is preferred when:
A. Neutral is required
B. Balanced two-phase load is required
C. Voltage regulation is critical
D. Load is highly inductive
Correct Option: B
Explanation:
Scott connection is used when a balanced two-phase load is required from a three-phase supply system.