Q1. Two 3-phase transformers are operated in parallel. Which condition is MOST critical to avoid circulating current?
A. Same kVA rating
B. Same impedance magnitude
C. Same voltage ratio and polarity
D. Same efficiency
Correct Option: C
Explanation:
If voltage ratio or polarity differs, induced secondary EMFs become unequal, causing circulating current even at no-load. This leads to overheating and extra copper loss, making this the most critical condition.
Q2. Two transformers with equal voltage ratio but different percentage impedances are connected in parallel. Load sharing depends mainly on:
A. Voltage ratio
B. Power factor
C. Percentage impedance
D. Core losses
Correct Option: C
Explanation:
With equal voltage ratio, circulating current is avoided. Load sharing is determined by percentage impedance; the transformer with lower %Z carries more load.
Q3. Two 3-phase transformers of 1000 kVA and 500 kVA have % impedances of 5% and 10% respectively. Total load is 900 kVA. Load carried by 1000 kVA transformer is:
A. 720 kVA
B. 450 kVA
C. 600 kVA
D. 750 kVA
Correct Option: A
Explanation:
Load sharing ∝ kVA / %Z.
For 1000 kVA: 1000/5 = 200
For 500 kVA: 500/10 = 50
Total = 250 → Share = (200/250) × 900 ≈ 720 kVA.
Q4. Which vector group combination is NOT suitable for parallel operation?
A. Yd11 with Yd11
B. Dyn11 with Dyn11
C. Dyn11 with Yd11
D. Yy0 with Yy0
Correct Option: C
Explanation:
Dyn11 and Yd11 have different phase displacement. Phase mismatch causes heavy circulating currents, making parallel operation unsafe.
Q5. Circulating current between parallel transformers occurs mainly due to:
A. Load current
B. Difference in efficiency
C. Difference in induced emf
D. Core losses
Correct Option: C
Explanation:
Circulating current is caused by unequal induced EMFs due to mismatch in voltage ratio, polarity, or phase angle, even at no-load.
Q6. If two transformers connected in parallel have unequal X/R ratios, the result is:
A. Unequal kVA sharing
B. Unequal kW sharing
C. Circulating reactive current
D. All of the above
Correct Option: D
Explanation:
Unequal X/R ratios cause unequal real and reactive power sharing and circulating reactive currents, affecting overall performance.
Q7. For ideal load sharing in parallel transformers, the condition is:
A. Equal impedance magnitude
B. Equal impedance angle
C. Impedance inversely proportional to kVA
D. Same copper loss
Correct Option: C
Explanation:
To ensure proportional load sharing, transformer impedance must be inversely proportional to its kVA rating.
Q8. Parallel operation of transformers increases:
A. Voltage regulation
B. Reliability of supply
C. Iron losses
D. No-load current
Correct Option: B
Explanation:
Parallel operation improves reliability and continuity of supply, allowing load sharing and uninterrupted service during maintenance.
Q9. Instrument transformers are used mainly to:
A. Reduce power loss
B. Improve efficiency
C. Extend measuring instrument range
D. Reduce harmonics
Correct Option: C
Explanation:
Instrument transformers allow high voltage/current to be measured safely using standard low-range instruments.
Q10. Which instrument transformer provides electrical isolation between high-voltage system and measuring instruments?
A. Auto-transformer
B. Current transformer
C. Potential transformer
D. Both CT and PT
Correct Option: D
Explanation:
Both current transformers (CT) and potential transformers (PT) provide electrical isolation and safety.
Q11. Burden of an instrument transformer is expressed in:
A. Ohms
B. Watts
C. VA at rated secondary
D. kVAR
Correct Option: C
Explanation:
Burden is the total volt-ampere (VA) load connected to the secondary at rated conditions.
Q12. A current transformer secondary should never be open-circuited because:
A. Copper loss increases
B. Core saturates causing overheating
C. Voltage becomes dangerously high
D. Accuracy improves
Correct Option: C
Explanation:
Open-circuiting CT secondary produces very high voltage, leading to insulation failure and serious safety hazards.
Q13. CT ratio is defined as:
A. Primary current / Secondary current
B. Secondary current / Primary current
C. Primary voltage / Secondary voltage
D. Turns ratio
Correct Option: A
Explanation:
CT ratio is the ratio of primary current to secondary current under rated conditions.
Q14. A 1000/5 A CT supplies a burden of 10 VA. Secondary current is 5 A. Secondary voltage is:
A. 1 V
B. 2 V
C. 5 V
D. 10 V
Correct Option: B
Explanation:
Burden = Vs × Is → Vs = 10 / 5 = 2 V.
Q15. Which error in CT depends on magnetizing current?
A. Ratio error
B. Phase angle error
C. Both A and B
D. Neither A nor B
Correct Option: C
Explanation:
Magnetizing current causes both ratio error and phase angle error in current transformers.
Q16. For protection CTs, the desirable feature is:
A. High accuracy at rated current
B. Low core saturation
C. High VA burden
D. Low knee-point voltage
Correct Option: B
Explanation:
Protection CTs must not saturate during faults; hence low saturation and high knee-point voltage are essential.
Q17. A potential transformer secondary is normally:
A. Short-circuited
B. Open-circuited
C. Connected to high burden
D. Connected to relay only
Correct Option: B
Explanation:
PT secondary operates close to no-load condition and is almost open-circuited due to very small burden.
Q18. Which error in PT is caused by magnetizing current?
A. Ratio error
B. Phase angle error
C. Both A and B
D. No error
Correct Option: C
Explanation:
Magnetizing current in PT causes both ratio error and phase angle error.
Q19. Why PT secondary is grounded?
A. Reduce losses
B. Improve accuracy
C. Operator safety
D. Reduce burden
Correct Option: C
Explanation:
Grounding PT secondary ensures operator safety by preventing dangerous voltages during insulation failure.
Q20. Compared to CT, PT operates closer to:
A. Short-circuit condition
B. Open-circuit condition
C. Saturation region
D. Knee-point region
Correct Option: B
Explanation:
PT operates near open-circuit condition, whereas CT operates near short-circuit condition.