351. The primary and secondary of a power transformer are separated by
A) Air gap
B) Insulation
C) Core
D) Copper
Answer: B
352. The efficiency of a distribution transformer is generally
A) 95%
B) 96–98%
C) 98–99%
D) 70–80%
Answer: B
353. A welding transformer has
A) High current, low voltage
B) High voltage, low current
C) High frequency
D) Constant voltage
Answer: A
354. A furnace transformer is used for
A) Supplying low voltage, high current
B) High voltage, low current
C) Lighting
D) Distribution
Answer: A
355. The auto-transformer is most economical when
A) Voltage ratio is small
B) Voltage ratio is large
C) For high voltage only
D) For low voltage only
Answer: A
356. In a current transformer, the primary winding carries
A) Line current
B) Meter current
C) Secondary current
D) None
Answer: A
357. In a potential transformer, the primary winding carries
A) Line voltage
B) Line current
C) Load current
D) None
Answer: A
358. The insulation level of CT secondary is
A) Low
B) High
C) Medium
D) Equal to primary
Answer: A
359. The core in an instrument transformer is designed to operate near
A) Saturation
B) Below saturation
C) Above saturation
D) At half saturation
Answer: B
360. The exciting current of CT is
A) 1–5% of primary current
B) 10%
C) 25%
D) 50%
Answer: A
361. A step-up transformer increases
A) Voltage
B) Decreases current
C) Constant kVA
D) All of these
Answer: D
362. An isolation transformer is used for
A) Safety and noise reduction
B) Increasing current
C) Voltage regulation
D) Power factor correction
Answer: A
363. A three-winding transformer is used in
A) Substations
B) HVDC systems
C) Converters
D) All of these
Answer: D
364. The tertiary winding in three-winding transformer is used to
A) Provide stabilization
B) Supply auxiliary load
C) Eliminate unbalance
D) All of these
Answer: D
365. The advantage of using tap changer is
A) Voltage regulation under varying load
B) Change in frequency
C) Improved insulation
D) Reduce current
Answer: A
366. Off-load tap changer is operated
A) When transformer is de-energized
B) During load
C) During fault
D) Any time
Answer: A
367. On-load tap changer is used to
A) Adjust voltage without interrupting supply
B) Adjust current
C) Regulate frequency
D) Change polarity
Answer: A
368. The function of silica gel breather is
A) Absorb moisture from air
B) Filter oil
C) Cool oil
D) Indicate temperature
Answer: A
369. The color of dry silica gel is
A) Blue
B) Pink
C) White
D) Green
Answer: A
370. When silica gel becomes moist, it turns
A) Pink
B) Blue
C) Brown
D) White
Answer: A
371. The main function of conservator tank in transformer is
A) To allow oil expansion
B) To store oil
C) To prevent oxidation
D) All
Answer: A
372. Transformer oil mainly acts as
A) Coolant and insulator
B) Lubricant
C) Fuel
D) Cleaning agent
Answer: A
373. The flash point of good transformer oil is about
A) 140°C
B) 100°C
C) 80°C
D) 60°C
Answer: A
374. The breakdown voltage (BDV) of new transformer oil is typically
A) 30–40 kV
B) 60–80 kV
C) 10–20 kV
D) 5–10 kV
Answer: B
375. Buchholz relay protects transformer from
A) Internal faults
B) External short-circuits
C) Overload
D) Overvoltage
Answer: A
376. Buchholz relay is provided between
A) Main tank and conservator
B) LV and HV winding
C) Core and winding
D) None
Answer: A
377. The differential protection of transformer is based on
A) Difference between input and output currents
B) Voltage difference
C) Magnetic difference
D) Temperature difference
Answer: A
378. The differential relay operates under
A) Internal fault
B) Overload
C) External fault
D) None
Answer: A
379. The earth fault protection of transformer is done by
A) Restricted earth fault relay
B) Buchholz relay
C) Differential relay
D) None
Answer: A
380. The overfluxing in transformer is caused by
A) High voltage, low frequency
B) Low voltage, high frequency
C) High frequency
D) High load
Answer: A
381. The magnetizing inrush current occurs when
A) Transformer is energized
B) Transformer is on load
C) Transformer is open-circuited
D) None
Answer: A
382. The waveform of inrush current is
A) Highly distorted
B) Pure sine wave
C) Square wave
D) Triangular
Answer: A
383. Harmonics in transformer current cause
A) Overheating
B) Noise
C) Vibration
D) All of these
Answer: D
384. A Scott connection is used for
A) Converting 3-phase to 2-phase
B) Converting 2-phase to 3-phase
C) Both A and B
D) None
Answer: C
385. The open-delta connection efficiency compared to closed delta is
A) 57.7%
B) 66%
C) 75%
D) 50%
Answer: A
386. The tertiary winding is generally designed for
A) 1/3 of main rating
B) 1/4 of main rating
C) Equal to main
D) Double
Answer: A
387. The vector group Yd11 means
A) Star primary, delta secondary, 30° lag
B) Delta primary, star secondary, 30° lead
C) Star primary, delta secondary, 30° lead
D) None
Answer: C
388. Zig-zag connection is used for
A) Earthing and harmonic suppression
B) Voltage regulation
C) Frequency conversion
D) Load sharing
Answer: A
389. A furnace transformer is designed for
A) High current, low voltage
B) High voltage
C) High frequency
D) Constant voltage
Answer: A
390. In welding transformer, the secondary is
A) High current, low voltage
B) High voltage
C) Same as primary
D) None
Answer: A
391. In rectifier transformer, the secondary winding is
A) Center-tapped
B) Star
C) Delta
D) Zig-zag
Answer: A
392. The regulating transformer is used to
A) Control voltage
B) Control current
C) Step down voltage
D) Provide isolation
Answer: A
393. The kVA rating of furnace transformer is
A) Large
B) Small
C) Very small
D) Medium
Answer: A
394. The pulse transformer is used in
A) Electronic circuits
B) Power transmission
C) DC motor drives
D) Welding
Answer: A
395. In an instrument transformer, ratio error occurs due to
A) Magnetizing current
B) Leakage flux
C) Core loss
D) All
Answer: D
396. Phase angle error in CT or PT is caused by
A) Excitation current
B) Frequency
C) Load
D) Copper loss
Answer: A
397. The turns ratio of a CT is
A) Inversely proportional to current ratio
B) Directly proportional to voltage ratio
C) Same as PT
D) None
Answer: A
398. The burden on instrument transformer should be
A) Small
B) High
C) Variable
D) None
Answer: A
399. The knee point voltage of CT is
A) The point where core saturates
B) Minimum excitation
C) Rated voltage
D) None
Answer: A
400. The polarity of CT and PT is marked for
A) Correct connection in protection circuits
B) Aesthetic purpose
C) Voltage rating
D) Current limiting
Answer: A
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