EETC Transformers 5 ā Questions and Answers
Question 1: Transformer inrush current occurs when a transformer is first energized and can be as high as:
- 8 to 12 times the rated full-load current (Correct answer)
- 2 to 3 times the rated full-load current
- 150% of rated current
- 50% above the short-circuit current
Correct answer: 8 to 12 times the rated full-load current
Inrush current can peak at 8ā12 times rated current due to residual flux in the core and the point on the voltage wave at which the transformer is energized.
Question 2: What is the purpose of a Buchholz relay installed on an oil-filled transformer?
- To detect internal faults by sensing gas accumulation and oil surge (Correct answer)
- To regulate the cooling oil flow rate
- To monitor winding temperature and trip on overtemperature
- To protect against overvoltage on the primary winding
Correct answer: To detect internal faults by sensing gas accumulation and oil surge
A Buchholz relay detects gases produced by incipient internal faults (arcing, overheating) and activates an alarm or trip signal.
Question 3: In a transformer, leakage flux refers to flux that:
- Links only one winding and does not contribute to energy transfer between windings (Correct answer)
- Escapes through the tank walls and causes stray losses
- Links both primary and secondary windings simultaneously
- Is absorbed by the core laminations
Correct answer: Links only one winding and does not contribute to energy transfer between windings
Leakage flux follows a path through air rather than the core, linking only its own winding and producing leakage reactance rather than mutual coupling.
Question 4: Which standard test is used to verify that transformer windings can withstand overvoltages from lightning or switching surges?
- Applied voltage (hi-pot) and impulse (BIL) tests (Correct answer)
- Short-circuit thermal test
- Temperature rise test
- Insulation resistance megger test
Correct answer: Applied voltage (hi-pot) and impulse (BIL) tests
The impulse test applies a high-voltage wave simulating lightning or switching surges to verify the Basic Insulation Level (BIL), while the hi-pot test checks turn-to-turn and winding-to-ground insulation.
Question 5: A distribution transformer is rated 25 kVA, 7200V/240V. What is the full-load secondary current?
- 104.2 A (Correct answer)
- 3.47 A
- 208.3 A
- 52.1 A
Correct answer: 104.2 A
I_secondary = kVA Ć 1000 / V_secondary = 25,000 / 240 = 104.2 A.
Question 6: The dielectric strength of transformer insulating oil is tested primarily to determine:
- Its ability to withstand electric stress without breaking down (Correct answer)
- Its viscosity at operating temperature
- The presence of carbon deposits from overloading
- Its moisture content by weight percentage
Correct answer: Its ability to withstand electric stress without breaking down
Dielectric strength testing applies increasing voltage between electrodes submerged in the oil to find the breakdown voltage, indicating insulation quality and moisture contamination.
Question 7: When a transformer is subjected to a sudden short circuit on the secondary, the mechanical forces on the windings are proportional to:
- The square of the fault current (Correct answer)
- The fault current linearly
- The rated kVA only
- The turns ratio squared
Correct answer: The square of the fault current
Magnetic forces between conductors follow F ā IĀ², so short-circuit forces can be hundreds of times greater than normal operating forces.
EETC Transformers 5
Transformer inrush current occurs when a transformer is first energized and can be as high as: