EETC Transformers 5

Question 1

Transformer inrush current occurs when a transformer is first energized and can be as high as:

Accepted Answer

8 to 12 times the rated full-load current

Answer

Inrush current can peak at 8–12 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?

Accepted Answer

To detect internal faults by sensing gas accumulation and oil surge

Answer

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:

Accepted Answer

Links only one winding and does not contribute to energy transfer between windings

Answer

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?

Accepted Answer

Applied voltage (hi-pot) and impulse (BIL) tests

Answer

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?

Accepted Answer

104.2 A

Answer

I_secondary = kVA × 1000 / V_secondary = 25,000 / 240 = 104.2 A.

(EETC) Electrical Technician Certification Practice Test

(EETC) Electrical Technician Certification Practice Test

EETC Transformers 5