309A Transformers and Power Distribution 4 — Questions and Answers
Question 1: What is the standard voltage for single-phase residential service in Canada?
- 120 V only
- 120/240 V single-phase, three-wire (Correct answer)
- 120/208 V single-phase, three-wire
- 240 V only
Correct answer: 120/240 V single-phase, three-wire
Canadian residential service is 120/240 V single-phase, three-wire, provided by a centre-tapped transformer. This gives 120 V line-to-neutral for general loads and 240 V line-to-line for large appliances.
Question 2: What is the purpose of a surge arrester on a transformer?
- To regulate output voltage
- To protect the transformer from lightning strikes and switching surges (Correct answer)
- To improve power factor
- To prevent overloading
Correct answer: To protect the transformer from lightning strikes and switching surges
Surge arresters divert high-voltage transients (from lightning or switching operations) to ground, protecting the transformer's insulation from damage caused by voltage spikes.
Question 3: What is a K-rated transformer designed to handle?
- High ambient temperatures
- Harmonic loads from non-linear loads such as VFDs and computers (Correct answer)
- Extremely cold environments
- Frequent overload conditions
Correct answer: Harmonic loads from non-linear loads such as VFDs and computers
K-rated transformers are designed with additional capacity in their windings and core to handle the extra heating caused by harmonic currents from non-linear loads like variable frequency drives, computers, and LED lighting.
Question 4: In a three-phase, four-wire wye system at 208/120 V, what is the voltage between any two line conductors?
- 120 V
- 208 V (Correct answer)
- 240 V
- 480 V
Correct answer: 208 V
In a wye system, the line-to-line voltage is √3 times the phase (line-to-neutral) voltage: 120 × 1.732 = 208 V. This is the voltage available for three-phase loads.
Question 5: What is the primary difference between a dry-type and liquid-filled transformer?
- Dry-type transformers are more efficient
- Dry-type uses air for cooling while liquid-filled uses oil or other fluids (Correct answer)
- Liquid-filled transformers are smaller
- Dry-type transformers can handle higher voltages
Correct answer: Dry-type uses air for cooling while liquid-filled uses oil or other fluids
Dry-type transformers use air circulation for cooling and are typically used indoors, while liquid-filled transformers use oil or other dielectric fluids for better cooling and insulation in higher-power outdoor applications.
Question 6: What is the purpose of the Buchholz relay on an oil-filled transformer?
- To regulate oil temperature
- To detect gas accumulation indicating internal faults (Correct answer)
- To control the cooling fans
- To monitor oil level only
Correct answer: To detect gas accumulation indicating internal faults
The Buchholz relay detects gases produced by internal faults (arcing, overheating) in oil-filled transformers. Minor gas accumulation triggers an alarm; rapid gas flow triggers a trip to disconnect the transformer.
309A Transformers and Power Distribution 4
What is the standard voltage for single-phase residential service in Canada?