309A Transformers and Power Distribution 1 — Questions and Answers
Question 1: What is the primary function of a distribution transformer in a power system?
- To generate electrical power
- To step down transmission voltage to utilization voltage levels (Correct answer)
- To store electrical energy
- To convert AC to DC
Correct answer: To step down transmission voltage to utilization voltage levels
Distribution transformers step down the higher transmission and subtransmission voltages (e.g., 13.8 kV, 27.6 kV) to utilization voltages (e.g., 120/240 V, 347/600 V) for end-user consumption.
Question 2: What is the turns ratio of a transformer with 2400 primary turns and 240 secondary turns?
- 1:10
- 5:1
- 10:1 (Correct answer)
- 24:1
Correct answer: 10:1
Turns ratio = Primary turns / Secondary turns = 2400/240 = 10:1. This is a step-down transformer with secondary voltage being 1/10th of the primary voltage.
Question 3: What type of transformer connection provides both three-phase power and a neutral for single-phase loads?
- Delta-delta
- Delta-wye (Correct answer)
- Wye-delta
- Open delta
Correct answer: Delta-wye
A delta-wye transformer connection provides a neutral point on the secondary wye winding, allowing both three-phase loads and single-phase line-to-neutral loads to be served from the same transformer bank.
Question 4: What is the purpose of transformer oil in a liquid-filled transformer?
- To lubricate the windings
- To provide insulation and cooling (Correct answer)
- To increase the turns ratio
- To filter harmonics
Correct answer: To provide insulation and cooling
Transformer oil serves dual purposes: it provides electrical insulation between windings and the tank, and it acts as a cooling medium by circulating through the transformer and dissipating heat.
Question 5: A 75 kVA, 480/120 V single-phase transformer has what full-load secondary current?
- 156.25 A
- 312.5 A
- 625 A (Correct answer)
- 937.5 A
Correct answer: 625 A
Secondary current = kVA × 1000 / V secondary = 75,000 / 120 = 625 A. The lower the secondary voltage, the higher the current for the same kVA rating.
Question 6: What is the purpose of the tap changer on a transformer?
- To change the frequency of the output
- To adjust the output voltage by changing the effective turns ratio (Correct answer)
- To switch between single-phase and three-phase operation
- To reverse the polarity of the output
Correct answer: To adjust the output voltage by changing the effective turns ratio
Tap changers adjust the effective turns ratio by connecting to different winding taps, allowing the output voltage to be fine-tuned to compensate for supply voltage variations or line voltage drops.
309A Transformers and Power Distribution 1
What is the primary function of a distribution transformer in a power system?