309A Transformers and Power Distribution

Question 1

What is the primary function of a distribution transformer in a power system?

Accepted Answer

To step down transmission voltage to utilization voltage levels

Answer

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?

Accepted Answer

10:1

Answer

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?

Accepted Answer

Delta-wye

Answer

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?

Accepted Answer

To provide insulation and cooling

Answer

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?

Accepted Answer

625 A

Answer

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.

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Transformers and Power Distribution