Transformers and Power Distribution Flashcards

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

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.

What is the turns ratio of a transformer with 2400 primary turns and 240 secondary turns?

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.

What type of transformer connection provides both three-phase power and a neutral for single-phase loads?

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.

What is the purpose of transformer oil in a liquid-filled transformer?

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.

A 75 kVA, 480/120 V single-phase transformer has what full-load secondary current?

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.

What is the purpose of the tap changer on a transformer?

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.