309A Transformers and Power Distribution 5

Question 1

What is the purpose of paralleling transformers in a distribution system?

Accepted Answer

To increase capacity and provide redundancy

Answer

Paralleling transformers increases the total available capacity of the system and provides redundancy — if one transformer fails, the other(s) can continue to supply the load at reduced capacity.

Question 2

What conditions must be met to parallel two transformers successfully?

Accepted Answer

Same voltage ratio, same impedance percentage, same polarity, and same phase relationship

Answer

Successful paralleling requires matching voltage ratios, impedance percentages (within 7.5%), polarity (for single-phase), and phase relationship (for three-phase) to prevent circulating currents.

Question 3

What is the total kVA of a three-phase transformer bank using three single-phase 50 kVA transformers?

Accepted Answer

150 kVA

Answer

The total three-phase kVA of a transformer bank equals the sum of the individual transformer ratings: 3 × 50 = 150 kVA.

Question 4

What test is performed on a transformer to determine its core losses (iron losses)?

Accepted Answer

Open-circuit test

Answer

The open-circuit (no-load) test is performed with the secondary open and rated voltage applied to the primary. The measured power represents core losses (hysteresis and eddy current losses).

Question 5

What is the percentage impedance of a transformer and why is it important?

Accepted Answer

The percentage of rated voltage needed to circulate rated current through a shorted secondary; determines fault current levels

Answer

Percentage impedance (%Z) is the percentage of rated primary voltage required to circulate rated full-load current through a short-circuited secondary. It determines maximum available fault current: Ifault = IFLA / (%Z/100).

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Transformers and Power Distribution 5