309A Transformers and Power Distribution 3

Question 1

What is the purpose of a potential transformer (PT) in a power distribution system?

Accepted Answer

To provide a scaled-down voltage signal for metering and protection relays

Answer

Potential transformers step down high system voltages to a standard secondary voltage (typically 120 V) for safe connection to voltage meters, watt-hour meters, and protective relays.

Question 2

What causes transformer humming noise during operation?

Accepted Answer

Magnetostriction — the expansion and contraction of the core laminations

Answer

Transformer hum is caused by magnetostriction — the physical expansion and contraction of the iron core laminations as the magnetic field alternates at 60 Hz, producing a 120 Hz mechanical vibration.

Question 3

What is the primary purpose of laminating a transformer core?

Accepted Answer

To reduce eddy current losses

Answer

Laminating the core into thin, insulated sheets reduces eddy current losses by limiting the paths available for circulating currents induced in the core by the alternating magnetic flux.

Question 4

In a three-phase transformer bank, what happens if one transformer fails in a delta-delta configuration?

Accepted Answer

It can operate as an open delta at reduced capacity (57.7%)

Answer

An open delta (V-V) connection uses only two transformers and can deliver approximately 57.7% (1/√3) of the original delta-delta bank capacity, providing emergency three-phase power.

Question 5

What is the voltage regulation of a transformer?

Accepted Answer

The percentage change in secondary voltage from no-load to full-load

Answer

Voltage regulation = ((VNL - VFL) / VFL) × 100%, where VNL is the no-load secondary voltage and VFL is the full-load secondary voltage. Lower regulation percentage indicates better performance.

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Transformers and Power Distribution 3